Scottish & Southern Energy Endowed Research Fellowship at ...€¦ · particular interest to the...

DISPOWER Distributed Generation with high penetration of renewable energy sources

ENK5-CT2001-00522

Title: Report describing and comparing opportunities and effectiveness of

generator control, load management, and additional storage and power electronic controller options for managing LV networks with distributed generation

Number: Deliverable D1.8 WP1-USTRAT-001 Date: 5th January, 2004 Authors: Paul Espie Company: University of Strathclyde

Colin Foote 204 George Street Graeme Burt Glasgow, UK Irena Wasiak Technical University of Lodz Ryszard Pawelek Rozmysław Mienski

Approved by: Joseph Mutale Company: University of Manchester Institute of Science and Technology

Distribution list

Status European Commission A I : for Information Project Participants DISPOWER I C : for Comment A : for Approval

Executive Summary This report has investigated the effectiveness of the various options for improving power quality in low voltage distribution networks. This assessment is of great value and relevance as the installation of small-scale new and renewable energy sources accelerates across Europe, while consumers continue to demand ever-higher standards of power quality. Definitions are first provided for power quality phenomena to develop a consistent foundation on which to base this work. The defined power quality phenomena are then prioritised to focus on the phenomena of particular interest to the DISPOWER project as well as the wider electricity industry. Voltage dips and swells, under- and over-voltages, and harmonic distortion were identified as the power quality phenomena of highest priority. The options considered within this report include distributed generation, energy storage devices, as well as traditional and novel power quality control devices. These options were evaluated in terms of their potential capability to mitigate the identified power quality phenomena. Technical studies were performed to validate the capabilities of the various options. The highest priority power quality phenomena were studied to determine what was required from a generator, energy storage device or control device to mitigate different disturbances. It was found that some disturbances could not be mitigated effectively if the magnitude of the disturbance was too large, but that smaller disturbances could be effectively addressed. Detailed study of pulse width modulated (PWM) inverters was performed to assess their potential role in improving power quality. An overall rating was derived for each of the identified options for improving power quality in low voltage grids. The overall ratings reflect the evaluation of technologies and the technical studies performed. It was found that the most promising technologies for improving power quality in low voltage grids are inverter-connected devices with controllable generators or flywheels. Amongst the other options, magnetic synthesisers were identified as having the greatest potential value. In reaching conclusions on the overall effectiveness of the various options at improving power quality, other factors that influence effectiveness were considered, including the impact on the local network, regulatory issues in different countries, and the future development of the technologies concerned.

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Table of Contents EXECUTIVE SUMMARY .............................................................................................................................................. 2 1. INTRODUCTION.................................................................................................................................................... 5

1.1 IMPROVING POWER QUALITY IN LV GRIDS............................................................................................................ 5 1.2 METHODOLOGY...................................................................................................................................................... 5 1.3 ACKNOWLEDGEMENTS........................................................................................................................................... 6

2. OVERVIEW OF POWER QUALITY PHENOMENA........................................................................................ 6 2.1 DEFINITIONS........................................................................................................................................................... 6 2.2 PRIORITIES ............................................................................................................................................................. 8

2.2.1 Assessment Criteria ...................................................................................................................................... 8 2.2.2 Criteria Ratings Adopted .............................................................................................................................. 8 2.2.3 Summary of Overall Priority Values............................................................................................................. 9

2.3 KEY OUTCOMES ................................................................................................................................................... 10 3. INVENTORY OF POWER QUALITY MANAGEMENT OPTIONS ............................................................. 11

3.1 POWER QUALITY PHENOMENA............................................................................................................................. 11 3.2 LOCAL GENERATION ............................................................................................................................................ 12

3.2.1 Summary of Local Generation Capability .................................................................................................. 12 3.2.2 Supporting Information for Local Generation Management Options......................................................... 12

3.2.2.1 Synchronous Machine .........................................................................................................................................12 3.2.2.2 Induction Machine ...............................................................................................................................................13 3.2.2.3 Doubly-Fed Induction Machine...........................................................................................................................13 3.2.2.4 PWM Inverter ......................................................................................................................................................14

3.3 ENERGY STORAGE................................................................................................................................................ 14 3.3.1 Summary of Energy Storage Capability...................................................................................................... 14 3.3.2 Supporting Information for Energy Storage Options.................................................................................. 15

3.3.2.1 Battery .................................................................................................................................................................15 3.3.2.2 Flywheel ..............................................................................................................................................................15 3.3.2.3 Micro-SMES........................................................................................................................................................15 3.3.2.4 Supercapacitor .....................................................................................................................................................16

3.4 TRADITIONAL CONTROLLERS............................................................................................................................... 16 3.4.1 Summary Information for Traditional Controllers ..................................................................................... 16 3.4.2 Supporting Information for Traditional Controller Options....................................................................... 16

3.4.2.1 Passive Filter........................................................................................................................................................17 3.4.2.2 Capacitor Banks...................................................................................................................................................17 3.4.2.3 Ferro-resonant Transformers ...............................................................................................................................17 3.4.2.4 Electronic Step Regulators...................................................................................................................................17 3.4.2.5 Static Var Compensator .......................................................................................................................................18 3.4.2.6 Saturable Reactors ...............................................................................................................................................19 3.4.2.7 Network Switching ..............................................................................................................................................19

3.5 NOVEL CONTROLLERS.......................................................................................................................................... 19 3.5.1 Summary of Novel Controller Capability ................................................................................................... 19 3.5.2 Supporting Information for Novel Controller Options ............................................................................... 20

3.5.2.1 Dynamic Voltage Restorer...................................................................................................................................20 3.5.2.2 Distribution STATCOM......................................................................................................................................20 3.5.2.3 Magnetic Synthesizer...........................................................................................................................................21 3.5.2.4 Shunt Active Power Filter....................................................................................................................................22 3.5.2.5 Series Active Power Filter ...................................................................................................................................23 3.5.2.6 Shunt / Series Conditioners..................................................................................................................................23

3.6 LOAD MANAGEMENT SCHEMES ........................................................................................................................... 24 3.6.1 Overview of Load Management Scheme Capability ................................................................................... 24 3.6.2 Additional Information for Load Management Schemes ............................................................................ 24

3.6.2.1 Interruptible Load ................................................................................................................................................24 3.6.2.2 Communication Schemes ....................................................................................................................................24 3.6.2.3 Incentive Based Schemes.....................................................................................................................................25 3.6.2.4 Load Aggregation Scheme...................................................................................................................................25 3.6.2.5 Energy Storage ....................................................................................................................................................25

3.7 KEY OUTCOMES ................................................................................................................................................... 25 4. ASSESSMENT BASED ON POWER QUALITY PHENOMENA ................................................................... 25

4.1 AIM OF STUDIES & ASSESSMENT METHODOLOGY ............................................................................................... 26 4.2 PRIORITISATION OF POWER QUALITY PHENOMENA.............................................................................................. 26 4.3 DEVELOPMENT OF ANALYTICAL STUDIES AND TEST SCENARIOS......................................................................... 27

4.3.1 Voltage Dips ............................................................................................................................................... 27 4.3.2 Under-Voltages........................................................................................................................................... 28 4.3.3 Voltage Swells............................................................................................................................................. 28

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4.3.4 Over-voltages.............................................................................................................................................. 30 4.3.5 Harmonic Distortion................................................................................................................................... 30

4.4 SELECTION OF ANALYTICAL SOFTWARE AND TEST NETWORK ............................................................................ 30 4.5 RESULTS FROM STUDIES....................................................................................................................................... 32

4.5.1 Voltage Dips and Under-Voltage................................................................................................................ 32 4.5.2 Voltage Swells and Over-Voltage ............................................................................................................... 34 4.5.3 Harmonic Current Distortion ..................................................................................................................... 35

4.6 UK AND EUROPEAN VOLTAGE LIMITS................................................................................................................. 36 4.7 OBSERVATIONS AND CONCLUSIONS ..................................................................................................................... 36 4.8 KEY OUTCOMES ................................................................................................................................................... 37

5. ASSESSMENT OF SPECIFIC MANAGEMENT OPTIONS ........................................................................... 37 5.1 AIM OF STUDIES ................................................................................................................................................... 37 5.2 DESCRIPTION OF STUDY NETWORK...................................................................................................................... 38 5.3 NETWORK MODELLING ........................................................................................................................................ 38

5.3.1 Lines and Transformers .............................................................................................................................. 39 5.3.2 Loads .......................................................................................................................................................... 39 5.3.3 Power Quality Management Options.......................................................................................................... 39

5.4 INVERTER CONTROL............................................................................................................................................. 39 5.4.1 Current Control Mode ................................................................................................................................ 40 5.4.2 Voltage Control Mode ................................................................................................................................ 41

5.5 STUDIES PERFORMED ........................................................................................................................................... 42 5.5.1 Load Compensation .................................................................................................................................... 42

5.5.1.1 Disturbing Loads .................................................................................................................................................43 5.5.1.2 Local Compensation for Harmonics and Asymmetry ..........................................................................................43 5.5.1.3 Local Compensation for Reactive Power and Reactive Power Control ...............................................................43 5.5.1.4 Local Compensation for Active Power and Active Power Control......................................................................44 5.5.1.5 Distant Compensation..........................................................................................................................................44 5.5.1.6 Comparison of Simulations .................................................................................................................................44

5.5.2 Dip Compensation ...................................................................................................................................... 45 5.5.2.1 Symmetrical Short-Circuits .................................................................................................................................45 5.5.2.2 Asymmetrical Short-Circuits ...............................................................................................................................48 5.5.2.3 Summary of Results.............................................................................................................................................50

5.6 OBSERVATION AND CONCLUSIONS....................................................................................................................... 51 6. SUMMARY OF OVERALL MANAGEMENT OPTION EFFECTIVENESS................................................ 52

6.1 OVERALL EFFECTIVENESS RATING SYSTEM......................................................................................................... 52 6.2 EXPLANATION OF MANAGEMENT OPTION RATINGS............................................................................................. 53

6.2.1 Grid Connection ......................................................................................................................................... 53 6.2.1.1 Rotating Machines ...............................................................................................................................................53 6.2.1.2 PWM Inverter ......................................................................................................................................................54

6.2.2 Energy Source............................................................................................................................................. 54 6.2.2.1 Controllable DG ..................................................................................................................................................54 6.2.2.2 Stochastic DG......................................................................................................................................................54 6.2.2.3 Battery .................................................................................................................................................................54 6.2.2.4 Flywheel ..............................................................................................................................................................55 6.2.2.5 Micro-SMES........................................................................................................................................................55 6.2.2.6 Supercapacitor .....................................................................................................................................................55

6.2.3 Other Options ............................................................................................................................................. 55 6.2.3.1 Dynamic Voltage Restorer...................................................................................................................................55 6.2.3.2 Distribution STATCOM......................................................................................................................................55 6.2.3.3 Magnetic Synthesizer...........................................................................................................................................55 6.2.3.4 Active Power Filter..............................................................................................................................................56 6.2.3.5 Load Management Schemes ................................................................................................................................56

6.3 SUPPORTING INFORMATION.................................................................................................................................. 56 6.3.1 Network Effects ........................................................................................................................................... 56 6.3.2 Regulatory Issues........................................................................................................................................ 57 6.3.3 Future Effectiveness.................................................................................................................................... 58

6.4 KEY OUTCOMES ................................................................................................................................................... 59 7. CONTRIBUTION FROM AND APPLICATION TO OTHER DISPOWER WORK PACKAGES............. 59

7.1 WP 2.6 – CONTRIBUTION TO GRID QUALITY IMPROVEMENT BY DECENTRALISED INVERTERS............................ 59 7.2 WP 2.7.2 – INVENTORY OF DISTRIBUTED GENERATION TECHNOLOGIES ............................................................. 59 7.3 WP 9 – DEVELOPMENT OF A POWER OPERATION AND POWER QUALITY MANAGEMENT SYSTEM (POMS) IN LOW VOLTAGE GRIDS ........................................................................................................................................................... 59

8. CONCLUSIONS AND SUMMARY .................................................................................................................... 60 9. REFERENCES....................................................................................................................................................... 62 APPENDIX...................................................................................................................................................................... 63

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1. Introduction

1.1 Improving Power Quality in LV Grids In the last decade there has been significant interest in the quality of power delivered through electricity supply systems as consumers become increasingly reliant on the use of sensitive electronic equipment. Although these disturbances have always existed to some degree within electricity supply systems they have traditionally been designed out of the system, something that was fairly ease to do before the advent and widespread use of power electronic equipment. If a customer wished to obtain a higher quality of power than that delivered by the supply system then very often they were forced to find a solution themselves (e.g. install back-up generator, ups system, etc). However, the significant expansion and usage of equipment containing power electronics in today's supply systems is rapidly making the task of designing out these system disturbances a problem of ever increasing difficulty. This has led to development of equipment to actively manage power quality phenomena. Such equipment has been designed for autonomous use at a particular load but the idea of utilising these resources for the wider mitigation of power quality phenomena at a network level with an active management system has also been explored. Such active management systems have been the subject of previous research efforts for medium and high voltages [1,2] and have shown considerable potential in managing power quality events at a network level. Following on from this research, task 1.8 takes this concept one step further to explore the possibilities of utilising distributed generation, energy storage, load management schemes and novel and traditional power quality devices to improve low voltage grid power quality. These resources, termed 'management options' throughout this report, could, as already mentioned, either be controlled centrally to provide an active management capability or could be given particular instructions to act in an autonomous manner in the event of power quality problems. However, before such improvements in LV grid power quality can be realised there is a clear need to assess the effectiveness of these different management options and identify their corresponding power quality capabilities. This is the objective of this report, and describes the work that has been performed within task 1.8 "Improving Power Quality in Low Voltage Grids" within DISPOWER Work Package 1. The results, observations and conclusions obtained from this assessment also provides a crucial input to other tasks within DISPOWER work packages, notably WP 8.4 (Transient Analysis) and WP 9.6 (PoMS Reaction Strategies) by providing a validated indication of the capabilities of particular technologies.

1.2 Methodology The primary aim of task 1.8 as stated above was to describe the opportunities for and the effectiveness of a range of options for improving LV grid power quality. To this end it was necessary to perform a number of tasks which collectively provide the means and opportunity to achieve this end-level goal. The sub-tasks implemented were:

• Develop definitions for commonly used power quality phenomena – described in section 2. • Consider the priorities of power quality phenomena to identify those of greatest concern to

utilities and consumers alike – also described in section 2. • Establish an inventory describing the capabilities of the considered options for power quality

management in low voltage distribution networks – presented in section 3. • Quantify the effectiveness capability of key management options through analytical assessment

studies – described in sections 4 and 5. • Collate technical study results and generate overall effectiveness ratings for key management

options – section 6.

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• Consider links and usage of this work with other work packages in the DISPOWER project – section 7.

• Conclusions relating to this work of improving power quality in low voltage grids are presented in section 8.

• References are presented in section 9, and a range of figures relevant to one set of studies presented in section 5 are shown in the appendix.

1.3 Acknowledgements This report has been created based on work performed by the University of Strathclyde and the Technical University of Lodz along with contributions from the Centro Elettrotecnico Sperimentale Italiano, Fraunhofer Institute for Solar Energy Systems, MVV Energie, Stadtwerke Karlsruhe and the Universita di Genova.

2. Overview of Power Quality Phenomena

2.1 Definitions The term "power quality" has been applied extensively as a high-level category to group a wide variety of disturbances that can occur within an electricity supply system. Although, specific definitions have been developed to describe each of these phenomena, in many cases the terminology adopted has been inconsistent. Consequently, there is a clear need, as a first step, to develop a clear definition for these power quality phenomena to ensure the reference to, and utilisation of a standard set of power quality terminology and definitions. Table 1 outlines the phenomena that can be considered as power quality disturbances and also describes how each phenomenon can be characterised and measured.

PQ Phenomenon

Description Characterisation Measures

Voltage Transient A ‘transient’ is some event that is momentary but undesirable in nature. The two types of transients that occur are impulsive and oscillatory.

Impulsive transients are characterised according to rise and decay times. For example, a 1.2 * 50-µs 2000V impulsive transient nominally rises from zero to 2000V in 1.2µs then decays to half its peak in 50µs. Oscillatory transients are characterised according to frequency content: High – primary frequency >500kHz. Medium – primary frequency >5kHz & <500kHz. Low – primary frequency <5kHz

Rise and decay time need to be measured for impulsive transients and frequency content measured for oscillatory transients.

Voltage Waveform Distortion

Waveform distortion is defined as a steady-state deviation from an ideal sine wave of power frequency.

Principally characterised by the spectral content of the deviation. Harmonics - these are sinusoidal voltages which have frequencies that are integer multiples of the frequency of the supply system. Interharmonics - voltages that have frequency components that are not integer multiples of the supply system frequency. Notching - this is a periodic voltage disturbance that occurs continuously and can be characterised through the harmonic spectrum of the affected voltage. However, it is generally treated as a special case as it cannot be characterised using measurement equipment normally used for harmonic analysis. DC offset - the presence of a DC voltage within an AC power system. Noise – any unwanted electrical signals with a broadband spectral content <200kHz superimposed upon the power system voltage.

A commonly used index for quantifying the impact of harmonic distortion is Total Harmonic Distortion (THD). Harmonic waveform distortion can also be quantified by examining the individual harmonic components within a waveform.

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Short-Duration Voltage Variation

A short-duration voltage variation describes a variation in the supply voltage (dip, swell or interruption) for a time period <1 min.

Characterised according to resultant p.u. voltage and duration. Dip - a decrease in the rms voltage generally to between 0.1 and 0.9 pu, at the power frequency for durations from 0.5 cycles to 1 min. Swell - an increase to between 1.1 and 1.8 pu in the rms voltage at the power frequency for durations from 0.5 cycles to 1 min. Interruptions - an interruption occurs when the supply voltage decreases to less than 0.1 pu for a period of time not exceeding 1 min.

To quantify the impact of short-duration voltage variations two indices must be measured, the duration of the voltage variation and the resultant p.u. voltage.

Long-Duration Voltage Variation

A voltage variation is considered to be long-duration when the variation is in excess of one minute.

Characterised according to resultant p.u. voltage and duration. Undervoltage - a decrease in the rms. AC voltage to less than 0.9 p.u. at the power frequency for a duration in excess of one minute. Overvoltage - an increase in the rms. AC voltage greater than 1.1 p.u. at the power frequency for a duration in excess of one minute. Sustained Interruption - a supply interruption occurs when the supply voltage has been at zero for a period in excess of one minute.

To quantify the impact of long-duration voltage variations two indices must be measured, the duration of the voltage variation and the resultant p.u. voltage.

Voltage Fluctuations

Systematic variations of the voltage envelope or a series of random voltage changes with a magnitude not normally outwith the range 0.9 to 1.1 pu can be described as voltage fluctuations. Loads which experience voltage fluctuations can cause voltage variations that are often referred to as ‘flicker’.

Characterised by voltage variation magnitude and frequency of occurrence

The severity of voltage flicker is commonly measured by two parameters, the short-term severity index, Pst, and a long-term severity index, Plt.

Voltage Phase Balance

Three-phase systems are designed to operate at maximum efficiency when the load on each phase is balanced. However, operating a three phase system with 100% phase balance is only theoretically possible. Consequently some level of voltage phase unbalance will usually exist.

Characterised by deviation from 100% voltage phase balance.

Frequency Variations

Any deviation from the nominal power system frequency can be defined as a power frequency variation.

Characterised by deviation from nominal power system frequency.

Power system frequency variations are measured by the determining the deviation from the nominal system frequency. The nominal power system frequency can be considered as the mean value of the fundamental frequency measured over a ten second period.

Current Transient Definition as for voltage transient. Definition as for voltage transient. Rise and decay time need to be measured for impulsive transients and frequency content measured for oscillatory transients.

Current Waveform Distortion

Definition as for voltage waveform distortion.

Principally characterised by the spectral content of the deviation. Harmonics - these are sinusoidal currents which have frequencies that are integer multiples of the frequency of the supply system. Interharmonics - currents that have frequency components that are not integer multiples of the supply system frequency. DC offset - the presence of a DC current within an AC power system. Noise - any unwanted electrical signals with a broadband spectral content <200kHz superimposed upon the power system current.

Although harmonic current distortion can be measured by a THD value this can produce misleading results. Consequently, a Total Demand Distortion (TDD) value is commonly calculated for harmonic currents. TDD is similar to THD except that the distortion is expressed as a percentage of rated load current.

Current Phase Balance

Three-phase systems operate at maximum efficiency when the load on each phase is balanced. However, operating a three phase system with 100% current balance is only theoretically possible. Some level of current phase unbalance will usually exist.

Characterised by deviation from 100% current phase balance. The current unbalance between phases can be calculated by dividing the maximum deviation from the average of the three-phase currents, by the average of the three-phase currents, expressed as a percentage.

Table 1: Summary of Power Quality Phenomena

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2.2 Priorities Having identified and defined the power quality phenomena of interest it was essential to identify which phenomena were of greatest concern to utilities and customers. This would ensure that the analytical studies that were subsequently performed to quantify the effectiveness of particular management options focussed on these particular high priority phenomena.

2.2.1 Assessment Criteria To derive priority values for the power quality phenomena outlined in section 2.1 a number of assessment criteria were considered. These were:

1. Frequency of Occurrence – the frequency of occurrence criterion1 was considered as some phenomena occur with greater frequency than the others.

2. Impact – the impact criterion represents the typical economic and technical impact associated with the occurrence of each phenomenon.

3. Monitoring Ability – this criterion has been included to allow consideration of the ease of monitoring of each phenomenon as some can be monitored relatively easily, while others may require the installation of permanent or expensive equipment.

4. Management Option Development – this criterion has been included to reflect the ease of mitigation of each phenomenon. .

2.2.2 Criteria Ratings Adopted Having selected an appropriate set of assessment criteria, the criteria ratings available to describe the effect of each power quality phenomenon on the chosen evaluation criteria must be specified. In order to maintain simplicity and avoid complicating the assessment process, five ratings were adopted for each evaluation criterion. These are outlined in Table 2 along with the associated points score (used to determine the overall priority) as well as an explanation for each of the five ratings in each of the four criteria.

Rating Frequency of Occurrence

Impact Monitoring Ability

Management Option Development

Points

Low Long time between events (e.g. several weeks or greater).

Low technical and economic impact

Difficult to monitor.

Difficult to develop options to manage PQ event.

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Low-Medium

Fairly long time between events (e.g. weekly).

Low technical and medium economic impact, or vice versa.

Quite difficult to monitor.

Quite difficult to develop options to manage PQ event.

2

Medium Intermediate time between events (e.g. days).

Medium technical and economic impact

Can be monitored with effort.

Options to manage PQ event can be developed with effort.

3

Medium-High

Fairly short time periods between events (e.g. hours).

Medium technical and high economic impact, or vice versa.

Quite easy to monitor.

Quite easy to develop options to manage PQ event.

4

High Short time periods between events (e.g. minutes or less).

High technical and economic impact.

Very easy to monitor.

Very easy to develop options to manage PQ event.

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Table 2: Ratings and Descriptions for Evaluation Criteria.

1 It should be noted that the frequency of occurrence criterion has still been included for the voltage and current

waveform distortion events even though these are, technically, continuously occurring events. However, in this case the frequency of occurrence criterion indicates how often these waveform distortion events become problematic.

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2.2.3 Summary of Overall Priority Values With the power quality phenomena defined and the available ratings identified to describe key aspects of each phenomenon (as outlined above in Table 2) the opinions of a range of industrial and academic experts in the field of electricity systems and power quality were elicited. Table 3 summarises the responses received concerning the priority of key aspects of the review power quality phenomena.

Power Quality Events Sub-Type Frequency of Occurrence

Impact Monitoring Ability

Management Option

Development

Overall Priority Value

Averaged Overall Priority

Low Medium High Medium 12 Medium Medium Low Low-Medium 9

Low Medium High Medium 12

Impulsive


11

Low Medium High Medium 12 Medium Medium Low Low-Medium 9


Voltage and Current Transient

Oscillatory


11

Medium Medium High High 16 Medium Low-Medium High Medium-High 14 Medium Medium High High 16

Harmonics

Medium Medium Medium High 14

15

Medium Not well known High High 13 Medium Low High Medium 12

Low-medium Not well known High High 12

Interharmonics

Low-Medium Not well known Medium High 10

12

Medium Medium Low Low 8 Low Low Low Low 4

Medium Medium Low Low 8

Notching

Low-Medium Medium Low Low 7

7

Low Medium High Medium 12 Low Low Medium Low 6 Low Medium-High High Low 11

DC Offset


10

Medium Medium Medium High 14 Low-Medium Low Medium Medium 9

Medium Low Medium High 12

Voltage and Current Waveform Distortion

Noise

Medium Medium Medium High 14

12

Medium Medium High High 16 Medium-High High High Low-Medium 16

Medium Medium High High 16

Dip


16

Medium Medium High High 16 Medium-High High High Low-Medium 16 Low-medium Medium High High 15

Short-Duration Voltage Variation

Swell


16

Medium Low High High 14 Low-Medium High High Medium-High 16

Medium Low High High 14

Undervoltage


15

Medium Low High High 14 Low-Medium High High Medium-High 16 Low-medium Low High High 13

Long-Duration Voltage Variations

Overvoltage


14

Table Continued on Next Page

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Power Quality Events Sub-Type Frequency of

Occurrence Impact Monitoring

Ability Management

Option Development

Overall Priority Value

Averaged Overall Priority

Low Low Medium Medium 8 Low Low-Medium Medium Medium 9 Low Medium Medium Medium 10

Voltage Fluctuations NA.

Low Low Medium Medium 8

9

Low Low High Medium 10 Low Low High Medium 10

Low-Medium Low High Medium 11

Voltage Phase Unbalance

NA.


11

Low Low High Medium 10 Medium-High Medium High Medium 15

Medium Low High Medium 12

Current Phase-Unbalance

NA.


12

Low Medium High Low 10 Low Medium Medium-High Low-Medium 10 Low Medium High Low 10

Frequency Variations NA.

Low Medium High Low 10

10

Low Medium High Low 10 Low-Medium High High Medium 15

Low High High High 16

Interruptions (Short and Long Duration)

NA.

Medium Medium-High High Medium 15

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Table 3: Priority Ratings and Overall Power Quality Priorities

The results of table 3 were then used to generate overall priority values by averaging the priority score for each power quality phenomenon. The resulting average overall priority values were then used to rank-order the phenomena in terms of priority. By following this process the following ranking is obtained:

1st – Short-duration voltage dips. – Short-duration voltage swells.

2nd – Harmonic current and voltage distortion. – Undervoltage.

3rd – Overvoltage. – Interruptions (short & long duration). 4th – Current phase balance.

– Interharmonics. – Noise.

5th – Voltage phase balance. – Impulsive transients. – Oscillatory transients. 6th – Frequency variations. – DC-offset. 7th – Voltage fluctuations. 8th – Notching. The priority values detailed above were used to guide the specification and selection of a range of studies to assess the effectiveness of key options detailed in the power quality management option inventory.

2.3 Key Outcomes The key outcomes of section 2 are:

• Commonly occurring power quality phenomena have been defined, characterised and their manner of measurement specified.

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• Assessment based on opinions from a range of stakeholders has identified voltage dips as having the greatest priority among the considered phenomena. Voltage swells, under- and over-voltage and harmonic distortion are also of high priority.

3. Inventory of Power Quality Management Options Alongside the definition and prioritisation of the reviewed power quality phenomena a comprehensive list of power quality management options was identified and information for each option collated and organised within an inventory structure. The capabilities shown for each option type have been identified through a combination of literature searches, textbooks, information from equipment manufacturers as well as from electricity industry and academic contributors. It should be noted that the devices and management options considered in this inventory are already used extremely successfully to manage power quality events for one or more specific loads. However, the aim of the management option inventory is to detail the potential use of these devices as a means by which to improve the power quality of a low voltage distribution network as a whole. As the use of these devices at a network level rather than at specific loads is not well documented, the power quality management option inventory has been developed as a starting point for this assessment. For each of the main areas considered in the inventory (e.g. local generation, energy storage devices, traditional controllers, novel controllers and load management schemes) an overview is first presented outlining the power quality events that can be managed by each management option. Supporting information pertaining to the provision and implementation of each management option to successfully mitigate particular phenomena are then presented where applicable, also obtained from the sources outlined above.

3.1 Power Quality Phenomena To facilitate the development and representation of the summary tables describing the capability of each management option, it is necessary to first review the power quality phenomena and assign an appropriate indicator to each phenomenon (Table 4). These indicators are used in this section to provide a succinct indication of the power quality phenomena that can be mitigated by a particular management option.

Power Quality Phenomena IndicatorImpulsive ImpulsiveVoltage Transient Oscillatory Oscillatory Harmonics Harmonics

Interharmonics Interharmonics Notching Notching

D.C. offset D.C. Offset

Voltage Waveform Distortion

Noise NoiseDip Dip

Swell SwellShort-Duration Voltage Variation

Interruption Interruption Undervoltage Undervoltage Overvoltage Overvoltage

Long-Duration Voltage Variation

Interuption Interuption (L) Voltage Fluctuations NA. Fluctuation

Voltage Phase Unbalance NA. Phase-Unbal. Frequency Variations NA. Frequency

Impulsive Impulsive (C) Current Transient Oscillatory Oscillatory (C) Harmonics Harmonics (C)

Interharmonics Interharmonics (C) Notching Notching (C)

D.C. offset D.C. Offset (C)

Current Waveform Distortion

Noise Noise (C)Current Phase Unbalance NA. Phase-Unbal.(C)

Table 4: Summary of PQ Phenomena and Associated Indicators

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The capabilities of each of the five primary management option groups, in terms of the power quality phenomena that they can mitigate, are presented in sections 3.2 – 3.5.

3.2 Local Generation

3.2.1 Summary of Local Generation Capability The ability of local distributed generation to manage power quality events is summarised in Table 5 with supporting information for each generator type presented in section 3.2.2.

Generator Type Management Option Phenomena Phenomena Ref. Back-Up Supply / Islanding Operation Interruption(L) Voltage Support Undervoltage Overvoltage

Synchronous Machine

Blackstart Capability Interruption(L) Back-Up Supply / Island Operation Interruption(L) Induction Machine Voltage Support Undervoltage Overvoltage Back-Up Supply / Island Operation Interruption(L) Doubly-Fed

Induction Machine Voltage Support Undervoltage Overvoltage Back-Up Supply / Island Operation Interruption(L) Voltage Support Undervoltage Overvoltage Blackstart Capability Interruption(L) Phase Unbalance Correction Phase-Unbal. Static Synchronous Compensator (STATCOM) Function

Dip Fluctuation 3

Dynamic Voltage Restorer (DVR) Function

Dip Swell 3

Pulse Width Modulation dc/ac Inverter

Active Power Filter Function Harmonics (C) 3

Table 5: Summary of Local Generation Capability

3.2.2 Supporting Information for Local Generation Management Options 3.2.2.1 Synchronous Machine DG devices utilising a synchronous machine do not have the capability necessary to respond to the short duration of voltage dips and swells or provide any harmonic distortion mitigation. They do however provide a particularly effective resource for use as a back-up supply or to supply an islanded section of a distribution network, and for use in mitigating long-duration voltage variations. Back-Up Supply / Islanding Operation The use of synchronous machines as a back-up supply for a particular load(s) and the operation of a synchronous machine to supply an islanded section of distribution network provide a similar benefit from a power quality perspective. That is, in both cases there will be a reduction in the number of customers affected (i.e. interruptions and duration) by a large-scale failure of the electricity supply system. The use of a synchronous generator to supply an islanded section of a DNO’s network brings with it added complications. Distributed generators utilising synchronous machines can be allowed to supply islanded sections of DNO networks in the UK as the decision is down to the DNO and not a legal requirement, [4]. The technical ability of a DG utilising a synchronous generator to operate in an islanding mode will depend on the generator transient stability and also if suitable black-start capability exists as well as appropriate frequency and voltage control. In addition, it will still be necessary to ensure that an adequate loss of mains protection system is installed to successfully detect the onset of islanding. For example, if a rate of change of

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frequency relay is used by the generator to detect loss of mains it is essential to select an appropriate rate of change of frequency, to minimise nuisance tripping. If an operating setting is chosen such that nuisance tripping occurs frequently, the generator may actually worsen the security of supply – if it is being relied upon to supply a number of network customers and is frequently disconnected due to frequency or voltage variations. Under- / Over-Voltage Synchronous machines can be used for managing under- or over-voltages by consuming or generating reactive power. Currently available synchronous machines can provide this management option but would require pre-set rules or control strategies to follow (e.g. constant power factor, constant reactive power output or constant voltage level) unless some external control and communication facility is utilised. Blackstart Capability Many DGs with a synchronous machine will have a restart capability without requiring connection to an external power supply. However, given that the focus of this task is DG in low voltage grids, although technically possible, it is unlikely that generators in an LV grid will be used to provide a blackstart capability. This is because: 1. Synchronous generators are not widely used in small-scale distributed generators due increased size and

cost over alternative grid connections. 2. Even with a group of LV synchronous generators there would still be insufficient total power to make a

significant blackstart contribution. 3. Using LV distributed generators for blackstart could result in significant reverse power flows back to

the higher voltage sections of the distribution network. 3.2.2.2 Induction Machine Given that induction machines require reactive power to magnetise the rotor it is unlikely that such generators will be able to provide much of a power quality benefit in an islanded distribution system. However, if an induction machine is equipped with suitable protection, frequency and voltage control facilities and sufficient reactive power existed, then it may be able to provide some benefit. Although an induction machine does not have the control of reactive power that a synchronous generator has, it may still be able to provide a benefit in terms of mitigation of under- and over-voltages by simply increasing or decreasing the real power output. 3.2.2.3 Doubly-Fed Induction Machine Although a doubly-fed induction generator (DFIG) is shown in Table 5 as having the same power quality benefits as a normal induction machine, in reality, its contribution is far greater. Doubly-fed induction machines utilise a power electronic interface to provide a variable frequency link to the rotor, which allows the prime mover to run at variable speeds. The power electronics also allows control of real and reactive power providing a significant effectiveness capability for mitigating under- and over-voltages. In addition, a DFIG is potentially of far greater use within an islanded section of a distribution network than a conventional induction machine since it is not limited by the available reactive power. If suitable modifications can be made to the DFIG power electronics, in a similar manner to those now being suggested for PWM inverters then the possibility exists that such machines could also provide a significant means by which to mitigate voltage dips and swells and possibly even harmonic distortion.

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3.2.2.4 PWM Inverter Pulse width modulation (PWM) based inverter connected DG devices have the ability to provide many of the power quality functions already achievable with synchronous generators, namely for use as a back-up supply, for operation in an islanded section of a DNO network as well as control of under- and over-voltages. PWM inverter connected DG also presents an opportunity to provide voltage phase unbalance correction, mitigation of voltage variations and fluctuations when configured as a STATCOM or DVR and the elimination of harmonic current distortion when configured as an active power filter. Static Synchronous Compensation (STATCOM) Function A PWM inverter can be configured to provide a STATCOM function for voltage dips but the utilisation of a shunt device requires a significant amount of reactive power. This may require that the rating of the inverter be increased beyond that required for generation. Other management options provided by the STATCOM function include managing voltage swells as well as under and over-voltages. Dynamic Voltage Restorer (DVR) Function Existing PWM connected generators could be operated in a DVR mode to manage dips and swells if a series winding associated with an inverter fed from the d.c. bus is included in the PWM system. However, the implementation of a DVR function may result in unbalanced currents and require a redesign of the PWM system. Active Power Filter Function Harmonic current loops could be added to existing interface inverters so that harmonic current components are injected into the grid system in phase opposition to those already present. However, in order to implement this function the d.c. bus voltage may have to be increased.

3.3 Energy Storage

3.3.1 Summary of Energy Storage Capability The ability of energy storage to manage power quality events is summarised in Table 6 with supporting information presented in section 3.3.2.

Storage Sub-Type Phenomena Phenomena Phenomena Phenomena Phenomena Ref.Stand-by UPS Dip Interruption 3Online UPS Dip Interruption 3

Battery

Line Interactive UPS Dip Interruption Undervoltage Overvoltage 3,5Stand-by UPS Dip Interruption 3Online UPS Dip Interruption 3

Flywheels

Line Interactive UPS Dip Interruption Undervoltage Overvoltage Harmonics 3Shunt Connected Dip Interruption Undervoltage Overvoltage 3Micro-SMES

Series Connected (continuous) Dip Interruption 3Supercapacitor Dip Interruption Harmonics Frequency C 6

Table 6: Summary of Energy Storage Capability

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3.3.2 Supporting Information for Energy Storage Options 3.3.2.1 Battery Battery systems have been used within uninterruptible power supply (UPS) systems for decades and are available in three configurations [3]. Online UPS An online UPS system feeds the load through the UPS. The incoming ac power is rectified into dc power which charges a bank of batteries. This dc power is then inverted back into ac power to feed the load. If the incoming ac power fails (i.e. through a voltage dip or supply interruption), the inverter is fed from the batteries and continues to supply the load. An online UPS can also provide a degree of isolation from all power system disturbances. Stand-by UPS A stand-by or offline UPS uses the normal line power to power the load until a disturbance (i.e. a voltage dip or supply interruption) is detected and a switch transfers the load to the battery-backed inverter. The transfer time from the normal source to the battery-backed inverter is important. Typically, a transfer time of 4 ms would ensure continuity of supply for a critical load [3]. A stand-by UPS does not provide any transient protection as does an online UPS. Hybrid UPS The hybrid (or line interactive) UPS is similar in design to the stand-by UPS but the utility power is not converted into dc rather it is fed directly to the critical load through an inductor or transformer. Regulation and continuous power to the critical load is achieved through the use of inverter switching elements in combination with inverter magnetic components such as inductors, linear transformers or ferro-resonant transformers. 3.3.2.2 Flywheel Flywheel UPS systems are now available as a replacement for battery based systems and are divided into two groups. High-speed flywheels made from composite carbon fibre, and low speed flywheels made from steel. Low speed flywheels rotate on low friction bearings in a vacuum but because of their mass, are limited to about 10000rpm. They can provide power from seconds to several minutes. Composite flywheels are made from different compositions of carbon and glass fibre and use either a passive magnet or electromagnetic bearings. They are capable of reaching speeds of 50000 rpm, with the potential to reach 100000 rpm. Discharge times range from minutes to hours. Flywheel systems have a number of advantages over batteries including a quick recharge ability, the ability to operate at low or high temperatures, no hazardous chemicals, low maintenance, high reliability and very high energy density. 3.3.2.3 Micro-SMES A series connection micro-SMES system is connected directly to the utility line and operates continuously. This configuration provides continuous support and permits the micro-SMES system to inject voltage and current without separating from the utility [7].

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In a shunt connection, the micro-SMES system is connected in parallel with the electric utility and responds rapidly (less than 4ms) to any disturbance in the quality of the utility power. This configuration provides better efficiency and lower costs with rapid response to customer needs [7]. Although, micro-SMES systems are available for use at MV / HV, it is questionable if a suitable system can be found for use within LV distribution networks. 3.3.2.4 Supercapacitor Supercapacitors exhibit a number of characteristics that make them suitable for mitigating power quality phenomena [8]. They offer a high power density and a rapid recharge capability. In comparison with batteries, supercapacitors have a wider operating temperature range. They require little or no maintenance, have a high reliability and typical have a lifespan of more than 100,000 cycles. They are however available in considerably smaller energy storage sizes than batteries or flywheels which may limit their use in LV distribution networks.

3.4 Traditional Controllers

3.4.1 Summary Information for Traditional Controllers The ability of traditional controllers to manage power quality events is summarised in Table 7 with supporting information presented in section 3

Technology Type Connection Phenomena Phenomena Phenomena Phenomena Ref.Passive Filter 1 & 3 Phase Harmonics

Harmonics Dip Undervoltage Overvoltage Ferroresonant (Constant Voltage) Transformers

3 Phase Fluctuations Noise Interruptions

3

Surge Arrestors 1 Phase Impulsive Transient Voltage Surge Suppressors (TVSS)

3 Phase Impulsive

Isolation Transformer 1 & 3 Phase Impulsive Oscillatory Noise 3Choke 1 Phase Harmonics Synchronous Compensators 3 Phase Undervoltage Overvoltage 9Ride-Through Transformer 3 Phase Dip Interruptions Electromechanical Voltage Regulator

3 Phase Undervoltage Overvoltage

Electronic Step Regulators 3 Phase Undervoltage Overvoltage Capacitor Bank Switching 3 Phase Undervoltage Overvoltage Static Var Compensator 3 Phase Undervoltage Overvoltage Fluctuation Phase-Unbal. 10Saturable Reactors 3 Phase Undervoltage Overvoltage Network Switching 3 Phase Interruption(L)

Table 7: Summary Traditional Controller Capability

3.4.2 Supporting Information for Traditional Controller Options Supporting information for sections 3.4.2.1 to 3.4.2.6 obtained from reference [11].

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3.4.2.1 Passive Filter Passive filters can reduce harmonics in LV networks by presenting varying impedances to different frequencies. Passive filters can be designed in either series or shunt configurations, blocking certain frequencies or providing a low-impedance path to earth. The resonant frequency and bandwidth of a filter is determined by the values of capacitance and inductance in the filter circuit. Filters would normally be designed to operate all of the time – reducing the impact of persistent sources of harmonic distortion – rather than be switched in and out. 3.4.2.2 Capacitor Banks Capacitor banks are generally designed to provide reactive power compensation for parameters that vary slowly. Since the capacitors are elements that have discrete variations, the techniques for shunt compensation may not allow the rapid variation in the reactive power to be followed. Consequently, capacitor banks are of use only for long-duration phenomena such as under- and over-voltages. 3.4.2.3 Ferro-resonant Transformers A well-known solution for electrical disturbance is the ferro-resonant or constant-voltage transformer (CVT). A CVT maintains two separate magnetic paths with limited coupling between them. The output contains a parallel resonant tank circuit and draws power from the primary to replace power delivered to the load. The transformer is designed so that the resonant path is in saturation while the other is not - a state known as ferro-resonance. As a result, a further change in the primary winding will not translate into changes in the saturated secondary winding, resulting in voltage regulation. The total secondary circuit is resonated at the third harmonic. This cancels out most of the harmonics generated by the saturation of the core and produces a reasonably clean sine-wave. The stability of the output is determined by the flux in the transformer core and the voltage generated by the compensating winding. Consequently, the output voltage can only be changed if there are taps on the transformer. The ferro-resonant transformer has a number of advantages. These are: the ability at low loads to have an exceptionally wide input range; the output of the CVT will automatically limit current in an overload situation; and CVTs are relatively maintenance free with no batteries or moving parts to replace. CVTs do however have some disadvantages not least being the automatic limiting of the current output which can prevent loads which require start-up surges from operating correctly unless the CVT is de-rated or designed specifically for the application. In addition, the CVT relies on resonance and consequently the output voltage will change by 1.5% for each 1% change in input frequency [11]. 3.4.2.4 Electronic Step Regulators Electronic step regulators operate by selecting separate taps on the input or the output of an auto-transformer (Figure 1). This tap selection can be performed by relays or a semiconductor device such as a thyristor. If relays are used, they only operate at the instance of a tap change. However, if a thyristor is used, it will operate 50 times a second by turning off and on each cycle of the 50 Hz supply. In this application, relays have proven themselves to be more reliable.

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Figure 1: Basic Circuit of a Step Regulator

[12]

Figure 2: Input / Output Characteristics For a Step

Regulator [12] Variations in the input voltage supply are monitored by an electronic sensor, which in turn automatically selects the appropriate tap on the transformer using a relay, thus maintaining the required output voltage. The instant of tap changing is phased by the electronic circuitry to occur very near the zero crossing of the supply voltage thus ensuring that any RF interference or switching transients are reduced to a minimum. The output voltage does however change in steps, as shown in Figure 2. Therefore, this type of voltage stabilization should not be used in lighting loads or other loads, which cannot accept step changes in input voltages. Electronic step regulators have a number of important advantages including very high efficiency, no sensitivity to frequency changes, small size and weight, insensitive to load changes and relatively low cost. The main disadvantages of the electronic step regulator are that the voltage regulation follows discrete steps and as a result the output voltage tolerance is no more accurate than ±3 %. In addition, the reliability can be limited when semiconductor devices are used to switch load current. 3.4.2.5 Static Var Compensator Static var compensators (SVCs) were developed in the late 1960s to provide fast reactive power (VAR) compensation for large, fluctuating industrial loads such as electric arc furnaces. The typical method of static VAR compensation utilises thyristor-switched capacitors (TSCs) and thyristor-controlled reactors (TCR) devices as shown in Figure 3. The leading reactive current necessary for VAR compensation is supplied by connecting capacitor banks across the ac lines in the form of TSC or more often from fixed capacitors.

Figure 3: Static VAR Compensation using TSC and TCR If the maximum lagging current drawn by the TCR is equal to the leading reactive VA of the capacitor, the two will cancel and the net reactive VA is zero. From this point, the lagging VAR of the TCR can be progressively decreased by phase control, thereby increasing the net leading VAR. After the maximum is

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reached, a further increase can be made by switching in another capacitor bank. In this manner, the TSCs provide VAR in steps, while the TCR will provide the continuous adjustment between steps. This scheme enables precise and fast automatic adjustment of the VAR by means of closed-loop control. In a practical high-voltage system the TSCs and TCRs may be connected to the secondary side of a transformer. In this way, the maximum voltage requirements of the thyristors and the capacitors can be lowered. 3.4.2.6 Saturable Reactors The saturable reactor is an electro-magnetic "switch". This switch controls current flow to the load by "switching" in and out of saturation during each half cycle. The resulting current waveform is rich in harmonics. The switch represents the ac coils and core of the saturable core reactor. The switch control represents the dc coil and dc drive circuitry used to saturate the core. The amount of current to the load is controlled by controlling the amount of dc current applied to the dc coil which in turn controls the amount of saturation in the core. Saturable reactors have the key advantage that they have no moving parts and can provide a smooth step-less control of voltage. However, even though the saturable reactor has no moving parts, its correction time can be as slow as 20 cycles (400 ms) due to high inductance. This is much slower than a comparable electro-mechanical stabilizer. In addition, saturable reactors also generate large magnetic fields, the voltage range is dependent on the load power factor and the output waveform can also be distorted depending on the supply frequency. 3.4.2.7 Network Switching Network switching may be used in the event of a system interruption to reconfigure a network and provide an alternative means of supply to customers. In addition, network switching may also be able to manage many other types of power quality phenomena (e.g. harmonics, noise, notching, voltage / current unbalance, etc) if these can be successfully eliminated by switching the source of the phenomena. However, it is questionable as to whether network switching will be able to provide a significant benefit in LV systems where many customers are supplied from a radial circuit and switching options may be limited.

3.5 Novel Controllers

3.5.1 Summary of Novel Controller Capability The ability of novel controllers to manage power quality events is summarised in Table 8 with supporting information presented in section 3.5.2.

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Technology Type Connection Phenomena Phenomena Phenomena Phenomena Ref.

Dip Swell Overvoltage Undervoltage Dynamic Voltage Restorer (DVR) 3 Phase Harmonics

6

Fluctuation Harmonics (C) Phase-Unbal. (C) Phase-Unbal. Distribution Static Synchronous Compensator (STATCOM)

3 Phase Overvoltage Undervoltage

6

Noise Dip Interruption Overvoltage Harmonics Swell Undervoltage Fluctuation

Magnetic Synthesizer 1 & 3 Phase

Phase-Unbal. Notching

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Avalanche Diode 1 Phase Swell Shunt Active Power Filter 1 & 3 Phase Harmonics Fluctuation Phase-Unbal. (C)

Dip Swell Phase-Unbal. Fluctuation Series Active Power Filter 3 Phase Harmonics Harmonics (C) Notching

Combined Shunt / Series Conditioner

3 Phase Harmonics Harmonics (C)

Table 8: Summary of Novel Controller Capability

3.5.2 Supporting Information for Novel Controller Options Supporting information for sections 3.5.2.1 to 3.5.2.6 obtained from reference [11]. 3.5.2.1 Dynamic Voltage Restorer The main component of a dynamic voltage restorer is the electronic power controller. Depending on the model, the power controller supplies a voltage to the primary of a buck/boost transformer, which is either in phase or out of phase. The power controller compares the 50 Hz stabilizer output voltage with that of a stable reference voltage and the error is used to control insulated gate bipolar transistor (IGBT) converter based bi-directional switches. The high frequency pulse-width-modulated (PWM) waveform is then filtered and supplied either to the primary of the buck/boost transformer where the secondary voltage adds or subtracts an appropriate voltage to provide a stable output voltage, or directly to the load through an auto-transformer. Although, dynamic voltage restorers have been shown to be very effective in dealing with short and long duration voltage variations as well as harmonic distortion, given that they are not widely available within low voltage grids they currently present only a potentially significant resource. 3.5.2.2 Distribution STATCOM The distribution static synchronous compensator (DSTATCOM) is a shunt-connected reactive power source that is coupled to the ac power system via a coupling reactance and designed for use in distribution systems. The coupling reactance is typically derived from the leakage impedance of a standard distribution-class transformer although some manufacturers are experimenting with direct-connected or transformerless equipment that is tied to the line via fixed reactors. Figure 4 depicts the functional arrangement of a modern DSTATCOM. Its operation is much like that of a synchronous compensator but with a significantly faster response time.

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Figure 4: DSTATCOM – Main Circuit

The DSTATCOM synthesizes output waveforms for insertion into the ac power system through high frequency switching action in its dc-to-ac inverter. In its simplest form, the inverter is composed of six semiconductor switches that possess inherent gate turnoff capability. The inverter generates a set of three synchronous voltage waveforms at fundamental frequency by connecting its dc terminals sequentially to the three output terminals via the appropriate inverter switches. Since the inverter supplies only reactive output power, the real input power provided by the dc source (charged capacitor) is theoretically zero. The dc capacitors plays no direct part in the reactive power generation process. In other words, the inverter simply interconnects the three ac terminals in such a way that the reactive output currents can flow freely between them to effect a circulating power exchange among the phases of the ac system. The DSTATCOM has the unique ability to absorb or generate dynamic VARs well in excess of its steady-state rating. Thus, it can effectively provide compensation during severe power system disturbances and yield superior benefits for the end user. In fact, the acting nature also enables the DSTATCOM to compensate for harmonic distortion [6] in addition to providing accurate dynamic voltage regulation. It achieves this by synthesizing and injecting harmonic currents required by the load thus freeing the power distribution system to supply only fundamental current at near unity power factor. The ability of the DSTATCOM to provide harmonic compensation occurs automatically and transparently up to the bandwidth limitation of the equipment regardless of any changes in the harmonic profile of the offending loads within the ac system. Although distribution DSTATCOMS have been applied successfully at MV to mitigate under- and over-voltages as well as harmonic distortion, in a similar manner to dynamic voltage restorers, they are not widely available for installation in low voltage grids. Consequently, they only represent a potentially significant resource for achieving power quality improvements in LV grids at the current time. 3.5.2.3 Magnetic Synthesizer

The magnetic synthesizer is a ferro-resonant device that consists of inductors and capacitors configured in a parallel resonant circuit with a network of six saturating pulse transformers. The output is synthesized by combining pulses of the saturating transformers into "building blocks" similar to the pseudo sine wave of many electronic inverters (see Figure 5). This device is typically used for applications of 50 kVA and larger, where voltage regulation, sag mitigation, and/or isolation are needed. Magnetic synthesizers are excellent for solving a wide variety of power quality problems, including voltage sags, sustained under-voltages, notches, transients, and even waveform distortions. It's also effective against voltage sags at full load; however, performance improves if loads are less than 100 %.

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Figure 5: Diagram of a Magnetic Synthesizer [14] 3.5.2.4 Shunt Active Power Filter Shunt active power filters compensate current harmonics by injecting equal-but-opposite harmonic compensating current. In this case, the shunt active power filter operates as a current source injecting the harmonic components generated by the load but phase shifted by 1800. As a result, components of harmonic currents contained in the load current are cancelled by the effect of the active filter, and the source current remains sinusoidal and in-phase with the respective phase to neutral voltage. This principle is applicable to any type of load considered as a harmonic source. Moreover, with an appropriate control scheme, the active power filter can also compensate the load power factor. In this way, the power distribution system sees the non-linear load and the active power filter as an ideal resistor. The compensation characteristics of the shunt active power filter are shown in Figure 6.

Figure 6: Compensation Characteristics of a Shunt Active Power Filter The load current is measured by a current transformer, the output of which is analyzed by a digital signal processor to determine the harmonic profile. This information is used by the current generator to produce exactly the harmonic current required by the load on the next cycle of the fundamental waveform. In practice, the harmonic current required from the supply is reduced by about 90 %. Because the APFs rely on the measurement from the current transformer, they adapt rapidly to changes in the load harmonics. Since the analysis and generation processes are controlled by software it is a simple matter to program the device to remove only certain harmonics in order to provide maximum benefit within the rating of the device.

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3.5.2.5 Series Active Power Filter Series active power filters were introduced at end of the 1980s, and operate mainly as a voltage regulator and as harmonic isolator between a nonlinear load and the utility system. The series connected active power filter is more preferable to protect the consumer from an inadequate supply voltage quality. This type of approach is especially recommended for compensation of voltage unbalances and voltage sags from the ac supply. It is also recommended for low power applications since it represents an economically attractive alternative to a UPS, since no energy storage is necessary and the overall rating of the components is smaller. The series active power filter injects a voltage component in series with the supply voltage and therefore can be regarded as a voltage harmonic filter, controlled voltage source, compensating voltage sags on the load side. In order to operate as a harmonic isolator, a parallel LC filter must be connected between the nonlinear loads and the coupling transformers. 3.5.2.6 Shunt / Series Conditioners This equipment is commonly referred to as either an Active Power Line Conditioner (APLC) or a Unified Power Quality Conditioner (UPQC). Historically, shunt and series active filters found widespread use in local power delivery systems such as in office buildings or small industrial facilities. Typically, these systems were designed to operate at the prevailing voltage levels of 480 V ac or less. The combined series/shunt conditioner consists of two switching, voltage-source converters operated from a common dc link. These inverters feature high operating bandwidth and consequently, wide dynamic range. One inverter is connected in shunt with the line via a standard coupling transformer while the other is connected in series. This arrangement functions as an ideal ac-to-ac power converter in which the real power can flow freely in either direction between the ac terminals of the two inverters. Each inverter, in turn, can independently generate or absorb reactive power at its own ac output terminals. This arrangement resembles the shunt and series active power conditioners. The series-connected inverter serves as a synchronous ac voltage in series with the line voltage having both controllable magnitude and phase angle. The line current flows through this voltage source resulting in real and reactive power exchange between it and the ac system. The real power that is exchanged at the ac terminal (the terminal of the insertion or coupling transformer), is converted by the inverter into dc power that appears at the dc link as positive or negative real power demand. Reactive power that is exchanged at the ac terminal, is generated internally by the inverter. The control system governing the operation of the combined shunt and series compensator dynamically controls the series voltage source to follow the harmonic voltage profile of the incoming ac supply lines, and removes unwanted voltage harmonics that may be present. The basic function of the shunt-connected inverter is to provide the harmonic current components demanded by the load, and to supply or absorb the real power demanded by the series-connected inverter at the common dc link. Like the STATCOM, this shunt inverter can generate or absorb controllable reactive power and thereby provide independent reactive compensation. It is the basic configuration that is applied in FACTS systems.

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3.6 Load Management Schemes

3.6.1 Overview of Load Management Scheme Capability Unlike the local generation, energy storage and novel / traditional controller groupings no distinction is made between the different load management schemes presented in this section in terms of the power quality phenomena managed. However, conceptually at least, load management schemes present a potential mechanism to manage the following phenomena:

• Long duration interruptions. • Undervoltages. • Overvoltages. • Frequency variations.

In addition, load management schemes could also provide a mechanism through which many of the other power quality phenomena (e.g. harmonics, noise, notching, voltage / current unbalance, etc) could also be mitigated if the load responsible for the production of the phenomena could be successfully reduced or disconnected at an appropriate time through a load management scheme. A number of load management schemes have been identified and are presented in Table 9.

Management Scheme Sub-Category Load Reduction Group Utility Control Ref. Interruptible Load Residential, Commercial, Industrial Direct

Radio Controlled Switches Residential, Commercial, Industrial Direct Ripple Control Residential, Commercial, Industrial Direct 15 Communication Schemes Telephone or Pilot Wire Residential, Commercial, Industrial Direct Customer Load Reduction Agreement Commercial, Industrial Indirect 16 Load Reduction Bid Scheme Commercial, Industrial Indirect 16 Incentive Based Schemes Load Reduction for Blackout Commercial, Industrial Indirect 16

Load Aggregation Scheme Residential, Commercial Industrial Indirect 17 Energy Storage Residential, Commercial, Industrial Direct / Indirect

Table 9: Overview of Load Management Schemes

3.6.2 Additional Information for Load Management Schemes 3.6.2.1 Interruptible Load Some loads can be temporarily interrupted without significantly affecting their functionality or performance, (e.g. electrical storage heating, irrigation pumping). Alternatively, some customers may allow selected loads to be interrupted in return for payment. Once the means of controlling these interruptible loads has been established, they offer a further option for the management of power quality phenomena. 3.6.2.2 Communication Schemes Interruptible loads may be identified but some means of communicating with and controlling these loads must then be established. There is a wide range of communication options available. Methods already employed in the electricity supply industry include radio controlled switching, power line ripple control, and telephone or dedicated pilot wires.

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3.6.2.3 Incentive Based Schemes Apart from the interruptible or controllable loads and the means for communicating with and controlling them, customers must be offered an incentive to participate in load management schemes. Incentives can be structured in a number of ways and will depend on the size of customer, the nature of the load and communication systems, and the regulatory and market structure established in each particular area. 3.6.2.4 Load Aggregation Scheme With this type of scheme small load reductions which could not individually participate or be supported by an electricity market are aggregated and presented to the market as a group. 3.6.2.5 Energy Storage Although energy storage schemes are dealt with in a separate section in this document, larger energy storage schemes can also be used as a load management scheme. Depending on the ownership of the scheme, a host distribution utility may have either direct or indirect control of the energy storage.

3.7 Key Outcomes The key outcomes from section 3 are:

• An inventory detailing the capabilities of the various available options for managing power quality in low voltage grids has been developed.

• Of the considered distributed generation options, generators utilising PWM inverters present the greatest opportunity for achieving power quality improvements – subject to suitable modifications being made to the inverter.

• Of the considered energy storage options, sophisticated hybrid (line-interactive) UPS systems based on battery or flywheel technologies provide the greatest power quality capabilities.

• Ferro-resonant transformers and static var compensators provide the greatest power quality functionality of the traditional controller devices.

• Of the available novel power technologies, magnetic synthesizers, DSTATCOMs and series active power filters show the greatest power quality capabilities.

4. Assessment Based on Power Quality Phenomena The inventory of management options for improving power quality in low voltage grids presented in section 3 simply details if a particular technology can be used to mitigate a particular power quality phenomenon. No indication was given of how successful a particular option is at mitigating a particular phenomenon. While this is not a problem for many of the options, since their use in improving power quality is fairly well known and understood, for other devices this is not the case. Consequently, a set of analytical studies have been performed to quantify the capabilities of a particular set of management options. Given the wide range of power quality phenomena considered, the set of studies have been limited to the subset of power quality phenomena identified in section 2 as presenting the greatest concern to customers and utilities.

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Although each of the DG and storage devices1 detailed in the option inventory were considered as a complete device, there are two basic components that can limit the use of the device for power quality improvement: the grid connection; and the energy source. The grid connection may be the limiting factor if it is not suitable for use in providing a particular power quality function. Alternatively, if the energy source of the device can vary or is finite then this may also be the factor limiting its' use in improving power quality. Consequently, the technical assessment studies were split into two groups: one relating to the energy source and one relating to the grid connection.

• The first set of studies (detailed in this section) considered the basic means by which power quality phenomena can be mitigated (i.e. injection of a current during a voltage dip). By comparing the underlying functionality of the options' energy source with the observations from the studies, conclusions related to the effectiveness of particular management options could be derived.

• The second set of technical studies (presented in section 5) considered the grid connection of DG and storage options. As the capabilities of the rotating machines are fairly well known and DFIG devices have found limited application at the present time, technical studies were performed for PWM inverter grid connections. PWM inverters are used on a variety of DG technologies and are also incorporated within many energy storage devices and novel power quality controllers.

4.1 Aim of Studies & Assessment Methodology The approach adopted to quantify the contribution of a management options' energy source to the mitigation of power quality phenomena was as follows:

1. Based on priority of the power quality phenomena, identify the individual phenomena to be considered within the ensuing analytical studies – information also required for the second assessment approach presented in section 5.

2. Develop analytical studies and scenarios to be performed to assess characteristics and requirements of each phenomenon.

3. Identify analysis software tool and select suitable distribution network for evaluation. 4. Perform analytical studies and obtain results. 5. Review functionality of individual management options and compare with results to develop

summary observations. Each of these five steps are now discussed.

4.2 Prioritisation of Power Quality Phenomena In section 3 the power quality management option inventory was presented and the capabilities of local generation, energy storage, load management schemes and other important technologies outlined. When all of these technologies are considered together the result is 25 different management options which provide mitigation capabilities across 24 different phenomena. In terms of actual mitigation functions detailed within the inventory there are more than 100 individual functions outlined. Given that resources did not exist within task 1.8 to perform studies for all 24 phenomena, the studies that were performed had to be carefully targeted to provide the greatest benefit and most relevant information. In order to prioritise the assessment studies the results of the power quality priority assessment (see section 2.2) were adopted. Thus, assessment studies were performed for the highest priority phenomena only, considering a combination of frequency of occurrence, likely impact, ease of monitoring and development of management options. The power quality

1 The technical studies have focussed on distributed generation and energy storage devices since DG is the primary focus of the DISPOWER project and energy storage devices have many similarities with DG.

26

phenomena resulting from this process with the highest priority were voltage dips / swells, under- and over-voltage and harmonic waveform distortion. By concentrating assessment resources on these particular phenomena the resulting effectiveness observations and conclusions would be of most relevance to the overall goal of achieving improvements in LV grid power quality.

4.3 Development of Analytical Studies and Test Scenarios The development of technical studies for voltage dips, under-voltages, voltage swells, over-voltages and harmonics are detailed in sections 4.3.1 to 4.3.5

4.3.1 Voltage Dips Although there are a number of causes of voltage dips within an electricity distribution network, in all cases the resultant effect is a reduction in the supply voltage to 0.9 p.u. or less. There are a number of approaches that can be adopted to reduce the severity of a voltage dip, including:

• Reduce network loads. • Increase distributed generation output. • Use of energy storage devices. • Switch on / energise capacitor bank.

Of these options, the energisation of capacitor banks will probably not be possible in the time-scale required and capacitor banks are also not used extensively at LV. Although switching off or reducing network loads is feasible in the time-scale associated with voltage dips, this type of action is not the primary aim of these studies. Consequently, only increasing DG output(s) and those of associated devices (e.g. storage units, active filters, magnetic synthesizers, etc) is considered within these studies as a mechanism by which to mitigate voltage dips. As far as increasing the output level of distributed generation and other technologies is concerned, what is actually required is to inject a current of appropriate amplitude to raise the voltage at the point of connection during the voltage dip. This may require an increase in the output of existing operational DG units or involve the use of dedicated power quality units simply to inject currents during voltage dip events (i.e. energy storage devices). Obviously there are a number of technical challenges with this solution, not least being the ability of a management option to successfully detect the onset of a voltage dip and subsequently detect the occurrence of the normal supply voltage when the dip ends. Many traditional generation units can be eliminated as potential solutions to voltage dips on such grounds as they cannot respond quickly enough to deal with the occurrence of the dip. Assuming that the needed response capability exists within the grid connection, what will mark out an individual management option as being particularly successful at mitigating a voltage dip is the ability to inject a large enough current during dip. This is directly related to the energy source used within the option. Also of importance when considering the mitigation of voltage dips using energy storage technologies is the duration of the dip as the device must have sufficient storage to last the duration of the dip. Thus, from the point of view of mitigating voltage dips, the key functions required by a management option are:

• Grid connection – device must be physically capable of injecting current. • Response time – must be able to respond to the onset and conclusion of dips.

27

• Energy source – must be able to inject a current of sufficient magnitude (may require multiple units to achieve this).

• Storage capabilities – for energy storage devices (multiple units may also be required here). Given that an assessment of the ability of the management options to physically inject an appropriate current during a voltage dip was out-with the scope of this set of studies and that the response time of the useable options is already fairly well known, the key unknown variables are the current injection magnitude plus the storage capacity of energy storage options. Thus, for this set of technical studies relating to voltage dips the approach adopted was to generate a voltage dip and assess how the injection of varying current levels impacted on the resultant network voltages during the dip. The required current injection magnitude was then compared with the general capabilities of the energy source of the currently available devices to develop effectiveness conclusions. In addition, by considering the required current injection and the time-scales involved, the approximate energy storage capacity for such options can also be estimated and compared with the capabilities of currently available devices. The studies performed for voltage dips considered a range of influencing factors:

• Magnitude of voltage dip – 10%, 30% and 50% considered. • Magnitude of current injection – 100A, 200A, 300A and 500A considered. • Multiple network injection points. • Load values – peak loads and half peak load values considered.

Although the duration of a voltage dip is also a factor influencing the severity and impact of the dip, from a mitigation perspective, the duration of the dip will not affect the resultant network voltage, unlike all of the factors outlined above. Consequently, there is no need to explicitly consider the duration of the voltage dip within the effectiveness assessment studies and the dip duration can remain as a set value. The duration of the dip will however be an important factor influencing the effectiveness of energy storage options and will be considered before assessing the capabilities of such devices.

4.3.2 Under-Voltages For under-voltages a similar approach was adopted as for voltage dips, as the only real difference between a dip and an under-voltage are the time-scales involved with the event. As the duration of the dip was the one variable that was not changed throughout the assessment studies, the studies carried out for voltage dips (see section 4.3.1) were also applicable for under-voltage mitigation. Note however that although an under-voltage could technically decrease to the same range of possible voltages as a voltage dip, in reality an under-voltage will typically be in the range of 0.8 – 0.9 p.u.[3]. Consequently, the studies performed for voltage dips of 10% and 30% will be of much greater importance that those performed for dips of 50%. It should also be noted that other options eliminated from consideration in voltage dips, such as more traditional DG technologies and load manipulation are potential solutions for reducing under-voltages given the significantly longer time-scales involved. However, for the purpose of the effectiveness studies these options have been not been considered as their use in managing and mitigating under-voltages are reasonably well understood. They are however included in the overall effectiveness rating process.

4.3.3 Voltage Swells As with dips there are a number of power system events which will give rise to a voltage swell, although in all cases the supply voltage will exceed 1.1 p.u. To provide a means to mitigate the occurrence of voltage swells a number of solutions can be considered. These include:

28

• Increase loads. • Reduce in-phase output of DG (and associated) technologies. • Switch off capacitor bank. • Anti-phase current injection.

In a similar manner to voltage dips, the use of capacitor banks can be discounted as a realistic option due to the time-scales involved and the sparse nature of their use at LV. Load manipulation can also be discounted as the focus of the studies is assessing the contribution of individual devices for power quality management. This leaves reducing DG and energy storage outputs plus the injection of current in anti-phase as the primary options for mitigating voltage swells. Reducing the in-phase output of distributed generation and storage technologies if currently running will certainly provide a mechanism by which to reduce the severity of voltage swells. Although there are numerous issues that need to be considered when manipulating the output of such devices in this way (e.g. acceptable time-scales, contractual, legal and regulatory implications, etc) there is little need for technical analysis as the concept of manipulating a generator’s output to decrease network voltages is well understood. For those devices that can be used to mitigate a voltage dip by injecting a current in in-phase with the normal current waveform, these devices also present a significant potential resource by which to mitigate voltage swells. To achieve this functionality a current would have to be injected at 180o out of phase, or in anti-phase with the normal current waveform. This will result in a reduction in network supply voltage. Given that the devices used to mitigate voltage dips through current injection already have a suitable grid connection and response capability, if we assume that they can also be modified to mitigate voltage swells, then the key functions required by a management option are:

• Energy source – must be able to inject a current of sufficient magnitude (may require multiple

units to achieve this) in anti-phase with normal current waveform. • Storage capabilities – for energy storage devices (multiple units may also be required here).

Thus, the aim of technical studies performed for voltage swells was to generate a voltage swell and assess how the injection of varying anti-phase current levels impacted on the resultant network voltages during the swell. The required anti-phase current injection magnitude was then compared with the general capabilities of the energy source of currently available devices to develop effectiveness conclusions. In addition, by considering the required anti-phase current injection magnitude and the time-scales involved, the approximate energy storage capacity for such options can also be estimated and compared with the capabilities of currently available devices. The studies performed for voltage swells considered the same range of influencing factors as for voltage dips. These were:

• Magnitude of voltage swell – 10%, and 30% considered. • Magnitude of anti-phase current injection – 100A, 200A, 300A and 500A considered. • Multiple network injection points. • Load values – peak loads and half peak load values considered.

As with voltage dips, the duration is also an important factor influencing the severity and impact of the voltage swell. However, as before, the duration will not affect the resultant network voltage, unlike all of the factors outlined above. Consequently, there is no need to consider varying the duration of the voltage swell within the effectiveness assessment studies, a single duration can be used for all studies. The duration of the swell will however be an important factor influencing the effectiveness of energy storage options and will be considered before assessing the capabilities of such devices.

29

4.3.4 Over-voltages For over-voltages a similar approach was adopted as for voltage swells, as the only real difference between a swell and an over-voltage is the time-scales involved with the event. As the duration of the swell was the one variable that was not changed throughout the assessment studies, the studies carried out for voltage swells (see section 4.3.3) were also applicable for over-voltage mitigation. Note that other options eliminated from consideration in voltage swells, such as more traditional DG technologies and network load manipulation are potential solutions for reducing over-voltages given the significantly longer time-scales typically involved. However, for the purpose of the effectiveness studies these options have been not been considered as their use in managing and mitigating over-voltages are reasonably well understood. They are however included in the overall effectiveness rating process.

4.3.5 Harmonic Distortion Unlike the studies performed for voltage dips / swells and under- / over-voltages the mitigation of harmonic current distortion requires the use of dedicated device models that can inject harmonic currents into the distribution network to cancel those already present1. The development and use of such models was not performed in this series of assessment studies but was considered in the assessment of the specific power quality management options as described in section 5. However, although the mitigation of harmonic distortion requires the use of a dedicated device model, studies were still performed to assess the impact of harmonic distortion on a typical distribution network. Conclusions could then be made relating to harmonic reduction devices simply by assessing the propagation of harmonics throughout the test distribution network. The approach adopted for this series of studies was to develop a simple harmonic current injection device and position it at a number of individual network locations as per the studies for voltage dips / swells and under- / over-voltages. The total harmonic distortion of the current waveform at the various measurement points within the test network was then noted. The level of harmonic distortion was then increased and its effect within the distribution network observed.

4.4 Selection of Analytical Software and Test Network In the previous section the details of the technical studies that were performed as part of the effectiveness assessment were presented. After careful consideration a test LV distribution network was selected which was appropriate given the nature of the studies being performed. That is, the network had to be sufficiently robust to accommodate the type of simulations and modifications required as part of the studies. In addition, the test network should also be typical of the type of network likely to be encountered within the DISPOWER project. The test network used for these studies was brought to the DISPOWER project as part of an earlier research effort and forms part of a UK LV distribution network. It has a configuration typical of UK systems and performs to an acceptable standard. The network is composed of a 33/11kV substation which supplies an 11kV distribution network with 10 secondary substations (11/0.4kV). This is shown in Figure 7. Of these, one secondary substation is of particular importance as the lower voltage network and loads have been specifically detailed here. This secondary substation supplies four individual 3 phase circuits to three different areas – an industrial estate, a residential area and a commercial area as shown in Figure 8.

1 Note that as the primary source of harmonic voltage distortion is the presence of harmonic currents, if

harmonic currents can be eliminated then this will also reduce harmonic voltage distortion.

30

SEC1

SUBTL SUBTHSEC2 SEC3

SEC5 SEC6

SEC7 SEC8

SEC9 SEC10

SUBTN

IND08

IND03

IND02 IND04

IND05

IND09 IND10 IND11

IND12

IND06 IND07

RES01 RES02 RES03 RES04 RES05

RES08

RES12

RES09

RES10

RES06

RES14 RES15 RES16 RES17 RES18 RES19 RES20

RES21

RES22

RES23

RES24

RES25

RES26

RES28 RES27

COM01 COM02 COM03 COM04 COM05 COM06 COM07 COM08 COM09

SEC4D

SEC4L

Figure 7: Test Network Adopted for Power Quality Phenomena Studies

Industrial Estate Residential Area Commercial Area

2

43

5

6

7

8

9

1011

S/S

IND01

IND02

IND04

IND05

IND06

IND07

IND08

IND09

IND12

IND10

IND11

IND03

1

S/S

RES20

1

7

8

9

10

11

12

13

22

23

14

15

16

17

18

19

21

20

23

4

56

24

2728

25

26

RES01RES14

RES15

RES16

RES17

RES18

RES19

RES13

RES02 RES03RES04

RES05

RES06

RES07

RES08

RES09

RES10 RES11

RES12

RES24

RES25

RES26

RES27RES28

RES21

RES22RES23

S/S

COM01 1

3

2

4

5

6

7

8

9

COM02COM03COM04

COM05

COM06COM07

COM08

COM09

Figure 8: Composition of Test LV Distribution Network

The test network outlined in Figure 7 was analysed in Alternative Transients Program (ATP) and created using the ATPDraw program of ATP. The version of ATP used was 3.7, which is the most recent version of this software. Within ATP there are a number of variables that are set as part of the analysis process. These are:

31

• End simulation time – set to 1 second. • Simulation time-step – set to 20µs

By using a shorter simulation time-step more detailed results can be obtained. However, given the time-scales associated with the various power quality phenomena, ranging from tens of milli-seconds to minutes, the adopted time-step is appropriate for the considered analysis. To perform the required analysis of the resulting voltage and current waveforms generated by the ATP software a power quality analyser developed using National Instruments Labview software was adopted. Note that as with the test network, this power quality analyser was developed as part of an earlier research effort but was entirely suitable for the effectiveness assessment studies required within the DISPOWER project. The power quality analyser provided the needed capability to assess voltage dips / swells (and also under- and over-voltages) as well as harmonic current distortion.

4.5 Results from Studies

4.5.1 Voltage Dips and Under-Voltage For voltage dips and under-voltages 22 separate simulations were performed in ATP assessing the impact of the various influencing factors outlined in section 4.3.1. For each simulation the rms voltage at a number of particular locations within the test network was monitored. A number of locations were selected to demonstrate the overall effect of the current injection, as follows:

• The 11kV/400V substation. • Each of the injection locations. • Bus RES26, which is at the end of one residential feeder so is most prone to under-voltage. • Bus RES05, which is at the end of the second residential circuit and most prone to under-voltage. • Bus IND12, which is at the end of the industrial circuit. • Bus COM09, which is at the end of the commercial circuit.

Table 10 outlines the variables considered in each of the simulations and presents the resultant network voltages at the various measuring locations. Note that the first two rows in Table 10 present the results of a 10% and 30% voltage dip with no current injection.

32

CO

M09

V

olta

ge

214.

0

166.

5

214.

2

166.

7

119.

1

214.

4

166.

9

119.

3

214.

6

167.

1

119.

5

215.

0

167.

5

120.

0

214.

2

167.

1

119.

5

167.

1

119.

5

170.

6

167.

1

119.

5

IND

12

Vol

tage

214.

8

167.

1

215.

0

167.

3

119.

5

215.

2

167.

5

119.

7

215.

4

167.

7

120.

0

215.

8

168.

1

120.

4

215.

0

167.

7

120.

0

167.

7

120.

0

170.

3

168.

1

120.

0

RE

S05

Vol

tage

217.

4

169.

1

217.

5

169.

2

120.

9

217.

7

169.

4

121.

1

217.

9

169.

7

121.

4

218.

4

170.

1

121.

8

217.

5

169.

7

121.

4

169.

6

121.

4

171.

9

170.

1

121.

4

Inje

ctio

n L

ocat

ion

Vol

tage

- -

225.

6

177.

6

129.

6

235.

2

187.

2

139.

2

244.

7

196.

8

148.

8

264.

0

216.

1

168.

3

223.

0

194.

4

150.

0

219.

1

171.

3

199.

7

196.

5/22

8.1

136.

0/14

3.6/

150.

9

RE

S26

Vol

tage

215.

9

167.

9

225.

3

177.

4

129.

4

234.

8

186.

9

139.

0

244.

4

196.

5

148.

6

263.

6

215.

8

168.

1

220.

2

181.

1

133.

2

201.

7

153.

9

199.

6

210.

8

145.

2

Subs

tatio

n V

olta

ge

219.

6

170.

8

219.

8

171.

0

122.

2

219.

9

171.

2

122.

4

220.

1

171.

4

122.

6

220.

6

171.

8

123.

1

219.

8

171.

4

122.

6

171.

4

122.

6

172.

8

171.

2

122.

6

Loa

ds

Peak

Loa

d

Peak

Loa

d

Peak

Loa

d

Peak

Loa

d

Peak

Loa

d

Peak

Loa

d

Peak

Loa

d

Peak

Loa

d

Peak

Loa

d

Peak

Loa

d

Peak

Loa

d

Peak

Loa

d

Peak

Loa

d

Peak

Loa

d

Peak

Loa

d

Peak

Loa

d

Peak

Loa

d

Peak

Loa

d

Peak

Loa

d

Hal

f Pea

k

Peak

Loa

d

Peak

Loa

d

Inje

ctio

n C

urre

nt (A

)

- -

100A

100A

100A

200A

200A

200A

300A

300A

300A

500A

500A

500A

100A

300A

300A

300A

300A

300A

200A

/300

A

100A

*3

Inje

ctio

n L

ocat

ion

- -

RES

21

RES

21

RES

21

RES

21

RES

21

RES

21

RES

21

RES

21

RES

21

RES

21

RES

21

RES

21

RES

16

RES

16

RES

16

RES

28

RES

28

RES

21

RES

16,2

8

RES

16,2

1,28

Dip

M

agni

tude

10%

30%

10%

30%

50%

10%

30%

50%

10%

30%

50%

10%

30%

50%

10%

30%

50%

30%

50%

30%

30%

50%

Table 10: Network RMS Voltages for Voltage Dip & Under-Voltage Studies

33

4.5.2 Voltage Swells and Over-Voltage For voltage swells and over-voltages 13 separate simulations were performed in ATP assessing the impact of the various influencing factors outlined in section 4.3.3. For each simulation the rms voltage at a number of particular locations within the test network were monitored. The same locations were used as for voltage dips and under-voltages. Table 11 outlines the variables considered in each of the simulations and presents the resultant network voltages at the various measuring locations.

CO

M09

V

olta

ge

261.

6

309.

2

261.

4

261.

1

308.

8

315.

1

261.

5

261.

4

261.

2

261.

1

308.

3

260.

8

260.

8

IND

12

Vol

tage

262.

6

310.

3

262.

4

262.

1

310.

0

314.

7

262.

5

262.

4

262.

1

262.

1

309.

5

261.

8

261.

8

RE

S05

Vol

tage

2165

.7

314.

0

265.

5

265.

1

313.

6

317.

6

265.

5

265.

5

265.

2

265.

1

313.

1

264.

9

264.

9

Inje

ctio

n L

ocat

ion

Vol

tage

- -

254.

8

236.

2

293.

5

288.

9

263.

4

246.

7

253.

0

213.

3

265.

8

252.

3/22

4.6

252.

3/24

0.5/

232.

2

RE

S26

Vol

tage

263.

9

311.

8

254.

5

235.

9

293.

1

288.

7

260.

1

252.

7

251.

4

230.

8

265.

4

236.

6

238.

3

Subs

tatio

n V

olta

ge

268.

4

317.

1

268.

2

267.

8

316.

8

319.

2

268.

2

268.

2

267.

9

267.

8

316.

3

267.

0

267.

6

Loa

ds

Peak

Loa

d

Peak

Loa

d

Peak

Loa

d

Peak

Loa

d

Peak

Loa

d

Hal

f Pea

k

Peak

Loa

d

Peak

Loa

d

Peak

Loa

d

Peak

Loa

d

Peak

Loa

d

Peak

Loa

d

Peak

Loa

d

Inje

ctio

n C

urre

nt (A

)

- -

100A

300A

200A

300A

100A

100A

300A

300A

500A

100A

/200

A

100A

*3

Inje

ctio

n L

ocat

ion

- -

RES

21

RES

21

RES

21

RES

21

RES

21

RES

21

RES

21

RES

21

RES

21

RES

21

RES

21

Swel

l M

agni

tude

10%

30%

10%

10%

30%

30%

10%

10%

10%

10%

30%

10%

10%

Table 11: Network RMS. Voltages for Voltage Swell & Over-Voltage Studies

34

4.5.3 Harmonic Current Distortion As discussed in section 4.3.5, the successful mitigation of harmonic current distortion requires the use of a detailed device model capable of injecting currents to cancel those present in the distribution system, something not possible within the scope of this particular set of assessment studies. A detailed device model for PWM inverter connected devices capable of eliminating harmonic distortion has however been developed as part of the second set of effectiveness assessment studies described in section 5. The approach adopted in this set of studies for harmonic distortion was to introduce varying levels of harmonic current distortion into the test distribution network at specific locations and monitor the total harmonic distortion at various points within the network. The results are shown in Table 12.

IND

08

Vol

tage

0.00

1

0.33

6

0.32

2

0.28

8

0.34

0

0.33

2

0.33

3

CO

M09

T

HD

0.00

1

0.34

8

0.33

2

0.29

8

0.35

1

0.34

4

0.34

5

RE

S05

TH

D

0.00

1

0.35

1

0.33

4

0.29

9

0.35

4

0.34

7

0.34

8

RE

S27

TH

D

0.00

1

0.95

6

0.90

0

0.80

0

0.67

3

1.30

7

1.18

1

RE

S24

TH

D

0.00

1

0.90

8

0.85

7

0.76

2

0.63

9

1.11

9

1.12

2

RE

S21

TH

D

0.00

1

0.95

5

0.89

9

0.80

1

0.67

3

1.04

6

1.04

8

RE

S18

TH

D

0.00

1

0.78

0

0.73

5

0.65

5

0.67

3

0.77

3

0.77

5

RE

S51

TH

D

0.00

1

0.62

3

0.58

8

0.52

4

0.62

8

0.61

7

0.61

9

RE

S14

TH

D

0.00

1

0.50

7

0.48

0

0.42

8

0.51

1

0.50

2

0.50

4

Dis

tort

ion

Sour

ce

Loc

atio

n

-

RES

20

RES

20

RES

20

RES

16

RES

28

RES

26

Table 12: Harmonic Current THD Values

35

4.6 UK and European Voltage Limits Before presentation of the effectiveness conclusions associated with this set of case studies special mention must be made of the voltage limits within the UK and the rest of Europe. The UK has historically used a single phase to neutral voltage of 240V with upper and lower limits of +-6% permitted. The rest of Europe has typically used a single phase to neutral voltage of 220V also with limits of +-6% permitted. As part of harmonisation of electrical systems European low voltage distribution networks are now all considered to be 230V. Indeed, the European standard EN50160 [18] which applies Europe wide specifies that low voltage systems have a phase to neutral voltage of 230V. However, there has never actually been a change of voltage in European LV distribution networks, the UK remains on 240V and the rest of Europe on 220V. What has changed are the upper and lower limits with reference to the specified nominal value. Currently, UK LV systems are specified as 230V +10% -6% and European systems as 230V -10% +6%. This has important consequences for the interpretation of the results presented in this section and also for the studies detailed in section 5, as the effectiveness of a particular management option will vary depending on whether it is installed in a UK or European network.

4.7 Observations and Conclusions The results of the studies performed for voltage dips / under-voltages, voltage swells / over-voltages and harmonic distortion have been presented and demonstrate how a number of factors influence the resultant voltages and harmonic current values. In order to facilitate the development of effectiveness ratings for particular power quality management devices it is important to distil from these results key conclusions. From the study results presented in section 4.5.1 a number of important conclusions can be made regarding the mitigation of voltage dips in the test distribution network:

• A current injection of 100A at any of the three connection points (in the main circuit) will raise

voltages of the points in the main circuit to a level such that a 10% dip can be eliminated. • Mitigation of a 30% dip required a current injection of approximately 500A (~120kW). • Mitigation of dips of 50% is not going to achievable even at the point of connection of an injection

source. • The voltage improvement gained in an additional circuit and at the substation was negligible even

with a 500A injection. • With the load values at half of the peak value network voltages only obtain a marginal increase.

From the study results presented in section 4.5.2 a number of important conclusion can be made regarding the mitigation of voltage swells in the test distribution network:

• An anti-phase current injection of 300A at any of the three connection points (in the main circuit) will reduce voltages in the main circuit to a level such that a 10% swell can be eliminated.

• Mitigation of a 30% swell is not going to be achievable even at the point of connection of an injection source.

• A negligible voltage increase was observed at the substation and additional circuit even when the current injection was 500A in the main circuit.

• With load values at half of the peak value, a 10V increase was observed at loads near and at the end of the main residential circuit.

For the study results presented in 4.5.3 a number of important observations can be made regarding the propagation of harmonic currents:

• Upstream from the disturbance the THD is attenuated and end loads in neighbouring circuits all have similar, but not exactly the same THD.

36

• As the source of the disturbance moves away from the substation and towards the end of the circuit the average THD across all circuit measuring points increases (although the decrease in THD towards the substation is still present).

The conclusions outlined in this section will be combined with those presented in section 5 to generate effectiveness ratings for specific management options.

4.8 Key Outcomes A number of important outcomes can be identified from the first set of analytical studies presented in this section:

• The requirements of the energy source of a distributed generation or energy storage device for the mitigation of voltage dips / swells as well as under- and over-voltages has been quantified through analytical studies.

• The propagation of harmonic current distortion through a distribution network has been examined to support the detailed assessment of harmonic distortion mitigation presented in section 5.

5. Assessment of Specific Management Options

5.1 Aim of Studies The primary goal of this set of analytical studies was assessment of the opportunities and effectiveness of power quality management options with grid connections based on voltage source inverters (VSI) with PWM control. Such options include:

• Small scale inverter connected distributed generators. • Inverter connected storage devices. • Novel power electronic controllers based on inverters, such as DSTATCOM and active power filters

(APF). Although these options have a number of significant differences their common feature is the inverter grid connection, which with suitable control allows them to be applied for mitigating power quality phenomena. VSI based devices can be used as shunt compensators working in current or voltage control mode. Reaction to harmonics, asymmetry and voltage fluctuation requires injection of specific currents into the grid to compensate undesirable components of load currents responsible for voltage waveform distortion. This is termed current compensation. Mitigation of slow voltage variations, dips and interruptions requires voltage support and is achieved by injecting real and reactive current components into the grid. This kind of power quality management operation is termed voltage compensation. The primary aim of the technical studies was to design the control requirements for VSI inverters and demonstrate the capabilities for voltage and current compensation. In addition, the studies also indicated the influencing factors and parameters related to this activity.

37

5.2 Description of Study Network

The network selected for this set of studies is a rural radial network with overhead lines making up the majority of the circuits. This type of network is typical in rural areas of Poland. The LV grid is radial and is supplied from an MV/LV distribution transformer, delta-wye connected. The MV grid forms a link arrangement but is operated radially. The HV network has a meshed configuration that is represented by two transmission lines supplying the distribution substation. The remaining part of the HV network is represented by an equivalent voltage source whose reactances result from a short-circuit power at the points of connection. The network configuration and element data are shown in Figure 9. Note stations A and B which are referenced in the ensuing studies.

Figur

A number of assump For loads:

• As the mainconnected tofluctuations,

20/0,4 kV 50 Hz400 kVA5 %

100 m

110/20 kV 50 Hz10 MVA10 %

Rk = 0,164 om/kmLk = 0,121 mH/kml = 1100 m

Rk = 0,164 om/kmLk = 0,121 mH/kml = 100 m

Rk = 0,164 om/kmLk = 0,121 mH/kml = 100 m

Rk = 0,2 om/kmXk = 0,4 om/kml = 5000 m

Rk = 0,2 om/kmXk = 0,4 om/kml = 20000 m

110 kV

110 kV

110 kV 20 kV 20 kV 0,4 kV

P = 3 MWtg O = 0,4

P = 3 MWtg O = 0,4

Load 1

100 m

Load 2 Load 3

100 m 100 m

Load 4

100 m 100 m 100 m

Load 5 Load 6 Load 7 Load 8B

For management op

• Two charact− for load− for volta

• The problemdisturbing lo

5.3 Network Mo The network outlinenumber of details assections.

B
A
e 9: Study Network for Specific Power Quality Management Options

tions have been made in relation to the study network:

aim of the studies is to assess compensation abilities, representative loads have been the grid, specially designed to introduce power quality disturbances such as voltage harmonics, and unbalance.

tion location:

eristic locations of the units have been assumed dependent on the compensation mode: compensation, locations close to the disturbing loads were selected. ge compensation, the LV busbars of the distribution substations were selected. of distant compensation when a compensation unit is connected some distance from a ad was also considered.

delling

d in Figure 9 has been modelled and simulated using the PSCAD/EMTDC program. A sociated with the modelling of the network elements are discussed in the following sub-

38

5.3.1 Lines and Transformers Transmission and distribution lines have been represented by the general π-type circuit which contains resistance, reactance and capacitance. Shunt capacitances for the LV lines are however neglected. The same scheme has been used for transformers.

5.3.2 Loads Two types of loads have been modelled:

• Dynamic nonlinear and symmetrical • Dynamic linear and asymmetrical.

The first load type is a converter which introduces harmonics of various numbers and values into the network. The second type represents a substitute load of fluctuating powers which is a source of voltage variations and unbalance. Variation of load currents and harmonic number has been achieved for the converter by means of a random number generator which produces the thyristor firing angle equal for all thyristors of the inverter bridge. For the loads of the second type, power variation and phase unbalance have been obtained by varying the values of delta connected impedances, independently in each phase, by means of three random number generators. The following loads have been assumed (refer to Figure 9):

• 1 to 6 – loads of the second type, of maximum power 3*10 kW each, nominal power factor 0,7 and medium range of power variations;

• 7 – load of the second type, of maximum power 3*50 kW each, nominal power factor 0,7 and full range of power variations;

• 8 – load of the first type, of nominal power 140 kW and full range of power variations.

5.3.3 Power Quality Management Options To analyse a power quality management option a common model was utilised. It contained two components: a dc voltage source (battery) or a capacitor and the PWM inverter connecting it to the ac power network through reactance of a coupling transformer or reactor. An additional shunt band-pass filter was also applied for compensating harmonics generated by the inverter.

5.4 Inverter Control The inverter control strategy assumed two objectives:

1. Active and reactive powers are generated to meet their reference values set by the network management system to optimise power flow and keep the voltage in the required limits.

2. Additionally, the inverter compensates power quality phenomena caused by disturbing loads or fault events.

The control scheme and specific tasks performed by the inverter depend on its operation mode, i.e. current control or voltage control mode.

39

5.4.1 Current Control Mode In current control mode, which is required for load compensation, the inverter must inject to the network currents such that undesirable components of the load current (e.g. harmonic and negative sequence ones) are cancelled. The network current will therefore contain the fundamental harmonic and positive sequence components only. At the same time the inverter is expected to generate active and reactive powers to meet requirements of the management system and / or maintain a constant voltage at the point of connection (PCC). Thus, the following tasks were specified for the inverter:

a. Load compensation. b. Generation of active and reactive powers on request of the external management system. c. Stabilisation of the voltage at the point of connection.

In general, in current control mode, the inverter reference current vector is described by the following formula:

refi

UstabQPcomref iiiii +++= (5.1)

where: icom - vector component which compensates selected components of the load current, iP - vector component responsible for active power generation, iQ - vector component responsible for reactive power generation, iUstab - vector component responsible for voltage stabilisation. The load current can be decomposed as:

∑ +++=h

QP2hload iiiii (5.2)

where:

hh∑i - harmonics vector,

2i - negative sequence component vector,

Pi - positive sequence active component vector,

Qi - positive sequence reactive component vector. If the inverter and the load are connected to the same busbar then the compensation is local. In the case where the inverter connection point is electrically distant from the load connection point, distant compensation is accomplished. From the point of view of active power losses in the network, optimal results in local compensation are obtained if all current components are cancelled. In practice the degree of compensation may be restricted by the inverter capability or imposed by the management system. Optimisation of power losses in the case of distant compensation is however a significantly more complicated task. This is because the injected currents from the inverter coupling point to the splitting point cause additional losses, which may be larger than the reduction in losses within the network as a result of compensation. This effect is likely to occur when the distance between the inverter and the load is significant. To solve the problem of distant compensation the following approach was applied. It can be observed that the minimum losses in the network occur for the natural current flow resulting from the network branch resistances. This assumes that:

• Branch resistances are constant in the range of the harmonics considered. • Current controlled inverters can generate any required current.

40

Consequently, the inverter reference currents resulting in minimum network losses can be determined using the model of the examined network in which each load is replaced with a current source and the supply network as well as all operating inverters are short-circuited. The reference current for the inverters is assumed to be the short-circuit current in the inverter location, taken with the opposite sign. With this approach the current flow in the real network should correspond with the optimal flow in the network model. The inverter compensating current thus becomes:

∑−=m

m loadmcom k ii (5.3)

The subscript m denotes individual loads and km is the share of a given inverter in compensation of the mth load. In both types of compensation it is possible to cancel all of the current components specified in (5.2) or only selected components. This does however depend on the inverter capacity and requirements of the management system. Consequently, the inverter control scheme should contain appropriate component filters. The next three components of the inverter reference current iP, iQ and iUstab are controlled by external signals, on request of the management system which decides what task the inverter should perform. It should be noted that from the two tasks associated with reactive power (i.e. iQ and iUstab) only one can implemented at a time. If the inverter is required to generate reactive power then the value of this power is sent as an input to the control circuit, otherwise the task of voltage stabilisation can be activated and the input to the control circuit is the voltage reference value. To stabilise the voltage at the point of connection the inverter must inject a reactive current of fundamental frequency and positive sequence which results in an appropriate voltage drop on the supply network. This current can be determined from:

[ ]dtUtU refRMS∫ −= )(T1Im Ustabi (5.4)

where: )(tURMS - instantaneous RMS voltage value measured at the PCC,

refU - set voltage value, T - integration constant in the process of control. For determination of the inverter reference current the theory of instantaneous active and reactive powers has been used. The closed-loop hysteresis switching control has been applied, in which the inverter tracks the current reference [19].

5.4.2 Voltage Control Mode In voltage control mode the inverter is required to produce an active power and stabilise the supplying voltage on the LV side of the transformer. The inverter operates with constant frequency of switching that is an odd multiple of the network frequency. Two loops were applied in the control circuit:

• Active power control can be obtained through varying the phase angle shift between the inverter voltage and the network.

• Voltage control can be obtained through varying the amplitudes of the inverter voltages in individual phases.

41

5.5 Studies Performed The simulation investigations were performed for two operation states of the network: normal (steady-state) and non-normal (fault conditions). Power quality phenomena occurring in normal operation conditions are monitored and have their associated indices limited. These limits should not be exceeded during steady-state conditions. As disturbing loads are the sources of such phenomena so the mitigation option requires load compensation. Power quality phenomena occurring in non-normal operation conditions are usually associated with fault events. Mitigation here results from the need to assure continuity of supply and load operation during and after the event. The mitigation option in this case is voltage support or islanding operation of DG sources (if it is allowed). The two categories of power quality phenomena are qualitatively different in their time-scale, method of assessment and approach to mitigation. However, for the purpose of improving LV grid power quality it is essentially the same in both cases: to ensure the compatibility between loads and the supplying network.

5.5.1 Load Compensation Part of the test network from Figure 9 was used for load compensation studies and is depicted in Figure 10 in the graphical form obtained from the PSCAD program. The inverter is connected with the capacitor or DC source (battery) of given characteristics and is controlled in current mode.

Vnom

IcomP

IcomQ

Qgen

Pgen

Vstab

Vstab

VcLVbLVaL

VaL

VcLVbL

IcLIbLIaL

IcLIbLIaL

IcR

efIb

Ref

IaR

ef

CurrentControl

Current ControlledPWM Inverter

LOAD 8

NOMINALPI

SECTION100 m

NOMINALPI

SECTION100 m

LOAD 7

LOAD 6

NOMINALPI

SECTION100 m

LOAD 5

NOMINALPI

SECTION100 m

LOAD 4

NOMINALPI

SECTION100 m

LOAD 3

NOMINALPI

SECTION100 m

LOAD 2LOAD 1

NOMINALPI

SECTION100 m

20/0,4 kV 50 Hz400 kVA5 %

VoltageParameter

Meter

Vref

Tmean

VcL

0,2

0,23

VbLVaL

Figure 10: Test Network For Load Compensation Studies Operation of the test network with disturbing loads was simulated in 20 second cycles. In each simulation case, different options of the inverter operation were investigated. The following simulation scenarios were investigated:

1. Disturbing loads connected and inverter disconnected. 2. Local load compensation for harmonics and asymmetrical currents.

42

3. Local load compensation for reactive power and reactive power control. 4. Local load compensation for active power and active power control. 5. Distant load compensation.

For each set of simulations two sets of results were obtained: voltage and current waveforms which demonstrate load variations and validate the assumed calculations and the quantitative impact of the simulation variables on the selected power quality indices. It should be noted that a special module was developed to quantify the power quality indices in each simulation. This module was developed as part of an earlier research effort [20] and was entirely suitable for use in the DISPOWER project. The specific details of each of the five scenarios are discussed in the following sub-sections. To maintain the clarity of this section of the report, the detailed illustration of voltage and current waveforms obtained from the five scenarios are presented in the Appendix. 5.5.1.1 Disturbing Loads This first set of studies was performed without any inverters connected to the test network and illustrated the impact of the disturbing load on the MV / LV transformer current and peak to peak voltage waveform at the point of connection (Figure A-1). The rectifier and impedance currents associated with the load are also shown in Figure A-1. The same waveforms have been magnified and presented in Figure A.2 to better show the inherent distortion and unbalance. For the same simulation case the power quality indices were determined and averaged in consecutive 0.2 second time periods in the way analogous to that given in the EN50160 Standard [18]. These indices are shown in Figures A-3 and A.4. The results presented in Figures A-1 and A-4 are essential to identify the improvements provided by the inverter system when operating in load compensation mode. 5.5.1.2 Local Compensation for Harmonics and Asymmetry The second set of studies was performed to illustrate the impact of switching in the inverter to compensate loads connected to the same busbar. The inverter was switched on at 5 seconds. Figure A-5 illustrates the rectifier and impedance currents of the load as well as the inverter current, MV / LV transformer current and inverter voltage (again peak to peak). A detailed examination of the waveforms at a particular time-scale are shown in Figure A-6. The values of the standard power quality indices are shown in Figures A-7 and A-8. It is evident from comparison of Figures A-1 and A-5 that the when the inverter was switched on at 5 seconds the MV / LV transformer current became smoother with fewer fluctuations. The voltages at the point of connection of the inverter are also considerably smoother when the inverter is operating. 5.5.1.3 Local Compensation for Reactive Power and Reactive Power Control This simulation was performed to illustrate the ability of the inverter to provide load reactive power compensation, reactive power control and voltage stabilisation in addition to compensation of harmonics and asymmetry. Harmonic and asymmetry compensation was provided at the beginning of the simulation. After 5 seconds reactive component compensation of the fundamental harmonic load current was simulated with the simulation of reactive power control by an external signal started after 10 seconds. At 15 seconds voltage stabilisation was simulated. The resulting waveforms are presented in Figures A-9 to A-12 of the Appendix.

43

It is evident from the results of the simulations that the provision of load reactive power compensation reduces the MV / LV transformer currents values (comparing Figure A-5 with A-9). The stabilisation of voltages after 15 seconds is also evident from comparison of Figures A-7 and A-11 where busbar voltages are now approximately 0.96 p.u. over the time-period compared to 0.93 p.u. before stabilisation. 5.5.1.4 Local Compensation for Active Power and Active Power Control The operation of the inverter for power quality improvement with a dc source (a battery) was the focus of this set of simulations. The simulation variables from the study of reactive power were assumed here and all available control options tested. In addition, the simulation of the compensation of the active component of the load currents was performed as well as active power control when receiving an external signal from a management system. The inverter was operated in full load compensation mode and the active load power compensation function added after 5 seconds. After 15 seconds of simulation the inverter generated additional active power on the request of the management system. The results obtained are shown in Figures A-13 to A-16. It is evident from comparison of Figures A-9 and A-13 that both load active power compensation and active power control result in a significantly smoother voltage waveform. In addition, the current required from the MV / LV transformer can be seen to be significantly reduced for active load compensation and control. Note however, that when active power control is performed the inverter current is significantly higher than with reactive power control only. Rms voltages are noticeably higher for compensation of load active power and active power control than for reactive power compensation and control only (see Figures A-10 and A-14). Harmonic distortion is also noticeably lower with active power compensation and control. 5.5.1.5 Distant Compensation The final set of simulations performed in relation to load compensation was to demonstrate how distant compensation can be performed for the same test network. To achieve this functionality the control scheme was modified according to the principle described in section 5.4.1. Results of simulation are presented in Figures A-17 to A-20 for the compensation of harmonics, unbalance, active and reactive components, respectively. From comparison of Figures A-17 to A-20 with those for the other studies it is evident that the voltage waveform at the point of connection is considerably smoother than with any of the other simulations and the MV / LV transformer current is also significantly lower. Rms busbar voltages are very stable at around 1 p.u., asymmetry between phases very low and harmonic distortion almost negligible. 5.5.1.6 Comparison of Simulations Having obtained the results of the various simulations of the network performance with the inverter both connected and disconnected, one can assess its effectiveness for power quality management in low voltage distribution networks. The overall assessment is based on the voltage characteristics at the point of connection which have been determined by means of a specially designed module. These are the results presented in Figures A-3, A-4, A-7, A-8, A-11, A-12, A-15, A-16, A-19, and A-20. From comparison of these results it is evident that in all cases there is a noticeable improvement in power quality and a reduction in severity of the PQ indices. It must be noted however that both the degree of compensation and the maximum value of the inverter current depend on the control option. Furthermore, the simulation model may be used in practice to design control systems for selected tasks for a given inverter or to select inverter characteristics to meet requirements of particular power quality standards.

44

5.5.2 Dip Compensation This set of studies focused on the assessment of inverter capabilities in reducing voltage dips that have occurred in an LV network due to short-circuits in MV and HV networks. The test network is shown in Figure 11 in a graphical form as obtained from the PSCAD software. The inverter based compensation unit was operated in voltage control mode and was connected to the LV busbars of the MV/LV transformer.

20/0,4 kV 50 Hz400 kVA5 %

NOMINALPI

SECTION100 m

LOAD 1 LOAD 2

NOMINALPI

SECTION100 m

LOAD 3

NOMINALPI

SECTION100 m

LOAD 4

NOMINALPI

SECTION100 m

LOAD 5

NOMINALPI

SECTION100 m

LOAD 6

LOAD 7

NOMINALPI

SECTION100 m

NOMINALPI

SECTION100 m

LOAD 8

Voltage ControlledPWM Inverter

VoltageControl

Fire

Puls

IaIbIc

IaIbIc

VbVc

Va

VaLVbLVcL

Vstab

Vstab

Pgen

VnomVoltageParameter

MeterVbVc

Va

VaVbVc

0,23Vref

0,2Tmean

12

Figure 11: Test Network for Voltage Dip Compensation Studies In the studies performed, consideration was given to the coupling between the LV and MV networks as, for example, the power capabilities of a distribution transformer will influence the compensation effects. Assuming the same short-circuit voltages (in %), a small transformer power will result in a weak connection between networks, whereas a large transformer power will result in a stiffer network connection. The capability of the distributed generation source was varied by changing the reactor interfacing the inverter with the grid. A range of values for the reactor inductance were obtained by considering the values of LDmin, representing an acceptable distortion of the inverter voltage, and LDmax, the needed slope of the inverter U/I characteristic. Both symmetrical and asymmetrical short-circuits on the HV network were considered. 5.5.2.1 Symmetrical Short-Circuits Simulations were performed with an inverter unit of large capacity (represented by a coupling reactance of 0.5 mH) and an inverter unit of small capacity (represented by a coupling reactance of 1.0 mH). In both cases, simulations were performed with a weak connection to the MV grid and with a stiff connection to the MV grid.

45

Figure 12 illustrates the effect of voltage dip compensation with a unit of large capacity, where the coupling reactance is LD = 0.5 mH. To obtain voltage dips of different depths, short-circuits were assumed in different locations on the 110 kV line. The line “a” on Figure 12 illustrates the voltage on the LV busbars under short-circuit conditions. Lines “b” and “c” on Figure 12 illustrate how voltage dips can be reduced by the inverter when the supplying transformer power is 160 kVA (a weak connection with MV grid) and 400 kVA (a stiff connection with MV grid), respectively. For both transformers, a short-circuit voltage of uz% = 10 % was assumed.

0

0.2

0.4

0.6

0.8

1

1.2

0 2 4 6 8 10 12 14 16 18 20Distance of short-circuit location from station A (Figure 9) [km]

u LV

[pu] a

b c

area of the efectivecompensation

Figure 12: Voltage Dip Compensation Using Large Capacity DG Inverter a) without compensation, b) with compensation; the supply transformer of 160 kVA.

c) with compensation; the supply transformer of 400 kVA. A similar process was followed to obtained results for a small capacity inverter unit (LD = 1.0 mH). Results are presented in Figure 13.

46

0

0.2

0.4

0.6

0.8

1

1.2


u LV

[pu] a b c


Figure 13: Voltage Dip Compensation Using Small Capacity DG inverter a) without compensation, b) with compensation; the supply transformer of 160 kVA.

c) with compensation; the supply transformer of 400 kVA. Figures 12 and 13 illustrate the significant impact of the distribution transformer on the ability of the inverter to compensate voltage dips. Using an inverter unit of relatively large capacity in a grid with a weak connection to the MV network will assure mitigation of voltage dips due to short-circuits on a significant area of the HV network. To further illustrate a voltage dip event, waveforms are presented in Figure 14 for the symmetrical short-circuit in the middle of a 110 kV line. In this case the transformer power was 160 kVA and the compensation reactor inductance was LD = 1.0 mH, representing a small capacity inverter. During a short-circuit the compensation unit produces a reactive power of up to 5-times the transformer nominal power, depending on the depth of the dip, and consequently maintains the active power production to the required level.

47

Voltage waveforms a)

b)

-0.50

-0.25

0.00

0.25

0.50

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4

time [s]

u(t)

[V]

-0.50

-0.25

0.00

0.25

0.50

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4

time [s]

u(t)

[V]

Voltage RMS values a) b)

0.0

0.2

0.4

0.6

0.8

1.0

1.2

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4

time [s]

Pc [M

W]

0.00

0.20

0.40

0.60

0.80

1.00

1.20

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4

time [s]

Pc [M

W]

Inverter reactive (left) and active (right) power

-0.1

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4

time [s]

Qc

[MVA

r]

0.00

0.05

0.10

0.15

0.20

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4

time [s]

Pc [M

W]

Figure 14: Voltage Dip at LV Busbars With Short-Circuit in HV Network. a) without compensation, b) with compensation

5.5.2.2 Asymmetrical Short-Circuits The same simulations performed for symmetrical short-circuits were also performed for asymmetrical short-circuits and both weak and stiff connections to the MV grid were considered.

48

The results of the simulations performed for asymmetrical short-circuits show that the compensation process is similar to the symmetrical voltage dip compensation, although the range of dip amplitudes is narrower. The minimum voltage value in the case of symmetrical dips is zero, whereas the minimum value for asymmetrical dips is 50% of the nominal voltage value. Additional results relevant to those presented in Figures 13 and 14 have been obtained for single-phase and two-phase short-circuits and are presented in Figures 15 and 16. In addition, a voltage dip due to a fault occurring at a distance of 2 km from station B (see Figure 9) is presented in Figure 17 for a single-phase fault and in Figure 18 for a two-phase fault.

0

0.2

0.4

0.6

0.8

1

1.2

0 2 4 6 8 10 12 14 16 18 20

Distance of short-circuit location from station A (Figure 9) [km]

u LV

[pu]

a

b

c


Figure 15: Single-Phase Voltage Dip Compensation With Small Capacity DG Inverter a) without compensation, b) with compensation; the supply transformer of 160 kVA,

c) with compensation; the supply transformer of 400 kVA

0

0.2

0.4

0.6

0.8

1

1.2


u LV [

pu] a

b c


Figure 16: Two-Phase Voltage Dip Compensation With Small Capacity DG inverter a) without compensation, b) with compensation; the supply transformer of 160 kVA,

c) with compensation; the supply transformer of 400 kVA

49

a) b)

1-phase fault (2 km)

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4

time

U [p

u]


0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4

time

U [p

u]

Figure 17: Voltage Dip at LV Busbars For Single-Phase Short-Circuit on HV Line 2 km from Station B.

a) without compensation, b) with compensation a) b)


0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4

time

U [p

u]


0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4

time

U [p

u]

Figure 18: Voltage Dip at LV Busbars For Two-Phase Short-Circuit on HV Line

2 km from Station B. a) without compensation, b) with compensation

5.5.2.3 Summary of Results The results of these simulations can be summarised to demonstrate the abilities of the inverter to reduce voltage dips in the test network due to any fault in the HV network and are shown in Figure 19.

50

0

0.2

0.4

0.6

0.8

1

1.2

0 0.2 0.4 0.6 0.8 1

voltage dips without compensation U[pu]

volta

ge d

ips

with

com

pens

atio

n U

[pu]

transformer160 kVA

transformer400 kVA

LD = 0.5 mHLD = 1.0 mH

Figure 19: Inverter Compensation Ability for Voltage Dips in Test Network

5.6 Observation and Conclusions The simulation studies presented in this section have focused on using current and voltage controlled PWM inverters as power quality management options. The studies were divided into two parts, dependent on the type of the inverter control to be realised. The first set of studies were designed to investigate the abilities of the inverter when operating in current control mode to effect an improvement in the power quality indices defined in the EN 50160 standard. Based on these studies a number of important conclusions were obtained:

1. Modifying the inverter control option resulted in noticeable changes in the current and voltage waveforms.

2. The inverter can compensate the undesirable components of load currents (e.g. harmonic and negative sequence components) resulting in a decrease in voltage drop at the supply transformer and an improvement in the voltage waveform on the LV network.

3. Through variation of the reactive power component of the fundamental harmonic and positive sequence current the inverter can stabilise the supply voltage. However, the provision of this function is highly dependent on the specific details of the LV network to which the inverter is applied. The reactive current value can also be varied in response to an external signal (i.e. from a network management system).

4. It is possible to control the active component of the inverter current. In the case of inverter connected dc energy sources the instantaneous power produced by the source is passed on to the system if the energy produced can not be stored in the capacitor because of it limited storage.

5. Due to the possible diverse location of disturbing loads and the compensation limits of the grid, the reference current determination algorithm should include the concept of distance compensation. This ensures minimisation of active power losses and significantly improves the effectiveness of the application.

Thus, these studies have demonstrated that it is possible to configure the inverter to compensate a variety of different power quality phenomena as well as provide control of active and reactive power production. The ability to provide these functions effectively allows the inverter to emulate the functions of active power filters and DSTATCOM devices. The second part of the simulation studies was concerned with voltage controlled inverter applications. These studies clearly demonstrated how inverter connected distributed generation (and also related devices such as energy storage systems) can provide an effective means by which to reduce voltage dips that occur due to faults in the HV network. For a given inverter, the compensation ability is dependent on the coupling

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impedance between the LV and MV networks, itself dependent on the power capabilities of the distribution transformer. In assessment of the management options to improve power quality in LV grids with distributed generation, the method of simulation has been applied effectively and PSCAD/EMTDC recognised as a beneficial tool for conducting this analysis. The simulations were developed to specifically represent the operation of the test distribution network and included representative disturbing loads and power quality management options. By conducting these simulations and considering the specifically developed module for measuring power quality indices a number of capabilities have been provided:

• To evaluate power quality at any point within a distribution network according to the EN 50160 standard.

• Assess to what extent power quality can be improved using DG inverters or other dedicated power quality devices.

• Design inverter control circuits and identify relevant parameter values for which the requirements of the EN 50160 standard can be fulfilled.

6. Summary of Overall Management Option Effectiveness

6.1 Overall Effectiveness Rating System To allow an overall effectiveness rating to be generated for the considered power quality management options it was necessary to combine the individual conclusions obtained from the two sets of technical studies (see section 4.7 and 5.6). A traffic light system has been adopted as the mechanism behind the overall effectiveness rating system whereby five key ratings are considered as shown in Table 13.

A device has a minor capability, subject to certainconditions and requirements being met.A device has a significant capability, subject to certainconditions and requirements being met.A device has a minor capability that is currentlyachievable.A device has a significant capability that is currentlyachievable.

A device cannot provide any capability in this particularPQ phenomenon.

Table 13: Overall Effectiveness Ratings The overall effectiveness rating system has been applied to distributed generation, energy storage and novel controller technologies that are potentially capable of mitigating voltage dips, swells, under- and over-voltages as well as harmonic current distortion1. Load management schemes have also been included as they are the only existing approach for managing technical issues at a network level. The overall effectiveness summary for these particular management options is based on the conclusions from the two sets of technical assessment studies and is illustrated in Table 14.

1 Note that harmonic current distortion is the primary focus here as harmonic voltages are primarily caused by harmonic currents. Consequently, if harmonic currents can be reduced then harmonic voltage distortion will also be reduced.

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Note that Table 14 has been structured slightly differently with regards to distributed generation and energy storage technologies than the earlier presentation of management option capabilities outlined in section 3. This is to better reflect where the key effectiveness contributions and also the limitations of these devices lie (i.e. within the grid connection or the energy source) – see section 4. Note also that in Table 14 several of the ratings have a question mark symbol. This is to indicate that the effectiveness of the management options in these phenomena has not been quantified based on the results of the technical assessment studies. A brief explanation will now be presented in the next section explaining how the overall ratings outlined in Table 14 have been derived.

Short-Duration Voltage Dip

Short-Duration Voltage Swells

Long-Duration Undervoltage

Long-Duration Overvoltage

Harmonic Current Distortion

Synchronous Machine

Induction Machine

Doubly-fed Induction Machine

PWM Inverter

Controllable DG

Stochastic DG

Battery

Flywheel

Micro-SMES

Supercapacitor

Dynamic Voltage Restorer (DVR)

Distribution STATCOM (DSTATCOM)

Magnetic Synthesizer

Active Power Filters

Load Management Schemes

Contribution

Grid Connection

Energy Source

Management Option

Other Options

?

?

?

? ?

?

?

? ?

? ?

Table 14: Overall Management Option Effectiveness for Mitigating Highest Priority PQ Phenomena

6.2 Explanation of Management Option Ratings

6.2.1 Grid Connection 6.2.1.1 Rotating Machines The capabilities of distributed generation and energy storage devices utilising synchronous machines, induction machines or doubly-fed induction machines has been described in sections 3.2.2.1 to 3.2.2.3.

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6.2.1.2 PWM Inverter The results of the technical studies presented in section 5 have shown how PWM inverters can be modified for load compensation providing a significant capability to address under- and over-voltages and harmonic distortion. In addition, through voltage support in abnormal conditions PWM inverters have also shown a significant capability to mitigate voltage dips. Such devices may also be able to provide a significant capability to mitigate voltage swells through anti-phase current injection, although this functionality was not considered in the detailed studies presented in section 5. Consequently it is only shown as a potential capability. The modifications required to PWM inverters to provide load compensation and voltage support in abnormal conditions are described in section 5.3.4.

6.2.2 Energy Source 6.2.2.1 Controllable DG Given that distributed generation units based on controllable energy sources have (from the point of view of power quality phenomena) an infinite availability of energy, if equipped with a suitable grid connection they could be used to successfully mitigate all of the phenomena outlined in Table 14. Note however that the ratings shown for controllable DG have not been shown with a question mark even though they have not been demonstrated through the associated technical studies. This is because no quantification is needed for this particular energy source given the wide variety and available sizes of controllable distributed generation units. 6.2.2.2 Stochastic DG Distributed generation devices that have a stochastic energy source (e.g. wind turbines, PV, solar thermal, wave power, etc) cannot be relied upon to provide a capability for mitigating steady-state phenomena such as under- and over-voltages or harmonic distortion. They do however present a potentially significant resource to mitigate short-duration phenomena such as voltage dips or swells (if equipped with a suitable grid connection) since the time-period over which the mitigation capability must be provided is fairly short. 6.2.2.3 Battery Based on the results of the technical assessment studies presented in section 4, battery devices have electrical systems capable of proving a minor capability as regards mitigation of voltage dips and swells. That is, mitigation of 10% dips or swells is achievable in both the UK and Europe but mitigation of 30% dips or swells is unlikely, particularly so in large networks, unless a very large battery system is installed. Battery systems also present a means by which to mitigate 10% under-voltages, but this is only a minor capability given the large energy storage requirements. Mitigation of over-voltages is also unlikely, but may be possible with very large battery systems. Although harmonic distortion is a steady-state phenomenon, the energy storage requirements when the grid connection (PWM inverter) is configured to provide harmonic mitigation are likely to be such batteries can present a significant active resource.

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6.2.2.4 Flywheel The capabilities of flywheel based storage systems are almost identical to that of battery systems except that flywheels present a significant capability for power quality mitigation given the high current injection capability and large energy storage possible with these devices. Multiple units will certainly have sufficient storage to deal with long duration voltage variations. Similarly to battery systems, a significant active capability exists for mitigation of harmonic distortion using flywheels as the energy storage required when the grid connection is configured as a STATCOM is currently achievable. 6.2.2.5 Micro-SMES Although micro-SMES devices have a significant capability to mitigate voltage dips and swells, given that they are not widely available for low voltage distribution networks they have been assigned a potential rating. The potential contribution for mitigation of under-voltages on the other hand is likely to be fairly minor given the fairly low stored energy (in comparison with other technologies) and there will almost certainly be insufficient energy storage for over-voltage mitigation. The low energy storage requirements for a PWM inverter when configured to provide harmonic distortion mitigation is achievable with current micro-SMES systems. 6.2.2.6 Supercapacitor Supercapacitors can be used to mitigate voltage dips but have only been given a minor potential effectiveness rating given that multiple units would have to be used. Although the energy storage capacity of supercapacitors is fairly limited it is still more than adequate to provide a significant capability for mitigating harmonic distortion when coupled with a grid connection configured to this mitigation function.

6.2.3 Other Options 6.2.3.1 Dynamic Voltage Restorer Capabilities described in section 3.5.2.1. 6.2.3.2 Distribution STATCOM Capabilities described in section 3.5.2.2. 6.2.3.3 Magnetic Synthesizer Based on the results of the technical assessment studies presented in section 4 significant voltage dip and swell mitigation could be achieved within a current injection of several 100A. Single magnetic synthesizers

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for use at LV are available which can provide this facility. A significant rating has been applied for under- and over-voltages as although a single unit may not have sufficient energy storage, these devices can easily be operated in parallel [13]. Magnetic synthesizers also exhibit a significant capability to prevent harmonic distortion from reaching a load and can be used to prevent harmonic distortion from a 'dirty' load propagating into the distribution network. Note however that this capability has not been confirmed through either set of technical assessment studies. 6.2.3.4 Active Power Filter Although there are a number of basic types of active power filter (e.g. shunt connected, series connected, hybrid, shunt/series conditioner) all have a significant ability to mitigate harmonic distortion. Given that the PWM inverter when providing load compensation was in fact operating as an active power filter, the harmonic distortion abilities of APFs have in effect been validated through the technical studies. Series active power filters can also be used to mitigate voltage dips and swells [11], but again the true effectiveness of the device as regards short-duration voltage variations has not been quantified through the technical studies. 6.2.3.5 Load Management Schemes Load management schemes are already used within LV distribution systems and provide utilities with a significant capability to manipulate steady state conditions through control of network loads. A significant effectiveness rating has been applied to load management schemes but since no technical assessment studies were performed to quantify the true contribution of such schemes, the overall effectiveness rating has been assigned a question mark.

6.3 Supporting Information The overall effectiveness ratings presented in the previous section were obtained by considering collectively the effectiveness conclusions obtained from the two sets of studies. However, there are a number of additional factors which although not considered explicitly within the overall effectiveness rating process are also of relevance. These are now discussed.

6.3.1 Network Effects In both sets of technical studies performed to quantify the effectiveness of the power quality management options, the impact on the local distribution network was explicitly considered. For the specific assessment of the phenomena and management option energy source, a number of important network related conclusions were observed, these were1:

1 It should be noted that the conclusions outlined concerning the network impact of power quality management devices were not considered within the effectiveness ratings since the studies applied equally and no distinction could be made between different types of devices.

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• The voltage increase observed at the substation and in an additional circuit during a voltage dip

was negligible even with a 500A current injection. • The voltage decrease observed at the substation and in an additional circuit during a voltage

swell was negligible even with a 500A current injection. • Upstream from the disturbance the THD is attenuated and end loads in neighbouring circuits all

have similar, but not exactly the same THD. • As the source of the disturbance moves towards the end of the circuit the average THD across all

circuit measuring points increases. It is evident from the first two conclusions that the effectiveness provided by a power quality management option in response to a voltage dip or swell (and under- / over-voltage) is limited to a fairly small network area. Consequently, the only way that such phenomena are likely to be mitigated for a particular network area comprising multiple substation circuits is to ensure that there are one or more capable devices in each substation circuit. For mitigation of harmonic current distortion there is however wider scope for utilising a management option to provide a benefit at a network level, assuming that the harmonic mitigation device is up-stream from the loads which are to be managed. Furthermore, if there are multiple devices capable of mitigating harmonic distortion then the nearest upstream device will provide the highest mitigation capability. Nevertheless, care will need to be taken to ensure that the mitigation of harmonic distortion produced by a load is not improved at the expense of introducing additional harmonic distortion to another part of the distribution network. From the technical assessment studies performed to quantify the effectiveness of PWM inverter connected technologies a number of network related conclusions were also obtained. These were:

• The inverter has shown the significant ability to reduced long duration voltage variations. However, the effectiveness of this inverter operation was found to be highly dependent on the specifics of the test network.

• Distributed generation and energy storage devices utilising voltage controlled inverters have been shown to have a significant effectiveness for mitigating voltage dips occurring as a result of MV network faults.

Note that as with the conclusions obtained from the specific study of the power quality phenomena these network conclusions are applicable to many other devices and consequently were not considered within the effectiveness rating system.

6.3.2 Regulatory Issues The utilisation of distributed generation and other technologies for power quality improvements at a network level raises a number of issues associated with the regulatory mechanisms. Such regulatory issues include DG ownership and power quality standards and although not considered within the overall effectiveness ratings are certainly of importance and consequently are now discussed. Given that in many countries distribution network operators (DNOs) are prohibited from owning distributed generation in their own networks (except under special circumstances, of which the provision of network support and power quality improvements is typically one) the large majority of DGs installed in a typical network will be privately own. Consequently, if DG is to be used to provide a power quality benefit then it will be necessary for the DNO to motivate the private owners of these devices to provide a specific power quality capability. Some form of incentivisation will almost certainly be necessary to convince them to use their devices to benefit the wider network. Energy storage devices fall into a similar category and again, a DNO will likely be prohibited from owning such devices in their own network unless there for a technical reason. There is likely to be less interest anyway for a DNO to invest in energy storage technologies since these devices are almost exclusively purchased by customers interested in protecting an individual load.

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Again, some form of incentivisation will likely be necessary to convince them to use their devices to benefit the wider network. As stated in the previous section, when mitigating harmonic current and voltage distortion through the injection of harmonic cancellation currents care must be taken to ensure that the applicable European standard EN50160 is still met. Situations where this may not be the case are when the harmonics generated by a load on one network circuit are eliminated through the injection of cancellation currents which subsequently propagate to and worsen the THD of a separate section of the network. In a network with multiple devices capable of mitigating harmonic current distortion this could become especially problematic unless an overview is maintained of the harmonic problems and mitigation sources to be activated within a network. In some countries, the active participation of LV-connected DG in power quality management may be forbidden. In particular the dynamic variation of harmonic contribution and power factor may be prohibited by regulations concerning the connection of DG and LV. In these circumstances, LV-connected DG cannot contribute to power quality management. However, as DG becomes more widespread and power quality becomes of greater importance, these regulations are likely to be revised to exploit the potential benefits outlined in this report.

6.3.3 Future Effectiveness The overall effectiveness ratings presented in section 6.3.1 have been obtained by considering results of the technical assessment studies alongside the functionality of currently available examples of the considered management options. While some of these options are fairly well developed and consequently their effectiveness rating will not change over the coming years, for other devices this will not be the case. The ability to modify PWM inverter grid connections to provide power quality capabilities to mitigate short and long-duration voltage variations was the specific focus of the second set of technical studies presented in section 5. As discussed in section 6.2.1.3, doubly fed induction machines also utilise a power electronic converter system as a grid connection interface. If the power electronics contained within such grid connections can be modified in a similar manner to PWM inverter interfaces then it is possible that devices utilising doubly-fed induction machines could also present a significant capability to mitigate voltage dips, swells and even harmonic current distortion. The rating shown in Table 14 for doubly-fed induction machine may therefore be changed to an active significant (large green) rating if the necessary modifications are made and such units become widely available. Conventional wisdom states that stochastic distributed generation devices (e.g. PV systems, wind generation, etc) are handicapped by the intermittent nature of their generation output and cannot be relied upon to provide a power quality function, even if the grid connection is suitable. An alternative view point is that although such stochastic energy sources cannot provide a guaranteed supply of energy and hence power quality functionality at any time of the day or night, the output of such systems is predictable to a certain degree. For example, although the energy output of a PV system will vary throughout a day and also throughout particular seasons, the system can almost certainly be used to provide a power quality output of some description (assuming a suitably configured grid connection) during daylight hours. What the actual level of output such systems will be able to provide is as yet unknown, but in the future when many more such systems are operating within low voltage distribution networks and there has been detailed analysis of the energy outputs, it may be possible to quantify the power quality contribution that such systems can make. Consequently, the potentially significant rating for stochastic DG technologies shown in Table 14 may be changed to an active significant (large green) rating. Dynamic voltage restorers and distribution STATCOMs are devices that have been applied with great success within medium and high voltage systems. As more and more of these devices are manufactured and installed at such voltages it is very likely that they will become more widely available and cost effective for low voltage distribution systems. Such a development would allow the potentially significant contribution of these devices shown in Table 14 to be changed to an active significant (large green) rating.

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6.4 Key Outcomes There are a number of important outcomes associated with the overall effectiveness ratings presented in this section.

• An overall effectiveness rating has been developed for the grid connection and energy source of distributed generation and energy storage devices. Together these ratings can be used to generate an overall effectiveness rating for an individual device by considering collectively its energy source and grid connection.

• An overall effectiveness rating has been developed for a selection of novel power quality controllers which have shown considerable potential for improving LV grid power quality. Load management schemes have also been included as they represent the only current option for managing power quality and technical issues at a network wide level.

• In terms of the effectiveness of the options across the five power quality phenomena considered, there are most options available for mitigation of harmonic current distortion, followed by under-voltage, over-voltage, voltage dips and finally voltage swells for which there are fewest available options.

• Of the available management options, inverter connected devices are the most promising technology for improving power quality in low voltage grids.

7. Contribution From and Application to Other DISPOWER Work Packages This section presents brief details of how Task 1.8 links to other activities within the DISPOWER project.

7.1 WP 2.6 – Contribution to Grid Quality Improvement by Decentralised Inverters Task 2.6 is concerned with assessing the contribution to grid quality improvement of decentralised inverters. The task includes the modelling of inverter topologies and the study of control algorithms to supply loads, provide grid voltage support, and compensate for slow-varying harmonic distortion. The potential for distributed power quality management with autonomous control is also being investigated. The findings from task 2.6 [21,22] helped inform the assessment of options for improving power quality in this task, particularly regarding the capabilities of inverter connected distributed generation and energy storage.

7.2 WP 2.7.2 – Inventory of Distributed Generation Technologies As part of task 2.7, which is concerned with energy flows and the classification of low voltage grid structures, an inventory of distributed generation technologies has been developed [23]. This provides a comprehensive list of distributed generation technologies in low voltage grids and provides easy access to information on installed devices, models and examples of commercial offerings for each technology. The inventory of technologies informed the assessment of the effectiveness of those different technologies in improving power quality.

7.3 WP 9 – Development of a Power Operation and Power Quality Management System (PoMS) in Low Voltage Grids

WP 9 is concerned with the development of a Power Operation and Power Quality Management System (PoMS) [24]. The PoMS system will facilitate economic optimisation of LV grids and connected devices for

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generation, storage and consumption of electrical energy. It will also seek to improve power quality on LV grids by exploiting the capabilities of distributed resources and developing reaction strategies to respond to certain grid conditions. Thus, the assessment conducted in this task of options for improving power quality and their effectiveness will directly support the development of PoMS. Likewise, the prioritisation of power quality phenomena will help direct the development of PoMS reactions strategies.

8. Conclusions and Summary Changing customer requirements mean power quality is an increasingly important aspect of the supply of electrical energy. The expansion of new and renewable energy sources as small generators connected to low voltage grids presents both threats and opportunities to power quality on those grids. As part of the DISPOWER project, the work reported here has investigated the effectiveness of various options in improving power quality in low voltage distribution networks. The wide range of power quality phenomena were introduced and defined. Prioritisation of these phenomena allowed the work to focus on the phenomena of particular interest. Prioritisation was based on the impact of each phenomenon on customers and the network, the frequency of occurrence, the ability to monitor each phenomenon and the availability of methods to mitigate the power quality problem. Voltage dips and swells, under- and over-voltages, and harmonic distortion were identified as the power quality phenomena of the highest priority. Options for managing power quality in low voltage distribution networks include distributed generation, energy storage devices, load management schemes, as well as traditional and novel power quality control devices. These options were evaluated in terms of their potential capability to mitigate the defined power quality phenomena. Technical studies were performed to validate the capabilities of the various options. The highest priority power quality phenomena were studied to determine what was required from a generator, energy storage device or control device to mitigate different disturbances. It was found that some significant disturbances could not be mitigated effectively but that smaller disturbances could be addressed. Detailed study of pulse width modulated (PWM) inverters was performed to assess their potential role in improving power quality. Important observations from the assessment of options for power quality improvements in low voltage grids are as follows:

• Mitigation of voltage dips is achievable with controllable DG as well as flywheel and battery storage systems with PWM inverter grid connections, although the inverter must employ advanced control techniques.

• The only devices available to actively manage voltage swells within low voltage grids are magnetic synthesisers and possibly PWM inverter connected devices.

• Mitigation of under- and over-voltages can be achieved using controllable DG – of several grid connection types – as well as magnetic synthesisers and load management schemes.

• Harmonic distortion mitigation is achievable using controllable DG devices and energy storage devices with PWM inverter grid connections as well as many existing control devices such as active power filters and magnetic synthesizers.

• Short and long duration voltage variations can only be mitigated in areas of the network close to where DG, storage or control devices are installed. Furthermore, relatively large or very many devices will be required to provide the necessary voltage support.

• In contrast, a relatively small capacity of devices is required to influence harmonic distortion across a low voltage grid.

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In reaching conclusions on the overall effectiveness of the various options for improving power quality, other factors that influence effectiveness were considered including the impact on the local network, regulatory issues in different countries, and the future development of the technologies concerned. An overall rating was derived for each of the identified options for improving power quality in low voltage grids. The overall ratings reflected the evaluation of technologies and the technical studies performed. It was found that the most promising technologies for improving power quality on low voltage grids are inverter-connected devices with controllable generators or flywheels. Amongst the other options, magnetic synthesisers were identified as having the greatest potential value. This report presented an assessment of the options for improving power quality in low voltage distribution networks with distributed generation, energy storage and power quality control devices. This assessment is of great value and relevance as the installation of small-scale new and renewable energy sources accelerates across Europe, while consumers continue to demand ever-higher standards of power quality. Within the DISPOWER project, this work will inform the further development of the Power Operation and Power Quality Management System (PoMS). In particular, the prioritisation of power quality phenomena and the identification of the mitigation options with greatest potential will be applied in the development of strategies for reacting to power quality disturbances in low voltage grids.

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9. References 1. Tone, Y., Mouri, K.: "A Concept of Energy Network Combined with Distributed Power Generation and

Internet Services", Proceedings of the 2002 International Joint Power Generation Conference, pp. 1019 – 1026, 2002.

2. Tsuij, K., Nara, K., Hasegawa, J., Oyama, T.: "Flexible, Reliable, and Intelligent Electric Energy Delivery System: Concepts and Perspective", Proceedings of the American Power Conference, pp. 504 – 511, 1999.

3. Dugan, R. C., McGranaghan, M. F., Beaty, H. W.: Electrical Power Systems Quality, McGraw-Hill, 1996.

4. Office of Gas and Electricity Markets (OFGEM).: "The Distribution Code of Licensed Distribution Network Operators of Great Britain", Issue 3, September 2003.

5. Information on battery systems – www.batterywholesale.com 6. Siemens Power Transmission and Distribution, Flexible AC Transmission System –

www.siemens.com/ptd 7. American SuperConductor – www.amsuper.com 8. Maxwell Technologies – www.maxwell.com 9. Weedy, B. M.: Electric Power Systems, John Wiley & Sons, 1987. 10. Toshiba Mitsubishi Transmission and Distribution (TMT&D) – www.tmt-d.com. 11. Hanzelka, Z.: "WP 1.8.2: Improving PQ in LV Grids with Different Options – Contribution of the

Technical University of Lodz", October 2002. 12. Maule D.: Voltage dips mitigation (5.3.2). Power Quality Application Guide, COPPER. 13. Liebert Datawave (Magnetic Synthesizer) – www.liebert.com. 14. Dorr, D., and Nastasi, D. EPRI PEAC Corp., Knoxville, Tenn. Power Quality, Jun 1, 2002. 15. INTRACOM Energy Management: Ripple Control Receiver – www.intracom.com. 16. Pacific Gas and Electric Load Management Programs. 17. Bonneville Power Administration: Demand Exchange (DEMX) Program – www.bpa.gov. 18. EN50160.: "Voltage Characteristics of Electricity Supplied by Public Distribution Systems", European

Committee for Electrotechnical Standardisation (CENELEC), November 1994. 19. Ghosh, A., Ledwich, G.: Power Quality Enhancement Using Custom Power Devices, Kluwer Academic

Publishers, 2002. 20. Mienski, R., Pawelek, R., Wasiak, I.: "Application of STATCOM Controllers for Power Quality

Improvement – Modeling and Simulation", 10th International Conference on Harmonics and Quality of Power, Rio de Janeiro, October 2002.

21. Jahn, J.; “Voltage-controlled inverters used for Power-Quality Improvement”; 3 October 2003; DISPOWER Work Package 2.6, Document no.: tech_2003_0018

22. Prodanovic, M.; Green, T.C.; “Control of power quality in inverter-based distributed generation”; 28th Annual Conference of the Industrial Electronics Society, IEEE 2002, IECON 02, 5-8 November 2002, volume 2, pages 1185 -1189

23. Foote, C.E.T., Espie, P., Burt, G.M.: “Inventory of Distributed Generation Technologies”; 16 January 2003; DISPOWER Work Package 2.7.2, Document no.: tech_2003_0019

24. Jantsch, M., Thoma, M., Puls, H-G., Benz, J., Erge, T., Vogel, M., Kroger-Vodde, A., and Sauer, D-U.: "WP9: General Concept for Hardware and Software including Control Algorithms for Power Quality Management", August 2003.

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http://www.batterywholesale.com/

http://www.siemens.com/ptd

http://www.amsuper.com/

http://www.maxwell.com/

http://www.tmt-d.com/

http://www.liebert.com/

http://www.intracom.com/

http://www.bpa.gov/

Appendix This appendix presents the detailed results and waveforms for the set of studies performed to quantify the power quality capabilities of inverter connected devices when providing load compensation. Figure A-1 details the current waveforms of the loads (rectifier and impedance), inverter and supply transformer as well as the voltage at the network point where the inverter will be connected – it is not connected in the results shown in Figure A-1.

Figure A-1: Loads, Inverter and Supply Transformer Current Waveforms and Voltages at the Inverter Connection Point

Figure A-2 shows the same waveforms outlined in Figure A-1 but focussing on the 5 – 6 second time period to provide additional detail.

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Figure A-2: Loads, Inverter and Supply Transformer Current Waveforms and Voltages at the Inverter Connection Point for Specific Time Period

Figure A-3 illustrates busbar voltages, voltage asymmetry between phases, voltage fluctuations (flicker) and total voltage harmonic distortion. Figure A-4 provides additional detail for the voltage harmonic distortion.

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Figure A-3: EN50160 Power Quality Indices for Network with no Inverter

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Figure A-4: Harmonic RMS Values for Network with no Inverter

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Figure A-5 details the current waveforms of the loads (rectifier and impedance), inverter and supply transformer as well as the voltage at the network point where the inverter is connected. Note that an inverter using a capacitor provides compensation for local load harmonics and phase unbalance after 5 seconds.

Figure A-5: Loads, Inverter and Supply Transformer Current Waveforms and Voltages at the Inverter Connection Point (Compensation After 5 Seconds)


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Figure A-6: Loads, Inverter and Supply Transformer Current Waveforms and Voltages at the Inverter Connection Point For Specific Time Period (Compensation After 5 Seconds)

Figure A-7 illustrates busbar voltages, voltage asymmetry between phases, voltage fluctuations (flicker) and total voltage harmonic distortion for the case where local load and unbalance compensation is provide after 5 seconds. Figure A-8 provides additional detail for the voltage harmonic distortion.

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Figure A-7: EN50160 Power Quality Indices for Network (Compensation After 5 Seconds)

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Figure A-8: Harmonic RMS Values for Network (Compensation After 5 Seconds)

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Figure A-9 details the current waveforms of the loads (rectifier and impedance), inverter and supply transformer as well as the voltage at the network point where the inverter is connected. Note that an inverter using a capacitor provides compensation for local load harmonics and phase unbalance at the beginning of the simulation. After 5 seconds load reactive power compensation is provided and after 10 seconds reactive power control. After 15 seconds voltage stabilisation is performed.

Figure A-9: Loads, Inverter and Supply Transformer Current Waveforms and Voltages at the Inverter Connection Point (Compensation After 5, 10 and 15 Seconds)


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Figure A-10: Loads, Inverter and Supply Transformer Current Waveforms and Voltages at the Inverter Connection Point For Specific Time Period (Compensation After 5, 10 and 15 Seconds)

Figure A-11 illustrates busbar voltages, voltage asymmetry between phases, voltage fluctuations (flicker) and total voltage harmonic distortion for the case where load reactive power compensation and reactive power control is provided. Figure A-12 provides additional detail for the voltage harmonic distortion.

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Figure A-11: EN50160 Power Quality Indices for Network (Compensation After 5, 10 and 15 Seconds)

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Figure A-12: Harmonic RMS Values for Network (Compensation After 5, 10 and 15 Seconds)

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Figure A-13 details the current waveforms of the loads (rectifier and impedance), inverter and supply transformer as well as the voltage at the network point where the inverter is connected. Note that an inverter using a battery provides compensation for local load harmonics, phase unbalance and load reactive power at the beginning of the simulation. After 5 seconds load active power compensation is provided and after 15 seconds active power control.

Figure A-13: Loads, Inverter and Supply Transformer Current Waveforms and Voltages at the Inverter Connection Point (Compensation After 5 and 15 Seconds)


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Figure A-14: Loads, Inverter and Supply Transformer Current Waveforms and Voltages at the Inverter Connection Point For Specific Time Period (Compensation After 5 and 15 Seconds)

Figure A-15 illustrates busbar voltages, voltage asymmetry between phases, voltage fluctuations (flicker) and total voltage harmonic distortion for the case where load active power consumption is provide after 5 seconds and active power control after 15 seconds. Figure A-16 provides additional detail for the voltage harmonic distortion.

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Figure A-15: EN50160 Power Quality Indices for Network (Compensation After 5 and 15 Seconds)

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Figure A-16: Harmonic RMS Values for Network (Compensation After 5 and 15 Seconds)

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Figure A-17 details the current waveforms of the loads (rectifier and impedance), inverter and supply transformer as well as the voltage at the network point where the inverter is connected. Note that the inverter uses a battery to provide distant compensation for all network loads.

Figure A-17: Loads, Inverter and Supply Transformer Current Waveforms and Voltages at the Inverter Connection Point (Distant Compensation)


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Figure A-18: Loads, Inverter and Supply Transformer Current Waveforms and Voltages at the Inverter Connection Point For Specific Time Period (Distant Compensation)

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Figure A-19 illustrates busbar voltages, voltage asymmetry between phases, voltage fluctuations (flicker) and total voltage harmonic distortion for the case where distant compensation is provided. Figure A-20 provides additional detail for the voltage harmonic distortion.

Figure A-19: EN50160 Power Quality Indices for Network (Distant Compensation)

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Figure A-20: Harmonic RMS Values for Network (Distant Compensation)

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Scottish & Southern Energy Endowed Research Fellowship at ...€¦ · particular interest to the...

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Transcript of Scottish & Southern Energy Endowed Research Fellowship at ...€¦ · particular interest to the...