Intermittency Mitigation and Power Quality …Intermittency Mitigation and Power Quality Improvement...

ISBN: 97-8-93-81195-82-6 Proceedings of NJCIET 2015

Canara Engineering College, Mangalore NJCIET-2015 433

Intermittency Mitigation and Power Quality Improvement of Grid Integrated with

Distributed Energy Resources

Prasanna

1, Shivalinga Swamy

2,

1P.G Scholar, Department of EEE, AMC Engineering College,Bangalore, India,

Email: [email protected] 2Assistant Professor, Department of EEE, AMC Engineering College,Bangalore, India,

Email: [email protected]

Abstract.Renewable energy generation is growing fast and ideas such as smart grid are trying to change the role of a consumer

from being a passive consumer to an active contributor who can supply stored excess power in various DERs, such as solar,

wind, hydro, Fuel cells and Storage technologies back to the distribution grid or the micro -grid. Increasing amounts of

distributed energy resources located on the customer side of the electric grid represent both a challenge and an opportunity for

grid operators. There is a corresponding increase in power quality problems and intermittencies on the dist ribution grid. In

order to reduce the intermittencies and improve the power quality of the distribution grid, an Ultracapacitor integrated powe r

conditioner is proposed in this paper. The simulation of Ultracapacitor supported Unified Power Quality Conditi oner system,

which consists of grid-tied inverter and DC/DC converter is performed using MATLAB/ Simulink.

Keywords: The Unified Power Quality Conditioner (UPQC) Active power filter (APF); DC/DC converter; d –q Control;

dynamic voltage restorer (DVR); Ultracapacitors(UCAP).

1 Introduction

The benefits associated with distributed energy resources (DERs) installations are often significant and numerous. DERs can be

defined as the concept of connecting generating units of small sizes, between several kW to a few MW. The primary source of

energy for these generators can be the renewable sources such as wind, solar, hydro, and biomass. These generators are

connected either to the medium voltage or low voltage sections of the electric grid [5]. Most often th ey are connected near the

load centers or the low voltage networks. They almost always provide tangible economic benefits, such as energy savings or

transmission and distribution upgrade deferrals, as well as intangible benefits, such as power quality impr ovements that lengthen

maintenance or repair intervals for power equipment.With the increase in the penetration of the distribution energy resources

(DERs), there is a corresponding increase in the power quality problems and intermittencies on the distribution grid in the

secondsto minutetime scale [1]. As more sensitive loads, such as computers, automation equipments, communication equipments

and manufacturing processes have come into wide use, the power quality is a big issue in both customer and utility company.

Since these equipments are very sensitive for the input voltage disturbances, the inadequate operation or the fault of these loads

brings about huge losses [3]. Also the insufficient power quality can be caused by failures and switching operations in the

network (voltage dips and transients) and by network disturbances from loads (flickers, harmonics and phase imbalance). The

elimination or mitigation of disturbances propagated from the source side and the other loads interconnected is critical for

improving the operational reliability of these critical loads. There have been increased focus in the recent years on the concept of

mitigating intermittent output of distributed energy resources (DERs) using Unified Power Quality Conditioner (UPQC) with

energy storage devices.The Unified Power Quality Conditioner (UPQC) is a distribution level controller which is to compensate

both source voltage sag and load current imperfections in power distribution system. Recently the Energy Storage System (ESS)

is widely applied in power electronics devices [11]. Despite the variety of the energy storage devices, the Ultracapacitor (UCAP)

or so-called electrochemical capacitor is one of the most popular choices for the energy storage system because of its distinct

characters. The reason behind overview of ultracapacitor energy storages system is that ultracapacitors charge very rapidly and it

can charge fully within few seconds. It hasextremely low internal resistance or effective series resistance and consequently high

efficiency (95% or more), with extremely low heating levels.It provides fast access to the stored energy and has very high rates

of charge and discharge.Ultracapacitors have wide operating temperature range (from -45°C to +55°C) and fast transient

response (~20 micro seconds).Ultracapacitors can be charged up to their full capacity and discharged all the down to a zero -

charge state.

mailto:[email protected]

mailto:[email protected]



The research work presented here aims at analyzing the technical issues and solutions of connecting distributed energy reso urces

to a grid/distribution system. The UPQC can compensate the reactive power, harmonic current, voltage sag and swell, voltage

unbalance, and the voltage interruption. The elimination or mitigation of disturbances propagated from the source side and th e

other loads interconnected is critical for improving the operational reliability of these critical loads. The performance of

proposed system was verified through model built in MATLAB/Simulink.

2.System Description

The system has two inverter and an ultracapacitor integrated DC/DC converter connected to distribution grid integrated with

DERs. Two inverters acts as series dynamic voltage restorer (DVR) and shunt active power filter (APF) respectively. DVR

prevents sensitive loads from experiencing voltage sags/swells, and APF prevents the grid from supplying non sinusoidal

currents when the load is nonlinear [1]. The design goal of the traditional UPQC was limited to improve the power quality of the

distribution grid by being able to provide sag, swell, and harmonic current compensation. In this paper, energy storage

integration into the power conditioner topology is being proposed, which will allow the integrated system to provide addition al

functionality.

Most DERs are connected to the grid through a three-phase grid-tied inverter which can be controlled to meet various control

objectives such as supplying active power, reactive power, harmonic compensation, and frequency regulation etc. A three phase

shunt active power filter (APF) can be controlled i n such a way that it meets all the same control objectives. Furthermore, a

three-phase grid-tied inverter is similar in topology to a three-phase shunt APF which opens up the possibility of exploring the

vast amount of literature available for shunt APFs and modifying the control strategies to suit the present control objectives of

the grid-tied inverter.Integration of active power filter (APF) with UCAP energy storage which gives the integrated APF -UCAP

system active power capability which aids in providing renewable intermittency smoothing, active and reactive power support to

the distribution grid. This cannot be achieved by a conventional APF with dc-link capacitor which does not have active power

capability.

Fig.1.One line diagram of grid integrated DERs system.

2.1Three Phase Inverters

The one-line diagram of the system is shown in Fig. 1. The power stage consists of two back-to-back three-phase voltage source

inverters connected through a DC-link capacitor. UCAP energy storage is connected to the dc-link capacitor through a

bidirectional dc–dc converter. The series inverter is responsible for compensating the voltage sags and swells; and the shunt

inverter is responsible for active/reactive power support and renewable intermittency smoothing. The complete circuit diagram

of the series DVR, shunt APF, and the bidirectional dc–dc converter is shown in Fig. 2. Both the inverter systems consist of



IGBT module, its gate-driver, LC filter, and an isolation transformer. The dc-link voltage Vdc is regulated at 260V for optimum

voltage and current compensation of the converter and the line–line voltage Vab is 208V. The goal of this project is to provide

the integrated power conditioner and UCAP system with active power capability 1) to compensate temporary voltage sag (0.1–

0.9p.u.) and swell (1.1–1.2p.u.), which last from 3s to 1min and to provide active/reactive support and renewable intermittency

smoothing, which is in the seconds to minutes time scale.

Fig. 2. Model of power conditioner with UCAP energy storage

Fig. 3. Proposed MATLAB Simulink model.

2.2 Bidirectional Dc–Dc Converter and Ultracapacitor

A bidirectional DC/DC converter is required as an interface between the UCAP and the dc-link, since the UCAP voltage varies with the amount of energy discharged, while the dc-link voltage has to be stiff. The dc–dc converter should operate in Discharge mode, while providing active/reactive power support and voltage sag compensation. The dc–dc converter should also be able to operate in bidirectional mode to be able to charge or absorb additional power from the grid during intermittency smoothing. In this



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paper, the bidirectional dc–dc converter acts as a boost converter, while discharging power from the UCAP and acts as a buck converter while charging the UCAP from the grid.

Ultracapacitors can deliver very high power in a short time span; they have higher power density and lowe r energy density when

compared with Li-ion batteries [9]. The major advantage UCAPs have over batteries is their power density characteristics, high

number of charge–discharge cycles over their lifetime, and higher terminal voltage per module. These are id eal characteristics

for providing active/reactive power support and intermittency smoothing to the distribution grid on a short -term basis. In [6], it is

proposed that UCAPs are currently viable as short-term energy storage for bridging power in kilowatt range in the seconds to

few minutes timescale. The choice of the number of UCAPs necessary for providing grid support depends on the amount of

support needed, terminal voltage of the UCAP, dc-link voltage, and distribution grid voltages. For a 260V dc-link voltage, it is

practical and cost effective to use three modules in the UCAP bank.

3 Controller Implementation

The series inverter controller implementation is based on the in-phase compensation method that requires PLL for estimating θ,

and this has been implemented using the fictitious power method described in [4]. Based on the estimated θ and the line –line

source, voltages Vab, Vbc, Vca (which are available for this delta -sourced system) are transformed into the d–q domain and the

line tral components ource v g Vs , Vsb, and Vsc which are not available can then be estimated using equations

⎡ ⎤ cos( sin( ⎢ ⎥ ⎢ ⎥ −sin os( ⎣ ⎦ √

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These voltages are normalized to unit sine waves using line–neutral system voltage of 120 Vrms as reference andcompared with unit sine waves in-phase with actual system voltages Vs from (2) to find the injected voltage references Vref necessary to

maintain a constant voltage at the load terminals, where m is the modulation index, which is 0.45 for this case. Therefore, whenever there is a voltage sag or swell on the source side, a corresponding voltage Vinj2 is injected in-phase by the DVR and

UCAP system to negate the effect and retain a constant voltage VLat the load end. The actual active and reactive power supplied

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The shunt in te ntroller implementation is based on the ‘id−iq’ method, which is modified to provide active and reactive power compensation, such that id controls the reactive power and iq controls the active power. Therefore, b ased on the references for active and reactive powers Pref and Qref, the reference currents iqref and idref in d–q domain can be calculated using

(4), where vsq is the system voltage in q-domain and the reference currents are calculated using (5)



4 Modes of Operation

Average current mode controlis used to regulate the output voltage of the bidirectional dc –dc converter in both Buck and Boost modes while charging and discharging the UCAP bank. This method tends to be more stable when compared with other methods like voltage mode control and peak current mode control. Average current mode control is used to regulate the output voltage of the bidirectional dc–dc converter in both Buckand Boost modes, while charging and discharging the UCAP bank.

While the UCAP-APF system is discharging power, the dc-link voltage Vout tends to be less than Vref , which causes the

reference current Iucref to be positive, thereby operating the dc–dc converter in Boost mode. Along similar lines, when the UCAP-

APF system is absorbing power from the grid, the dc-link voltage Vout tends to be greater than Vref , which causes the reference

current Iucref to be negative and thereby operating the dc–dc converter in Buck mode. Average current mode control technique

was found as the ideal method for UCAP-APF integration as it tends to be more stable when compared with other methods like voltage mode control and peak current mode control. This is a major advantage in the present topology, where the stability of the dc–dc converter has to be ensured over a wide operating range and in both Buck and Boost modes of operation.

The higher level integrated controller is designed to make system level decisions on the inverter and DC/DC converter controllers. Based on various system parameters like Pload, Qload, Pgrid, Qgrid, Vucap, Vdc, Idclnk, and Iucap, the higher level integrated

controller will decide on operating in one of the following modes: active power support mode, reactivepower support mode, renewable intermittency smoothingmode, sag/swell compensation mode, and UCAP chargemode.

In active power support mode and renewable intermittency smoothing mode, the UCAP-UPQC system must provide activepower to the grid. Therefore, the active power capabilityof the UCAP-UPQC system must be assessed by the higherlevel integrated controller. Based on the Pgridand Ploadvalues, the reference Prefis calculated in the higher levelintegrated controller, and it will

decide if the UCAP hasenough energy to respond to the Prefcommand based on theUCAP state of charge. If the UCAP has

enough capacity to respond to the request, then the dc–dc converter controller is operated in grid support mode; otherwise, it is operated in charge mode, where the UCAP is recharged and the power request is met at a later time. In grid support mode, the dc–dc converter will operate in a bidirectional fashion in both Buck and Boost modes to respond to the active power requests and regulate the dc-link voltage in a stable fashion, while the inverter controller should respond such that the commanded Prefis

supplied by the inverter through current control.

In reactive power support mode, the UCAP-UPQC system must provide reactive power to the grid. In this mode, the UCAP-

UPQC does not provide any active power to the grid and even the UPQC losses are supplied by the grid. Based on the Qgridand

Qloadvalues, the reference Qrefis calculated in the higher level integrated controller. In this mode, the dc–dc converter controller

can be programmed to operate in grid support mode directly because the active power requirement for operating in this mode is minimal. Therefore, the goal of the dc–dc converter controller is to regulate the dc-link voltage in a stable fashion, while the

inverter controller should respond such that the commanded Qrefis supplied by the inverter through current control.In sag/swell

compensation mode, the UCAP-PC system is programmed to prevent sensitive loads from disturbances on the supply-side like voltage sag or voltage swell. These disturbances require short-term energy storage, and in this mode, the dc–dc converter controller can be programmed to operate in grid support mode. Therefore, the goal of the dc–dc converter controller is to regulate the dc-link voltage in a stable fashion during both sag/swell events. It is also required that the dc–dc converter be able to discharge and meet the active power requirements during a voltage sag and to be able absorb active power in a stable fashion during a voltage swell event.

In charge mode, the UCAP is recharged by absorbing active power from the grid when the UCAP state of charge falls below

50%. The rate at which the UCAP can be charged is assessed by the higher level integrated controller based on the Pgridand

Ploadvalues and the reference Prefis calculated. Then the dc–dc converter controller is commanded to operate in charge mode,

wherein the dc–dc converter will operate in Buck Mode to absorb the power from the grid and the inverter controller must respond to supply commanded Pref..



5 Simulation Result An extensive simulation study is carried out using MATLAB/Simulink in order to verify the proposed control strategy. The

simulation of the proposed UCAP integrated power conditioner system is carried out in Simulink for a 208V, 50Hz system,

where 208V is 1p.u. The system response for a three-phase voltage sag which lasts for 0.1s and has a depth of 0.64p.u is shown

Fig. 4 (a) Source and load rms voltages Vsrms and VLrms during sag.

(b) Source voltages Vsab (blue), Vsbc (red), and Vsca (green) during sag.

in Fig. 4(a-d). During voltage sag, the source voltage Vsrms is reduced to 0.36p.u, whiletheload voltage VLrms is maintained

constant at around 1.01p.u due to voltages injected in-phase by the series inverter. This can also be observed from the plots of

the line–line source voltages (Vsab, Vsbc, and Vsca) [Fig. 4(b)], the line–line load voltages (VLab, VLbc, and VLca) [Fig. 4(c)],

and the line–neutral injected voltages of the series inverter (Vinj2a, Vinj2b, and Vinj2c) [Fig. 4(d)].

Fig 4. (c) Injected voltages Vinj2a blue), Vinj2b (red), and Vinj2c (green) during sag.

(d) Load voltages VLab (blue), VLbc (red), and VLca (green) during sag.



Fig. 5. (a) Currents and voltages of dc–dc converter.

In Fig 5(a), the plots of the bidirectional dc–dc converter are presented, and it can be observed that the dc-link voltage Vfdc is

regulated at 260V, the average dc link current Idclnkav and the average UCAP current Iucav increase to provide the active power

required by the load during the sag. This can also be observed from various active power plots shown in Fig 5(b), where the

power supplied to the load Pload remains constant even during the voltage sag when the grid power Pgrid is decreasing. The

active power deficit of the grid is met by the DVR power Pdvr, which is almost equal to the input power to the inverter Pdc in

available from the UCAP. Therefore, it can be concluded from the plots that the active power deficit between the grid and load

during the voltage sag event is being met by the UCAP-based energy storage system through bidirectional dc–dc converter and

the inverter. It can also be noticed that the grid reactive power Qgrid reduces during the voltage sag while Q dvr increases to

compensate for the reactive power loss in the system. Similar analysis can also be carried out for voltage sags that occur in one

of the phases (A, B, or C) or in two of the phases (AB, BC, or CA); however, three -phase voltage sag case requires the

maximum active power support and is presented here.

Fig.5 (b) Active and reactive power of grid, load, and inverter during voltage sag



6Conclusion

In this paper, the concept of integrating UCAP-based rechargeable energy storage to a power conditioner system to improve the

power quality of the distribution grid is presented. With this integration, the DVR portion of the power conditioner will be able

to independently compensate voltage sags and swells and the APF portion of the power conditioner will be able to provide

active/reactive power support and renewable intermittency smoothing to the distribution grid. UCAP integration through a

bidirectional dc–dc converter at the dc-link of the power conditioner is proposed. The control strategy of the series inverter

(DVR) is based on in phase compensation and the control strategy of the shunt inverter (APF) is based on ‘id−iq’ method.

Designs of major components in the power stage of the bidirectional dc–dc converter are discussed. Average current mode

control is used to regulate the output voltage of the dc–dc converter due to its inherently stable characteristic. A higher level

integrated controller that takes decisions based on the system parameters provides inputs to the inverters and dc –dc converter

controllers to carry out their control actions. The simulation of the integrated UCAP-UPQC system which consists of the UCAP,

bidirectional dc–dc converter, and the series and shunt inverters is carried out using MATLAB/Simulink. Results from

simulation successfully mitigate intermittency due to DERs and UPQC compensate for both volt age sag/swell and reactive

power, thereby verifying the concepts introduced in this paper. Similarly UCAP based energy storages can be deployed in the

future in a microgrid or a low-voltage distribution grid to respond to dynamic changes in the voltage profiles and power profiles

on the distribution grid.

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