Long Overhead Electric Power Transmission Line Design With Assistance of High Voltage Direct Current(HVDC) System Ahmed Rafique Aziz June,2011

Department of Electrical & Electronic Engineering,Khulna University of Engineering

&Technology(KUET),Khulna-920300 Long Overhead Electric Power Transmission Line Design with assistance of High Voltage Direct Current(HVDC) System

This thesis papers is submitted to the Department of Electrical & Electronic Engineering,Khulna University of Engineering & Technology(KUET),Bangladesh,in partial fulfillment of the requirements

for the Degree of “Bachelor of Science in Electrical &Electronic Engineering”. Prepared by Ahmed Rafique Aziz Roll No:0203079 Thesis Supervisor Md.SalahUddin Yusuf Assistant Professor Department of Electrical & Electronic Engineering

Department of Electrical & Electronic Engineering,Khulna University of Engineering & Technology(KUET),Khulna-920300,Bangladesh.

Dedicated to my Parents

Acknowledgement First and Foremost,I would like to reveal profound gratitude to omniscient Allah who give me the knowledge for composing this Thesis papers.Then,I would like to thank always inspiring, enthusiastic and very supportive supervisor Md.SalahUddin Yusuf,Assistant Professor,Department of Electrical & Electronic Engineering,KUET.He has always been extremly generous with his time,knowledge and ideas and allowed me great freedom in this research.His enthusiastic approach to research,his endless excitement for Long Overhead Electric Transmission Line Design with assistance of HVDC System and his effervescent personality has made this experience all the more enjoyable and I am greatly appreciative. I also thank all the teachers of the Department of Electrical & Electronic Engineering,KUET,who all gave valuable advice and particularly Mr.A.N.M Enamul Kabir,Associate Professor,EEE,KUET.He had kept attention on the progress of my work and was always available when I needed to consult with him.His encouragement,motivation and expert guidance has provided a good basis for my entire thesis work. Special thanks for Prof.Dr.Md. Abdur Rafiq ,Head of the Dept. of EEE,KUET, for his valuable advices and guidance and providing the supports and lab facilities for the practical works. I am indebted to all my class mates of 2k6 batch,EEE,for their cooperation.I am specially grateful to my maternal uncle for financial cooperations in the whole thesis works.Many thanks to all those who help to create a nice environment in spite of many obstacles at Khulna University of Engineering & Technology. Author June,2011

Abstract

It is well recognized that direct current and direct voltage offer special advantages for both land and sea cable systems, both with regard to power transmission capability, losses, as well as possible transmission length due to no capacitive currents. As cable systems were used very early in large cities, one of the first applications considered for HVDC was to use it for city infeed and some schemes were also built. However, it turned out that the cost for the stations was too high and that the savings on the cable part were not high enough to justify the high costs of the converter stations, even considering other possible benefits of the HVDC techniques such as fast control of active power and almost no contribution to fault currents.

During the 1990s new HVDC Voltage Source Converters, VSC, and new HVDC cables with solid insulation have been developed and the relative cost for the converters has been steadily decreasing. It was, therefore, found justifiable to reexamine the feasibility of using HVDC, especially based on the new VSC technique, for long transmission line.

Although the transmission and distribution of electrical power will be preferably made with conventional AC technique, but HVDC transmission would offer special advantages for long transmission cable systems with especial requirements with regard to power flow control, systems with restrictions to short circuit currents, and other relevant issues. HVDC transmission would have advantages over the conventional AC solution, simplifying the operation of the system or resulting in a more economical solution. The Author has designed a overhead long transmission line from Khulna to Dhaka district.The Author would like to make a compromise between expensive HVDC and comparatively inefficient 3-phase AC Transmission.Instance of HVDC, terminal equipment design is very expensive and sophisticated and complex.On the other instance,for 3-phase AC transmission considerable demerits arises. However,3-phase High Voltage (230KV) AC transmission have been existed and have been operating for few decades.So,HVDC is a certainly new considerations for transmitting a bulk amount of electric power . Moreover,HVDC has several considerable advantages over AC and HVDC has future prospects for new development in Electric Power Sector of Bangladesh.

List of Figures Page Figure2.1. The twelve pulse valve group configuration with two converter transformers. One in star-star connection and the other in star-delta connection. . . . . . 5 Figure 2.2. Example of an HVDC substation. . . . . 7 Figure 2.3. Monopolar and bipolar connection of HVDC converter bridges. . . . . .9 Figure 2.4. Voltage and current waveshapes associated with d.c. converter bridges.. . . 13 Figure 4.1:Mass Impregnated Cable . . . .29 Figure 4.2:Self Contained Fluid –Filled Cables . . . . 29 Figure 4.3:Extruded Cables (XLPE Cables) . . . 30 Figure 5.1 Typical HVDC 500KV Lattice Tower . . . . . 35 Figure6.1:Mass Impregnated Cable . . . . 47 Figure6.2:VSC based HVDC Substation . . . . 48 Figure6.3:VSC based HVDC Converter Arrangement . . . . 48 Figure 6.4:VSC based HVDC Indoor-Outdoor View . . . . 49 Figure 6.5:HVDC transmission with VSC . . . . 49 Figure 6.6 :Control of VSC Based HVDC Transmission . . . . 50 Figure 6.7:HVDC Overall View. . . . 50 Figure 6.8 :Efficiency Vs Transmission System Curve .. . .69 Figure 6.9:Voltage Regulation Vs Transmission System Curve. . . . . 70 Figure 6.10:Transmission Loss Vs Transmission System Curve. . . . . 71

Contents Acknowledgement . . . . . . . . . . . . . . . . .(i) Abstract . . . . . . . . . . . . . . . . .(ii) List of Figures . . . . . . . . . . . . . . . . . (iii) Chapter 1 Page Introduction 1.1 General background and overall aim of the study . . . . . . . . . . . . 1 1.2 Contribution in the study . . . . . . . . . . . . 2 1.3 General overview of the thesis . . . . . . . . . . . . 2 Chapter 2 HVDC Overview 2.1 Intoduction . . . . . . . . . . . . . 3 2.2 Why Use DC Transmission . . . . . . . . . . . . . 3 2.3 Configurations . . . . . . . . . . . . . 4 2.4 Twelve Pulse Valve Group . . . . . . . . . . . . . 5 2.5 Thyristor Module . . . . . . . . . . . . . 6 2.6 Substation Configuration . . . . . . . . . . . . . 6 2.7Applications Of HVDC Converters . . . . . . . . . . . . . 8 2.8 HVDC Converter Arrangements . . . . . . . . . . . . . 10 2.9 Environmental Considerations . . . . . . . . . . . . . 11 2.10 D.C Converter Operation . . . . . . . . . . . . . .12 2.11Commutation Failure . . . . . . . . . . . . . 15 2.12 Series Capacitors With D.C. Converter Substations . . . . . . . . . . . . . 16 2.13 Control And Protection . . . . . . . . . . . . . 17 2.14 A.C. Voltage Control . . . . . . . . . . . . . 20 2.15 Special Purpose Controls . . . . . . . . . . . . . 21

Chapter 3 Page VSC Based HVDC Transmission System 3.1 Introduction . . . . . . . . . . . . 23 3.2 Advantages and Applications for VSC Based HVDC . . . . . . . . . . . . 23 3.3VSC-based HVDC Transmission System Configurations . . . . . . . . . . . . 25 3.4 Voltage Source Converter . . . . . . . . . . . . 25 3.5 Transformer . . . . . . . . . . . . 25 3.6 Phase Reactor . . . . . . . . . . . . 25 3.7 AC Filter . . . . . . . . . . . . 26 3.8 DC-link Capacitor . . . . . . . . . . . . 26 3.9 DC Cable . . . . . . . . . . . . 26 Chapter 4 HVDC Cables 4.1 Introduction . . . . . . . . . . . 27 4.2 Main Characteristics of an HVDC Cable System . . . . . . . . . . . .28 4.3 Classification of HVDC Cables . . . . . . . . . . . .28 Chapter 5 HVDC Tower 5.1 Construction Process and Costs . . . . . . . . . . . . 31 5.2 Detailed Planning of The Transmission Route . . . . . . . . . . . . 31 5.3 Detailed Design and Execution Drawing . . . . . . . . . . . . 32 5.4 Construction Phases / Time Schedule . . . . . . . . . . . . 32 5.5 Preliminary Work On Construction . . . . . . . . . . . . 32 5.6 Foundations . . . . . . . . . . . . 32 5.7 Pylon Assembly / Switch Yard Erection . . . . . . . . . . . . 33 5.8 Cables Hanging . . . . . . . . . . . . 33 5.9 Tests and Acceptance . . . . . . . . . . . . 34 5.10 Recultivation . . . . . . . . . . . . 34 5.11Values of T & D Lines . . . . . . . . . . . . 34 5.12Typical 500 KV HVDC Lattice Tower . . . . . . . . . . . . 35

Chapter 6 Page HVDC +/-260 KV Transmission Line Project 6.1Tentative Title . . . . . . . . . 36 6.2Introduction . . . . . . . . . 36 6.3 Long Term Significance . . . . . . . . . 37 6.4 Proposed Plan and Methodology . . . . . . . . . 37 6.5Existing Transmission Network(as on June,2010) . . . . . . . . . 38 6.6 Existing Transmission Lines . . . . . . . . . 38 6.7Existing Power Generation in Ghorasal . . . . . . . . . 39 6.8 Generation Voltage . . . . . . . . . 39 6.9Typical Electrical Parameters for a 230KV Overhead Line . . . . . . . . . 39 6.10 Data of Existing Transmission Line . . . . . . . . . 40 6.11 Typical High Voltage Direct Current (HVDC) Transmission Line Between Khulna-Ishurdi-Ghorasal . . . . . . . . . 45 6.12 Typical Tower Structure . . . . . . . . .. 46 6.13 HVDC Project Cables . . . . . . . . . 47 6.14 Typical HVDC Circuit Diagram . . . . . . . . . 48 6.15 HVDC +/-260 KV Project Economics . . . . . . . . . 51 6.16 Cost ratios for DC and AC Transmission Line construction . . . . . . . . . 51 6.17 HVDC System Reliability . . . . . . . . . 52 6.18 Cost Structure of Converter Stations . . . . . . . . . 52 6.19 HVDC Current,Voltage,Insulation Level,Power Transmission and Percentage Loss comparision with HVAC . . . . . . . . . 53 6.20Tower Calculation . . . . . . . . . 55 6.21Preliminary Design of Tower . . . . . . . . . 57 6.22 Corona Loss . . . . . . . . . 58 6.23 MATLAB Program for comparison of HVAC and HVDC . . . . . . . . . 59 6.24 Present HVAC Power Grid of Bangladesh . . . . . . . . . 61 6.25Electrical Design of Typical Existing HVAC Transmission Line . . . . . . . . . 62 6.26 Proposed HVDC Project Electrical Design . . . . . . . . . 67 6.27Typical Performance Curves . . . . . . . . . 69

Chapter 7 Page HVDC Transmission-Opportunities and Challenges 7.1Developments in Energy Policies . . . . . . . . . .72 7.2 Developments in Transmission Networks . . . . . . . . . . 72 7.3Challenges and Opportunities . . . . . . . . . .74 7.3.1Wind Power and Energy Diversity 7.3.2 AC Network Enhancement 7.4 HVDC System Challenges . . . . . . . . . . 75 7.4.1Cost and Value of HVDC 7.4.2 Power Loss 7.4.3 Complexity of HVDC Schemes 7.4.4 Dispatch and Control of HVDC Scheme 7.4.5 Integration of HVDC Scheme in AC Network 7.4.6 Harmonics 7.4.7 Operation of HVDC Scheme With Ground Return 7.4.8 Stability of Network With Multi-Infeed of HVDC 7.5 Conclusion .. . . . . . . . .78 Chapter 8 Conclusion . . . . . .79-80 REFERENCES

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Chapter 1 Introduction

1.1 General background and overall aim of the study

It is well recognized that direct current and direct voltage offer special advan- tages for both land and sea cable systems, both with regard to power transmis- sion capability, losses, as well as possible transmission length due to no capaci- tive currents.However, it turned out that the cost for the stations was too high and that the savings on the cable part were not high enough to justify the high costs of the converter stations, even considering other possible benefits of the HVDC techniques such as fast control of active power and almost no contribution to fault currents.

During the 1990´s, with the development of new HVDC converters using Volt- age Source Converters, VSC, new HVDC cables with solid insulation and with the relative cost for the converters steadily decreasing, it was found justifiable to again study the feasibility of using HVDC, especially based on the new VSC technique, for feeding electrical power to large cities. This new HVDC-VSC technique will, for instance, make it possible to control both active and reactive power and will be more suitable for cable multi-terminal systems.

.From the specific studies performed in close cooperation with utilities, the ma- jor driving forces and evaluating criteria used to decide whether to rebuilt or expand an existing electrical power or built a complete new system, were iden- tified. Specific criteria such as thermal security, voltage security, short circuit current security, reliability of supply, and capability for power flow control were found to be the major driving forces in the review of the existing infra- structure. Each of these criteria was evaluated in a systematic way and a comparison was made between the existing or expected possible improved AC technique and an alternative HVDC solution. The comparison was made from both a technical and an economical point of view.

Finally a more generic study was performed in order to evaluate the expected break-even distance for a HVDC o v e r h e a d transmission system by com- parison with an equivalent HVAC transmission. The break-even distance was in this case the distance in which the saving in capital cost and lower losses with a DC overhead transmission cable may be enough to pay for the two converters, one at either end. This distance depends on several factors, and most of these factors are related to the specific characteristic of the network. Some parametric study of these factors was also made in the calculation of the break-even distance.

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1.2 Contribution in the study

The present study provides the following main contributions:

• A systematic overview of evaluation criteria and values of HVDC solutions including comparison with the best HVAC alternative.

• Generic conclusions regarding when HVDC could be an alternative for power transmission to large cities.

• Suggestion and motivation of new Hybrid HVDC topology

• Extension of the concept of ‘break-even distance’ widely mentioned in the literature when comparing HVDC and HVAC transmission with overhead lines.

1.3 General overview of the thesis

Chapter 2 describes the technical,economical and environmental aspects of HVDC. Chapter 3 describes the characteristics of HVDC Voltage Source Converters(VSC).

Chapter 4 describes the HVDC cable system.During the last years very cost effective extruded DC cables have been devel- oped which can fit in the existing cables ducts. These cables have

considerably higher power transmission capability than the corresponding AC cables. In chapter 5 involves HVDC towers.

In chapter 6 describes the conveniences of using HVDC for bulk amount of electric power transmission over Ghorasal,Narsinghdi and Khulna district.

In Chapter 7 describes the future challenges and opportunities of HVDC.

Finally in chapter 8 ,generic conclusion is presented for long overhead electric power

transmission.

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Chapter 2 HVDC Overview 2.1 Intoduction Electric power transmission was originally developed with direct current. The availability of transformers and the development and improvement of induction motors at the beginning of the 20th Century, led to greater appeal and use of a.c. transmission. Through research and development in Sweden at Allmana Svenska Electriska Aktiebolaget (ASEA), an improved multi-electrode grid controlled mercury arc valve for high powers and voltages was developed from 1929. Experimental plants were set up in the 1930’s in Sweden and the USA to investigate the use of mercury arc valves in conversion processes for transmission and frequency changing.

D.c. transmission now became practical when long distances were to be covered or where cables were required. The increase in need for electricity after the Second World War stimulated research, particularly in Sweden and in Russia. In 1950, a 116 km experimental transmission line was commissioned from Moscow to Kasira at 200 kV. The first commercial HVDC line built in 1954 was a 98 km submarine cable with ground return between the island of Gotland and the Swedish mainland.

Thyristors were applied to d.c. transmission in the late 1960’s and solid state valves became a reality. In 1969, a contract for the Eel River d.c. link in Canada was awarded as the first application of sold state valves for HVDC transmission. Today, the highest functional d.c. voltage for d.c. transmission is +/- 600 kV for the 785 km transmission line of the Itaipu scheme in Brazil. D.c. transmission is now an integral part of the delivery of electricity in many countries throughout the world 2.2 Why Use DC Transmission The question is often asked, “Why use d.c. transmission?” One response is that losses are lower, but this is not correct. The level of losses is designed into a transmission system and is regulated by the size of conductor selected. D.c. and a.c. conductors, either as overhead transmission lines or submarine cables can have lower losses but at higher expense since the larger cross-sectional area will generally result in lower losses but cost more. When converters are used for d.c. transmission in preference to a.c. transmission, it is generally by economic choice driven by one of the following reasons: 1. An overhead d.c. transmission line with its towers can be designed to be less costly per unit

of length than an equivalent a.c. line designed to transmit the same level of electric power. However the d.c. converter stations at each end are more costly than the terminating stations of an a.c. line and so there is a breakeven distance above which the total cost of d.c. transmission is less than its a.c. transmission alternative. The d.c. transmission line can have a lower visual profile than an equivalent a.c. line and so contributes to a lower environmental impact. There are other environmental advantages to

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a d.c. transmission line through the electric and magnetic fields being d.c. instead of ac.

2. If transmission is by submarine or underground cable, the breakeven distance is much less than overhead transmission. It is not practical to consider a.c. cable systems exceeding 50 km but d.c. cable transmission systems are in service whose length is in the hundreds of kilometers and even distances of 600 km or greater have been considered feasible. 3. Some a.c. electric power systems are not synchronized to neighboring networks even though their physical distances between them is quite small. This occurs in Japan where half the country is a 60 hz network and the other is a 50 hz system. It is physically impossible to connect the two together by direct a.c. methods in order to exchange electric power between them. However, if a d.c. converter station is located in each system with an interconnecting d.c. link between them, it is possible to transfer the required power flow even though the a.c. systems so connected remain asynchronous. 2.3 Configurations The integral part of an HVDC power converter is the valve or valve arm. It may be non- controllable if constructed from one or more power diodes in series or controllable if constructed from one or more thyristors in series. Figure 1 depicts the International Electrotechnical Commission (IEC) graphical symbols for valves and bridges (1). The standard bridge or converter connection is defined as a double-way connection comprising six valves or valve arms which are connected as illustrated in Figure 2. Electric power flowing between the HVDC valve group and the a.c. system is three phase. When electric power flows into the d.c. valve group from the a.c. system then it is considered a rectifier. If power flows from the d.c. valve group into the a.c. system, it is an inverter. Each valve consists of many series connected thyristors in thyristor modules. Figure 2 represents the electric circuit network depiction for the six pulse valve group configuration. The six pulse valve group was usual when the valves were mercury arc.

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2.4 Twelve Pulse Valve Group Nearly all HVDC power converters with thyristor valves are assembled in a converter bridge of twelve pulse configuration. Figure 3 demonstrates the use of two three phase converter transformers with one d.c. side winding as an ungrounded star connection and the other a delta configuration. Consequently the a.c. voltages applied to each six pulse valve group which make up the twelve pulse valve group have a phase difference of 30 degrees which is utilized to cancel the a.c. side 5th and 7th harmonic currents and d.c. side 6th harmonic voltage, thus resulting in a significant saving in harmonic filters. Figure 3 also shows the outline around each of the three groups of four valves in a single vertical stack. These are known as “quadrivalves” and are assembled as one valve structure by stacking four valves in series. Since the voltage rating of thyristors is several kV, a 500 kV quadrivalve may have hundreds of individual thyristors connected in series groups of valve or thyristor modules. A quadrivalve for a high voltage converter is mechanically quite tall and may be suspended from the ceiling of the valve hall, especially in locations susceptible to earthquakes.

3 Quadrivalves

Ac Side

a

b

Dc c Side

a

Figure2.1. The twelve pulse valve group configuration with two converter transformers. One in star-star connection and the other in star-delta connection.

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2.5 Thyristor Module A thyristor or valve module is that part of a valve in a mechanical assembly of series connected thyristors and their immediate auxiliaries including heat sinks cooled by air, water or glycol, damping circuits and valve firing electronics. A thyristor module is usually interchangeable for maintenance purposes and consists of electric components as shown in Figure 4. 2.6 Substation Configuration

The central equipment of a d.c. substation (2) are the thyristor converters which are usually housed inside a valve hall. Outdoor valves have been applied such as in the Cahora Bassa d.c. transmission line between Mozambique and South Africa. Figure 5 shows an example of the electrical equipment required for a d.c. substation. In this example, two poles are represented which is the usual case and is known as the “bipole” configuration. Some d.c. cable systems only have one pole or “monopole” configuration and may either use the ground as a return path when permitted or use an additional cable to avoid earth currents.

From Figure 5, essential equipment in a d.c. substation in addition to the valve groups include the converter transformers. Their purpose is to transform the a.c. system voltage to which the d.c. system is connected so that the correct d.c. voltage is derived by the converter bridges. For higher rated d.c. substations, converter transformers for 12 pulse operation are usually comprised of single phase units which is a cost effective way to provide spare units for increased reliability.

The secondary or d.c. side windings of the converter transformers are connected to the converter bridges. The converter transformer is located in the switchyard, and if the converter bridges are located in the valve hall, the connection has to be made through its wall. This is accomplished in either of two ways. Firstly, with phase isolated busbars

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where the bus conductors are housed within insulated bus ducts with oil or SF6 as the insulating medium or secondly, with wall bushings. When applied at d.c. voltages at 400 kV or greater, wall bushings require considerable design and care to avoid external or internal insulation breakdown. Harmonic filters are required on the a.c. side and usually on the d.c. side. The characteristic a.c. side current harmonics generated by 6 pulse converters are 6n +/- 1 and 12n +/- 1 for 12 pulse converters where n equals all positive integers. A.c. filters are typically tuned to 11th, 13th, 23rd and 25th harmonics for 12 pulse converters. Tuning to the 5th and 7th harmonics is required if the converters can be configured into 6 pulse operation. A.c. side harmonic filters may be switched with circuit breakers or circuit switches to accommodate reactive power requirement strategies since these filters generate reactive power at fundamental frequency. A parallel resonance is naturally created between the capacitance of the a.c. filters and the inductive impedance of the a.c. system. For the special case where such a resonance is lightly damped and tuned to a frequency between the 2nd and 4th harmonic, then a low order harmonic filter at the 2nd or 3rd harmonic may be required, even for 12 pulse converter operation.

Converter

Dc reactor and arrester

Dc

Dc surge capacitor

Converter unit 6 pulse

Converter transformer

bridge filters

Earth return transfer breaker

Metallic return transfer breaker

Neutral bus arrester

Neutral bus surge capacitor

Ac filter

Earth electrode and line

Converter unit 12 pulse

Midpoint dc bus arrester

Dc bus arrester

Dc bus arrester

Dc line arrester

Figure 2.2. Example of an HVDC substation.

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Characteristic d.c. side voltage harmonics generated by a 6 pulse converter are of the order 6n and when generated by a 12 pulse converter, are of the order 12n. D.c. side filters reduce harmonic current flow on d.c. transmission lines to minimize coupling and interference to adjacent voice frequency communication circuits. Where there is no d.c. line such as in the back-to-back configuration, d.c. side filters may not be required.

D.c. reactors are usually included in each pole of a converter station. They assist the d.c. filters in filtering harmonic currents and smooth the d.c. side current so that a discontinuous current mode is not reached at low load current operation. Because rate of change of d.c. side current is limited by the d.c. reactor, the commutation process of the d.c. converter is made more robust.

Surge arresters across each valve in the converter bridge, across each converter bridge and in the d.c. and a.c. switchyard are coordinated to protect the equipment from all overvoltages regardless of their source. They may be used in non-standard applications such as filter protection. Modern HVDC substations use metal-oxide arresters and their rating and selection is made with careful insulation coordination design.

2.7APPLICATIONS OF HVDC CONVERTERS The first application for HVDC converters was to provide point to point electrical power interconnections between asynchronous a.c. power networks. There are other applications which can be met by HVDC converter transmission which include: 1. Interconnections between asynchronous systems. Some continental electric power

systems consist of asynchronous networks such as the East, West, Texas and Quebec networks in North America and island loads such as the Island of Gotland in the Baltic Sea make good use of HVDC interconnections.

2. Deliver energy from remote energy sources. Where generation has been developed at remote sites of available energy, HVDC transmission has been an economical means to bring the electricity to load centers. Gas fired thermal generation can be located close to load centers and may delay development of isolated energy sources in the near term.

3. Import electric energy into congested load areas. In areas where new generation is impossible to bring into service to meet load growth or replace inefficient or decommissioned plant, underground d.c. cable transmission is a viable means to import electricity.

4. Increasing the capacity of existing a.c. transmission by conversion to d.c. transmission. New transmission rights-of-way may be impossible to obtain. Existing overhead a.c. transmission lines if upgraded to or overbuilt with d.c. transmission can substantially increase the power transfer capability on the existing right-of-way.

5. Power flow control. A.c. networks do not easily accommodate desired power flow control. Power marketers and system operators may require the power flow control capability provided by HVDC transmission.

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6. Stabilization of electric power networks. Some wide spread a.c. power system

networks operate at stability limits well below the thermal capacity of their transmission conductors. HVDC transmission is an option to consider to increase utilization of network conductors along with the various power electronic controllers which can be applied on a.c. transmission.

(a) Monopolar configuration Figure 2.3. Monopolar and bipolar connection of HVDC converter bridges.

(b) Bipolar configuration

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2.8 HVDC Converter Arrangements HVDC converter bridges and lines or cables can be arranged into a number of configurations for effective utilization. Converter bridges may be arranged either monopolar or bipolar as shown in 12 pulse arrangement in Figure 6. Various ways HVDC transmission is used are shown in simplified form in Figure 7 and include the following: 1. Back-to-Back. There are some applications where the two a.c. systems to be

interconnected are physically in the same location or substation. No transmission line or cable is required between the converter bridges in this case and the connection may be monopolar or bipolar. Back-to-back d.c. links are used in Japan for interconnections between power system networks of different frequencies (50 and 60 Hz). They are also used as interconnections between adjacent asynchronous networks.

2. Transmission Between Two Substations. When it is economical to transfer electric power through d.c. transmission or cables from one geographical location to another, a two-terminal or point-to-point HVDC transmission is used. In other words, d.c. power from a d.c. rectifier terminal is dedicated to one other terminal operating as an inverter. This is typical of most HVDC transmission systems.

3. Multiterminal HVDC Transmission System. When three or more HVDC substations are geographically separated with interconnecting transmission lines or cables, the HVDC transmission system is multiterminal. If all substations are connected to the

same voltage then the system is parallel multiterminal d.c. If one or more converter bridges are added in series in one or both poles, then the system is series multiterminal d.c. Parallel multiterminal d.c. transmission has been applied when the substation capacity exceeds 10% of the total rectifier substation capacity. It is expected a series multiterminal substation would be applied when its capacity is small (less than 10%) compared to the total rectifier substation capacity. A combination of parallel and series connections of converter bridges is a hybrid multiterminal system. Multiterminal d.c. systems are more difficult to justify economically because of the cost of the additional substations. 4. Unit Connection. When d.c. transmission is applied right at the point of generation, it

is possible to connect the converter transformer of the rectifier directly to the generator terminals so the generated power feeds into the d.c. transmission lines. This might be applied with hydro and wind turbine driven generators so that maximum efficiency of the turbine can be achieved with speed control. Regardless of the turbine speed, the power is delivered through the inverter terminal to the a.c. receiving system at its fundamental frequency of 50 or 60 hz.

5. Diode Rectifier. It has been proposed that in some applications where d.c. power transmission is in one direction only, the valves in the rectifier converter bridges can be constructed from diodes instead of thyristors. Power flow control would be achieved at the inverter, and in the case where the unit connection is used, a.c. voltage control by the generator field exciter could be applied to regulate d.c. power. This connection may require high speed a.c. circuit breakers between the generator and the rectifier converter bridges to protect the diodes from overcurrents resulting from a sustained d.c. transmission line short circuit.

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2.9 ENVIRONMENTAL CONSIDERATIONS

The electrical environmental effects from HVDC. transmission lines can be characterized by field and ion effects as well as corona effects (4), (5). The electric field arises from both the electrical charge on the conductors and for a HVDC overhead transmission line, from charges on air ions and aerosols surrounding the conductor. These give rise to d.c. electric fields due to the ion current density flowing through the air from or to the conductors as well as due to the ion density in the air. A d.c. magnetic field is produced by d.c. current flowing through the conductors. Air ions produced by HVDC lines form clouds which drift away from the line when blown by the wind and may come in contact with humans, animals and plants outside the transmission line right-of -way or corridor. The corona effects may produce low levels of radio interference, audible noise and ozone generation.

Field and corona effects

The field and corona effects of transmission lines largely favor d.c. transmission over a.c. transmission. The significant considerations are as follows:

1. For a given power transfer requiring extra high voltage transmission, the d.c. transmission line will have a smaller tower profile than the equivalent a.c. tower carrying the same level of power. This can also lead to less width of right-of-way for the d.c. transmission option.

2. The steady and direct magnetic field of a d.c. transmission line near or at the edge of the transmission right-of-way will be about the same value in magnitude as the earth’s naturally occurring magnetic field. For this reason alone, it seems unlikely that this small contribution by HVDC transmission lines to the background geomagnetic field would be a basis for concern.

3. The static and steady electric field from d.c. transmission at the levels experienced beneath lines or at the edge of the right-of-way have no known adverse biological effects. There is no theory or mechanism to explain how a static electric field at the levels produced by d.c. transmission lines could effect human health. The electric field level beneath a HVDC transmission line is of similar magnitude as the naturally occurring static field which exists beneath thunder clouds. Electric fields from a.c. transmission lines have been under more intense scrutiny than fields generated from d.c. transmission lines.

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4. The ion and corona effects of d.c. transmission lines lead to a small contribution of

ozone production to higher naturally occurring background concentrations. Exacting long term measurements are required to detect such concentrations. The measurements taken at cross-sections across the Nelson River d.c. lines in Canada failed to distinguish background from downwind levels (4). While solar radiation influences the production of ozone even in a rural environment, thereby maintaining its level, any incremental contribution from a d.c. line source is subject to breakdown, leading to a resumption of background levels downwind from the line. Investigations of ozone for indoor conditions indicate that in well mixed air, the half-life of ozone is 1.5 minutes to 7.9 minutes. Increases in temperature and humidity increase the rate of decay (4).

5. If ground return is used with monopolar operation, the resulting d.c. magnetic field can cause error in magnetic compass readings taken in the vicinity of the d.c. line or cable. This impact is minimized by providing a conductor or cable return path (known as metallic return) in close proximity to the main conductor or cable for magnetic field cancellation. Another concern with continuous ground current is that some of the return current may flow in metallic structures such as pipelines and intensify corrosion if cathodic protection is not provided. When pipelines or other continuous metallic grounded structures are in the vicinity of a d.c. link, metallic return may be necessary.

2.10 D.C CONVERTER OPERATION The six pulse converter bridge of Figure 2 as the basic converter unit of HVDC transmission is used equally well for rectification where electric power flows from the a.c. side to the d.c. side and inversion where the power flow is from the d.c. side to the a.c. side. Thyristor valves operate as switches which turn on and conduct current when fired on receiving a gate pulse and are forward biased. A thyristor valve will conduct current in one direction and once it conducts, will only turn off when it is reverse biased and the current falls to zero. This process is known as line commutation.

An important property of the thyristor valve is that once its conducting current falls to zero when it is reverse biased and the gate pulse is removed, too rapid an increase in the magnitude of the forward biased voltage will cause the thyristor to inadvertently turn on and conduct. The design of the thyristor valve and converter bridge must ensure such a condition is avoided for useful inverter operation.

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X X

Commutation

Rectification or inversion for HVDC converters is accomplished through a process known as line or natural commutation. The valves act as switches so that the a.c. voltage is sequentially switched to always provide a d.c. voltage. With line commutation, the a.c. voltage at both the rectifier and inverter must be provided by the a.c. networks at each end and should be three phase and relatively free of harmonics as depicted in Figure 8. As each valve switches on, it will begin to conduct current while the current begins to fall to zero in the next valve to turn off. Commutation is the process of transfer of current between any two converter valves with both valves carrying current simultaneously during this process.

Consider the rectification process. Each valve will switch on when it receives a firing pulse to its gate and its forward bias voltage becomes more positive than the forward bias voltage of the conducting valve. The current flow through a conducting valve does not change instantaneously as it commutates to another valve because the transfer is through transformer windings. The leakage reactance of the transformer windings is also the commutation reactance so long as the a.c. filters are located on the primary or a.c. side of the converter transformer. The commutation reactance at the rectifier and inverter is shown as an equivalent reactance XC in Figure 8. The sum of all the valve currents transferred the d.c. side and through the d.c. reactor is the direct current and it is relatively flat because of the inductance of the d.c. reactor and converter transformer

Rectifier

Id

Inverter

Ivr Ivi

Udr Udi

C

ULr Uvr

Uvi

C

ULi

Commutation voltage at rectifier

Commutation Voltage at invert

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Figure 2.4. Voltage and current waveshapes associated with d.c. converter bridges. At the inverter, the three phase a.c. voltage supplied by the a.c. system provides the forward and reverse bias conditions of each valve in the converter bridge to allow commutation of current between valves the same as in the rectifier. The inverter valve can only turn on and conduct when the positive direct voltage from the d.c. line is greater than the back negative voltage derived from the a.c. commutation voltage of the a.c. system at the inverter.

Due to the line commutation valve switching process, a non-sinusoidal current is taken from the a.c. system at the rectifier (Ivr in Figure 8) and is delivered to the a.c. system at the inverter (Ivi in Figure 8). Both Ivr and Ivi are lagging to the alternating voltage. This non-sinusoidal current waveform consists of the fundamental frequency a.c. component plus higher harmonics being taken from, and injected into, each a.c. system. The a.c. filters divert the harmonics from entering the a.c. system by offering a low impedance by- pass path allowing the commutation voltage to be relatively harmonic free(ULr and ULi in Figure 8).

Reversal of power flow in a line commutated d.c. link is not possible by reversing the direction of the direct current. The valves will allow conduction in one direction only. Power flow can only be reversed in line commutated d.c. converter bridges by changing the polarity of the direct voltage. The dual operation of the converter bridges as either a rectifier or inverter is achieved through firing control of the grid pulses.

Short circuit ratio

The strength of the a.c. network at the bus of the HVDC substation can be expressed by the short circuit ratio (SCR), defined as the relation between the short circuit level in MVA at the HVDC substation bus at 1.0 per-unit a.c. voltage and the d.c. power in MW.

The capacitors and a.c. filters connected to the a.c. bus reduce the short circuit level. The expression effective short circuit ratio (ESCR) is used for the ratio between the short circuit level reduced by the reactive power of the shunt capacitor banks and a.c. filters connected to the a.c. bus at 1.0 per-unit voltage and the rated d.c. power.

Lower ESCR or SCR means more pronounced interaction between the HVDC substation and the a.c. network (9), (10). A.c. networks can be classified in the following catagories according to strength:

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strong systems with high ESCR: ESCR > 3.0

systems of low ESCR: 3.0 > ESCR > 2.0

weak systems with very low ESCR: ESCR < 2.0 In the case of high ESCR systems, changes in the active/reactive power from the HVDC substation lead to small or moderate a.c. voltage changes. Therefore the additional transient voltage control at the busbar is not normally required. The reactive power balance between the a.c. network and the HVDC substation can be achieved by switched reactive power elements.

In the case of low and very low ESCR systems, the changes in the a.c. network or in the HVDC transmission power could lead to voltage oscillations and a need for special control strategies. Dynamic reactive power control at the a.c. bus at or near the HVDC substation by some form of power electronic reactive power controller such as a static var compensator (SVC) or static synchronous compensator (STATCOM) may be necessary (12). In earlier times, dynamic reactive power control was achieved with synchronous compensators.

2.11Commutation Failure

When a converter bridge is operating as an inverter as represented at the receiving end of the d.c. link in Figure 8, a valve will turn off when its forward current commutates to zero and the voltage across the valve remains negative. The period for which the valve stays negatively biased is the extinction angle , the duration beyond which the valve then becomes forward biased. Without a firing pulse, the valve will ideally stay non conductive or blocked, even though it experiences a forward bias.

All d.c. valves require removal of the internal stored charges produced during the forward conducting period (defined by period + at the inverter in Figure 8) before the valve can successfully establish its ability to block a forward bias. The d.c. inverter therefor requires a minimum period of negative bias or minimum extinction angle for forward blocking to be successful. If forward blocking fails and conduction is initiated without a firing pulse, commutation failure occurs. This also results in an immediate failure to maintain current in the succeeding converter arm as the d.c. line current returns to the valve which was previously conducting and which has failed to sustain forward blocking (13).

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Commutation failure at a converter bridge operating as an inverter is caused by any of the following reasons:

1. When the d.c. current entering the inverter experiences an increase in magnitude which

causes the overlap angle to increase, the extinction angle is reduced and may reach the point where the valve is unable to maintain forward blocking. Increasing the inductance of the d.c. current path through the converter by means of the d.c. smoothing reactor and commutating reactance reduces the rate of change of d.c. current. This has the greatest effect on commutation failure onset.

2. When the magnitude of the a.c. side voltage on one or more phases reduces or is distorted causing the extinction angle to be inadequate as commutation is attempted.

3. A phase angle shift in the a.c. commutating voltage can cause commutation failure. However, the a.c. voltage magnitude reduction and not the corresponding phase shift is the most dominant factor determining the onset of commutation failures for single phase faults.

4. The value of the pre-disturbance steady state extinction angle also effects the sensitivity of the inverter to commutation failure. A value of = 18O is usual for most inverters. Increasing to values of 25O, 30O or higher will reduce the possibility of commutation failure (at the expense of increasing the reactive power demand of the inverter).

5. The value of valve current prior to the commutation failure also effects the conditions at which a commutation failure may occur. A commutation failure may more readily happen if the pre-disturbance current is at full load compared to light load current operation.

In general, the more rigid the a.c. voltage to which the inverter feeds into and with an absence of a.c. system disturbances, the less likelihood there will be commutation failures.

2.12 Series Capacitors With D.C. Converter Substations

HVDC transmission systems with long d.c. cables are prone to commutation failure when there is a drop in d.c. voltage Ud at the inverter. The d.c. cable has very large capacitance which will discharge current towards the voltage drop at the inverter. The discharge current is limited by the d.c. voltage derived from the a.c. voltage of the commutating bus as well as the d.c. smoothing reactor and the commutating reactance. If the discharge current of the cable increases too quickly, commutation failure will occur causing complete discharge of the cable. To recharge the cable back to its normal operating voltage will delay recovery.

The converter bridge firing controls can be designed to increase the delay angle when an increase in d.c. current is detected. This may be effective until the limit of the minimum allowable extinction angle is reached.

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Another way to limit the cable discharge current is to operate the inverter bridge with a three phase series capacitor located in the a.c. system on either side of the converter transformer. Any discharge current from the d.c. cable will pass into the a.c. system through the normally functioning converter bridge and in doing so, will pass through the series capacitor and add charge to it. As a consequence, the voltage of the series capacitor will increase to oppose the cable discharge and be reflected through the converter bridge as an increase in d.c. voltage Ud. This will act as a back emf and limit the discharge current of the cable, thereby avoiding the commutation failure.

The proposed locations of the series capacitor are shown in Figure 9 in single line diagram form (14), (15). With the capacitor located between the converter transformer and the valve group, it is known as a capacitor commutated converter (CCC). With the capacitor located on the a.c. system side of the converter transformer, it is known as a controlled series capacitor converter (CSCC). Each configuration will improve commutation performance of the inverter but the CSCC requires design features to eliminate ferroresonance between the series capacitor and the converter transformer if it should be instigated.

2.13 CONTROL AND PROTECTION

HVDC transmission systems must transport very large amounts of electric power which can only be accomplished under tightly controlled conditions. D.c. current and voltage is precisely controlled to effect the desired power transfer. It is necessary therefor to continuously and precisely measure system quantities which include at each converter bridge, the d.c. current, its d.c. side voltage, the delay angle and for an inverter, its extinction angle .

Two terminal d.c. transmission systems are the more usual and they have in common a preferred mode of control during normal operation. Under steady state conditions, the inverter is assigned the task of controlling the d.c. voltage. This it may do by maintaining a constant extinction angle which causes the d.c. voltage Ud to droop with increasing d.c. current Id as shown in the minimum constant extinction angle characteristic A-B-C- D in Figure 10. The weaker the a.c. system at the inverter, the steeper the droop.

Alternatively, the inverter may normally operate in a d.c. voltage controlling mode which is the constant Ud characteristic B-H-E in Figure 10. This means that the extinction angle

must increase beyond its minimum setting depicted in Figure 10 as 18O.

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If the inverter is operating in a minimum constant or constant Ud characteristic, than the rectifier must control the d.c. current Id. This it can do so long as the delay angle is not at its minimum limit (usually 5O). The steady state constant current characteristic of the rectifier is shown in Figure 10 as the vertical section Q-C-H-R. Where the rectifier and inverter characteristic intersect, either at points C or H, is the operating point of the HVDC system.

The operating point is reached by action of the on-line tap changers of the converter transformers. The inverter must establish the d.c. voltage Ud by adjusting its on-line tap changer to achieve the desired operating level if it is in constant minimum control. If in constant Ud control, the on-line tap changer must adjust its tap to allow the controlled level of Ud be achieved with an extinction angle equal to or slightly larger than its minimum setting of 18O in this case.

The on-line tap changers on the converter transformers of the rectifier are controlled to adjust their tap settings so that the delay angle has a working range at a level between approximately 10O and 15Ofor maintaining the constant current setting Iorder (see Figure 10). If the inverter is operating in constant d.c. voltage control at the operating point H, and if the d.c. current order Iorder is increased so that the operating point H moves towards and beyond point B, the inverter mode of control will revert to constant extinction angle control and operate on characteristic A-B. D.c. voltage Ud will be less than the desired

value, and so the converter transformer on-line tap changer at the inverter will boost its d.c. side voltage until d.c. voltage control is resumed. Not all HVDC transmission system controls have a constant d.c. voltage control such as is depicted by the horizontal characteristic B-H-E in Figure 10. Instead, the constant extinction angle control of characteristic A-B-C-D and the tap changer will provide the d.c. voltage control

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. Current margin

The d.c. current order Iorder is sent to both the rectifier and inverter. It is usual to subtract a small value of current order from the Iorder sent to the inverter. This is known as the current margin Imargin and is depicted in Figure 10. The inverter also has a current controller and it attempts to control the d.c. current Id to the value Iorder - Imargin but the current controller at the rectifier normally overrides it to maintain the d.c. current at Iorder. This discrepancy is resolved at the inverter in normal steady state operation as its current controller is not able to keep the d.c. current to the desired value of Iorder - Imargin and is forced out of action. The current control at the inverter becomes active only when the current control at the rectifier ceases when its delay angle is pegged against its minimum delay angle limit. This is readily observed in the operating characteristics of Figure 10 where the minimum delay angle limit at the rectifier is characteristic P-Q. If for

some reason or other such as a low a.c. commutating voltage at the rectifier end, the P-Q characteristic falls below points D or E, the operating point will shift from point H to somewhere on the vertical characteristic D-E-F where it is intersected by the lowered P-Q characteristic. The inverter reverts to current control, controlling the d.c. current Id to the value Iorder - Imargin and the rectifier is effectively controlling d.c. voltage so long as it is operating at its minimum delay angle characteristic P-Q. The controls can be designed such that the transition from the rectifier controlling current to the inverter controlling current is automatic and smooth. Voltage dependent current order limit (VDCOL)

During disturbances where the a.c. voltage at the rectifier or inverter is depressed, it will not be helpful to a weak a.c. system if the HVDC transmission system attempts to maintain full load current. A sag in a.c. voltage at either end will result in a lowered d.c. voltage too. The d.c. control characteristics shown in Figure 10 indicates the d.c. current order is reduced if the d.c. voltage is lowered. This can be observed in the rectifier characteristic R-S-T and in the inverter characteristic F-G in Figure 10. The controller which reduces the maximum current order is known as a voltage dependent current order limit or VDCOL (sometimes referred to as a VDCL). The VDCOL control, if invoked by an a.c. system disturbance will keep the d.c. current Id to the lowered limit during recovery which aids the corresponding recovery of the d.c. system. Only when d.c. voltage Ud has recovered sufficiently will the d.c. current return to its original Iorder level.

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2.14 A.C. Voltage Control

It is desirable to rigidly maintain the a.c. system and commutating bus voltage to a constant value for best operation of the HVDC transmission system. This is more easily achieved when the short circuit ratio is high. With low or very low short circuit ratio systems, difficulties may arise following load changes. With fast load variation, there can be an excess or deficiency of reactive power at the a.c. commutating bus which results in over and undervoltages respectively. When the a.c. system is weak, the changes in converter a.c. bus voltage following a disturbance may be beyond permissible limits. In such cases, an a.c. voltage controller is required for the following reasons:

1. To limit dynamic and transient overvoltage to within permissible limits defined by

substation equipment specifications and standards. 2. To prevent a.c. voltage flicker and commutation failure due to a.c. voltage fluctuations

when load and filter switching occurs. 3. To enhance HVDC transmission system recovery following severe a.c. system

disturbances. 4. To avoid control system instability, particularly when operating in the extinction angle

control mode at the inverter.

The synchronous compensator has been the preferred means of a.c. voltage control as it increases the short circuit ratio and serves as a variable reactive power source. Its

disadvantages include high losses and maintenance which add to its overall cost. Additional a.c. voltage controllers are available and include: 1. Static compensators which utilize thyristors to control current through inductors and switch

in or out various levels of capacitors. By this means, fast control of reactive power is possible to maintain a.c. voltage within desired limits. The main disadvantage is that it does not add to the short circuit ratio.

2. Converter control through delay angle control is possible to regulate the reactive power demand of the converter bridges. This requires that the measured a.c. voltage be used as a feedback signal in the d.c. controls, and delay angle is transiently modulated to regulate the a.c. commutating bus voltage. This form of control is limited in its effectiveness, particularly when there is little or no d.c. current in the converter when voltage control is required.

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3. Use of specially cooled metal oxide varistors together with fast mechanical switching

of shunt reactors, capacitors and filters. The metal oxide varistors will protect the HVDC substation equipment against the transient overvoltages, and the switchings of reactive power components will achieve the reactive power balance. Its disadvantage is that voltage control is not continuous, reactive power control is delayed by the slowness of mechanical switching, and short circuit ratio is not increased.

4. Saturated reactors have been applied to limit overvoltages and achieve reactive power balance. Shunt capacitors and filters are required to maintain the reactors in saturation. A.c. voltage control is achieved without controls on a droop characteristic. Short

circuit ratio is not increased. 5. Series capacitors in the form of CCC or CSCC can increase the short circuit ratio and

improve the regulation of a.c. commutating bus voltage. 6. The static compensator or STATCOM makes use of gate turn-off thyristors in the

configuration of the voltage source converter bridge. This is the fastest responding voltage controller available and may offer limited capability for increased short circuit ratio.

Since each a.c. system with its HVDC application is unique, the voltage control method applied is subject to study and design. 2.15 Special Purpose Controls

There are a number of special purpose controllers which can be added to HVDC controls to take advantage of the fast response of a d.c. link and help the performance of the a.c. system. These include:

A.c. system damping controls. An a.c. system is subject to power swings due to electromechanical oscillations. A controller can be added to modulate the d.c. power order or d.c. current order to add damping. The frequency or voltage phase angle of the a.c. system is measured at one or both ends of the d.c. link, and the controller is designed to adjust the power of the d.c. link accordingly.

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A.c. system frequency control. A slow responding controller can also adjust the power of the d.c. link to help regulate power system frequency. If the rectifier and inverter are in asynchronous power systems, the d.c. controller can draw power from one system to the other to assist in frequency stabilization of each.

Step change power adjustment. A non-continuous power adjustment can be implemented to take advantage of the ability of a HVDC transmission system to rapidly reduce or increase power. If a.c. system protection determines that a generator or a.c. transmission line is to be tripped, a signal can be sent to the d.c. controls to change its power or current order by an amount which will compensate the loss. This feature is useful in helping maintain a.c. system stability and to ease the shock of a disturbance over a wider area.

A.c. undervoltage compensation. Some portions of an electric power system are prone to a.c. voltage collapse. If a HVDC transmission system is in such an area, a control can be implemented which on detecting the a.c. voltage drop and the rate at which it is dropping, a fast power or current order reduction of the d.c. link can be affected. The reduction in power and reactive power can remove the undervoltage stress on the a.c. system and restore its voltage to normal.

Subsynchronous oscillation damping. A steam turbine and electric generator can have mechanical subsynchronous oscillation modes between the various turbine stages and the generator. If such a generator feeds into the rectifier of a d.c. link, supplementary control may be required on the d.c. link to ensure the subsynchronous oscillation modes of concern are positively damped to limit torsional stresses on the turbine shaft.

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Chapter 3 VSC Based HVDC Transmission System 3.1 Introduction The HVDC transmission technology can be realized by using current source converters (CSCs) commutated thyristor switches, known as traditional HVDC or classic HVDC, or by using voltage source converters (VSC-based HVDC). Due to the rapid development of power electronic devices with turn-off capability and of DSPs, which are generating the appropriate firing patterns, the VSC are getting more and more attractive for HVDC transmission .Usually, the VSCs are using insulated gate bipolar transistor (IGBT) valves and pulse width modulation (PWM) for creating the desired voltage wave form. The first HVDC transmission using VSC was installed in 1997 in Gotland (Sweden) .

3.2 Advantages and Applications for VSC Based HVDC By analyzing the operation of both classic HVDC technology and VSC-based HVDC technology, the main difference between these two technologies can be highlighted: the controllability. Thus, the controllability in the case of VSC-based HVDC technology is higher compared with the one of the earlier developed technology. Thereby, if VSCs are used instead of line-commutated CSCs several advantages can be stated, some of them being presented below: (i)VSC converter technology provides rapid and independent control of active and reac- tive power without needing extra compensating equipment; the reactive power can be controlled at both terminals independently of the DC transmission voltage level .

On the market, mainly two manufacture refer to the technology of DC transmission using VSC; these are: ABB under the name HVDC Light R [14], with a power rating from tenths of megawatts up to over 1000 MW, and the second manufacturer is Siemens under the name HVDC Plus (”Plus” - Power Link Universal Systems).

(ii) the commutation failures due to disturbances in the AC network can be reduced or even avoided if VSC-HVDC technology is used.

(iii)the VSC-HVDC system can be connected to a ”weak” AC network or to a network where no generation source is available (the VSC can work independently of any AC source), so the short circuit level is low .

(iv)self (forced) commutation with voltage source converters permits black start, which means that the VSC is used to synthesize a balanced set of three phase voltages as a virtual synchronous generator.

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(i)Power supply to insular loads:

new units can be easily added if the expand of the WF is desired . (iii)Underground/underwater cables: The use of HVDC cable systems is not constraint by any distance limitations as in the case of AC cable systems. Moreover, the losses are reduced when an HVDC cable system is used. The XLPE (Cross Linked Poly-Ethylene) extruded HVDC cables can overcome RoW constrains and the power transfer capacity is increased at the same time . (iv)Urban Infeed : Mainly due to RoW constraints and land use constraints, the compact VSC-based HVDC technology represents a feasible solution to feed the city centers. Thus, the underground transmission circuits are placed on already existing dual-use RoWs in or- der to bring in power as well as to provide voltage support . This process is realized without compromising reliability and it is an economical way of power supply.

(v)due to its modular, compact and standardized construction, the converter can be easily and rapidly installed/commissioned at the desired site .

(vi)in comparison with the classic HVDC transmission, the VSCs don not have any reactive power demand and moreover, they can control their reactive power to regulate the AC system voltage like a generator .

However, the VSC-based HVDC technology has some drawbacks, which include poten- tially high power losses and high cost (caused by the converter stations) compared with traditional HVDC technology. Because of its advantages, some of them presented above, the VSC-based HVDC transmission suits very well in certain application. An enumeration of these applications is presented below:

Due to some of its advantages such as: dynamic voltage control, black start capability or forced-commutation the VSC-HVDC transmission is capable to supply remote locations (i.e. islands) using submarine cables and without any need of running expensive local An example of this application is the Gotland Island System.

(ii)Offshore Application: The VSC-based HVDC technology represents a very suitable way of transmitting power from wind farms to the main AC grid. The ability of controlling reactive power as well as the AC voltage and its contribution to the grid stability makes the VSC-HVDC technology very popular for such applications. Moreover, the technology is flexible and

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3.3VSC-based HVDC Transmission System Configurations Such a transmission system consists of: two voltage source converters, transformers, phase reactors, AC filters, DC-link capacitors and DC cables. In the upcoming paragraphs each of these components will be briefly discussed. 3.4 Voltage Source Converter The two VSCs may be seen as the core of this transmission system topology. One of the VSCs works as rectifier, while the other one works as an inverter, and both of them are based on IGBT power semiconductors. The two VSC stations are connected through a DCtransmission line or an overhead line. Mainly, two basic configurations of VSCs are used on HVDC transmission system. Theseare the two-level VSC converter, presented in Figure 3.2(a), and the three-level VSC converter, which is presented Figure 3.2(b) . The two-level VSC, also known as the three phase, two level, six-pulses bridge, is the simplest configuration suitable for HVDC transmission. Such a converter consists of six valves (each valve consist of an IGBT and an anti-parallel diode) and is capable of generating two voltage levels −0.5 · UDCn and +0.5 · UDCn . In high power applications, the three-level VSC configuration , repre- sents a reliable alternative to the two-level VSC configuration, because the phase potentials can be modulated between three levels, −0.5·UDCn , 0 and +0.5·UDCn . In this configuration,one arm of the converter consists of four valves. 3.5 Transformer The transformers are used to interconnect the VSC with the AC network. The main function of the transformers is to adapt the voltage level of the AC network to a voltage level suitable to the converter. This voltage level can be controlled using a tap changer, which will maximize the reactive power flow. 3.6 Phase Reactor The phase reactors, known also as converter reactors, are used to continuously control the active and reactive power flow.The phase reactors have three main functions:

the last function is to limit the short-circuit currents. Typically, the short-circuit voltage of the phase reactor is 15%.

the second function is to provide active and reactive power control; the active and reactive power flow between the AC and the DC side is defined by the fundamental frequency voltage across the reactors.

• the first one is to provide low-pass filtering of the PWM pattern in order to provide the desired fundamental frequency voltage,

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3.7 AC Filter

3.8 DC-link Capacitor As presented in Figure , on the DC side, there are two capacitor stacks of the same power rating. The main goal of the DC-link capacitor is to provide a low-inductance path for the the harmonics ripple on the DC voltage. Depending on the size of the DC side capacitor, DC voltage variations caused by distur- bances in the system (e.g. AC faults) can be limited . 3.9 DC Cable

the self contained fluid filled(oil filled, gas pressurized) cables, the solid cables and XLPE polymer extruded cables. Lately, the last mentioned type seems to be the the preferred choice for VSC-based HVDC transmission system, because of their mechanical strength, flexibility and low weight

The main goal of the AC filters is to eliminate the harmonic content - which was created by using the PWM technique - of the output AC voltage. Otherwise, if these harmonic components are not eliminated or reduced, malfunctioning in the AC grid will appear. Typical requirements for AC filters are: individual harmonic distortion level (Dh ≈ 1%), total harmonic distortion (THD) level may vary between 1.5% and 2.5% and telephone influence factor (TIF) between 40 and 50% . Depending on the desired filter performances or requirements, the filter configuration is varying from application to application. In a typical HVDC Light scheme, the AC filter consists of two or three grounded /ungrounded tuned filter branches .

Mainly, three types of DC cables are suitable for HVDC transmission systems. These are:

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Chapter 4 HVDC Cables 4.1 Introduction Firsts of all, Cables are used when Overhead Line(that are simple and cheap but with a significant impact on ambient) cannot be built for environmental reasons or when power shall be transmitted underwater (through sea, lakes or rivers). In first case we have the so called Underground High Voltage Cable systems, in the second case Submarine

Cable systems. In general the power is transmitted using Alternating Current (AC) by simply connecting the two networks. The two networks must be SYNCHRONOUS: same frequency, same phasing(different voltages can be managed with transformers). Disturbances are also transmitted between the two networks. A cable under AC voltage is subject to a capacitive current that is proportional to the frequency f[Hz], to the voltage V[V], to the unitary capacitance C [μF/km] and to the cable length L[km]:I = 2·π· f · C · V · L Cables for HV-AC transmission typically have a capacitance of the order of 0,2-0,3 [μF/km] therefore require capacitive currents of 10 to 25 [A/km], depending on system voltage and frequency. For short lengths (few kilometers) this is not a problem, but for long lengths, e.g. above 60-80 km the capacitive current become similar in magnitude (even if in quadrature) to the active current that the cable is asked to transmit: losses are very much increased and consequently actual cable rating is reduced. With DC transmission, the things for the cable system are much simpler: f = 0; Consequently, capacitive current and main effects relevant to reactances are eliminated.Only conductor resistance plays the major role.

Transmission (Joule) losses are:W [W] = R · L · I 2

(+ W Earth Return)

and Voltage Drop:ΔV [V] = R · L · I(+ ΔV Earth Return) Practically, there are no limits for the Transmission Length, quite independently from transmission Voltage and Power.

However, systems are operated in AC; therefore DC transmission requires Converter Stations at both ends to convert AC to DC at sending point and DC to AC at receiving end. The two networks are not required to be syncronised; they can have different frequency and voltage. The power flow is simply controlled by voltage drop. The system, overall, acts like a Generating Power Station that is injecting power into the receiving network. Conventional High-Power Converters use Tyristors (controlled Diodes): the current must flow in one direction only. Therefore, when the power flow is reversed, also the polarity on the HVDC cable is reversed:

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4.2 Main Characteristics of an HVDC Cable System In general, an HVDC system can be composed by various sections, sometime including OHL lines, land and submarine cable. The HVDC Cable system is typically made by: (i)Cables ,(ii)Intermediate Joints and (iii)End Terminations. In the Land (Underground) sections, Installation is generally done from large drums, in excavated trenches, being the cable directly buried or pulled in plastic pipes. For Submarine Cables, the Installation is done by laying the cable on the sea bottom by using suitable Ships, that can accomodate large quantity of cable on board, stored on rotating platforms. Very often, the cable is protected on the sea bottom against possible damages caused by fishing tools and anchors by various methods. 4.3 Classification of HVDC Cables

Cables used for HVDC transmission are mainly of three types:

1. MI: Insulated with special paper, impregnated with high viscosity compound. 2. SCFF: Insulated with special paper, impregnated with low viscosity oil

3.Extruded: Insulated with extruded polyethylene-based compound These HVDC cables are briefly explained as follows: 1.Mass Impregnated Cables : Mass Impregnated Cables are the most used; they are in service for more than 40 years and have been proven to be highly reliable. At present used for Voltages up to 500 kV DC.Conductor sizes up to 2500 mm2. Typical Manufacturing Flow Diagram of a Mass Impregnated Cables.

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Figure 4.1:Mass Impregnated Cable 2.Self Contained Fluid-Filled Cables : Self Contained Fluid-Filled Cables are used for very high voltages (they are qualified for 600 kV DC) and for short connections, where there are no hydraulic limitations in order to feed the cable during thermal transients; at present used for Voltages up to 500 kV DC.Conductor sizes up to 3000 mm2.

Figure 4.2:Self Contained Fluid –Filled Cables

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3. Extruded Cables : Extruded Cables for HVDC applications are still under development; at present they are used for relatively low voltages (up to 150 kV DC), mainly associated with Voltage Source Converters, that permit to reverse the power flow without reversing the polarity on the cable. In fact, an Extruded Insulation (generally PE based) can be subjected to an uneven distribution of the charges, that can migrate inside the insulation due to the effect of the electrical field. It is therefore possible to have an accumulation of charges in localised areas inside the insulation( space charges) that, in particular during rapid polarity reversals, can give rise to localised high stress and bring to accelerated ageing of the insulation.

Figure 4.3:Extruded Cables (XLPE Cables)

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Chapter 5 HVDC Tower 5.1 Construction Process and Costs One of the major problems of the lines are the using of the area below. The first step for the project is to define according to the possible consequences the best routing in term of costs and result for the crossed areas. Use of the area below an overhead line is restricted because objects must not come too close to the energized conductors. Overhead lines and structures may shed ice, creating a hazard. Radio reception can be impaired under a power line, due both to shielding of a receiver antenna by the overhead conductors, and by partial discharge at insulators and sharp points of the conductors which creates radio noise. In the area surrounding overhead lines it is dangerous to risk interference; e.g. flying kites or balloons, using ladders or operating machinery. In add some studies are showing that life of organism can be influenced by the electrical field. The view of the lines can also be another difficulty due to the tourism presence and real estate area in the vicinity. Overhead distribution and transmission lines near airfields are often marked on maps, and the lines themselves marked with conspicuous plastic reflectors, to warn pilots of the presence of conductors. All these subjects shall be anticipated in the first phase of the project, then we can summarize as follow taking account of quality and reliability requirements: Rough determination of the route, taking account of the following criteria: Environmental compatibility Low impact on nature Most cost-effective construction possible Efficient operation (small losses) Consideration of natural or man-made obstacles (e.g. lakes, mountains and mountain ranges, cities, conservation areas, etc.) Possible locations of transformer substations Possible locations of assembly yards Maintenance costs in the operating phase 5.2 Detailed Planning of The Transmission Route For the detailed planning, routing is carried out – an operation which involves recording and assessing the features of the terrain in particular. This routing is carried out in stages, in ever more detail.

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5.3 Detailed Design and Execution Drawing Taking account of the results of the routing, a detailed execution plan for the overhead line is worked out. Besides a detailed geological survey (soil testing), this also includes the design planning of the pylons. This essentially depends on topological conditions (minimum clearances from objects and trees), scenic aspects (low mast height in built-up areas wherever possible) and meteorological effects (influence of wind, ice load, avalanche hazard), as well as on the number of conductor systems. In order to ensure the highest possible level of operational safety, a thorough study of wind conditions is carried out along the entire route. Individual wind zones are established in the course of this study and the pylons are dimensioned accordingly. Particular attention must be paid to critical sections in which the topography is such that it can give rise to "funnel effects"characterised by high wind speeds. The crossing of mountain tops is also to be regarded as critical. Sectioning into ice-load zones (if there are) is likewise carried out. In Europe this is based on a pan-European standard with individual national appendices. In critical areas, the design of pylons and conductors should be reinforced. In areas at risk from avalanches, pylons must be provided with special protection (e.g. by means of avalanche wedges, intended to steer the avalanche forces around the pylon). A project flowchart is drawn up for the realisation of the project. With longer lengths of transmission lines, the project as a whole is divided into individual lots (e.g. 20 km). 5.4 Construction Phases / Time Schedule In this chapter we will focus only on the T&D lines itself erection, the construction of other element as substations are well-known. 5.5 Preliminary Work On Construction Once the detailed planning has been carried out and the approval process completed, a start can be made with the actual on-site construction work. However, considerable preliminary work is needed before the actual work of erecting overhead lines can begin. This preliminary work includes: Tree-felling work on routes running through forests Road building work Site facilities (usually about every 20 km) 5.6 Foundations Foundations for tower structures may be large and costly, particularly if the ground conditions are poor, such as in wetlands. Each structure may be considerably strengthened by the use of guy wires to resist some of the forces due to the conductors. In case the earth is extremely aggressive, special concrete must be used to avoid damage in the foundation. In extreme climatic circumstances a foundation must be stronger and bigger. If you are to build closely to the coast, you must consider that the wind conditions are stronger there than in the middle of the land mass. Where you are, determines the terrain class. The size of the pylon is also an important factor in the evaluation of the load

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on the foundation and consequently the size of the foundation. For this purpose the following methods can be applied: The earth is dug up normally and in keeping with the size of the foundation, after which the foundation is cast. Bunging/ Sheet piling method is applied in narrow spaces. Interlocking sheets of steel are pressed down at all four corners and the cast of the foundation starts step by step from there. The earth will not fall into the pit during the dig, since it is held by plates. Piling method this method is used for building an especially strong foundation. The method is suitable for places where the ground does not have a strong adhesion (sandy earth).Concrete piles are thrust into the ground into e.g. 10 metres depth with approximately half a meter to one meter above the ground. The upper part of the concrete pile is then blasted off and the iron inside the pile bent into the top layer of the foundation, which is being cast on top. Thus the foundation is anchored in the best

possible way into the ground and has great static carrying capacity. The time frame depends on the size of the foundation, but it typically takes one week to cast a foundation of 5x5 metres. The drying of foundation depends on the time of the year and the weather. In Summer the foundation is ready for use after 1 – 2 weeks, whereas in Winter the foundation dries for about 3 – 4 weeks. 5.7 Pylon Assembly / Switch Yard Erection Whereas concrete and round steel masts are supplied complete, lattice pylons are usually delivered in individual pieces and assembled into segments on site – on the ground. The pylon segments and arms are then fixed together (pylon assembly). Depending on the local conditions, this is done using either cranes or – especially in rough terrain – helicopters. According to the type and size of the elements the preassembling is scheduled. The location of the T&D line and weight of the elements can drive to a mixed solution. 5.8 Cables Hanging After the erection of the steel structure and the fitting of the surge arresters, isolators, and cable reels are preassembled on the ground then they are attached to the pylon. The cable reels allow the pilot rope, pulling rope and conductors to be installed. Parallel to this, the cable-drum and winch sites are constructed and anchored appropriately. The cable reels and cable winches are then fastened onto them. The usual and simple method is the use of drawing machine tool. Where the transmission route crosses transportation routes such as motorways or railway lines,safety scaffolding is set up in the crossing area to prevent danger to the traffic running below in the event of any cables falling. These pilot ropes are up to 6 km long and are used for attaching the cable pulling ropes. It will allow initiating the drawing of the conductor at its place. After the pulling rope, finally the (operating) conductors and, depending on the voltage level and lightning protection, one or two earthing conductors are hoisted up. The pulling rope is a steel-wire rope with enough tensile strength to be able to hoist up the final conductor and the earthing conductor (lightning protection cable).

The pilot ropes (usually nylon ropes 10-15 mm in diameter) are then hoisted up, using helicopters.

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The cables are then adjusted. This involves tensioning the cables to the relevant tension and adjusting to provide the necessary sag. The cables are then braced in the case of angle pylons and clamped in the case of support pylons. The final work consists of fitting the spacers of the individual conductor bundles (field spacers),installing the bird warning and aircraft warning spheres, and attaching the cable loops on the pylons. 5.9 Tests and Acceptance

operation following a precisely specified start-up programme. 5.10 Recultivation .

5.11Values of T & D Lines Investment cost. A high-voltage, direct current (HVDC) transmission line costs less than an AC line for the same transmission capacity. However, the terminal stations are more expensive in the HVDC case due to the fact that they must perform the conversion from AC to DC and vice versa. On the other hand, the costs of transmission medium (overhead lines and cables), land acquisition/right-of-way costs are lower in the HVDC case. The here below scheme summarize the cost comparison between DC and AC line. It appear that some technical trend, such as material, diameters, and other parameters can influence the diagram, but as they are linked to the mechanical characteristics of the materials, the choice can be driven through the global parameters as mentioned. This fact explains partially the big differences which can occur between price of tow projects.

The test phase is very important, as it should simulate every possible operating condition. Besides visual and mechanical inspections (clamped and screwed connections), earth-fault tests are also carried out, as well as technical tests in the transformer stations. The line section is then taken into

Once all the work has been completed, the relevant road removal, reforestation and recultivation work is carried out.

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5.12. Typical 500 KV HVDC Lattice Tower:

Transmission Line Quick Facts

(All numbers are typical and approximate, and will vary with final route and design.)

Total length: 500 kilometres Total towers: 1500 Span between towers: 365 metres (1200 feet) Tower height: 39 metres (128 feet) Tower width (at arms): 27 to 29 metres (89 to 95 feet) Max. tower base (square): 13 metres (43 feet) Min. conductor height: 12 metres (39 feet) Total wires: 2 sets of 4 conductor wires, 1 set of 2 neutral return wires, 2 sets of overhead shield wires Right-of-way width: 55 to 60 metres (180 to 197 feet) Total right-of-way: 2750 hectares (6800 acres)

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Chapter 6

HVDC +/-260 KV Transmission Line Project

6.1Tentative Title: Long Overhead Electric Power Transmission Line Design with assistance of High Voltage Direct Current (HVDC) System.

6.2Introduction:

HVDC transmission has been in use for more than 50 years.It has proved to be a reliable and valuable transmission media for electrical energy and has a number of technical advantages compared with HVAC transmission. Nonetheless, a comprehensive HVDC/HVAC system planning approach is not commonly found within utilities, and therefore full advantage is not being taken of the HVDC technology. of electrical power transmission.Recent developments in energy policies and stronger environmental lobbies have a significant impact on the design and construction networks, and could provide a number of opportunities for HVDC transmission. However, HVDC transmission is perceived to be expensive, difficult to integrate in an ac network, to require highly skilled personnel to operate and maintain, and to have high power losses. In today electricity industry, in view of the liberalisation and increased effects to conserve the environment, HVDC solutions have become more desirable for the following reasons:

1.Environmental advantages 2.Economical (cheapest solution) 3.Asynchronous interconnections 4.Power flow control 5.Added benefits to the transmission (stability, power quality etc.)

High voltage DC (HVDC) Transmission system consists of three basic parts: 1) converter station to convert AC to DC 2) transmission line 3) second converter station to convert back to AC. HVDC transmission systems can be configured in many ways on the basis of cost,flexibility and operational requirements.The simplest one is the back-t-back interconnection and it has two converters on the same site and there is no transmission line.This type of connection is used as an inter tie between two different AC transmission systems.The monopolar link connect two converter stations by a single conductor line and earth or,sea is used as a returned path.The most common HVDC link is bipolar ,where two converter stations are connected by bipolar conductors and each conductor has its own ground return.The multi-terminal HVDC transmission systems have more than two converters stations which could be connected is series or,parallel.

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6.3 Long Term Significance: There are noteable advantages of HVDC transmission which are as follows: Advantages:

1.Greater power per conductor. 2.Simpler line construction. 3.Ground return can be used. 4.Hence each conductor can be operated as an independent circuit. 5.No charging current. 6.No Skin effect. 7.Cables can be worked at a higher voltage gradient.

8.Low short- Line power factor is always unity: line does not require reactive compensation.

9.Less corona loss and radio interference, especially in foul weather, for a certain conductor diameter and rms voltage.

10.Synchronous operation is not required. 11.Hence distance is not limited by stability. 12.May interconnect A.C systems of different frequencies. circuit current on D.C line. 13.Does not contribute to short-circuit current of a A.C system. 14.Tie-line power is easily controlled. However,there are unavoidable disadvantages of HVDC system which are as follows:

Disadvantages: 1.Converters are expensive. 2.Converters require much reactive power. 3.Converters generate harmonic, require filters. 4.Multiterminal or network operation is not easy. Considering advantages,HVDC is a preferable method for transmission of bulk amount of power over long distances.HVDC is reliable method.For plenty of advantages and technical and economical reasons,HVDC provides long standing potential of enhancing the compensating of the rapidly growth demand of electric power in Bangladesh undoubtedly .In far future,the electric power sector of Bangladesh will enjoy the technical and economical advantages for employing the HVDC transmission instead of HVAC. 6.4 Proposed Plan and Methodology: The Author has observed the inefficient existing HVAC transmission with respect to the growing demand of electric power under the lack of generation of electric power corresponding to load in Bangladesh.The electric energy shortage has been existing for 20 years in Bangladesh.One of the possible solution of this problem is adoption of HVDC transmission.Obviously,we know that due to considerable amount of line loss,total generated power is not entirely transmitted to the receiving load center.So,we have lossed huge amount of power for transmission line.But,if we implement the HVDC system for transmission of electric power,technically ,in transmission line less power loss will be occurred.Therefore,extra power would be added to the receiving load center.Thes added extra

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power will serve a load which are deprieved of service instance of HVAC.However,HVDC transmission is economically quite cheap. Experimentally,the Author has proposed long overhead HVDC transmission line from Ghorasal –Ishurdi-Khulna.It is a typical model for transmission in Bangladesh.Generated power in Ghorasal will be converted to DC by a converter and then transmitted through the bipolar DC line including ground return and in Khulna converted to AC by a converter.So,AC generation,DC transmission and AC distribution is adopted.The Author has typically proposed for a +/-260KV,350KM,500MW Bipolar Earth Return overhead HVDC transmission system between Ghorasal and Khulna.This is Voltage Source Converter (VSC) based overhead HVDC system. 6.5Existing Transmission Network(as on June,2010): PGCB owns and operates the high voltage transmission network throughout Bangladesh.The national gfrid operates at 230KV,132KV and 66KV and controls and manages the second to second operation of electricity transmission system ,balancing electricity generation to meet the demand. Salient features of PGCB and BPDB is transmission network is: Grid Substations Capacity:16749 MVA Total No. of substations :108 Nos.(7 nos.BPDB,PGCB 88 nos and DPDC 13 nos.)

6.6 Existing Transmission Lines: *230 KV Transmission Lines: Serial No. Name of Lines Length in

Route ,KM Length in CKT,KM

No. of Circuits Conductor

1. Ghorasal-Ishurdi

178.00 356.00 Double Mallard,795MCM

2. Khulna-Ishurdi 185.00 370.00 Double Twin AAAC,37/4.176mm

*132 KV transmission Lines:

Serial No. Name of Lines Length in Route ,KM

Length in CKT,KM

No. of Circuits Conductor

1. Goalpara-Ishurdi

169.00 338.00 Double AAAC,804MCM

2. Khulna-Khulna

9.00 18.00 Double Twin AAAC,37/4.176 mm

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6.7Existing Power Generation in Ghorasal: 1.Ghorasal Unit -1,2:2X55MW(Installed Capacity)85 MW (Present Capacity) 2.Ghorasal Unit-3,4,5,6 :4X210MW(Installed Capacity)760 MW (Present Capacity) 3.Ghorasal 100MW (Aggreko)HSD QRPP:100MW(Installed Capacity)100MW(Present Capacity) 4.Ghorasal 45MW(Aggreko)HSD QRPP:45MW(Installed Capacity)45MW(Present Capacity) 5.Ghorasal 78.5 MW(Max.)Gas QRPP Proposed Power Plant: 1.Ghorasal 200-300MW Gas Turbine Peaking Power Plant Project 2.Invitation Notice for Ghorasal 100+/-10% MW Gas Fired Power Project At Ghorasal,Narsingdi,Bangladesh.

6.8 Generation Voltage:

Terminal voltage of different generators are 11KV,11.5KV and 15.75KV. Bangladesh Power Development Board Installed Capacity: As on June,2010,the total installed capacity including IPP consists of the following mix: *Hydro-230MW(3.95%) *Steam-2638MW(45.31%) *Gas Turbine-1466MW(25.18%) *Combined Cycle-1263MW(21.69%) *Diesel-226MW(3.87%) Total-5823MW(100%)

6.9Typical Electrical Parameters for a 230KV Overhead Line:

Parameters Quantity

R[ohm/KM] 0.050

XL[ohm/KM] 0.488

Bc[micros/KM] 3.371

α [nepers/KM] 0.000067

β [rad/KM] 0.00128

Z0 [ohm] 380

SIL[MW] 140

Charging[MVA/KM] 0.18

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6.10 Data of Existing Transmission Line

Table 7.1:Transmission Line Data

ID Database ID Status From Bus

To Bus

KV Level

Length Length Unit

R1

[P.U]

X1

[P.U]

B1

[P.U]

R1*

[P.U]

2010_2020_1 Ghorasal_Ishurdi ON 2010 2020 230 178 KM .00015 .00077 .0004 0

2020_2032_1 Ishurdi_Khulna New_1

ON 2020 2032 230 172 KM .00008 .00055 .0006 0

2020_2032_2 Ishurdi_Khulna New_2

ON 2020 2032 230 172 KM .00008 .00055 .0006 0

ID Database ID R0[P.U] X0[P.U] B0[P.U] LOADING LIMIT[A]

EMERGENCY LOADING LIMIT[A]

No. of Conductor Per phase

Tower

Structure

N1 Neutral Status

2010_2020_1 Ghorasal_Ishurdi .00065 .0023 .0009 753.07 1129.61 1 Double

Circuit_Lattice

Eliminated

2020_2032_1 Ishurdi_Khulna New_1

.0006 .0021 .0003 1500 2250 1 Double

Circuit_Lattice

Eliminated

2020_2032_2 Ishurdi_Khulna New_2

.0006 .0021 .0003 1500 2250 1 Double

Circuit_Lattice

Eliminated

Table 7.2 :Static Load

ID DataBase ID Status Duplic Duplic Info From Bus P load [MW]

Q Load

[MVAR]

Ghora_Sl 1 Ghora_SL1 ON 100 Combined Data

1130 1.8 0.9

Ghora_Sl2 Ghora_Sl2 ON 100 Combined Data

2010 40.8 20

Ghora 1 Ghora 50 ON 100 Combined Data

Ghor1 24.67 12.3

Ghora2 Ghora25 ON 100 Combined Data

Ghora2 12.335 6.1

Ishurdi 1 Ishurdi10 ON 100 Combined Data

Ishurdi1 5.333 2.5

Ishurdi 2 Ishurdi 10 ON 100 Combined Ishurdi 2 5.333 2.5

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Data

Ishurdi 3 Ishurdi 10 ON 100 Combined Data

Ishurdi 3 5.333 2.5

KhulnaC1 KhulnaC48 ON 100 Combined Data

KhulnaC1 20 6.5

KhulnaC2 KhulnaC48 ON 100 CombinedData KhulnaC2 20 6.5

KhulnaC3 KhulnaC48 ON 100 Combined Data

KhulnaC3 20 6.5

7.3 Three Winding Transformer:

Location Status Duplic Duplic Info From Bus Secondary Bus ID

Tertiary Bus ID

Ghorasal ON 1 N/A 2010 1130 4

Ghorasal ON 1 N/A 2010 1130 4

Ishurdi ON 1 N/A 2020 1401 9

Ishurdi ON 1 N/A 2020 1401 9

Ishurdi ON 1 N/A 2020 1401 9

Khulna ON 1 N/A 2032 1332 11

Khulna ON 1 N/A 2032 1332 11

Location Total min(%) Total max(%) Primary Voltage(KV)

Secondary Voltage(KV)

Tertiary Voltage(KV)

Primary (MVA)

Ghorasal -2 2 230 132 33 125

Ghorasal -2 2 230 132 33 125

Ishurdi -2 2 230 132 33 125

Ishurdi -2 2 230 132 33 125

Ishurdi -2 2 230 132 33 125

Khulna -2 2 230 132 33 125

Khulna -2 2 230 132 33 125

Location Secondary (MVA)

Tertiary(MVA) Primary Winding

Secondary Winding

Tertiary Winding

Z1 P_S[P.U]

Ghorasal 125 25 YG YG D .077

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Ghorasal 125 25 YG YG D .077

Ishurdi 225 25 YG YG D .0585

Ishurdi 225 25 YG YG D .0585

Ishurdi 225 25 YG YG D .0585

Khulna 225 25 YG YG D .0585

Khulna 225 25 YG YG D .0585

Location Z1 P_T[P.U] Z1 S_T[P.U] X/R Positive P_S

X/R Positive P_T

X/R positive S_T

Z0 P_S

Ghorasal .24 .16 50 50 50 .077

Ghorasal .24 .16 50 50 50 .077

Ishurdi .0762 .0516 42 50 42 .0585

Ishurdi .0762 .0516 42 42 42 .0585

Ishurdi .0762 .0516 42 42 42 .0585

Khulna .0762 .0516 42 42 42 .0585

Khulna .0762 .0516 42 42 42 .0585

Location Z0 P_T[P.U] Z0 S_T[P.U] X/R zero P_S X/R Zero P_T P_S Phase shift P_T Phase Shift

Ghorasal .24 .16 50 50 0 -30

Ghorasal .24 .16 50 50 0 -30

Ishurdi .0762 .0516 42 42 0 -30

Ishurdi .0762 .0516 42 42 0 -30

Ishurdi .0762 .0516 42 42 0 -30

Khulna .0762 .0516 42 42 0 -30

Khulna .0762 .0516 42 42 0 -30

Location No. of taps Loading Limiting[MVA]

Emergency loading limit[MVA]

V min Tap[%] V max tap[%] Control Bus voltage

Ghorasal 17 125 150 90 110 132

Ghorasal 17 125 150 90 110 132

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Ishurdi 17 225 275 90 110 132

Ishurdi 17 225 275 90 110 132

Ishurdi 17 225 275 90 110 132

Khulna 17 225 275 90 110 132

Khulna 17 225 275 90 110 132

7.4 Fixed tap transformer

ID Status Duplic Duplic Info. From bus To Bus Rated S [MVA]

1130_GHORA_1 ON 1 N/A 1130 GHOR_1 100

1130_GHORA_1 ON 1 N/A 1130 GHOR2 100

1130_GHORA5U ON 1 N/A GHOR1U132 100 100

1130_GHORA6U ON 1 N/A GHOR2U132 100 100

1302_KHULN_0 ON 1 N/A 1302 KHULNAC2 100

1302_KHULN_1 ON 1 N/A 1302 KHULNAC3 100

1302_KHULN_2 ON 1 N/A 1302 KHULNAC1 100

1401_ISHUR_0 ON 1 N/A 1401 ISHURDI1 100

1401_ISHUR_1 ON 1 N/A 1401 ISHURDI2 100

1401_ISHUR_2 ON 1 N/A 1401 ISHURDI3 100

ID Primary [KV] Secondary[KV] Primary Winding

Secondary Winding

Phase Shift Z1[P.U]

1130_GHORA_1 132 33 D YG 30 .152

1130_GHORA_1 132 33 D YG 30 .152

1130_GHORA5U 132 33 D YG 30 .1429

1130_GHORA6U 132 33 D YG 30 .1429

1302_KHULN_0 132 33 D YG 30 .1187

1302_KHULN_1 132 33 D YG 30 .1187

1302_KHULN_2 132 33 D YG 30 .1187

1401_ISHUR_0 132 33 D YG 30 .7662

1401_ISHUR_1 132 33 D YG 30 .7662

1401_ISHUR_2 132 33 D YG 30 .7662

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ID Z0[P.U] X/R Positive X/R Zero Type Loading limit[MVA]

Emergency Loading limit[MVA]

Primary tap[%}

1130_GHORA_1 .116 42 42 Shell 75 90 100

1130_GHORA_1 .116 42 42 Shell 41 49 100

1130_GHORA5U .3761 999.9 999.9 Core 115 140 100

1130_GHORA6U .3761 999.9 999.9 Core 115 140 100

1302_KHULN_0 .0906 42 42 Shell 64 76.8 100

1302_KHULN_1 .0906 42 42 Shell 64 76.8 100

1302_KHULN_2 .0906 42 42 Shell 64 76.8 100

1401_ISHUR_0 .4361 42 42 Core 13.3 16 100

1401_ISHUR_1 .4361 42 42 Core 13.3 16 100

1401_ISHUR_2 .4361 42 42 Core 13.3 16 100

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6.11 Typical High Voltage Direct Current (HVDC) Transmission Line Between Khulna-Ishurdi-Ghorasal:

Scheme Ratings(Typical)

Commissioning Year 2011(Suppose)

Power Transmitted ,MW 500

Direct Voltage,KV +/-260

Configuration Bipole,Ground Return

Converter Type Force Commutated Voltage Source Converter

Converter Transformer One star/star at rated 203 MVA and 415/111.5 KV;One star/delta at rated 203 MVA and 415/111.5 KV.

Converter per stations 2

Direct Voltage per converter,KV 260

Direct Current,A 40

Reactive Power Supply Capacitors,Synchronous Condensers

Converter Station Location and AC Grid Voltage

Goalpara,260 KV and Ghorasal ,260KV

132/230 KV

Length of Overhead DC Line 350 KM

Cable Arrangement 2 Cable,ground return

Cable Route Length 350 KM

Grounding of the DC Circuit Full current in two ground electrode stations

AC grids at both ends Synchronous

Control Constant Power in either Direction

Emergency change of power flow On manual or,automatic order to preset value

Main reason for choosing HVDC System Overhead Transmission of bulk amount of power.

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6.12 Typical Tower Structure

Typical +/- 260 KV Transmission Line Tower Structure:(All numbers are typical and approximate and will vary with final route and design)

Tower entities Typical Ratings

Total Length 350 KM

Total towers 1050

Space between towers 365 metres(1200 feet)

Tower height 39 metres(128 feet)

Tower width (at arms) 27 to 29 metres( 89 to 95 feet)

Max. tower base (square) 40 metres ( 135 feet)

Min. conductor height 12 metres( 39 feet)

Total wires 1 sets of 2 conductor wires

Right of way width 55 to 60 metres(180 to 197 feet)

Total right of way 2750 hectares(6800 acres)

Insulator arrangement Conventional cross arms

Typical foundation dimensions 4 off,4X4 m square pad,1 m deep (above ground) 0.9 m diameterX4m deep pier(below ground)pad protrudes 400mm above ground.

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6.13 HVDC Project Cables:

Figure6.1:Mass Impregnated Cable

Mass Impregnated Cables are the most used; they are in service for more than 40 years and have been proven to be highly reliable. At present used for Voltages up to 500 kV DC.Conductor sizes up to 2500 mm2.

Typical Weight= 30 to 60 kg/m

Typical Diameter = 110 to 140 mm

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6.14 Typical HVDC Circuit Diagram

Figure6.2:VSC based HVDC Substation

Figure6.3:VSC based HVDC Converter Arrangement

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Figure 6.4:VSC based HVDC Indoor-Outdoor View

Figure 6.5:HVDC transmission with VSC

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Figure 6.6 :Control of VSC Based HVDC Transmission

Figure 6.7:HVDC Overall View.

51

6.15 HVDC +/-260 KV Project Economics:

HVDC cost values given in year 2011 US $/KW (both ends inclusive) for one valve group per pole.For Bipole +/-260 KV,500 MW Khulna_Ishurdi_Ghorasal Project,costs are typically as follows:

Schemes Cost (Typical),US $

Valve Groups 21

Converter Transformer 22

DC Switchyard and Filtering 6

AC Switchyard and filtering 9.5

Control/Protection/Communication 8

Civil/Mechanical works 14

Auxiliary Power 2.5

Engineering & Administration 17

Total 100

Total cost per KW US$ US$ 170

6.16 Cost ratios for DC and AC Transmission Line construction.

AC equivalent line Cost P.U. HVDC Bipolar line ratings

Range of costs P.U.

230 KV,Double Circuit

1.00 +/-260KV,500MW 0.68 to 0.95

52

6.17 HVDC System Reliability

HVDC system composed of a bipole

Schemes Forced outage rate Energy unavailability Energy Availability

Duplicated filter .010798 .004098 99.9959

Converter .731940 .138605 99.8614

DC Yard .067643 .038882 99.9611

Converter .731940 .138605 99.8614

Duplicated filter .010798 .004098 99.9959

One Pole 1.599167 .354550 99.6454

Common equipment .169400 .262838 99.7372

Two Poles .202297 .624602 99.354

6.18 Cost Structure of Converter Stations:

The cost of an HVDC transmission system depends on many factors,such as power capacity to be transmitted,type of transmission medium,environmental conditions and other safety,regulatory requirements etc.Even when these are available ,the options available for optimal design (different commutation techniques,variety of filters,transformers etc) render it is difficult to give a cost figure for an HVDC system.The main equipment of the D.C station is converters and more than 50% cost of HVDC transmission system are related to the converters.The line cost for the HVDC transmission system is $320-$370 KV-mile for +/-400 to +/-700KV.Nevertheless,a typical cost structure for the converter stations could be as follows:

53

HVDC System costs as a percentage of Total Project Cost

Equipments Percentage of Total Cost

Valves 20%

Converters Transformers 16%

AC Filters 10%

Control 7%

Other equipments 10%

Engineering 10%

Civil works,buildings 14%

Erection,commissioning 8%

Freight,Insurance 5%

6.19 HVDC Current,Voltage,Insulation Level,Power Transmission and Percentage Loss comparision with HVAC:For the following assumptions,the current,voltage,insulation level,power transmission and percentage loss have showed between HVAC and HVDC.The assumptions are:

(i)Same Power Transmitted

(ii)Same Power Loss

(iii)Same Conductor Size.

a.Current:

For the above assumptions,HVDC current and HVAC current has the following relationship: Ia(Alternating Current)/Id(Direct Current)=√(2/3)

54

b.Voltage:

For the above assumptions,HVDC voltage and HVAC voltage has the following relationship: Vd (Direct Voltage)/ Va (Alternating voltage)=√6

c.Insulation level:

For the above assumptions,HVDC insulation and HVAC insulation has the following relationship:

DC insulation level/AC insulation level=.867

d.Power Transmission:

For the above assumptions ,power transmission of HVDC and HVAC has the following relationship:

Power transmitted by DC/Power transmitted by AC=2

e.Percentage loss:

For the above assumption,percentage loss of power between HVDC and HVAC has the following relationship:

Percentage loss by DC/Percentage loss by AC=0.707

55

6.20Tower Calculation Load due to wind pressure on projected area of the conductor={(1.4+2)/100}X39 kgm-2

=1.326kgm-1

Weight of conductor=0.728 kg/m

Wind pressure=1.326 kg/m

The vertical loading,v=.728 kg/m

The Total load,W=

=1.512 kg/m

The breaking load=10570 kg

The permissible tension is assumed to be 3500 kg.

(a) Maximum loading,W=1.512 kg/m T=3500 kg Span=2l=365 m Or, Half span,l=182.5 m

Horizontal tension, H . . . . . . . . .(1)

Putting values in the eqn(1),we have,H=3500-10.88=3489.12 kg

The total sag, d= (cosh -1) . . . . . . . . .(2)

Putting values in eqn(2),we have,d=2307.62(cosh 0.0789-1)=7.2 m

Half length of conductor= sinh( ). . . . . . . . (3)

Putting values in eqn(3),we have,l=182.69 m

Length of the conductor,L=365.38 m

(b) The vertical sag is d X =7.2 X =3.47 m

56

(c) (i) d=7.2 m and 2l=365 m

The length of the conductor L is given by,

L=2l(1+ . - . +. . . . . . ). . . . . . . . . .(4)

Putting values in eqn(4),we have,L=365(1+0.001)=365.365 m

(ii) Unstretched length at 00 C with no ice and wind :

Luo=L- (1+ . +. . . . . . . .). . . . . . . . . (5)

Area of conductor,A= (d2)

= (1.4)2

=1.54 cm2

Modulus of Elasticity,e=12.65X105 kg/cm2

Vertical load,v=0.728 kg/m

Half span,l=182.5 m

Sag,d=7.2 m

Putting values in eqn(5),we have,Luo=(365.365-0.315)=365.05 m

(iii) Unstretched length at 600 C with no ice:

When only weight of the conductor is taken into account for loading and there is no ice loading ,the loading will be 0.728 kg/m.

Luo=365.365-(0.315X1)=365.05 m

If this is at 60 C ,to convert it to temperature 0 C ,unstreched length at 0 C when the sag occurs at 60 C.

Luo=365.05(1-60X16.5X10-6)=365.05(1-0.00099)=364.69 m

(d)Taking unstreched length at 0 C,find L with maximum load on the line,viz,weight of conductor ,no ice and wind pressure:

L=Luo+ (1+ . + . +. . . . . . .). . . . . . . . . .(6)

Putting values in eqn(6),we have,L=365.05+0.00359=365.05 m

Assuming maximum tension to be 3450 kg,

L=2l{1+ ( 2+ . . . . . . . .}. . . . . . . . . .(7)

Putting values in eqn(7),we have,L=365 m

57

This length is less than 365.05 m.Therefore,the tension used is less than 3450 kg.

Assuming a tension of 3400 kg, from eqn(7),L=365 m

Therefore,the tension is more than 3400 kg.

Assuming T=3425 kg,from eqn(7),L=365 m

The tension on the line thus lies between 3425 and 3450 kg and assumed to be 3440 kg.

Breaking strength of the conductor= 10570 kg Therefore,the factor of safety=10570/3440=3.07

6.21Preliminary Design of Tower: For a +/- 260 KV line, a steel lattice tower is chosen.The spacing of the conductor is horizontal ,the equivalent spacing being 16 m or,horizontal distance between adjacent conductors is 16 m .The span is 365 m and the minimum height of the conductor at mid span is as 23.8 m .The supports are assumed at same level.The sag is calculated as 7.2 m.

For 260 KV line,18 suspension type insulators are used,each of 25 cm diameter,disc spacing 14.6 cm with respect to each other.Length of the insulator string=18X14.6 cm = 262.8 cm or,about 2.63 m .

For insulation level of 18000 KV ,the tower footing ground impedance should be about 10 ohm.

Minimum height of conductor at mid span=23.8 m

Sag=7.2 m

Insulator string length=2.63 m

Minimum height upto cross-arm=(23.8+7.2+2.63)m=33.63 m

Height of earth-wire location above conductor=8 m

Overall height of tower=(33.63 +8-2.63) m = 39 m

58

6.22 Corona Loss: The density of air at b cm (Hg) barometric pressure and t0 C temperature is given by,

. . . . . . . . . .(1)

Here,b=76 cm

t=400C

So,=0.952

The disruptive critical voltage Ed is given by, Ed=21.1mrln kv. . . . . . . . .(2) Here, m=surface factor

r=radius of the conductor

d=distance between conductors

=Density of air

Here,m=1

r=.7X10-2 m

d=16 m

=0.952

Ed=21.1X1X.7X10-2X0.952ln(16/.7X10-2)=1.08 KV

The corona loss, P= (f+25) (E-Ed)X10-5 KW . . . . . . . . .(3) Here,E=260 KV

Ed=1.08 KV

f=0 Hz

1

r=.7X10-2 m

d=16 m

Putting values in eqn(3),P=0.313 KW

59

6.23 MATLAB Program for Comparison of HVDC and HVAC (a)MATLAB Program for cost comparison of HVAC and HVDC clc; cost=[128.7 340.34 300.3 750.75 107.61 170.17 560 1120 1096.61 2381.26]; bar(cost,'group'); set(gca,'XTickLabel',{'Substation';'Cable'; 'Cable Installation';'Transmission Line';'Total'}); xlabel('Field of Cost'); ylabel('Million (US $)'); title('Cost for HVDC & HVAC system'); legend('HVDC','HVAC'); set(legend,'Position',[0.5 0.8 0.2 0.1]); grid ; Output Figure:

Substation Cable Cable Installation Transmission Line Total0

500

1000

1500

2000

2500

Field of Cost

Mill

ion

(US

$)

Cost for HVDC & HVAC system

HVDCHVAC

60

(b) MATLAB Program for losses of power between HVAC and HVDC: clc; TL=[2 0 0 2 2 0 8 8 2 15 13 3 22 22 6 32 28 10 40 34 15 ]; bar(TL,'group'); set(gca,'XTickLabel',{'0';'50';'200';'400'; '600';'800';'1000'}) ylim([0 50]); xlabel('Power Flow (MW)'); ylabel('Losses in (MW)') legend('Total Loss','Converter Loss','AC loss') set(legend,'Position',[0.5 0.8 0.2 0.1]); grid on; Output Figure:

0 50 200 400 600 800 10000

5

10

15

20

25

30

35

40

45

50Losses Vs Power flow curve

Power Flow (MW)

Loss

es in

(MW

)

Total LossConverter LossAC loss

61

6.24 Present HVAC Power Grid of Bangladesh

62

6.25Electrical Design of Typical Existing HVAC Transmission Line:

At present, two HVAC transmission line is feeding Khulna district via Ishurdi.One line is 178 KM long from Ghorasal to Ishurdi which is operated at 230 KV. Other from Ishurdi to Khulna which is 185 KM long operated at also 230 KV.Moreover,there is another transmission line from Ishurdi to Khulna which is 169KM long operated at 132KV.Therefore,there is no direct HVAC Transmission line from Ghorasal to Khulna district. At Goalpara,at present 110 MW power plant is operating which supplies the local load.There are few small amount of plants also operating and feeding local loads of Khulna district.So,from the observation,the Author has concluded that one 100 MW extra transmission line is enough to meet up the excess demand of load of Khulna district.This HVAC transmission line would be 350 KM long operated at 230 KV directly from Ghorasal,Narsingdi to Khulna district.This transmission line would be 50Hz and worked at 0.9 power factor lagging.In the following section ,The Author has devised a proposed HVAC Transmission Line and compare the HVAC line with proposed HVDC +/-260KV Transmission line.Instance of HVDC,500MW could be transmitted from Ghorasal to Khulna district.

Choosing a conductor of equivalent copper section .968 cm2.

Then diameter of conductor =1.814 cm

Outer radius ,R =0.907 cm

ACSR conductor 30/.259 aluminium conductor strands ,7/.259 steel strands.

Resistance of the conductor per KM=0.1832 ohm

Current carrying capacity =300 amp

Vr per phase =230/√3=133 KV

For line voltage of 230 KV ,the current at

Receiving end ,Ir=100000/√3X230X.9=278.91∠-25.830

Resistance of the line per phase =.1832X350=64.12 ohm

The spacing of conductor ,Dm=10.2 m

Ds=.768R=(.768X0.907)cm =0.70 cm

Inductance per phase per metre =2X 10-7Xln Dm/Ds

Inductance per phase for 350 KM line =2X10-7X350X103Xln 1020/.7

=.51 H

63

Reactance per phase ,XL=2πfL=314X.51=160.14 ohm

Impedance per phase,Z=64.12+j 160.14

=172.50∠68.180

The outer radius of the conductor,R=0.907 cm

The ratio Dm/R =1020/.907=1124.59

The capacitance per phase per metre =1/18X109Xln 1124.59

The capacitance of 350 KM line per phase ,CN=350X1000/18X109Xln 1124.59

=2.77X10-6F

Y per phase=2πfCN =314X2.77X10-6 mho = .000869∠900 mho

ZY=172.50∠68.180X.000869∠900=0.15∠158.180

Z2Y2=0.022∠-43.640

ZY/2=0.075∠158.180

ZY/6=0.025∠158.180

Z2Y2/120=0.000183∠-43.640

A=D=1+ZY/2=1+0.075∠158.180=0.93∠1.720

B=Z(1+ZY/6+Z 2Y 2/120+. . . . . . .)

(1+ZY/6+Z 2Y 2/120+. . . . . . .)

=1+0.025∠158.180+0.000183∠-43.640=.98∠0.540

B=172.50∠68.180X.98∠0.540=169.05∠68.720

C=Y(1+ZY/6+Z 2Y 2/120+. . . . . . .)

= .000869∠900X.98∠0.540=0.000852∠90.540

Sending End Voltage,VS=AVr+BIr

=0.93∠1.720X133000∠00 +169.05∠68.720 X 278.91∠-25.830

=162180.23∠12.750

=158181.24+j 35792.76

64

Mod VS=162180.228 V = 162.18 KV

Voltage Regulation= (VS-Vr)/Vr X 100%

=(162.18-133)/133 X 100%

=21.94%

Sending End Current,IS=CVr+DIr

=0.000852∠90.540X 133000∠0 + 0.93∠1.720 X 278.91∠-25.830

=235.80∠1.790

=235.68+j 7.37

Mod IS =235.80 amp

Cos φS= Power factor at the sending end = Cos ( 12.75 + 1.79) = Cos 14.540 =.097 lagging .

Sending End Power,PS= 3VSIS Cos φS = 3X 162.18 X 235.80 X 0.97

=111284.348 KW =111.284 MW

Corona Loss:

With the conductor radius 0.907 cm and spacing 10.2 m ,the disruptive voltage ,Ed is given by , Ed = 21.1 X m X r X ln D/r KV

=21.1 X 0.82 X 0.907 X ln 1020/0.907 =110 KV

Line –to-Neutral voltage,E rms =133 KV

E/Ed = 133/110 =1.21

The value of the constant in Peterson’s formula for the ratio 1.21 is F = 0.08 and the corona loss is given by ,Pc =( 21 X 10-6 X f X E2 X F X 3)/ (log D/r)2 KW per KM for a three phase line.

Pc =(21 X 10-6 X 50 X 1332 X .08 X 3)/(3.0512)2 KW

=0.48 KW per KM of the three phase line.

Total corona loss for this line =(.48X 350)=168 KW

Transmission line efficiency = Output /Input X 100%

=100000/(111284.348+ 168) X 100%

=100000/ 111452.348 X 100%

65

=.897 X 100%

=.90X 100%

=90%

The charging current of the line per phase =VY

=133000∠00 X 0.000869∠900

=115.57 ∠900

Charging KVA per phase = 133 X 115.58 = 15372.14 KVA

The surge impedance of the line =√(L/C)

Here ,L = .51 H

C=2.77X 10-6 F

L/C=.51/2.77X 10-6 = 184115.52 =18.41 X 104

Surge Impedance =√(L/C)=4.29X 102 =429 ohm

Surge Impedance loading =(line KV)2/429 =(230)2/429=123.31 MW

When the receiving end load is 100000 KW and power factor is .9 lagging ,

The reactive KVA =48432.2 KVAr

When the power factor is improved to .93 lagging , φ =21.50 ,the reactive KVA would be 100000tan 21.5 = 39391.05 KVAr.

The reactive KVA that must be added at the receiving end by installation of synchronous compensator would be given by the difference of the two reactive KVA at the receiving end ,i.e, =( 48432.2 – 39391.05)KVA = 9041.15 KVAr.

Therefore,the capacity of the synchronous compensator required at the receiving end for improving the power factor is given by 9041.15 KVA.

Insulator:

We use 16 insulators in suspension string ,each insulator being of 25 cm disc diameter .If the capacitance between the insulator and the earth is taken as 0.0625 times the capacitance between the units ,ie, Ce=0.0625 C and then the voltage ,e1, across the insulator nearest to the conductor is given by ,

E1 = E 2 Sinh (1/2 √ 0.0625) cosh ((√ 0.0625 (16 –1/2))/ Sinh (16√ 0.0625)

66

Where E is the maximum conductor to earth voltage .From this,the ratio E1/E and then the string efficiency E/(nE1) can be found out

E1/E = 2 Sinh (1/2 √ 0.0625) cosh ((√ 0.0625 (16 –1/2))/ Sinh (16√ 0.0625)

=(2X .1253 X 21.9 )/27.29 =20%

and string efficiency = E/nE1 =1/(16 X 0.2 ) = 31.5%

Each insulator can stand 40000 V ,the string of 16 insulators can stand 40000X 100/.2=200000 V .Line-to-line voltage =√3 X 200 KV = 346 KV maximum while the line-to-line voltage in this case=√2 X 230 KV = 324 KV maximum.The number of insulators chosen is suitable.

Existing Typical Transmission Line Performance:

1. Sending End Voltage,VS =162180.23∠12.750 V

2. Sending End Line-to-Line Voltage,VS=280904.40∠12.750 V

3. Sending End Current,IS=235.80∠1.790 amp

4. Sending End Power Factor,CosφS=0.97 lagging

5. Receiving End Voltage,Vr=133000∠00

6. Receiving End Current,Ir=278.91∠-25.830 amp

7. Receiving End Line-to-Line Voltage, Vr=230000∠00 V

8. Receiving End Power,Pr =100000 KW

9. Sending End Power,PS=111284.348 KW

10. Voltage Regulation=21.94%

11. Transmission Line Loss=11452.35 KW

12. Transmission Line Efficiency=90%

13. Charging Current =115.57 ∠900 amp

14. Surge Impedance Loading =123.31 MW

15. Synchronous Reactive Compensation=9041.15 KVAr

67

6.26 Proposed HVDC Project Electrical Design:

+/-260 KV ,Bipolar Ground Return,500 MW ,350 KM Transmission line.

Mass Impregnated Cable is chosen as transmission cable.

Diameter of cable:110 to 140 mm

Typical Weight=30 to 60 Kg/m

Copper is the conductor.

Conductor diameter,d=110 mm = .11 m

Conductor Cross Section,A=πd2/4=π(.11)2/4=9.5 X 10-3 m2

Specific Resistivity of Copper,ρ=1.723 X 10-6 ohm-cm = 1.723 X 10-4 ohm-m

Transmission line,ie,Cable length,l=350 KM =350 X 103 m

Total Resistance of Transmission Line,Rt=ρXl/A=1.723 X 10-4X 350 X 103/(9.5 x 10-3)

=6.35 Kohm

Transmission Voltage,V=260 KV DC

Transmission Line Resistance=6.35 K ohm

Transmission line Current,I=260/6.35 =40.94 amp ≈ 41 amp

Total Transmission line loss,W=I2Rt

=(40.94)2 X 6.35 KW

=10643.13 KW Corona Loss=0.313 KW

Voltage Drop =IRt = 40.94 X 6.35 = 259.97 KV

Sending End Voltage=260 KV

Receiving End Voltage=259.97 KV

Voltage Regulation=( 260-259.97)/259.97 X 100% = 0.03/259.97 X 100% = 0.01%

Transmission line efficiency=(500000-(10643.13+.313))/500000 X 100%

=489356.557/500000 X 100%

=.978 X100% =98%

68

As there is no reactance of HVDC circuits,so there is no charging current and charging KVA.No synchronous condenser is required.

Transmission line has no inductance and capacitance.So,there is no surge impedance and surge impedance loading.

Proposed Typical HVDC Transmission line performance:

1.Sending End (Ghorasal Rectifier End) Voltage=+/-260 KV

2.Receiving End (Khulna Inverter End) Voltage =259.97 KV

3.Transmission Line Current=40.94 amp

4.Transmission Line Total Resistance=6.35 K ohm

5.Transmission line total loss=10643.44 KW

6.Voltage Regulation=.01%

7.Transmission Line Efficiency=98%

8.Charging Current=0 amp

9.Charging KVA=0 KVA

10.Surge Impedance=0 ohm

11.Surge Impedance loading=0 MW

12.Synchronous Reactive Compensation=0 KVAr

69

6.27 Typical Performance Curves:

(a)Efficiency Vs Transmission System Curve:

The following Bar Graph represents the calculated efficiency between proposed HVAC and proposed alternative HVDC system of Ghorasal to Khulna Transmission Line:

Efficiency Vs Transmission System Curve

98

90

86

88

90

92

94

96

98

100

HVDC HVAC

Transmission System

Tran

smis

sion

Effi

cien

cy in

%

HVDCHVAC

Figure 6.8 :Efficiency Vs Transmission System Curve

70

(b)Voltage Regulation Vs Transmission System Curve:

The following Bar Graph represents the calculated voltage regulation between proposed HVAC and proposed HVDC system of Ghorasal to Khulna Transmission Line:

Voltage Regulation Vs Transmission System Curve

0.01

21.94

0

5

10

15

20

25

HVDC HVAC

Transmission System

Volta

ge R

egul

atio

n in

%

Series1

Figure 6.9:Voltage Regulation Vs Transmission System Curve.

71

(c ) Transmmission Loss Vs Transmission System Curve:

The following Bar Graph represents the calculated transmission loss between proposed HVAC and proposed HVDC system of Ghorasal to Khulna Transmission Line:

Transmission Loss Curve

10.64

11.45

10.2

10.4

10.6

10.8

11

11.2

11.4

11.6

HVDC HVAC

Transmission System

Loss

in M

ega

Wat

ts

HVDCHVAC

Figure 6.10:Transmission Loss Vs Transmission System Curve.

72

Chapter 7 HVDC Transmission-Opportunities and Challenges 7.1Developments in Energy Policies

The first effects of the unbundling of the electricity supply industry, has typically been a move towards generation providing a quick return on investment. In the UK, and in many other places, this resulted in the so-called “dash for gas” and consequently to the closure of older less efficient thermal power stations. In the unbundled and competitive environment little investment was made in more risky andlonger term generation plant, such as Nuclear or Hydro power stations. Nevertheless, the use of more efficient generating plant was beneficial in the short term for the environment, resulting in a reduction in CO2 gas.The siting of new generation plant was determined largely by the access to the gas, and in spite of the connection charging structure encouraging siting of new generation to suit network loading, ac networks still came under increasing stress.With growing concerns over Global Climate Change, many governments decided to encourage the development of renewable energy sources through subsidies and preferential price levels within the energy market. At this juncture wind generation is the most efficient and cheapest source of renewable power, and the political support has resulted in a dramatic growth in this source of energy.The best wind resource is often located at considerable distance from major load centres. For example, in the UK excellent wind resources can be found in the northern parts of Scotland, where a generation factor of >45% can be found in many land based locations. Transport of remote wind energy puts additional stress on the transmission network and enhancement of the transmission infrastructure is required. Recent events, such as the energy dispute in early January2005 between the Russian Confederation and Ukraine, has highlighted the need for a balanced energy portfolio. Whilst wave and tidal current generation, bio-mass, bio-crops and other renewable energy sources could play a significant role in obtaining the desired diversity, other sources such as Nuclear Power and clean generation from coal with CO2 recovery are also being seriously considered. However, the new generation resources are unlikely to be located at theoptimum point from the perspective of network loads and power flows.Interconnections between national networks are being recognised as being essential for mutual support and to provide economic access to diverse energy sources. Some of the links now being considered involve considerable transmission distance over land or as submarine links.

7.2 Developments in Transmission Networks Cost of potential black outs caused by delay in network In Europe the public resistance to overhead lines has grown steadily during the last couple of decades. The objections are caused by fear of detrimental health effects from magnetic fields.

73

Furthermore, objections are raised on environmental grounds, including visual impact, audible noise, impact on birds and other wildlife. With electricity being considered to be cheap, the public seems prepared to pay the extra cost of mitigation of the environmental issues. Approval for a new transmission line requires a lengthy and time-consuming public enquiry, even when an existing line is to be replaced by a higher voltage line. The full exploitation of wind resources in North Scotland is dependent on the strengthening of the transmission infrastructure, to enable the energy to be exported to load centres in England. However, permission to build new or upgrade existing ac lines, if granted, could be delayed by at 5 and possibly 10 years because of the objections and necessary public enquiries. Because of the difficulty of constructing new lines, the network operators use a number of measures to increase the capacity of the existing lines. Such measures include uprating of the line’s current capability, e.g. by re-conductoring or permitting higher operating temperature. Achieving higher controllability of the power flow in the ac network is also an important part of increasing the overall transmission capacity. This is achieved by the installation of series compensation (controlled or fixed), phase angle control (e.g. quadrature boosters) and shunt reactive power control (fixed, breaker switched or dynamic). Increasing the transmission capability of an ac network without additional lines or increasing the ac voltage will of course increase the power loss in the transmission network. In some cases the addition of an underground HVDC cable solution may prove to be more economic, than an ac overhead line when taking all factors into account. The issues to be considered includes:

• Capital Cost of stations (HVDC converters or HVAC substations, including installation and cost of land)

• Capital Cost of land for line ( overhead line requires much more than that required for a cable route)

• Cost of Consent Process (EIA, Consultation, Legal, etc) • Cost of Delay, including loss of opportunities • Power losses, both for scheme itself and its impact on the existing ac network • Reliability & Availability, including strengthening.

• Maintenance & Operation Cost • Auxiliary service benefits, e.g. impact on overall network

transmission capability In the past cable links have been considered to cost up to 10 times as much as an overhead line. However, when taking into account developments in cable and converter technologythe HVDC cable option may be the most economic option! In Sweden such a comparison is presently being carried out for the “Snitt 4” grid re-inforcement project [4]. The comparison is between a 400km long 400kV ac overhead line and a±300kVdc option.Network owners and operators will be forced to use cables for more and more stretches of transmission, particularly in urban areas, because of growing opposition to overhead lines. For long distances, say >100km, dc cable transmission could be the most economic option.

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7.3Challenges and Opportunities 7.3.1Wind Power and Energy Diversity

One of the challenges of wind power is its intermittency. In order to ensure continuation of power supply during periods of calm, other sources of power generation must be provided to take over power supply during such periods. Today long term energy storage is available only as pumped storage or reservoir storage for hydro power. Hydrogen energy storage needs substantial R&D to reduce cost. Strong interconnections from strategic points in the ac network to geographically remote points in other networks could provide both energy diversity and alleviate the intermittency of wind power. By overlaying the existing networks with “Super Interconnectors”, bottlenecks could be avoided and power losses reduced. Super Interconnectors would enable mutual support between networks, and would thus allow more intermittent sources to be applied overall. In principle, Super Interconnectors could be merchant links and used for energy trading during normal conditions, but would need to provide support to the underlying ac networks, when required. However, public/ governmental ownership would seem more appropriate, showing the commitment to renewable power generation and a means of increasing energy diversity and system security. Super Interconnectors would be long distance and with high rating of individual power blocks, and would be terminated at strong points in the ac network. Very high voltage HVDC links, say 800kVdc or even 1000kVdc, could provide an economic solution to Super Interconnectors. At this juncture the largest power HVDC scheme is 2 x3150MW, ±600kVdc at Itaipu, Brazil. However, the need for projects with higher rating and dc voltage has been identified in China, India, Africa and Brazil, and some manufacturers have started R&D activities linked to such projects. The challenge of the higher voltage has to be met for the HVDC converter and for overhead lines and cables. For very high power land based applications, it might be interesting to develop Gas Insulated dc Lines as an alternative to cables. 7.3.2 AC Network Enhancement The capacity of an ac network could be increased by addition of HVDC overhead lines or dc cables, or by the conversion of ac lines to dc operation. Where the main objection to the construction of overhead lines is the fear of its impact on health, there could be an argument for the use of dc transmission instead of ac transmission. This is because the magnetic field from a bipolar dc line is static and generally transmission capacity of up to 100% of the existing capacity [3]. Such an increase in capacity could be very worthwhile. During the B4 session at the 2004 CIGRE meeting a new method for the conversion of ac lines to HVDC operation was outlined [5]. The idea is to use all three conductors of the ac line, with three converters, one being capable of bi-directional current operation,

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and to cycle the current duty between the three converters as shown in Figure 4. The idea has the advantage of utilisation of the full current capability of the three existing conductors, and does not require a metallic earth return. Further development work would be necessary to make it a realistic option. less than the earth’s magnetic field at any location accessible to the public. However, to categorically prove that a dc line has no negative health impact would be a challenge.

7.4 HVDC System Challenges 7.4.1Cost and Value of HVDC The market for HVDC has traditionally been relatively small, and there are only very few manufacturers capable of providing such systems. With few projects, almost all of which require bespoke engineering, the benefit of mass production are not available, and costs become relatively high. Naturally, cost levels could never become similar to those for an ordinary ac substation of similar rating, because the advantageous technical performance of a HVDC system necessitates the use of many more components than an ac substation. The challenge for proponents of HVDC is to ensure that the value of the technical characteristics of an HVDC system are fully recognised:

• Full and fast control of the power flow • Enhancement of ac networks (power oscillation damping capability, increased

transmission capacity of parallel lines, etc) • No contribution to system short circuit level • AC voltage control (smooth control with VSC Transmission)

Additionally, the HVDC manufacturers must continue their R&D to improve performance and continue to drive down prices, as has been seen during the last decade or so, such that all opportunities can be targeted and the market can grow. Finally, increases in volume would result in a reduction of price levels. 7.4.2 Power Loss The power loss in a HVDC converter station is higher than that in an ac substation, because of the conversion between ac and dc and the harmonics produced by this process. However, the power loss in a HVDC transmission line can be 50 to 70% of that in an equivalent HVAC transmission line. Thus for large distances, an HVDC solution may have lower loss. Nevertheless, it would be desirable to reduce the power loss in the converter stations. Thyristors are highly efficient conversion devices, and the efficiency of each LCC HVDC converter station is typically about 99.3%. The efficiency of the converter stations in a VSC Transmission scheme is today typically around 98.2%, a value which is significantly higher than that for the first generation of the technology. R&D for VSC Transmission has power loss reduction as a high priority. However, whilst a further significant reduction is likely, the power loss is unlikely to become as low as that of a LCC HVDC scheme, because of the use of transistors, rather than thyristors. A significant reduction in the power loss of a HVDC scheme might result from new generations of semi-conductors, e.g. the use of Silicon Carbide, diamond or other materials. Meanwhile, proponents of HVDC must elaborate on the overall power loss comparison. This should take

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into account the loss in converters, dc lines and any power loss reduction in the ac network, e.g. elimination of loop power flows and balancing of power flow in ac lines. 7.4.3 Complexity of HVDC Schemes

An HVDC system is relatively complex, but is in fact easy to operate, since the control system executes the necessary detailed sequences and control commands to achieve high level objectives given by the operator, e.g. power order and reactive power exchange with the ac network.Specially trained personnel are required for maintenance and fault finding of HVDC scheme equipment. These activities occur infrequently, but time has to be allocated to enable the personnel to keep their skills up to date, and this cost needs to be taken into account by the Owner.This situation is not that different from the maintenance and fault finding of the complex SCADA systems in ac substations. In fact, Vendors of the equipment or specialist companies often provide this service. By providing long term service contracts as an extension to the HVDC scheme contract, the costs become known, and the network operator can take these costs into account in his comparisons.The maintenance and fault finding requirements could be reduced by further development of the monitoring system. Self diagnosis of problems is performed by some HVDC solutions, providing the personnel with step by step instruction for its rectification. Such systems remove the need for day to day involvement of specialists. However,specialists are likely to still be required to solve the more rare problems. 7.4.4 Dispatch and Control of HVDC Scheme Network operators wish to dispatch an HVDC scheme as if it were a generator or a large controllable load. One of the great benefits of any type of HVDC scheme is that its active power can be controlled irrespective of the ac voltage phase angle or angle at its terminals.Grid codes typically stipulate that a generator has to be able to operate with a controllable power factor, and that the reactive power capability has to be available throughout most of its operating range. Typically, ac voltage controllability is also required. The ability of a VSC Transmission scheme to control the reactive power at its two terminals independently of each other and independently of the active power transmission is a valuable technical benefit in this respect. A LCC HVDC scheme can change its power factor by the switching of ac harmonic filters and shunt capacitors/reactors. The resulting control of reactive power/ac voltage is in steps, which is generally acceptable to the ac network, particularly if the ac network is relatively strong. Smooth control of the reactive power by a LCC HVDC scheme could be achieved by the addition of a SVC at the ac terminals. In principle, the reactive power could also be controlled by the insertion of a TCSC in series with the converter transformer impedance. The reactive power could also be controlled by the converter firing angle, and the steady state impact at the other terminal could be eliminated through converter transformer tapchanger action. Further developments in this area could improve the performance and acceptability of LCC HVDC. 7.4.5 Integration of HVDC Scheme in AC Network Integration of a HVDC terminal into an ac system requires some specialist engineering. The large ac harmonic filters, particularly for LCC HVDC, can cause significant overvoltages during fault recovery, if the ac network strength is relatively weak. Development in HVDC control has resulted in improved performance during and after faults

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in the ac network, and the performance can be optimised to suit particular network requirements. Nonetheless, the performance is different from that of an ac connection, and network planners have a natural tendency to use the more familiar ac options, even though the system performance could, in some cases, be improved with an HVDC scheme.The dynamic and transient performance of an HVDC scheme can be improved by the incorporation of dynamic reactive power control capability. This capability is already available with VSC Transmission, and could be added to LCC HVDC, either through new circuit topologies and control algorithms, or by the addition of new components, such as shunt or series reactive power compensation. 7.4.6 Harmonics All power electronic converters produce harmonics as a by- product of the conversion process. In order to prevent these harmonics spreading into the ac network, where they could cause problems, ac harmonic filters are used at the ac terminals of the HVDC scheme.As the number of converters connected to an ac network increases, the harmonic pollution in the network increases, as filtering is not perfect. Therefore, the harmonic pollution that a new scheme is permitted to contribute is reduced, making ac harmonic filtering increasingly difficult, and therefore expensive. Since LCC HVDC produces harmonics at relatively low frequencies (primarily 550Hz and above), the problem is worse for this type of HVDC than it is for VSC Transmission (usually >1kHz). Another issue is that the ac harmonic filters and any shunt capacitor banks used for reactive power compensation can actually cause magnification of the distortion caused by other remote harmonic sources.HVDC manufacturers need to consider new converter topologies and the commercialisation of low-cost active ac harmonic filters, which would provide adaptable filtering of harmonics over a broad range.

7.4.7 Operation of HVDC Scheme With Ground Return The cost of an HVDC system can be significantly if it is permissible to operate with a single/HV metallic conductor. Furthermore, the power loss in the transmission line during earth return operation is almost half of that applicable to operation with a LV metallic return conductor. Early HVDC schemes routinely used earth or sea electrodes for the neutral return current, when operating in mono-polar mode.Naturally, care must be taken in the design and location of electrodes, since the direct current flowing between them could result in corrosion of metallic structures. During the last decade environmentalists have increasingly expressed concerns for the wellbeing of organisms and creatures in the vicinity of the electrodes. No detrimental effects have been proven, but planning permission for electrodes has become difficult to obtain. Deep earth electrodes were tested with mixed success on the Baltic cable scheme, and more R&D would be necessary to achieve a satisfactory solution.CIGRE Working Group B4-44 “Planning Guidelines Dealing with HVDC Environmental Issues” is looking at the issues of earth electrodes, as well as a number of other environmental factors, e.g. audible noise, magnetic fields, etc. 7.4.8 Stability of Network With Multi-Infeed of HVDC If HVDC were used for many more applications in a network, then the issue of interaction between multiple HVDC schemes would become increasingly important.

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Commutation failures, which are typically caused by large voltage dips or sudden ac voltage phase angle changes, could be caused by disturbances on another HVDC scheme, and interaction between schemes could potentially cause instability, unless appropriate steps were taken. The problems are not insurmountable as witnessed by several examples where HVDC converters terminate electrically close to each other, and where good performance has been experienced.It should be noticed that VSC Transmission does not suffer from commutation failures, and is therefore not likely to suffer from instability, even if several HVDC terminate in close proximity to each other. 7.5 Conclusion Many technical papers have explained the technical advantages resulting from the use of HVDC transmission. , However, HVDC is not suitable for all transmission applications.Rather than writing yet another paper focusing on all the beneficial features of HVDC, the author decided to use the opportunity of this keynote paper to discuss some of the technical challenges which have to be faced when applying an HVDC scheme. The author strongly believes that the growth in environmental opposition and the need for energy diversity will result in a dramatic growth in the application of HVDC schemes, as a solution to future power transmission challenges. To enable the full potential for HVDC schemes to be exploited, it is necessary to take into account the issues which have been highlighted in this paper. Some aspects requires education of the public, some training of planners and the advisors of investors, and some requires R & D, primarily by the HVDC manufacturers.

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Chapter 8 Conclusion

Today, most of the transmission and distribution of electrical energy is made with alternating current, since it is relatively simple to convert hierarchical voltage levels using simple and reliable transformers. This is a consolidated technology and it has been used for more than century. Only in special applications the transmission of electric power is made using direct current instead of alternating current. Examples of such applications are connections of asynchronous systems (e.g. systems operating at different frequencies) and very long transmission lines or long transmission cables. The important conclusion obtained in this work is that, in general, the transmis- sion and distribution of electrical power will be preferably made with conventional AC technique. The use of HVDC transmission for long overhead electric power transmission will only be justified when severe restrictions exist using conventional AC technique that demand significant additional measures to mitigate. In those cases, the involved cost of these additional measures must be significantly high to justify the use of an alternative technique. Or, the implementation of those measures will make the system too complex to operate. In these cases, HVDC transmission would have advantages over the conventional AC solution, simplifying the operation of the system or resulting in a more economical solution. The followings are the advantages of HVDC:

1.Greater power per conductor. 2.Simpler line construction. 3.Ground return can be used. 4.Hence each conductor can be operated as an independent circuit. 5.No charging current. 6.No Skin effect. 7.Cables can be worked at a higher voltage gradient.

8.Line power factor is always unity: line does not require reactive compensation. 9.Less corona loss and radio interference, especially in foul weather, for a certain

conductor diameter and rms voltage. 10.Synchronous operation is not required. Hence distance is not limited by stability. 11.May interconnect A.C systems of different frequencies.

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12.Low short-circuit current on D.C line. 13.Does not contribute to short-circuit current of a A.C system. 14.Tie-line power is easily controlled.

Evidently,HVDC is profitably prefarable over HVAC transmission for long overhead bulk amount of electric power transmission.At present in Bangladesh,generation of electric energy is less than total load demand.So,loadshedding arises.If we transmit power with HVDC then we can obviously capable to provide more power in the grid which is not possible instance of HVAC.Moreover,HVDC has future development potential. The Authors has exhibited an example of Transmission line in Khulna-Ishurdi-Ghorasal as a model of HVDC quite successfully. Considering all merits and few demerits of HVDC,the Author would conclude that to compensate the existing scarcity of electric power,HVDC should be chosen as bulk amount of overhead electric poer transmission media.

List of References

[1] HVDC Systems and their Planning, Siemens (1999). [2] Economic Assessment of HVDC links, ELT_196_4, ELECTRA. [3] Cheju-Haenam HVDC Manual, AREVA (1996). [4] Barker, C.D. and Sykes, A.M. (1998) Design HVDC Transmission Schemes for Defined Availability. Proceedings of Generation and Transmission, IEE, 1(1), 4/1–4/11.

[5] Hammad, A.E. and Long, W.F. (1990) Performance and economic comparisons between point-to-point HVDC transmission and hybrid back-to-back HVDC/AC transmission. IEEE Transactions on Power Delivery, 5(2),1137–1144.

[6] Hammons, T.J., Olsen, A. and Gudnundsson, T. (1989) Feasibility of Iceland/United

Kingdom HVDC submarine cable link. IEEE Transactions on Energy Conversion, 4(3), 414–424.

[7] Diemond, C.C., Bowles, J.P., Burtnyk, V. et al. (1990) AC–DC economics and alternatives – 1987 panel session report. IEEE Transactions on Volume Power Delivery, 5(4), 1956–1979.

[8] Andersen, B. and Barker, C. (2000) A new era in HVDC? IEE Review, 46(2), 33–39.

[9] Bakken, B.H. and Faanes, H.H. (1997) Technical and economic aspects of using a long submarine HVDC connection for frequency control. IEEE Transactions on Power Systems, 12(3), 1252–1258.

[10] Povh, D. (2000) Use of HVDC and FACTS. Proceedings of the IEEE, 88(2), 235–245.

[11] Kuruganty, S. (1995) Comparison of reliability performance of group connected and conventional HVDC transmission systems. IEEE Transactions on Power Delivery, 10(4), 1889–1895.

[12] Billinton, R., Fotuhi-Firuzabad, M. and Faried, S.O. (2002) Reliability evaluation of multiterminal HVDC subtransmission systems. Generation, Transmission and Distribution, IEE Proceedings, 149(5), 571–577.

[13] Hingorani, N.G. (1996) High-voltage DC transmission: a power electronics workhorse. Spectrum, IEEE, 33(4),63–72.

[14] Dialynas, E.N., Koskolos, N.C. and Agoris, D. (1996) Reliability assessment of autonomous power systems incorporating HVDC interconnection links. IEEE Transactions on Power Delivery, 11(1), 519–525.

[15] Kuruganty, S. (1994) Effect of HVDC component enhancement on the overall system reliability performance.IEEE Transactions on Power Delivery, 9(1), 343–351. [16] Dialynas, E.N. and Koskolos, N.C. (1994) Reliability modeling and evaluation of HVDC power transmission systems. IEEE Transactions on Power Delivery, 9(2), 872–878. [17] Melvold, D.J. (1992) HVDC converter terminal maintenance/spare parts philosophy and comparison with performance. IEEE Transactions on Power Delivery, 7(2), 869–875. [18] Baker, A.C., Zaffanella, L.E., Anzivino, L.D. et al. (1989)

A comparison of HVAC and HVDC contamination performance of station

post insulators. IEEE Transactions on Power Delivery, 4(2), 1486–1491.

[19] Kuruganty, P.R.S. and Woodford, D.A. (1988) A reliability cost-benefit analysis for HVDC transmission expansion planning. IEEE Transactions on Power Delivery, 3(3), 1241–1248.

[20] Hingorani, N.G. (1988) Power electronics in electric utilities: role of

power electronics in future power systems.Proceedings of the IEEE, 76(4),

481–482.

[21] El-Amin, I.M., Yacamini, R. and Brameller, A. (1979) AC–HVDC solution and security assessment using a diakoptical method. International Journal of Electrical Power and Energy Systems, 1(3), 175–179. [22] Kalra, P.K. (1987) Feasibility study for development of expert systems for power system control. Electric Power Systems Research, 12(2), 125–130.

[23] Sood, V.K. (2007) HVDC Transmission, Power Electronics Handbook, 2nd Edn, pp. 769–795. [24] Cochrane, J.J., Emerson, M.P., Donahue, J.A. et al. (1996) A survey of HVDC operating and maintenance practices and their impact on reliability and performance. IEEE Transactions on Power Delivery, 11(1), 514–518. [25] Kunder, P. (1996) Power System Stability and Control, McGraw-Hill, New York. [26] High-Voltage Direct Current Handbook (1994) EPRI TR-104166S.