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IN

DEGREE PROJECT ELECTRICAL ENGINEERING, SECOND CYCLE, 30 CREDITS

STOCKHOLM SWEDEN 2016,

The Impacts of UHV AC

Transmission Lines on Traditional Line Differential Protection

Functions

MD ZAKARIA HABIB

KTH ROYAL INSTITUTE OF TECHNOLOGY SCHOOL OF ELECTRICAL ENGINEERING

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KTH ROYAL INSTITUTE OF TECHNOLOGY

The Impacts of UHV AC

Transmission Lines on Traditional Line Differential Protection

Functions

Master’s Thesis for the Electric Power Engineering Program

MD ZAKARIA HABIB

August, 2016

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i

Sammanfattning

UHV-transmissionsledningar har ett antal fördelar när det gäller överföringskapacitet av effekt över långa avstånd. Även om tekniken har varit tillgänglig sedan 1980-talet är det inte förrän under det senaste årtiondet som utbyggnaden har tagit fart ordentligt. Detta har skett för att möta den höga efterfrågan på elektricitet. De närmaste åren väntas utbyggnaden öka ytterligare. Mot denna bakgrund är det relevant att studera karaktäristiken av UHV samt att uppdatera kraftsystemutrustning såväl som driften.

UHV-tramsmissionslinor är förenade med vissa fenomen som inte förekommer för transmissionsledningar med lägre spänning. Vissa av dessa fenomen är har stor inverkan på skyddsutrustning för transmissionslinorna. Syftet med denna uppsats är att studera inverkan från UHV-transmissionsledningar på differentialskydd samt att föreslå lösningar för att överkomma inverkan.

Differentialskydd är populärt tack vare goda selektiva egenskaper och enkelhet så länge det finns ett pålitligt kommunikationssystem. Hög kapacitans och stor fasskiftning mellan strömmen på avsändar- och mottagarsidan är två viktiga egenskaper hos UHV-transmissionsledningar vilka har stor inverkan på differentialskyddet. Det är väldigt viktigt att skyddsutrustningen kan upprätthålla god sensitivitet samt säkerhet. Av denna anledning är kompensation för den höga kapacitansen viktig. Konstant kompensation används för att kompensera för hör kapacitans hos långa transmissionslinor med lägre spänning. Denna metod är däremot inte fungerande för UHV.

Det är därför nödvändigt att söka efter en annan lösning.

I denna uppsats föreslås en lösning på behovet att kompensera för hög kapacitans genom att använda adaptiv fasskiftkompensation. Flertalet simuleringar har genomförts för att utvärdera karaktäristiken av den utvecklade metoden. Det konstateras att metoden väldigt god känslighet och säkerhet för differentialskydd av UHV-transmissionsledningar.

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Abstract

UHV transmission lines have several advantages when it comes to transferring of bulk amount of power in long distances. Although the technology is available from 1980’s, the number of UHV transmission lines around the world has been increased in the beginning of this century in order to supply the high demand of electricity. The number is going to increase even more in the next few years. Thus, it is important to study the UHV line characteristics and update the power system equipment as well as the operation procedure accordingly.

UHV transmission lines exhibits some distinct phenomena which are not present in the transmission lines with lower voltage levels e.g. high amount of charging current, non-linear increase of apparent fault impedance with the increase in fault distance, longer time constant for the DC component in fault current etc. Some of these are very important for different protection schemes of the transmission line. The aim of this thesis work is to study the impact of UHV line characteristics on line differential protection and propose solutions to overcome them.

Line differential protection is popular for its good selectivity and simplicity as long as there is a dependable reliable communication system between the two ends of the line. High amount of capacitive charging current and large phase shift between sending and receiving end currents are two important characteristics of UHV lines which have severe impact on the line differential protection. It becomes very critical for the protection scheme to maintain good sensitivity and security at the same time. As a result, compensation of the charging current becomes essential.

The fixed compensation method is used to compensate the charging current of long lines with lower voltage levels. However, it cannot satisfy the sensitivity requirements for line differential protection scheme on UHV lines. Hence, it is necessary to search for other compensation methods.

In this thesis, a solution related with charging current compensation method is proposed with the use of adaptive phase shift compensation. Several simulations have been done to examine the characteristics of the developed method in the worst case scenarios. It is found that the method exhibits very good sensitivity as well as security for line differential protection on UHV lines.

Keywords: UHV lines phenomenon, Line differential protection, Capacitive charging current compensation.

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Acknowledgement

This thesis work is a part of master program in Electric Power Engineering at KTH Royal Institute of Technology. The work was carried out at ABB Corporate Research Center, Västerås, Sweden under the Power Systems Development Team of Electrical Systems department. I am grateful to get an opportunity to work in such a nice environment.

First, I would like to express my deepest gratitude to the Swedish Institute for selecting me for the ‘SI Study Scholarship’. Otherwise, it would have been impossible for me to stay in Sweden and pursue the master program at KTH.

My heartfelt thanks go to my supervisor Dr. Jianping Wang for his valuable guidance throughout the thesis work and introducing me to the research world. I learned a lot about the practical world of power system protection from him. I am also thankful to Dr. YouYi Li for sharing his knowledge and his continuous encouragement throughout the entire work.

I would like to thank Dr. Nathaniel Taylor, my supervisor at KTH; for his kind review on the report and valuable comments on the work. I am also grateful to Assoc. Prof. Dr. Hans Edin for accepting to become my examiner at KTH for my thesis work.

Last but not the least; I am grateful to my parents, family members and all of my friends who encouraged me a lot during the whole master’s program.

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Contents

1 Introduction ... 1

1.1 Problem Definition ... 2

1.2 Objectives of the Thesis ... 2

1.3 Method ... 2

1.4 Outline of the Thesis ... 3

2 History and Development of UHV Transmission Technology ... 5

2.1 Development of Transmission Grid and Voltage Upgrade ... 5

2.2 Development of UHV Transmission System Technology ... 6

2.3 Advantages of UHV AC Lines ... 8

2.4 Existing and upcoming UHV Transmission Lines Worldwide ... 8

3 Modeling of UHV Transmission Line... 11

3.1 Need for a line model ... 11

3.2 Choice of the model ... 11

3.2.1 Frequency dependent line model ... 11

3.3 Line model in PSCAD ... 13

3.3.1 Type of conductor ... 14

3.3.2 Surge Impedance loading ... 16

3.3.3 Overhead transmission line parameter ... 17

4 Typical Features of UHV Lines ... 19

4.1 Description of the test system ... 19

4.2 UHV line phenomena... 19

4.2.1 Line impedance ... 19

4.2.2 Line charging current ... 21

4.2.3 DC component in the current during fault ... 23

4.2.4 Phase shift in the current ... 25

4.2.5 Over voltage during energization of the line ... 26

5 Challenges and Solutions for Line Differential Protection on UHV lines ... 29

5.1 Principle of Current Differential Protection ... 29

5.2 Restraint characteristics ... 30

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5.3 Methods to set the operating criteria for the relay ... 31

5.3.1 Phasor based differential protection ... 31

5.3.2 Fault component differential protection ... 31

5.4 Effect of charging current on the protection setting ... 32

5.5 Study on line differential protection scheme ... 35

5.5.1 Types of faults ... 35

5.5.2 Internal/External faults with different types ... 35

5.6 Challenges for the differential protection ... 38

5.6.1 Check on the restraint characteristics during external faults ... 42

5.7 Proposed solutions to the challenges ... 43

5.7.1 Phase shift compensation method ... 44

5.7.2 Inclusion of shunt reactors and comparison between the methods ... 49

6 Closure ... 51

6.1 Discussion ... 51

6.2 Conclusions ... 51

7 Future work ... 53

8 Appendix 1 ... 55

9 References ... 57

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vi

List of figures

Figure 3-1: Distributed line parameter model. ... 12

Figure 3-2: Line geometry. ... 15

Figure 3-3: Sending and receiving end voltage relationship under SIL. ... 16

Figure 4-1: UHV transmission line system for the analysis. ... 19

Figure 4-2: Impedance during ABC-G bolted fault for different fault locations on the line. ... 21

Figure 4-3: Charging Current along with sending and receiving end current when there is no load connected to the line. ... 23

Figure 4-4: Fundamental and DC components of the sending end current after fault inception on 765 kV and 230 kV line. ... 24

Figure 4-5: Sending and receiving end currents for surge impedance loading condition. ... 25

Figure 4-6: Sending and receiving end voltages during line energization with a load angle of 30o; (a) without shunt reactors; (b) shunt reactors are connected at the both ends. ... 26

Figure 5-1: Sending and receiving end currents during internal and external faults. ... 29

Figure 5-2: Operate and restrain characteristics of current differential relays. ... 30

Figure 5-3: Line energization with a load angle of 30o and only one side connected; (a) sending and receiving end currents; (b) characteristics of differential and bias currents according to phasor differential method; (c) characteristics of differential and bias currents according to fault component method. ... 33

Figure 5-4: Line energization with a load angle of 30o and both side connected simultaneously; (a) sending and receiving end currents; (b) characteristics of differential and bias currents according to phasor differential method; (c) characteristics of differential and bias currents according to fault component method. ... 34

Figure 5-5: Phasor differential currents for different types of internal faults. ... 36

Figure 5-6: Absolute values of the phase currents for different types of external faults; (a) phase-A currents of the receiving end for external faults at the receiving end side; (b) phase-A currents of the sending end for external faults at the sending end side. ... 37

Figure 5-7: Differential current during different types of external faults. ... 37

Figure 5-8: Operate and restrain characteristics at the maximum detectable fault resistances; (a) fault component method (200 Ω); (b) phasor based differential method (160 Ω). ... 39

Figure 5-9: Fault detection time required by phasor differential method and fault component method for different fault resistances. ... 40

Figure 5-10: Restraint curves for prolonged internal faults; (a) phasor differential method; (b) fault component method. ... 41

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vii Figure 5-11: Restraint curve for fixed compensation method when 225 Ω single phase to ground fault is applied. ... 42 Figure 5-12 : Restraint curves for external faults; (a) Phasor differential method; (b) Fault component method. ... 43 Figure 5-13: Situation of sending and receiving end currents-before and after the inception of external bolted faults; (a) actual currents; (b) currents after phase shift compensation. ... 44 Figure 5-14: Behavior of differential current for the developed method after inception of external three phase to ground bolted faults; (a) sending and receiving end currents; (b) restraint curve after fault inception. ... 45 Figure 5-15: Situation during external line to line bolted faults; (a) differential currents before and after phase shift compensation; (b) restraint curve during the same time. ... 46 Figure 5-16: Situation during external three-phase to ground bolted faults; (a) differential currents before and after phase shift compensation; (b) restraint curve during the time. ... 47 Figure 5-17: Restrain curves for phase shift compensation method when single phase to ground 340 faults are applied at (a) 10% , (b) 50% and (c) 90% distance of the line length. ... 48 Figure 8-1: Bundle arrangement for a six-conductor bundle. ... 55

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viii

List of Tables

Table 2-1: Some important events on the development of transmission line voltage. ... 6

Table 2-2: Information of some existing UHV lines around the world [19]–[28]. ... 9

Table 3-1: Conductor parameters used for UHV line model ... 15

Table 3-2: Typical overhead transmission line parameter [36] ... 17

Table 3-3: Line parameters for the simulated Line model in PSCAD ... 17

Table 4-1: Line charging currents for transmission lines of different voltage level [39]. ... 22

Table 5-1: Probability of occurrence for different types of faults [46]. ... 35

Table 5-2 : Maximum detectable fault resistances for different line differential protection schemes ... 39

Table 5-3: Maximum time required to detect 150 Ω internal faults for different methods. ... 50

Table 5-4: Maximum detectable fault resistances and fault detection time for different methods ... 50

Notations

UHV Ultra-High Voltage

EHV Extra-High Voltage

HV High Voltage

GMD Geometric Mean Distance

GMR Geometric Mean Radius

SIL Surge Impedance Loading

A-G Fault Line to Ground Fault

AB-G Fault Line to Line to Ground Fault ABC-G Fault Three-phase to Ground Fault AB Fault Line to Line Fault

PSCAD Power System Computer Aided Design

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1

Chapter 1

1 I

NTRODUCTION

One of the distinct features of a modern society is the high use of electricity. Electric power systems are responsible to ensure the reliable and secured supply of electricity. No matter how well designed and maintained the system is, there is always risk of faults and failures. These faults can lead to severe damage or even system failure. Thus, a protection system is required with a focus on the corrective steps after fault occurrence. This protection system plays a significant role to detect the faults in the system and isolate the faulted parts.

An electric power system is typically differentiated into three parts: generation, transmission and distribution. Most of the time the generation part is located in remote places and the load density is high in the industrial and residential areas. Transmission lines or the transmission grid transfer the generated electricity from the generation side to the load side. In case of any fault occurrence it is very important to isolate that section of the grid to ensure fast power restoration and continuous flow of electricity to the end user. Thus, study of transmission line protection has been always an essential part from the early edge of electricity.

The power system has been evolving to meet the increasing demand of electricity. It has evolved from isolated generators feeding their own loads to a huge interconnected system covering the whole country. The voltage level and the power handling capability have been increased due to the increasing demands. Side by side, the protection requirements have been changed based on the evolvement of the transmission system. As a result, the protection system advanced through an era of electromechanical relays and/over static relays to the era of numerical protection with the advancement of available technology and system requirements [1].

The necessity of upgrading to the Ultra High Voltage (UHV) level comes from the requirement of increasing the capability of transmitting more power but reducing the transmission losses at the same time. It also helps to connect different energy sources from long distance and thus improve the system security. Recently, engineers have become more focused on environment friendly solutions. Increase in renewable energy sources is a demand of time for a sustainable world. But for the most of the renewable energy sources are situated at remote places and the geographical location depends on the availability of the natural resource. This requires the interconnection between sources and loads. UHV lines can play an important role to transfer the renewable energy from one side to another side within a long distance. The motivation behind establishing UHV lines can lie behind some other beneficial aspects subjected to the country’s perspective. However, voltage upgrading requires the upgrading of the ancillary parts such as the substation technology[2],[3], insulation technology[4], the transmission tower configuration etc..

A study on the impact of UHV transmission line on the traditional protection schemes is equally important to ensure a reliable and secured power system.

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2

1.1 Problem Definition

Ultra-high voltage lines have a low resistance value per unit length which results in low energy loss i.e. beneficiary from economic perspective. In order to meet the low inductance and less corona loss, multiple conductors are bundled in UHV transmission lines [5]. This leads to a higher distributed capacitance value. The impact of this distributed capacitance is not significant if the line length is not much long or the voltage level is not very high. However, for UHV transmission lines with a long line length the effect of this distributed capacitance is significant for the traditional line protection scheme. The long lines result in smaller equivalent capacitive reactance which leads to larger capacitive charging current. However, the traditional protection algorithms and analysis are still based on the classical lumped parameters, which may lead to big error or even problems for the protections of UHV transmission lines. As a result, a study should be done on how significant the impact is and how to overcome them in order to maintaining the reliability, sensitivity and security of the system.

1.2 Objectives of the Thesis

The thesis is focused on the impacts of UHV AC transmission lines on traditional protective relays. The objective of the thesis can be described as follows,

• Market survey on the existing UHV AC transmission lines and its applications

• Modeling of UHV AC transmission line in PSCAD and validate with practical data.

• Transient fault analysis for UHV AC transmission line and find the special fault phenomena and their impact on the line differential protection solutions.

• Propose solutions to overcome the challenges found in the transient fault analysis.

1.3 Method

A 765 kV UHV AC transmission line is modeled in PSCAD. A frequency dependent model is selected from PSCAD library to get the most precise results during transients. The required input parameters for the tower configuration and conductor specification are given according to the guidance of U.S. Department of Energy [6] and other available open source information. Then the line parameters i.e. the sequence components of the modeled line are cross verified with the line parameters of an existing line to make sure the validity of the modeled line. Different types of faults along the line are introduced in order to study the line behavior during the transients.

Later the study is done to find out the impacts on specific protection scheme and how to overcome the challenges for that protection scheme.

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1.4 Outline of the Thesis

Chapter 1 of the thesis describes the motivation, objective and method of the thesis work.

Chapter 2 describes the development of the transmission grid, existing situation of UHV AC transmission lines.

Chapter 3 focuses on the modeling of UHV AC transmission line and their operation.

Chapter 4 presents the UHV AC transmission line phenomena those are important for the protection system.

Chapter 5 contains the impacts of the UHV AC line characteristics on existing line differential protection and solutions to overcome them.

Chapter 6 provides discussions on the thesis, the conclusions and suggestions for the future works.

This thesis is done in parallel with another master thesis [7] which deals with the challenges and solutions regarding the distance protection for UHV lines. The topics for the first three chapters are the common for both of the thesis works and the study is done together. Thus these chapters are common in both of the thesis reports. However, the rest of the works in each thesis are done individually focusing on either the differential protection or the distance protection scheme. This thesis focuses on the differential protection for the UHV AC transmission lines.

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5

Chapter 2

2 H

ISTORY AND

D

EVELOPMENT OF

UHV T

RANSMISSION

T

ECHNOLOGY

2.1 Development of Transmission Grid and Voltage Upgrade

Electricity is one of the major contributors to the development of human civilization. It has become the part and parcel of modern life. A very complex system consisting of generation, transmission and distribution is being operated continuously to supply the electricity to the consumers.

The first commercial AC transmission grid was built by Westinghouse and started operation in 1896 [8]. It was a 40 km long, three phase AC transmission line from Niagara Falls generating station to Buffalo. Although DC transmission system was prominent at that time, it was dominated by AC technology to its lower transmission loss. The successful implementation of transformers in AC transmission technology made it possible to increase the transmission voltage level which allowed transferring the same amount of power but with lower current i.e. low loss.

Since then, the development of the transmission grid has been done depending on the generation capacity, available technology as well as economic feasibility. Until Second World War, single unit generation capacity was not more than 200 MW and the dominant transmission voltage was maximum up to 220 kV [9]. Typically the grid size was small and isolated, thus long distance transmission line was not required. After the Second World War, there was rapid increase in demand of electricity due to the industrialization. The generating capacity also increased and the transmission line voltage went up in order to transmit large amount of power in long distance [10].

Another important phenomenon is the integration of the grid. The grid interconnection took place to balance loads and improve load factors between interconnected central stations.

Interconnection became increasingly desirable in order to ensure the availability and reliability that cross border interconnection started to take place in later of the twentieth century. Thus transmission lines interconnecting two countries or two regions required to transmit huge amount of power in very long distances with maximum efficiency. As a result, transmission voltages also climbed up in steps from high voltage to ultra-high voltage. Some significant landmarks in the history of AC transmission grid are depicted in Table 2-1 [9].

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Table 2-1: Some important events on the development of transmission line voltage.

Year Event

1895 The first commercial AC transmission line from Niagara Falls to Buffalo was built in USA

1923 USA built the first 230 kV transmission line in the world

1952 The first Extra-High Voltage line was built in Sweden. It was 620 km long and the voltage level was 380 kV.

1956 1000 km long, 400 kV transmission line was put in to operation in Soviet Union 1965 The first 735 kV transmission line was completed in Canada

1969 USA built a transmission line of 765 kV

1985 USSR built the first 1150 kV transmission line.

2.2 Development of UHV Transmission System Technology

A rapid expansion of electricity dependent development caused increasing demand in the load centers. Both generation and transmission capacity had to increase with the same pace as the generation and load sites are situated in a far distant due to various practical reasons. To cover the power mismatch between countries with large geographical area it is needed to push their transmission capacity up.

Russia, Japan, USA, Canada, Italy are the pioneer countries in the study of Ultra-High Voltage transmission line technology [10]. The recent successful operation of 1000 kV AC transmission line in China has proved the feasibility of this technology and boost up the interest in it.

However, the technology has been developing since 1960’s by the contribution of different researches around the world.

World’s first commercial UHV transmission line was built by former USSR [11]. The need of interconnection and transferring large quantity of power over long distance was identified in 1970’s. As a result, a three phase test line of 1.17 km and 1500 kV voltage was constructed for the in-depth study. Several tests were carried out to obtain data about insulation of the equipment, switching over-voltage, audible noise, radio interference, electric fields in a substation, corona performance of the conductor bundle etc. [12]. The constructed UHV line was in operation at the rated voltage for few years until the demand reduced due to the dissolution of USSR. Since then the lines have been operated at reduced voltage.

In 1967 the Quebec Government in Canada funded to establish the HV laboratory of Hydro- Quebec Institute of Research (IREQ) for the studies related to high voltages [13]. It was anticipated that transmission lines over 1000 kV would be required to transfer bulk amount of electricity from the remote hydropower stations to the load centers. The research laboratory was

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7 facilitated to study up 1500 kV and several studies were performed about corona, electric field and ion current performance, phase to phase and phase to ground air insulations for 1500 kV AC line. IREQ also build a test line in Magdalen islands to study the vibration performance. These studies provided necessary data for developing the spacer dampers and determining the air-gap clearance for 1500 kV transmission systems [11].

USA also started to study UHV transmission technology in 1967. The American Electric Power Company (AEP), General Electric Company (GE), Electric Power Research Institute (EPRI) and Bonneville Power Administration (BPA) began the study and three separate research and test facilities were built by these companies. GE and EPRI conducted the project jointly where the others built test facilities individually. Enormous data regarding the corona performance of several different conductor bundles with different sub-conductor diameters was generated at these research facilities [10],[12].

In 1973, Japan began study on UHV transmission line with the intention to overcome the problem of excessive short-circuit current and to improve the stability of the existing network [14]. A double-circuit test line was built by Tokyo Electric Power Company (TEPCO) and research facility including a large fog chamber was developed by Central Research Institute of Electric Power Industry (CRIEPI). In addition to the study on corona, insulation, effect of wind and earthquake on the conductor bundle; audible noise and television interference, the effect of pollution, snow on polluted insulators at line to ground voltage up to 900 kV, were tested.

Valuable information about the withstand voltage of contaminated and snow covered insulator strings was obtained due to the carried investigations [12].

Italy also began the study of UHV transmission line to increase the transmission capacity from the larger power plant situated in remote location during 1970’s. Two test lines and an outdoor cage were built for UHV studies at Suvereto 1000 kV project and Pradarena Pass. Researches were also carried out at Centro Elettrotecnico Sperimentale Italiano (known as CESI laboratories) in Milan. Italy generated significant amount of data regarding determination of phase to ground and phase to phase air clearance, selection of ceramic and non-ceramic insulator strings, selection of conductor bundles for 1050 kV line, development of vibration dampers, spacers, non-conventional tower structure and their foundations [10].

The efforts made by various countries on key UHV transmission technologies and equipment manufactures over the years laid the foundation for the subsequent development and application of the technology.

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2.3 Advantages of UHV AC Lines

As mentioned in the previous sections, the most important advantage of UHV AC lines is the higher transmission capacity and lower losses. Besides this, UHV AC lines have several advantages over the Extra High Voltage (EHV) or High Voltage (HV) lines. Few of them are mentioned as follows

• A single circuit 765 kV transmission line can transfer the equivalent amount of energy by using three single-circuit 500 kV lines, three double-circuit 345 kV lines or six single- circuit 345 kV line. Thus, it requires less amount of land acquisition to deliver the same amount of energy [15].

• The construction cost is also less in case of transferring the similar amount of energy through transmission lines with the reduced voltage levels. According to a study done by Electric Transmission America (ETA), construction of 765 kV line requires about only 38% of the cost of equivalent 500 kV line or 29% of the cost of equivalent 345 kV line [16].

• UHV lines have less thermal overloading risks due its small resistive value. It reduces the risks during the schedules and unscheduled parallel transmission lines of lower voltage levels.

• The overall losses of the system can be reduced in a significant amount after shifting bulk power transfers from the underlying lower voltage transmission network. Moreover, the increase in the efficiency will reduce the transmission loss which might reduce burning of fossil fuels i.e. reduction of carbon emission in the annual basis.

• Addition of UHV lines to the grid enables the easy integration with the overlying existing lower voltage system. The combination results in a strong network which enables comfortable integration situation for the renewable sources.

2.4 Existing and upcoming UHV Transmission Lines Worldwide

There are difference views in consideration of UHV transmission lines. Some literatures consider voltage of and over 800 kV as UHV and some consider 750 kV. In case of protection, if the UHV line is very long and the voltage level is higher than 750 kV, the line behavior will change obviously due to the increase in capacitive reactance. With this motivation this thesis considers transmission lines over 750 kV as UHV transmission line. Table 2-2 shows the information regarding some of the existing UHV lines sorted according to the line voltage levels. It can be observed that out of 11,000 km of existing UHV lines, about 8,600 km are already in operation at their rated voltages. The rest are operating at lower voltage levels due to insufficient load requirement or some other issues. However, China is going to include more UHV AC lines in their grid which will act as the backbone of their grid in the future [17]. According to the ten year development plan of ESKOM in South Africa, there will be approximately 3700 km of 765 kV line going to be installed in the country by 2022 [18]. India is at the verge of installing a 1200

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9 kV line. The existing 400 km long 400 kV line between Wardha and Aurangabad will be upgraded to 1200 kV in the next few years [19]. The increasing number of UHV lines around the world enhances the necessity of in-depth study on UHV transmission line protection system.

Table 2-2: Information of some existing UHV lines around the world [20]–[29].

UHV Line Country Voltage Level

(kV)

Operational Voltage

(kV)

Distance (km)

Year of Establishment

Kokchetav-kustanai USSR 1150 500 410 1988

Jingdongan-Nanyang-Jingmen China 1100 1000 640 2009

Huain-Zhebei-Shanghai China 1100 1000 780 2014

Minami-Nigata/Nishi-Gunma Japan 1100 550 200 1993

Kita-Tochigi/Minami-lwaki Japan 1100 550 250 1999

Dangjin Line Korea 765 765 178 2002

Sin Taebaek Line Korea 765 765 162 2002

American Electric Power USA 765 765 3400 1969

New York Power Authority USA 765 345 249 1978

Foz do lguacu-Sao Paulo Brazil 765 765 900 1986

Caracas-Maracay-Valencia Venezuela 765 765 1250 1986

Valencia-Yaracuy Venezuela 765 765 850 1991

ESKOM South Africa 765 400 1600 2013

Rachipur-Solapur India 765 765 208 2014

Jabalpur Bhopal Line India 765 765 285 2015

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11

Chapter 3

3 M

ODELING OF

UHV T

RANSMISSION

L

INE

3.1 Need for a line model

The accurate measurement of the fault currents, voltages and the impedance is of primary importance for the proper operation of the power system protection to detect faults along the lines and to locate the zones.

In general, the transmission line can be either represented as distributed parameter model or lumped line model depending on the length of a line. A lumped line model with the impedances of the entire line is assumed by multiplying the series impedance per unit length for most of short lines. Also the fault current and impedance values calculated are based on this simple model.

3.2 Choice of the model

In case of a long Ultra High Voltage (UHV) transmission line, the lumped parameter model is less useful, due to the effect of shunt capacitance distributed over the entire line together with corona effect around the conductor. In an UHV line, the transmitted power is increased with the reduction in the characteristic impedance of the line with the increase in the distributed capacitance effect. Thus the losses in the line along with corona losses and the electric field strength constraints are accounted [30]. In order to study the transient behavior of UHV transmission lines, an accurate model of the power system transmission network is necessary. In the lumped parameter modeling, the transmission line is represented by resistance, inductance and a parallel capacitance. The modeling is developed in the time domain.

A PI model can also be used to represent the long transmission line. Though the impedance of the line is represented, it is only effective for the fundamental component while the frequencies other than fundamental are not represented accurately. On the other hand, PI model cannot reflect the transient signal transmission along the line because it is not frequency dependent with distributed parameters and it could not create travelling wave reflections together with its time delays. On the other hand, the frequency dependent model is developed by distributed parameters of the line in frequency domain. All the simulations with frequency dependent model is carried out in the frequency domain. It is analyzed in [31] & [32] that the error caused in the estimation of the fault distance calculation with the lumped parameter line model accounts to 17%.

3.2.1 Frequency dependent line model

The frequency dependent model of transmission line used for modeling the UHV lines in PSCAD is obtained based on the theory proposed in [33].

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12 UHV power transmission is associated with long distance transmission and the representation of the long-line model with distributed parameters is presented in Figure 3-1. Considering the long- line model with transposition, the voltage and current relationships can be obtained equation (3.1) and (3.2).

Figure 3-1: Distributed line parameter model.

. .

2 2

R R

R c x R c x

V Z I V Z I

V = + eγ + − eγ (3.1)

. .

2 2

R R

R c x R c x

V Z I V Z I

I = + eγ − − eγ (3.2)

Where,

Zc = z y is the characteristic impedance of the line

zy j

γ = = +α β is the propagation constant.

α is the attenuation constant and βis the phase constant.

In this model the current and voltage signals at any point along the line are equal to the summation of the incident and the reflected wave.

Equation (3.1) and (3.2) can be rewritten as

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13

. .

2 2

. .

2 2

x x x x

R

R c

x x x x

R c R

e e e e

V V Z I

e e e e

I V Z I

γ γ γ γ

γ γ γ γ

+ −

= +

− +

= +

(3.3)

By the hyperbolic trigonometric functions, equation (3.3) can be expressed as (3.5)

cosh( ) 2

sinh( ) 2

x x

x x

e e

x

e e

x

γ γ

γ γ

γ γ

+ =

− =

(3.4)

( ) ( )

( ) ( )

cosh sinh

cosh sinh

R c R

R R c

V V x Z I x

I I x V Z x

γ γ

γ γ

= +

= + (3.5)

For the case of a lossless line,

Zc L C j LC

γ ω

=

= (3.6)

From (3.6), the expression (3.5) can be written as (3.7) & (3.8) .cos( ) R.sin( )

R c

V =V βx + jZ I βx (3.7)

( )

.cos( ) / .sin( )

R R c

I =I βx + j V Z βx (3.8)

The frequency dependency is considered with distributed R, L and C parameters along the line modeled as travelling waves. Also the parameters modeled represent the frequency dependence.

In the studies for the line characteristics for the UHV lines, the transient behavior is considered.

In this consideration, the frequency dependent model is the most suited model, as it gives an accurate representation of all frequencies. They can be simulated either by modal techniques (Mode model) or phase domain techniques (phase model).

3.3 Line model in PSCAD

In order to simulate the real time application of the UHV transmission system, various data were analyzed from the practical cases of UHV transmission around the world. The parameters thus obtained are used in the modeling of the UHV line in PSCAD.

In this case, the 765 kV simulation system is chosen for the detailed study and simulation purposes. The line is represented as a frequency dependent model in order to obtain the

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14 information of all the frequency components during the transient state (faults). In line with the transmission line frequency dependent model, the parameters for tower configuration, conductor data, and ground wire data are to be provided in PSCAD. All the data obtained from different sources were compared and validated with the typical data and practical data [34]. In this section both the values and the calculated parameters used in PSCAD model are discussed.

3.3.1 Type of conductor

After thorough investigation of different 765 kV transmission lines operated in countries like China, India, Japan, South Korea and USA, it is observed that the conductors used in UHV transmission lines are cardinal, curlew or rail.

Owing to the minimum resistance of the conductor and the higher amp, Cardinal 54/7 is chosen for the studies. Table 3-1 shows the typical conductor characteristics. For UHV lines the power transmitted is high and a bundled conductor is considered in this case. The cardinal 54/7 conductor is in practical usage in the Korea Electric Power Company (KEPCO) 765 kV transmission lines [35].

Advantages of having a bundle will reduce the losses in the line and the skin effect and the corona.

The 765 kV lines are in general designed for a power rating of more than 4000MVA transmission capacity. In line with the theory and the standard practice, a transmission line with a 6 sub-conductor bundle is considered

The maximum power transmission in the 765kV line is calculated as 8000 MVA considering the 6 conductors with 996 A current capacity and it is given by (3.9) below

3. 3 996 6 765

8000

S IV

S MVA

= = × × ×

= (3.9)

The transmission line towers are available in PSCAD line model library.

The tower components are used to define the geometric configuration of the transmission line.

The configuration editor for the overhead line towers considers the input parameter description for tower data, circuit conductor data, and circuit ground data. The following Figure 3-2 provides the information of the line geometry used in modeling the UHV transmission line in PSCAD.

As it can be seen from the Figure 3-2, a three-phase line model is arranged in a triangular configuration with equally spaced distance of 14 m.

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15

Figure 3-2: Line geometry.

The line is considered ideally transposed and considered with 2 ground wires. The circuit conductor data includes the geometric mean radius of the conductor which is the individual GMR of each sub conductor in the bundle. Each phase has 6 bundled conductors which have been selected based on most practical 765 kV line parameters. The GMR for the sub conductor is calculated as equation (3.10)

Individual sub- conductor diameter = 1.196 inch [36]

Considering the conversion factor in meters,

Diameter =0.03078 m

1 1

( ) ( )

' 4 4

'

0.03048

. .

2 2

0.0119857 diameter

r e e

r m

= =

=

(3.10)

The conductor and ground wire parameters used for the model of the line is presented in Table 3- 1 for reference. The values in the table are obtained by calculation based on the data provided in PSCAD.

Table 3-1: Conductor parameters used for UHV line model

Parameter Data used

Sub -Conductor Radius r' (m) 0.0119857

Conductor DC resistance (Ω/km) 0.058727

Ground wire radius 0.0055245

Ground wire DC resistance 2.5

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16 3.3.2 Surge Impedance loading

In the UHV lines the value of R is small and negligible, and the line is assumed to be lossless subject to high voltage surges. The impedance associated with the line without losses and resistance effect is called surge impedance and the power transmitted through the line is called surge impedance loading (SIL) given by equation (3.12).

C

( )

Z = L C Ω (3.11)

2

( )

0

C

SIL V W

= Z (3.12)

Where, V0 is the rated line voltage.

At SIL, it is observed that

1. The voltage and current at the sending and receiving end are constant and equal in amplitude.

2. Voltage and current signals are in phase along the line length.

3. There is a phase shift between the sending end and receiving voltage signals, owing to the length of transmission lines. This phase difference is given byβl. Where,β is the phase constant and l is the line length.

The phase shift between the sending end and receiving end parameters is a phenomenon due to the long transmission line and it is not due to UHV. But UHV lines are associated with long distance power transmission and it is significant to observe.

Figure 3-3: Sending and receiving end voltage relationship under SIL.

Surge impedance loading in MW for the three phases and the surge impedance of the UHV transmission line in PSCAD is given by equation (3.17) and (3.18) in the next section. The detailed analysis of the phase difference between the sending and receiving end voltages for the UHV transmission line used for simulation in presented in Chapter-4 of this document.

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17 3.3.3 Overhead transmission line parameter

The parameters such as the line sequence resistance, reactance and the surge impedance are obtained from the simulated model of the UHV line in PSCAD. The values obtained are thus compared with the typical parameter values obtained from book on power system stability and control as tabulated in

Table 3-2 [37]. Also to be noted that the data in the tabulation is calculated based on a rated frequency of 60 Hz.

Table 3-2: Typical overhead transmission line parameter [37]

With the data for the UHV transmission line the value of sequence impedance and the line capacitance are tabulated in Table 3-3.

Table 3-3: Line parameters for the simulated Line model in PSCAD

Sequence component Positive sequence Negative sequence Zero sequence Resistance

(

/ km

)

0.0107623935 0.0107623935 0.272553164

Reactance

(

/ km

)

0.243559208 0.243559208 0.947583086 Capacitance

(

mF km/

)

0.014978 0.014978 0.00891309

Nominal Voltage 230 kV 345 kV 500 kV 765 kV 1100 kV

(

/ /

)

Rkm phase 0.050 0.037 0.028 0.012 0.005

(

/ /

)

xLLkm phase 0.488 0.367 0.325 0.329 0.292

(

1/ /

)

bCC m Ω km phase 3.371 4.518 5.200 4.978 5.544

(

nepers km/

)

α 0.000067 0.000066 0.000057 0.000025 0.000012

(

rad km/

)

β 0.00128 0.00129 0.00130 0.00128 0.00127

( )

ZC Ω 380 285 250 257 230

( )

SIL MW 140 420 1000 2280 5260

2 0

Charging

MVA/km=V bc 0.18 0.54 1.30 2.92 6.71

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18 Based on the parameters obtained for the PSCAD simulation model for the UHV line, the line impedance can be calculated as equation (3.13). The UHV line model results of sequence components are calculated based on a frequency of 50 Hz.

( )

1 1

0.01076 0.2435 /

Z R jX

j km

= +

= + Ω (3.13)

For the 500 km transmission line, the impedance is calculated as (3.14)

( )

1 (0.01076 0.2435) 500 5.38 121.47

Z j

j

= + ×

= + Ω (3.14)

( )

0 (0.2725 0 ) 500

136.28 473.7 5 .94758

9

Z j

j

= + ×

= + Ω (3.15)

Surge impedance loading is obtained from (3.12)

( )

6

( )

0.3876 7.4892 10

Surge impedance ZC =

× (3.16)

( )

C 227

( )

Surge impedance Z = Ω (3.17)

( )

( )

7652

227 2572

SIL MW

MW

=

=

(3.18)

It can be observed that the values thus obtained are in line with the practical data provided in Table

Table3-2 and the values are also validated with the practical line parameters which are in line with the model.

The model parameters which are described in this chapter is used for the fault transient simulation of the UHV transmission line and for the further studies which are detailed in the following chapters.

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19

Chapter 4

4 T

YPICAL

F

EATURES OF

UHV L

INES

After successful modeling of an UHV line, it is required to study both the steady state operations and the transient behavior of the line during fault conditions to distinguish the challenges that might appear to the protection solution schemes. This chapter aims to distinguish the phenomena which are distinctive to UHV line.

4.1 Description of the test system

Two sources are connected to the transmission line which is modeled in Chapter 3 in order to analyze the line behavior during transient faults as shown in Figure 4-1. The sending end source is considered as stronger than the receiving end source. The angle difference (𝛿𝛿1− 𝛿𝛿2) between the sources indicates the load magnitude. The reference direction of the current is considered toward the line during the measurement recordings. It is considered that the current and voltage transformers are ideal without measurement errors in order to concentrate totally on the UHV line behavior. The line length is 500 km and there is no switching substation or tapped line between the sending and receiving end sources.

iS iR

Source 1

(Sending end) Source 2

(Receiving end) 500 km UHV line

Figure 4-1: UHV transmission line system for the analysis.

4.2 UHV line phenomena

UHV transmission line exhibits some characteristics which are different from the lower voltage (<750 kV) line. Thus necessary steps should be taken in the existing protection schemes to overcome its negative influences including mal-trips. The following characteristics were observed during the analysis.

4.2.1 Line impedance

The apparent impedance is calculated from the recorded voltages and currents at the local side of the transmission line. Faults applied at different locations of the line. The voltages and currents at the sending end are recorded when they become stable after the fault inception. The aim is to determine how the line resistance varies with the increase of fault distance from the local side.

For lower voltage transmission line, a lumped circuit model provides a good assumption for the

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20 determination of fault impedance. However, for UHV line, the increase in fault impedance is not linear to the increase in the fault distance from the local side. Thus, the lumped circuit model of transmission line is no longer valid to represent the operating conditions or characteristics of UHV line here.

For a three-phase solid fault, the voltage at the fault location becomes zero. If the distance between the sending end and the fault location is ‘𝑥𝑥’, the equations for the voltage and current at the sending end can be written as equation (4.1) and (4.2) according to the distributed line parameter model described in chapter 3. It is mentionable that positive sequence values of the inductance and capacitance are used to calculate the surge impedance (𝑍𝑍𝑐𝑐) and propagation constant (𝛾𝛾) of the line. The combined effect of self and mutual inductances are accounted in this way.

𝑉𝑉𝑆𝑆 = 𝑉𝑉𝑅𝑅𝑅𝑅𝑐𝑐𝑐𝑐𝑐𝑐ℎ(𝛾𝛾𝑥𝑥) + 𝑗𝑗𝑍𝑍𝑐𝑐𝑐𝑐𝑠𝑠𝑠𝑠ℎ(𝛾𝛾𝑥𝑥) × 𝐼𝐼𝑅𝑅𝑅𝑅 (4.1) 𝐼𝐼𝑆𝑆 = 𝑐𝑐𝑐𝑐𝑐𝑐ℎ(𝛾𝛾𝑥𝑥) × 𝐼𝐼𝑅𝑅𝑅𝑅+ 𝑉𝑉𝑅𝑅𝑅𝑅� 𝑐𝑐𝑠𝑠𝑠𝑠ℎ(𝛾𝛾𝑥𝑥) 𝑍𝑍𝑐𝑐 (4.2) Placing 𝑉𝑉𝑅𝑅𝑅𝑅= 0 in equation yields,

𝑉𝑉𝑆𝑆 = 𝑗𝑗𝑍𝑍𝑐𝑐𝑐𝑐𝑠𝑠𝑠𝑠ℎ(𝛾𝛾𝑥𝑥) × 𝐼𝐼𝑅𝑅𝑅𝑅 (4.3) 𝐼𝐼𝑆𝑆 = 𝑐𝑐𝑐𝑐𝑐𝑐ℎ(𝛾𝛾𝑥𝑥) × 𝐼𝐼𝑅𝑅𝑅𝑅 (4.4) Thus the apparent impedance during the fault yields

𝑍𝑍𝑓𝑓 =𝑉𝑉𝑆𝑆

𝐼𝐼𝑆𝑆 = 𝑗𝑗𝑍𝑍𝑐𝑐 𝑡𝑡𝑡𝑡𝑠𝑠ℎ(𝛾𝛾𝑥𝑥) (4.5) Figure 4-2 shows that the apparent impedance for a three-phase to ground bolted fault varies according to the equation mentioned above. It is also visible that the increase of apparent impedance is not linear to the increase in the distance of fault location. This nonlinear characteristic brings trouble to the accurate calculation of the apparent fault by classical methods.

For detailed analysis regarding the impedance characteristics during all type of faults, please read the master thesis [7] which was made in parallel with this thesis as the main focus in this thesis is the line differential protection.

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21

Figure 4-2: Impedance during ABC-G bolted fault for different fault locations on the line.

4.2.2 Line charging current

Transmission line exhibits capacitance effect due to the potential difference between two conductors where air acts as an insulator between them. A similar effect is visible between each conductor and the ground. The total resultant capacitance can be assumed as uniformly distributed throughout the line. The current that flows between the conductors due to this capacitance effect is called line charging current. It contributes to cancel out a portion of the lagging component in the load current and thus reduce the current flowing through the line. As a result there is improvement in the transmission efficiency, voltage regulation and power factor.

The magnitude of the charging current depends on the voltage level, capacitance of the line, frequency and the length of the line. The effect of charging current in a UHV transmission line is prominent as it has a very high value compared to the lower voltage line. The charging current can be calculated using the equation (4.6).

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22

𝐼𝐼𝑐𝑐ℎ𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎= 𝑗𝑗2𝜋𝜋𝜋𝜋𝜋𝜋𝑉𝑉 (4.6)

Where, 𝜋𝜋 is the frequency of the system, 𝜋𝜋 is the capacitance of the line and 𝑉𝑉 is the line to ground voltage of the system. The capacitance can be calculated by the following equation (4.7) [38].

𝜋𝜋 = 0.0556 ln �𝐺𝐺𝐺𝐺𝐺𝐺𝐺𝐺𝐺𝐺𝐺𝐺�

𝜇𝜇𝜇𝜇/𝑘𝑘𝑘𝑘 (4.7)

Here, 𝐺𝐺𝐺𝐺𝐺𝐺 is the geometric mean distance between the conductors which is 14 m in case of the simulation to the guidance of U.S. Department of Energy [6] for equilateral triangular configuration. 𝐺𝐺𝐺𝐺𝐺𝐺 is the geometric mean radius of the conductor bundle. The UHV line is modeled with a six conductor bundle. The 𝐺𝐺𝐺𝐺𝐺𝐺 value for a six conductor bundle can be calculated with the use of equation (4.8). Appendix 1 discusses this equation in details.

𝐺𝐺𝐺𝐺𝐺𝐺 = 1.348�(𝑟𝑟 × 𝑑𝑑6 5) (4.8)

Here, 𝑟𝑟 is the radius of each sub-conductor in the bundle and 𝑑𝑑 is the bundle spacing. Putting the corresponding values in the above equations the calculated capacitance value of the modeled 765 kV line is 0.0146 𝜇𝜇𝜇𝜇/𝑘𝑘𝑘𝑘. This yields that for each hundred kilometer distance of transmission line, the amount of charging current is 206.7 A. On the other hand, a 230 kV line has about 35.3 A as charging current for each hundred kilometer distance which is very less compared to the UHV line.

Table 4-1: Line charging currents for transmission lines of different voltage level [39].

Voltage Level (kV) 230 500 765 1000

Charging Current

per 100 km (A) 35.3 126.97 206.7 367.95

Figure 4-3 shows the charging current of the modeled line in PSCAD. The angle difference between the sources (𝛿𝛿1− 𝛿𝛿2) is put zero which exhibits that no load is connected to the line.

Therefore, the phasor summation of the currents of the both end should indicate the charging current. Figure 4-3 shows the absolute value of the line charging current when simulation is done in this way. For a 500 km line the total charging current is 1.052 kA which is very close to the calculated value. This high value of charging current has a severe impact on the sensitivity and security of the line differential protection scheme. Chapter 5 discusses in-depth analysis of this effect. Figure 4-3 also shows absolute value of sending and receiving end charging current that is fed by the sources. It is noticeable that the portion of charging current that is fed by the sending end source is a bit higher than the receiving end source. This is because the sending end source is

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23 modeled a bit stronger than the receiving end source. Both of the values would be same if the sources were identical. It also indicates that there is no load connected to the line and the charging current comes from the angle difference between the sending and receiving end currents.

Figure 4-3: Charging Current along with sending and receiving end current when there is no load connected to the line.

4.2.3 DC component in the current during fault

The inductance of a transmission line is dominant than the resistance or capacitance. Thus a fault current can be asymmetrical depending on the fault inception angle. An asymmetrical fault current consists of an AC part and a DC part. The DC part decays to zero over time and then the fault current becomes symmetrical after that. The amplitude of the DC component depends on the inception angle of the fault and the decay time depends on the ratio of reactance and resistance of the line (i.e. 𝑋𝑋 𝐺𝐺 ⁄ ratio). The exponential decay of the DC component occurs according to equation (4.9) [40].

𝑠𝑠𝐷𝐷𝐷𝐷 = √2𝐼𝐼𝑘𝑘𝑒𝑒−�𝑡𝑡×2𝜋𝜋𝑓𝑓𝑋𝑋𝑅𝑅

(4.9)

Where, 𝐼𝐼𝑘𝑘 is the symmetrical short circuit current (RMS), 𝜋𝜋 is the system frequency and 𝑡𝑡 is the time after fault inception. 𝑋𝑋 and 𝐺𝐺 are the reactance and resistance of the line respectively. It is

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24 clear from the equation that the higher value of 𝑋𝑋 𝐺𝐺 ⁄ ratio, longer the time for the DC component to decay to zero.

UHV lines have a very high ratio between the reactance and resistance. This is important information for the protection engineer to determine the size of the current transformer, withstand capability of the circuit breaker, delay setting in the protection relays [41]. A longer duration of DC component might require special consideration for these cases.

Figure 4-4 shows the DC decays for a three phase to ground fault at the middle of a 765 kV and a 230 kV transmission line. It is clearly observable that the UHV line exhibits a longer decaying time compared to the lower voltage line. The figure also shows the RMS and fundamental currents for both lines. It is noticeable that it requires longer time for the RMS and fundamental quantity to become equal.

Figure 4-4: Fundamental and DC components of the sending end current after fault inception on 765 kV and 230 kV line.

2 2.05 2.1 2.15 2.2 2.25 2.3 2.35

Current (kA)

0 5 10

I

SaRMS

| ISa1

|

I

SaDC

Time (s)

2 2.05 2.1 2.15 2.2 2.25 2.3 2.35

Current (kA)

0 1 2 3 4 5

765 kV line

230 kV line

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25 4.2.4 Phase shift in the current

Typically the UHV lines have very long distance as they can efficiently transfer high amount of energy in long distance. Because of the long length, there is a phase shift between the sending and receiving end current of UHV line. The amount of phase shift is determined by the following equation (4.10).

𝜃𝜃(𝑟𝑟𝑡𝑡𝑑𝑑𝑠𝑠𝑡𝑡𝑠𝑠) = 𝛽𝛽𝛽𝛽 (4.10)

Where, 𝛽𝛽 is the phase constant i.e. the imaginary part of the propagation constant of the line as described in chapter 3. 𝛽𝛽 is the length of the line. As a result, the longer the line is the higher phase difference between sending and receiving end currents. It has an effect on the line differential protection scheme as the phasor summation of sending and receiving end currents does not become zero during steady state operation. However, the effect of phase shift is not that prominent if the line length is short.

Figure 4-5 shows the angle difference between sending and receiving end currents when the load is set to the surge impedance loading. From the picture below, the angle difference is approximately 30.6o for a 500 km length line The UHV line modeled in PSCAD has a phase constant value of 0.0011 rad/km (0.06 degree/km) which indicates approximately 30.67o phase shift. The calculated and the simulated results approved each other.

Figure 4-5: Sending and receiving end currents for surge impedance loading condition.

References

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