OVERCURRENT RELAY COORDINATION IN DISTRIBUTION SYSTEM. (A CASE STUDY ON PHUENTSHOLING LOW VOLTAGE DISTRIBUTION NETWORK)

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1 OVERCURRENT RELAY COORDINATION IN DISTRIBUTION SYSTEM. (A CASE STUDY ON PHUENTSHOLING LOW VOLTAGE DISTRIBUTION NETWORK) Project Report Submitted in partial fulfillment of the requirements For the award of Bachelor of Degree In Electrical Engineering Submitted By: Ms. Rinzin Choden Mr. Tshewang Sither Mr. Tashi Namgyel Under the guidance of: Mr. Cheku Dorji DEPARTMENT OF ELECTRCIAL ENGINEERING COLLEGE OF SCIENCE AND TECHNOLOGY PHUENTSHOLING, BHUTAN June, 2017

2 ROYAL UNIVERSITY OF BHUTAN COLLEGE OF SCIENCE AND TECHNOLOGY DEPARTMENT OF ELECTRICAL ENGINEERING CERTIFICATE This is to certify that the project entitled Overcurrent Relay Coordination in Distribution System. (A case study on Phuentsholing low voltage distribution system), which is being submitted by Ms. Rinzin Choden ( ), Mr. Tshewang Sither ( ) and Tashi Namgyel (EDE ) in partial fulfilment of the requirement for the award of the Degree of Bachelor of Engineering in Electrical Engineering is a record of student work carried out at College of Science and Technology, Phuentsholing under my supervision and guidance. Mr. Cheku Dorji Project Guide

3 ABSTRACT Electrical Supply Division (ESD) is responsible for the distribution of power supply safely and efficiently from the low voltage (LV) substation to consumer end points. However, with the growth in population density and the demand for energy, the electric distribution system has become more complex and congested. It makes difficult for the utility (BPC) to maintain continues power supply in parallel with the fast growing energy demand. Therefore this project is an attempt to study the appraisal of LV distribution network, the protection schemes and particularly how to improve the coordination of protective relays of Phuentsholing LV networks. The study consist of detail network systems, single line diagram, followed by load flow and short circuit analysis for the protection schemes and the coordination of Overcurrent Relays. The simulation and analysis were carried out in Dig Silent Power Factory and the results obtained were compared with the data provided by ESD (BPC), Phuentsholing. In general any power system would comprises number of important equipment to be protected, and the complete control-protective gears (relay, circuit breakers) are necessary to ensure the reliability of power supply all the time. The protective relays placed in the network either in radial mode or ring system are normally coordinated based on time discrimination, current discrimination or the combination of both. The primary protection relay must operate within its predetermined time period, In case of failure of primary relay, the next relay called back-up protection has to react after the stipulated delay. The relay coordination and the time-current characteristics of three over current relays placed in 33kV/11kV distribution network were simulated using Dig SILENT Power Factory. The simulated results, the operating times of relay are found to be bit higher values while comparing with the practical time settings. However, the time settings of the existing relays could not be ascertained. The Inverse Definite Minimum time (IDMT) over current relay characteristics is used for the time-current discrimination of three relays, from load to source. The maximum three phase short circuit and single phase to ground fault were considered for short circuit analysis. ii

4 ACKNOWLEDGEMENT We, the project students would like to thank the Royal University of Bhutan, the College of Science and Technology for providing an invaluable opportunity to study under graduate course and facilitating the platform for our Final year Project Work. The project would not be achieved without the support and assistance of our project guide Mr. Cheku Dorji, who has been a mentor and constant source of aspiration in accomplishing our project. We also would like to thank all the other tutors for their generous feedback and suggestions during the reviews. Lastly, we would also like to particularly thank Mr. Sherab Tenzin (Assistant Engineer), Bhutan Power Cooperation, Phuntsholing for providing information and made available for us whenever we approached. Finally, a special thanks to all the individual who directly or indirectly helped in the completion of this project in every little capacity possible. Ms. Rinzin Choden Mr. Tshewang Sither Mr. Tashi Namgyel iii

5 TABLE OF CONTENTS Abstract... ii ACKNOWLEDGEMENT...iii Table of Contents... iv List of Figures... vii List of Tables... ix List of Abbreviations... xi CHAPTER ONE: INTRODUCTION Background and Motivation Objectives Proposed Methodology... 2 CHAPTER TWO: LITRERATURE REVIEW Overcurrent Relay Plug Setting Multiplier (PSM) & Time Multiplier Setting (TMS) Standard formula for overcurrent relay Load flow analysis Short Circuit Analysis Types of fault Overcurrent Protection system Types of an overcurrent relay CHAPTER THREE: STUDY OF THE EXISTING DISTRIBUTION SYSTEM of PHUENTSHOLING Appraisal of low voltage distribution system of Phuentsholing Main Distribution System Current Protection Scheme and Relay Settings in the network CHAPTER FOUR: METHODOLOGY iv

6 4.1 Line loss in Water Booster and PWD feeder Loss in the distribution line Transformer loss calculations Relay Coordination Method of relay coordination Theoretical Calculation for overcurrent Relay Setting Water booster feeder (Time Current graded system) PWD feeder (Time current graded system) CHAPTER FIVE: SIMULATION USING DIGSILENT POWER FACTORY SOFTWARE Feeder 1: Water Booster Feeder Line loss in water booster feeder Transformer Losses Feeder 2: PWD Feeder Line losses in PWD feeder Transformer losses Observation from the system loss Voltage Profile Relay Coordination result from DIgSILENT PowerFactory Water Booster Feeder PWD feeder Earth fault relay setting CHAPTER SIX: RESULT VALIDATION System loss validation Water booster feeder PWD feeder Relay setting validation Water booster feeder v

7 6.2.2 PWD feeder CHAPTER SEVEN: RECOMMENDATION AND FUTURE WORK CHAPTER EIGHT: CONCLUSION References Appendix vi

8 LIST OF FIGURES Figure 1. Showing Symmetrical faullts... 8 Figure 2. Open circuit faults... 9 Figure 3. Double line to groung faults Figure 4. Instantaneous overcurrent relay Figure 5. Definite time overcurrent relay Figure 6.Inverse time overcurrent relay Figure 7. Inverse definite minimum time relay Figure 8. Very inverse overcurrent relay Figure 9.Extremely inverse curve Figure 10. Relay Location of the existing network Figure 11. Discrimination by time Figure 12.Discrimination by current Figure 13: Water booster feeder model in DIgSILENT PowerFactory software Figure 14. Line losses in feeder 1 for the month of February 2015 using DIgSILENT PowerFactory software Figure 15.Transformer Losses in feeder 1 for the month of February 2015 using DIgSILENT PowerFactory software Figure 16: PWD Feeder model in DIgSILENT PowerFactory software Figure 17. Line losses in feeder 2 for the month of February 2015 using DIgSILENT PowerFactory software Figure 18. Transformer losses in feeder 2 for the month of February 2015 using DIgSILENT PowerFactory software Figure 19. Voltage profile of Water Booster feeder for the month of February vii

9 Figure 20. Voltage profile of PWD feeder for the month of February Figure 21. Max 3 phase short circuit fault created at Water booster feeder Figure 22. IDMT curve plotted for the 3 phase fault at Water booster feeder Figure phase short circuit fault at the RSTA bus Figure 24. IDMT curve polted for the fault at RSTA bus Figure phase short circuit fault created at the incoming line Figure 26. IDMT curve ploted for the fault Figure phase short circuit fault created at the Water Booster Feeder Figure 28. IDMT curve plotted for the fault Figure phase short circuit fault created at the PWD feeder Figure 30. IDMT curve plotted for the fault at PWD feeder Figure phase short circuit fault created at the RRCO feeder Figure 32. IDMT curve plotted for the fault at RRCO feeder Figure 33. Single phase to groung fault created at the PWD feeder Figure 34. IDMT curve plotted for the fault at PWD feeder Figure 35. Single phase to ground fault created at the PWD feeder Figure 36. IDMT curve plotted for the fault viii

10 LIST OF TABLES Table 1. IEC standard table... 6 Table 2.Theoretical calculation of Line losses for the month of February 2015 in feeder Table 3.Theoretical calculation of line losses for the month of February 2015 in feeder Table 4. Feeder 1 Transformer Losses for the month of February Table 5. Feeder 2 Transformer Losses in the month of February Table 6. Theoretical Results of Overcurrent relay setting for 3 phase fault performed Table 7. Theoretical Result of Earth fault relay setting for single phase to ground fault performed Table 8. Theoritical Results of Overcurrent relay setting for 3 phase fault performed Table 9. Theoretical Result of Earth fault relay setting for single phase to ground fault performed Table 11. Plug setting and Time of operation of the relays(time current graded system) Table 12. Plug setting and Time of opereation of the relays(timecurrent graded system) Table 13. Plug Setting and Time of Operation of the relay(timecurrent graded system) Table 14. Plug Setting and Time of Operation for time graded Table 15. Plug Setting and Time of Operation of relay in feeder 2 for time current graded system Table 16. Plug Setting and Time of Operation of relay in feeder 2 for time current graded system Table 17. Plug Setting and Time of Operation of relay in feeder 2(Time current graded system) Table 18. Plug Setting and Time of Operationof relay in feeder 2(Time graded system) Table 19. Comparison of Losses of Water Booster feeder Table 20. Comparison of Losses of PWD feeder ix

11 Table 21. Relay setting comparison for Water booster feeder (Time current graded system). 46 Table 22. Relay setting comparison for PDW feeder(timecurrent graded system) x

12 LIST OF ABBREVIATIONS Sl. No. Terms Descriptions 1 CST College of Science and Technology 2 IEEE Institute of Electrical and Electronics Engineers 3 BPC Bhutan Power Corporation 4 AC Alternating Current 5 CT Current transformer xi

13 CHAPTER ONE: INTRODUCTION 1.1 Background and Motivation The power system comprises of generating station, transmission and distribution system. In Bhutan, the generating stations are located at different parts of the country which are interconnected by the transmission network and ultimately connected to the distribution station. Distribution system is the link between the distribution station and the customer. Electrical Supply Division (ESD) is responsible for the power distribution from the low voltage distribution substation (66/33/11kV) to various consumer in Bhutan. For the distribution of power, the distribution network requires distribution station, distribution feeder, distribution transformer and service mains. Power system network in Bhutan has been expanding yearly and it is expected to increase further after the completion of the ongoing hydropower project. With the increase in power system network there is a need of upgrading the equipment and protection setting for the reliable power supply of the Bhutan network. Therefore, transmission and distribution feeders should be protected by a comprehensive protection scheme. The protection scheme designed for the system should be fast and selective. Also if main protection fails to operate, there should be a backup protection for which proper relay coordination is necessary. There are three types of protection scheme namely the overcurrent protection, distance protection and the differential protection. The overcurrent and distance protection is widely used in the power system network. In the transmission network, the distance protection is the primary protection and the overcurrent protection as the backup protection whereas in distribution system the overcurrent is the main primary protection scheme. Overcurrent protection follows different time current characteristics for the relay coordination. In this project, the overcurrent relay coordination for the distribution network of Phuentsholing using DIgSILENT PowerFactory software is considered for the studies. The inverse definite minimum time (IDMT) overcurrent relay and its time current characteristics are used for the entire distribution network. 1.2 Objectives The following are the main objectives of the project; Review on the various types of relay and relay coordination in distribution system. 1

14 Appraisal of the protection setting of the existing Phuentsholing distribution network. Learning the basic features of DigSILENT powerfactory software and how to simulate the system using the software. Analysis of load flow and the short circuit on various voltage levels in a network. Modelling of Phuentsholing distribution network and protection schemes in DIgSILENT PowerFactory. To analyse the coordination of overcurrent relay with different possible faults. Validation of simulated results with the practical set values. 1.3 Proposed Methodology 2 Literature review Data collection from BPC Modelling of the system in DIgSILENT PowerFactory software Load flow analysis Short circuit analysis Overcurrent relay coordination Compare the Simulated results with the actual relay setting 2

15 Literature review The knowledge for load flow analysis, short circuit analysis and the parameters required for the relay settings to be implemented in the DIgSILENT PowerFactory software for the relay coordination will be acquired by doing the literature review. Data collection from BPC For the overcurrent relay coordination of the Phuentsholing distribution network, the single line diagram of the network will be obtained from BPC. The information on the type of relay used, location of the relays, numbers of relays installed and the parameters for relay setting will be also acquired from BPC. Modelling of the system in DIgSILENT PowerFactory software Phuentsholing distribution network will be modelled in the DIgSILENT PowerFactory software using the data obtained from the BPC. Load flow analysis After obtaining the data from BPC, load flow analysis will be done theoretically. Then will perform load flow analysis in DIgSILENT PowerFactory software. Short circuit analysis For the protection setting, three phase short circuit and single phase to ground fault will be performed in the software to obtain the fault current level. Overcurrent relay coordination Based on the short circuit analysis, the overcurrent relay will be coordinated in the software and various analysis is to be made. Compare the simulated result with actual relay setting The simulated result of overcurrent relay coordination will be compared with the actual relay setting of the Phuentsholing distribution network. 3

16 CHAPTER TWO: LITRERATURE REVIEW 2.1 Overcurrent Relay The power system network should be protected for the reliable power supply. The protection is done by relays and circuit breakers. The design of sizing and number depends upon the power distribution system and it varies from system to system, however the fault is isolated by the relay [1]. Over current phase and earth fault relay coordination is necessary to achieve proper fault identification and fault clearance sequence. The load flow analysis gives the current, voltage and power flow of line, bus, transformer, circuit breakers, motors and other equipment s. Using the load flow study, we can decide the plug setting of relay. Same as load flow study, the short circuit study is essential to find PSM of relay. Then using this PSM, we can find the TMS of back up relay. Thus, load flow and short circuit study must be required in relay coordination [2]. Overcurrent protection is the predominant protection method used for distribution feeders. The standard time-current curves, pickup values, and time dial coordinate the operation of multiple protective relays on radial feeders. The objective is to operate as fast as possible for faults in the primary zone, while delaying operations for faults in the backup zone. The engineer derives available short circuit current and the desired coordination time interval between relays [3]. The relays shall reach at least up to the end of the next protected zone. This is required to ensure the back-up protection. whenever possible, use relays with the same operating characteristic in series with each other and make sure that the relay farthest from the source has current settings equal to or less than the relays behind it, that is, that the primary current required to operate the relay in front is always equal to or less than the primary current required to operate the relay behind it [4]. The relay current setting is given by Plug Setting Multiplier (PSM) and the time settings are given by the Time Dial Settings. The plug-setting must not be less than the maximum normal load including permissible continuous overload unless monitor by under voltage relay, otherwise the relay will not allow the normal load to be delivered. In estimating the plug-setting, an allowance must be made for the fact that the relay pick-up varies from 1.05 to 1.3 times pugsettings, as per standards [2]. The overcurrent relay coordination curve for the feeder must lie below the feeder overload and feeder short circuit damage curve on the time current characteristics graph. Also the 4

17 overcurrent relay coordination curve for the feeder must lie above the capacity curve of the feeder [4]. Overcurrent relay (OCR) is a type of protective relay which operates when the load current exceeds a pre-set value. Overcurrent relays generally have current setting multipliers ranging from 50 to 200% in steps of 25% which is referred to as plug setting (PS). 2.2 Plug Setting Multiplier (PSM) & Time Multiplier Setting (TMS) PS for each relay is determined by two parameters; the minimum fault current and the maximum load current. The coordination of this protective relay is set up during the process of system design based on the fault current calculation. In the coordination problem of overcurrent relays, the objective is to determine the time setting multiplier (TSM) and plug setting multiplier (PSM) of each relay, so that the overall operating time of the primary relays is minimized properly [5]. An overcurrent relay has a minimum operating current, known as the current setting of the relay. The current setting must be chosen so that the relay does not operate for the maximum load current in the circuit being protected, but does operate for a current equal or greater to the minimum expected fault current. The current setting of a relay nearer the source must always be higher than the setting of the preceding relay. The relays must have current settings which are higher than any current which can flow through the relays under normal conditions i.e. 110% of the rated current. Electronic and microprocessor-based relays have current setting steps of 5% [6]. In order to apply the relay in the power system it is necessary to be able to modify the time scale of time-current characteristic. The time-multiplier setting must be chosen to give lowest possible time for the relay at the end of the radial feeder. In the preceding sections towards the source, the time multiplier should be chosen to give desire selective interval from the downstream relay at maximum fault conditions. The time multiplier setting should allow not only for the time of the breaker but also for the overshoot of the relay and allowable time-errors in the time of operation of successive relays. 2.3 Standard formula for overcurrent relay By using the general equation of IEC (International Electro Technical Commission) standard: C Tp= [ ] TMS) (I/Ip) α 1 Tp = Operating time in second. 5

18 (I/Ip) α = Applied multiples of set current value. C and α = Constant of Relay Constants for IEC Standard Time Overcurrent Characteristics IEC Standard [2] Table 1. IEC standard table IEC standard table Type of characteristics C α Normal inverse Very inverse Extremely inverse 80 2 Long-time inverse Short time inverse Inverse Load flow analysis Load flow analysis is necessary to obtain how much the voltages, currents, and power (active and reactive) are flowing in the power system network under steady state conditions.it also provides power losses in the system, the voltage profile and the percentage loading of line and transformer. From the load flow analysis the plug setting required for the relay setting is acquired.load flow studies can also be used to determine the optimum size and location of capacitors for power factor correction. [7] The other importance of the load flow analysis are as following: To plan ahead and account for various hypothetical situations that may occur in the system. The impact of increased load on the system. Solutions for loss reduction in the system. Improvement of voltage profile 6

19 2.5 Short Circuit Analysis A short circuit is an abnormal connection of very low impedance between two points of different potential, whether made intentionally or accidentally. Due to short circuit it causes the flow of excessive current in the power system leading to the interruption of power supply. Although the power system is being designed to protect from various faults but somehow the system gets damaged. Fault current depends on the power circuit voltage and configuration, method of neutral connections (solidly grounded, resistance grounded, reactance grounded and ungrounded), presence of the regulating devices (such as shunt reactor, series reactor, shunt and series capacitors and FACT devices), and the speed of disconnection of the faulted circuit section. Determination of fault current in power system for various fault such as 3 phase to ground,2 phase to ground, phase to phase,single phase to ground and phase to neutral ground can be done through the short circuit analysis. Apart from the short circuit current, the interrupting ratings of protective devices such as circuit breaker and fuses for ensuring the protection of equipment installed in the power system and as well as coordination of protective devices can be known. If an electrical fault exceeds the interrupting rate of the protective device, extensive damage of equipment will occur. Therefore any electrical equipment should not be installed without the knowledge of the complete short circuit study for the power distribution. The short circuit in the power system cannot always be prevented but its effect can only be reduced at the time of planning and design stage of the system. The electrical equipment such as conductor, transformer, switchgear equipment (relay, circuit breaker, fuses) should be designed with the capability of withstanding the system fault current rating. A power system is not static but changes during operation (switching on or off of generators and transmission lines) and during planning (addition of generators and transmission lines).thus short circuit analysis should be done timely for the proper protection. The short circuit is caused by the following: Internal effects: Breakdown of equipment, transmission or distribution lines from deterioration of insulation in generator, transformer etc. Inadequate design such as selecting improper equipment s rating and improper installation. 7

20 External effects: Overloading of the equipment. Weather conditions: It includes the insulation failure due to lightning surges. The damage of equipment due to heavy rains, heavy winds, salt deposition on overhead lines and conductors, snow and ice accumulation on transmission lines. Smoke: If the smoke is present around the overhead lines, ionization between air and smoke particles will take place causing a flashover between the conductors or between the conductor and insulator.this flashover causes insulators to lose their insulting capacity due to high voltage Types of fault Symmetrical /balanced faults Symmetrical faults occur when all the three phase are simultaneously short circuited and give rise to symmetrical fault current having different magnitudes with equal phase displacement (120 degree from each other). These faults rarely occur in practice as compared with unsymmetrical faults and causes severe damage to the electrical equipment. Types: there are two types namely line to line to line (L-L-L) and Line to line to line to ground (L-L-L-G).. Figure 1. Showing Symmetrical faullts 8

21 Unsymmetrical /unbalanced Faults Unsymmetrical faults are the most common faults that occur in the power system network giving rise to unsymmetrical fault current having different magnitudes with unequal phase displacement. Types: open circuit faults (single and two phase open circuit faults) line to ground fault(l-g) phase to phase fault(l-l) double line to ground fault(l-l-g) Open Circuit Faults/series fault The failure of one or more conductors, circuit breaker in one or more phase and melting of fuse in or more phase caused the open circuit faults in the power system network. Figure 2. Open circuit faults The single phase and two phase are the open circuit fault except for three phase open circuit fault. Single phase to ground fault The short circuit path between the line and ground is called single phase to ground.it is the one of the most common fault that occur in the power system network.it being less severe than other faults, it causes less damage to the electrical equipment. 9

22 Line to line fault The fault occur when a live conductor comes in contact with another live conductor in the network. The falling of tree over the two line and the heavy winds which makes the conductor to swing and touch each other caused the line to line fault. Double line to ground fault The short circuit between two lines as well as with the ground is called double line to ground fault. 2.6 Overcurrent Protection system Figure 3. Double line to groung faults Overcurrent relay is a sensing relay which operates when the current exceeds a predetermined value. Overcurrent relay is used to protect the electrical power system element such as transmission line, transformers, generators and motor from excessive current caused by short circuit, corona discharge, overloading of the system and other faults. Various fault.in transmission network overcurrent relay act as the backup relay.it is the primary protection in distribution network protecting mainly the feeders. For a feeder protection there would be more than one overcurrent relay to protect different sections of the feeder Types of an overcurrent relay Depending upon the time of operation of the relay there are different types of overcurrent relay. 1. Instantaneous overcurrent relay 2. Definite time overcurrent relay 10

23 3. Inverse time overcurrent relay a) Moderately inverse relay b) Inverse definite minimum time relay c) Very inverse relay d) Extremely inverse relay 4. Directional overcurrent relay I. Instantaneous overcurrent relay The relay operates in definite time when the current exceeds its pick up value. The operation of relay only depends upon the magnitude of the current where the operating time is constant. There is no time delay. The principle for the coordination of the instantaneous overcurrent relay is that the fault current varies with the location of the fault in the system due to the difference in the impedance between the fault and the source. The operating current of the relay progressively increased for the other relays when moving towards the source whereas the relay located away from the source operate for a low current value. Figure 4. Instantaneous overcurrent relay II. Definite time overcurrent relay For the operation of definite time overcurrent relay, the current should exceed the predetermined value and the fault must be continuous at least a time equal to time setting of the relay. Its operation is independent of the magnitude of current above the pickup value. 11

24 Drawback of the Relay Figure 5. Definite time overcurrent relay The continuity in the supply cannot be maintained at the load end in the event of fault. Time lag is provided which is not desirable in on short circuits. It is difficult to coordinate and requires changes with the addition of load. It is not suitable for long distance transmission lines where rapid fault clearance is necessary for stability. Relay have difficulties in distinguishing between faults currents at one point or another when fault impedances between these points are small, thus poor discrimination. [8] Application It act as a backup protection to distance relay in transmission line with time delay. It also act as back up protection to differential relay of power transformer with time relay. For the protection of outgoing feeders and bus couplers. III. Inverse time overcurrent relay Inverse time over-current Relay is one in which the time of operation of Relay decreases as the fault current increases. The more the fault current the lesser will be the time of operation of the Relay and vice versa. If fault current is equal to pick-up value then the relay will take infinite time to operate. 12

25 Figure 6.Inverse time overcurrent relay a. Inverse definite minimum time relay For this relay the operating time is inversely proportional to the fault current. The operating time of the relay can be made less by adjusting the time dial setting. The relay operates when current exceeds its pick up value and the operating time depends on the magnitude of current. For the lower values of fault current the relay gives the inverse time current characteristics and for higher values of fault current it gives definite time characteristics.it is used for the protection of distribution lines. Figure 7. Inverse definite minimum time relay b. Very inverse relay In this relay the range of operating time is inversely proportional to the fault current over a wide range.it is effective for the protection from ground fault.it protects the feeders and long sub transmission lines. 13

26 Application of the very inverse relay Suitable for the application if there is reduction in fault current as the fault distance from the power source increases. Used when the fault current is dependent on the fault location. Used when the fault current is independent of normal changes in generating capacity. Figure 8. Very inverse overcurrent relay c. Extremely Inverse Relay: The operating time of this relay is inversely proportional to the square of the current.it gives more inverse characteristics than that of IDMT and very inverse overcurrent relay. Figure 9.Extremely inverse curve 14

27 CHAPTER THREE: STUDY OF THE EXISTING DISTRIBUTION SYSTEM of PHUENTSHOLING 3.1 Appraisal of low voltage distribution system of Phuentsholing. Apart from being one of the biggest towns in Bhutan, Phuentsholing is a prominent business centre as well, pertaining to the fact that the town shares its border with India. With increased business opportunities and easy access to Jaigaon, the town has experienced increasing number of people and houses every year, for the statistics being according to NSB. It is therefore, very important to proclaim a better understanding of power distribution system of the town for convenient installation of the distribution lines and aid in other related prospects of the power system The Phuentsholing city get the power supply from 66/33/11kV substation located at Dhamdara. It is known to be one of the oldest substation in Bhutan built in 1980s to import power from India during the construction of chukka Hydro power project. After the completion of Chukka Hydro power project, the substation was used for exporting the power generated from CHP to India. Now it is catering power supply only to its locality. The substation has two incoming lines, 66kV from malbase substation and another 66kV directly from Chukha. There are three 66/33kV power transformer of 10MVA and one 3MVA transformer. From this substation there are three 33kV and six 11kV outgoing feeders. 3.2 Main Distribution System Two parallel 33kV lines are supplied from 66/33/11/kV to Phuentsholing substation located at the ground floor of the BPCL office. There are three 11kV outgoing feeders from this substation namely Water Booster feeder, RSA feeder and PWD feeder. 1. Water booster feeder Water Booster Feeder supplies power to half of the Phuentsholing area namely Lower market, Dhoti khola, NPPF colony, RSTA,Choden Engineering, Tashi engineering, AWP and Dratshang covering total conductor length of m and the conductor used are of dog and CBL type. There are 11 distribution transformers connected to this main feeder namely six 750kVA, three 500kVA, one 315kVA and one 5MVA transformer. 15

28 2. PWD feeder This feeder supply power to another half area of the Phuentsholing area namely RICB, Lhaki Hotel, Telecom, Pemaling, Bank of Bhutan, Druk hotel, Imtrat, Water treatment area and Gompa area covering total conductor length of m and there are mixed of conductors namely HVABC, CBL and Dog. There are 14 distribution transformers connected to this feeder namely three 75okVA, three 500kVA, three 250kVA, 5MVA, 1000kVA, 63kVA, 125kVA and 16kVA. 3.3 Current Protection Scheme and Relay Settings in the network Figure 10. Relay Location of the existing network There are two parallel 33kV line coming from 66/33/11/kV Phuentsholing substation. The overcurrent numerical relays (micom P122) are installed only in the feeders going to the core town area. One relay each at 33kV incoming (Relay 3) and 11kV outgoing (Relay 2) of Water Booster and PWD Feeders as shown in the figure (09) and Relay 1 at the outgoing feeder. The CT ratio of relays are 150/1 A, 400/1A and 300/1A for Relay 3, Relay 2 and Relay 1 respectively. Earth fault relays are installed as the backup protection for each relay. 16

29 CHAPTER FOUR: METHODOLOGY Having known the details of existing network and data obtained from BPC, theoretical calculation for the load flow is carried out and using the same data it is simulated later in the DIgSILENT PowerFactory software. 4.1 Line loss in Water Booster and PWD feeder Loss in the distribution line Power loss in MV lines is proportional to square of the current flowing through it and can be determined by the formula [9]: W = N I 2 R L(watt) Where W= power loss in Watt N= no. of phases (2 for single phase two wire, 3 for 3 phase or 3 phase four wire) I = current in Amps flowing through the section under consideration r = resistance of the section in ohm/meter L = length of section in meter Since the Phuentsholing distribution is a radial type system, section wise current is being calculated as shown in appendix [A]. 17

30 Table 2.Theoretical calculation of Line losses for the month of February 2015 in feeder 1. Section Length Conductor Resistance Transformer Current Losses (W) (m) (ohm/km) Connected at the end of the section(kva) (A) Dog CBL CBL CBL CBL CBL CBL CBL CBL CBL CBL CBL CBL CBL Total

31 Table 3.Theoretical calculation of line losses for the month of February 2015 in feeder 2. Section Length(m) Conductor Resistance Transformer Current(A) Loss(W) (ohm/km) Connected at the end of the section(kva) Dog CBL CBL CBL CBL HVABC HVABC HVABC HVABC CBL CBL CBL CBL CBL CBL CBL CBL CBL Total Transformer loss calculations The transformer is the most efficient of electrical machines, with efficiencies typically in the high range of %. In spite of this, the cost of losses is an important factor in specifying and purchasing transformers, especially distribution transformers which play the main role in the power grid losses. Although their efficiency is high when compare to other electrical apparatus the number of distribution transformers used in the LV network are more and kept 19

32 adding with rise in electrical load. And a cumulative amount of power loss is much more than other devices. [10] There are two types of losses in the distribution transformer. They are: No-load loss It is also called core loss or constant loss.it is caused by the time varying nature of the magnetizing force and the eddy current and hysteresis in the core materials. No load loss includes dielectric loss and conductor loss due to excitation currents as well but the dominant no load loss is the core loss [11].The loss is directly proportional to the frequency and maximum flux density but independent of the load. Variable Load loss It is also called as copper loss or winding loss.it is caused by the windings of the transformer which is made up of copper. The load loss is not constant but it varies with the square of the current carried by the transformer, the resistive heating losses in the windings due to both load and eddy currents, stray losses due to leakage fluxes in the windings, core lamps, and other parts, and the loss due to circulating currents in parallel windings and parallel winding strands. [10] The copper loss can be calculated by using the following formula [9]: Wc = ( P )^2 Wcr(watt) T Wt = Wi + Wc Where Wt = total loss in transformer Wi = fixed loss W CR = coppper loss at rated output W c = coppper loss proportional to the loading P = output of the transformer 20

33 T = rated kva of the transformer Iron loss is a fixed loss (as prescribed by the manufacture or as per IS/Relevant standard) FEEDER 1:Water booster feeder Since we didn t get the loading of each transformer from BPC, we calculated in the following manner Peak load in kva Total connected transformer in kva = 11.59% So 11.59%, the loading of overall transformer connected in the feeder Therefore to find the loading of each transformer: For 750kVA transformer: For 500 kva transformer: = kw 315 = 36.5 kw 21

34 Table 4. Feeder 1 Transformer Losses for the month of February 2015 Sl. No Transfor Qty Total Iron loss Full load output of Wc (W) Wi (W) mer Rating (kva) (kva) (Wi) in Watt. copper loss (Wcr) in Watts the trans (P) (kw) Total FEEDER 2:PWD Feeder Peak load in kva Total connected transformer in kva = 23.65% So 23.65% is the loading of overall transformer connected in the feeder 2. Therefore to find the loading of each transformer: For 750kVA transformer: For 500 kva transformer: For 63 kva transformer: = kW 500 = 93.4 kw = kw For 125 kva transformer: 22

35 = kw For 250 kva transformer: = kw For 1000 kva transformer: For 16 kva transformer: = kw Sl. No Trans- former rating (kva) = kw Table 5. Feeder 2 Transformer Losses in the month of February 2015 Qty Total (kva) Iron loss (Wi) in Watts. Full load copper loss in Watts. (Wcr) Output of the trans. P(kW) Wc (W) Wi(W) TOTAL Iron loss and copper losses are obtained from standard. 2 23

36 4.2 Relay Coordination The coordination of the protective relay is done during the process of system design based on the short circuit current level.it is the process of determining the sequence of relay operation for various faults in power system so that the faulted section is cleared in minimum time. For the proper relay coordination it is necessary to determine an appropriate time setting multiplier(tsm) and plug setting multiplier(psm) for each relay so that the operating time of the relay is minimized. Besides the TSM and PSM, the type of network either radial or interconnected system play a vital role for the optimum relay coordination. Primary and back up protection The first line of protection providing a quick and selective clearing of faults in the system is called the primary protection. The protection given to the system when the main protection fails is called back up protection.. Failure of the main protection may be due to any of the following reasons [12]:- A) D.C supply to the tripping circuit fails B) Current or voltage supply to the relay fails C) Tripping mechanism of the circuit breaker fails D) Circuit breaker fails to operate E) Main protective relay fails Method of relay coordination Discrimination by time, discrimination by current and discrimination both by current and time are the three methods used for a correct relay coordination. Though the methods are different from each other but they follow the same aim of isolating only the faulty section of system and leaving the rest of the system undisturbed 1. Discrimination by time In this method, an appropriate time setting keeping the same fault current level is given to each relay controlling the circuit breakers in power system to ensure that the relay nearest to the fault operates first. The relay near the source will have the maximum time compared to the relay at far end from the source. 24

37 Figure 11. Discrimination by time Overcurrent protection is provided at B, C, D and E, that is at the in feed end of each section of the power system. If the fault F occurs the relay B will have least operating time compared to other relay. If relay B is able to clear the fault there is no need of the operation of other relays but if it failed to clear the fault in a given time then the relay C will act as the backup relay for it. 2. Discrimination by current Discrimination by current relies on the fact that the fault current varies with the position of the fault because of the difference in impedance values between the source and the fault. Therefore the relays controlling the various circuit breakers are set to operate at suitably tapered values of current such that only the relay nearest to the fault trips its breaker. [13] Figure 12.Discrimination by current 3. Discrimination by both time and current With this characteristic, the time of operation is inversely proportional to the current level and the actual characteristics is a function of both time and current settings. For a large variation in fault current between the two ends of the feeder, faster operating times can be achieved by the relays nearest to the source, where the fault level is the highest. The selection of overcurrent relay characteristics generally starts with selection of the correct characteristics to be used for each relay, followed by choice of the relay current settings. Finally the grading margins and hence time settings of the relays are determined. 25

38 Requirements for proper relay coordination Relay current setting The minimum current required for the relay to operate is known as relay current setting. Determination of current setting should be in such a way that the relay does not operate for the maximum fault current level but does operate for a minimum fault current level. If the current setting is set for the maximum fault current level in the power system, an overcurrent relay can provide small degree of protection against overload and as well as for fault but the main function of an overcurrent relay is to isolate primary system faults not for the overload protection. [13] Relay time grading margin The minimum time interval between the primary and backup protective relay to achieve proper discrimination between them is known as the time grading margin. If the grading margin is not provided then more than one relay will operate for the same fault leading in failure of the determination of fault location and occurring of blackout in the power system. Time Multiplier setting The operating time of an electrical relay mainly depends upon two factors [14]: 1. How long distance to be traveled by the moving parts of the relay for closing relay contacts and 2. How fast the moving parts of the relay cover this distance. The adjustment of travelling distance of a relay is commonly known as time setting. This adjustment is commonly known as time setting multiplier of relay Theoretical Calculation for overcurrent Relay Setting By using the following algorithm, the relay setting was done. Assuming Plug Setting (PS) of relay 1=100% and TMS=0.025 PS of relay 2> 1.3 PS of relay PS of relay 3> 1.3 PS of relay PSM= I f PS CT ratio 26

39 T= 0.14 PSM TMS Water booster feeder (Time Current graded system) For the time current graded coordination of the overcurrent relay, three phase short-circuit and single phase to ground fault was created on the water booster feeder bus in the software. Using this fault current level the relay setting was done theoretically using the above mentioned formula. Table 6. Theoretical Results of Overcurrent relay setting for 3 phase fault performed Relay Fault current (ka) Plug setting (%) PSM Operation time(t) Relay Relay Relay TMS Table 7. Theoretical Result of Earth fault relay setting for single phase to ground fault Relay Fault current (ka) performed Plug setting (%) PSM Operation time(t) TMS Relay Relay Relay PWD feeder (Time current graded system) For the time current graded coordination of the overcurrent relay, three phase short-circuit and single phase to ground fault was created on the PWD feeder bus in the software. Using this fault current level the relay setting was done theoretically using the above mentioned 27

40 Table 8. Theoritical Results of Overcurrent relay setting for 3 phase fault performed Relay Fault current Plug setting PSM Operation TMS (ka) (%) time(sec) Relay Relay Relay Table 9. Theoretical Result of Earth fault relay setting for single phase to ground fault performed Relay Fault current Plug setting PSM Operation TMS (ka) (%) time(sec) Relay Relay Relay

41 CHAPTER FIVE: SIMULATION USING DIGSILENT POWER FACTORY SOFTWARE DIgSILENT Powerfactory is the most economical solution, as data handling and modelling capabilities which replaces overall functionality of other software systems. It has all-in-one powerfactory solution which promotes highly optimized work flow. It is the tool which combines reliability and flexibility. The PowerFactory data base environment fully integrates all data required for defining cases, scenarios, single line graphics, outputs, run conditions, calculation options, graphics, user defined models, etc. as an example, the power flow, fault analysis, and harmonic load flow analysis tools can be executed sequentially without resetting the program. For the load flow analysis, the method chosen was AC balanced, positive sequence and the formulation Newton-Raphson with power equations. 5.1 Feeder 1: Water Booster Feeder Line loss in water booster feeder Figure 13: Water booster feeder model in DIgSILENT PowerFactory software 29

42 Loss(kW) Figure 14. Line losses in feeder 1 for the month of February 2015 using DIgSILENT PowerFactory software. From the graph it was observed that the 11kv outgoing line 2 has maximum line losses of 0.17kW and RSTA line with minimum loss of kW.It is due to the length and resistance of the line Transformer Losses Since actual loading of the transformer was not available from BPC, the load capacity based on peak load (February 2015) was taken. Losses(kW) 8% 6% 6% 8% 4% 8% 8% 8% 30% 8% 6% sector 2 Awp waterbooster USS Dratshang lower market Nppf RSTA Norgay Dhuti khola Choden engg Tashi engg Figure 15.Transformer Losses in feeder 1 for the month of February 2015 using DIgSILENT PowerFactory software. 30

43 The maximum losses was found in sector 2 transformer(5mva) and minimum in Choden enng transformer (315kVA).The transformer with high capacity has maximum losses as it has high fixed and copper losses. 5.2 Feeder 2: PWD Feeder Line losses in PWD feeder Figure 16: PWD Feeder model in DIgSILENT PowerFactory software 31

44 Losses (kw) Figure 17. Line losses in feeder 2 for the month of February 2015 using DIgSILENT PowerFactory software. The maximum losses is found in PWD outgoing line with 0.39kW and minimum in RRCO line with kW Transformer losses Losses(kW) Figure 18. Transformer losses in feeder 2 for the month of February 2015 using DIgSILENT PowerFactory software. 32