Comparison Between Distance And Differential

COMPARISON BETWEEN DISTANCE AND DIFFERENTIAL LINE PROTECTION IN NEW NUMERICAL RELAY

By MAGID ABDELHALIM AWAD ABDELHALIM MOHAMMED MAHDI ABDELRAHEEM

Supervisor Dr. Kamal Ramadan

A REPORT SUBMITTED TO University Of Khartoum In partial fulfillment for the degree of B.Sc. (HONS) Electrical and Electronics Engineering (POWER SYSTEMS ENGINEERING)

Faculty of Engineering Department of Electrical and Electronics Engineering October, 2017

DECLARATION OF ORGINALITY

I declare this report entitled “Comparison between Distance and Differential Line Protection in New Numerical Relay” is my own work except as cited in references. The report has been not accepted for any degree and it is not being submitted currently in candidature for any degree or other reward. Signature: ____________________ Name: _______________________ Date: _______________________

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ACKNOWLEDGEMENT First and foremost, all praise and thanks to Allah for providing me with this opportunity and granting me the strength to proceed successfully. To my mother and father, who grew me up, fed me and guided me through life. I take immense pleasure to express my sincere and deep sense of gratitude to my supervisor, Dr. Kamal Ramadan, for his guidance and support through this project. This thesis would not have been completed without his expert advice and unfailing patience. One must appreciate good people; they are hard to come by, Eng. Elsheikh kamal, Eng. Abdullah Elnour and Eng. Azza Khidir . Thank you for sharing your precious time with us. Many thanks to my colleagues Mohammed Alkhatim and all 0124 for the pleasant working atmosphere and your friendship. Last word of thanks to my family and everyone who supported me during the completion of the project.

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ABSTRACT Growth and urbanization in Sudan have - over the years - resulted in the demand for electricity’s rising to higher proportions. This results in load bases being centralized in certain geographical areas. Due to firm supply requirements of these loads, HV power lines are interconnected to form a ring network. HV lines are traditionally protected by distance schemes, due to their performance over the years. However the distance schemes have underperformed in some instances. Numerical relays are the result of the application of microprocessor technology in relay industry. Numerical relays have the ability to communicate with their peers, economical, easy to operate adjust and repair. Modeling of numerical relays is important to adjust and settle protection equipment in electrical facilities. Electrical protection systems are one of the most important elements for the continuity of supply. Due to the networks that are connected in the power systems by the transmission lines, transmission lines must be especially protected. In this study, differential protection and distance protection, two of the most important principles in the protection of transmission lines, are examined and the working principles of these protection structures are presented. The work of these two protection schemes are compared in different feature faults created on “Algadarif-Alfau” transmission line by means of MATLAB/SIMULINK program.

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‫المستخلص‬ ‫أدى النمو والتحضر في السودان ‪ -‬على مر السنين ‪ -‬إلى ارتفاع الطلب على الكهرباء إلى نسب أعلى‪ .‬ويؤدي ذلك إلى تمركز‬ ‫األحمال في بعض المناطق الجغرافية‪ .‬ونظرا لمتطلبات اإلمداد الصارمة لهذه األحمال‪ ،‬تكون خطوط النقل ذات الجهد العالي‬ ‫مترابطة لتشكل حلقة‪ .‬خطوط الجهد العالي محمية تقليديا من قبل مرحالت المسافة‪ ،‬وذلك بسبب أدائها على مر السنين‪ .‬غير أن‬ ‫مرحالت المسافة لم تحقق أداء جيدا في بعض الحاالت‪.‬‬ ‫المرحالت العددية هي نتيجة لتطبيق تكنولوجيا المعالجات الدقيقة في صناعة المرحالت‪ .‬المرحالت العددية لديها القدرة على‬ ‫التواصل مع أقرانها‪ ،‬اقتصادية و سهلة التشغيل‪ ،‬الضبط واإلصال‪ ..‬تعد نمذجة المرحالت العددية مهمة لضبط وتسوية معدات‬ ‫الحماية في المنشآت الكهربائية‪.‬‬ ‫أنظمة الحماية الكهربائية هي واحدة من أهم العناصر الستمرارية اإلمداد‪ .‬نظرا للشبكات المتصلة في أنظمة الطاقة بواسطة‬ ‫خطوط النقل‪ ،‬يجب أن تكون خطوط النقل محمية بشكل خاص‪ .‬في هذه الدراسة‪ ،‬تم فحص الحماية التفاضلية وحماية المسافة‪،‬‬ ‫وهما من أهم المبادئ في حماية خطوط النقل‪ ،‬و تم عرض مبادئ عمل هياكل هذه الحمايات‪ .‬تمت مقارنة عمل هذين النظامين‬ ‫للحماية في أخطاء مختلفة تم إنشاؤها على خط نقل "القضارف‪-‬الفاو" من خالل برنامج ‪.MATLAB/SIMULINK‬‬

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TABLE OF CONTENTS DECLARATION OF ORGINALITY ............................................................................................. i ACKNOWLEDGEMENT .............................................................................................................. ii Abstract .......................................................................................................................................... iii ‫ المستخلص‬........................................................................................................................................... iv

List of Figures ................................................................................................................................ ix List of Tables ................................................................................................................................. xi List of equations............................................................................................................................ xii List of Abbreviation ..................................................................................................................... xiii CHAPTER ONE ............................................................................................................................. 1 Introduction ..................................................................................................................................... 1 Overview ......................................................................................................................... 1 Problem Statement .......................................................................................................... 1 Objectives ....................................................................................................................... 1 Thesis Layout .................................................................................................................. 2 CHAPTER TWO ............................................................................................................................ 3 Literature Review ........................................................................................................................... 3 Introduction ..................................................................................................................... 3 Fault Types...................................................................................................................... 3 Protection Definitions ..................................................................................................... 6 Protection System ..................................................................................................... 6 Protection Equipment................................................................................................ 6 Protection Scheme .................................................................................................... 7

v

Instrument Transformers ................................................................................................. 7 Current Transformer ................................................................................................. 7 Voltage Transformer ................................................................................................. 8 Protection Quality ......................................................................................................... 11 Zones of Protection ................................................................................................. 11 Design ..................................................................................................................... 13 Settings .................................................................................................................... 14 Installation............................................................................................................... 14 Testing..................................................................................................................... 14 Deterioration in Service .......................................................................................... 14 Protection Performance ........................................................................................... 15 Selectivity ............................................................................................................... 16 Stability ................................................................................................................... 16 Speed ....................................................................................................................... 17 Sensitivity ............................................................................................................... 17 Primary and Back-Up Protection .................................................................................. 17 Fuses ............................................................................................................................. 18 Over Current Protection ................................................................................................ 18 Non-directional Over-current Protection ................................................................ 18 Directional Over-current Protection ....................................................................... 18 Differential Protection .................................................................................................. 19 Distance Protection ....................................................................................................... 20 Introduction ............................................................................................................. 20

vi

Principle of Distance Relay .................................................................................... 20 Distance Protection for Phase Faults ...................................................................... 20 Relays ............................................................................................................................ 22 Electromechanical Relays ....................................................................................... 23 Static Relays............................................................................................................ 23 Digital Relays.......................................................................................................... 24 Numerical Relay ..................................................................................................... 24 CHAPTER THREE ...................................................................................................................... 26 Methodology ................................................................................................................................. 26 Introduction ................................................................................................................... 26 Transmission Line Protection Schemes ........................................................................ 26 Distance Protection ................................................................................................. 26 Differential Protection ............................................................................................ 34 CHAPTER FOURE ...................................................................................................................... 40 Implementation and Results.......................................................................................................... 40 Introduction ................................................................................................................... 40 MATLAB Software ...................................................................................................... 40 Algadarif-Alfau Transmission Line Review ................................................................. 41 Distance Protection ....................................................................................................... 43 Relay Settings ......................................................................................................... 43 Simulink Model ...................................................................................................... 43 Case One: Three Phase to Ground Faults at Different Distances from The Relay Location ……………………………………………………………………………………..45 Case Two: Three Phase to Ground Faults with Fault Resistance ........................... 47

vii

Differential Protection .................................................................................................. 48 Relay Settings ......................................................................................................... 48 Simulink Model ...................................................................................................... 48 Case One: Three Phase to Ground Faults at Different Distances from The Relay Location ……………………………………………………………………………………..50 Case Two: Three Phase to Ground Faults with Fault Resistance ........................... 51 Results Discussion ........................................................................................................ 52 CHAPTER FIVE .......................................................................................................................... 55 Conclusion and recommendations ................................................................................................ 55 Conclusion .................................................................................................................... 55 Recommendations ......................................................................................................... 55 Refreneces ..................................................................................................................................... 57 APPENDEX A .............................................................................................................................. 58

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LIST OF FIGURES Figure 2. 1 shows the main types of faults on a three-phase system .............................................. 4 Figure 2. 2 wound primary.............................................................................................................. 8 Figure 2. 3 Bar primary................................................................................................................... 8 Figure 2. 4 Electromagnetic Type ................................................................................................... 9 Figure 2. 5 Capacitor-Type VT ..................................................................................................... 10 Figure 2. 6 Division of power system into protection zones ........................................................ 11 Figure 2. 7 CT locations................................................................................................................ 12 Figure 2. 8 overlapping zones of protection system ..................................................................... 13 Figure 2. 9 Simple differential protections ................................................................................... 19 Figure 2. 10 Protection zones with distance relays ....................................................................... 22

Figure 3. 1 MHO Distance Relay Algorithm ................................................................................ 28 Figure 3. 2Fault detection block ................................................................................................... 29 Figure 3. 3 Logical fault detection scheme ................................................................................... 30 Figure 3. 4 Apparent impedance model for SLG Fault ................................................................ 32 Figure 3. 5 Apparent impedance model for DLG Fault ................................................................ 32 Figure 3. 6 Zone coordination Subsystem .................................................................................... 33 Figure 3. 7 Mho shape characteristics........................................................................................... 34 Figure 3. 8 The current entering and exiting a node ..................................................................... 35 Figure 3. 9 External faults ............................................................................................................. 36 Figure 3. 10 Internal faults ............................................................................................................ 37 Figure 3. 11 Protected area and tele-protection interface ............................................................. 38

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Figure 3. 12 Differential Relay tripping characteristic ................................................................. 39

Figure 4. 1 Single line diagram of Algadarif-Alfao transmission line ......................................... 41 Figure 4. 2 Overall simulation model ........................................................................................... 44 Figure 4. 3 Distance relay model .................................................................................................. 44 Figure 4. 4 R-jX plot Impedance for a fault at 51 km distance .................................................... 45 Figure 4. 5 R-jX plot Impedance for a fault at 102 km distance .................................................. 46 Figure 4. 6 R-jX plot Impedance for a fault at 153 km distance .................................................. 46 Figure 4. 7 R-jX plot Impedance for a fault at 51 km distance (33%) with 20 Ω resistance ........ 47 Figure 4. 8 R-jX plot Impedance for a fault at 51 km distance (33%) with 20 Ω resistance ........ 48 Figure 4. 9 Overall simulation model ........................................................................................... 49 Figure 4. 10 Differential relay model ........................................................................................... 50 Figure 4. 11 current wave form at bus A and B for three phase to ground fault .......................... 50 Figure 4. 12 trip signal as a correct reaction to the fault............................................................... 51 Figure 4. 13 current wave forms at bus A and B for three phase to ground fault with fault resistances 20 and 50 Ω ................................................................................................................ 51 Figure 4. 14 trip signal as a correct reaction to the fault............................................................... 52

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LIST OF TABLES Table 2. 1 Fault Statistics With Reference to Type of Fault ........................................................... 5 Table 2. 2 Fault Statistics With Reference to Power System Elements .......................................... 5

Table 3. 1 Fault impedance Algorithm for various fault types ..................................................... 31

Table 4. 1 Algadarif- Alfao Line Parameters ............................................................................... 42 Table 4. 2 Relay Settings .............................................................................................................. 43 Table 4. 3 Simulation results summery ........................................................................................ 53

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LIST OF EQUATIONS k0 = (Z0-Z+)/KZ+

3. 1 .................................................................................................... 32

I0 = (Vs / Z0+2Z+)

3. 2 .................................................................................................... 32

IS1 = a1 * IP1 – IS1e

3. 3 ..................................................................................................... 35

IS2 = a2 * IP2 – IS2e

3. 4 ..................................................................................................... 35

IDIFF = | IP1 + IP2| = |IF - IF |

3. 5 ..................................................................................................... 36

IDIFF = | IP1 + IP2| = |I1F + I2F | 3. 6 .................................................................................................... 37

xii

LIST OF ABBREVIATION

A

Ampere

A/D

Analogue To Digital Conversion

ABC

Three Phase Fault

ABCG

Three Phase To Ground Fault

CB

Circuit breaker

CT

Current Transformer

CVT

Capacitor Voltage Transformer

DC

Direct Current

DFT

Discrete Fourier Transform

DLG

Double Line To Ground

DSP

Digital Signal Processor

EHV

Extra High Voltage

HV

High Voltage

I/O

Input Output

IA

Phase A Current

IS

Differential relay restrain current

xiii

K0

Residual compensation factor

kV

Kilo Volt

L-G

Line To Ground

LV

Low Voltage

M-file

MATLAB File

R

Resistance

SCADA

Supervisory control and data acquisition

SLG

Single Line To Ground

VA

Phase A Voltage

Vs

Phase voltage during the phase to ground fault

VT

Voltage transformer

X

Reactance

Z

Line Impedance

Z1

Zone 1 Impedance

Z+

Line positive-sequence impedance

Z2

Zone 2 Impedance

Z3

Zone 3 Impedance

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CHAPTER ONE CHAPTER ONE INTRODUCTION Overview This chapter discusses project’s problem Statement, motivation, objectives and methodology used. In addition, an overview of the thesis layout is provided.

Problem Statement Transmission lines are situated in wide area these lines unfortunately struck with severe geographical as well as atmospheric conditions; due to this several faults took place. EHV lines are the very most important aspect in power transportation and to keep healthy operation continuously becomes very crucial task for maintaining system reliability. That is why prevention of transmission system is necessary in modern power system. Due to the occurrence of more than 80% of disturbances or short-circuit faults in an overhead line, this section has become the most vulnerable part of the electrical system. Therefore, it is necessary having designed a power system protection include speed, selectivity, sensitivity, security, dependability and reliability to protect against disturbances.

Objectives The main objective of this project is to minimize damage to transmission lines that would be caused by system faults, if residues, and maintain the delivery of electrical energy to the consumers. In order to achieve these objectives MATLAB/SIMULINK model of both distance and differential relays is developed, the same test scenarios were performed on both models in order to compare between their performances to decide which protection scheme between them is the most efficient to implement.

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Introduction Thesis Layout This report is organized in 5 chapters including this introduction as follows: Chapter 2 (Literature Review): This chapter provides overall discussion of all protection systems and schemes. Chapter 3 (Methodology): This chapter explains the building process of Simulink model for distance and differential relays. Chapter 4(Implementation and Results): This chapter carries out the overall design and the simulation model of both distance and differential relays in case of Algadarif-Alfau transmission line in MATLAB/SIMULINK. Also contains results of the simulation and results discussion. Chapter 5 (Conclusion): This chapter shows a conclusion for the result obtained, features and limitations of the implementation and recommendations.

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CHAPTER TWO CHAPTER TWO LITERATURE REVIEW

Introduction The purpose of an electrical power system is to generate and supply electrical energy to consumers. The system should be designed and managed to deliver this energy to the utilization points with both reliability and economy. Severe disruption to the normal routine of modern society is likely if power outages are frequent or prolonged, placing an increasing emphasis on reliability and security of supply. As the requirements of reliability and economy are largely opposed, power system design is inevitably a compromise. Many items of equipment are very expensive, and so the complete power system represents a very large capital investment. To maximize the return on this outlay, the system must be utilized as much as possible within the applicable constraints of security and reliability of supply. More fundamental, however, is that the power system should operate in a safe manner at all times. No matter how well designed, faults will always occur on a power system, and these faults may represent a risk to life and/or property. The destructive power of a fault arc carrying a high current is very great; it can burn through copper conductors or weld together core laminations in a transformer or machine in a very short time – some tens or hundreds of milliseconds. This is the measure of the importance of protection systems as applied in power system practice and of the responsibility vested in the Protection Engineer. The provision of adequate protection to detect and disconnect elements of the power system in the event of fault is therefore an integral part of power system design. Only by so doing can the objectives of the power system be met and the investment protected [1]

Fault Types In electrical power system faults are classified as: 1) Three phase fault

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Literature Review 2) Three phase to ground fault 3) Single phase to ground fault 4) Phase to Phase fault 5) Double Phase to earth fault

Figure 2. 1 shows the main types of faults on a three-phase system

(A) Phase-to-earth fault; (B) Phase-to-phase fault; (C) Phase-to phase- to-earth fault; (D) Threephase fault; (E) Three-phase-to-earth fault. Power systems have been in operation for over a hundred years now. Accumulated experience shows that all faults are not equally likely. Single line to ground faults (L-G)are the most likely whereas the fault due to simultaneous short circuit between all the three lines, known as the threephase fault(L-L-L), is the least likely. This is depicted in Table 2.1[2]

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Literature Review

Table 2. 1 Fault Statistics With Reference to Type of Fault

Fault

Probability of occurrence (%)

Severity

Line-to-Ground

85

Least severe

Line-to-line

8

Line-to-line-to-Ground

5

Line-to-line-to-line

2

Most severe

The probabilities of faults on different elements of the power system are different. The transmission lines which are exposed to the vagaries of the atmosphere are the most likely to be subjected to faults. Table 2.2: Fault Statistics With Reference to Power System Elements.

Table 2. 2 Fault Statistics With Reference to Power System Elements

Power system element

Probability of faults (%)

Overhead lines

50

Underground cables

9

5

Literature Review

Transformers

10

Generators

7

Switchgear

12

CT, PT relays, control equipment, etc.

12

The severity of the fault can be expressed in terms of the magnitude of the fault current and hence it’s potential for causing damage. In the power system, the three-phase fault is the most severe whereas the single line-to-ground fault is the least severe. [2]

Protection Definitions The definitions that follow are generally used in relation to power system protection:

Protection System A complete arrangement of protection equipment and other devices required to achieve a specified function based on a protection principal (IEC 60255-20)

Protection Equipment A collection of protection devices (relays, fuses, etc.).Excluded are devices such as CT’s, CB’s, Contactors, etc.

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Literature Review Protection Scheme A collection of protection equipment providing a defied function and including all equipment required to make the scheme work (i.e. relays, CT’s, CB’s, batteries, etc.) In order to fulfill the requirements of protection with the optimum speed for the many different configurations, operating conditions and construction features of power systems, it has been necessary to develop many types of relay that respond to various functions of the power system quantities. [1]

Instrument Transformers Current transformers and voltage transformers form a very important link between the power system and the protective system. These instrument transformers basically extract the information regarding current and voltage from the power system under protection and pass it on to the protective relays. While doing this, they insulate the low-voltage protective system (both personnel and protective apparatus) from the high-voltage power system. [3]

Current Transformer All current transformers used in protection are basically similar in construction to standard transformers in that they consist of magnetically coupled primary and secondary windings, wound on a common iron core, the primary winding being connected in series with the network unlike voltage transformers. They must therefore withstand the networks short-circuit current. There are two types of current transformers: 1. Wound primary type 2. Bar primary type. Wound type CT is shown in Figure 2.2. The wound primary is used for the smaller currents, but it can only be applied on low fault level installations due to thermal limitations as well as structural requirements due to high magnetic forces. For currents greater than 100 A, the bar primary type is used as shown in Figure 2.6. If the

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Literature Review secondary winding is evenly distributed around the complete iron core, its leakage reactance is eliminated. [2]

Figure 2. 2 Wound primary

Figure 2. 3 Bar primary

Voltage Transformer There are basically, two types of voltage transformers used for protection equipment: 1) Electromagnetic type (commonly referred to as a VT)

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Literature Review 2) Capacitor type (referred to as a CVT). The electromagnetic type is a step down transformer whose primary (HV) and secondary (LV) windings are connected as shown in figure 2.4

Figure 2. 4 Electromagnetic Type

The number of turns in a winding is directly proportional to the open-circuit voltage being measured or produced across it. The above diagram is a single-phase VT. In the three-phase system it is necessary to use three VTs at one per phase and they being connected in star or delta depending on the method of connection of the main power source being monitored. This type of electromagnetic transformers are used in voltage circuits up to 110/132 kV. For still higher voltages, it is common to adopt the second type namely the capacitor voltage transformer (CVT). Figure 2.5 below gives the basic connection adopted in this type. Here the primary portion consists of capacitors connected in series to split the primary voltage to convenient values.

The magnetic voltage transformer is similar to a power transformer and differs only so far as a different emphasis is placed on cooling, insulating and mechanical aspects. The primary winding has larger number of turns and is connected across the line voltage; either phase-to-phase or phase-to-neutral. The secondary has lesser turns however, the volts per turn on both primary and secondary remains same.

9

Literature Review The capacitor VT is more commonly used on extra high-voltage (EHV) networks. The capacitors also allow the injection of a high-frequency signals onto the power line conductors to provide endto-end communications between substations for distance relays, telemetry/supervisory and voice communications. Hence, in EHV national grid networks of utilities, the CVTs are commonly used for both protection and communication purposes.

Figure 2. 5 Capacitor-Type VT

Where fuses are unsuitable or inadequate, protective relays and circuit breakers are used in combination to detect and isolate faults. Circuit breakers are the main making and breaking devices in an electrical circuit to allow or disallow flow of power from source to the load. These carry the load currents continuously and are expected to be switched ON with loads (making capacity). These should also be capable of breaking a live circuit under normal switching OFF conditions as well as under fault conditions carrying the expected fault current until completely isolating the fault side (rupturing/breaking capacity). Under fault conditions, the breakers should be able to open by instructions from monitoring devices like relays. The relay contacts are used in the making and breaking control circuits of a

10

Literature Review circuit breaker, to prevent breakers getting closed or to trip breaker under fault conditions as well as for some other interlocks. The types of breakers basically refer to the medium in which the breaker opens and closes. The medium could be oil, air, vacuum or SF6. The further classification is single break and double break. In a single break type only the bus bar end is isolated but in a double break type, both bus bar (source) and cable (load) ends are broken. However, the double break is the most common and accepted type in modern installations. [2]

Protection Quality In this section we will mention power system protection – basic requirements.

Zones of Protection To limit the extent of the power system that is disconnected when a fault occurs, protection is arranged in zones. The principle is shown in Figure 2.6.

Figure 2. 6 Division of power system into protection zones

11

Literature Review

Ideally, the zones of protection should overlap, so that no part of the power system is left unprotected. The circuit breaker being included in both zones. For practical physical and economic reasons, this ideal is not always achieved, accommodation for current transformers being in some cases available only on one side of the circuit breakers. This leaves a section between the current transformers and the circuit breaker that is not completely protected against faults. In Figure 2.7(b) a fault at F would cause the bus bar protection to operate and open the circuit breaker but the fault may continue to be fed through the feeder. The feeder protection, if of the unit type, would not operate, since the fault is outside its zone. This problem is dealt with by inter-tripping or some form of zone extension, to ensure that the remote end of the feeder is tripped also.

Figure 2. 7 CT locations

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Literature Review The point of connection of the protection with the power system usually defines the zone and corresponds to the location of the current transformers. Unit type protection will result in the boundary being a clearly defined closed loop. Figure 2.8 illustrates a typical arrangement of overlapping zones. [1]

Figure 2. 8 Overlapping zones of protection system

Incorrect operation can be attributed to one of the following classifications:

i.

Incorrect design/settings.

ii.

Incorrect installation/testing.

iii.

Deterioration in service.

Design The design of a protection scheme is of paramount importance. This is to ensure that the system will operate under all required conditions, and (equally important) refrain from operating when so required (including, where appropriate, being restrained from operating for faults external to the zone being protected).

13

Literature Review Settings It is essential to ensure that settings are chosen for protection relays and systems which take into account the parameters of the primary system, including fault and load levels, and dynamic performance requirements etc. The characteristics of power systems change with time, due to changes in loads, location, type and amount of generation, etc. Therefore, setting values of relays may need to be checked at suitable intervals to ensure that they are still appropriate. Otherwise, unwanted operation or failure to operate when required may occur.

Installation The need for correct installation of protection systems is obvious, but the complexity of the interconnections of many systems and their relationship to the remainder of the installation may make checking difficult. Site testing is therefore necessary; since it will be difficult to reproduce all fault conditions correctly, these tests must be directed to proving the installation. The tests should be limited to such simple and direct tests as will prove the correctness of the connections, relay settings, and freedom from damage of the equipment.

Testing Comprehensive testing is just as important, and this testing should cover all aspects of the protection scheme, as well as reproducing operational and environmental conditions as closely as possible. Type testing of protection equipment to recognized standards fulfills many of these requirements, but it may still be necessary to test the complete protection scheme (relays, current transformers and other ancillary items) and the tests must simulate fault conditions realistically.

Deterioration in Service Subsequent to installation in perfect condition, deterioration of equipment will take place and may eventually interfere with correct functioning. For example, contacts may become rough or burnt owing to frequent operation, or tarnished owing to atmospheric contamination; coils and other

14

Literature Review circuits may become open-circuited, electronic components and auxiliary devices may fail, and mechanical parts may seize up. The time between operations of protection relays may be years rather than days. During this period defects may have developed unnoticed until revealed by the failure of the protection to respond to a power system fault. For this reason, relays should be regularly tested in order to check for correct functioning.

Protection Performance Protection system performance is frequently assessed statistically. For this purpose each system fault is classed as an incident and only those that are cleared by the tripping of the correct circuit breakers are classed as ‘correct’. The percentage of correct clearances can then be determined. This principle of assessment gives an accurate evaluation of the protection of the system as a whole, but it is severe in its judgment of relay performance. If the level of reliability achieved by a single device is not acceptable, improvement can be achieved through redundancy, e.g. duplication of equipment. Two complete, independent, main protection systems are provided, and arranged so that either by itself can carry out the required function. If the probability of each equipment failing is X/unit, the resultant probability of both equipments failing simultaneously, allowing for redundancy, is (X2). Where X is small the resultant risk (X2) may be negligible. Where multiple protection systems are used, the tripping signal can be provided in a number of different ways. The two most common methods are:

i.

All protection systems must operate for a tripping operation to occur (e.g. ‘two-out-of-two’ arrangement).

ii.

Only one protection system need operate to cause a trip (e.g. ‘one out of two’ arrangement) the former method guards against mal-operation while the latter guards against failure to operate due to an unrevealed fault in a protection system. Rarely, three main protection systems are provided, configured in a ‘two out of three’ tripping arrangement, to provide both reliability of tripping, and security against unwanted tripping.

15

Literature Review Selectivity When a fault occurs, the protection scheme is required to trip only those circuit breakers whose operation is required to isolate the fault. This property of selective tripping is also called ‘discrimination’ and is achieved by two general methods.

Time Grading Protection systems in successive zones are arranged to operate in times that are graded through the sequence of equipments so that upon the occurrence of a fault, although a number of protection equipments respond, only those relevant to the faulty zone complete the tripping function. The others make incomplete operations and then reset. The speed of response will often depend on the severity of the fault, and will generally be slower than for a unit system.

Unit Systems It is possible to design protection systems that respond only to fault conditions occurring within a clearly defined zone. This type of protection system is known as ‘unit protection’. Certain types of unit protection are known by specified names, e.g. restricted earth fault and differential protection. Unit protection can be applied throughout a power system and, since it does not involve time grading, is relatively fast in operation. The speed of response is substantially independent of fault severity.

Stability The term ‘stability’ is usually associated with unit protection schemes and refers to the ability of the protection system to remain unaffected by conditions external to the protected zone, for example through load current and external fault conditions.

16

Literature Review Speed The function of protection systems is to isolate faults on the power system as rapidly as possible. The main objective is to safeguard continuity of supply by removing each disturbance before it leads to widespread loss of synchronism and consequent collapse of the power system.

Sensitivity Sensitivity is a term frequently used when referring to the minimum operating level (current, voltage, power etc.) of relays or complete protection schemes. The relay or scheme is said to be sensitive if the primary operating parameter(s) is low. With older electromechanical relays, sensitivity was considered in terms of the sensitivity of the measuring movement and was measured in terms of its volt-ampere consumption to cause operation. With modern digital and numerical relays the achievable sensitivity is seldom limited by the device design but by its application and CT/VT parameters. [1]

Primary and Back-Up Protection The reliability of a power system has been discussed earlier, including the use of more than one primary (or ‘main’) protection system operating in parallel. In the event of failure or nonavailability of the primary protection some other means of ensuring that the fault is isolated must be provided. These secondary systems are referred to as ‘back-up protection’. Back-up protection may be considered as either being ‘local’ or ‘remote’. Local back-up protection is achieved by protection which detects an un-cleared primary system fault at its own location and which then trips its own circuit breakers, e.g. time graded overcurrent relays. Remote back-up protection is provided by protection that detects an un-cleared primary system fault at a remote location and then issues a local trip command, e.g. the second or third zones of a distance relay. In both cases the main and back-up protection systems detect a fault simultaneously, operation of the back-up protection being delayed to ensure that the primary protection clears the fault if possible. [3]

17

Literature Review Fuses Probably the oldest, simplest, cheapest, and most-often used type of protection device is the fuse. The operation of a fuse is very straightforward: The thermal energy of the excessive current causes the fuse-element to melt and the current path is interrupted. Technological developments have made fuses more predictable, faster, and safer (not to explode). [4]

Over Current Protection The term “overcurrent” refers to abnormal current flow higher than the normal value of current flow in an electrical circuit. Uncorrected “overcurrent” can cause serious safety hazards and costly damage to electrical equipment and property. There are three basic types of current flow in an electrical circuit: i.

Normal intended current flow to operate electrical equipment.

ii.

Abnormal overcurrent flow with a value of up to 10 times normal current flow. This is known as an “overload”.

iii.

Abnormal overcurrent flow with a value more than 10 times the normal current flow is known as “short-circuit” or “fault” current flow

Non-directional Over-current Protection It depends on only the magnitude of the current, without taking any cognizance of its phase angle. [5]

Directional Over-current Protection It takes into account, not only the magnitude of the current but also its phase with respect to the voltage at the relay location. It discriminates between faults in front of the breaker and faults behind the breaker (better selectivity). [5]

18

Literature Review Differential Protection Differential protection is based on the fact that any fault within an electrical equipment would cause the current entering it, to be different, from that leaving it. Thus, we can compare the two currents either in magnitude or in phase or both and issue a trip output if the difference exceeds a predetermined set value. This method of detecting faults is very attractive when both ends of the apparatus are physically located near each other. A typical situation, where this is true, is in the case of a transformer, a generator or a bus bar. In the case of transmission lines, the ends are too far apart for conventional differential relaying to be directly applied. For the operating condition of normal load flow shown in Figure 2.14, the currents transformed by the two CTs, being equal in magnitude as well as in phase, just circulate on the secondary side. There is no tendency for the current to spill into the over-current relay. The overcurrent relay connected in the spill path is wired to trip the two circuit breakers on either side of the equipment being protected. [5]

Figure 2. 9 Simple differential protections

19

Literature Review Distance Protection This section is general discussion about distance protection Introduction Distance protection is mainly used in transmission applications as an alternative, and often complementary, to differential protection, especially for very long lines.

Distance protection basically operates on the principle that the impedance of a line is stable under healthy line conditions. An electrical fault on the line will cause the impedance to alter. This will result in a shift of the voltage and current phasors with respect to one another. This shift will be detected by the relay. [4]

Principle of Distance Relay The basic principle of distance protection involves the division of the voltage at the relaying point by the measured current. The apparent impedance so calculated is compared with the reach point impedance. If the measured impedance is less than the reach point impedance, it is assumed that a fault exists on the line between the relay and the reach point. [5]

Distance Protection for Phase Faults Zone 1 at each end of the line provides the most desirable protection simultaneous high- speed operation for the middle 80 % of the line section. This can be increased to 100 % only with pilot relaying. Zone 2 set for 50 % of line HR will cover only a small percentage of line HS. Setting for 50 % of line HS would result in possibly overreaching and miss coordinating with Z2 of line HR, unless T2 time was increased. Undesired operation of zone 3 distance relays, applied for remote backup protection during major system disturbances, has caused the magnitude of the scope of such disturbance s to be

20

Literature Review expanded. Large ohmic settings had been typically applied to the zone 3 relaying, in order to obtain the desired backup protection. Power swings and low voltage conditions that often exist during system disturbance s resulted in the impedance seen by the zone 3 relay to be within its operating characteristic for a sufficient length of time for it to initiate a trip command. Such experiences have resulted in utilities restricting the use or reach applied to zone 3 relaying. These zones and typical settings are illustrated in Figure 12.15. Figure 12.15a shows the zones at several locations. Figure 12.15b shows the operating circles for the three zone at bus G, breaker 1 (solid line) and at bus H, breaker 2 (bro ken line), plot ted on the R–X diagram. The several lines are shown at their respective (r+jx) positions. [3]

(a) Time–distance plot

21

Literature Review

(b) R–X diagram plot

Figure 2. 10 Protection zones with distance relays

Relays The most versatile and sophisticated type of protection available today, is undoubtedly the relay/circuit-breaker combination. The relay receives information regarding the network mainly from the instrument transformers (voltage and current transformers), detects an abnormal condition by comparing this information to pre-set values, and gives a tripping command to the circuit-breaker when such an abnormal condition has been detected. The relay may also be operated by an external tripping signal, either from other instruments, from a SCADA master, or by human intervention. [4]

22

Literature Review

Electromechanical Relays These relays were the earliest forms of relay used for the protection of power systems, and they date back nearly 100 years. They work on the principle of a mechanical force causing operation of a relay contact in response to a stimulus. The mechanical force is generated through current flow in one or more windings on a magnetic core or cores, hence the term electromechanical relay The principle advantage of such relays is that they provide galvanic isolation between the inputs and outputs in a simple, cheap and reliable form – therefore for simple on/off switching functions where the output contacts have to carry substantial currents, they are still used. Electromechanical relays can be classified into several different types as follows: i.

Attracted armature.

ii.

Moving coil.

iii.

Induction.

iv.

Thermal.

v.

Motor operated.

vi.

Mechanical.

However, only attracted armature types have significant application at this time, all other types having been superseded by more modern equivalents. [1]

Static Relays The term ‘static’ implies that the relay has no moving parts. This is not strictly the case for a static relay, as the output contacts are still generally attracted armature relays. In a protection relay, the term ‘static’ refers to the absence of moving parts to create the relay characteristic. Introduction of static relays began in the early 1960’s. Their design is based on the use of analogue electronic devices instead of coil sand magnets to create the relay characteristic. Early versions used discrete devices such as transistors and diodes in conjunction with resistors,

23

Literature Review capacitors, inductors, etc., but advances in electronics enabled the use of linear and digital integrated circuits in later versions for signal processing and implementation of logic functions. They therefore can be viewed in simple terms as an analogue electronic replacement for electromechanical relays, with some additional flexibility in settings and some saving in space requirements. In some cases, relay burden is reduced, making for reduced CT/VT output requirements. [1]

Digital Relays Digital protection relays introduced a step change in technology. Microprocessors and microcontrollers replaced analogue circuits used in static relays to implement relay functions. Early examples began to be introduced into service around 1980, and, with improvements in processing capacity, can still be regarded as current technology for many relay applications. However, such technology will be completely superseded within the next five years by numerical relays. Compared to static relays, digital relays introduce A/D conversion of all measured analogue quantities and use a microprocessor to implement the protection algorithm. The microprocessor may use some kind of counting technique, or use the Discrete Fourier Transform (DFT) to implement the algorithm. However, the typical microprocessors used have limited processing capacity and memory compared to that provided in numerical relays. The functionality tends therefore to be limited and restricted largely to the protection function itself. Additional functionality compared to that provided by an electromechanical or static relay is usually available, typically taking the form of a wider range of settings, and greater accuracy. A communications link to a remote computer may also be provided. [1]

Numerical Relay The distinction between digital and numerical relay rests on points of fie technical detail, and is rarely found in areas other than Protection. They can be viewed as natural developments of digital relays as a result of advances in technology. Typically, they use a specialized digital signal processor (DSP) as the computational hardware, together with the associated software

24

Literature Review tools. The input analogue signals are converted into a digital representation and processed according to the appropriate mathematical algorithm. Processing is carried out using a specialized microprocessor that is optimized for signal processing applications, known as a digital signal processor or DSP for short. Digital processing of signals in real time requires a very high power microprocessor. In addition, the continuing reduction in the cost of microprocessors and related digital devices (memory, I/O, etc.) naturally leads to an approach where a single item of hardware is used to provide a range of functions (‘one-box solution’ approach). By using multiple microprocessors to provide the necessary computational performance, a large number of functions previously implemented in separate items of hardware can now be included within a single item. [1]

25

CHAPTER THREE CHAPTER THREE METHODOLOGY

Introduction Power transmission lines being attached with number of components of electrical power system Transmission line are situated in wide area these lines unfortunately struck with severe geographical as well as atmospheric condition due to this several faults took place. EHV lines are the very most important aspect in mass power transportation and to keep healthy operation continuously becomes very crucial task for maintaining system reliability; maintain control voltage and other factor also. That is why prevention of transmission system is necessary in modern power system [6].

Transmission Line Protection Schemes There are several protection schemes for transmission lines the study was carried out to compare between distance and differential protection showing their pros and cons therefor Algadarif-Alfau transmission line was taken as the case study.

Distance Protection Distance protection is the most widely used method to protect transmission lines. The fundamental principle of distance Relying is based on the local measurement of voltages and currents, where the Relay responds to the impedance between the relay terminal and the fault location .There are many types of distance relay characteristic such as mho, reactance, admittance, quadrilateral polarized-mho, offset mho etc. Every type of characteristics has different intended function and theories behind [7].

26

Methodology Distance Protection Methodology Distance relays are designed to protect power system against two general types of faults symmetrical and unsymmetrical. This both types of faults further divide into L-G, L-L-G, L-L and L-L-L. In order to perceive any of the above faults, every one of the zone of distance relay required six units’. Three units for detecting faults between the phase and the remaining three units for detecting phase to ground fault. The distance relays setting for line protection are done on the substratum of the positive sequence impedance between the relay unit and the fault point. On the other hand, the settings of ground distance relays are carried out on the substructure of the zerophase-sequence impedance. [8] Mho type distance characteristic was chosen to be as the protection scheme for this relay in the developed model.

Algorithm of Mho Distance Relay When a fault occurs on transmission lines, the voltage and current signals are severely distorted and there is a possibility of containing of dc components available in voltage and current signals, higher order frequency components and lower order frequency components. The higher frequency components can be eliminated using low pass anti-aliasing filters with appropriate cut-off frequency, but the anti-aliasing filters cannot remove decaying dc components and reject low frequency components. This makes the phasors very difficult to be quickly estimated and affects the performance of digital relaying. Therefore, the Discrete Fourier transform is usually used to remove the dc-offset components. [8][9] Flow chart shown in figure .3.2 explains the mho distance relay methodology.

27

Methodology

Figure 3. 1 MHO Distance Relay Algorithm Building Distance Relay Model The main functions included in the numerical relay model are: 1- Fault detection 2- Impedance measurement 3- Zone protection coordination

28

Methodology 3.2.1.3.1 Fault Detection Block The relay permit direct detection of the phases involved in a fault or called faulted phase selection, which then permits the appropriate distance-measuring zone to trip. Without phase selection, the relay risks having over or under reach problems, or tripping three phases when single-pole fault clearance is required. The ‘Delta’ algorithm techniques, was selected for a phase selection, which comparing the step change of level between pre-fault load, and fault current, this is achieved by a logic circuit.

Figure 3.3 shows the fault detection block built in MATLAB, it is clear the relay can discriminate all types of fault. While figure 3.4 represents the scheme logic designed in MATLAB/SIMULINK, where a function block parameter if, the If blocks, along with If Action subsystems containing Action Port blocks were used to achieve this logic circuit. .

Figure 3. 2 Fault detection block

29

Methodology

Figure 3. 3 Logical fault detection scheme

3.2.1.3.2 Impedance Measurement Block

The fault detection block, determines the fault type, and then sends a signal to the impedance measurement block to determine which impedance measurement algorithm must be used. The impedance measurement block consists of different subsystems used to compute the fault impedance for different types of fault.

30

Methodology Table 3.1 presents the different algorithm used to compute the apparent impedance at the relay location for a various types of fault. An illustration of computed impedance for a single phase to ground fault, and double phase to ground fault, being developed in SIMULINK environment, are in figure 3.5 and 3.6 respectively.

Table 3. 1 Fault impedance Algorithm for various fault types

Fault Type

Algorithm For Fault Impedance

ABC or ABCG

Z=(VA / IA) or (VB / IB) (VC / IC)

AB or ABG

Z=(VA – VB)/(IA – IB)

AC or ACG

Z=(VA – VC)/(IA – IC)

BC or BCG

Z=(VB –VC)/(IB –IC )

AG

Z=VA/(IA + 3 k0I0)

BG

Z=VB/(IB + 3k0I0)

CG

Z=VC/(IC + 3k0I0)

31

Methodology

k0 = (Z0-Z+)/KZ+.

3. 1

K can be 1 or 3 depend on the relay design. I0 = (Vs / Z0+2Z+)

3. 2

Figure 3. 4 Apparent impedance model for SLG Fault

Figure 3. 5 Apparent impedance model for DLG Fault

32

Methodology

3.2.1.3.3 Zone Protection Coordination Careful selection of the reach settings and tripping times for the various zones of measurement enables correct coordination between distance relays on a power system. Subsystem zone coordination model was created which comprise time settings for a 3- zone distance protection as shown in figure 3.7.

Figure 3. 6 Zone coordination Subsystem

33

Methodology 3.2.1.3.4 Building Shape Mho Characteristics

The final stage of the model is to develop the Mho characteristics of the distance relay. This stage enhances the understanding of the distance relay behavior. To obtain the shape of mho characteristic by using M-file MTALAB, the calculations of the setting impedance for each zone has to be performed first, and then attaching the corresponding results in a specific code in M-file MTALAB , which draws the shape of each zone of Mho relay characteristic, as presented in figure3.8.[7]

Figure 3. 7 Mho shape characteristics

Differential Protection Differential protection operates with speed therefore suited as fast main protection for all important plant items. The differential protection is a selective type of protection that’s why it only responds to faults within its protected zone.

34

Methodology Working principle of the operation of Differential protection based o Kirchhoff’s current law that is current entering and leaving a zone of protection will be equal. The line differential protection relays detect the leakage currents that will occur on the line according to this law. The limits of the protected zone is exactly defined by the location of the current transformers (CTs).synchronizing with other protection systems is therefore not required allowing for tripping without additional delay. As shown in figure3.9 the currents entering a node are positive, and the currents leaving the node are negative. Here the node point is the protection element and the protected element is the transmission lines.

Figure 3. 8 The current entering and exiting a node

Behavior of Differential Relay in External and Internal Faults By considering current transformer 1 the current that enter the relay is equal to: IS1 = a1 * IP1 – IS1e

3. 3

Similarly for current transformer 2: IS2 = a2 * IP2 – IS2e

3. 4

3.2.2.1.1 External Faults The behavior of differential relay when fault occurs outside the protected area is shown in figure.3.10.

35

Methodology

Figure 3. 9 External faults In case of no fault between the two current transformers the current entering the protected unit Ip1 is the same as the current leaving the protected the zone. If the current transformer ratio is assumed to be 1 to 1 in order to simplify the operations. Also both CTs secondary excitation currents are ignored then:

IP1 = IF IP2 = - IF IDIFF = | IP1 + IP2| = |IF - IF |

3. 5

Circuit breaker is not tripped

3.2.2.1.2 Internal Faults The behavior of differential relay when fault occurs inside the protected area is shown in figure.3.11.

36

Methodology

Figure 3. 10 Internal faults

In case of occurrence of fault between the two current transformers and same assumption above is made then:

IP1 = I1F IP2 = I2F IDIFF = | IP1 + IP2| = |I1F + I2F |

3. 6

Circuit breaker is tripped.

Relay Communication and Protection Zone The protection zone is the area between the current transformers connected to relevant relays at both ends. As shown in figure 3.12. Line differential protection is not possible with a single relay since there exists distance in the protected zone since the case is transmission line protection. Two relays located at the two ends provide instant communication and information sharing. The

37

Methodology communication system between two relays is called tele-protection communication. Both relays simultaneously send current values to the other one and both relays measure the differential current. At the moment of failure, when the relay sees the differential current, it sends trip to its own circuit breaker and the circuit breaker of the remote end. Both relays actually work like a single relay.

Figure 3. 11 Protected area and tele-protection interface

Differential Protection Tripping Characteristics In the case of normal operation differential currents can occur for several reasons: 1. Differential current due to line capacity. 2. Differential current due to linear errors of current transformers. 3. Differential current due to nonlinear fault of current transformers. As can be seen in figure 3.13 the tripping characteristics of the differential relay are affected by the occurrence of these three conditions. As the current values (Is) increase the error margin increases. With this in mind unwanted tripping is prevented. A graph is created by adjusting the Kl and K2 values and the slope between the tripping area and restraint area. Any kind of failure in the system will cause the differential current value to increase too much, so the operating area enters the tripping area and the relay clears the fault from the system by sending a trip to the circuit breaker instantaneously. [10]

38

Methodology

Figure 3. 12 Differential Relay tripping characteristic

39

CHAPTER FOURE CHAPTER FOURE IMPLEMENTATION AND RESULTS

Introduction Transmission line protection was simulated using power system software called MATLAB for Algadarif-Alfau transmission line.

MATLAB Software Millions of engineers and scientists worldwide use MATLAB® to analyze and design the systems and products transforming our world. MATLAB is in automobile active safety systems, interplanetary spacecraft, health monitoring devices, smart power grids, and LTE cellular networks. It is used for machine learning, signal processing, image processing, computer vision, communications, computational finance, control design, robotics, and much more. Math. Graphics. Programming. The MATLAB platform is optimized for solving engineering and scientific problems. The matrixbased MATLAB language is the world’s most natural way to express computational mathematics. Built-in graphics make it easy to visualize and gain insights from data. A vast library of prebuilt toolboxes lets you get started right away with algorithms essential to your domain. The desktop environment invites experimentation, exploration, and discovery. These MATLAB tools and capabilities are all rigorously tested and designed to work together. Simulation and Model-Based Design Simulink® is a block diagram environment for multidomain simulation and Model-Based Design. It supports simulation, automatic code generation, and continuous test and verification of embedded systems.

40

Implementation and Results Simulink provides a graphical editor, customizable block libraries, and solvers for modelling and simulating dynamic systems. It is integrated with MATLAB®, enabling you to incorporate MATLAB algorithms into models and export simulation results to MATLAB for further analysis. From Concept to Code Engineers everywhere use Simulink to get their ideas off the ground, including reducing fuel emissions, developing safety-critical autopilot software, and designing wireless LTE systems. Discover how Simulink can help you complete your projects."[11]

Algadarif-Alfau Transmission Line Review The line between Algadarif and Alfau substations is a 110 KV transmission line. Algadarif substation is considered as the generation unit feeding the load (Alfau substation). The line consists of one three phase power supply as a power station supplying 110 KV transmission line, the line is modeled as three separate transmission lines each 51 km, is designed to deliver the power to the load of 10 MW at the end of transmission line. The bus bars are equipped by current measurement and voltage measurement, each line is equipped separately by two main circuit breakers at the sending and receiving ends as in figure 4.1. The data of the line is given on table 4.1.

Figure 4. 1 Single line diagram of Algadarif-Alfao transmission line

41

Implementation and Results Table 4. 1 Algadarif- Alfau Line Parameters Parameter Name

Value

Length (km)

153

Nominal voltage (kV)

110

No of circuits

1

R1(Ω/km)

0.48

X1 (Ω/km)

0.421

C1 (nf/km)

8.6

Rо (Ω/km)

0.546

Xо(Ω/km)

1.38

Cо (nf/km)

5.3

Z

83.57∟50.42

Z0

227.0654∟68.4137

f (Hz)

50

In order to compare between distance and line differential protections, faults are created at different locations on the 110kV, 153km transmission line and fault resistances of different values were used in MATLAB / SIMULINK software.

42

Implementation and Results Distance Protection In this section Settings of the distance relay along with Simulation model. Relay Settings The settings of the relay model used are in Table 4.2.

Table 4. 2 Relay Settings Zone

Settings

Values (Ω)

Time setting s

Zone1

80% from A

66.8561

instantaneously

Zone2

120% from A

100.2841

0.3

Zone3

180% from A

150.4261

0.6

Simulink Model The MAATLAB / SIMULINK view of the distance relay is reviewed, which consists of two parts (blocks) as The overall model of transmission line with the distance relay and the distance relay model shown in figures 4.2 and 4.3. The Three-Phase V-I Measurement block measures the voltage and current in the transmission line and send them as inputs to the distance relay which uses discrete Fourier transform to get the magnitude and phase angle of the voltage and current. In this model only three phase to ground fault was implemented. After calculations of the fault impedance using table 3.1, MATLAB function was implemented in order to plot the mho characteristic of the fault which In this case the relay will determine the correct zone of the measured impedance for different locations, also it will determine the fault location on the line. Two cases of faults were introduced in order to analyze the behavior of the implemented mho relay characteristics:

43

Implementation and Results 1- Three phase to ground fault at three different locations 2- Three phase to ground fault in zone 1 with fault impedance

Figure 4. 2 Overall simulation model

Figure 4. 3 Distance relay model

44

Implementation and Results Case One: Three Phase to Ground Faults at Different Distances from The Relay Location Figures 4.4 and 4.5 show trace of apparent impedance as seen by the Mho distance relay due to three phase to ground faults at 51 km and 102 km respectively. The distances representing 33% and 66% from bus A. the impedance trajectory fall in the first zone of R-jX plain which is correct function of the relay. While figure 4.6 demonstrates the impedance measured by the Mho distance relay under three phase to ground fault at the distance of 153 km from the relay location (100% from bus A). The result shows that the relay has indicated impedance in the second zone (correct function).

Figure 4. 4 R-jX plot Impedance for a fault at 51 km distance

45

Implementation and Results

Figure 4. 5 R-jX plot Impedance for a fault at 102 km distance

Figure 4. 6 R-jX plot Impedance for a fault at 153 km distance

46

Implementation and Results

Case Two: Three Phase to Ground Faults with Fault Resistance Three phase to ground faults with different fault resistances were set at 0.5 cycles, Figures 4.7 and 4.8 show the impedance trajectory for two cases. The shown cases illustrate the behavior of the relay when fault resistance is 20Ω and 50Ω. In the first case the relay detects the fault in zone 1 as the resistance value were not enough to change the reach of the relay, while in the second case the value of the resistance was enough to make the impedance presented to the relay lies in zone two, even though the fault were set in zone one. So, increment in magnitude of fault resistance, impedance seen by the relay lies in the different zones as shown in the below Figure 4.7 and 4.8 here, we can say that mho distance relay under reaches because of increment in fault resistance magnitudes.

Figure 4. 7 R-jX plot Impedance for a fault at 51 km distance (33%) with 20 Ω resistance

47

Implementation and Results

Figure 4. 8 R-jX plot Impedance for a fault at 51 km distance (33%) with 20 Ω resistance

Differential Protection Settings of the differential relay along with Simulation model of the relay.

Relay Settings The settings of the relay model used given is IS = 2.

Simulink Model The MATLAB / SIMULINK view of the distance relay is reviewed, which consists of two parts (blocks) as the overall model of transmission line with the differential relay and the differential relay model shown in Figure 4.9 and Figure 4.10.

48

Implementation and Results The two Three-Phase current measurement blocks measure the current at both ends of the transmission line (bus A and B respectively), and sends them as inputs to the differential relay which uses the discrete RMS value block to get the measured currents RMS value, then it compares the absolute values of their vector sum with the set restraint current. If there is a fault in the transmission line the comparison result will be 1 and the S-R latch sends trip signal to the circuit breaker. But if there is no fault the comparison result will be 0 and no trip signal is sent. Two cases of faults were introduced in order to analyze the behavior of the implemented mho relay characteristics: 1- Three phase to ground fault at three different locations 2- Three phase to ground fault with fault impedance

Figure 4. 9 Overall simulation model

49

Implementation and Results

Figure 4. 10 Differential relay model

Case One: Three Phase to Ground Faults at Different Distances from The Relay Location Figure 4.11 shows the currents at bus A and B in case of three phase to ground fault at time 0.5 cycles for any internal fault. Figure 4.12 shows the relay tripping signal as correct reaction to the internal faults.

Figure 4. 11 Current wave form at bus A and B for three phase to ground fault

50

Implementation and Results

Figure 4. 12 Trip signal as a correct reaction to the fault

Case Two: Three Phase to Ground Faults with Fault Resistance Three phase to ground faults with different fault resistances were set at 0.5 cycles, Figures 4.13 and 4.14 show the current waveforms for the two cases and there tripping signal respectively. The shown cases illustrate the behavior of the relay when fault resistance is 20Ω and 50Ω. For both cases the relay sends the correct trip signal.

Figure 4. 13 Current wave forms at bus A and B for three phase to ground fault with fault resistances 20 and 50 Ω

51

Implementation and Results

Figure 4. 14 Trip signal as a correct reaction to the fault

Results Discussion In three phase to ground fault event the current will increase above the set value: 

Distance relay performs correct reaction showing the fault at the correct zone and fault location in its mho characteristic except for faults with fault impedance the mho distance relay under reaches because of increment in fault resistance magnitudes.



Differential relay operates correctly by sending instantaneous trip signal weather there is fault resistance or not.

The results of the simulation of both distance and differential relays is summarized in table 4.4 along with the tripping time for different fault conditions.

52

Implementation and Results

Table 4. 3 Simulation results summery

Fault discerption

Type of fault

Distance

Differential

Relay

Relay

Trip time

Trip time

Zone 1 Fault at 51 km

Three phase to ground

Instantaneously

Instantaneously

fault Zone 1 Fault at 102 km

Instantaneously

Instantaneously

Fault at 153 km

Zone 2 with delay 0.3

Instantaneously

Three phase to ground With resistance 20 Ω

With resistance 50 Ω

fault

Zone 1 Instantaneously

Instantaneously

Zone 2 with delay 0.3

Instantaneously

However both distance and differential protections have advantages and disadvantages.

5.1.1 Distance Relay Distance relay has the following advantages and disadvantages.

53

Implementation and Results 5.1.1.1 Advantages 

The relay has the ability of indicating the correct zone of operation in all cases of faults without impedance.



The relay identifies the fault locations as expected.



The relay provides backup protection for the adjacent transmission lines (zone 2 and zone 3).



Less expensive because it often doesn’t require communication link.

5.1.1.2 Disadvantages 

Requires more input data (voltage and current), thus requires more computation time.



Resistive fault causes the relay to under-reaches and misjudges the fault location which reflects the accuracy of the developed model. The effect is severe in case of short transmission lines where the impedance of the line is small compared with the fault resistance.

5.1.2 Differential Relay Differential relay has the following advantages and disadvantages. 5.1.2.1 Advantages 

The relay covers any type of faults even the resistive one and performs instantaneous trip.



Requires less input data (current only), thus requires less computation time.

5.1.2.2 Disadvantages 

The relay doesn’t identify the fault locations which is a major problem for long transmission lines.



The relay doesn’t provide backup protection for the adjacent transmission lines.



More expensive because it requires communication link and the expense increases in case of long lines.

54

CHAPTER FIVE CHAPTER FIVE CONCLUSION AND RECOMMENDATIONS

Conclusion  Transmission lines are one of the most important elements of the power system because they carry electrical energy from the source of production to the consumers, any breakdown in the transmission line reduces the power transmission capability of the power system for this reason faults must be identified, located and cleared as quickly as possible in order to maintain the power transmission capacity of a power system and ensure the safety of the population. The previous objective was achieved.  A 110 KV, 153 km Algdarif-Alfau transmission line is taken into consideration. A Mho type distance relay and differential relay were successfully developed based on MATLAB/SIMULINK package. Each function has been created using special blocks of SIMULINK. A three phase to ground fault is considered.  The simulation results shows that if the transmission line is protected with distance or differential protection relays, the related faults in the lines can be cleaned from the system as designed  Advantages and disadvantages of distance and differential relays were obtained. Finally both models have decent margin to evolve in the future.

Recommendations 1. Transmission lines with length approximately ≥ 15 km is recommended to be protected using distance protection. 2. Lines with length approximately ≤ 5 km is recommended to be protected using differential protection. 3. This leaves the power line length range between 5km and 15 km to trading off between them.

55

Conclusion and recommendations Since the developed model of distance relay was not provided with all types of fault detection schemes also was not provided with decision scheme, work may be extended to include a comprehensive relay model including all types of faults and trip scheme. Work may be extended to incorporate algorithms used to improve the relay behavior when overhead lines is subjected to more than one generation unit.

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Refreneces REFRENECES [1] Alstom (Schneider Electric). "Network protection and automation guide”. 2012 [2] L.G Hewitson Mark Brown Ramesh Balakrisnan “Practical Power Systems Protection”, 2004 [3] J. Lewis Blackburn and Thomas J. Domin."Protective relaying principle and applications". Third edition. 2006 [4] Cobusstrauss."Practical-Electrical-Network-Automation-and-Communication-Systems". [5] Y.G. Paithanka &S.R. Bhide, “Fundamentals of Power System Protection”.Visvesvaraya National Institute of Technology Nagpur, 2003. [6] Anderson. P.M.”Power System Protection”, ISBN 0-07-134323-7 McGraw-Hill, 1999 [7] Omar G. Mrehel, Hassan B. Elfetori, Abdallah O. Hawal “Implementation and Evaluation a SIMULINK Model of a Distance Relay in MATLAB/SIMULINK”, ISBN: 978-0-9891305-3-0 ©2013 SDIWC [8] Mr. Kunal K. Joshi, Prof. M. R. Hans “Development of Mho Type Distance Relay for Protection of Long Transmission Line using Matlab Simulink Environment” International Journal of Engineering Research & Technology (IJERT) Vol. 5 Issue 04, April-2016 [9] Abdlmnam A. Abdlrahem and Hamid H Sher Modeling of, IEEE Symposium on Industrial Electronics and Applications (ISIEA 2009), October 4-6, 2009, Kuala Lumpur, Malaysia. [10] Gokhan Degerli, Recep Y umurtacl, “The Comparison of Distance Protection and Differential Protection Techniques for T -Connected Transmission Lines”, 978-1-5090-6789-3/ 17/$31 .00 ©20 17 IEEE [11] Mathworks, “Matlab Help,” 2010.

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APPENDEX A

APPENDEX A

function Y = fcn(u,th)%Defines a function which recieves the magnitude and phase angle of the %fault impedence theta=linspace(0,2*pi,360);%Defines zones boundary th0=th/180*pi; %to convert the imaginary input into radian l1=1.3401e-3; %positive sequence inductance in km r1=0.348;%positive sequence resistance in km Zpu=(((110*10^3)^2)/(15*10^6));%the perunit impedence of the line m=0:0.1:153; % factor that increments by 1 meter until the length of the line xL=l1*2*pi*50*m;% the imaginary part of the line rL=r1*m;% the real part of the line x3=u*cos(th0)*Zpu;%the imaginary part of fault impedence that comes from the relay y3=u*sin(th0)*Zpu;%the real part of fault impedence that comes from the relay x=abs(153*.80*(l1*2*pi*50i+r1))/2*cos(2*pi*50+theta)+(153*.85*r1)/2;%Boundary of zone 1 in the X axis y=abs(153*.80*(l1*2*pi*50i+r1))/2*sin(2*pi*50+theta)+(153*.80*l1*2*pi*50)/2;%Boundary zone 1 in the Y axis a=abs(153*1.2*(l1*2*pi*50i+r1))/2*cos(2*pi*50+theta)+(153*1.2*r1)/2;%Boundary zone 2 in the X axis b=abs(153*1.2*(l1*2*pi*50i+r1))/2*sin(2*pi*50+theta)+(153*1.2*l1*2*pi*50)/2;%Boundary zone 2 in the Y axis c=abs(153*1.8*(l1*2*pi*50i+r1))/2*cos(2*pi*50+theta)+(153*1.8*r1)/2;%Boundary zone 3 in the X axis d=abs(153*1.8*(l1*2*pi*50i+r1))/2*sin(2*pi*50+theta)+(153*1.8*l1*2*pi*50)/2;%Boundary zone 3 in the Y axis

A1

APPENDEX A

plot(rL,xL,'.b')%Draws the line in R+jX plane from 0 to 153 km if abs(x3) <200 && abs(y3) < 200 plot(x3,y3,'.r')%Draws the fault loction in R+jX plane with red dotted end plot(x,y,'green')%Draws Zone 1 in R+jX plane with green color plot(a,b,'black')%Draws Zone 2 in R+jX plane with black color plot(c,d,'magenta')%Draws Zone 3 in R+jX plane with magenta color axis('equal')%Defines two equal axis x and y hold on % draws the relay results second by second figure ( 1 );%Figure num 1 set (gca, 'FontSize', 12)%Sets the font size whitebg('white')%Sets the color of the page grid on %Draws grid on the page xlabel ('Resistance (R)','FontSize',9);% X axis is the real part R ylabel ('Reactance (jX)','FontSize',9);% Y axis is the Imaginary jX Y=2; end Figure A. 1 Distance relay mho MATLAB function

Figure A. 2 SR flip flop description

A2