fata McGraw-Hill © 1995, Tata McGraw-Hill Publishing Company Limited 22" reprint 2007 RCLYYRLYRZLDL No part of this publication can be reproduced in any form or by any means without the prior written permission of the publishers This edition can be exported from India only by the publishers, Tata McGraw-Hill Publishing Company Limited ISBN 0-07-462350-8 Published by Tata McGraw-Hill Publishing Company Limited, 7 West Patel Nagar, New Delhi 110 008, typeset at Anvi Composers, and printed at Pushp Print Services, Delhi 110 053 iaue eeComtemts Preface Introduction 1.1 Need for Protective Systems 7 1.2 Nature and Causes of Faults 2 1.4 Effects of Faults_4 15 Fault Statistics 5 1.7 Zones of Protection 8 1.9 Essential Qualities of Protection 17 1.10 _ Classification of Protective Relays 12 1.11 Classification of Protective Schemes 74 1.12 Automatic Reclosing _15 113 Current Transformers for Protection 16 1.14 Potential Transformer 23 1,15 Summation Transformer _25 1,16 Phase-sequence Current-segregating Network 27 4.17 _ Basic Relay Terminology 27 Exercises 35 Operating Principles and Relay Construction 2.1 Electro: netic Relays 36 22 Thermal Relays 53 23 StaticRelays 54 2.4 _ Microprocessor-based Protective Relays 66 Exercises 67 Qvercurrent Protection 3.1___ Time-current Characteristics 68 3.2 Current Setting 72 3.3 Time Setting 74 3.4 Overcurrent Protective Schemes 75 36Contents 3.6 Protection of Parallel Feeders _82 3.7___ Protection of Ring Mains 83 3.9 Combined Earth Fault and Phase Fau Pen Scheme 86 : 3.11_Directional Earth Fault Relay 86 3.12 Static Overcurrent Relays 87 : 3.13 Micro) cessor-based Overcurrent Relays 97 Exercises 92 3.5 ‘everse Power or Directional Relay_79 Distance Protection 94 4.1__Impedance Relay _94 4.2 Reactance Relay 104 - 4.3 MHO (Admittance or Angle Admittance) Relay 708 4.4 Angle Impedance (Ohm) Relay 775 4.5 Input Quantities for Various Types of Distance Relays 117 4.6 Sampling Comparator_117 Distance Relays 119 Performance of Distance Relays 122 4.9 _ Effectof Line Length and Source Impedance on Distance Relays 126 4.10 _ Selection of Distance Relays 128 4.11 _MHO Relay with Blinders 129 4.12 Quadrilateral Relay 730 4.13 Elliptical Relay 132 4.14 Restricted MHO Relay 134 4.15 Restricted Impedance Relay 735 4.16 Restricted Directional Relay 136 4.17 Restricted Reactance Relay 736 4.18 Some Other Distance Relay Characteristics 137 4.19 Swivelling Characteristics 140 4.20 Choice of Characteristics for Different Zones of Protection 147 4.21 Compensation for Correct Distance Measurement 14] 4.22 _ Reduction of Measuring Units 146 Copyrighted material4.24 Auto-reclosing 150 Appendix 153 * Contents bx Exercises 155 Pilot Relaying Schemes 159 5 Wire Pita. ° - 59 5.2___Carrier Current Protection 165. Exercises 177 AC Machines and Bus-zone Protection 179 6.1 Protection of Generators 179 6.2 Transformer Protection 193 6.3 Bus-zone Protection 203 64 Frame Leakage Protection 205 Exercises 205 Microprocessor and Interfacing 207 71 Introduction 207 7.2 Microprocessors 209 7.3 Input/Output Devices 224 7.4 Semiconductor Memories 224 7.5 Single Chip Microcomputers 226 7.6 I/O Ports and Programmable Peripheral Interface 227 7.7 Programmable Interval Timer (Intel 8253) 231 7.8 IC Elements and Circuits for Interfaces 237 7.9 __ A/D Converter, Analog Multiplexer, S/H Circuit 243 Exercises 258 Microprocessor-based Protective Relays 260 8.1 ___Introduction 260 8.2 Overcurrent Relays 267 8.3 Impedance Relay 268 8.4 Directional Relay 273 8.5 Reactance Relay 277 8.6 Generalised Mathematical Expression for Distance Relays 279 8.7 Measurement of RandXY 283 8.8 | Mho and Offset Mho Relays 285x Contents 8.9 Quadrilateral Relay 294 8.10 Generalised Interface for Distance Relays 296 8.11 Digital Relaying Algorithms 296 8.12 Differential Equation Technique 299 8.13 Discrete Fourier Transform Technique 300 8.14 Walsh-Hadamard Transform Technique 3171 8.15 Rationalised Haar Transform Technique 327 8.16 Removal of the dc Offset 339 8.17 Microprocessor Implementation of Digital Distance Relaying Algorithms 342 Exercises 345 9 Cirenit Break 346 10 91 Are Voltage 346 9.2 ArcInterruption 347 9.3 Restriking Voltage and Recovery Voltage 348 9.4 Resistance Switching 352 9.5 Current Chopping 355 9.6 Interruption of Capacitive Current 356 9.7 __ Classification of Circuit Breakers 358 98 Oil Circuit Brea 9.9 Air Blast Circuit Breaker 364 9.11 SE, Circuit Breakers: 368 9.12 Vacuum Circuit Breakers 370 9.13 Operating Mechanism 372 9.15 High Voltage dc Circuit Breakers 375 9:16 Rating of Circuit Breakers 376 9.17 Testing of Circuit Breakers 379 Exercises 385 Fuses 387 10.1 Introduction 387 10.2 Definitions 387 10.3. Fuse Characteristics 397 10.4 Typesof Fuses 392 10.5 Applications of HRC Fuses 397 10.6 Selection of Fuses 398 10.7 Discrimination 399 Exercises 401Contents xd 11 Protection against Overvoltages 402 11.1 Causes of Overvoltages .402 11.2 Lightning Phenomena 403 11.3. Wave Shape of Voltage due to Lightning 406 11.4 Overvoltages duc to Lightning 406 11.5 Klydonograph and Magnetic Link 408 11.6 Protection of Transmission Lines against Direct Lightning Strokes 410 17 __ Protection of Stations and Sub-stations from Direct Strokes 414 11.8 Protection against Travelling Waves 416 11.9__ Peterson Coil 428 11.10 Insulation Coordination 430 11.11 Basic Impulse Insulation Level (B.I.L.) 432 Exercises 432 Appendix A: 8086 Assembly Language Programming 434 Appendix B: Orthogonal and Orthonormal Functions 441 Appendix C: Gray-code to Binary Conversion 442 Appendix D; Kronecker (or Direci) Product of Matrices 443 References Ada Index. 453Onz Introduction 1.1 Need for Protective Systems An electrical power system consists of generators, transformers, transmission and distribution lines, etc. Short circuits and other abnormal conditions often occur on a power system. The heavy current associated with short circuits is likely to cause damage to equipment if suitable protective relays and circuit breakers are not provided for the protection of each section of the power system. Short circuits are usually called faults by power engineers. Strictly speaking, the term ‘fault’ simply means a ‘defect’. Some defects, other than short circuits, are also termed as faults. For example, the failure of conducting path due to a break in a conductor is a type of fault. Tf a fault occurs in an element of a power system, an automatic protective device is needed to isolate the faulty element as quickly as possible to keep the healthy section of the system in normal operation. The fault must be cleared within a fraction of a second. Ifa short circuit persists on a system fora longer period, it may cause damage to some important sections of the system. A heavy short circuit current may cause a fire. It may spread in the system and damage a part of it. The system voltage may reduce to a low level and individual generators in a power station or groups of gencrators in different power stations may lose synchronism. Thus, an uncleared heavy short circuit may cause the total failure of the system. A protective scheme includes circuit breakers and protective relays to isolate the faulty section of the system from the healthy sections. A circuit breaker can disconnect the faulty element of the system when it is called upon to do so by the protective relay. The function of a protective relay is to detect and locate a fault and issue a command to the circuit breaker to disconnect the faulty element. It.is a device which senses abnormal conditions on a power system by constantly monitoring electrical quantities of the system, which differ under normal and abnormal conditions. The basic electrical quantities which are likely to change during abnormal conditions are current, voltage, phase-angle (direction) and frequency: Protective relays utilise one or more of2 Power System Protection and Switchgear these quantities to detect abnormal conditions on a power system. Protection is needed not only against short circuits but also against any other abnormal conditions which may arise on a power system. A few ex- amples of other abnormal conditions are overspeed of generators and motors, over-voltage, under-frequency, loss of excitation, overheating of stator and rotor of an alternator, etc. Protective relays are also provided to detect such abnormal conditions and issue alarm signals to alert operators or trip circuit breaker. A protective relay does not anticipate or prevent the occurrence of a fault, rather it takes action only after a fault has occurred. However, one exception to this is the Buchholz relay, a gas actuated relay, which is used for the protection of power transformers. Sometimes, a slow breakdown of insulation due to a minor arc may take place in a transformer, resulting in the generation of heat and decomposition of the transformer’s oil and solid insulation. Such a condition produces a gas which is collected in a gas chamber of the Buchholz relay. When a specified amount of gas is accumulated, the Buchholz relay operates an alarm. This gives an early warming of incipient faults. The transformer is taken out of service for repair before the incipient fault grows into a serious one. Thus, the occurrence of a major, fault is prevented. If the gas evolves rapidly, the Buchholz relay trips the circuit breaker instantly. The cost of the protective equipment generally works out to be about 5% of the total cost of the system. 1.2 Nature and Causes of Faults Faults are caused cither by insulation failures or by conducting path failures. The failure of insulation results in short circuits which are very harmful as they may damage some equipment of the power system. Most of the faults on transmission and distribution lines are caused by overvoltages due to lightning or switching surges, or by external conducting objects falling on overhead lines. Overvoltages due to lighting or switching surges cause flashover on the surface of insulators resulting in short circuits. Sometimes, insulators get punctured or break. Sometimes, certain foreign particles, such as fine cement dust or soot in industrial areas or salt in coastal areas or any dirt in general accumulates on the surface of string and pin insulators. This reduces their insulation strength and causes flashovers. Short circuits are also caused by tree branches or other conducting objects falling on the overhead lines. Birds also may cause faults on overhead lines if their bodies touch onc of the phases and the carth wire (or the metallic supporting structure which is at earth potential). If the conductors are broken, there is a failure of the conducting path and the conductor becomes open-circuited. If the broken conductor falls to the ground, it results in a short circuit. Joint failures on cables or overhead lines are also a cause of failure of the conducting path. The opening of one orIntroduction 3 two of the three phases makes the system unbalanced. Unbalanced currents flowing in rotating machines set up harmonics, thereby heating the machines in short periods of time. Therefore, unbalancing of the lines is not allowed in the normal operation of a power system. Other causes of faults on overhead lines are: direct lightning strokes, aircraft, snakes, ice and snow loading, abnormal loading, storms, earthquakes, creepers etc. In the case of cables, transformers, generators and other equipment, the causes of faults are: failure of the solid insulation due to aging, heat, moisture or overvoltage, mechanical damage, accidental contact with earth or earthed screens, flashover due to over- voltages, etc. Sometimes, circuit breakers may trip due to errors in the switching opera- tion, testing or maintenance work, wrong connections, defects in protective devices, etc. Certain faults occur due to the poor quality of system components or because of a faulty system design. Hence the occurrence of such faults can be reduced by improving the system design, by using components and materials of good quality and by better operation and maintenance. 1.3 Types of Faults Two broad classifications of faults are: (i) Symmetrical faults (ii) Unsymmetrical faults 1.3.1 Symmetrical Faults A three-phase (3-9) fault is called a symmetrical type of fault. In a3-¢ fault, all the three phases are short circuited. There may be two situations—ail the three phases may be short circuited to the ground or they may be short-circuited without involving the ground. A 3-9 short circuit is generally treated as a standard fault to determine the system fault level. 1.3.2 Unsymmetrical Faults Single phase to ground, two phase to ground, phase to phase short circuits; single phase open circuit and two phase open circuit are unsymmetrical types of faults. Single phase to ground (L-G) fault A short circuit between any onc of the phase conductors and earth is called a single phasc to ground fault. It may be duc fo the failure of the insulation between a phase conductor and the earth, or due to a phase conductor breaking and falling to the ground. Two phase to ground (2L-G) fauit A short circuit between any two phases and the earth is called a double line to4 Power System Protection and Switchgear ground or a two phase to ground fault. Phase to phase (L-L) fault Ashort circuit between any two phases is called a line to line or phase to phase fault. Open circulted phases: This type of fault is caused by a break in the conducting path. Such faults occur when one or more phase conductors break or a cable joint or a joint on the overhead lines fails. Such situations may also arise when circuit breakers orisolators open but fail to close one or more phases. Due to the opening of one or two phases, unbalanced currents flow in the system, thereby heating rotating machines, Protective schemes must be provided to deal with such abnormal Situations. Winding faults .All types of faults discussed above also occur on the alternator, motor and transformer windings. In addition to these types of faults, there is one more type of fault, namely the short circuiting of turms which occurs on machine windings. 1.3.3 Simultaneous Faults Two or more faults occurring simultaneously on a system are known as multiple or simultaneous faults. In simultaneous faults, the same or different types of faults may occur at the same or different points of the system. An example of two different types of faults occurring at the same point is a single line to ground fault on one phase and breaking of the conductor of another phase, both simultaneously present at the same point. The simultaneous presence of an L-G fault at one point and a second L-G fault on another phase atsome other point is an example of two faults of the same type at two different points. If these two L-G faults are on the same section of the line, they are treated as a double line to ground fault. If they occur in different line sections, it is known as a cross-country earth fault. Cross-country faults are common on systems grounded thrcugh high impedance or Peterson coil but they are rare on solidly grounded systems. 1.4 Effects of Faults The most dangerous type of fault is a short circuit as it may have the following effects on a power system, if it remains uncleared. (i) Heavy short circuit current may cause damage to equipment or any other element of the system due to overheating and high mechanical forces set up due to heavy current. (ii) Arcs associated with short circuits may cause fire hazards. Such fires,Introduction 5 resulting from arcing, may destroy the faulty element of the system. There is also a possibility of the fire spreading to other parts of the system if the fault is not isolated quickly. (iii) There may be reduction in the supply voltage of the healthy feeders, Tesulting in the loss of industrial loads. (iv) Short circuits may cause the unbalancing of supply voltages and cur- Tents, thereby heating rotating machines. (v) There may be a loss of system stability. Individual generators in a power station may lose synchronism, resulting in a complete shutdown of the system. Loss of stability of interconnected systems may also result, Subsystems may maintain supply for their iridividual zones but load shedding would have to be resorted in the sub-system which was receiving power from the other subsystem before the occurrence of the fault. (vi) The above faults may cause an interruption of supply to consumers, thereby causing a loss of revenue. High grade, high speed, reliable protective devices are the essential require- ments of a power system to minimise the effects of faults and other abnormali- ties. 1.5 Fault Statistics For the design and application of a protective scheme, it is very useful to have an idea of the frequency of occurrence of faults on various elements of a power system. Usually the power stations are situated far away from the load centres, resulting in hundreds of kilometres’ length of overhead lines being exposed to atmospheric conditions. The chances of faults occurring due to storms, falling of external objects on the lines, flashovers resulting from dirt deposits on insulators, etc., are greater for overhead lines than for other parts of the power system. Table 1.1 gives an approximate idea of the fault statistics. TABLE 1.1 Percentage Distribution of Faults in Various Elements of a Power System Element % of Total Faults Overhead Lines 50 - 7 Underground Cables 9 ‘Transformers 10 Generators: 7 Switchgears 12 CTs, PTs, Relays Control Equipment, etc. 126 Power System Protection and Switchgear From Table 1.1, it is evident that 50% of the total faults occur on overhead lines. Hence it is overhead lines that require more attention while planning and designing protective schemes for a power system. Table 1.2 shows the frequency of occurrence of different types of faults (mainly the different types of short circuits) on overhead lines. From the table it is evident that the frequency of line to ground faults is more than any other type of fault, and hence the protection against L-G fault requires greater attention in planning and design of protective schemes for overhead lines. TABLE 1.2 Frequency of Occurrence of Different Types of Faults on Overhead Lines Types of Faults Fault Symbol % of Total Faults Line to Ground LG 85 Line to Line LL 8 Double Line to Ground 2L-G 5 Three Phase 3 2 In the case of cables, 50% of the faults occur in cables and 50% at end junctions. Cable faults are usually of a permanent nature and hence, automatic reclosures are not recommended for cables. 1.6 Evolution of Protective Relays In the very early days of the power industry, small generators were used to supply local loads and fuses were the only automatic devices to isolate the faulty equipment. They were effective and their performance was quite satis- factory for small systems. However, they suffered from the disadvantage of requiring replacement before the supply could be restored. For important lines, frequent interruption in power supply is undesirable. This inconvenicnce was overcome with the introduction of circuit breakers and protective relays. Attracted armature type electromagnetic relays were first introduced. They were fast, simple and economical. Their use will continue even in future as auxiliary relays duc to their simplicity and low cost. Later on, induction disc type inverse time-current telays were developed in the carly 1920s to meet the selectivity requirement. They were used for over-current protection. For directional and distance relays, induction cup type units were widely used throughout the world. An induction cup type unit was fast and accurate due to its higher torque/inertia ratio. For greater sensitivity and accuracy, polarised dc relays are being used since 1939. In 1947, rectifier bridge type comparators were developed in Norway and Germany. Polarised de relays, energised from rectifier bridge comparatorsIntroduction 7 challenged the position of the induction cup type relays. At present they are widely used for the realisation of distance relay characteristics. Electronic relays using vacuum tubes first appeared in the literature in 1928 and continued up to 1956. They were not accepted because of their complexity, short life of vacuum tubes and incorrect operation under transient conditions. But electronic valves were used in carrier equipment. There was automatic checking of the carrier channel. An alarm was sounded if any tube became defective, and it was replaced immediately. Magnetic amplifiers were also used in protective relays in the past. A magnetic amplifier consists of a transformer and a separate dc winding. As the transformer action is controlled by the de winding, the device is also known as transductor. This type of relay is rugged but slow in action. At present such relays are not used. Hall crystals were also used to construct phase comparators. Because of their low output and high temperature errors, such relays have not been widely adopted except in the USSR. The first transistorised relay was developed in 1949, soon after the innova- tion of the transistor. Various kinds of static relays using solid state devices were developed in the fifties. Multi-input comparators giving quadrilateral characteristics were developed in the sixties. Static relays possess the advan- tages of low burden on the C.T. and P.T., fast operation, absence of mechani- cal inertia and contact troubles, long life and less maintenance. As static relays proved to be superior to electromagnetic relays, they were used for the protection of important lines, power stations and sub-stations. But they did not replace electromagnetic relays. Static relays were treated as an addition to the family of relays. In most static relays, the output or slave relay is a polarised d.c. relay which is an electromagnetic relay. This can be replaced by a thyristor circuit, but it is used because of its low cost. Electromagnetic relays have continued to be used because of their simplicity and low cost. Their mainte- nance can be donc by less qualified personnel, whereas the maintenance and repair of static relays requires personnel trained in solid state devices, Static relays using digital techniques have also been developed recently. ‘With the growing size and complexity of modern power networks, fast, accurate and reliable protective schemes will be the demand of the future. Increasing interest is being shown in the use of on-line digital computers for protection. But their cost is 15 to 20 times more than that of conventional protective schemes. The latest fascinating innovation in the field of computer technology is the development of the microprocessor which is making in-roads in every activity of mankind. With the developments in VLSI technology, microprocessors that appeared in the seventies have evolved and have made remarkable progress in recent years. The mass production of inexpensive microprocessors constitutes a major event in electrical as well as computer technology. In the former area, microprocessors are used to replace conventional8 Power System Protection and Switchgear digital and electromechanical logic systems. In the latter area they are being used as the basis of all types computers. With the recent developments in VLSI technology, sophisticated microprocessors and single-chip microcomputers are being developed. The power industry is also expected to be affected to a great extent by this marvel of technology. Power engineers are presently interested in the application of this powerful tool to the problem of protective Telays, The inherent advantage of microprocessor-based protective schemes, over the existing static relays with one or a very limited range of applications, is their flexibility. The application of microprocessor to protective relays will also result in the availability of faster, more accurate and reliable relaying units. A microprocessor increases the flexibility of a relay due to its program- mable approach. It can provide protection at low cost and compete with conventional relays. A number of relaying characteristics can be realised using the same interface. Using a multiplexer, the microprocessor can obtain the required input signals for the realisation of a particular relaying characteristic. Different programs can be used for different characteristics. Individual types and number of relaying units is reduced to a great extent, resulting in a very compact protective scheme. Field tests have demonstrated their feasibility and some schemes are under investigation. A number of schemes have been put into service and their performance is being observed. Microprocessor-based protective schemes are under the active research and development stage. 1.7 Zones of Protection A power system contains generators, transformers, bus bars, transmission and distribution lines, etc. There is a separate protective scheme for each piece of equipment or element of the power system, such as generator protection, transformer protection, transmission line protection, bus bar protection, etc. Thus, a power system is divided into a number of zones for protection. A protective zone covers one or at the most two elements of a power system. The protective zones are planned in such a way that the entire power system is collectively covered by them, and thus, no part of the system is left unpro- tected. The various protective zones of a typical power system are shown in Fig. 1.1. Adjacent protective zones must overlap cach other, failing which a fault on the boundry of the zones may not lic .in any of the zones (this may be due to errors in the measurement of actuating quantities, etc.), and hence no Circuit breaker would trip. Thus, the overlapping between the adjacent zones is unavoidable. If a fault occurs in the overlapping zone in a properly protected scheme, more circuit breakers than the minimum necessary to isolate the faulty element of the system would trip. A relatively low extent of overlap reduces the probability of faults in this region and consequently, tripping of too many breakers does not occur frequently.Introduction 9 ) (<—— Generator Protection t assis t 4 Circuit Breaker a H.V. Switchgear Protection Transformer Protection “\ ke— EHV Switchgear Protection ' 74! ' i 1 '«———— Transmission Line Protection FIGURE 1.1 Zones of protection 1.8 Primary and Back-up Protection Ithas already been explained that a power system is divided into various zones for its protection. There is a suitable protective scheme for each zone. Ifa fault occurs in a particular zone, it is the duty of the primary relays of that zone to isolate the faulty element. The primary relay is the first line of defence. If due to any reason, the primary relay fails to operate, there is a back-up protective scheme to clear the fault as a second line of defence. The causes of failures of a protective scheme may be due to the failure of various elements, as mentioned in Table 1.3. The probability of failures is shown against each item. The reliability of a protective scheme should at least be 95%. With proper design, installation and’ maintenance of the relays, circuit breakers, trip mechanisms, ac and de wiring, etc. a very high degree of reliability can be achieved. The back-up relays are made independent of those factors which might cause primary relays to fail. A back-up relay operates after a time delay to give the primary relay sufficient time to operate. When a back-up relay operates, a larger part of the power system is disconnected from the power source, but this is unavoidable. As far as possible, a back-up relay should be placed at a different station. Sometimes, a local back-up is also used. It should be located insuch a way that it does not employ components (P.T., C.T., measuring unit,10 Power System Protection and Switchgear TABLE 1.3 Name of Equipment % of Total Failures (a) Relays 44 (b) Circuit breaker interrupters 14 (© AC wiring 2 (@) Breaker trip mechanisms 8 (c) Current transformers 7 () DC wiring 5 (g) BT. 3 (h) Breaker auxiliary switches 3 (i) Breaker tripcoils 3 (j) DC supply 1 etc.) common with the primary relays which are to be backed up. There are three types of back-up relays: (a) Remote back-up (b) Relay back-up (©) Breaker back-up 1.8.1. Remote Back-up When back-up relays are located at a neighbouring station,they back-up the entire primary protective scheme which includes the relay, circuit breaker, P.T., C.T. and other elements, in case of a failure of the primary protective scheme. It is the cheapest and the simplest form of back-up protection and is a widely used back-up protection for transmission lines. It is most desirable because of the fact that it will not fail due to the factors causing the failure of the primary protection. 1.8.2 Relay Back-up This is a kind of a local back-up in which an additional relay is provided for back-up protection. It trips the same circuit breaker if the primary relay fails and this operation takes place without delay. Though such a back-up is costly, it can be recommended where a remote back-up is not possible. For back-up Telays, principles of operation that are different from those of the primary protection are desirable. They should be supplied from separate current and potential transformers. 1.83 Breaker Back-up This is also a kind of a local back-up. This type of a back-up is necessary for a bus bar system where a number of circuit breakers are connected to it. When a protective relay operates in response to a fault but the circuit breaker fails to trip, the fault is treated as a bus bar fault. In such a situation, it becomes necessary that all other circuit breakers on that bus bar should trip. After aIntroduction 11 time-delay, the main relay closes the contact of a back-up relay which trips all other circuit breakers on the bus if the proper breaker does not trip within a specified time after its trip coil is energised. 1.9 Essential Qualities of Protection The basic requirements of a protective system are as follows: (i) Selectivity or discrimination Reliability (iii) Sensitivity (iv) Stability (v) Fast Operation 1.9.1 Selectivity or Discrimination Selectivity, is the quality of a protective relay by which it is able to discrimi- nate between a fault in the protected section and the normal condition. Also, it should be able to distinguish whether a fault lies within its zone of protection or outside the zone. Sometimes, this quality of the relay is also called discrimina- tion. When a fault occurs on a power system, only the faulty part of the system should be isolated. No healthy part of the system should be deprived of electric supply and hence should be left intact. The relay should also be able to discriminate between a fault and transient conditions like power surges or inrush of a transformer’s magnetising current. The magnetising current of a large transformer is comparable to a fault current, which may be 5 to 7 times the full load current. When generators of two interconnected power plants lose synchronism because of disturbances, heavy currents flow through the equip- ment and lines. This condition is like a short circuit. The flow of heavy currents is known as a power surge. The protective relay should be able to distinguish between a fault or power surge either by its inherent characteristic or with the help of an auxiliary relay. Thus, we see that a protective relay must be able to discriminate between those conditions for which instantaneous tripping is required and those for which no operation or a time-delay operation is required. 1.9.2 Reliability A protective system must operate reliably when a fault occurs in its zone of protection. The failure of a protective system may be due to the failure of any one or more elements of the protective system. Its important elements are the protective relay, circuit breaker, P.T., C.T., wiring, battery, etc. To achieve a high degree of reliability, greater attention should be given to the design; installation, maintenance and testing of the various elements of the protective system: Robustness and simplicity of the relaying equipment also contribute to reliability. The contact pressure, the contact material of the relay, and the prevention of contact contamination are also very important from the12 Power System Protection and Switchgear reliability point of view. A typical value of reliability of a protective scheme is 95%. 1.9.3 Sensitivity A protective relay should operate when the magnitude of the current exceeds the preset value. This value is called the pick-up current. The relay should not operate when the current is below its pick-up value. A relay should be sufficiently sensitive to operate when the operating current just exceeds its pick-up value. 1.9.4 Stability A protective system should remain stable even when a large current is flowing through its protective zone due to an external fault, which does not lie in its zone. The concermed circuit breaker is supposed to clear the fault. But the protective system will not wait indefinitely if the protective scheme of the zone in which fault has occurred fails to operate. After a preset delay the relay will operate to trip the circuit breaker. 1.9.5 Fast Operation A protective system should be fast enough to isolate the faulty element of the system as quickly as possible to minimise damage to the equipment and to maintain the system stability. For a modern power system, the stability criterion is very important and hence, the operating time of the protective system should not exceed the critical clearing time to avoid the loss of synchronism. Other points under consideration for quick operation are protec- tion of the equipment from burning due to heavy fault currents, interruption of supply to consumers and the fall in system voltage which may result in the loss of industrial loads. The operating. time of a protective relay is usually one cycle. Half-cycle relays are also available. For distribution systems the operat- ing time may be more than one cycle. 1.10 Classification of Protective Relays Protective relays can be classified in various ways depending on their con- struction, function, etc. and will be discussed in more details in the following chapters. 1.10.1 Classification of Protective Relays Based on Technology Protective relays can be broadly classified into the following categories, depending on the technology they use for their construction and operation. (i) Electromagnetic Relays (ii) Static Relays (iii) Microprocessor-Based RelaysIntroduction 13 Electromagnetic relays Electromagnetic relays include attracted armature, moving coil, induction disc and induction cup type relays. Electromagnetic relays contain an electromag- net (or a permanent magnet) and a moving part. When the actuating quantity exceeds a certain predetermined valuc, an operating torque is developed which is applicd on the moving part. This causes the moving part to travel and to finally close a contact to energise the trip coil of the circuit breaker. Static relays Static relays contain electronic circuitry which may include transistors, ICs, diodes and other electronic components. There is a comparator circuit in the relay, which compares two or more currents or voltages and gives an output which is applied to either a slave relay or a thyristor circuit. The slave relay is an electromagnetic relay which finally closes the contact. A static relay containing a slave relay is a semi-static relay. A relay using a thyristor circuit is a wholly static relay. Static relays possess the advantages of having low burden on the C.T. and P.T., fast operation, absence of mechanical inertia and contact trouble, long life and less maintenance. Static relays have proved to be superior to electromagnetic relays and they are being used for the protection of important lines, power stations and sub-stations. Yet they have not completely replaced electromagnetic relays. Static relays are treated as an addition to the family of relays. Electromagnetic relays continue to be in use because of their simplicity and low cost. Their maintenance can be done by less qualified personnel, whereas the maintenance and repair of static relays requires person- nel trained in solid state devices. Microprocessor-based protective relays Microprocessor-based protective relays are the latest development in this area. With the developments in VLSI technology, sophisticated and fast micropro- cessors are coming up. Their applications to the problems of protective relaying schemes are of current interest to power engineers. The inherent advantages of microprocessor-based relays over static relays with or a very limited range of applications, are attractive flexibility due to their program- mable approach. Microprocessor-based protective relays can provide protec- tion at low cost and compete with conventional relays. The present downward trend in the cost of large scale integrated circuits will encourage wide applica- tions of microprocessor-based relays for the protection of modern complex power networks. 1.10.2 Classification of Protective Relays Based on Their Function Protective relays can be classified into the following categories, depending on the duty they are required to perform.14 Power System Protection and Switchgear @ Overcurrent relays ii) Undervoltage relays (iii) Impedance relays (iv) Underfrequency relays (v) Directional relays, etc. These are some important relays. Many other relays specifying their duty they perform can be put under this type of classification. The duty which a relay performs is evident from its name. For example, an overcurrent relay operates when the current exceeds a certain limit, an impedance relay measures the line impedance between the relay location and the point of fault and operates if the point of fault lies within the protected section. Directional relays check whether the point of fault lies in the forward or reverse direction. The above relays may be electromagnetic, static or microprocessor-based relays. 1.11 Classification of Protective Schemes A protective scheme is used to protect an equipment or a section of the line. It includes one or more relays of the same or different types. The following are the most common protective schemes which are usually used for the protection of a modern power system. (i) Overcurrent Protection (ii) Distance Protection (iii) Carrier-Current Protection (iv) Differential Protection 1.11.1 Overcurrent Protection This scheme of protection is used for the protection of distribution lines, large motors, equipment, etc. It includes one or more overcurrent relays. An overcurrent relay operates when the current exceeds its pick-up valuc. 1.11.2 Distance Protection Distance protection is used for the protection of transmission or sub-transmis- sion lines; usually 33 KV, 66 kV and 132 kV lines. It includes a number of distance relays of the same or different types. A distance relay measures the distance between the relay location and the point of fault in terms of imped- ance, reactance, etc. The relay operates if the point of fault lies within the protected section of the line. There are various kinds of distance relays. The important types are impedance, reactance and mho type. An impedance relay measures the line impedance between the fault point and relay location; a reactance relay measures reactance, and a mho relay measures a component of admittance,