A arte & ciencia da protecção de transmissão, Manuais, Projetos, Pesquisas de Eletrotécnica

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I N D E X 1

Abnormal conditions other than short circuits,, 8 A-c tripping, 335 Angle-impedance relay, 79 for tripping on 1088 of synchronism, 362 Angle of maximum torque, adjustment,, 57 of power relays, 52, 55 of shortcircuit relays, 55 Arcs, effect on distance relays, 345 resistance, 302 Arc - furnace - transformer protection, 270 ASA accuracy classification,, of current transformers,, 121 of potential transformers, 133 Attenuation, carrier-current, 378 Automatic reclosing, see Reclosing, automatic Auxiliaries,, station, protection of, 229 see also Motor protection Back-up relaying, defined, 6 effect of intermediate current sources, 347 for bus protection, 275, 291 generator external fault, reversed third zone, 175, 349 transformer external fault, 264, 268 with pilot relaying, 378, 386 Blind spot in pilot relaying, 94 Blinder with directional-comparison relaying, 386, 390 Blocking pilot, 88, 91 Blocking terminal, with a-c wire-pilot relaying, 375

with directionalcomparison pilot relaying, 392 with phase-comparison pilot relaying, 383 Broken-delta connection, burden calculation, 140 for detecting grounds in ungrounded systems,, 320 for polarizing directionalground relays, 151 of capacitance potential devices, 135, 140 Buchholz relay, 281 Burden, current-transformer, 114 potential transformer, 133 Bus protection, automatic reclosing, 292 by back-up relays, 275, 291 circuitbreaker by-psasing, 292 combined power transformer and bus, 287 current differential with overcurrent relays, 278 current differential with overvoltage relays, 288 current differential with percentage relays, 284 directional comparison, 277 fault bus,, 275 grounding secondaries of differentially connected CT’s, 291 partial-differential, 282 ring-bus,, 290 testing, 292 voltage differential with linear cou, 284

Bushing potential device, see Capacitance potential device

Capacitance potential device, broken burden, 140 comparison with instrument PT’s, 144 couplingcapacitor insulation coordination, 142 effect of overloading, 138

I N D E X

I N D E X 3

Directional relays,, a-c types, 33, 52 connections,, 52 power, 52 short circuits, 56 characteristics on R-X diagram, 74 d-c types,, 49 use of shunts, 52 effect of mutual induction on ground relays, 393 electromagnetic-attraction type, 24 ground- relay polarization, 151, 326 misoperating tendencies, 314 negative-phase-aequence type, 330 operating characteristics,, 37 response of polyphase relays to positive and negative phase sequence,, 183 response of ainglephase relays to short circuits,, 187

Distance relays,, current and voltage switching, 368 effect of power swings and lose of synchronism,, 181 effect of wye-delta transformer between relay and fault, 172 electronic type, 369 ground-relay connections, 360 impedance seen during faults,, 167

Generator protection, overvoltage, 217 potential-transformer fuse blowing, 228 prime mover, 230 station auxiliary, 229 stator overheating, 216 stator short circuit, 195 calculation of CT errors, 198 ground faults, sensitive, 208 ground faults in unit generators, 209 overcurrent relays for, 215 turn-to-turn faults, 204 unbalanced phase currents, 221 vibration, 225

Ground-distance-relay connections, 360

Ground-fault neutralizer, effect on line relaying, 321 to mitigate the effect of a fault, 2

Grounding protective relay for trans former protection, 263

Ground preference, 91, 108

Ground resistance, 303

Grounding-tranaformer protection, 268

Hsrmonic-current restraint, for distance relays,, 357 for transformer differential relays, 257

Holding coil, 18

Impedance diagram, see R-X diagram

Impedance relay, characteristic on R-X diagram, 72 for line protection, 340 general, 70 see also Distance relays

Induction-cup and induction-loop structures, 30, 31

Induction-type relay, directional, 31 general characteristics,, 26 single-quantity, 31 structures, 29 torque production, 26

Insulating transformer for pilot-wire circuits,, 98

Intermittent pilot, 109

Line protection with distance relays,, adjustment of distance relays,, 341 area, effect of, 345 blocking tripping on 1088 of synchro , 304 choice between impedance, reactance, and mho,, 340 connections of ground distance relays,, 360

Distance relays,, use of low-tension voltage, 145, 148

see also Line protection with distance relays

Distribution-circuit protection, see Line protection with overcurrent relays

Drop-out defined, 17

Electric arc-furnace-transformer protection, 270

Electromagnetic-attraction relay, directional, 24

general characteristics,, 16 single-quantity, 22

Electronic relay, directionalcomparison pilot, 396

distance, 369

4 I N D E X

Evaluation of protective relaying, 12

Expulsion protective gaps, effect of, on distance relays, 367

External-fault back-up relaying, see Back-up relaying

Fail ures,, electrical, see Faults

False residual current, 318

Fault bus, 275

Faults, mitigation of effects of, 2 prevention of, 2 probability of, effect on practice, 11 see also Short circuits

Fire, protection against, 230

Fire-pump-motor protection, 230

Footing resistance, tower-, 303

Frequency, compensation of relays for changes in, 49

effect on induction relays,, 32, 39

Frequency-converter protection, see Generator protection

Fundamental principles of protective relaying, 4

Fuse, coordinating with a, 335

Fuse blowing, potential-tranaformer, effect on distance relays,, 361

effect on generator relays,, 228

Generator protection, bearing overheating, 228

external-fault back-up, m field ground, 218

loss of excitation, 223

1088 of synchronism,, 218 miscellaneous,, 228

motoring, 225

open circuits,, 215 over excitation,, 225 over speed,, 226

Line protection with distance relays, current and voltage switching, 368 expulsion protective gaps, effect of, 367 fuse blowing, effect of, 361 electronic relays, 369 intermediate current sources, effect of, 347 low-tension current, use of, 356 low-tension voltage, use of, 352 magnetizing inrush, effect of, 359 overreach, 351, 360 purposeful tripping on loss of synchronism, 361 reclosing, automatic, 366 series capacitor, effect of, 367 see also Distance relays

Line protection with overcurrent relays, a-c and capacitor tripping, 335 adjustment of ground vs. phase re lays, 316 adjustment of inverse-time-overcur rent relays, 297 arc and ground resistance, 302 directional feature, 310 fuses, coordination with, 335 ground faults in ungrounded systems, detection of, 319 ground-fault neutralizers, effect of, 321 instantaneous overcurrent relays,, use of, 306 inverseness,, choice of, in relay char acteristics, 305 limiting ground-fault-current magni tude, effect of, 317 loop circuits,, effect on relay adjust , 303 misoperation prevention of aingle directional-overcurrent re during ground faults, 314 negative-phase sequence ground di

6 I N D E X

Modified-impendance relay, 77

Motor protection, field ground, 237 fire-pump, 230 1088 of excitation, 237 1088 of synchronism, 236 rotor overheating, 236 stator overheating, 232 stator short circuit, 230 unattended motors,, 230 under voltage,, 237

Multiterminal-line protection, with a-c wire-pilot relaying, 376 with directional-comparieon pilot relaying, 387 with phasecomparison pilot relaying, 382

Mutual induction, effect of, on direc relays,, 393 from power circuit to pilot wires,, 88

Neutralizing transformers for wire-pilot circuits,, 99

Normally blocked trip circuit, 109

Open phase, effect of, on directional relays, 323, 325 equivalent circuits for, 323 protection of generators against, 215

Operating principles,, basic, electromagnetic- attraction relays,, 16

Operating principles, directional type, 24 single- quantity type, 22 induction relays: directional type, 33 singlequantity type, 31

Operation indicator, 17

Operator vs. protective relays, 11

Opposed-voltage pilot relaying, 92, 95

Out of step, see Loss of synchronism

Overcurrent relays, combination of instantaneous and time delay, 49 pickup or reset, 45 time delay, 45, 46 see also Line protection with overcurrent relays

Overreach, of distance relays, 82, 350, 351 of

instantaneous overcurrent relays, 308

Over travel,, defined, 48 effect of, on overcurrent-relay adjustment, 301

Overvoltage, in CI secondaries, 124 see also Generator protection

Overvoltage relays, pickup or reset,, 45 time delay, 45, 46

Percentage-differential relays, description, 65 for bus protection, 284 for generator protection, 195 for transformer protection, 241 locking-in for internal faults,, 202, 251 product restraint, 204 variable percent slope, 187, 203

Petersen coil, see Ground-fault neutralizer

Phase-comparison relaying, principle of operation, 101 see also Line protection with pilot relays

Pilot relaying, circulating current, 92, 94 general, 86 line protection, 373, 374 opposed voltage, 92, 95 pilot-wire protection, 98 pilot- wire requirements, 97 pilot-wire supervision, 97 remote tripping, 97 see also Line protection with pilot relays

Pilot relaying, carrier-current-, directional comparison, 106 general, 86 intermittent or continuous, 109 phase comparison, 101 pilot described, 100 supervision of pilot channel, 378 see also Line protection with pilot relays ;

Pilot relaying,-d-c wire-, basic principles, 89 description of pilot 86

Pilot relaying, micro-wave-, description of channel, 397 general, 86, 101, 396 remote tripping, 398 see also Line protection with pilot relays

Pilot relaying, principles of, ground preference, 91 purpose of a pilot, 87 sensitivity levels, 91, 106 tripping and blocking pilots, 88, 90

I N D E X 7

Polarizing quantity, of a-c directional relays’ 37 of d-c directional relays, 61 of directional units of ground relays, 151, 326

Polyphase directional relays, advantages and disadvantages, 313 response to positive- and negative volt-amps, 183

Potential device, see Capacitance potential device

Potential transformers, accuracy, 133 connections, 146 effect of fuse blowing, on distance relaying, 361 on generator relaying, 228 low-tension voltage for distance relays, 148

Power-balance relaying for line protection, 330

Power-rectifier-transformer protection, 271

Power swings, see Loss of synchronism

Power transformer and autotransformer protection, external-fault backup, 264 gasaccumulator and pressure relays, 261 grounding protective relay, 263 overcurrent relaying, 261 percentagedifferential relaying, 241 CT accuracy requirements, 251 CT connections, 242 CT ratios, 250 magnetizing inrush, effect of, 254 parallel banks, 258 percent slope, choice of, 252 two-winding relay for threewinding transformer, 252 zero- phasesequence-current shunt, 249 remote tripping, 263

Primary relaying defined, 4

Product restraint for generator differential relays, 204

Protective relaying, evaluation of, 12 function of, 3 functional characteristics of, 9 fundamental principles of, 4

Ratings, relay, contact, 43 current and voltage,

42 holding coil, seal-in coil, and target, 43

Ratio correction factor, see Current transformers; Voltage trans formers

Reactance relay, characteristic on R-X diagram, 79 operating characteristic, 78 partial- differential bus protection, 282 see also Line protection with distance relays

Reclosing, automatic, affected by remote tripping, 367 effect on blocking-terminal pilot equipment, 376 mitigation of fault effects, 2 of bus breakers,, 292 single-phase switching, 399 synchronism check, 333, 366 with distance relaying, 366 with overcurrent relaying, 333 with pilot relaying, 399

Rectifier-transformer protection, 271

Regulating-transformer protection, ex fault back-up, 208 in-phase type, 265 phase-shifting type, 267

Reliability, a functional relay characteristic, 9

Remote tripping, by carrier current, 384 by microwave, 398 by pilot wire, 97 for power- transformer protection, 263 frequency-shift system, 264

Replica-type relays, for generator protection, 216 for motor protection, 232, 235

Reset, adjustment of, 19 defined, 17 ratio to pickup, electromagnetic relays, 22 induction relays, 32 time of induction relays, 33, 49

Reversed third zone, for back-up with carrier pilot, 349 where a line includes a transformer, 175

R-X diagram, los-of-synchronism char , 176 appearance to distance relays,, 181 principle of, 72, 156 short-circuit characteristics, 160 appearance to distance relays,, 167 wyedelta transformer between relay and fault, 172

I N D E X 9

Transient shunt, for distance relays, 351

for overcurrent relays, 310

Transmission-line protection, with distance relaying, 340

with overcurrent relaying, 296

Tranemission-line protection, with pilot relaying, 373

Trap, line, 100

Tripping pilot, 88, 90

Tripping suppressor for transformer differential relays, 256

Tripping, undesired, vs. failure to trip, 11

Undercurrent and under voltage relays, pickup or reset, 45 time, 46

Universal relay torque equation, 39

Variable restraint for generator differential relays, 203

Vector conventions, 53

Vibration, of electromagnetic-attrao relays, 23 protection for generators, 210, 225

Voltage-balance relays, 61

Voltage compensation for ground distance relays, 360

Voltage-regulator protection, 268 see also Regulating-transformer protection; Step- voltage-regulator protection

Voltage switching for distance relays, 368

Voltage transformers, see Capacitance potential devices; Potential transformers

Watthour-meter structure, 30

Wire-pilot relaying, see Pilot relaying, a-c wire-; Pilot relaying, d-c wire

Zero-phase-sequence-current shunt, current- transformer connections, 130 used with transformer differential relays, 249

THE PHILOSOPHY OF PROTECTIVE RELAYING 1

1 THE PHILOSOPHY OF PROTECTIVE RELAYING

WHAT IS PROTECTIVE RELAYING?

We usually think of an electric power system in terms of its more impressive parts–the big generating stations, transformers, high-voltage lines, etc. While these are some of the basic elements, there are many other necessary and fascinating components. Protective relaying is one of these. The role of protective relaying in electric-power-system design and operation is explained by a brief examination of the over-all background. There are three aspects of a power system that will serve the purposes of this examination. These aspects are as follows: A. Normal operation B. Prevention of electrical failure. C. Mitigation of the effects of electrical failure. The term “normal operation” assumes no failures of equipment, no mistakes of personnel, nor “acts of God.” It involves the minimum requirements for supplying the existing load and a certain amount of anticipated future load. Some of the considerations are: A. Choice between hydro, steam, or other sources of power. B. Location of generating stations. C. Transmission of power to the load. D. Study of the load characteristics and planning for its future growth. E. Metering F. Voltage and frequency regulation. G. System operation. E. Normal maintenance. The provisions for normal operation involve the major expense for equipment and operation, but a system designed according to this aspect alone could not possibly meet present-day requirements. Electrical equipment failures would cause intolerable outages. There must be additional provisions to minimize damage to equipment and interruptions to the service when failures occur. Two recourses are open: (1) to incorporate features of design aimed at preventing failures, and (2) to include provisions for mitigating the effects of failure when it occurs. Modern

THE PHILOSOPHY OF PROTECTIVE RELAYING 3

D. Features that operate throughout the period from the inception of the fault until after its removal, to maintain voltage and stability.

1. Automatic voltage regulation.
2. Stability characteristics of generators. E. Means for observing the electiveness of the foregoing features.
3. Automatic oscillographs.
4. Efficient human observation and record keeping. F. Frequent surveys as system changes or additions are made, to be sure that the foregoing features are still adequate. Thus, protective relaying is one of several features of system design concerned with minimizing damage to equipment and interruptions to service when electrical failures occur. When we say that relays “protect,” we mean that, together with other equipment, the relays help to minimize damage and improve service. It will be evident that all the mitigation features are dependent on one another for successfully minimizing the effects of failure. Therefore, the capabilities and the application requirements of protective-relaying equipments should be considered concurrently with the other features.^2 This statement is emphasized because there is sometimes a tendency to think of the protective-relaying equipment after all other design considerations are irrevocably settled. Within economic limits, an electric power system should be designed so that it can be adequately protected.

THE FUNCTION OF PROTECTIVE RELAYING

The function of protective relaying is to cause the prompt removal from service of any element of a power system when it suffers a short circuit, or when it starts to operate in any abnormal manner that might cause damage or otherwise interfere with the effective operation of the rest of the system. The relaying equipment is aided in this task by circuit breakers that are capable of disconnecting the faulty element when they are called upon to do so by the relaying equipment. Circuit breakers are generally located so that each generator, transformer, bus, transmission line, etc., can be completely disconnected from the rest of the system. These circuit breakers must have sufficient capacity so that they can carry momentarily the maximum short-circuit current that can flow through them, and then interrupt this current; they must also withstand closing in on such a short circuit and then interrupting it according to certain prescribed standards. 3 Fusing is employed where protective relays and circuit breakers are not economically justifiable. Although the principal function of protective relaying is to mitigate the effects of short circuits, other abnormal operating conditions arise that also require the services of protective relaying. This is particularly true of generators and motors. A secondary function of protective relaying is to provide indication of the location and type of failure. Such data not only assist in expediting repair but also, by comparison with

4 THE PHILOSOPHY OF PROTECTIVE RELAYING

human observation and automatic oscillograph records, they provide means for analyzing the effectiveness of the fault-prevention and mitigation features including the protective relaying itself.

FUNDAMENTAL PRINCIPLES OF PROTECTIVE RELAYING

Let us consider for the moment only the relaying equipment for the protection against short circuits. There are two groups of such equipment–one which we shall call “primary” relaying, and the other “back-up” relaying. Primary relaying is the first line of defense, whereas back-up relaying functions only when primary relaying fails.

PRIMARY RELAYING

Fig. 1. One-line diagram of a portion of an electric power system illustrating primary relaying.

6 THE PHILOSOPHY OF PROTECTIVE RELAYING

A clear understanding of the possible causes of primary-relaying failure is necessary for a better appreciation of the practices involved in back-up relaying. When we say that primary relaying may fail, we mean that any of several things may happen to prevent primary relaying from causing the disconnection of a power-system fault. Primary relaying may fail because of failure in any of the following: A. Current or voltage supply to the relays. B. D-c tripping-voltage supply. C. Protective relays. D. Tripping circuit or breaker mechanism. E. Circuit breaker. It is highly desirable that back-up relaying be arranged so that anything that might cause primary relaying to fail will not also cause failure of back-up relaying. It will be evident that this requirement is completely satisfied only if the back-up relays are located so that they do not employ or control anything in common with the primary relays that are to be backed up. So far as possible, the practice is to locate the back-up relays at a different station. Consider, for example, the back-up relaying for the transmission line section EF of Fig. 3. The back-up relays for this line section are normally arranged to trip breakers A, B, I, and J. Should breaker E fail to trip for a fault on the line section EF, breakers A and B are tripped; breakers A and B and their associated back-up equipment, being physically apart from the equipment that has failed, are not likely to be simultaneously affected as might be the case if breakers C and D were chosen instead.

The back-up relays at locations A, B, and F provide back-up protection if bus faults occur at station K. Also, the back-up relays at A and F provide back-up protection for faults in the line DB. In other words, the zone of protection of back-up relaying extends in one direction from the location of any back-up relay and at least overlaps each adjacent system element. Where adjacent line sections are of different length, the back-up relays must overreach some line sections more than others in order to provide back-up protection for the longest line. A given set of back-up relays will provide incidental back-up protection of sorts for faults in the circuit whose breaker the back-up relays control. For example, the back-up relays that trip breaker A of Fig. 3 may also act as back-up for faults in the line section AC. However,

Fig. 3. Illustration for back-up protection of transmission line section EF.

THE PHILOSOPHY OF PROTECTIVE RELAYING 7

this duplication of protection is only an incidental benefit and is not to be relied on to the exclusion of a conventional back-up arrangement when such arrangement is possible; to differentiate between the two, this type might be called “duplicate primary relaying.” A second function of back-up relaying is often to provide primary protection when the primary-relaying equipment is out of service for maintenance or repair. It is perhaps evident that, when back-up relaying functions, a larger part of the system is disconnected than when primary relaying operates correctly. This is inevitable if back-up relaying is to be made independent of those factors that might cause primary relaying to fail. However, it emphasizes the importance of the second requirement of back-up relaying, that it must operate with sufficient time delay so that primary relaying will be given enough time to function if it is able to. In other words, when a short circuit occurs, both primary relaying and back-up relaying will normally start to operate, but primary relaying is expected to trip the necessary breakers to remove the short-circuited element from the system, and back-up relaying will then reset without having had time to complete its function. When a given set of relays provides back-up protection for several adjacent system elements, the slowest primary relaying of any of those adjacent elements will determine the necessary time delay of the given back-up relays. For many applications, it is impossible to abide by the principle of complete segregation of the back-up relays. Then one tries to supply the back-up relays from sources other than those that supply the primary relays of the system element in question, and to trip other breakers. This can usually be accomplished; however, the same tripping battery may be employed in common, to save money and because it is considered only a minor risk. This subject will be treated in more detail in Chapter 14. In extreme cases, it may even be impossible to provide any back-up protection; in such cases, greater emphasis is placed on the need for better maintenance. In fact, even with complete back-up relaying, there is still much to be gained by proper maintenance. When primary relaying fails, even though back-up relaying functions properly, the service will generally suffer more or less. Consequently, back-up relaying is not a proper substitute for good maintenance.

PROTECTION AGAINST OTHER ABNORMAL CONDITIONS

Protective relaying for other than short circuits is included in the category of primary relaying. However, since the abnormal conditions requiring protection are different for each system element, no universal overlapping arrangement of relaying is used as in short protection. Instead, each system element is independently provided with whatever relaying is required, and this relaying is arranged to trip the necessary circuit breakers which may in some cases be different from those tripped by the short-circuit relaying. As previously mentioned, back-up relaying is not employed because experience has not shown it to be economically justifiable. Frequently, however, back-up relaying for short circuits will function when other abnormal conditions occur that produce abnormal currents or voltages, and back-up protection of sorts is thereby incidentally provided.

THE PHILOSOPHY OF PROTECTIVE RELAYING 9

types of relays are purchased. Such a manual specifies minimum test and maintenance procedure that experience has shown to be desirable. The manual is prepared in part from manufacturers’ publications and in part from the utility’s experience. As a consequence of standardized techniques, the results of periodic tests can be compared to detect changes or deterioration in the relays and their associated devices. Testers are encouraged to make other tests as they see fit so long as they make the tests required by the manual. If a better testing technique is devised, it is incorporated into the manual. Some organizations include information on the purpose of the relays, to give their people better appreciation of the importance of their work. Courses may be given, also. Such activity is highly recommended. Unless a person is thoroughly acquainted with relay testing and maintenance, he can do more harm than good, and he might better leave the equipment alone. In some cases, actual field tests are made after installation and after careful preliminary testing of the individual relays. These field tests provide an excellent means for checking the over-all operation of all equipment involved. Careful maintenance and record keeping, not only of tests during maintenance but also of relay operation during actual service, are the best assurance that the relaying equipment is in proper condition. Field testing is the best-known way of checking the equipment prior to putting it in service, but conditions may arise in actual service that were not anticipated in the tests. The best assurance that the relays are properly applied and adjusted is a record of correct operation through a sufficiently long period to include the various operating conditions that can exist. It is assuring not only when a particular relaying equipment trips the proper breakers when it should for a given fault but also when other relaying equipments properly refrain from tripping.

ARE PROTECTIVE PRACTICES BASED ON THE

PROBABILITY OF FAILURE?

Protective practices are based on the probability of failure to the extent that present-day practices are the result of years of experience in which the frequency of failure undoubtedly has played a part. However, the probability of failure seldom if ever enters directly into the choice of a particular type of relaying equipment except when, for one reason or another, one finds it most difficult to apply the type that otherwise would be used. In any event, the probability of failure should be considered only together with the consequences of failure should it occur. It has been said that the justification for a given practice equals the likelihood of trouble times the cost of the trouble. Regardless of the probability of failure, no portion of a system should be entirely without protection, even if it is only back-up relaying.

PROTECTIVE RELAYING VERSUS A STATION OPERATOR

Protective relaying sometimes finds itself in competition with station operators or attendants. This is the case for protection against abnormal conditions that develop slowly enough for an operator to have time to correct the situation before any harmful consequences develop. Sometimes, an alert and skillful operator can thereby avoid having

10 THE PHILOSOPHY OF PROTECTIVE RELAYING

to remove from service an important piece of equipment when its removal might be embarrassing; if protective relaying is used in such a situation, it is merely to sound an alarm. To some extent, the preference of relying on an operator has a background of some unfortunate experience with protective relaying whereby improper relay operation caused embarrassment; such an attitude is understandable, but it cannot be supported logically. Where quick and accurate action is required for the protection of important equipment, it is unwise to rely on an operator. Moreover, when trouble occurs, the operator usually has other things to do for which he is better fitted.

UNDESIRED TRIPPING VERSUS FAILURE TO TRIP WHEN

DESIRED

Regardless of the rules of good relaying practice, one will occasionally have to choose which rule may be broken with the least embarrassment. When one must choose between the chance of undesired or unnecessary tripping and failure to trip when tripping is desired, the best practice is generally to choose the former. Experience has shown that, where major system shutdowns have resulted from one or the other, the failure to trip–or excessive delay in tripping-has been by far the worse offender.

THE EVALUATION OF PROTECTIVE RELAYING

Although a modern power system could not operate without protective relaying, this does not make it priceless. As in all good engineering, economics plays a large part. Although the protection engineer can usually justify expenditures for protective relaying on the basis of standard practice, circumstances may alter such concepts, and it often becomes necessary to evaluate the benefits to be gained. It is generally not a question of whether protective relaying can be justified, but of how far one should go toward investing in the best relaying available. Like all other parts of a power system, protective relaying should be evaluated on the basis of its contribution to the best economically possible service to the customers. The contribution of protective relaying is to help the rest of the power system to function as efficiently and as effectively as possible in the face of trouble. 2 How protective relaying does this is as foIlows. By minimizing damage when failures occur, protective relaying minimizes: A. The cost of repairing the damage. B. The likelihood that the trouble may spread and involve other equipment. C. The time that the equipment is out of service. D. The loss in revenue and the strained public relations while the equipment is out of service. By expediting the equipment’s return to service, protective relaying helps to minimize the amount of equipment reserve required, since there is less likelihood of another failure before the first failure can be repaired.