AC generator working Principles

a.c generator construction and what are the a.c generator components and what are the difference between a.c generator and d.c motor
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HelenBoyle,Greece,Researcher
Published Date:14-07-2017
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10A-C GENERATOR AND MOTOR PROTECTION The remaining chapters deal with the application of protective relays to each of the several elements that make up the electric power system. Although there is quite good agreement among protection engineers as to what constitutes the necessary protection and how to provide it, there are still many differences of opinion in certain areas. This book describes the general practice, giving the pros and cons where there are differences of opinion. Four 1,2,3,4 standard-practice publications deal with the application of protective relays. 5,6,20 ManufacturersÕ publications are also available. Bibliographies of relaying literature prepared by an AIEE committee provide convenient reference to a wealth of information 7 for more detailed study. Frequent reference will be made here to publications that have been found most informative. The fact that this book recognizes differences of opinion should not be interpreted as complete approval of the various parallel practices. Although it is recognized that there may sometimes be special economic and technical considerations, nevertheless, much can still be done in the way of standardization. GENERATOR PROTECTION Except where specifically stated otherwise, the following will deal with generators in attended stations, including the generators of frequency converters. The protection of generators involves the consideration of more possible abnormal operating conditions than the protection of any other system element. In unattended stations, automatic protection against all harmful abnormal conditions should be 1 provided. But much difference of opinion exists as to what constitutes sufficient protection of generators in attended stations. Such difference of opinion is mostly concerning the protection against abnormal operating conditions, other than short circuits, that do not necessarily require the immediate removal from service of a machine and that might be left to the control of an attendant. The arguments that are advanced in favor of a minimum amount of automatic protective equipment are as follows: (a) the more automatic equipment there is to maintain, the poorer maintenance it will get, and hence it will be less reliable; (b) automatic equipment might operate incorrectly and trip a generator undesirably; (c) an operator can sometimes avoid removing a generator from service when its removal would be embarrassing. Most of the objection to automatic protective equipment is not so much that a relay will fail to operate when it should, but that it might remove a generator from service unnecessarily. Part of the basis for this attitude is simply fear. Each additional device adds another A-C GENERATOR AND MOTOR PROTECTION 171contact that can trip the generator. The more such contacts there are, the greater is the possibility that one might somehow close when it should not. There is some justification for such fears. Relays have operated improperly. Such improper operation is most likely in new installations before the installation ÒkinksÓ have been straightened out. Occasionally, an abnormal operating condition arises that was not anticipated in the design or application of the equipment, and a relay operates undesirably. Cases are on record where cleaning or maintenance personnel accidentally caused a relay to trip a generator. But, if something is known to be basically wrong with a protective relay so that it cannot be relied on to operate properly, it should not be applied or it should be corrected one way or another. Otherwise, fear alone is not a proper basis for omitting needed protection. Admittedly, an alert and skillful operator can sometimes avoid removing a generator from service. In general, however, and with all due respect to operators, the natural fear of removing a machine from service unnecessarily could result in serious damage. Operators have been known to make mistakes during emergencies and to trip generators 8 unnecessarily as well as to fail to trip when necessary. Furthermore, during an emergency, an operator has other important things to do for which he is better fitted. An unnecessary generator outage is undesirable, but one should not try to avoid it by the omission of otherwise desirable automatic protection. It is generally agreed that any well- designed and well-operated system should be able to withstand a short unscheduled 9 outage of the largest generating unit. It is realized that sometimes it may take several hours to make sure that there is nothing wrong with the unit and to return it to service. Nevertheless, if this is the price one has to pay to avoid the possibility of a unitÕs being out of service several months for repair, it is worth it. The protection of certain generators against the possibility of extensive damage may be more important than the protection of 9 the service of the system. The practice is increasing of using centralized control, which requires more automatic equipment and less manual Òon the spotÓ supervision, in order to provide higher standards 10 of service with still greater efficiency. Such practice requires more automatic protective- relaying equipment to provide the protection that was formerly the responsibility of 8 attendants. SHORT-CIRCUIT PROTECTION OF STATOR WINDINGS BY PERCENTAGE-DIFFERENTIAL RELAYS It is the standardized practice of manufacturers to recommend differential protection for 2 generators rated l000 kva or higher, and most of such generators are protected by 11 differential relays. Above 10,000 kva, it is almost universally the practice to use differential 9 relays. Percentage-differential relaying is the best for the purpose, and it should be used wherever it can be justified economically. It is not necessarily the size of a generator that determines how good the protection should be; the important thing is the effect on the rest of the system of a prolonged fault in the generator, and how great the hardship would be if the generator was badly damaged and was out of service for a long time. The arrangement of CTÕs and percentage-differential relays is shown in Fig. 1 for a wye- connected machine, and in Fig. 2 for a delta machine. If the neutral connection is made inside the generator and only the neutral lead is brought out and grounded through low 172 A-C GENERATOR AND MOTOR PROTECTIONimpedance, percentage-differential relaying for ground faults only can be provided, as in Fig. 3. The connections for a so-called ÒunitÓ generator-transformer arrangement are shown in Fig. 4; notice that the CTÕs on the neutral side may be used in common by the differential-relaying equipments of the generator and the transformer. For greatest sensitivity of differential relaying, the CT primary-current rating would have to be equal to the generatorÕs rated full-load current. However, in practice the CT primary- current rating is as much as about 25% higher than full load, so that if ammeters are connected to the CTÕs their deflections will be less than full scale at rated load. It may be impossible to abide by this rule in Fig. 5; here, the primary-current rating of the CTÕs may have to be considerably higher than the generatorÕs rated current, because of the higher system current that may flow through the CTÕs at the breakers. Fig. 1. Percentage-differential relaying for a wye-connected generator. The way in which the generator neutral is grounded does not influence the choice of percentage-differential relaying equipment when both ends of all windings are brought out. But, if the neutral is not grounded, or if it is grounded through high enough impedance, the differential relays should be supplemented by sensitive ground-fault relaying, which will be described later. Such supplementary equipment is generally provided when the ground-fault current that the generator can supply to a single-phase-to- ground fault at its terminals is limited to less than about rated full-load current. Otherwise, the differential relays are sensitive enough to operate for ground faults anywhere from the A-C GENERATOR AND MOTOR PROTECTION 173terminals down to somewhat less than about 20% of the winding away from the neutral, depending on the magnitude of fault current and load current, as shown in Fig. 6, which was obtained from calculations for certain assumed equipment. This is generally considered sensitive enough because, with less than 20% of rated voltage stressing the insulation, a ground fault is most unlikely; in the rare event that a fault did occur, it would simply have to spread until it involved enough of the winding to operate a relay. To make the percentage-differential relays much more sensitive than they are would make them likely to operate undesirably on transient CT errors during external disturbances. Fig. 2. Percentage-differential relaying for a delta-connected generator. The foregoing raises the question of CT accuracy and loading. It is generally felt that CTÕs having an ASA accuracy classification of 10H200 or 10L200 are satisfactory if the burdens imposed on the CTÕs during external faults are not excessive. If variable-percentage relays (to be discussed later) are used, CTÕs of even lower accuracy classification may be permissible, or higher burdens may be applied. It is the difference in accuracy between the CTÕs (usually of the same type) at opposite ends of the windings that really counts. The difference between their ratio errors should not exceed about one-half of the percent slope of the differential relays for any external fault beyond the generator terminals. Such things as unequal CT secondary lead lengths, or the addition of other burdens in the leads on one side or the other, tend to make the CTÕs have different errors. A technique for calculating the steady-state errors of CTÕs in a differential circuit will be described for the circuit of Fig. 7, where a single-phase-to-ground external fault is assumed to have occurred 174 A-C GENERATOR AND MOTOR PROTECTIONon the phase shown. The equivalent circuit of each CT is shown in order to illustrate the method of solution. The fact that (I Ð I ) is flowing through the relayÕs operating coil S1 S2 in the direction shown is the result of assuming that CT is more accurate than CT , or in 1 2 other words that I is greater than I . We shall assume that I and I are in phase, and, S1 S2 S1 S2 by KirchhoffÕs laws, we can write the voltages for the circuit a-b-c-d-a as follows: E Ð I (Z + 2Z + Z ) Ð (I Ð I )Z = 0 1 S1 S1 L1 R S1 S2 0 or (I Ð I )Z = E Ð I (Z + 2Z + Z ) (1) S1 S2 0 1 S1 S1 L1 R Fig. 3. Percentage-differential relaying for a wye-connected generator with only four leads brought out. Similarly, for the circuit e-f-d-c-e, we can write: E2 Ð I (Z + 2Z + Z ) + (I Ð I )Z = 0 S2 S2 L2 R S1 S2 0 or (I Ð I )Z = I (Z + 2Z + Z ) Ð E (2) S1 S2 0 S2 S2 L2 R 2 For each of the two equations 1 and 2, if we assume a value of the socondary-excitation voltage E, we can obtain a corresponding value of the secondary-excitation current I from e the secondary-excitation curve. Having I we can get I from the relation I = I/N Ð I ,where e S S e A-C GENERATOR AND MOTOR PROTECTION 175Fig. 4. Percentage-differential relaying for a unit generator and transformer. Note: phase sequence is a-b-c. 176 A-C GENERATOR AND MOTOR PROTECTIONFig. 5. Generator differential relaying with a double-breaker bus. I is the initial rms magnitude of the fundamental component of primary current. This enables us to calculate the value of (I Ð I ). Finally, the curves of I and I versus S1 S2 S1 S2 (I Ð I ) for each of the two CTÕs is plotted on the same graph, as in Fig. 8. For only one S1 S2 value of the abscissa (I Ð I ) will the difference between the two ordinates I and I be S1 S2 S1 S2 equal to that value of the abscissa and this is the point that gives us the solution to the problem. Once we know the values of I and I , we can quickly determine whether the S1 S2 differential relay will operate for the maximum external fault current. From the example of Fig. 7, it will become evident that, for an external fault, if there is a tendency for one CT to be more accurate than the other, any current that flows through the operating coil of the relay imposes added burden on the more accurate CT and reduces the burden on the less accurate CT. Thus, there is a natural tendency in a current- differential circuit to resist CT unbalances, and this tendency is greater the more impedance there is in the relayÕs operating coil. This is not to say, however, that there may not sometimes be enough unbalance to cause incorrect differential-relay operation when the burden of leads or other devices in series with one CT is sufficiently greater than the burden in series with the other. If so, it becomes necessary to add compensating burden on one side to more nearly balance the burdens. If the CTÕs on one side are inherently considerably more accurate than on the other side, shunt burden, having saturation A-C GENERATOR AND MOTOR PROTECTION 177Fig. 6. Percent of winding unprotected for ground faults. Fig. 7. Equivalent circuit for calculating CT errors in a generator differential circuit. characteristics more or less like the secondary curve of the less accurate CTÕs, can be connected across the terminals of the more accurate CTÕs; this has the effect of making the two sets of CTÕs equally poor, but, at any rate, more nearly alike. 178 A-C GENERATOR AND MOTOR PROTECTIONAnother consequence of widely differing CT secondary-excitation characteristics may be Òlocking-inÓ for internal faults. In such a case, the inferior CTÕs may be incapable of inducing sufficient rms voltage in their secondaries to keep the good CTÕs from forcing current through the inferior CTÕs secondaries in opposition to their induced voltage, thereby providing a shunt around the differential relayÕs operating coil and preventing operation. Adding a shunt burden across the good CTÕs, as was described in the foregoing paragraph, is a solution to this difficulty. The larger the impedance of the operating coil, the more likely locking-in is to happen. However, the circumstances that make it possible are rare. Fig. 8. Plot of the relations of the currents of Fig. 7 for various assumed values of E and E . 1 2 Chapter 7 described the possibility of harmfully high overvoltages in CT secondary circuits of generator-differential relays when the system is capable of supplying to a generator fault short-circuit current whose magnitude is many times the rating of the CTÕs. In such cases, it is necessary to use overvoltage limiters, as treated in more detail in Chapter 7. Whether to use high-speed relays or only the somewhat slower ÒinstantaneousÓ relays is sometimes a point of contention. If system stability is involved, there may be no question but that high-speed relays must be used. Otherwise, the question is how much damage will be prevented by high-speed relays. The difference in the damage caused by the current supplied by the generator will probably be negligible in view of the continuing flow of fault current because of the slow decay of the field flux. But, if the system fault-current contribution is very large, considerable damage may be prevented by the use of high-speed relays and main circuit breaker. It is easy enough to compare the capabilities of doing 2 damage in terms of I t, but the cost of repair is not necessarily directly proportional, and there are no good data in this respect. The savings to be made in the cost of slower-speed relays are insignificant compared to the cost of generators, and there cannot fail to be some benefits with high-speed relays. Except for Òmatch and line-upÓ considerations, the slower-speed relays might well be eliminated. Generally, the practice is to have the percentage-differential relays trip a hand-reset multi- contact auxiliary relay. This auxiliary relay simultaneously initiates the following: (1) trip main breaker, (2) trip field breaker, (3) trip neutral breaker if provided, (4) shut down the prime mover, (5) turn on CO if provided, (6) operate an alarm and/or annunciator. The 2 auxiliary relay may also initiate the transfer of station auxiliaries from the generator terminals to the reserve source, by tripping the auxiliary breaker. Whether to provide a A-C GENERATOR AND MOTOR PROTECTION 179main breaker or only an exciter-field breaker is a point of contention. The tripping of a main-field breaker instead of only an exciter-field breaker will minimize the damage, but there is insufficient evidence to prove whether it is worth the additional expense where other important factors urge the omission of a main-field breaker. Consequently, the practice is divided. THE VARIABLE-PERCENTAGE-DIFFERENTIAL RELAY High-speed percentage-differential relays having variable ratioÐor percent-slope characteristics are preferred. At low values of through current, the slope is about 5%, increasing to well over 50% at the high values of through current existing during external faults. This characteristic permits the application of sensitive high-speed relaying equipment using conventional current transformers, with no danger of undesired tripping because of transient inaccuracies in the CTÕs. To a certain extent, poorer CTÕs may be usedÐor higher burdens may be appliedÐthan with fixed-percent-slope relays. Two different operating principles are employed to obtain the variable characteristic. In both, saturation of the operating element is responsible for a certain amount of increase 13 in the percent slope. In one equipment, saturation alone causes the slope to increase to about 20%; further increase is caused by the effect on the relay response of angular differences between the operating and restraining currents that occur owing to CT errors at high values of external short-circuit current. The net effect of both saturation and phase angle is to increase the slope to more than 50%. l4 The other equipment obtains a slope greater than 50% for large values of through current entirely by saturation of the operating element. A principle called Òproduct restraintÓ is used to assure operation for internal short circuits. Product restraint provides restraint sufficient to overcome the effect of any CT errors for external short circuits; for internal short circuits when the system supplies very large currents to a fault, there is no restraint. PROTECTION AGAINST TURN-TO-TURN-FAULTS IN STATOR WINDINGS Differential relaying, as illustrated in Figs. 1-5, will not respond to faults between turns because there is no difference in the currents at the ends of a winding with shorted turns; a turn fault would have to burn through the major insulation to ground or to another phase before it could be detected. Some of the resulting damage would be prevented if protective-relaying equipment were provided to function for turn faults. Turn-fault protection has been devised for multicircuit generators, and is used quite 15 extensively, particularly in Canada. In the United States, the government-operated hydroelectric generating stations are the largest users. Because the coils of modern large steam-turbine generators usually have only one turn, they do not need turn-fault protection because turn faults cannot occur without involving ground. Even though the benefits of turn-fault protection would apply equally well to single-circuit generators, equipment for providing this protection for such generators has not been used, l5,18 although methods have been suggested; as will be seen later, the equipment used for multicircuit generators is not applicable to single-circuit generators. 180 A-C GENERATOR AND MOTOR PROTECTIONTo justify turn-fault protection, apart from what value it may have as duplicate protection, one must evaluate the savings in damage and outage time that it will provide. In a unit generator-transformer arrangement, considerable saving is possible where the generator operates ungrounded, or where high-resistance grounding or ground-fault-neutralizer grounding is used; if the ground-detecting equipment is not permitted to trip the generator breakers, a turn fault could burn much iron before the fault could spread to another phase and operate the differential relay. Even if the ground-detecting equipment is arranged to trip the generator breakers, it would probably be too slow to prevent considerable iron burning. (The foregoing leads to the further conclusion that if the generator has single-circuit windings with no turn-fault protection, the ground-fault detector should operate as quickly as possible to trip the generator breakers.) For other than unit generator-transformer arrangements, and where the generator neutral is grounded through low impedance, the justification for turn-fault protection is not so apparent. The amount of iron burning that it would save would not be significant because 19 conventional differential relaying will prevent excessive iron burning. Consequently, the principal saving would be in the cost of the coil-repair job, and it is questionable whether there would be a significant saving there. The conventional method for providing turn-fault protection is called Òsplit-phaseÓ relaying, and is illustrated in Fig. 9. If there are more than two circuits per phase, they are divided into two equal groups of parallel circuits with a CT for each group. If there is an odd number of circuits, the number of circuits in each of the two groups will not be equal, and the CTÕs must have different primary-current ratings so that under normal conditions their secondary currents will be equal. Split-phase relaying will operate for any type of short circuit in the generator windings, although it does not provide as good protection as differential relaying for some faults. The split-phase relays should operate the same hand- reset auxiliary tripping relay that is operated by the differential relays. An inverse-time overcurrent relay is used for split-phase relaying rather than an instantaneous percentage-differential relay, in order to get the required sensitivity. For its use to be justified, split-phase relaying must respond when a single turn is short-circuited. Moreover, the relay equipment must not respond to any transient unbalance that there may be when external faults occur. If percentage restraint were used to prevent such undesired operation, the restraint caused by load current would make the relay too insensitive at full load. Consequently, time delay is relied on to prevent operation on transients. Time delay tends somewhat to nullify the principal advantage of turn-fault protection, namely, that of tripping the generator breakers before the fault has had time to develop serious proportions. A supplementary instantaneous overcurrent unit is used together with the inverse-time unit, but the pickup of the instantaneous unit has to be so high to avoid undesired operation on transients that it will not respond unless several turns are short- circuited. Faster and more sensitive protection can be provided if a double-primary, single-secondary 16 CT, as shown in Fig. 10, is used rather than the two separate CTÕs shown in Fig. 9. Such a double-primary CT eliminates all transient unbalances except those existing in the primary currents themselves. With such CTÕs and with close attention to the generator 17 design to minimize normal unbalance, very sensitive instantaneous protection is possible. Such practices have been limited to Canada. A-C GENERATOR AND MOTOR PROTECTION 181Fig. 9. Split-phase relaying for a multicircuit generator. Split-phase relaying at its best could not completely replace over-all differential relaying which is required for protection of the generator circuit beyond the junctions of the paralleled windings. However, some people feel that if split-phase relaying is used with unit generator-transformer arrangements, it is not necessary to have separate generator- differential relaying if the transformer differential relaying includes the generator in its protective zone. This would be true if the split-phase relaying was instantaneous. The greater sensitivity of generator-differential relaying is not needed for faults beyond the generator windings. The principal advantage of retaining the generator-differential relaying, apart from the duplicate protection that it affords, is the value of its target indication in helping to locate a fault. However, if split-phase relaying is provided by inverse-time overcurrent relays, generator-differential relaying is recommended because of its higher speed for all except turn faults. COMBINED SPLIT-PHASE AND OVER-ALL DIFFERENTIAL RELAYING Figure 11 shows an arrangement that has been used to try to get the benefits of split-phase and over-all differential protection at a saving in current transformers and in relays. However, this arrangement is not as sensitive as the separate conventional split-phase and over-all differential equipments. Sensitivity for turn faults is sacrificed with a percentage- differential relay; with full-load secondary current flowing through the restraining coil, the pickup is considerably higher than with the conventional split-phase equipment, and the equipment will not operate if a single turn is shorted. With the main generator breaker open, the sensitivity for ground faults near the neutral end of the winding that does not have a current transformer may be considerably poorer than with the conventional over- 182 A-C GENERATOR AND MOTOR PROTECTIONFig. 10. Split-phase relaying using double-primary current transformers. all differential; the current flowing in the winding having the CT is much smaller than one-half of the current flowing in the neutral lead where the over-all differential CT would be. For this reason, the modification shown in Fig. 12 is sometimes used. Fig. 11. Combined split-phase and differential relaying (shown for one phase only). A-C GENERATOR AND MOTOR PROTECTION 183SENSITIVE STATOR GROUND-FAULT RELAYING The protection of unit generator-transformer arrangements is described under the next heading. Here, we are concerned with other than unit arrangements, where the generatorÕs neutral is grounded through such high impedance that conventional percentage-differential relaying equipment is not sensitive enough. The problem here is to get the required sensitivity and at the same time to avoid the possibility of undesired Fig. 12. Modification of Fig. 11 for greater sensitivity. operation because of CT errors with large external-fault currents. Figure 13 shows a solution to the problem: a current-current directional relay is shown whose operating coil is in the neutral of the differential-relay circuit and whose polarizing coil is energized from a CT in the generator neutral. A polarized relay provides greater sensitivity without excessive operating-coil burden; the polarizing CT may have a low enough ratio so that the polarizing coil will be ÒsoakedÓ for the short duration of the fault. Supplementary equipment may sometimes be required to prevent undesired operation because of CT errors during external two-phase-to-ground faults. 184 A-C GENERATOR AND MOTOR PROTECTIONSTATOR GROUND-FAULT PROTECTION OF UNIT GENERATORS. 19 Figure 14 shows the preferred way to provide ground-fault protection for a generator that is operated as a unit with its power transformer. The generator neutral is grounded through the high-voltage winding of a distribution transformer. A resistor and an overvoltage relay are connected across the low-voltage winding It has been found by test that, to avoid the possibility of harmfully high transient overvoltages because of ferroresonance, the resistance of the resistor should be no higher, approximately, than: X C R = ÑÐ ohms 2 3N where X is the total phase-to-ground-capacity reactance per phase of the generator stator C windings, the surge protective capacitors or lightning arresters, if used, the leads to the main and station-service power transformers, and the power-transformer windings on the generator side; and N is the open-circuit voltage ratio (or turns ratio) of the high-voltage 20 to the low-voltage windings of the distribution transformer. The value of R may be less than that given by the foregoing equation. The value of resistance given by the equation will limit the maximum instantaneous value of the transient voltage to ground to about 260% of normal line-to-ground crest value. Further reduction in resistance will not appreciably reduce the magnitude of the transient voltage. The lower the value of R, the more damage will be done by a ground fault, particularly if the relay is not connected to trip the generator breakers. The relaying sensitivity will decrease as R is decreased, because, as can be seen in Fig. 15, more of the available voltage will be consumed in the positive and negative-phase-sequence impedances and less in the zero-phase-sequence impedance which determines the magnitude of the relay voltage. This decrease in sensitivity is considered by some people to be an advantage because the relay will be less likely to operate for faults on the low-voltage side of the generator 21 potential transformers, as discussed later. In fact, it has been suggested that for this reason, and also to simplify the calculations by making it unnecessary to determine X a C resistor be chosen that will limit the fault current to approximately 15 amperes, neglecting the effect of X In other words, C 3 10 V G R = ÐÐÐÐÐÐ ohms Ð 2 15Ã3 N where V is the phase-to-phase-voltage rating of the generator in kilovolts. G It has been suggested that, to avoid large magnetizing-current flow to the distribution transformer when a ground fault occurs, the high-voltage rating of the distribution transformer should be at least 1.5 times the phase-to-neutral-voltage rating of the 20 generator. Its insulation class must fulfill the standard requirements for neutral 22 grounding devices. The low-voltage rating may be 120, 240, or 480 volts, depending on the available or desired voltage rating of the protective relay. A-C GENERATOR AND MOTOR PROTECTION 185Fig. 13. Sensitive stator ground-fault relaying for generators. The kva rating to choose for the distribution transformer and for the resistor will depend on whether the user intends to let the overvoltage relay trip the generator main and field breakers or merely sound an alarm. If the relay wilI merely sound an alarm, the transformer should be continuously rated for at least: 3 10 V V G T kva = ÑÑÑÐ Ð 2 Ã3N R Fig. 14. Stator ground-fault protection of unit generators. 186 A-C GENERATOR AND MOTOR PROTECTIONwhere V is the high-voltage rating of the distribution transformer in kilovolts. Similarly, T the continuous rating of the resistor should be at least: 3 2 10 V G kw = ÑÑÑ 2 3N R If the relay is arranged to trip the generator breakers, short-time transformer and resistor 23 ratings may be used. For example, a 1-minute-rating transformer would have only 21% of the continuous kva rating, and a 10-minute rating would have 40%. However, the lower the transformer rating, the more inductive reactance the transformer will introduce in series with the grounding resistance; for this reason, the 1-minute rating is the lowest considered desirable. The resistor may have either a 10-second or a 1-minute rating, but the 1-minute rating is generally preferred because it is more conservative and not much more expensive. In fact, continuous-rated resistors may even be economical enough. It is preferred to have the relay trip the generator main and field breakers. Even though the fault current is very low, some welding of the stator laminations may occur if the 24 generator is permitted to continue operating with a ground fault in its winding. Also, in the presence of the fault, the voltage to ground of other parts of the stator windings will Ð rise to Ã3 times normal; should this cause another ground fault to develop, a phase-to- phase fault might result, and additional damage would be done that would have been avoided had the generator breakers been tripped when the first fault occurred. Furthermore, if split-phase protection is not provided and if the ground relay is not permitted to trip, the generator could develop a turn-to-turn fault and there might be considerable iron burning before the fault could spread to another phase and cause the differential relays to operate. In spite of the foregoing, many power companies are willing to risk the possibility of additional damage until they can conveniently remove the faulty generator from service. A number of power companies simply connect a potential transformer between the generator neutral and ground without any loading resistor. They have operated this way 19 for years, apparently without any difficulty. An overvoltage relay is used as with the distribution-transformer arrangement. The maximum current that can flow in a ground fault is 71% of that with the distribution-transformer-and-resistor combination if the maximum allowable value of R is used, which is not a significant difference. In either case, 25 the arc energy is sufficient to cause damage if immediate tripping is not done. The potential transformer is considerably smaller and cheaper than the distribution- transformer-and-resistor combination, although either one is relatively inexpensive compared with the equipment protected. The principal disadvantage is that the user cannot be certain whether harmfully high transient overvoltages will occur. The only evidence we have is that such overvoltages can occurÐat least under laboratory-controlled circumstances. Therefore, until positive evidence to the contrary is presented, the distribution-transformer-and-resistor combination seems to be safer. If one prefers to let a generator operate with a ground fault in its stator winding, a ground- fault neutralizer will limit the fault current to the smallest value of any of the 25 arrangements, and at the same time will hold the transient voltage to a lower level. However, it is necessary to be sure that surge-protective capacitors, if used, cannot cause harmfully high overvoltages should a defect cause the capacitances to ground to become 26 unbalanced. A-C GENERATOR AND MOTOR PROTECTION 187Fig. 15. (a) One-line diagram of a system with a unit generator and a phase-to ground fault. (b) Phase-sequence diagram for (a). Reference 27 describes a modification of the foregoing methods whereby voltage is introduced between the generator neutral and ground for the purpose of obtaining greater sensitivity. Little, if any, application of this principle exists presently in the United States. The same overvoltage-relay characteristics are required for any of the foregoing generator- neutral-grounding methods. The relay must be sensitive to fundamental-frequency voltages and insensitive to the third-harmonic and multiples of third-harmonic voltages. The relay may require adjustable time delay so as to be selective with other relays for ground faults on the high-voltage side of the main power transformer for which there may be a tendency to operate owing to capacitance coupling between the power-transformer windings, particularly if the high-voltage winding of the power transformer does not have 19,20,25 its neutral solidly grounded; the ground-fault-neutralizer-grounding arrangement has the greatest operating tendency under such circumstances, and the distribution- transformer arrangement has the least. Time delay is desirable also to provide as good 188 A-C GENERATOR AND MOTOR PROTECTIONselectivity as possible with potential-transformer fuses for faults on the secondary side of potential transformers connected wye-wye. If the relaying equipment is used only to sound an alarm, a combination of relays may be required to get both good sensitivity and a high continuous-voltage rating. If grounded-neutral wye-wye potential transformers are connected to the generator leads, it may be impossible to get complete selectivity between the relay and the PT (potential- transformer) fuses for certain ground faults on the low-voltage side of the PTÕs, depending 21 on the fuse ratings and on the relay sensitivity. In other words, the relay may sometimes operate when there is not enough fault current to blow a fuse. Such lack of coordination might be considered an advantage; the relay will protect the PTÕs from thermal damage for which the fuses could not protect. To make the relay insensitive enough so that it would not operate for low-voltage ground faults would sacrifice too much sensitivity for generator faults. Of course, if the relay has time delay, it will not operate for a momentary short circuit such as might be caused inadvertently during testing. If the relay is used only to sound an alarm, some selectivity may be sacrificed in the interests of sensitivity; the relay will not operate frequently enough to be a nuisance. SHORT CIRCUIT PROTECTION OF STATOR WINDINGS BY OVERCURRENT RELAYS If current transformers are not connected in the neutral ends of wye-connected generator windings, or if only the outgoing leads are brought out, protective devices can be actuated, as in Fig. 16, only by the short-circuit current supplied by the system. Such protection is when the main circuit breaker is open, or when it is closed if the system has no other generating source, and the following discussion assumes that short-circuit current is available from the system. If the generatorÕs neutral is not grounded, sensitive and fast ground overcurrent protection can be provided; but, if the neutral is grounded, directional overcurrent relaying should be used for the greatest sensitivity and speed. In either event, directional overcurrent relays should be used for phase-fault protection for the greatest sensitivity and speed. If non-directional voltage-restrained or -controlled overcurrent relays are used for external- fault back-up protection, they could also serve to protect against generator phase faults. None of the foregoing forms of relaying will provide nearly as good protection as percentage-differential relaying equipment, and they should not be used except when the cost of bringing out the generator leads and installing current transformers and differential relays cannot be justified. PROTECTION AGAINST STATOR OPEN CIRCUITS An open circuit or a high-resistance joint in a stator winding is very difficult to detect before it has caused considerable damage. Split-phase relaying may provide such 17 protection, but only the most sensitive equipment will detect the trouble in its early stages. Negative-phase-sequence-relaying equipment for protection against unbalanced phase currents contains a sensitive alarm unit that will alert an operator to the abnormal condition. A-C GENERATOR AND MOTOR PROTECTION 189It is not the practice to provide protective-relaying equipment purposely for open circuits. Open circuits are most unlikely in well-constructed machines. STATOR-OVERHEATING PROTECTION General stator overheating is caused by overloading or by failure of the cooling system, and it can be detected quite easily. Overheating because of short-circuited laminations is very localized, and it is just a matter of chance whether it can be detected before serious damage is done. The practice is to embed resistance temperature-detector coils or thermocouples in the slots with the stator windings of generators larger than about 500 to 1500 kva. Enough of these detectors are located at different places in the windings so that an indication can be 28 obtained of the temperature conditions throughout the stator. Several of the detectors that give the highest temperature indication are selected for use with a temperature indicator or recorder, usually having alarm contacts; or the detector giving the highest indication may be arranged to operate a temperature relay to sound an alarm. Supplementary temperature devices may monitor the cooling system; such equipment would give the earliest alarm in the event of cooling-system failure, but it is generally felt that the stator temperature detectors and alarm devices are sufficient. Figure 17 shows one form of detector-operated relaying equipment using a Wheatstone-bridge circuit and a directional relay. In another form of equipment, the stator current is used to energize the bridge. ÒReplicaÓ-type temperature relays may be used with small generators that do not have temperature Fig. 16. Generator stator detectors. Such a relay is energized either directly overcurrent relay. by the current flowing in one of the stator windings of the machine or indirectly from current transformers in the stator circuit. The relay is arranged with heating and heat-storage elements so as to heat up and cool down as nearly as possible at the same rate as the machine in response to the same variations in the current. A thermostatic element closes contacts at a selected temperature. It will be evident that such a relay will not operate for failure of the cooling system. The temperature-detector-operated devices are preferred because they respond more nearly to the actual temperature of the stator. The fact that the actual stator copper 29 temperature is higher than the temperature at the detector should be taken into account in the adjustment of the temperature relay. This difference in temperature may be 25¡C or more in hydrogen-cooled machines, being greater at the higher hydrogen pressures. Thus, if the permissible copper temperature is assumed to remain constant with higher loading at higher hydrogen pressure, the temperature-relay setting must be lower. 190 A-C GENERATOR AND MOTOR PROTECTION

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