GET-6497AGenerating Station        Protection
INTRODUCTION                           - Tripping modes                                                          - Generat...
12. The Art and Science of Protective Relaying, C.    15. IEEE Recommended Practice for Protection     R. Mason.          ...
GENERATING-STATION         PROTECTIONUNIT GENERATOR-TRANSFORMER                             to limit the current for a sin...
151 IN                                                                                                s u                 ...
TABLE 1-___. __                                            Function, Relay                                                ...
GENERATORS BUSSED AT                                      GENERATOR VOLTAGE   Figure 2 shows a typical installation where ...
Step-Up                     Trans.                                                  Grounding                             ...
TABLE 2                                             Function, Relay                                            Function, R...
EXCITATION SYSTEM PROTECTIONALTERRAEX EXCITATION SYSTEM                                       cialized protective function...
TABLE 3                                            ALTERREX EXCITATION SYSTEM                                             ...
GENERREX EXCITATION SYSTEM   Figure 4 shows the GENERREX excitation sys-                              required for protect...
TABLE 4                                    Notes:           GENERREX    EXCITATION   SYSTEM                         * This...
INDIVIDUAL FORMS OF RELAYING                          about 96 percent of the generator winding will                      ...
LOSS OF EXCITATION PROTECTION   Two types of relays are available for this protec-tion. These are:0    CEHS1. This relay h...
when the relay is operating near pickup in a loca-      phase conditions have also been caused by brokention where there i...
Relay 94G2 only initiates tripping of the main      Ground Fault Backupgenerator breaker(s). This mode is used where,durin...
Bus                 Bus                 (S)                                                                               ...
in case the current level is below pickup of the         ance during the high slip interval can be repre-fault detector re...
Reverse Power Relays (GGP, ICW)                          Supplementary Protection   The power into the machine during this...
In general, current transformer performance will                                                                not be a p...
is used, the PJC coil would have to be short cir-       series with the time overcurrent relay normallycuited before the m...
8    r                                            .                                             .                         ...
Main      Transformer                                  PT                0                                                ...
In addition, LSTG CTs rated 15,000 primary            ing this mode of operatio n, this can be accomp ishedamperes and hig...
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Generating stationprotection ge

  1. 1. GET-6497AGenerating Station Protection
  2. 2. INTRODUCTION - Tripping modes - Generator back up protection This guide is concerned primarily with shortcircuit protection and protection for abnormal - Protection for accidentally energizing aoperating conditions for generating stations. It generator on turning gearspecifies the minimum recommended protection - Protection during start-up or shutdownfor steam turbine generators, for hydro generators, - Current transformersand for gas turbine generators. Additional desiredprotection should be provided if it is economically - Potential transformersjustified. Unless otherwise stated, these recom- - Sub-synchronous resonancemendations apply to either attended or unattendedstations having generators rated as follows: REFERENCES l 1000 kva and higher, any voltage 1. Protection for Accidentally Energizing a Gen- l 5 kv and higher, any kva erator While on Turning Gear, J. Berdy, 1977. l 2.2 kv and higher, and 501 kva and higher 2. Out of Step Protection for Generators, J. Berdy, 1976. The guide contains the following sections: 3. Protection of Synchronous Generators During Unbalanced System Conditions, J. Berdy, P. G. Recommended Generating Station Protection Brown, 1975. - presents recommended and optional protec- tion for specific generator applications and 4. Protective Relaying for Pumped Storage Hydro arrangements. Units, IEEE PSRC Report, PAS, May/June 1975. - Unit generator-transformer installation - Generators bussed at generator voltage 5. Loss of Excitation Protection for Modern Syn- chronous Generators, J. Berdy, IEEE Trans- Excitation System Protection - presents rec- actions, PAS, Sept./Oct. 1975. ommended and optional protection for avail- able excitation systems. 6 Protection of Steam Turbine Generators During Abnormal Frequency Conditions, J. - Alterrex Berdy, P. G. Brown, L. E. Goff, 1974. - Generrex 7 Protection of Large Steam Turbine Generators Considerations Involving Individual Forms of During Abnormal Conditions, J. Berdy, M. Relaying - considers special problem areas, Crenshaw, M. Temoshok, CIGRE, 1972. setting information and application rules not provided elsewhere, or any other special con- 8. Protection of Large Tandem Generators, J. siderations pertinent to a particular type of Berdy, 1972. relay or protection. 9. Potential Transformer Applications on Unit - Stator short-circuit protection Connected Generators, I E E E Committee Re- port, PAS, Jan./Feb. 1972. - Loss-of-excitation protection - Reverse power (anti-motoring) protection 10. Electrical Design Considerations in Pumped - Unbalanced current (single phasing) protec- Storage Hydro Plants, P. G. Brown, G. W. tion Otte, IEEE Transactions, Vol. S82, 1963. Other Protective Considerations - considers 11 Recent Practices and Trends in Protective supplementary protection and other forms of Relaying, IEEE Committee Report, PAS, Feb. protection not covered earlier. 1960. 3
  3. 3. 12. The Art and Science of Protective Relaying, C. 15. IEEE Recommended Practice for Protection R. Mason. and Coordination of Industrial and Commercial Power Systems, I EE E Standard 242-l 975.13. Operation of Apparatus Protective Relaying at Reduced Frequencies, AIEE-Committee Re- 16. Transmission Line Reclosing - Turbine Gener- port, PAS Feb. 1959. ator Duties and Stability Considerations, P. G. Brown, R. Quay, Texas A&M Relay Confer-14. Design and Test of the Navajo Plant Subsyn- ence, April 1976. chronous Resonance Protection Filters, C.E.J. Bowler, C. H. Holley, T. M. Morong, J. B. Tice, CIGRE Paper 31-06, 1976.
  4. 4. GENERATING-STATION PROTECTIONUNIT GENERATOR-TRANSFORMER to limit the current for a single line-to-ground faultINSTALLATION at the terminals of the generator to 10 primary amperes or less. Figure 1 shows graphically the Figure 1 shows a typical unit generator-transfor- various available forms of protection, while relaymer installation with high impedance grounding recommendations are given in Table 1 where devicethrough a distribution transformer. In this arrange- numbers correspond to the encircled numbers inment, the grounding impedance is usually selected Fig. 1.
  5. 5. 151 IN s u Volts/Hz Over Volt Tran Diff Sta. Service Tran o-87 s u Tran. Diff Stator Overheating Aux Pt Sta. Tran Aux Diff Back-Up Sta. Aux Back-Up " Ct t Feeders Figure 1. Unit generator transformer arrangement.6
  6. 6. TABLE 1-___. __ Function, Relay Function, Relay Quantity Control led or Quantity Controlled or Device and Type Breakers Tripped Device and Type Breakers Tripped21 3-CEB5lB (a) (w) 21x 78 1 -GSY51 A (p) 94G2 1-CEBl3C (a) (w) and 1-CEX57E (p)21x 1-SAM14 86G 81 1-SFF21A (q) 94G232 1 -GGP53C (b) 94G 1 86B 1-HEA61 D,E,F,G40 1-CEH51A (c) 94G 1 86G 1 -HEA61 A,C,D,H, shutdown 1 -CEH52A (c) (BFI)*46 1 -SGC12A (d) 94G 1 86T/SU 1 -H EA61 B,E49 1-IRT51A (e) Alarm (f) 87B 3-PVD11 C 86B51 3-IAC53A (g) 86G or 3-PVD21 (r) or 3-l FC53A 87G 3-CFD22 (s) 86G, 87X51 /GN 1-IAC53A (g) (y) 86G 87T 3-BDD 15B (t) 86G or 1-IFC53A or 3-STD15C (t)51 /TN 1-IAC53A (g) (w) 86G 87T/SS 3-STD 15C 86G or 1 -I FC53A or 3-STD 15C51TN/SS 1-IAC53A (g) 86G 87T/SU 3-BDD1 5B 86T/SU or 1-IFC53A (g) or 3-STD 15C51v 3-IJCV51A (h) (w) 86G 87X 1-HGA14A CO 2 (z) or 3-IGCV51A (h) (w) 94G 1 1 or 2-HFA53K A,C,H (x) (BFI)*59 1-IAV71B (i) 94G 1 9462 2 or 2-HFA53K A (BFI)*59V/HZ 2-STV11 A (j) 94G 1 151 3-IAC53A (g) 86T/SU60 1-CFVB11B (k) Alarm o r 3-IFC53A61 3-IAC53B (Q) 86G, 87X 151TN 1 -IAC53A (g) 86T/SU or 3-IFC53B (a) o r 1-IFC53A63 l-Type 900-1A (m) 86G 151 TN/SU 1-IAC53A (g) 86T/SU64F 1-PJG12 (n) 86G o r 1-IFC53A (g)64G 1 -IAV51 K 86G, 87X AUX CT 3-JARO (u)71 1 -Gas Detector (0) Alarm AUX PT 3-JE27 (v)*(BFI) = Breaker failure initiate (k) Used to open trip circuits of devices 21, 32, 40, 51V, 78. Can also be used to remove regulator from service.(a) (Q) For multi-circuit hydro generators.(b) 1;’ Type 900-1A fault pressure relay. The PJG12 can be set with zero or 2 seconds time delay. In general, it is recommended that time delay be used with this protection.(c) (o) Gas Detector Relay. Only applicable on conservator or atmo- seal type of transformers. (P) Optional. Used when during loss of synchronism, the electrical center is in either the step up transformer or in the generator. (q) Underfrequency protection for the turbine. One or more relays may be required to provide protection. See reference 6. The PVD21 is faster version of the PVD1 1.(d) For generators smaller than 2000 kVA, the IJD52 may be sub- stituted. The IJD52 is also used on class 1 E standby generators.(e) If HV bus arrangement is such that two HV breakers are In- volved, use 3-BDD16B or 3STD16C. Use with 87T.(f) Use with 21 and 51V. For two step tripping with external fault backup relays, devices(g) 51TN, 51V or 21X would first trip breaker A and then after a short time delay, control device 86G. This would require an(h) additional RPM1 IA or SAM11A timer to be controlled by 51TN and 51V. A generator may have a main field breaker (C) or an exciter field breaker (H) or a generator may have both. If both field breakers are used, both should be tripped by 94G1 and 86G. 51GN is backup for stator ground faults. Two possible loca- tions for this protection are shown. Used only on enclosed air cooled machines. 7
  7. 7. GENERATORS BUSSED AT GENERATOR VOLTAGE Figure 2 shows a typical installation where gen- than rated full load current. Figure 2 shows graph-erators are bussed at generator voltage. In this ically the available forms of protection, and relayarrangement, the generator is generally low resist- recommendations are given in Table 2, where theante grounded. The grounding resistance is usually device numbers correspond to the encircled num-selected to limit the generator’s contribution to a bers in Fig. 2.single line-to-ground fault at its terminals to less8
  8. 8. Step-Up Trans. Grounding Bus Diff LU Line HU Phase Relays VoltStator Over- -4t PT Loss of Sync 7* 0” n BalHeating 59 UHZ LA Loss of Volts/Hz "Y Exc. Sta Trans Service Diff LV Trans Phase Back- Jp n 46 Curr Unbal 51T ss - JO 87 Gen GN GND Y Grounding Impedance Figure 2. Generators bussed at generator voltage. 9
  9. 9. TABLE 2 Function, Relay Function, Relay Quantity Controlled or Quantity Controlled or Device and Type Breakers Tripped Device and Type Breakers Tripped ___-___-21 3CEB51 B (a) 21X 64F 1 -PJG 12 (n) 86G 1-CEB13C (a) 71 1-Gas Detector (o) Alarm21x 1-SAM11A B 78 1-GSY51A (p) 94G232 1 -GGP53C (b) 94G1 and 1-CEX57E (p)40 1 -CEH51 A (c) 94G1 81 1-SFF21A (q) 94G2 1 -CEH52A (c) 86B1 1-HEA61 B,C,D,E. (BFI)*46 1 SGC12A (d) 94G1 86B2 1-HEA61 G,H,K49 1-IRT51A (e) Alarm (f) 86G 1 -HEA61 D, L,F, shutdown50/5 1 SS 3-IAC53B (g) E (BFI)* or 3-IFC53B (g) 86T 1 -HEA61 A,B51GN 1-IAC53A (g) (w) 86G 86T/SS 1 -HEA61 E,G or 1-IFC53A (g) 87Bl 3-PVD11C 86B 151 N/GT 1-IAC53A (g) 86T o r 3-PVD21 (r) or 1-IFC53A (g) 87B2 3-PVD1 1C 86B251T 3-IAC53A (g) 86T o r 3 - P V D 2 1 (r) or 3-IFC53A (g) 87G 3-CFD22 (s) 86G, 87X51TN 1-IAC53A (g) 86T 87GN 1-ICC52A (t) 86G, 87X or 1-IFC53A (g) o r 1-IFC53A ( t )51TN/SS 1-IAC53A (g) 86T/SS 87T 3-BDD15B (u) 86T o r 1-IFC53A (g) o r 3STD15C ( u )51V 3-IJCV51A (h) (w) 86G 87TN 1-ICC52A 86T or 3-IGCV51A (h) (w) 1-IFC53A59 1 -IAV71 B (i) 94G 1 87T/SS 3-BDD15B 86T/SS59VHZ 2-STV11A (j) 94G 1 or 3-STD 15C60 1 -CFVB11B (k) Alarm 87X 1-HGA14A CO2 (y)61 3-IAC53B ((i) 86G, 87X 94G 1 1 o r 2-HFA53K D,F,L (x) (BFI)* o r 3-IFC53B ((0 94G2 1 o r 2-HFA53K A, (BFI)*63 I-Type 900-IA (m) 86T AUX PT 3-JE27 (v) GS Ground Sensor (z) 86T/SS (k)*(BFI) B r e a k e r Failure lnitiate(a)(b) (o)(c) (P) (q) II! (t) (U)(f) iii,(9)(h)
  10. 10. EXCITATION SYSTEM PROTECTIONALTERRAEX EXCITATION SYSTEM cialized protective functions are incorporated as required for protection of the specific excitation Figure 3 shows the ALTERREX excitation sys- system components unique to this particular de-tern as used on large steam turbine generators. It sign. Figure 3 shows schematically the variousshould be noted that a number of generator pro- forms of protection, while the device designationstective functions are incorporated as standard are given in Table 3.features of the excitation equipment. Many spe- DiodeExt. Exciter Bridge Main MainField Alternator Field GeneratorBrk’r > n Field O Overvoltage PT Exciter 6 4 Field Voltage F Ground Unbalance Exc. Diff =_ Exc. - Field Ground I ) ) ( 59E V/Hz 0 Generator I * Volts/Hertz Voltage * 1 1 Regulators A * De-Exe Figure 3. Alterrex excitation system protective equipment. 11
  11. 11. TABLE 3 ALTERREX EXCITATION SYSTEM Function, Notes: Relay * This equipment supplied as an integral part of excitation equip- Controlled ment. Device or Device Device Identifi- Breakers ** These protective functions are optional. The 60E provides moreFunction Description cation Tripped c o m p l e t e protection t h a n 8 7 E w h i c h c o v e r s o n l y t h e exciter alternator.59F Field Overvoltage JA114* 86Gt i All excitation systems will be equipped with t w o independent, redundant shutdown methods:50E/76E Exciter Field Over- 83Rt current Instantaneous/ Main field breaker or equivalent. (41 F) * Inverse Time l Exciter field breaker or equivalent. (41 E)59E Generator Volts Per MA336* 94G-1 t V/HZ Hertz l De-excitation thru control action of voltage regulators. (DE-EXC)60E Exciter Voltage MA305** 8 6 G t Balance64E Exciter Field Ground YA122* Alarm64F Generator Field YA122* 86Gt Ground87E Exciter Differential 3-CFD 86G-t 22””83R Regulator Tripping AC to Control Relay DC Mode12
  12. 12. GENERREX EXCITATION SYSTEM Figure 4 shows the GENERREX excitation sys- required for protection of the specific excitationtern as used on large steam turbine generators. It system components u n i q u e t o t h i s p a r t i c u l a rshould be noted that a number of generator pro- design. Figure 4 shows schematically the varioustective functions are incorporated as standard forms of protection, while the device designationsfeatures of the excitation equipment. Many spe- are given in Table 4.cialized protective functions are incorporated as Generator Stator Wdg. “C” I, P” CT PT’S m "P" Bar r ,1 F” Field Wdg. I Field “z O v e r Exc. Voltage Gnd. Field 6 4 GND F Loss of Coolant z Flow Voltage Regulators Volt Generator 0--j-g-- De-Exe Bal Volts/Hertz Field Brk’r. Figure 4. Generrex excitation system protective equipment. 13
  13. 13. TABLE 4 Notes: GENERREX EXCITATION SYSTEM * This equipment supplled a s a n i n t e g r a l part of the excitation-_- equipment as printed circuit b o a r d s . I- Function, I Relay ** T h e s e r e l a y s supplied as standard feature of the excitation I systern. I Controlled Device or t All excitation systems wiII be supplied with two independent, redundant Device Device Identifi- Breakers shutdown methods:Function Description cation Tripped __-_____ l Main field breaker or equivalent. (41 F) Field Overvoltage * t 86G t59F l E x c i t e r s h o r t i n g b r e a k e r . (52E) Generator Volts Per * 94G-1 t59E l De-excitation thru control action of voltage regulators. V/HZ Hertz (DE-EXC) Exciter Voltage * 86G t60E Unbalance * Alarm60R Voltage Balance64E Exciter Winding IAV 86G t Ground 51K**64F Generator Field YA1 22” 86G t Ground74 Rectifier Over Tem- Alarm perature80 Generator/Exciter Turbine Winding Loss of Runback Coolant Flow Then 86Gt / - I14
  14. 14. INDIVIDUAL FORMS OF RELAYING about 96 percent of the generator winding will be protected for grou nd faults. The primary purpose of this section is to presentspecial considerations involving individual forms ofrelaying. Items under this heading are intended to l When grounded wye-grounded wye PTS are usedindicate special problem areas, application rules on the generator terminals, the IAV51K will notnot provided elsewhere, changes in application always coordinate with the PT fuses for groundrules, or any other special considerations pertinent faults on the PT secondaries. Since the IAV pro-to a particular type of relay or protection. vides better thermal protection than the fuses, some utilities accept this shortcoming and con- tinue to use the recommended settings. The No.STATOR SHORT-CIRCUIT PROTECTION 10 time-dial setting provides maximum obtain- able coordination with the low side PT fuses.Differential Protection (Type CFD22B) Where this situation is not acceptable, utilities The only problem which may require consider- will unground the PT secondary neutral andation with this type of protection is the possibility ground any one of the secondary phase wires.of high voltages in the CT circuits during internal With this approach, a ground fault on one of thefaults, and therefore the need for Thyrite limiters other phase wires would create a phase-to-phaseacross each phase of the CT secondaries. It shou Id fault which would be cleared by the fuses andbe noted that this problem is of primary concern which would not affect the IAV51K.when a group of generators are bussed at generatorvoltage and there are no external impedances tolimit the current flow into a fault within a differ-ential zone. This is not generally a problem with l A problem not often recognized by users is thethe unit generator-transformer arrangement since possibility of incorrect operation of the IAV fortransformer impedance will limit fault current. ground faults on the high-voltage side of the unit transformer. When a high-tension ground fault Current transformer secondary voltages are a occurs, a voltage may appear at the generatorfunction of secondary fault current, impedance of neutral due to the capacitance coupling betweenthe differential operate circuit, and CT tap being transformer windings. The magnitude of thisused. Since a rigorous calculation of this voltage voltage, on the distribution transformer second-is complex, two simple rules have been evolved to ary basis, can be as high as 20 volts which is welldetermine the need for Thyrite limiters. These above the sensitive pick-up setting (5.4 volts) ofrules are: the IAV. The No. 10 time dial setting generally used provides more than enough time to ridel When the full CT winding is being used, limiters over the clearing of system ground faults and are not required when the secondary currents thereby prevents undesired IAV operation. are below 84 amperes.l When a lower tap is used on the CT, limiters l Where PT secondary neutrals are not grounded are not required when the current is less than and coordination with PT fuses is not a consid- 84 x (active turns/total turns)2. eration, some utilities will want to use a lower time dial setting. If this is the case, the time dial It should be noted that when an IJD differential setting should be based on the voltage magni-relay is used, the limiting current is 50 amperes. tude which can appear across the IAV during high-side ground faults, and the time it takes toGround Fault Protection (Type IAV51K) clear such faults. If this voltage is not known or cannot be calculated, a reasonable assumption There are seve ral points which might be of would be that this voltage will be about fourinterest w ith this p rotection. times pickup (4 x 5.4 volts). With this multiple of pick-up, select a time dial setting whichl The recommended setting for this relay is 5.4 would provide enough time with margin to ride volts pickup, No. 10 time dial. With this setting, over clearing of high-side faults. 15
  15. 15. LOSS OF EXCITATION PROTECTION Two types of relays are available for this protec-tion. These are:0 CEHS1. This relay has a single mho function which operates with no external time delay.0 CEH52. This relay has two independent mho functions and a built in timer which operates in conjunction with one of the mho units. The CEH51 would be used on small, less impor-tant machines on a power system. Its setting wouldbe as shown in Fig. 5. This setting will detect a lossof excitation from full load down to no load.Normally, no external time delay would be usedwith this relay. If it is possible, however, for stableswings to enter the relay characteristic (see refer- Figure 5. Application of the CEH51.ence 5), an external time delay of up to 0.5 setsmay be added to prevent incorrect tripping. The CEH52 would be used on the importantsystem generators where optimum protection andsecurity are essential. The settings for this relay are +x Ishown in Fig. 6. The unit set with 1.0 per unit im-pedance on machine base, provides loss of excita-tion protection from full load down to about 30percent load. Since a loss of excitation in thisloading range has the greatest adverse effects onthe generator and system, this unit should be per-mitted to trip in high-speed. The second unit, with a diameter setting equalto synchronous reactance Xd, would detect a lossof excitation from full load down to no load. Sinceit is possible this unit may operate on stable Diameter = Xswings, however, a time delay of up to 0.5 setsmay be used to prevent incorrect tripping.REVERSE POWER (ANTI -MOTORING)PROTECTION Figure 6. Application of the CEH52. Reverse power or ant -motoring protection isrecommended for all steam turbine generators. It is recommended that a 30-second time delay The relay recommended for this function is the be used with this relay to prevent operation duringGGP53C. This relay has a current pickup level of power swings caused by system disturbances.10 milliamperes and should be applicable in mostcases. The user, however, should obtain the ex- It should be noted, this relay has a holding coilpected no load motoring losses from the steam which, if not properly adjusted, may cause unde-turbine generator manufacturer to ascertain that sired relay operation. The purpose of this holdingthere will be sufficient pickup margin. coil is to hold the directional unit contacts closed16
  16. 16. when the relay is operating near pickup in a loca- phase conditions have also been caused by brokention where there is vibration. This holding coil can tine conductors or the misoperation of one pole ofbe adjusted to prevent incorrect operations, or if a line circuit breaker.vibration at the relay location is not severe, theholding coil can be shorted out and eliminated. With one or two phases open on the high voltage system, the negative sequence current levels in a generator will be a function of generator and sys-UNBALANCED CURRENT (SINGLE tem impedances, generator or line loading, andPHASING) PROTECTION system configuration. While specific values of generator negative sequence currents will vary from The relay recommended for this function for all system to system, these negative sequence currenttypes and sizes of generators is the SGC1 2, static levels can vary over a range from .05 per unit up tonegative sequence time overcurrent relay. The 0.6 per unit of generator rated current. The SGC12characteristics of this relay are: relay, with its sensitive pickup range, can be set to either alarm or to trip for these low levels of nega-l Tripping Unit tive sequence currents. - A negative sequence current pick-up range of It should be noted that the electromechanical 9 to 20 per cent of generator rated current. negative sequence relay, type INC, with an I2 tripping level of 0.63 per unit, will not detect an - A time-current characteristic which exactly open conductor or single phasing condition in most matches the generator I2t capability curve. 2 instances. The relay I2t characteristic is adjustable over 2 a range of 2-40. OTHER PROTECTIVE CONSIDERATIONS - A reset characteristic which approximates This section considers supplementary protection generator rotor cooling rates. and other forms of protection not covered earlier. In addition, it considers equipment and devices - An optional direct reading meter, calibrated used with protective relaying in the generator zone. in per cent negative sequence current.l Alarm Unit TRIPPING MODES - Negative sequence current pick-up range: 3 to Figures 1 and 2 show three methods of tripping 20 percent of generator rated current. with the electrical protection in the generator zone: tripping hand-reset lockout relay 86G; - A three (3) second time delay. tripping self reset auxiliary relays 94G1 and 94G2. With this sensitivity, the SGC12 is not only Tripping via the lockout relay 86G trips thecapable of providing protection for uncleared un- main and field breakers, the turbine, and the boiler.balanced system faults but it can also provide pro- This mode is used where generator and/or trans-tection for open conductor faults and/or single former faults are involved and for backup oper-phasing of generators. ations. Of particular concern is the need for open con- Relay 94G1 initiates tripping of the main gen-ductor and single phasing protection for gener- erator breaker(s) and the field breaker(s). Thisators. There have been a number of recent cases mode is used where it may be possible to quicklywhere generator-transformer units were operated correct the abnormality and therefore would per-with one or two phases open for prolonged periods mit reconnecting the machine to the system in aof time. In most instances, these open phase condi- short period of time.tions were caused by the failure of one pole of ahigh-voltage circuit breaker to close or to open In both of the above instances, it will be neces-when the generator-transformer unit was con- sary to transfer the station auxiliaries to the stand-nected to or disconnected from the system. Open- by source. 17
  17. 17. Relay 94G2 only initiates tripping of the main Ground Fault Backupgenerator breaker(s). This mode is used where,during some system disturbances, it is desirable to Generator ground backup protection may bekeep the station auxiliaries connected to the gen- provided in several ways.erator. For example, during a loss of synchronismor a disturbance which produces low frequency 1. One approach is to use a simple time overcurrenttripping, the standby source may be out of phase relay 51GN connected to CTS in either the neu-with the generator or non-existent. This tripping tral of the primary winding of the distributionmode would permit reconnecting the machine to transformer or in the distribution transformerthe system with a minimum of delay. secondary as shown in Fig. 1. With either method, the CT ratio and relay pickup setting are selected with the intent of providing the These are the only tripping modes for electrical same degree of protection as with the overvolt-protection discussed in this guide. age relay IAV51K. In general, it is difficult to achieve this sensitivity, since the relay must be set so that it will not pick up on the zero se- For other turbine protective functions, a so- quence harmonic currents and the normal 60 Hzcalled sequential tripping mode is used. In this unbalance currents that flow in the neutral. Themode, the turbine valves are tripped first, then aux- time setting for this relay must be coordinatediliary contacts on these valves are used to initiate with the IAV51K and the potential transformertripping of the main and field breakers. With this fuses.approach, backup protection for possible failuresin the valve auxiliary contacts, the control cir- 2. Another approach is to use potential transform-cuitry, and/or the breakers is provided by the ers connected grounded wye-broken delta at thereverse power relay. To provide this backup func- machine terminals with an IAV51K connectedtion, the reverse power relay (32) must be con- across the broken delta. This method providesnected to initiate tripping of the main and field the same degree of protection as with the over-breakers through 94G1 as shown in Figures 1 and voltage relay across the generator neutral re-2. sistor. It should be noted that this approach acts as a high impedance grounding bank and there- fore its effect on generator grounding and fault Another tripping mode used is a simultaneous current levels should be determined.trip. In this approach, the protective relays used totrip the turbine valves would also initiate a simul- 3. Some utilities use a voltmeter connected acrosstaneous trip of the generator main and field the grounding resistor to check the integrity ofbreakers. In some instances, a time delay is used in the grounding system and the availability of thethe breaker tripping chain. If such time delay is ground protection. Under normal conditions,used, the effect of this time delay on the generator zero sequence harmonic voltages (mostly 3rdand/or system should be determined. harmonic) will be present across the resistor. The absence of these harmonic voltages would be an indication of possible problems with theGENERATOR BACK-UP PROTECTION grounding system and/or relays.Phase Fault Backup Backup protection against relay failure is not Breaker Failure Relayingnormally applied to unit generator transformers.Their operation is closely supervised, and the It is recommended that breaker failure relayingprotection listed under 2A1 provides enough over- be incorporated as an integral part of the overalllap and duplication of the various protective func- protection for all types of generators.tions to make separate backup relaying consideredto be unnecessary. It should be noted that the sys- Protection for failure of the H.V. main generatortem phase fault backup relays, type CEB or IJCV, breaker(s) can be accomplished through the use ofprovide some measure of backup protection for electromechanical or static current detectors andboth the generator and the main transformer. timers as in the breaker failure schemes for line18
  18. 18. Bus Bus (S) (N) Trip Trip Bkr. #I Device Type Function *50BF CHC Fault Detector *62BF SAM11 Timer 62X NGA1 5A Bkr Failure Initiate E/M Line Relays 86BF HEA61 Lockout Relay 86G HEA61 Gen. Zone Prot. Lockout Relay 86SB HEA61 Bus (S) Lockout Relay 94G1,94G2 HGA14 or Gen. Zone Prot. HFA53 Trip Relays BFI Bkr Failure Initiate Static Line Relays “As an alternate use SBC21 static breaker failure relay. Figure A simplified breaker failure arrange- necessary to clear the fault. In this case, if breakerment is shown in Figure 7. Since two main break- No. 3 fails, bus (S) must be tripped; if breakerers are involved, there is a current detector and No. 2 fails, breaker No. 1 and the remote end oftimer associated with each breaker. line (A) must be tripped. Since the remote relays on line (A) will not be able to detect a low level Operation of the scheme is simple and straight- fault in the generator-transformer zone, some formforward, Like all such schemes, when the primary of transferred tripping will have to be used.and backup protection detect an internal fault,they will attempt to trip the necessary breakers The only thing unusual about the arrangementand at the same time start the breaker failure in Figure 7 is the use of breaker auxiliary “a” con-timers. If a breaker does not clear the fauIt in a tacts in the circuit. These breaker auxiliary con-specified time, the timer wilt trip other breakers tacts are necessary for controlling the backup timer 19
  19. 19. in case the current level is below pickup of the ance during the high slip interval can be repre-fault detector relays (50B R/2, 50B F/3) as could be sented by negative sequence reactance (X2) inpossible for a low-level transformer fault, a motor- series with negative sequence resistance (R2).ing condition, or low level unbalanced currents. If (Note: Negative sequence reactance of a steam tur-independent pole control is used, breaker “a” con- bine generator equals subtransient reactancetacts from each pole (three required) would have X”dv.) The machine terminal voltage and currentto be placed in parallel with the current detector during this interval will be a function of generator,50BF contacts for each breaker. It shouId be noted transformer and system impedances. If the gen-auxiliary switches on circuit breakers are not other- erator-transformer is connected to an infinitewise used in breaker failure relaying because (1) system the machine currents will be high (severalthe auxiliary switch or mechanism may have been per unit) and conversely if the unit is connectedthe cause of the breaker failing to clear the fault or to a weak system, the machine current could be(2) the breaker may have opened mechanically low (1-2 p.u.).without clearing the fault. In this case, the breaker“a” switches must be trusted for low-level faults. During the period the machine is accelerating, high currents will be induced in the rotor and the It should be emphasized that the intent of the time to damage may be on the order of a fewsimplified scheme shown in Fig. 7 is to illustrate s e c o n d s . To prevent damage to the rotor, stator,only the general approach used for achieving breaker bearings, etc., it is desirable that high-speed pro-failure backup. The user should design and imple- tection be provided for this contingency.ment the scheme to obtain the required level ofreliability. For example, Figures 1, 2, and 7 show There are several relays used in the generatora single lockout relay 86G. It would be desirable zone that may detect or can be set to detect thisto split the generator zone protection into groups condition. These are:and have each group operate a separate lockoutrelay. For instance, separate the primary and back- l Loss of excitation relay, CEH.up relays and have each operate a separate lockoutrelay. In this way, a single lockout relay failure l Reverse power relay, GGP, ICW.will not eliminate all protection. l System backup relays CEB, IJCV. Another factor to consider is the operating pro-cedure when a machine is shut down for mainte-nance. When a ring bus, or a breaker and a half, or CEH Relaya double breaker-double bus arrangement is usedon the high side, it is common practice for some W ith normal settings, subtransient reactanceutilities to isolate the unit generator and close the (X”dv) may be just inside at the top of the relayhigh-voltage breakers to close the ring or tie the characteristic (see reference 1). Depending on thetwo busses together. Under these conditions, it offset used and the relay and impedance toler-will be necessary to isolate the lockout and trip ances, CEH operation may be marginal in somerelay contacts to prevent unnecessary breaker instances. Therefore, each application should befailure backup operation during relay testing. checked. In any event, it is not recommended thatSome utilities use knife switches for this function. the offset be reduced to obtain more margin sinceWhatever approach is used, the protection provided the CEH may then operate incorrectly on stablefor accidentally energizing a generator on turning swings.gear should never be removed from service whenthe machine is shut down for maintenance. It should be noted, the CEH is usually taken outGENERATOR ENERGIZED ACCIDENTALLY of service by breaker “a” switches when the ma-ON TURNING GEAR chine is shut down. When the breaker is closed, the I, II a switch may fail to close and place the relay When a generator is energized three phase while back in service and therefore there may be a ques-on turning gear, it will behave and accelerate as an tion as to the dependability of the CEH for thisinduction motor. The equivalent machine imped- protection.20
  20. 20. Reverse Power Relays (GGP, ICW) Supplementary Protection The power into the machine during this contin- While there are several generator zone relaysgency can be approximated by using machine cur- which may provide protection for this contingency,rent and the negative sequence resistance R2. It the performance of this protection may be marg-would appear, the resulting power levels will be in inal and requires close checking. Therefore, thethe pickup range (3 percent or higher of machine preferred approach is to provide supplementaryrating) of the GGP and the ICW. If the terminal protection designed for this specific purpose.voltage is low during this contingency, however,the GGP may not produce an output. Figure 8 shows a protective scheme which has been suggested for this contingency. This approach The GGP relay has an ac voltage timer (IAV) uses a frequency relay-current relay combinationwhose pickup level is about 48 percent of rated which would only be in service when the machinevolts. If the terminal voltage is below this level, as is shut may well be, the relay will never time out. The current relay could be a sensitively set Both types of relays involve time delay and instantaneous relay of the type CHC12. It shouldtherefore are less desirable. have a continuous rating which would permit it to be picked up continuously when the machine isSystem Backup Relays (CEB13C, IJCV) on-line and carrying full load. The pickup setting of this relay should be 50 percent or less of the In many instances, the CEB relay can be ad- minimum generator current seen during thisjusted to provide protection. The relay would have be connected at the machine terminals and thereverse offset adjusted to encompass subtransient The frequency relay would be an IJF51C2A,reactance. In general, a 4 ohm offset would be with a pickup setting range of 48-55 Hz. This relayrequired. This relay has time delay associated with would be set well below any emergency operatingit which would be undesirable in this instance. frequency. The IJCV should be able to detect this condi- The voltage balance relay CFVB prevents incor-tion with normal settings. Its pickup, however, rect operations for an IJF loss of potential undershould be checked with the expected terminal normal operating conditions.conditions. This relay has an advantage in that itcan operate as a simple overcurrent relay in case When the generator is shut down and the fre-the potential supply is disconnected when the quency drops below the IJF setting, the IJF resetsmachine is down for maintenance. Again, the time and energizes auxiliary relay 83 which in turn armsdelay associated with this relay would be undesir- the current relay circuit. If the generator is acci-able. dentally energized, the time delay dropout of 83 permits 50 to pick up and trip the unit in high It should be noted that if the potential supply is speed.disconnected during maintenance, the GGP, theICW, and the CEH will become inoperative. The Whichever scheme is used to provide protectionCEB, with its offset setting, will have a voltage pro- for accidentally energizing a generator on turningportional to current which may produce sufficient gear, the protection should be connected to triptorque to cause relay operation. the main breaker, initiate breaker failure backup, and be so implemented that it is never taken out of service when the machine is shut down for main- tenance.Speed Relay Gas turbines have an auxiliary relay in the START-UP OR SHUTDOWN PROTECTIONSpeedtronic Governor which will continuallyattempt to trip the unit during underspeed condi- During start-up or shutdown of a generator, thetions. unit may be operated at reduced and/or decreasing 21
  21. 21. In general, current transformer performance will not be a problem at reduced frequency (Reference 13). While CT voltage output decreases with fre- quency, the reactive component of most relay bur- dens also decreases with frequency. Therefore, the reduction in CT capability is compensated for by a reduction in relay burden. +DC Supplementary protection for a unit generator transformer during the start-up or shutdown period can be provided through the use of plunger- type relays as shown in Figure 10. Supplementary ground fault protection can be provided by using a PJV voltage relay connected in parallel with the IAV51 K. Supplementary phase fault protection can be provided by using PJC overcurrent relays in either one of two methods: 1. Placing PJC relays in series with the operate cir- cuits of the transformer differential relay.50 --Instantaneous Overcurrent Relay, Type CHC1250BF - Breaker Failure Current Detector60 - Voltage Balance Relay, Type CFVB 2. Place PJC relays in the CT phase leads which81 - Frequency Relay, Type IJF51C2A connect to the generator backup relays or (Depending on the aux. relay used (83), it may metering. be necessary to bypass seal-in circuit. The HGA17A specified for 83 will not pick up this Method (I), the preferred approach, is capable seal-in).83 - Auxiliary Relay Time Drop Out, HGAl7A, (15 of providing sensitive supplementary protection. cycle drop out time.) The PJC relay would be set above the difference86 - Lock Out Relay - HEA current that will flow in the differential circuit during normal 60 Hz operation. This is to preventFigure 8. Supplementary protection f o r accidentally ener- operating the PJC relay continuously energized.gizing a generator on turning gear. In general, the difference current will be small and it will be possible in most instances to set the PJC at its minimum 2 amp pickup.frequency with field applied for a period of time.When operating frequency decreases below rated, One factor which must be considered whenthe sensitivity of most generator zone protective using this method is the effect of the PJC burdenrelays will be adversely affected. The sensitivity of on the 60 Hz operation of the transformer differ-a few relays will only be slightly reduced while ential relay. The PJC current coil will be in the dif-other relays will not provide adequate protection ferential circuit at all times and will present addi-or become inoperative. Reference 13 and Fig. 9 tional burden to the CTS. The effect of added bur-show the effects of frequency on the pickup of den on the low voltage CTS will generally berelays which may be used in the generator zone. It negligible but it may be necessary to check theshould be noted that the transformer differential ratio error of the high voltage CTS.relay (BDD and STD) and generator ground relay(IAV51 K) both being tuned relays, lose sensitivityrather rapidly below 60 Hz. The generator differ- When method (2) is used, the PJC would have toential CFD is less affected by frequency but be set above maximum full load current so that thebecomes insensitive below 30 Hz. Induction-disk, relay would not be picked up continuously whencurrent-type relays could provide adequate protec- the machine is on line. This setting would not pro-tion down to 20 Hz, while plunger-type relays are vide sensitive protection during start-up or shut-not adversely affected by off-frequency operation. down. If a pickup setting below full load current22
  22. 22. is used, the PJC coil would have to be short cir- series with the time overcurrent relay normallycuited before the machine is connected to the sys- used for protection. Supplemental phase faulttem since plunger relays cannot be operated picked protection could be provided by method (2). Inup continuously.In general, short circuiting cur- this case, it would be necessary to short circuitrent coils is not considered a desirable practice. both the phase and ground PJC current coils whenMethod (2) has an advantage in that it may also be the machine is connected to the system if bothused to provide protection for accidentally ener- relays could be picked up continuously.gizing a generator on turning gear. In this instance,it could be used in place of the CHC as discussed The supplementary protection for both typesearlier. of generator arrangements is usually inactivated when the units are connected to the system. This When a generator is bussed at low vottage (see can be accomplished by opening the trip circuitsFig. 2), supplemental ground protection could be with a breaker“b” switch or with an underfre-provided by using a sensitive PJC current relay in quency relay as shown in Fig. 10. 23
  23. 23. 8 r . . . . 7 .. 1 1 .. . 6 . l .. .. . 5 .. .. (D) I . ‘lAV51K . I . . I 4 1 (C) l . i(E) CFD l , BDD . I ETD , .. t .s . .. .. .. / /’ _0’ (A) PJC I 1 r/-m- -. /- (F) PJV 0 0 10 20 30 40 50 60 70 80 Frequency In Hz _ - - _ (A) ) Plunger Type Current Relay (B) Induction Overcurrent Relay (C) . . . . . . . . Generator Differential Relay (D) - - - Generator Ground Relay (E) - - - - Harmonic, Restraint Transformer Differential Relay (F) Jc--)cI Plunger Type Voltage Relay Figure 9. Relay pick-up vs. frequency.24
  24. 24. Main Transformer PT 0 STD BDD Gen Alternate Position a For PJC I ’ Station Service Bus Control Power Alternate Connections Relays PJC31C45A Overcurrent, Relay 1 TpJV ;e, tkks PJV Relay 8-1/2 to 17-1/2 volts Adjustable Pick Up, 12 Volt Continuous IJF51C2A Frequency Relay, 48-55 Hz Figure 10. Protection during low frequency operation. CURRENT TRANSFORMERS Current transformers having small air gaps in the these CTS provide added security against incorrectcore are available on large steam turbine generators. differential relay operation for both internal andThe purpose of these air gaps in the CT cores is to external faults. Where this added security isdesired,reduce residual magnetism to a point where it will air gap CTS would be used for the generator differ-not have any adverse effect on the transient per- ential protection and for the generator CTS used informance of the CTS (see Reference 8). Basically the overall (transformer) differential scheme. 25
  25. 25. In addition, LSTG CTs rated 15,000 primary ing this mode of operatio n, this can be accomp ishedamperes and higher are provided with shield wind- in one of two methods:ings. The purpose of these windings is to shield thecore from extraneous flux fields (proximity effects) 1. Connect an overvoltage-undervoltage relay, Typewhich might cause localized saturation in the core IAV53M, across one phase of the secondaryand thereby cause excessive CT error during a fault winding.condition. 2. Connect a secondary winding in broken delta and use an IAV51 K across the broken delta.POTENTIAL TRANSFORMERS With both methods, it will be necessary to check the effects of the loading resistance on the relay For many years, it has been and still is recom- voltages during a ground fault.mended that line-to-line rated PTs connected line- To avoid coordination problems, it may beto-ground be used on high resistance grounded necessary to remove this supplementary protectiongenerators. This practice minimizes the possibility when the unit is operated in normal mode. In addi-of ferro-non-linear resonance when the generator tion, the resistance loading applied to suppressand transformer are connected as a unit. ferro-resonance should be removed when the generator is reconnected. It should be noted it is possible to have a PTferro-resonance problem with the unit generator-transformer arrangement if the generator is dis- SUB-SYNCHRONOUS RESONANCEconnected and the PTs are left connected tothe delta winding of the GSU transformer, which When a generator is connected to a system thatis then used to serve station auxiliary load. With has series capacitor compensation, it is possible tothe PTs connected to an ungrounded system, the develop sub-synchronous frequency oscillationspossibility of ferro-non-linear resonance is almost and shaft torques which can be damaging to thea certainty. To suppress ferro-resonance for this generator. Therefore, when a generator will beoperating condition, resistance loading should be operating on a series-compensated system, it isapplied across each phase of the secondary wind- recommended that a protective system be provideding. Resistance loading equal to PT thermal rating to control the sub-synchronous resonance duty onmay be required. the generator. This complex subject and the pro- tection recommendations are discussed in Refer- If the detection of ground faults is desired dur- ence 14.26