The protective functions noted in the various generating station configurations provide both primary and backup protection for the generating station as well as additional protection schemes which could also be applied.

These protective functions are listed below with a reference to the subclause in the text that discusses their application in detail. Also included is a discussion of the various tripping modes used in generating stations.

Protective Devices

Device Function                      Subclause
21 Distance relay. Backup for system and generator zone phase faults. Device 21
requires a time delay for coordination.
24 Volts/hertz overexcitation protection for the generator and its associated step-up 
and auxiliary transformers.
27 Undervoltage relay. 
32 Reverse-power relay. Motoring protection. 
40 Loss of field protection
46 Stator unbalanced current protection. Negative sequence relay. 
49 Stator thermal protection. 
50N Instantaneous overcurrent relay used as current detector in a breaker failure scheme.
50/51 Time overcurrent relays with instantaneous element. High-side bank overcurrent relays providing phase-fault protection for unit auxiliary transformer and backup protection for failure of UAT low-side bank breaker.
50/51GN Time overcurrent relay with instantaneous element. Primary and/or backup protection for generator ground faults.
51 Time overcurrent relay. Detection of turn-to-turn faults in generator windings. 
51TN1 Time overcurrent relay. Provides backup protection for transmission ground faults when applied to GSU neutral. Protects for ground faults on the unit  auxiliary bus when applied to UAT neutral.
51TN2 Time overcurrent relay. Provides backup protection for GSU ground faults when applied to GSU neutral. Protects for faults in the low-side of the UAT down to the low-side bank breaker when applied to UAT neutral. Provides backup for failure of low-side breaker to trip.
Device Function Subclause
51 UAT Time overcurrent relays connected to current transformers in UAT low-side bank breaker. Protects for phase faults on unit auxiliary bus.
51V Voltage controlled or voltage-restrained time overcurrent relay. Backup for system and generator zone phase faults.
53 Exciter or dc generator relay. 
59 Overvoltage protection.
59BG Zero-sequence voltage relay. Ground fault protection for an ungrounded bus.
59GN Voltage relay. Primary ground fault protection for a generator. 
60 Voltage balance relay. Detection of blown potential transformer fuses. 
62B Breaker failure timer. 
63 Fault pressure relay. Detects transformer faults. 
64F Voltage relay. Primary protection for rotor ground faults. 
71 Transformer oil or gas level. 
78 Loss of synchronism protection. This protection is optional. Applied when, during a loss of synchronism, the electrical center is in the step-up transformer or in the generator zone. Alternate locations are shown for this protection. A study should be made to determine which location is best for the detection of an out-of-step condition.
81 Frequency relay. Both under frequency and overfrequency protection may be required.
86 Hand-reset lockout auxiliary relay.
87B Differential relay used for bus protection.
87G Differential relay. Primary phase-fault protection for the generator. 
87GN Differential relay. Sensitive ground-fault protection for the generator. 
87T Differential relay. Primary protection for the GSU or UAT transformer. May be used to provide phase fault backup for the generator in some station arrangements. The zone may be extended to cover the generator bus using cts from the generator and Unit Auxiliary Transformer when lowside cts are not available.
87U Differential relay for overall unit and transformer.

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