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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