Overcurrent Alarm; Offline Overcurrent - GE 489 Instruction Manual

4.6 S5 CURRENT ELEMENTS
T

OVERCURRENT ALARM

T
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If enabled as Latched or Unlatched, the Overcurrent Alarm will function as follows: If the average generator current (RMS)
measured at the output CTs exceeds the level programmed for the period of time specified, an alarm will occur. If pro-
grammed as unlatched, the alarm will reset itself when the overcurrent condition is no longer present. If programmed as
4 latched, once the overcurrent condition is gone, the reset key must be pressed to reset the alarm. The generator FLA is cal-
culated as:
Generator FLA
=
T
OFFLINE O/C
T
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When a synchronous generator is offline, there should be no measurable current flow in any of the three phases unless the
unit is supplying its own station load. Also, since the generator is not yet online, differentiation between system faults and
machine faults is easier. The offline overcurrent feature is active only when the generator is offline and uses the neutral end
CT measurements (Ia, Ib, Ic). It may be set much more sensitive than the differential element to detect high impedance
phase faults. Since the breaker auxiliary contacts wired to the 489 Breaker Status input may not operate at exactly the
same time as the main breaker contacts, the time delay should be coordinated with the difference of the operation times. In
the event of a low impedance fault, the differential element will still shutdown the generator quickly.
If the unit auxiliary transformer is on the generator side of the breaker, the pickup level must be set greater
than the unit auxiliary load.
NOTE
4-20
ð
OVERCURRENT
ENTER
ALARM: Off
ESCAPE
ASSIGN ALARM
ESCAPE
RELAYS (2-5): ---5
MESSAGE
OVERCURRENT ALARM
ESCAPE
LEVEL: 1.01 x FLA
MESSAGE
OVERCURRENT ALARM
ESCAPE
DELAY: 0.1 s
MESSAGE
OVERCURRENT ALARM
ESCAPE
EVENTS: Off
MESSAGE
×
3 rated generator phase-phase voltage
ð

OFFLINE OVERCURRENT

ENTER
TRIP: Off
ESCAPE
ASSIGN TRIP
ESCAPE
RELAYS (1-4): 1---
MESSAGE
OFFLINE OVERCURRENT
ESCAPE
PICKUP: 0.05 x CT
MESSAGE
OFFLINE OVERCURRENT
ESCAPE
TRIP DELAY: 5 cycles
MESSAGE
489 Generator Management Relay
4 SETPOINT PROGRAMMING
4.6.2 OVERCURRENT ALARM
Range: Off, Latched, Unlatched
Range: Any combination of Relays 2 to 5
Range: 0.10 to 1.50 × FLA in steps of 0.01
Range: 0.1 to 250.0 s in steps of 0.1
Range: On, Off
4.6.3 OFFLINE OVERCURRENT
Range: Off, Latched, Unlatched
Range: Any combination of Relays 1 to 4
Range: 0.05 to 1.00 × CT in steps of 0.01
Range: 3 to 99 cycles in steps of 1
GE Power Management