GE 489 - Manual (Page 76)

4-26 489 Generator Management Relay GE Multilin
4.6 S5 CURRENT ELEMENTS 4 SETPOINTS
4
4.6.5 VOLTAGE RESTRAINED PHASE OVERCURRENT
PATH: SETPOINTS ÖØ S5 CURRENT ELEMENTS ÖØ PHASE OVERCURRENT
If the primary system protection fails to properly isolate phase faults, the voltage restrained overcurrent acts as system
backup protection. The magnitude of each phase current measured at the output CTs is used to time out against an inverse
time curve. The 489 inverse time curve for this element may be either ANSI, IEC, or GE Type IAC standard curve shapes.
This allows for simplified coordination with downstream devices. If these curve shapes are not adequate, FlexCurves™
may be used to customize the inverse time curve characteristics.
The voltage restraint feature lowers the pickup value of each phase time overcurrent element in a fixed relationship (see
figure below) with the corresponding input voltage to a minimum pickup of 0.15
× CT. The VOLTAGE LOWER LIMIT setpoint
prevents very rapid tripping prior to primary protection clearing a fault when voltage restraint is enabled and severe close-in
fault has occurred. If voltage restraint is not required, select "No" for this setpoint. If the VT type is selected as "None" or a
VT fuse loss is detected, the voltage restraint is ignored and the element operates as simple phase overcurrent.
A fuse failure is detected within 99 ms; therefore, any voltage restrained overcurrent trip should have a
time delay of 100 ms or more or nuisance tripping on fuse loss could occur.
For example, to determine the voltage restrained phase overcurrent pickup level under the following situation:
• PHASE OVERCURRENT PICKUP: "2.00 × CT"
• ENABLE VOLTAGE RESTRAINT: "Yes"
• Phase-Phase Voltage / Rated Phase-Phase Voltage = 0.4 p.u. V
The voltage restrained phase overcurrent pickup level is calculated as follows:
(EQ 4.9)
„ PHASE OVERCURRENT
„ [ENTER] for more
PHASE OVERCURRENT
TRIP: Off
Range: Off, Latched, Unlatched
ASSIGN TRIP
RELAYS (1-4): 1---
Range: Any combination of Relays 1 to 4
ENABLE VOLTAGE
RESTRAINT: No
Range: No, Yes
VOLTAGE LOWER
LIMIT: 10%
Range: 10 to 60%. Seen only if ENABLE VOLTAGE
RESTRAINT is "Yes"
PHASE OVERCURRENT
PICKUP: 10.00 x CT
Range: 0.15 to 20.00 × CT in steps of 0.01
CURVE SHAPE:
ANSI Extremely Inv.
Range: See Table 4–1: 489 Overcurrent Curve Types on
page 4–21.
FLEXCURVE TRIP TIME
AT 1.03 x PU: 65535 ms
Range: 0 to 65535 ms
Seen only if CURVE SHAPE is set to "Flexcurve"
↓
FLEXCURVE TRIP TIME
AT 20.0 x PU: 65535 ms
Range: 0 to 65535 ms
Seen only if CURVE SHAPE is set to "Flexcurve"
OVERCURRENT CURVE
MULTIPLIER: 1.00
Range: 0.00 to 1000.00 in steps of 0.01
OVERCURRENT CURVE
RESET: Instantaneous
Range: Instantaneous, Linear
ENTER
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NOTE
Voltage Restrained Phase OC Pickup Phase OC Pickup Voltage Restrained Pickup Curve Multiplier CT Ч Ч
=
2 0.4 Ч ( ) CT ×
= 0.8 CT Ч
=