Voltage Restrained Phase Overcurrent - GE 489 Instruction Manual

4.6 S5 CURRENT ELEMENTS
PATH: SETPOINTS S5 CURRENT ELEMENTS
PHASE OVERCURRENT
[ENTER] for more
4
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
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.
NOTE
For example, to determine the voltage restrained phase overcurrent pickup level under the following situation:
•
PHASE OVERCURRENT PICKUP:
•
ENABLE VOLTAGE RESTRAINT:
• Phase-Phase Voltage / Rated Phase-Phase Voltage = 0.4 p.u. V
The voltage restrained phase overcurrent pickup level is calculated as follows:
Voltage Restrained Phase OC Pickup
4-26
PHASE OVERCURRENT
ð
PHASE OVERCURRENT
ENTER
ð
ESCAPE TRIP: Off
ASSIGN TRIP
ESCAPE
RELAYS (1-4): 1---
MESSAGE
ENABLE VOLTAGE
ESCAPE
RESTRAINT: No
MESSAGE
VOLTAGE LOWER
ESCAPE
LIMIT: 10%
MESSAGE
PHASE OVERCURRENT
ESCAPE
PICKUP: 10.00 x CT
MESSAGE
CURVE SHAPE:
ESCAPE
ANSI Extremely Inv.
MESSAGE
FLEXCURVE TRIP TIME
ESCAPE
AT 1.03 x PU: 65535 ms
MESSAGE
↓
FLEXCURVE TRIP TIME
ESCAPE
AT 20.0 x PU: 65535 ms
MESSAGE
OVERCURRENT CURVE
ESCAPE
MULTIPLIER: 1.00
MESSAGE
OVERCURRENT CURVE
ESCAPE
RESET: Instantaneous
MESSAGE
"2.00 × CT"
"Yes"
Phase OC Pickup
=
( × ) CT ×
2 0.4 0.8
= =
489 Generator Management Relay

4.6.5 VOLTAGE RESTRAINED PHASE OVERCURRENT

Range: Off, Latched, Unlatched
Range: Any combination of Relays 1 to 4
Range: No, Yes
Range: 10 to 60%. Seen only if
RESTRAINT
Range: 0.15 to 20.00 × CT in steps of 0.01
Range: See Table 4–1: 489 Overcurrent Curve Types on
page 4–21.
Range: 0 to 65535 ms
Seen only if CURVE SHAPE is set to "Flexcurve"
Range: 0 to 65535 ms
Seen only if
Range: 0.00 to 1000.00 in steps of 0.01
Range: Instantaneous, Linear
×
Voltage Restrained Pickup Curve Multiplier CT
×
CT
4 SETPOINTS
ENABLE VOLTAGE
is "Yes"
CURVE SHAPE is set to "Flexcurve"
setpoint
VOLTAGE LOWER LIMIT
×
(EQ 4.9)
GE Multilin