Phase Differential - GE Multilin 489 Instruction Manual

CHAPTER 5: SETPOINTS
5.6.8

Phase Differential

1 PHASE
DIFFERENTIAL
489 GENERATOR MANAGEMENT RELAY – INSTRUCTION MANUAL
Courtesy of NationalSwitchgear.com
For example, if the ground CT is 50:0.025, a pickup of 0.20 would be 0.20 × 50 = 10 A
primary. If the ground CT is 50:0.025, a pickup of 0.05 would be 0.05 × 50 = 2.5 A primary.
PATH: SETPOINTS S5 CURRENT ELEM.
[ ] PHASE DIFFERENTIAL
TRIP: Off
ASSIGN TRIP
MESSAGE
RELAYS (1-4): 1---
DIFFERENTIAL TRIP
MESSAGE
MIN. PICKUP: 0.10 x
DIFFERENTIAL TRIP
MESSAGE
SLOPE 1: 10%
DIFFERENTIAL TRIP
MESSAGE
SLOPE 2: 20%
DIFFERENTIAL TRIP
MESSAGE
DELAY: 0 cycles
The 489 differential element consists of the well known, dual slope, percent restraint
characteristic. A differential signal is derived from the phasor sum of the currents on either
side of the machine. A restraint signal is derived from the average of the magnitudes of
these two currents. An internal flag (DIFF) is asserted when the differential signal crosses
the operating characteristic as defined by the magnitude of the restraint signal. The DIFF
flag produces a relay operation.
External faults near generators typically result in very large time constants of dc
components in the fault currents. This creates a real danger of CT saturation.
The external fault currents will be large and the CTs will initially reproduce the fault current
without distortion. Consequently the relay will see a large restraint signal coupled with a
small differential signal. This condition is used as an indication of the possible onset of ac
saturation of the CTs.
Magnetizing Inrush current due to the energizing of a step-up transformer or a sudden
change of load, could cause a large dc offset on even very small currents that would
saturate poor quality or mismatched CTs within a few power system cycles.
In order to provide additional security against maloperations during these events, an
internal flag, SC, is set when either an ac or a dc saturation condition is indicated. Once the
SC flag has been set, a comparison of the phase angles of the currents on either side of the
generator is carried out. An external fault is inferred if the phase comparison indicates
both currents are flowing in the same direction. An internal fault is inferred if the phase
comparison indicates that the currents are flowing in opposite directions. In this case an
internal flag, DIR, is set.
If the SC flag is not set, then the relay will operate for a DIFF flag alone. If the SC flag is set
then the directional flag supervises the differential flag. The requirement for both the DIFF
flag and the DIR flag during the period where CT saturation is likely therefore enhances the
security of the scheme.
PHASE DIFFERENTIAL
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: 1 to 100% in steps of 1
Range: 1 to 100% in steps of 1
Range: 0 to 100 cycles in steps of 1
5–39