GE 889 Instruction Manual page 319

CHAPTER 4: SETPOINTS
NOTE:
NOTE
889 GENERATOR PROTECTION SYSTEM – INSTRUCTION MANUAL
The calculated V_0 can be used as polarizing voltage only if the voltage transformers
are connected in Wye. The auxiliary voltage can be used as the polarizing voltage if
the auxiliary voltage is connected to a zero-sequence voltage source (such as the
open delta connected secondary of VTs).
The zero-sequence (V_0) or auxiliary voltage (V_X), accordingly, must be greater than
0.02 x VT to be validated for use as a polarizing signal. If the polarizing signal is invalid,
neither forward nor reverse indication is given.
• If Current polarizing mode is selected, the element uses the angle of the polarizing
current measured on the ground current input K1-CT (if available). The polarizing
current must be greater than 0.05 x CT to be validated as a polarizing signal. If the
polarizing signal is not valid, neither forward nor reverse indication is given.
For a choice of current polarizing, it is recommended that the polarizing signal be
analyzed to ensure that a known direction is maintained irrespective of the fault
location.
• If Dual polarizing mode is selected, the element performs both directional
comparisons as described above. A given direction is confirmed if either voltage or
current comparators indicate so. If a conflicting (simultaneous forward and reverse)
indication occurs, the forward direction overrides the reverse direction.
POLARIZING VOLTAGE
Range: Calculated V0, Measured VX
Default: Calculated V0
Selects the polarizing voltage used by the directional unit when "Voltage" or "Dual"
polarizing mode is set. The polarizing voltage can be programmed to be either the zero-
sequence voltage calculated from the phase voltages ("Calculated V_0") or supplied
externally as an auxiliary voltage ("Measured VX")
FORWARD ECA
Range: –90° to 90° in steps of 1°
Default: 75°
This setting defines the element characteristic angle (ECA) for the forward direction in
"Voltage" polarizing mode. "Current" polarizing mode uses a fixed ECA of 0°. The ECA in
the reverse direction is the angle set for the forward direction shifted by 180°.
FORWARD LIMIT ANGLE
Range: 40° to 90° in steps of 1°
Default: 90°
This setting defines a symmetrical (in both directions from the ECA) limit angle for the
forward direction.
FORWARD PICKUP
Range: 0.050 to 30.000 x CT in steps of 0.001 x CT
Default: 0.050 x CT
This setting defines the Pickup level for the overcurrent unit of the element in the
forward direction.
When Signal Input is selected as K1 ground input from 50:0.0.25 CT (B1/B5 order code), the
pickup setting must be done considering 1:1 CT ratio. The relay applies a ratio of 2000
internally to directly measure primary side ground current and apply into this function. For
example, a pickup setting 1.000 x CT translates to the pickup of the element when CT
primary current exceeds 1.000 Ampere (i.e. 0.5 mA injected into the relay from CT
secondary.)
PROTECTION
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