GE 845 Instruction Manual page 305

CHAPTER 4: SETPOINTS
845 TRANSFORMER PROTECTION SYSTEM – INSTRUCTION MANUAL
Path:
Setpoints > Protection > Group 1(6) > Current > Neutral Directional OC 1(X)
FUNCTION
Range: Disabled, Enabled
Default: Disabled
POLARIZING MODE
Range: Voltage, Current, Dual
Default: Voltage
This setting selects the polarizing mode for the directional unit.
• If Voltage polarizing mode is selected, the element uses the zero-sequence voltage
angle for polarization. The user can select either the zero-sequence voltage V_0,
calculated from the phase voltages, or the zero-sequence voltage supplied externally
as the auxiliary voltage V_X.
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 ground
current measured on the ground current input. The ground CT must be connected
between the ground and neutral point of an adequate source of ground current. The
ground 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. For example, if using an autotransformer neutral current as a polarizing
source, it should be ensured that a reversal of the ground current does not occur for a
high-side fault. The low-side system impedance should be assumed minimal when
checking for this condition. A similar situation arises for a wye/delta/wye transformer,
where current in one transformer winding neutral may reverse when faults on both
sides of the transformer are considered.
• 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")
POS SEQ RESTRAINT
Range: 0.000 to 0.500 in steps of 0.001
Default: 0.063
This setting controls the amount of the positive-sequence restraint. Set to zero to
remove the restraint. Set higher if large system unbalances or poor CT performance are
expected.
PROTECTION
4–177