GE G30 Instruction Manual page 313

5 SETTINGS
The zero-sequence voltage must be greater than 0.02 pu to be validated for use as a polarizing signal. Addition-
ally, when offset impedance is applied and zero-sequence current is above 0.2 pu, compensated zero-sequence
voltage -V_0 + Z_offset x I_0 has to be above 0.02 pu in order to discriminate fault direction; otherwise when zero-
sequence current is less than 0.2 pu, -V_0 is then used as the polarizing signal. If the polarizing signal is invalid,
neither forward nor reverse indication is given.
– If "Current" polarizing is selected, the element uses the ground current angle connected externally and configured
under
NEUTRAL OC1 SOURCE
point of an adequate local source of ground current. The ground current must be greater than 0.05 pu to be vali-
dated as a polarizing signal. If the polarizing signal is not valid, neither forward nor reverse indication is given. In
addition, the zero-sequence current (I_0) must be greater than the
CURRENT CUT-OFF LEVEL
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 sit-
uation 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 is selected, the element performs both directional comparisons as described. 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.
– If "Dual-V" polarizing is selected, "Voltage" polarizing is performed and "Current" polarizing is ignored if the voltage
polarizing signal is valid; otherwise "Current" polarizing is performed if the current polarizing signal is valid. If nei-
ther of them is valid, neither forward nor reverse indication is given.
– If "Dual-I" polarizing is selected, "Current" polarizing is performed and "Voltage" polarizing is ignored if the current
polarizing signal is valid; otherwise "Voltage" polarizing is performed if the voltage polarizing signal is valid. If nei-
ther of them is valid, neither forward nor reverse indication is given.
• NEUTRAL DIR OC1 POL VOLT: Selects the polarizing voltage used by the directional unit when "Voltage," "Dual,"
"Dual-V," or "Dual-I" polarizing mode is set. The polarizing voltage can be programmed to be either the zero-sequence
voltage calculated from the phase voltages ("Calculated V0") or supplied externally as an auxiliary voltage ("Measured
VX").
• NEUTRAL DIR OC1 OP CURR: This setting indicates whether the 3I_0 current calculated from the phase currents, or
the ground current shall be used by this protection. This setting acts as a switch between the neutral and ground
modes of operation (67N and 67G). If set to "Calculated 3I0" the element uses the phase currents and applies the pos-
itive-sequence restraint; if set to "Measured IG" the element uses ground current supplied to the ground CT of the CT
bank configured as
NEUTRAL DIR OC1 SOURCE
setting must be "Voltage", as it is not possible to use the ground current as an operating and polarizing signal simulta-
neously. IG current has to be above 0.05 ps to be used as operate quantity.
• NEUTRAL DIR OC1 POS-SEQ RESTRAINT: This setting controls the amount of the positive-sequence restraint. Set
to 0.063 for backward compatibility with firmware revision 3.40 and older. Set to zero to remove the restraint. Set
higher if large system unbalances or poor CT performance are expected.
• NEUTRAL DIR OC1 OFFSET: This setting specifies the offset impedance used by this protection. The primary appli-
cation for the offset impedance is to guarantee correct identification of fault direction on series compensated lines. In
regular applications, the offset impedance ensures proper operation even if the zero-sequence voltage at the relaying
point is very small. If this is the intent, the offset impedance shall not be larger than the zero-sequence impedance of
the protected circuit. Practically, it shall be several times smaller. The offset impedance shall be entered in secondary
ohms.
• NEUTRAL DIR OC1 FWD ECA: This setting defines the characteristic angle (ECA) for the forward direction in the
"Voltage" polarizing mode. The "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°.
• NEUTRAL DIR OC1 FWD LIMIT ANGLE: This setting defines a symmetrical (in both directions from the ECA) limit
angle for the forward direction.
GE Multilin
for polarization. The ground CT must be connected between the ground and neutral
setting value.
. If this setting is "Measured IG", then the
G30 Generator Protection System
5.6 GROUPED ELEMENTS

PRODUCT SETUP DISPLAY PROPERTIES
NEUTRAL DIR OC1 POLARIZING

5
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