GE D90 Plus Instruction Manual page 261

CHAPTER 7: PROTECTION
PLUS
D90 LINE DISTANCE PROTECTION SYSTEM – INSTRUCTION MANUAL
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,
ensure that a reversal of the ground current does not occur for a high-side fault. Assume
that the low-side system impedance is minimal when checking for this condition. A
similar situation arises for a wye/delta/wye transformer, where current in one
transformer winding neutral can 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.
Polarizing Voltage
Range: Calculated V0, Measured VX
Default: Calculated V0
This setting selects the polarizing voltage used by the directional unit when the "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 V0") or
supplied externally as an auxiliary voltage ("Measured VX").
Operating Current
Range: Calculated 3I0, Measured IG
Default: Calculated 3I0
This setting indicates whether the 3I_0 current calculated from the phase currents, or
the ground current is used by this protection. This setting acts as a switch between the
neutral and ground modes of operation (ANSI devices 67N and 67G). If set to "Calculated
3I0," the element uses the phase currents and applies the positive-sequence restraint. If
set to "Measured IG," the element uses ground current supplied to the ground CT of the
CT bank configured as the neutral directional "Source." If this setting is "Measured IG,"
then the setting must be "Voltage," as it is not possible to use the ground
Polarizing
current as an operating and polarizing signal simultaneously.
Positive-Sequence 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 it to 0.063 for
backward compatibility with firmware revisions 3.40 and older. Set it to zero to remove
the restraint. Set it higher if large system unbalances or poor CT performance are
expected.
Offset
Range: 0.00 to 250.00 ohms in steps of 0.01
Default: 0.00 ohms
This setting specifies the offset impedance used by this protection. The primary
application 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 is not to be larger than the zero-
sequence impedance of the protected circuit. Practically, it is several times smaller. Enter
the offset impedance in secondary ohms.
GROUPED PROTECTION ELEMENTS
251