GE D90 Plus Instruction Manual page 258

GROUPED PROTECTION ELEMENTS
248
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 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
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 shall be 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
the ground 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 to 0.063 for
backward compatibility with firmware revisions 3.40 and earlier. Set to zero to remove
the restraint. Set 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 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.
setting must be "Voltage", as it is not possible to use
Polarizing
PLUS
D90 LINE DISTANCE PROTECTION SYSTEM – INSTRUCTION MANUAL
CHAPTER 7: PROTECTION