Phase Selection - GE D90 Plus Instruction Manual

CHAPTER 15: THEORY OF OPERATION

Phase selection

PLUS
D90 LINE DISTANCE PROTECTION SYSTEM – INSTRUCTION MANUAL
If the fault evolves slowly, the sequence is different: The relay trips phase A as in the
previous example. The phase selector resets, the open pole detector is activated and
forces the zone 1 and zone 2 AG, AB, CA and negative-sequence overcurrent elements to
reset. If the zone 1 BG element picks up, or the zone 2 BG element picks up resulting in
operation of the POTT scheme, no trip command will be issued until the
asserted. This happens 1.25 cycles after the first trip. If at this time or any time later a
request for trip is placed (due to an evolving fault), a three-pole trip is initiated. The
operand is de-asserted by the
POLE
Shortly all three-poles are opened.
When the dead time expires, the recloser signals the breaker control to close the breaker.
At this time all the protection elements are operational, as the open pole detector is not
blocking any elements. If the line-side VTs are used, the line pickup element is armed as
well. If there is a fault on the line, these elements will pickup the fault and issue next
request for trip. This request results in three-pole trip as the
asserted.
The response of the system from this point is as described above for the second trip,
except the recloser will go to lockout upon the next initiation (depending on the number of
shots programmed).
Plus
The D90 uses phase relations between current symmetrical components for phase
selection. First, the algorithm validates if there is enough zero-sequence current, positive-
sequence current, and negative-sequence current for reliable analysis. The comparison is
adaptive; that is, the magnitudes of the three symmetrical components used mutually as
restraints confirm if a given component is large enough to be used for phase selection.
Once the current magnitudes are validated, the algorithm analyzes phase relations
between the negative-sequence and positive-sequence currents and negative-sequence
and zero-sequence currents (when applicable) as illustrated below.
Figure 476: Phase selection principle (ABC phase rotation)
Due to dual comparisons, the algorithm is very secure. For increased accuracy and to
facilitate operation in weak systems, the pre-fault components are removed from the
analyzed currents. The algorithm is very fast and ensures proper phase selection before
any of the correctly set protection elements operates.
Under unusual circumstances such as weak-infeed conditions with the zero-sequence
current dominating during any ground fault, or during cross-country faults, the current-
based phase selector may not recognize any of the known fault pattern. If this is the case,
voltages are used for phase selection. The voltage algorithm is the same as the current-
based algorithm; for example, phase angles between the zero-sequence, negative-
sequence, and positive-sequence voltages are used. The pre-fault values are subtracted
prior to any calculations.
operand, resetting the open pole detector.
TRIP 3-POLE
AR FORCE 3-P TRIP
SINGLE-POLE TRIPPING
is
AR FORCE 3-P TRIP
TRIP 1-
is still
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