GE L90 Instruction Manual page 781

Line current differential system
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CHAPTER 10: THEORY OF OPERATION
block phase distance AB and CA elements, respectively (all zones); the
elements (all zones). As a result, the
distance elements remain operational guarding the line against evolving faults: BG, CG, and BC.
As zone 2 and/or negative-sequence directional elements pick up due to the fault, the permission to trip is keyed to the
remote end. Assume here that a single-bit channel is used. If so, no extra information is sent to the remote end, just
permission to trip sent over the TX1 operand. Upon receiving permission to trip over RX1, the POTT scheme decides to trip.
The scheme checks the phase selector for phase type identification and issues a trip for phase A by asserting the
operand. This operand is passed to the trip output and results in the same action as described previously for zone 1.
A
Depending on response times, the actual trip is initiated either by zone 1 or by the POTT scheme. At the moment that the
operand is asserted, the phase selector resets and no other trip action can take place. After the trip command
TRIP 1-POLE
is issued, all the picked up elements are forced to reset by the open pole detector.
The
operand initiates automatically a single-pole autoreclose. The autoreclose is started and asserts the
TRIP 1-POLE
operand. This operand keeps blocking the phase selector so that it does not respond to any subsequent events. At the
same time, the operand removes zero-sequence directional supervision from ground distance zones 2 and 3 so that they
can respond to a single-line-to-ground fault during open pole conditions.
The
operand is asserted 1.25 cycles following autoreclose initiation. This operand acts as an enabler for
AR FORCE 3-P TRIP
any existing trip request. In this case, none of the protection elements is picked up at this time, therefore no more trips are
initiated.
When the recloser dead time interval is complete, it signals the breaker control element to close the breaker. The breaker
control element operates output relays to close the breaker.
When pole A of the breaker closes, this new status is reported to the breaker control element, which transfers this data to
the breaker failure, autorecloser, open pole detector and trip output elements. The response at breaker failure depends on
the programming of that element. The response at the autorecloser is not relevant to this discussion. At the open pole
detector, the blocking signals to protection elements are de-asserted.
If the fault was transient, then the reset time expires at the autorecloser and the
asserted, returning all features to the state described at the beginning of this description.
If the fault was permanent, appropriate protection elements detect this and place a trip request for the trip output
element. As the
AR FORCE 3-P TRIP
The response of the system from this point is as described earlier for the second trip, except that the autorecloser locks out
upon the next initiation (depending on the number of shots programmed).
10.5.1.3 SLG fault evolving into LLG
When an AG fault occurs, the events unfold initially as in the previous example. If the fault evolves quickly, the phase
selector changes its initial assessment from AG to ABG fault and when the trip request is placed either by the zone 1 or the
POTT scheme, a three-pole trip is initiated. When this is the case, all three
command is passed to the breaker control element and results in a three-pole trip. At the same time, the recloser is
initiated as per settings of the trip output. As the
pole is not activated. Because the
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 is issued until 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
TRIP 1-POLE
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 pick up the fault and issue the next request
for trip. This request results in a three-pole trip as the
The response of the system from this point is as described earlier for the second trip, except that the recloser goes to
lockout upon the next initiation (depending on the number of shots programmed).
L90 LINE CURRENT DIFFERENTIAL SYSTEM – INSTRUCTION MANUAL
and
operands that were picked-up reset immediately. The following
Z1 OP
Z2 PKP
is still asserted, the request is executed as a three-pole trip.
operand is asserted (not the
TRIP 3-POLE
in progress is asserted, the phase selector is blocked as well.
AR RIP
AR FORCE 3-P TRIP
operand, resetting the open pole detector. Shortly all three-poles are
TRIP 3-POLE
AR FORCE 3-P TRIP
blocks the ground distance AG
OPEN POLE FA OP
and
AR FORCE 3-P TRIP
A, B, and
operands are asserted. The
TRIP PHASE
C
TRIP 1-POLE
is asserted. This happens 1.25 cycles after the
is still asserted.
SINGLE-POLE TRIPPING
POTT TRIP
AR RIP
outputs are de-
RIP
operand), the open
10
10-39

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