CHAPTER 15: THEORY OF OPERATION SINGLE-POLE TRIPPING
D90
PLUS
LINE DISTANCE PROTECTION SYSTEM – INSTRUCTION MANUAL 655
zone 1 operate and zone 2 pickup operands that were picked up reset immediately. The
BG, CG, and BC distance elements remain operational guarding the line against evolving
faults.
As zone 2 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 POTT TRIP A operand. This operand is passed to the trip output
and results in exactly same action as described above for zone 1.
Depending on response times, the actual trip is initiated either by zone 1 or by the POTT
scheme. At the moment the TRIP 1- POLE operand is asserted, the phase selector resets and
no other trip action can take place. After the trip command is issued, all the picked up
elements are forced to reset by the open pole detector.
The TRIP 1-POLE operand initiates automatically a single-pole autoreclose. The autoreclose
is started and asserts the AR RIP 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 AR FORCE 3-P TRIP operand is asserted 1.25 cycles following autoreclose initiation. This
operand acts as an enabler for 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, the reset time then expires at the autorecloser and the AR FORCE
3-P TRIP and AR RIP outputs are de-asserted, returning all features to the state described at
the beginning of this section.
If the fault was permanent, appropriate protection elements detect it and place a trip
request for the trip output element. As the AR FORCE 3-P TRIP is still asserted, the request is
executed as a three-pole trip.
The response of the system from this point is as described earlier for the second trip,
except that the autorecloser goes to lockout upon the next initiation (depending on the
number of shots programmed).
SLG fault evolving into an LLG fault scenario for single-pole tripping
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. If this is the case, the TRIP PHASE A, TRIP PHASE B, and TRIP PHASE C operands are
asserted. The 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
TRIP 3-POLE operand is asserted (not the TRIP 1-POLE operand), the open pole is not
activated. Because the AR RIP in progress is asserted, the phase selector is blocked as well.
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
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