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5.121
Date Code 20171021
Instruction Manual
SEL-421 Relay
Protection Functions
Directional Comparison Blocking Scheme
Directional Comparison Blocking Scheme
The Directional Comparison Blocking (DCB) trip scheme performs the follow-
ing tasks:
➤
Provides carrier coordination timers that allow time for the block trip
signal to arrive from the remote terminal. The 21SD timer is for the
Zone 2 distance elements Z2P and Z2G. The 67SD timer is for the
Level 2 overcurrent elements 67Q2 and 67G2.
➤
Instantaneously keys the communications equipment to transmit
block trip for reverse faults and extends this signal for a settable time
(Z3XD) following the dropout of all Zone 3 distance and Level 3
directional overcurrent elements.
➤
Latches block trip send condition by the phase-distance elements
following a close-in zero voltage three-phase fault when the
polarizing memory expires; return of polarizing memory voltage or
interruption of fault current removes the latch.
➤
Extends the received block trip signal by a settable time (BTXD).
The DCB scheme consists of four sections:
➤
Coordination timers
➤
Starting elements
➤
Extension of the blocking signal
➤
Stopping elements
Coordination Timers
NOTE:
The TRCOMM SEL
OGIC
control
equation determines which protection
elements cause the relay to trip via
the communications-assisted tripping
scheme logic. In DCB schemes, set
delayed Zone 2 mho phase and
ground-distance protection (Z2PGS)
plus delayed Level 2 negative-
sequence residual ground directional
overcurrent element (67QGS2) in the
TRCOMM SEL
OGIC
control equation.
Transmission Line Example on
page 6.53
Momentarily delaying the forward-looking Zone 2 and Level 2 elements that pro-
vide high-speed tripping at the local terminal ensures that the local circuit breaker
does not trip for external faults behind the remote terminal. This delay provides
time for the nondirectional and reverse-looking elements at the remote terminal
to send a blocking signal to the local terminal during out-of-section faults. This
particular time delay is the coordination time for the DCB scheme. There are sep-
arate coordination timers for Zone 2 distance elements (21SD) and Level 2 resid-
ual directional overcurrent elements (67SD).
The recommended setting for the 21SD timer is the sum of the following three
times:
➤
Control input recognition time (including debounce timer)
➤
Remote Zone 3 distance protection maximum operating time
➤
Maximum communications channel time
The output of Zone 2 delay timer 21SD is Relay Word bit Z2PGS (Zone 2 Phase
and Ground Short Delay).
The recommended setting for the 67SD timer is the sum of the following three
times:
➤
Control input recognition time (including debounce timer)
➤
Remote Level 3 nondirectional low-set overcurrent element
maximum operating time
➤
Maximum communications channel time
The output of Level 2 delay timer 67SD is Relay Word bit 67QG2S (Negative-
Sequence and Residual Directional Overcurrent Short Delay).
Summary of Contents for SEL-421-4
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