Sel SEL-421 - Directional Comparison Blocking Scheme; Coordination Timers

R.1.88
SEL-421 Relay Reference Manual Date Code 20111215
Protection Functions
Directional Comparison Blocking Scheme
Directional Comparison Blocking Scheme
The Directional Comparison Blocking (DCB) trip scheme performs the
following 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 M2P 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
Momentarily delaying the forward-looking Zone 2 and Level 2 elements that
provide 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 separate coordination timers for Zone 2 distance elements
(21SD) and Level 2 residual 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).