Description of Operation
GARD 8000 Distance Relay
RFL Electronics Inc.
Transient block
ECHO distance
Carrier Receive
Forward pilot zone
Three Poles Open
Power swing block
Block pilot trip
Weak infeed trip
distance
Zone 1 Fault
100ms
0
Enable open breaker
carrier sending
0
T
Distance carrier
send extend time
Enable Power
swing block pilot
Distance pilot
protection trip
Distance
protection channel
start
Loss-of-guard
10
0
Unblock delay time
200
0
Carrier fail time
150
0
Unblock window
Carrier Fail
Figure 9-21. Directional Comparison Unblocking Diagram
During normal operation, a ‘guard’ carrier signal is transmitted and received at the remote terminal.
When a terminal detects a forward fault within its pilot zone, it issues a signal for the carrier to switch
to trip frequency (Distance protection channel start). When the shift is recognized at the remote end, a
‘carrier receive’ signal is given to the receiving relay, and a ‘Distance protection pilot trip’ is issued as
for a permissive overreach scheme. As for the permissive schemes, the carrier signal is continued for
the time set by ‘Distance carrier send extend time’.
The transient block logic ensures that overtripping is not taking place for current reversals on parallel
lines.
In case the carrier trip signal is lost in the fault on the line, the unblock logic comes into play. A ‘loss-
of-guard’ AND no ‘Carrier receive’ provides a permissive trip signal to the distance relay after a short
time delay; 10 ms. This delay is to avoid enabling the unblock logic for normal carrier switching from
guard to trip operation.
The permissive signal is held during the ‘Unblock window’ (150 ms).
August 1, 2009
9-24
973.334.3100
