Page 222 - Practical Power System and Protective Relays Commissioning

222 Practical Power System and Protective Relays Commissioning

will trip on Zone 2 with 500 ms, but if a received signal comes from end B
then the relay will accelerate and trip instantaneously, as shown in
Fig. 18.6.17, which shows the logic of trip for relay at end A.

FIGURE 18.6.17 Trip logic for a direct transfer trip scheme.

The trip will occur instantaneously and includes only the time to receive
the signal from the other end.
The disadvantage of this scheme is the possibility of false tripping by
accidental or maloperation of signaling equipment.

18.6.5.2 Permissive Underreach Scheme
This more reliable and more secure system is a modification of the direct
transfer trip scheme.
As shown in Fig. 18.6.18, a fault F is seen by relay B at end B of the
transmission line, then a permissive trip signal is sent to end A, at end A if
the relay A in Zone 2 sees the fault then the relay will accelerate Zone 2 and
trip instantaneously, as shown in Fig. 18.6.19, which shows the trip logic of
the permissive underreach transfer trip scheme.

FIGURE 18.6.18 Permissive underreach transfer trip scheme.