Page 220 - Practical Power System and Protective Relays Commissioning

220 Practical Power System and Protective Relays Commissioning

FIGURE 18.6.14 Effect of two parallel line sections (A and B) in transmission line A Con a
distance relay setting at end A.

As can be seen from Fig. 18.6.14, in the case of two parallel sections in a
transmission line and one line connected to them as shown when we set the
distance protection, the relay will be set as follows to avoid underreaching:

I A
The actual relay measurement isZ R 5 Z A 1 : Z C
I A 1 I B
As we set the relay to Z A 1 Z C only the relay will underreach.

I A
Therefore; Z R SET5 Z A 1 : Z C :
I A 1 I B

18.6.4.3 Weak Infeed in a Permissive Overreach Scheme

As explained in the permissive overreach transfer trip (POTT) scheme
described in Section 18.6.5.3, in this scheme the POTT signal is sent by
Zone 2, which is set to overreach the protected transmission line, but in the
case of a weak infeed at one end of the transmission line, the relay R 1 at the
weak infeed end will not see the close-up fault F 1 and will not trip and send
the POTT signal to the strong end feed, as shown in Fig. 18.6.15.
To overcome this problem we set Z 3 R 1 to cover the reach of Z 2 R 2 in the
reverse direction at end A, the weak infeed circuit will operate if these con-
ditions occur:
1. Zone 3 of relay R1 (Z 3 R 1 ) will not operate for fault F 1 , this will confirm
that there is no fault in the reverse direction at end A.
2. The voltage-measuring unit at relay A has lost voltage in one phase at
least. Then the weak infeed circuit will operate and echo the POTT signal
received from R 2 Z 2 again to end B to accelerate R 2 to trip instantaneously
and to trip R 1 instantaneously.