Page 270 - Practical Power System and Protective Relays Commissioning

266 Practical Power System and Protective Relays Commissioning

Subchapter 18.11

Backup Protections

18.11.1 INTRODUCTION

The basic design of protective relays of a power system is designed as
follows.
All elements of the power system are protected by at least one sensitive
relay called a primary relay, which trips as quickly as possible, in the order
of 100 ms when a fault occurs, as shown in Fig. 18.11.1.
These elements of the power system are protected by another set of
relays, which are called backup protection, in the event of failure of the pri-
mary protection, as shown in Fig. 18.11.2.
Backup protection is divided into two types as follows:
1. Remote backup relays are located one bus away from the primary
protection;
2. Local backup relays are at the same location as the primary protection.

18.11.2 REMOTE BACKUP PROTECTION

As shown in Fig. 18.11.2, the primary protection will trip breaker 1 and
breaker 2 for a fault on line A instantaneously, but if the breaker 1 is not
tripped the backup protection at breaker 3 will trip breaker 3, but with a time
delay usually of 300 ms, the same applies if breaker 2 does not trip the
backup protection at breaker 6, which will trip breaker 6 but with a time
delay of 300 ms, this give a second protection to clear the fault with a time
delay if the primary protection did not clear it instantaneously.

18.11.3 LOCAL BACKUP AND BREAKER FAILURE
PROTECTION
In the case of relay failure, two sets of protection are installed at the local
ends of the protected line. One is the primary and the second is the backup,
alternatively these are called main 1 and main 2, in the case of one set fail-
ing to trip the breaker, the other relay trips. This normally utilizes two relays
from different manufacturers, and if these need a communication channel
they use separate communication channels for each relay.
However, in the case of a breaker failing to trip we use a BF scheme, as
shown in Fig. 18.11.3.
When a fault occurs on the line controlled by breaker 1, the distance
relay at the breaker 1 end sends a trip signal to breaker 1 at the same time it
will send a start signal to the BF relay then The timer (T) will starts and the