TIME GRADED OVER CURRENT PROTECTION IN PARALLEL FEEDERS
PARALLEL FEEDER – Where continuity of supply is particularly necessary, two parallel feeders may be installed. If a fault occurs on one feeder, it can be disconnected from the system and continuity of supply can be maintained from the other feeder. The parallel feeder cannot be protected by non directional relay over current relays only. It is necessary to use directional relay also and to grade the time setting of relays for selective tripping.
Fig above shows the system where two feeders are connected in parallel between the generating station and the sub-station. The protection of this system requires that
(i) Each feeder has a non directional over current relay at the generator end. These relays should have inverse -time characteristic.
(ii) Each feeder has a reverse power or directional relay at the sub station end. These relays should be instantaneous type and operate only when power flows in the reverse direction i.e. in the direction of arrow at P and Q.
Suppose an earth fault occurs on feeder 1 as shown in Fig 5. It is desired that only circuit breakers at A and P should open to clear the fault whereas feeder 2 should remain intact to maintain the continuity of supply. The fault is fed through two routes
(a) directly from feeder l via the relay A
(b) from feeder 2 via B, Q, sub-station and P
Therefore, power flow in relay will be in normal direction but is reversed in relay P. This causes the opening of circuit breaker at P. Also the relay will operate while relay B remains inoperative. It is because these relays have inverse-time characteristics and current flowing in relay A is in excess of that flowing in relay B. In this way the faulty feeder is isolated.
Written by John on March 24th, 2009 with
7 comments.
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Dayan
sujana
#1. May 13th, 2009, at 3:59 AM.
Its very useful topics