TIME GRADED OVER CURRENT PROTECTION IN RADIAL FEEDERS

In this type of protection time settings of relays is so graded that in the event of fault, the smallest possible part of the system is isolated. Following are the important cases.

RADIAL FEEDER

The main characteristics of this system is that power can flow only one direction, from generator or supply end to the load .It has the disadvantage that continuity of the supply cannot be maintained at the receiving end in the event of fault .Time graded protection of a radial feeder can be achieved by using (i) Definite time relay (ii) Inverse time relay

(i) Using Definite time relay

Fig .2 shows the over current protection of radial feeder by definite time relays .The time of operation of each relay is fixed and is independent of the operating current

Thus relay D has an operating time of 0.5 seconds while for other relays, time delay is successively increased by 0.5 second. If a fault occurs in the section DE, it will be cleared in 0.5 second by the relay and circuit breaker at D because all other relays have higher operating time. In this way only section DE of the system will be isolated. If the relay at D fails to trip, the relay at C will operate after a time delay of 0.5 second i.e. after 1 second from the occurrence of fault. The disadvantage of this system is that if there are a number of feeders in series, the tripping time for faults near the supply end becomes high (2 seconds in this case) but in most cases, it is necessary to limit the maximum tripping time to 2seconds. This disadvantage can be overcome toe reasonable extent by using Inverse time relays

(ii) Using inverse time relays – Fig 3 shows over current protection of a radial feeder using inverse time relays in which operating time is inversely proportional to the operating current. With this arrangement, the farther the circuit breaker from the generating station, the shorter is its relay operating time. The three relays at A, B and C are assumed to have inverse-rime characteristics. A fault in section will give relay times which will allow breaker to trip out before the breaker at A

Written by John on March 24th, 2009 with 1 comment.
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