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PokemonFan asked Question? Please help?
Explain why a time grading time/time margin is important to relay protection and state how long time grading is usually implemented for relay coordination
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Explain why a time grading time/time margin is important to relay protection and state how long time grading is usually implemented for relay coordination • [--------1i+2o---------A] [--------1i+4o------B1/2] [-—---1i+4o-----B3/4] [----1i+4o---C1] [----1i+4o---C2] [-----1i+4o---C3] [ -----1i+4o----C4] 2o/4o= 2/4 nos of outgoing breakers 1i = 1 nos of incoming breakers Let us assume we have a power distribution system, in 3 steps, with 7 nos of distribution boards (db), each designated as A, B1/2, B3/4 etc and as shown in the above figure. Each db consists of 1 incomer breaker and 2 or 4 out going breakers.. All the dbs are interconnected. Say db A is connected to B1/2 and B3/4 , and B1/2 is connected to C1 and C2 , and db B3/4 is connected to C3 and C4 through the respective incoming(i/c) and outgoing(o/g) breakers. It is assume that the o/g breakers are if not supplying power to the down stream db, then they are supplying to some other consumers or motors etc. Also for the protection of the distribution system, protective relays are in place, associated with every i/c and o/g breaker. In a distribution system such as this, say a fault occurs in any of the o/g feeders of db C2. The fault current will be flowing through all the corresponding i/c and o/g breakers of dbs; A, B1/2 , and C2. Now to achieve correct and proper operation of protection relay system, first the affected o/g breaker of dbC2 ( nearest to the fault) should operate and clear the fault. If it fails to operate within certain allowable time, then the next immediate upstream breaker (in the path of fault current) i.e i/c of C2 should clear. If this also fails to clear within certain time then next upstream breaker in the path of fault current , i.e corresponding o/g breaker of B1/2 should clear. And so on.. This principle of time dependent operation of the relay protection system is called TIME GRADING / TIME DISCRIMINATION. Now, to achieve time grading two important time aspects are to be known (1)RELY OPERATING Time and (2) BREAKER OPERATING time. And (3) TOTAL OPERATING TIME. (1)Relay operating time (Tr), is the time interval between appearances of fault to the appearance of signal at relay output end. This will be in terms of few milliseconds (2)Breaker operating time (Tbr) is the time interval between exercitation of trip coil to the complete clearance of breaker. This will in terms of some tens of milliseconds. (3)Total operating time (Top), is the sum total of time under (1) and (2) For proper achievement of time grading, on principal the following should be maintained:- (Top) of Down Stream breaker is less than (Tr) of immediate Upstream Breaker. Additional aspects Short circuit ratings of bus bars and other items of power distribution systems are with two parameters. (a) Magnitude of Short Circuit current and (b) Withstand Time Capability. Typical value for (a) i.e magnitude of short circuit current is say 50 KA, And for the With Stand time capability (b) two typical values are given, ie 1sec and 3sec. 1sec is normally applicable for 415 V distribution systems, where numbers of steps are generally minimum. (in our figure it is 3) . 3 sec is normally applicable for distribution system of higher voltages where number of steps are generally more.. It is necessary that within this given withstand time capability (1 sec or 3 sec) complete time discrimination for of all the downstream and upstream breakers of the system under consideration are implemented.