6. For ground faults at four locations, 3lo from bus G: Fault F1-fault on bus H,
ID: 2265869 • Letter: 6
Question
6. For ground faults at four locations, 3lo from bus G: Fault F1-fault on bus H, all ties closed 1219A Fault F2 -close-in fault just on line-side of breaker - 3247A Fault F3-fault on bus H, maximum 3lo contribution from bus G 1462A Fault F4-resistive fault on bus H, minimum 3lo contribution from bus G 753A . Using these values: Set the pickup for a ground instantaneous element and determine if it is useful (5pts) (F1) 310-1219A (F4) 3lo 753A (F3) 310-1462A (F2) 3lo 4247A H bus Q bus 161kV X F1 F2 J busExplanation / Answer
In generl power system get in to fault duo line to grount fault. ofcourse the line to ground faut is frequently occurs and less severe whereas 3 phase line fault is most sevre and accurance of fault is least. so the maximum fault current will flow through the system when 3 phase fault occurs. to nterrupt the fault the circuit breaker has capable to extinct such large fault current.so in power systems the circuit breaking capacity was designed based on the 3 phae short MVA capacity.
power system is relable when the fault current just above the predetermined value it can be detectable by the relay. hence circuit breaker trips the circuit. we know that in power sytem the least fault currrent is due to the line to ground fault. so that thsis current should sense bye the relay. so in order to trip the circuit due to the fault current the predetermined value or cutoff value or trip should be based on the line to fault curent.
in line to ground fault the fault current is Ir=3I0
the fault curent due to short circuit is last in case the fault occuer on H bus at 4thn location. therefore 753A is taken as the set value for great relaibility
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