Directional time graded protection for parallel feeders

Directional time graded protection for parallel feeders

~ Two parallel feeders are often used for the reliability of the power supply.

~ so that in case of a fault on one feeder , a certain amount of power flow is turned on by the other feeder and complete shutdown dose not occur.

~ non-directional over current relay can  not be used for protection of this parallel feeder as it dose not contain discrimination.

~ below this understood from the example shown. figure 

~ parallel feeders ab and cd suppose A,B,C coke has a non-directional relay

~ And the operating time of the relay near D is 0.5 second and the operating time of the bag near A and C is 1.0 seconds.

~ suppose the feeder has a fault above f.

~ fault current will be fed via CP and ABDF.

~ the relay near both have low time setting.

~ hence the relay on B and D will operates first.

~ the wife relay will then operate.

~ now this fault belong to the feeder on the CD so the single CD feeder is also isolated from B

~ to overcome this problem a non-directional relay with inverse time characteristic is placed on the generator side of the feeder and a revers power types relay with instantaneous time characteristic is placed on the load side.

~ now suppose F is a fault on the feeder CD. the fault will feed in two ways..

~ the first CPR route and the second ABDI route.

~ The power flow of section AB is in the right directional ( dose not revers).

~ there for the relay on B cannot be operated.

~ when the directional of power is reversed in section DF. relay characteristic is kind of instantaneous.

~ so the relay on D will operated immediately and trip the circuit breaker.

~ this will stop feeding the fault ABDF route. 

~ so the flow flowing the fault ABDF route.

~ therefor the relay on a will not operated.

~ the fault is still fed by C so the relay on C will trip.

~ this the faulty feeder CD will be isolated and the healthy feeder will be in the circuit protection of ring mains