What is Bus differential Relay ?

The principle of differential busbar protection is very simple. Here, secondaries of CTs are connected parallel. That means, S1 terminals of all CTs connected together and forms a bus wire. Similarly S2 terminals of all CTs connected together to form another bus wire. A tripping relay is connected across these two bus wires.

The Relay which is used to protects the Bus Bar in the substation is called Bus Differential Relay. This relay acts on Differential current measurement principle. As per the Kirchoff's current law, at any point, the sum of currents entering the junction is equal to the sum of currents leaving the junction. The Bus Differential Relay acts on the same principle. The Current Transformer (CT) installed in every feeders  measures the current entering or leaving the bus bar and the summation of the secondary current's of all CT's is given as input to the relay coil. So when any unbalance occurs, the relay will be activated and the total feeds to the bus bar will be cut off.

In 33 KV common bus-A having 5 Feeders:

a) 2 Incoming Feeders( 1000/1 A CT 2 numbers)( 1st one is Generator, 2nd one is GRID, all Synchronising Breaker) 

b) 2 Outgoing Feeders ( 1000/ 1 A CT 2 nos)( 1st one is Load, 2nd one is Station Auxiliary Transformer Upstream-Breaker)

C) 1 Buscoupler Feeder.( 1000/1 A CT 1 Nos).

All 5 CT's are connected Parallel with Bus-Differential Relay.

Now in normal Condition According to  Kirchhoff's Current law , Summation of all 5 CT'S current will be Zero/ . OR Ia+Ib+Ic+Id+Ie = 0.

In case of fault/ unbalance Current in any of these individual CT's will make Differential Current in Relay Current Coil OR ( Ia+Ib+Ic+Id+Ie >= Trip Current Setting in Relay). Trip coil will energize. 

It will trip the individual Faulty Breaker to save-guard the 33 KV Bus........through Bus-Differential Relay.