Assume the following system shown in figure (53) which consists of two bus bars and four equipments two on each bus and a bus coupler between the two buses.
Each equipment contains one current transformer. Connected on it except the bus coupler which contains two current transformers one on each side of the circuit breaker. For equilibrium, each bus is stable in load condition (without faults).
There are two cases:-
1- Case bus coupler is disconnected:-
I3+ I4=0
I5+ I6=0
2- Case2 bus coupler is connected
I1+ I3+I4=0
I2+ I5+I6=0
Note:
CT1 (of the coupler) is connected between circuit breaker and bus1 and for equilibrium it is added to CT3, and Ct4 which are connected to bus2, and CT2 (of the coupler) is connected between circuit breaker and bus2 and for equilibrium it is connected to CT5, CT6 which are connected to bus1.
The advantage of these connections is to protect the zone between circuit breaker and current transformer of the coupler. If one CT is installed for the coupler, the zone between circuit breaker and current transformer is not protected by bus protection.