How to Core Balance Transformer (CBCT)

The CBCT or the Core Balance Current Transformer is a current transformer is used for earth fault protection in grounded three phase systems. It is also known as the zero-sequence current transformer. The CBCT is a current transformer through all the three phases are made to pass as in the diagram. Thus the magnetic fluxes caused by the three phase currents cancel each other. The net resultant flux being zero does not induce any current in the secondary of the transformer. Thus the secondary current of the core balance current transformer when all the three phases are healthy is zero.

When an earth fault occurs in one of the phases, the zero-sequence fault current which flows is not cancelled by the flux of the other two phases and hence induces a current in the secondary.

The core balance current transformer can be connected to an earth fault relay which can be used to generate the tripping signal.