Core Balance Current Transformers

The CBCT, or Core Balance Current Transformer, is a current transformer 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 which all 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 three phases are healthy is zero.

When an earth fault occurs in one of the phases, the zero-sequence fault current that 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.