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.TheCBCT 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 transformerwhen 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 beused to generate the tripping signal.


Summation C.T.′s are designed to summarize several synchronous A.C. currents of equal phase relation with any angle of phase difference, i.e. summarize the secondary currents of a number of main C.T.’s. The secondary circuits of the main C.T.’s are to be connected to the corresponding marked primary terminals of the summation C.T., i.e. each main C.T. feeds with its secondary current a specific portion of the primary winding of the summation C.T. The number of turns of the particular sections of the primary winding must fit in with ratios of the main C.T’s. If all main C.T.’s have identical ratios, it is irrelevant for their secondary circuits to which section of the primary winding of the summation C.T. they are connected.
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