An Asynchronous Method Based Saturation Criterion for CT in Busbar Protection

The reliability of busbar protection will be improved if the reason causing serious saturation of CT in busbar protection, i.e., internal fault or external fault, could be correctly distinguished. When internal or external fault occurs, the CT signal can be linearly transmitted and transduced before CT saturation, and within effective data window (usually taking the data in half-period as the effective data for criterion of differential protection) the variations of magnitudes and phases of differential current and restraint current are not synchronous before and after the saturation of the CT, namely the moment when the CT is saturated is in advance of the moment when the differential protection acts, and the variation amount of differential current and that of restraint current are non-uniform. Using such asynchronous relation the authors propose a new saturation criterion of CT by which the maloperation of differential protection caused by serious saturation of CT can be effectively avoided. The reliability of this criterion is verified by digital and dynamic simulations.