Improvement of Security of Transformer Differential Protection in Breaker-and-a-Half Substations

The differential protection of power transformers is prone to incorrect operation in breaker-and-a-half substations if the transformer winding current is provided by paralleling the bay current transformers (CTs). In this paper, a real event of undesired tripping without the presence of CT saturation under fault current flowing through the bay CTs between the substation buses is analyzed and an approach is proposed to remedy the problem. Simulation studies conducted in PSCAD/EMTDC program show that the nominal accuracy errors of CTs under fault currents below the CTs accuracy limit factor (ALF) can cause false differential currents higher than the usual differential relay setting. Using the proposed approach, the restraint current would be able to desirably stabilize the relay during external faults.