Simulation analysis of transformer main protection maloperation caused by current transformer saturation

Abstract A BC-phase short-circuit grounding fault occurred on the 35kV Xiaai line at the 110kV Xiajiaka substation in a specific power grid. This fault caused an incorrect operation of the differential instantaneous protection of the 110kV Xiajiaka #1 transformer. Theoretical analysis suggests that this maloperation may have been caused by the simultaneous action of the line protection during the fault, which led to the saturation of the protection CT on the medium-voltage side of the transformer. To thoroughly investigate the cause of the incident, a detailed electromagnetic transient simulation model was developed using PSCAD. The simulation results revealed that the maloperation of the #1 transformer differential protection was primarily due to two factors: the differing voltage states at the time of the two fault occurrences and the remanence in the CT. The overlap of the line protection action with the fault triggered the saturation of the medium-voltage side CT, leading to the maloperation of the differential protection. These simulation results, based on a realistic grid fault scenario, are crucial for analyzing similar incidents and enhancing the safe and reliable operation of power grids.