Root Cause Analysis of Over-Current Ground Relay Tripping During Energizing Parallel Autotransformers

In this paper, the root cause analysis of excessive zero-sequence currents is conducted when energizing parallel autotransformers in a power system. The studied system is a large international airport in Canada, where its power is supplied by both utility and local distributed generation (DG) feeding the load through two parallel autotransformers. Recent field records indicated that when the utility, local generators and the first autotransformer were in operation, and the second autotransformer was already energized from its low voltage side, at this moment, closing the circuit breaker on the high voltage side of the second autotransformer caused rich third and ninth order harmonic currents and the subsequent over-current ground relay (51G) tripping. Through this research, it is determined that the root cause of the problem is the saturation of the two autotransformers. The PSCAD/EMTDC simulation is conducted for the system, and the simulation model is benchmarked using field measurement data. Two methods are developed in this paper to calculate the maximum RMS zero-sequence current flowing through the two autotransformers: 1) an analytical based simplified equivalent circuit method and 2) a curve fitting based mathematical equation method. The solutions to prevent misoperation of the ground relay are proposed and their effectiveness is validated.