Root cause investigation of over-current ground relay tripping during energizing parallel autotransformers

Abstract

In this paper, the root cause of excessive zero-sequence currents observed when energizing parallel autotransformers in a power system is investigated. 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 monitoring data indicated that when the utility, local generators and the first autotransformer were already in operation, there were rich 3^rd and 9^th order harmonic currents, which caused the over-current ground relay (51G) to trip, during the second autotransformer energizing through its high voltage side while its low voltage side had already been electrically connected to the system. Through this research, the root cause of the problem is determined to be saturation of the two autotransformers. It is found that this is a special phenomenon that only occurs to parallel 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 mis-operation of the ground relay are proposed and their effectiveness are validated by simulation, and one method has been currently implemented in the field.