Ferroresonance Causing Sustained High Voltage at A De-energized 138 kV Bus: A Case Study

Ferroresonance causes overvoltage and excessive current flow on potential transformers (PT), which can cause substantial damage to PTs and interrupt power system normal operation. In this paper, a sustained overvoltage event on a de-energized 138 kV bus after circuit breakers were opened to clear a phase to ground fault at a Canadian transmission utility power grid is investigated. The PSCAD simulation model is created and validated using field measurement data. The PSCAD simulation results indicate that the ferroresonance due to a magnetic PT interacting with grading capacitors of circuit breakers is the root cause of the problem. The effective mitigation methods for the ferroresonance are proposed, case and sensitivity analysis using these methods for this specific event is provided to demonstrate the effectiveness.