Investigation of 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 the power system normal operation. In this article, a sustained overvoltage event on a de-energized 138 kV bus after the circuit breakers were opened to clear a phase A to ground fault at a Canadian transmission utility power grid is investigated. The power systems computer aided design (PSCAD) simulation model is created and validated using the field measurement data. The PSCAD simulation results indicate that the ferroresonance due to a limb-type open-core magnetic PT interacting with the grading capacitors of circuit breakers is the root cause of the problem. Due to the linearized magnetizing characteristic, the limb-type open-core PT is generally believed to be immune from ferroresonance; however, the event and simulation in this article demonstrate that the ferroresonance can still occur using this type of PT. The mitigation methods for the ferroresonance in this system are proposed; case and sensitivity studies using these methods are conducted to validate their effectiveness. In this article, the criterion on how to properly size a damping resistor or a damping reactor at the PT secondary is proposed, and the performance comparison using a damping resistor or a damping reactor for this system is demonstrated through case and sensitivity studies.