Tertiary Shunt Reactor Grounding Impact on Collateral Damage to Autotransformer

BC Hydro experienced a catastrophic fire involving an autotransformer, initiated by a ground fault on its delta tertiary winding. Typically, delta tertiary ground faults are a low immediate risk and are managed through alarms, allowing for controlled transformer outages.  However, this transformer had shunt reactors connected to the delta winding. A decision was made to ground the shunt reactor neutrals to mitigate transient overvoltages on the switching breakers, which had been prone to failures. Despite the activation of high-speed ground fault protection, which responded within a quarter-cycle of the fault’s occurrence, the fault rapidly escalated, resulting in a severe transformer fire with significant collateral damage. This paper explores the sequence of events leading up to the fire, discusses the lessons learned, and underscores the necessity of comprehensive disturbance analysis to enhance the safety and reliability of electrical systems.