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On March 31, 2019, a 500 kV air-core, shunt reactor at a large generating station in BC Hydro was damaged shortly after four unit transformers were simultaneously energized from a 500 kV series and shunt compensated line source. Forensic investigations revealed that the controlled switching system, controlling the instant at which each breaker phase closes during energization to minimize inrush current, had failed. Uncontrolled closing led to under-damped, high-magnitude and sub-synchronous currents. Protection tripped and isolated the four transformers, followed shortly by shunt reactor protection tripping. Before the reactor could be isolated by circuit breakers, a damaging internal fault developed. This paper presents forensic investigation describing sequence of events leading up to reactor damage and shares the lessons learned with the peer utilities having similar installations.

Blackouts usually occur when a heavilyloaded transmission interconnection is subjected to a disturbance that pushes it over the stability limit. In this case, a blackout was caused by multiple lightning strikes to a transmission link consisting of three parallel 500 kV power lines carrying a small fraction of their rated load. The incident caused the islanding of seven power stations, the shedding of 663 MW of generation, a regional blackout in British Columbia, Canada that affected 34,500 customers and damage to the equipment of a major customer. One of the subject lines experienced a double line-to-ground fault, and the other two lines experienced single line-to-ground faults. It took about 3.25 hours to end the blackout and totally restore the system to its pre-outage condition. The following unusual phenomena were encountered during this incident: a) 9 Hz ferroresonance currents that triggered the relay of the shunt reactor bank of the 500 kV line which experienced the double line-to-ground fault, and; b) Ferranti Effect overvoltages that reached 148% at the remote end of a chain of 138 kV & 230 kV lightly-loaded lines that became electrically connected in series, with no voltage control at the intermediate points. This paper gives a review of both this incident and a prior unreported one which also caused a regional blackout, and presents the lessons learned from them.