Shunt reactors are applied to long extra high voltage transmission lines to compensate for their natural capacitance which otherwise could cause over-voltages under light load conditions. However, when the level of compensation approaches around 65% or higher, these reactors themselves can, due to contingency, become the cause of hazardous over-voltages during unbalanced openphase conditions on the healthy line. This paper reports on an over-voltage incident that occurred on a 500 kV 72% shunt compensated line in the BC Hydro System. The overvoltage occurred when one of the breaker poles failed to interrupt the current following an inadvertent three-phase trip of the healthy line. The uninterrupted current kept one phase energized for an extended time. Temporary overvoltages during this condition and restrike transients eventually led to surge arrester failures at both terminals.Analysis of recordings of voltage and current provided useful insight into the mechanism responsible for the damaging over-voltages as well as the protection and control responses during the disturbance. The intent of this paper is to share the knowledge gained from analyses of the recorded waveforms with other utilities either having or planning for highly compensated lines.

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