1. Damaging Open-Phase Overvoltage Disturbance on a Shunt-
Compensated 500-kV Line Initiated by Unintended Trip
Abstract:
Shunt reactors are applied to long extra high-voltage (HV) transmission
lines to compensate for their natural capacitance which otherwise could
cause overvoltages 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 overvoltages
during unbalanced open-phase conditions on the healthy line. This paper
reports on an overvoltage 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 voltage and current
recordings provided useful insight into the mechanism responsible for the
damaging overvoltages 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.