Overvoltages on EHV transmission lines due to faults and subsequent bypassing of series capacitors

It is shown that overvoltages exceeding 2.0 per unit (pu) can occur on unfaulted lines in series-compensated systems, following fault initiation on other lines in the system and subsequent bypassing of series capacitors. The cause of these overvoltages is an oscillatory discharge of the series capacitor banks initiated by sparkover of the protective gaps. As a consequence, a voltage standing wave will appear on the unfaulted lines, superimposed on the 60-Hz quantities, leading to high overvoltages in certain sections of the transmission lines. This phenomenon can result in cascading outage of more than one transmission line, and is particularly serious in case of long highly series compensated transmission lines. The theoretical investigation indicates that a particularly bad case will result when the length of the line is approximately equal to 3/4 wave length of the series capacitor discharge ringing frequency. Overvoltages of this type, rather than switching surges, may dictate transmission line insulation requirements in series-compensated systems. The effect of series capacitor discharge on system transients has not been reported previously in the literature and, therefore, it is the purpose of this paper to discuss key results of recent digital and Transient Network Analyzer (TNA) studies of this phenomena. Increased damping of the series capacitor discharge circuit is recommended to control this type of overvoltages on transmission lines.