Trapped Charge Effect
Trapped charges occur on the transmission lines in three-pole out closure operations. The contact-making of three poles of a circuit breaker is non-simultaneous. Consider breakers at the sending end and receiving ends of a line and a transient ground fault, which needs to be cleared by an auto closure operation. The opening of two breakers is nonsimultaneous and the one which opens later must clear two-line phases at no load. These two phrases can, therefore remain charged at the peak of the power frequency voltage, which is still present when the closure takes place. After the dead time, one breaker has to close with two phases still charged.
If the closing instant happens to be such that the trapped charge and power frequency voltage are of opposite polarity, maximum transient overvoltage will occur. And these overvoltages will result in the over-current m transmission line.