Capacitance Switching of Equipment

Capacitance is the phenomenon of holding electrostatic charge. In electrical systems, long transmission lines, power cables and capacitor banks can have large amounts of capacitance. In a circuit containing capacitance, the current will lead the voltage by 90 degrees. This means that at the instant of the current zero crossing, the voltage across the breaker contacts will be the maximum. If a circuit is isolated at this instant, the high system voltage will be retained by the line capacitances. If the breaker is opened when the current is zero and the voltage is maximum, half a cycle later when the supply voltage reaches maximum in the opposite direction, the voltage across the breaker contacts will be 2V. This can result in a restriking voltage being developed and a flashover occurring across the circuit breaker. Once the flashover due to the restrike occurs, oscillations are set up in the line between the system inductance and the capacitance. These oscillations and the restrikes they cause can result in the line voltage reaching up to 4 times the voltage (4V). Hence, in lines with high capacitances, air blast circuit breakers or multi break circuit breakers are used for isolation.