Capacitor Switching

Capacitors represent an effective and low cost alternative for improving the power flow of transmission and distribution systems. As one might expect, it is not unusual for shunt capacitors to be switched daily as the system loads vary during the day. Connecting and disconnecting shunt capacitor banks from the system presents some unique challenges since the voltage across the capacitor cannot change instantaneously. This switching causes unwanted, high frequency, voltage and current transients that may cause nuisance tripping of adjustable-speed drives, computer network problems, as well as customer equipment damage or failure. In addition, restrikes in the switching device (especially vacuum devices) due to fast voltage recovery may cause failure of the switching device and/or the capacitor bank.

Fig. 1 Switching of a 72.5 kV Capacitor Bank with no Transient Suppression

Several techniques have been developed to mitigate these transients including application of arrestors, inrush reactors, controlled voltage switching devices, pre-insertion resistors, and pre-insertion inductors.

An ideal capacitive switching device would be designed with a higher transient recovery voltage capability than needed to eliminate or minimize the likelihood of a restrike or reignition occurring. In addition, the switching device would incorporate a feature that would reduce the high frequency voltage and current transients when the capacitor bank is connected to the system.

In 2003, Southern States introduced the only switching device designed specifically for capacitor switching, the CapSwitcher®. The CapSwitcher® is capable of a high number of operations (10,000), utilizes closing resistors to mitigate voltage transients and inrush current. By designing specifically for this application, it was possible to improve the performance and reliability over general purpose devices (such as vacuum switches, circuit switchers, and circuit breakers) while maintaining a competitive cost.

Fig. 2 Actual recording of a 44 kV capacitor bank being switched in with a CapSwitcher with closing resistors.

In the CapSwitcher® design, the resistor, resistor contact design, nozzle design, and resistor insertion method were the keys to achieving a reliable long term solution for mitigating the transients that occur when switching in a capacitor bank. Additionally, the contacts and nozzle were designed to virtually eliminate the possibility of restrikes during the disconnection of the capacitor bank from the system.