Capacitor bank can hold dangerous voltage after disconnecting from power system unless discharging devices are connected to the capacitor terminals.
The capacitor bank may be subjected to overvoltages resulting from abnormal system operating conditions. If the system voltage exceeds the capacitor capability the bank should be removed from service. The removal of the capacitor bank lowers the voltage in the vicinity of the bank reducing the overvoltage on other system equipment.
An unbalance in the capacitor bank will cause an unbalance in the voltages at the tap point of the three phases. The protection scheme consists of voltage sensing device connected between the capacitor intermediate point and ground on each phase. A time delay voltage relay with third harmonic filter is connected to the broken delta secondaries.
To discharge the bank, each individual capacitor unit has a resistor to discharge the trapped charge within 5 minutes. Undervoltage or undercurrent protection function with a time delay is used to detect the bank going out of service and prevent closing the breaker until the set time has elapsed.
The unbalance protection should coordinate with the individual capacitor unit fuses so that the fuses operate to isolate the faulty capacitor unit before the protection trips the whole bank. The alarm level is selected according to the first blown fuse giving an early warning of a potential bank failure.
Studies show that a flat voltage profile on the system can significantly reduce line losses. Shunt capacitor banks are relatively inexpensive and can be easily installed anywhere on the network. This paper reviews principles of shunt capacitor bank design for substation installation and basic protection techniques.