Power capacitors are those capacitors which are used in power transmission system in order to deal with lagging power factor load. Capacitors are generally rated at a frequency of 60 Hertz (Hz), however, they are also suitable for operation at frequencies below 60 Hz. There is no physical limit to the under-frequency operation of the capacitors. The limit is economic, in that the capacitor kVar output is directly proportional to frequency and applied voltage. If a capacitor is operated at a frequency lower than rated, consequently, its kVar rating is reduced. Since capacitors are installed in theory to utilize their rated capacity, utilization at reduced frequencies is not economical, as the unit's design rating can never be achieved.
They can be categorised in following types :-
· Fixed Capacitors
Fixed capacitor installations are those that are continuously on-line. Fixed capacitor banks are connected to the system through a disconnecting device that is capable of interrupting the capacitor current, allowing removal of the capacitors for desired time. Fixed capacitor banks should be applied to give a voltage boost to the system during heavy load periods. To isolate or deenergize a fixed capacitor installation, the disconnecting switches should be opened. It is more difficult to deenergize a capacitor bank than it is to energize it, because deenergizing the capacitor bank depends on the point on the voltage wave when the switch contacts separate i.e. wheather it is at zero point or not After the disconnecting device has been opened, the capacitor installation is isolated but still charged. The capacitors should be left open from the line for at least five minutes before they are returned to service. This precaution will prevent a build up of the line voltage above normal, which may occur if a fully charged capacitor bank was closed on a line.
· Switched Capacitors
Switched capacitor installations are those where the capacitor bank is switched in and out of service depending upon system operating conditions. They are usually switched on when the load requirements are the greatest and switched off during light-load conditions. Sometimes the capacitor banks are installed to enable incremental switching, depending on the system reactive requirements and the amount of system voltage required. To remove a switched capacitor bank from service, the control box should be opened and the automatic control switch should be placed in the by-pass position. The switching device should then be tripped. To ensure the switching device remains open, the fuses should be removed from the control circuit.