In Low Voltage installations, when a Capacitor is switched
‘ON’, it results in resonant circuit damped to a greater
degree. In addition to the rated current, over current of high
amplitude (>180 In) and high frequencies (3~15 KHz) occur
during transit period (1 to 2 ms). The resultant high
in-rush current peaks, caused due to capacitor switching,
depends upon following factors:
Network inductances
Transformer power and short circuit voltage
Type of power factor correction: fixed or automatic
Harmonics presence in the system
The in-rush current of such high magnitudes is undesirable
and it is likely to weld main poles of any standard contactor.
Therefore, contactor for capacitor bank switching must be
designed to withstand :
Permanent current that can reach 1.5 time the nominal
current of capacitor bank. Short but high peak current on pole closing Hence, selection of capacitor duty switching device requires careful product selection. It is always recommended to use dedicated Capacitor Duty Switching Contactor for switching Capacitor bank, which optimizes the switchgear cost &
enhances the equipment life.
Capacitor contactor are specially designed to meet stringent requirements of capacitor switching as deliberated above. These contactors are fitted with front- mounted block of 3 early make auxiliary contact in series with quick discharge damping six-resistors-2 per phase to limit peak current to value within contactor making capacity such that normal rated capacitor current is carried by main contacts which, after closing, effectively short out the resistors.
