Capacitors can be applied anywhere on the electrical system downstream of the metering point and have the same effect on the plant's power factor. However, it is generally advisable to place them as close to the inductive loads as feasible to minimize power losses and voltage drops on the feeder and branch circuits. For motor applications, the most effective location is at the motor terminals where the capacitors will be switched on and off with the motor. The system would not be left with excessive kVAR correction when the motors are not running. However, the amount of capacitance switched in this way is limited by the size and speed of the motor. The motor manufacturer should be consulted when this is done. Too much capacitance across the motor terminals can result in transient overvoltages when the motor is disconnected and is coasting to a stop, and excessive shaft torque can result during reclosure operations of utility line circuit breakers while the motor shaft is still turning.
The cost of these capacitors is higher when many small units are used instead of larger units at a central location with the capacity to serve the kilovar needs of several motors. When these centralized capacitors are not switched with the load, the voltage on the system can rise during periods of light loading because of the partial resonance with the inductance of the lines and transformers. If this becomes troublesome, it can be corrected by lowering the voltage tap setting on the supply transformer or by switching some of the capacitors off the line. This can be accomplished using automatic controls if the situation justifies that complexity. Such a rise in voltage rarely exceeds 5% in modern systems.
A convenient formula for sizing capacitor banks is given below. The required reactance is the total reactance per phase as seen from the place the capacitor bank is installed, looking back toward the source of power. Use the phase voltage in this formula.
