The intelligence required to switch banks of shunt capacitors automatically depends upon the reason for
their use. If they are used primarily to control voltage, then the capacitors can be switched on when the voltage is low or off when the voltage is high, and a voltage relay supplies the control.
If the system voltage is regulated by other means and the capacitors are used for power factor correction, then the load kvar or total current must be used as the means for control. It is always desirable to use the simplest type of control that will accomplish the desired result.
Current control is commonly used where the voltage is regulated by other means and the power factor is practically constant through wide variations in load. Kvar control is used where t,he load power factor varies over a wide range as the load changes.
Whether the control is accomplished by voltage, current, or kvar, the control systems are similar. In addition to the master control relay, other devices are required in the control scheme such as time-delay relays, control switches, etc.
For one-step automatic control the master relay energizes the “closing” element of a time-delay relay, and if the master-relay contacts stay closed for the time required for the time-delay relay contacts to make, then the operating circuit is energized and the capacitor breaker closes.
A similar process in reverse trips the capacitor breaker. For a two-step control the sequence is the same as for one-step control except that auxiliary contacts on the No. 1 breaker set up the circuits for the control of the second step.
If the No. 1 breaker is closed, the circuit is set up to either trip No. 1 or to close No. 2. The sequence of
operation is the same in all cases, that is, No. 1 breaker always closes first and trips last.
For more than two-step control, each additional breaker, by means of auxiliary contacts, sets up the control circuits for the next operation whether it be to add or remove capacitor kvar.
The control circuits become numerous and involved, but their operation is accurate, reliable, and thoroughly proved by many applications. Where the need for capacitor kvar follows a fixed schedule, the capacitors cab be switched by a time relay that initiates on or off at predetermined times.