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 3.7.7.4 Load Interrupter Switches. A load interrupter switch is employed for capacitor
switching. The most common types are air, gas-filled, and vacuum switches. These devices do
not provide fault protection and are backed up by fuses or circuit breakers.
3.7.7.5 Fused Cutouts. On small fixed installations, fused cutouts are sometimes used to
perform the switching operation.
3.7.8 Control Devices. Switched capacitor bank control systems initiate the switching of the
capacitors into, or out of, the circuit at predetermined conditions. The auxiliary components of
the controls include sensors, circuit breakers or switches, and the control power source. The
controls normally include an automatic control lever or control switch that permits the operation
of each capacitor switch either by manual operation of its control switch or in response to a
signal initiated by a sensor. The most commonly used inputs for capacitor bank control are time,
voltage, current, and load kVar.
3.7.8.1 Time Switch Control. Time switch or time clock control is often used with small
switched capacitor banks. Time control switches the capacitor bank on at a certain time of the
day and off at a later time. A carry-over device is normally used for each time clock to keep the
clock running during temporary power outages. If a carry-over device is not used, it will be
necessary for the operator to go to each capacitor location that is affected and reset the clock after
a power outage.
3.7.8.2 Voltage Control. Voltage control is widely used for capacitor switching in
substation applications. It is generally used to correct steady-state undervoltage conditions
caused by heavy circuit loading, or inadequate voltage regulation from upstream transformers
and line regulators. Voltage control includes a voltage-sensitive relay, and time-delay and
auxiliary relays to open and close the capacitor switching device in response to predetermined
values of steady-state circuit voltage. The time-delay relay is incorporated so that the switching
operation is not initiated for momentary line voltage dips, that may be caused by a lightning
strike, a momentary dip caused by a line fault, or by a large motor load starting on a downstream
bus.
3.7.8.3 Current Control. Current control is often used on regulated circuits where voltage
cannot be used for capacitor switching. Voltage control includes a current-sensitive relay plus
time-delay and auxiliary relays to open and close the capacitor switching device in response to
predetermined values of load current.
3.7.8.4 Kilovar Control. The capacitor bank will be energized at a certain value of lagging
power factor. It will be switched off at a lower value or when there is leading power factor.
Kilovar control is used to reduce peak kVar loads.
3.7.8.5 Combined Controls. Combined controls can be used for more complex situations.
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