Voltage regulation can also be improved by the application of
shunt capacitors at the substation, out on the primary feeder, or both.
The current drawn by a capacitor has a leading power factor characteristic
and will cause a voltage rise from the location of the capacitor back to
the current source.
The voltage rise will be equal to the reactance of the
circuit (back to the source) multiplied by the capacitor current (taking
into account their vector relationship). The rise in voltage is independent
of the load on the circuit and is greatest at the location of the capacitors
and decreases to the source.
Capacitors provide a constant increase in the level of voltage at the
location of the capacitor that is the same under any load condition of
the feeder, from light to heavy loading. If capacitors are installed so that
they may be switched on during heavy load periods and off at light load
periods, voltage regulation can be improved. If a bank of capacitors is
so arranged that some of its units can be switched on and off separately,
voltage regulation can be improved even further.
Primary Feeder
When they are installed out on a primary feeder, the capacity of
the capacitors (in kVA) and the location on the feeder where they are
to be installed depends on the manner in which the loads are distributed
on the feeder, the power factor of the loads, the feeder conductor
size and spacing between conductors, and the voltage conditions
along the feeder.
Like the line voltage regulator, capacitors should be
installed approximately at the point where the voltage at heavy load
is at the minimum permissible level (with some consideration given
to load growth). The conditions under light load will determine what
portion of the capacitance installed may be fixed and what may be
switched.
Substations
Capacitors may also be installed at substations on the bus supplying
the outgoing distribution feeders. They are usually installed in
relatively large-capacity banks, and it is usually necessary to switch off
portions of them at periods of light load to prevent excessively high
outgoing voltage
The voltage drops along the feeders supplied from
this substation bus remain the same as do their power factors, since the
relationship between the voltage and current flowing through each of
the feeders supplying their loads is unaffected by the capacitors added
to the substation bus. The voltage level of each of the entire feeders is
raised depending on the capacitance added at the substation, but the
voltage spread on each feeder remains the same.
In many instances, the principal reason for the capacitors at the substation bus is not necessarily
to control the bus voltage, but, by counteracting the effect of induction
(or reactance), to reduce the current to that necessary to supply the load
at approximately unity power factor, thereby permitting larger loads to
be supplied by the same transmission and substation facilities.
Series Capacitors
Capacitors can also be installed in series with primary feeders to
reduce voltage drop, but they are rarely employed in this fashion. Where
shunt capacitors, connected in parallel with the load, correct the component
of the current due to the inductive reactance of the circuit, series
capacitors compensate for the reactive voltage drop in the feeder.
A capacitor in series in a primary feeder serving a lagging-power
factor load will cause a rise in voltage as the load increases. The power
factor of the load through the series capacitor and feeder must be lagging
if the voltage drop is to decrease appreciably. The voltage on the
load side of the series capacitor is raised above the source side, acting
to improve the voltage regulation of the feeder. Since the voltage rise
or drop is produced instantaneously with the variations in the load, the
series capacitor response as a voltage regulator is faster and smoother
than the induction or TCUL-type regulator; moreover, no contact-making
voltmeter and load compensator are required for its operation.
During fault conditions, however, the large fault current passing
through the series capacitor can develop excessive voltage across the
capacitor, sufficient to cause its destruction. It is essential, therefore,
that it be taken out of service as quickly as possible. A resistor and
air gap are connected between the terminals of the series capacitor.
When the voltage becomes sufficiently high, the gap breaks down and
permits the capacitor to be short-circuited through the resistor; the
resistor dampens out any oscillatory discharge current so the gap can
break down and restrike repetitively without damaging the capacitor.
Auxiliary relays operate to short-circuit and bypass the capacitor if the
fault persists.
Because of the potential hazard, series capacitors as voltage regulators
are usually restricted to supplying single large consumers where
flicker may result from frequent motor starts or from electric welders,
furnaces, and similar devices that may cause rapid and repetitive load
fluctuations.
