The function of a shunt capacitor applied as a single unit
or in groups of units is to supply lagging kilovars to the system at the point
where they are connected. A shunt capacitor has the same effect as an
overexcited synchronous condenser, generator or motor.
Shunt capacitors applied on the load end of a circuit
supplying a load of lagging power factor have several effects, one or more of
which may be the reason for the application:
1. Reduces lagging component of circuit current.
2. Increases voltage level at the load.
3. Improves voltage regulation if the capacitor units are
properly switched.
4. Reduces Z^2R power loss in the system because of
reduction in current.
5. Reduces Z2X kilovar loss in the system because of
reductionin current.
6. Increases power factor of the source generators.
7. Decreases kva loading on the source generators and
circuits to relieve an overloaded condition or release capacity for additional
load growth.
8. By reducing kva load on the source generators additional
kilowatt loading may be placed on the generators if turbine capacity is
available.
9. To reduce demand kva where power is purchased. Correction
to 100 percent power factor may be economical in some cases.
10. Reduces investment in system facilities per kilowatt of
load supplied.
The shunt capacitor affects all electrical equipment and
circuits on the source side of where they are installed. If the capacitor kvar
is small, say ten percent of the circuit rating, it is usually sufficient to
make an analysis on the circuit involved for the application.
However, where the capacitor kvar is large, its effect on
each part of the system back to and including the source should be considered.
In determining the amount of shunt capacitor kvar required, it must be
recognized that a voltage rise increases the lagging kvar in the exciting
currents of transformer and motors.
Thus, to get the desired correction some additional
capacitor kvar may be required above that based on
initial conditions without capacitors. If the load includes
synchronous motors, it may be desirable, if possible, to increase the field
currents to these motors.
Shunt capacitors are applied in groups ranging from one
capacitor unit of 15 kvar to large banks of these standard units totaling as
much as 20000 kvar. Many small banks of 45 kvar to 360 kva are installed on
distribution circuits.
Banks of 520 kvar to about 3000 kvar are common on distribution
substations of moderate size. Larger banks of 5000, 10 000 and 15 000 kvar are
in service in a number of larger substations.
Usual voltage ratings of capacitor banks start at 2400 volts
and range upward for groups of capacitors connected in series for 46 kv.
Consideration is being given to voltages up to and including 138 kv. This is
feasible provided the bank is sufficiently large in kvar.
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