SHUNT CAPACITORS FUNDAMENTALS AND TUTORIALS

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.

It supplies the kind of kilovars or current to counteract the out-of phase component of current required by an induction motor as illustrated in Fig. 1.


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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