Define Shunt Capacitors and Reactors
Shunt capacitors absorb leading VArs (i.e. they are used to supply lagging VArs) whereas reactors are used to absorbs lagging VArs. Capacitors and reactors can be either directly connected to the busbar or connected through switches (mechanical or thyristors). In the latter, the number of elements connected to the system may be controlled in order to supply the required VAr demand to maintain the specified value of the voltage.
It has been shown that by using shunt VAr compensators to control the voltage at key points along a transmission line, it is possible to increase the power transmission capability of the line. When using shunt capacitors, it is important to note that at high loads, the voltage falls and consequently the VArs produced by the capacitors falls as the square of the voltage (Q=V2/Xc). That is, when they are most needed, their effectiveness falls. Also, on light loads when the voltage is high the capacitor output is large and can produce voltage rises.