Shunt Capacitors and Reactors
Shunt capacitors are used to compensate lagging power factor loads, whereas reactors are used on circuits that generate VArs such as lightly loaded cables. The effect of these shunt devices is to supply or absorb the requisite reactive power to maintain the magnitude of the voltage. Capacitors are connected either directly to a busbar or to the tertiary winding of a main transformer. In the USA they are often installed along the routes of distribution circuits to minimize the losses and voltage drops. Unfortunately, as the voltage reduces, the VArs produced by a shunt capacitor or absorbed by a reactor fall as the square of the voltage; thus, when needed most, their effectiveness drops. Also, with light network load when the voltage is high, the capacitor output is large and the voltage tends to rise to excessive levels, requiring some capacitors or cable circuits to be switched out by local overvoltage relays.
Capacitors can be connected in series with overhead lines and are then used to reduce the inductive reactance between the supply point and the load. One major drawback is the high overvoltage produced across the capacitor when a short-circuit current flows through the circuit, and special protective devices need to be incorporated (e.g. spark gaps) and non-linear resistors. The phasor diagram for a line with a series capacitor is shown in Figure 5.6(b).
The relative merits between shunt and series capacitors may be summarized as follows:
- 1. If the load VAr requirement is small, series capacitors are of little use.
- 2. With series capacitors the reduction in line current is small; hence if thermal considerations limit the current, little advantage is obtained and shunt compensation should be used.
- 3. If voltage drop is the limiting factor, series capacitors are effective; also, voltage fluctuations due to arc furnaces, and so on, are evened out.
Figure 5.6 (a) Line with series capacitor, (b) Phasor diagram for fixed VR
4. If the total line reactance is high, series capacitors are very effective in reducing
voltage drops and stability is improved.
Both shunt and series capacitors need to be applied with care as they can both lead to resonance with the inductive reactance of the power system. Shunt capacitors are benign as long as their network is connected to the main power system and the voltage is controlled. However, if a section of network containing both shunt capacitors and induction generators is isolated then self-excitation of the induction generators can lead to very high resonant voltages. The use of series capacitors, although very effective in reducing voltage drop on heavily loaded circuits, can lead to sub-synchronous resonance with rotating machines. Capacitors are not commonly used in distribution systems in the UK, partly because of concerns over resonance.