Need and methods of reactive power compensation

Reactive power compensation means supplying or absorbing reactive power at suitable points of the system, so that voltage stays within limits and transmission capacity is used well.

Why compensation is required

1. Voltage control: the voltage drop in a line is mainly $\Delta V \approx \frac{QX}{V}$. Heavy lagging loads lower the receiving voltage; light loads raise it (Ferranti effect). Compensation keeps voltage within ±5–10 %.
2. Line behaviour changes with load: below SIL a line generates Mvar (which must be absorbed by reactors); above SIL it absorbs Mvar (which needs capacitive support).
3. Reduce losses: reactive current increases line current and $I^2R$ loss. Supplying Q locally reduces it.
4. Increase power transfer: in $P = \frac{V_SV_R}{X}\sin\delta$, series capacitors reduce $X$ and shunt support holds up $V$, raising the stability limit.
5. Improve power factor and release capacity of generators, transformers and lines.
6. Voltage stability: prevents voltage collapse in heavily loaded systems.

Methods

1. Shunt compensation

• Shunt capacitors: connected at load buses or substations to supply lagging Mvar during heavy load; they raise voltage and p.f. They are cheap, but their output falls with $V^2$.
• Shunt reactors: connected at the ends of long EHV lines to absorb the line's charging Mvar at light load; they control the Ferranti rise and switching overvoltages.

2. Series compensation

• Series capacitors in the line cancel part of the line inductance ($X_{eff} = X_L - X_C$, typically 25–70 % compensation). They increase power transfer and stability and reduce voltage drop. Risk: subsynchronous resonance.

3. Synchronous condensers

• Synchronous motors running without mechanical load: when over-excited they supply Mvar, and when under-excited they absorb Mvar. They give smooth, continuous control but are costly and need high maintenance.

4. Static VAR compensators (SVC) and FACTS

• SVC: a thyristor-controlled reactor (TCR) plus thyristor-switched or fixed capacitors give fast, variable Q.
• STATCOM (voltage-source converter), TCSC (thyristor-controlled series capacitor) and UPFC give faster and wider control.

5. Other means: tap-changing transformers (redistribute Q and adjust voltage) and generator excitation control.