Reactive power compensation and its benefits

Reactive power compensation is the control of reactive power flow in a power system by adding devices that supply reactive power (capacitors, over-excited synchronous condensers, STATCOM in capacitive mode) or absorb it (reactors, under-excited condensers, SVC in inductive mode). The aim is to keep bus voltages within limits and to reduce reactive power flow through lines.

Reactive power does no useful work but is needed for magnetic and electric fields of motors, transformers and lines. If it is carried over long distances it causes voltage drop and losses. Compensation supplies it close to where it is needed.

Main types:

• Shunt compensation: capacitors or reactors connected in parallel at buses.
• Series compensation: capacitors connected in series with the line to cancel part of its reactance.
• Dynamic compensation: synchronous condensers, SVC, STATCOM.

Benefits to the system

1. Better voltage profile: voltage drop $\Delta V \approx (PR + QX)/V$ is reduced; overvoltage at light load (Ferranti effect) is controlled.
2. Lower losses: line current falls, so $I^2R$ losses in lines and transformers fall.
3. Improved power factor, avoiding low-p.f. penalties for consumers.
4. More transmission capacity: the same line and transformers can carry more active power (MW) because less of their rating is used by Mvar.
5. Higher stability limit: series capacitors reduce $X$ in $P = \frac{V_SV_R}{X}\sin\delta$, and shunt devices hold voltage up, improving steady-state and transient stability.
6. Voltage stability: prevents voltage collapse under heavy loading or contingencies.
7. Released generation capacity: generators supply less Mvar and can deliver more MW.
8. Economic saving: lower losses, deferred investment in new lines and equipment.