Explaining reactive power

In an ac circuit, the current generally leads or lags the voltage. Thus, the current consists of an active component, in phase with the voltage, and an out-of-phase, or reactive, component.

The active component, measured in watts (voltage times in-phase current), is the part that powers equipment. The reactive power, measured in volt-amperes reactive (VAr), represents the energy stored in electric or magnetic fields. (These relations often are depicted as a right-angle triangle, where apparent power is the hypotenuse and active and reactive power are the other two sides.)

Reactive power is consumed or absorbed in the magnetic field of inductive equipment; it is produced or generated in capacitive equipment. Induction motors consume it because of their magnetizing current requirements (consumption even at no load) and the inductive reactance in their windings. Shunt capacitor banks produce it. Field winding excitation control makes synchronous generators either produce or absorb it.

Reactive power consumption tends to depress transmission voltage, while its production or injection tends to support voltage. Transmission lines both consume it (because of their series inductance) and produce it (from their shunt capacitance).

Because transmission line voltage is held relatively constant, the production of reactive power is nearly constant. Its consumption, however, is low at light load and high at heavy load.

The variable net reactive-power requirements of a transmission line give rise to a voltage control problem. Generators and reactive-power compensation equipment must absorb reactive power during light load, and produce it during heavy load.

In a general emergency, when there are outages and high loading on re-maining transmission lines, those lines consume reactive power that must be supplied by nearby generators and shunt capacitor banks. (Reactive power can be transmitted only over relatively short distances.)

If reactive power cannot be supplied promptly enough in an area of decaying voltage, voltage may in effect collapse. Insufficient voltage support may in addition contribute to synchronous instability. —C.W.T.