A set of devices, control actions and placement strategies that locally supply or absorb reactive power (VARs) to regulate voltage profile, improve power factor and reduce current‑related losses on an AC power system without delivering net real energy.

Principle

Supplying or absorbing reactive power close to the load changes local voltage angles and magnitudes, thereby reducing line currents and apparent power required from upstream sources; for linear sinusoidal systems the effect follows Q (reactive) and P (real) vector relationships, so local VAR injection lowers required I and transmission losses for the same real power demand.

Demonstration

Illustrative scenario → Situation: A distribution feeder supplies a large inductive load producing lagging current and low terminal voltage. Recognition: Measurements show low power factor and elevated line current. Action: Install and switch a shunt capacitor bank near the load. Consequence: The capacitor supplies leading VARs that reduce the net reactive current drawn from the feeder, terminal voltage rises toward nominal, feeder current magnitude falls and measured power factor improves.

Misapplication

Treating reactive power compensation as a source of real energy or assuming any added capacitance always improves system performance. The semantic error is conflating energy delivery (real power) with phase‑shifted energy exchange (reactive power); overcompensation or inappropriate placement can create overvoltages or resonance with network inductances.

Consequence

When correctly applied: reduced feeder currents, lower I²R losses, increased transfer capacity and improved voltage regulation; when misapplied: possible resonance amplification of harmonics, unacceptable overvoltage at light load, increased short‑circuit currents and coordination complications with control‑based devices.

Reversal

The expected benefit can fail or reverse when network impedance and harmonic content create conditions for parallel resonance, when loads vary rapidly such that switched compensation causes oscillatory switching losses, or when power‑electronic inverters with fast voltage control render static shunt devices ineffective or destabilizing.

Boundary

Clearly within: switched or fixed shunt capacitors, series reactors for compensation, synchronous condensers, STATCOMs and other power‑electronic VAR devices placed to alter local Q. Boundary case: inverter‑based generation providing dynamic VARs — functionally compensation but also supply real power. Clearly outside: generators and storage systems whose primary purpose is to supply net real energy (though they may also provide VARs as ancillary services).

Semantic Tension

Tension between local VAR provision (which optimizes voltage and losses locally) and centralized grid reactive control (which optimizes system stability and dispatch); tension between reducing apparent power flow and avoiding harmonic amplification.

Synthesis

Reactive power compensation is an exchange of stored field energy used to shape voltage and current phasors rather than to supply energy; its benefits (reduced current, improved voltage) depend on correct device type, placement and consideration of network impedance and harmonics.