Reactive Power Explained

Reactive power is the energy that flows back and forth between the source and the load in AC systems. It doesn’t perform useful work but is essential for maintaining voltage and power flow. It supports inductive loads like motors and transformers in electrical systems.

What is Reactive Power?

Reactive power is a type of power that does no real work and is generally associated with reactive elements (inductors and capacitors).

✅ Maintains voltage levels in AC power systems

✅ Powers magnetic fields in motors and transformers

✅ Measured in volt-amperes reactive (VAR), not watts

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For example, the inductance of a load such as a motor causes the load current to lag behind the voltage. Power appearing across the inductance sloshes back and forth between the inductance itself and the power sources, producing no net work. For this reason, it is referred to as imaginary or reactive power, as no power is dissipated or expended. It is expressed in units of volt-ampere-reactive or var. In the sinusoidal case, the reactive power is defined as:

 which is the portion of power in quadrature with the active power and demonstrates the relationship between P, Q, and S in sinusoidal conditions. Reactive power is essential in maintaining system voltage, which directly relates to what is voltage and the overall efficiency of AC power distribution.

There is some disagreement among harmonics analysts on how to define Q in the presence of harmonic distortion. If it were not for the fact that many utilities, which produce magnetic fields in transmission lines, measure Q and compute demand billing from the power factor computed by Q, it might be a moot point. It is more important to determine P and S; P defines the amount of active power being consumed, while S defines the capacity of the power system required to deliver P. Q is not particularly useful by itself. However, Q1, the traditional reactive power component at fundamental frequency, may be used to size shunt capacitors. Unlike reactive power, which does not perform real work, active power represents the actual energy consumed by resistive components to perform useful tasks in electrical systems. 

Relationship between P, Q, and S in a sinusoidal condition

The reactive power, when distortion is present, has another interesting peculiarity. In fact, it may not be entirely accurate to refer to it as reactive power. The concept of VAR flow in the power system is deeply ingrained in the minds of most power engineers. What many do not realize is that this concept is valid only in the sinusoidal steady state. When distortion is present (as in the case of induction motors), the component of S that remains after P is taken out is not conserved—that is, it does not sum to zero at a node. Power quantities are presumed to flow around the system in a conservative manner. It supports inductive loads by powering magnetic fields, a concept closely tied to what is inductance and inductive load behavior in electrical systems.

This does not imply that P is not conserved or that current is not conserved because the conservation of energy and Kirchhoff's current laws are still applicable for any waveform. The reactive components actually sum in quadrature (i.e., the square root of the sum of the squares). This has prompted some analysts to propose that Q be used to denote the reactive components that are conserved and introduce a new quantity for the components that are not. Many refer to this quantity as D, for distortion power or, simply, distortion voltamperes. The interaction between real and reactive power is best understood through the lens of power factor, which measures how effectively electrical power is converted into useful work.

It has units of volt-amperes, but it may not be strictly appropriate to refer to this quantity as power, because it does not flow through the system as power is assumed to do. In this concept, Q consists of the sum of the traditional reactive power values at each frequency. D represents all cross products of voltage and current at different frequencies, which yield no average power. P, Q, D, and S are related as follows, using the definitions for S and P above as a starting point:

Some prefer to use a three-dimensional vector chart to illustrate the relationships between reactive power components. P and Q contribute the traditional sinusoidal components to S, while R represents the additional contribution to the apparent power by the harmonics.

There are many factors to consider when determining and measuring reactive power in an AC circuit in any power plant:

• Real power

• Voltage level

• Purely resistive true power

• Active, reactive, and apparent power

• Active and reactive powers

• How to absorb reactive power

To better understand current flow disruptions in AC systems, compare real vs reactive power and how each influences overall electrical safety.

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