Reactive Power – Definition, Units, Importance, and Relationship Between P, Q, and S

What is Reactive Power?

Reactive power is the type of electrical power that flows back and forth between the source and the load. Unlike active power, it does not perform any useful work but is essential for maintaining the voltage levels required for power system operation. Reactive power continuously alternates direction in the circuit and is mainly associated with reactive components such as inductors and capacitors.

Unit of Reactive Power

Reactive power is measured in volt-ampere reactive (VAR). For large power systems, it is commonly expressed in:

• kilovolt-ampere reactive (kVAR)
• megavolt-ampere reactive (MVAR)

Nature of Reactive Power

Reactive power does not perform real or mechanical work. Instead, it supports the creation of electric and magnetic fields required for the proper operation of AC machines, transformers, and transmission lines. In sinusoidal AC systems, reactive power represents the component of power that is 90° out of phase (in quadrature) with the active power.

Mathematical Representation

In a sinusoidal AC system, reactive power is defined as: Q = V × I × sinφ

where φ is the phase angle between voltage and current. Reactive power demonstrates the phase relationship between voltage and current and forms an essential part of the power triangle along with active power and apparent power.

Relationship Between Active Power (P), Reactive Power (Q), and Apparent Power (S)

In sinusoidal conditions, the relationship between active power (P), reactive power (Q), and apparent power (S) is represented by the power triangle:

S² = P² + Q²

Here:

• P represents the active power that performs useful work
• Q represents the reactive power that sustains magnetic and electric fields
• S represents the apparent power that defines the total power capacity required

Importance of Reactive Power

Although reactive power does not perform useful work, it plays a vital role in power system operation. Power utilities often measure reactive power because it affects the system power factor and overall efficiency.

Many utility companies calculate demand charges based on the power factor, which depends on reactive power. Therefore, managing reactive power is essential for reducing losses and improving system performance. While active power (P) determines actual energy consumption and apparent power (S) defines system capacity, reactive power (Q) is mainly useful for:

• Power factor correction
• Voltage regulation
• Sizing shunt capacitors and reactors

Reactive Power in Distorted Conditions

When waveform distortion is present, the concept of reactive power becomes more complex. In such cases, it may not be entirely accurate to describe it as traditional reactive power. However, the fundamental frequency component of reactive power, often denoted as Q₁, is still useful for practical applications such as sizing shunt capacitors.