Reactive power is an abstract quantity, typically used by power engineers to describe a certain type of energy flow in an electric distribution system. Reactive power is the unwanted flow-reversing portion of the total energy flow present in a transmission line. It is measured in reactive volt-amperes (VAr) not watts.

Engineers use three types of power to describe energy flow in a system:

Real power (P)

Apparent power (S)

Reactive power (Q)

In an alternating current (AC) circuit, both the current and voltage are sinusoidal. If the two quantities reverse their polarity in phase, the direction of energy flow does not reverse. However, if there is a phase separation between the two quantities, part of the transmitted energy is sent back to the source during the next half-cycle of AC. This leads to additional currents in the transmission lines and unwanted energy losses because of conductor heating.

Reactive power is described as the amount of power required to overcome the phase shift between the current and voltage. It is generally regarded as waste energy as it moves back and forth through the system rather than being absorbed by the load and used to perform useful work.

In an alternating current circuit both the current and voltage are sinusoidal, and energy flow (power) is expressed as the product of current and voltage. In a simple resistive circuit the voltage and current are in phase; the real power is equal to the apparent power, and no reactive power appears.

However when the voltage and current are applied to certain types of loads (inductive or capacitive) the load absorbs and then returns energy to the network, and voltage and current become out of phase. This phase separation is usually called the Power factor and the current is said to lag or lead the voltage. Power factor equals unity (1) when the voltage and current is in phase, and is zero when the current leads or lags the voltage by 90 degrees.

Capacitive circuits create reactive power with the current waveform leading the voltage wave by 90 degrees, while inductive circuits create reactive power with the current waveform lagging the voltage waveform by 90 degrees.

In power transmission and distribution, significant effort is made to control the reactive power levels and keep it to a minimum. This is typically done automatically by switching in/out inductors or capacitor banks. Electricity retailers may use electricity meters which measure reactive power to financially penalise customers with low power factor loads.

Understanding the relationship between these three quantities lies at the heart of understanding power engineering. The mathematical relationship between them is a vector and is typically expressed using complex numbers

As the quantities are vectors

The reactive power is the complex value of the total apparent power in the system.

External links

• Article on reactive power (http://www.redherring.com/Article.aspx?a=10760)