IN Electrical engineering Reactive power is a quantity describe that component of total power flow which is due to the circulating flow of energy between the source and the inductive and capacitive energy storage components of a load. Since this energy flows back and forth between the load and source on each cycle, no average transfer of energy occurs to the load. It is measured in reactive volt-amperes (VAr) not watts, and is sometimes called "wattless" power to distinguish it from the transfer of energy that can do work at the load.
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 when the voltage and current are applied to inductive or capacitive loads, the load absorbs and then returns energy to the network, and voltage and current become out of phase. The current flow and voltage no longer reverse in step. This leads to additional currents in the transmission lines and unwanted energy losses because of conductor heating.
The ratio between real power and apparent power in a circuit is called the Power factor, and is equal to the cosine of the phase angle between the current and voltage sine waveforms. 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 flow. This is typically done automatically by switching in/out inductors or capacitor banks, by adjusting generator excitation, and by other means. 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
which defines apparent power S as a complex number with real component P (real power) and imaginary component Q (reactive power).
As the quantities are vectors
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