Reactive power is the oscillating exchange of energy between the supply and the magnetic fields of motors and transformers or the electric fields of capacitors and cables, measured in var or Mvar. It averages to zero over a cycle but still loads conductors with current and causes voltage drop. Grid operators control voltage largely by moving reactive power with generator excitation, capacitor banks, reactors and STATCOMs, because voltage and reactive flow are tightly coupled on inductive networks.
Voltage at the east sub is sagging again, switch in the second cap bank and ask Unit 3 for another 40 Mvar of reactive power.
Inductive loads absorb vars and capacitors supply them. A generator running overexcited is said to be pushing or exporting vars.
Also heard as
- vars
- Q
- wattless power
Related terms
Active power
The part of electrical power that does real work, averaged over each cycle and measured in watts.
var
Unit of reactive power, the volt-ampere reactive, written lower case and usually scaled to kvar or Mvar.
Power factor
Ratio of active power to apparent power, showing how much of the current drawn does useful work.
Apparent power
Product of RMS voltage and RMS current, the figure transformers, generators and UPS units are rated on.