With an appropriate connection of conventional wattmeter and phase-shifting autotransformer, reactive power can be accurately measured.
A transformer used for the application is termed as phaseformer or reactiformer.
Before proceeding further, it is good to go back to formulas for watts and vars.
The mathematical formulas for watts and vars is:
Watts = |V| |I| cos θ
Vars = |V| |I| sin θ
∵ sin θ = cos (θ – 90°)
The autotransformer shifts the phase angle of the potential by 90° (lagging) before supplying it to the wattmeter.
When potential phase angle is shifted back 90° the wattmeter measures VARs.
Good to know: The conventional VAR metering techniques must be used to measure reactive power if real power flows in both directions.
In case of radial power distribution systems, relay power flows only in one direction, in such case VARs are measured using Q-meter.
The Q-meter is a special meter that measures power flow in one direction.