Strictly speaking, you can't. Watt meters measure real power (strictly speaking, power is the real component of the V-I product).
If you know what the magnitude of the voltage and current, and you know the power, you can calculate the reactive component by subtraction.
Knowing the power, and the VA calculation, you can also determine the power factor, which will give you the relative phase angle between the voltage and current.
You use a highly reactive coil in series with the voltage coils of the Watthour meter to shift the voltage almost 90 degrees. In the days before digital, electronic meters, standard multi-phase Watthour meters were used along with the commercially available phase shifting modules to measure KVARHrs commercially. In such an installation, the meters used to record kWHrs and KVARHrs were interchangeable.
The kH factor may be used to read the average Watts or Vars over a short period of time on revenue meters. (5, 10, or more seconds.)
To measure single phase VARs, connect "A" phase current to the meter and connect "B" to "C" phase voltage to the meter.
You have a 90 degree voltage shift so the meter will respond to VARs. Divide the reading by 1.73 to adjust for the ratio between line to neutral and line to line.
I will leave it to you to expand this to three phase. I don't have the time just now.
Bill
--------------------
"Why not the best?"
Jimmy Carter
