Measuring MVARs in the Field: EMS Systems Explained

[traijan]

How are MVARs actually measured in the field? How do EMS systems know the difference between MW and MVARS as opposed to an absolute value of MVA? Thanks!

 

[psparky]

How are MVARs actually measured in the field? How do EMS systems know the difference between MW and MVARS as opposed to an absolute value of MVA? Thanks!

Since watts, Vars and VA are all related, if you measure your actual line current, you can then get your VA.

If you then have your power factor, you can get easily find your watts or Kvars from there.

Or if you have two pieces from your triangle, like watts and Kvars, you can easily calculate VA and power factor from there.

Today's meter's calculate this stuff easily with their "loop" sensors. I believe the loop sensors (magnetic field sensors) can see the lag between voltage and current kinda like a oscilliscope, once you have that, you pretty much have it all.

 

[jim hardy]

Watts = VICos(Θ)
Vars = VISin(Θ)
and sin is just cos shifted by 90 degrees...

The analog ones I'm accustomed to pick from among the three-phase quantities available convenient ones to measure.
For example - draw your individual phasors
notice that with everything balanced and unity power factor, Voltage between phases A and C is 90 degrees out of phase with phase B current.

Aha ! there's the 90 degrees phase shift we need to calculate Vars.
An analog multiplier IC with inputs of Va-c and Ib will produce a signal that can be wired straight to a DC voltmeter having scale calibrated in Vars. It should be a zero-in-the-center type meter.