Electrical – What would cause a power meter to report negative vars


I have some digital power meters at work that we installed due to previous concerns of voltage instability. The voltage instability has been corrected and these are now primarily used for monitoring of energy usage. I have noticed some strange reactive power measurements. These meters report a lagging power factor as positive vars (inductive) and a leading power factor as negative vars (capacitive). At no load, or very little load, when we have many idle VFDs, large transformers and voltage regulators online but without any load, the meters report a lagging power factor or positive vars for the most part, which I believe. When we start bringing loads online, motor loads to be exact, the vars start to go negative and the meters report a capacitive load. This seems to be the opposite of what would be expected. Wouldn’t you expect the power factor to be lagging even more as motor loads are brought online? What would cause these meters to report a capacitive load as inductive loads are brought online? Thanks!

Best Answer

The peculiar readings may be the result of harmonic currents drawn by the VFDs. You need to investigate how the meters show that. The reactive component of the motors that are powered through VFDs is supplied entirely by the DC bus capacitors in the VFDs. The current supplied to the VFDs has a substantial harmonic component unless they contain harmonic mitigation provisions. The current for a system with harmonic current content has three orthogonal components, X representing the real component, Y representing the reactive component and Z representing the harmonic component.

You should determine how much of the motor load in the facility is due to motors powered directly by the three-phase power source and how much is powered through VFDs. Also determine the harmonic content.