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!
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\$\begingroup\$ Is this a single or split-phase system? Or 3-phase? It may be helpful to say. \$\endgroup\$– jonkJan 15, 2020 at 4:31
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\$\begingroup\$ Three phase wye. 480/277. \$\endgroup\$– AllenCJan 15, 2020 at 5:23
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\$\begingroup\$ "What would cause these meters to report a capacitive load as inductive loads are brought online?" Reversal of the current transformer connections or mounted in reverse direction? There should be a directional arrow on the CTs. \$\endgroup\$– TransistorJan 15, 2020 at 7:05
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\$\begingroup\$ We checked for reversal of the CTs. We even tried reversing them and when we did it made our watts go negative. We can’t have negative watts as we are not generating power. I’m pretty sure we ruled out improper CT connections or reversal of the CTs. \$\endgroup\$– AllenCJan 15, 2020 at 7:22
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\$\begingroup\$ Are they three phase meters? \$\endgroup\$– Andy akaJan 15, 2020 at 8:13
1 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.
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\$\begingroup\$ It is pretty much a 50/50 split of VFDs and across the line motor starters. We were considering that the harmonic content from the VFDs may have had something to do with these readings. What gets me is that when we start the motors with contactors, the vars still go negative. As we load up these motors, the vars go even more negative. I could understand this if these motors were powered by VFDs but they're powered directly from the three phase source. \$\endgroup\$– AllenCJan 15, 2020 at 18:29
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\$\begingroup\$ If the unloaded system has idle VFDs, the harmonic content might properly be indicated as negative vars rather than positive. If that is the case, the situation may be a simple sign reversal. That would seem a little less weird, but it still doesn't provide an explanation. \$\endgroup\$– user80875Jan 15, 2020 at 19:31