VSD effect to energy meter?

[yogesh]

We have 10 energy meter in that 4x of them are giving the negative reading. The events shown are two 1is Negative Power Element 2 Warnings & 2. Negative Total Power Warning. The plant is not generating power & we are not sure where this export power coming from on site We have verified the voltage connection, CT reversal & alignment but all is fine. Supplied a new meter which shows the same readings. We have concluded the negative reading could be associated with the equipment connected. The “issues” with the meters are on DB boards where the board is connected to air handling units which are full of variable speed drives and I think that some of these may be DC drives. If that is the case, this may be causing the reverse power flow,??? What effect VSD’s will have on the meter readings?

 

[Simon47]

There won't be reverse power flow - unless you've discovered a new infinite power source !
What could be happeningis that the VSDs are creating a load of noise and/or a complex current waveform - and the meters are rubbish.
Possibly the quickest test would be to fit a Ferarris disk meter (the spinning wheel tyoe) between one of the dodgy meters and the load. I'm fairly sure they handle complex loads without problems and you'll be able to compare readings.

I read an article a few years ago where some electronic meters were found to misread badly with complex loads. Found when a large solar install gave odd results due to noise from the inverters interfering with the poorly designed meter.

 

[James]

Some inverter drives can generate when slowing down as the kinetic energy has To go somewhere.

 

[Simon47]

Not for very long though.
None of the drives I've worked with (admittedly not a large sample) have had that ability - those that could actively slow a load had an option for a braking resistor to bleed off excess energy from the internal DC bus. But with the rapid development/maturity of such systems, I could see regenerative braking becoming more common - which I guess also means interesting safety questions.

But it does raise an interesting question. How quickly do modern meters "trip" when presented with a momentary power reversal ?

 

[Marvo]

A VFD under normal operation converts AC to DC internally which is a one-way system and if energy is fed backwards into a VFD it causes an elevated reverse voltage on the DC bus which would usually cause an alarm condition. Reverse power flow through the drive isn't possible and regen from motor braking also shouldn't cause a back EMF but this may be possible under certain conditions on certain syastems.

I would use a power quality analyzer to obtain a profile of the exported waveform including is size, duration, frequency of occurance and waveform characteristics. Once you have that info and you've proved it's origin is the VFD's I'd contact the drive manufacturer (rather than the AHU manufacturer) for technical support