CT meter going nuts under load

[Rockingit]

So, I've had one of those days of chasing my tail...

What should have been a simple hour's job of fitting a 3ph CT consumption meter.....
no / little load = fine, behaves as expected.
load (ashp compressors) = meter claims incorrect polarities, meter claims it's generating... much weirdness.

From a wiring perspective it's all good, metered, tested, signed off.

I'm wondering if there might be a harmonics issue??

 

[plugsandsparks]

Should not be harmonics as things like ASHP have to be compliant to standards for that. Could it be the CT's are saturating due to high start currents ?

 

[Rockingit]

Should not be harmonics as things like ASHP have to be compliant to standards for that. Could it be the CT's are saturating due to high start currents ?

I don’t think it‘s that. I’ve played around with various combinations downstream and nothing makes any sense. I’m going back in the morning just to double check my work but not expecting to find anything. It’s truly bizarre.

 

[James]

Might seem obvious, but do check the ct’s are on the right cables and the right way round.

 

[plugsandsparks]

OK, another guess, lol - Assuming James' bad day point, could it be that the CT's are not compatible with the meter, i.e. the ratio is outside its range, in terms of either voltage or current. This would show up more as you load it up. If its going haywire it could be outputting to the meter too high mV.

 

[Rockingit]

So, the mystery thickens but I think I've resolved it..... Definitely going to chalk this one up to some kind of PF/Reactive/Harmonics issue. I got there today (not having done so since I posted this) to find all three phase indicators sat happily (they were randomly blinking when I'd left before) and it's measured 50kWh useage which is about what I'd expect. I ripped it all apart and rebuilt from scratch again anyway, just in case, and it repeats the behaviour - at turn on it's like watching Riverdance but then as soon as some respectable load gets put on it seems to settle. My clamp meter wasn't showing anything on the neutral that wasn't also on the phases.

So my guess is that the suppression / built in PF isn't quite in synch across three variable motor controllers. The meter's own kVar was definitely running as fast as the kWh so without plumbing in a pq meter for a while and studying it then we're not to know but I'm happy that it's measuring energy consumption as intended, not blowing the neutral and the level of PF (whatever that may be) isn't going to cause an issue to the rest of the building (it's a care home, not a factory!) or undue bills. Not sure what else I could have done, tbh.

 

[Pete999]

So, I've had one of those days of chasing my tail...

What should have been a simple hour's job of fitting a 3ph CT consumption meter.....
no / little load = fine, behaves as expected.
load (ashp compressors) = meter claims incorrect polarities, meter claims it's generating... much weirdness.

From a wiring perspective it's all good, metered, tested, signed off.

I'm wondering if there might be a harmonics issue??

e Are the CTs matched with the meters ratios?

 

[Rockingit]

e Are the CTs matched with the meters ratios?

Yes, all supplied as a package by the supplier. Besides, if the ratios were out wouldn’t they still be in ‘balance’ just inaccurate??

 

[Pete999]

Yes, all supplied as a package by the supplier. Besides, if the ratios were out wouldn’t they still be in ‘balance’ just inaccurate??

possible I suppose