capacitor problem..help appreciated

[meddy]

we have been experiencing some weird issues concerning exported energy into the distribution grid ..the meters show readings for exported active energy at customers having capcitors installed for power factor correction
when the capacitors are disconnected the meters show 0 KW exported
i m struggling finding a logical explanation..i checked the polarities of the CT's connected to the meters and they seem to be connected as the should

many thanks in advance

 

[patrick7]

Have you taken your test set out and verified that the meters are with calibration and functioning as they should?
When was the last time the meters were tested?
When was the last time the test set was calibrated.
then you may want to go to the where the ct's are at disconnect all the inputs to the meter and test the metering circuit from the start by injecting known voltage and current from a certified test set.

 

[meddy]

Actually a the meters were tested and calibrated today ..doesnt change anything ..still shows exprot or active energy

THis phenom happens only with customers equipped with capacitor banks

i know he reqding on the meter is related to the capacitors banks just cannot find a logical explanation

all the wiring have been checked and double checked down from the secondary or the PT and so was the calibration of the meter and also the windings on the CT's

 

[dpc]

Based on what you've been able to test, the most likely explanation (although certainly not the only one) is that the CT polarities are wrong for some reason.

Do the meters appear to accurately indicate real power flowing into loads that do not have capacitors?

And how many meters do you have that are showing this behavior?

 

[Marke]

I have come acros this problem reasonable frequently in a past life where the CTs were not on the same pahse as the voltage sensing. This resulted in an effective 60 degre phase shift in one installation (depends on ct polarity as well) and the meter read low, but not sufficiently to be picked up.

Try using a protable clip on meter and see if the readigns stack up with the installed meter.

Best regards,

Mark Empson

http://www.lmphotonics.com

 

[meddy]

i have been working on the problem and checked all the sequences from the secon dary of the distribution transformer to no avail
The electronic meter is connected to 3 CT's one per phase the voltages are taken from the secondary of the transformer (240V L-N) and connected to the meter with the phase sequence being respected the neutral is taken also from the secondary of the transformer so the meter connectors go something like this
I1 V1 I2 V2 I3 V3 N G

althought all the connected were veriefiedmany time all all levers from the transformar down

the Index for energy export in KWh is quited substantial for customers with a big PF corrective capacitors or with those who have ne of the poles (for one phase) of the capacitors dead
i have spoken to a profeesor in the distribution and have done a lot of research but i havent been able to find a convincing explanation let alone solution
any further help wll be greatly appreciated

 

[waross]

I think that the next step in troubleshooting theory is when everything has been checked and rechecked, and nothing is wrong, try re-evaluating your basic assumptions. Also try replacing and isolating.
One assumption we have made is that the meters are ok. Rather than challenge that assumption directly perhaps we could substitute another type of meter and see if the problem is meter specific. Also try one of these meters in another circuit that does not have problems (yet?). Try putting three simple single phase meters in the system, one per phase.
Just for fun, try reversing all the ct connections on the test bench in the meter shop and applying a leading PF load.
Another assumption we are making that may possible be in error is that the connection diagram is correct. Is it possible that part of the diagram is a front view that should be a back view? I know these sound like silly questions, but at this point I start wondering if the answer is so simple that i will be embarrased when it's found.

Is it possible that the meters are internally programed to meter the current/voltage phase displacement in a delta system?

 

[wfowfo]

Quote:
"The electronic meter is connected to 3 CT's one per phase the voltages are taken from the secondary of the transformer (240V L-N)"

What exactly is your PT ratio? This 240 to N looks unusual?

 

[waross]

Is this a 240/480 center-tapped delta system?

 

[LionelHutz]

You're describing what sounds like a Y connected PT arrangement. It's possible the meter expects an open delta arrangement or is set to use an open delta arrangement.

What does the manufacturer say about the problem?

 

[meddy]

the tranformer in use is called the H61mostly 50 KVA rating
delta to wye
22kv-400v

the meter expects a wye connection thus the neutral connection in the meter
thank you guys for ur suggestions

 

[meddy]

the manufacturer doesnt comment on the problem and says it could be cause by an unbalance between the poles of the correction capacitors..yet they fail to give a convincing explanation

 

[Marke]

You could measure the current into each phase of the capacitors. It should be equal. If there is a variation in the capacitor currents, then you have a problem with the capacitors and perhaps this is causing the problem.

Best regards,

Mark Empson

http://www.lmphotonics.com

 

[LionelHutz]

So, this meter is properly reading imported power and inductive reactive power but not capacitive reactive power? If so, then I'd expect it to be a problem with the meter itself.