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capacitor problem..help appreciated

capacitor problem..help appreciated

capacitor problem..help appreciated

(OP)
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

RE: capacitor problem..help appreciated

Improperly connected metering seems the most likely cause.  You need to check voltage and current polarities and phasing and verify phase sequence assumptions.  

 

RE: capacitor problem..help appreciated

(OP)
Already have done that ..and that still wouldnt explain the presence of an export only when a capacitor is connected

RE: capacitor problem..help appreciated

Sure it would.  If the meters are miswired such that they are metering kVArHr rather than kWHr, there would be export only when the capacitors are connected.

RE: capacitor problem..help appreciated

I would verify that the phase sequence matches what you think it is.  I've seen a lot of screwy metering results when phases got swapped somewhere between here and there.

You probably need to go up to the primary CT and PT connections and work your way back, assuming, as you go, that everything is hooked up wrong.  A donut CT installed "backwards" will flip the polarity, even when the secondary wiring looks fine.

 

RE: capacitor problem..help appreciated

(OP)
already did that before..but ill try that again tomorrow i ll get back to you tomorrow
thanks for the suggestions

RE: capacitor problem..help appreciated

Is it an electromechanical or an electronic meter?

Gunnar Englund
www.gke.org

RE: capacitor problem..help appreciated

(OP)
it s a digital meter(electronic)

RE: capacitor problem..help appreciated

Can you display or record the voltage and current phasors (magnitude and angle)?  

RE: capacitor problem..help appreciated

(OP)
sure... and the all have the desired amplitudes and angles

RE: capacitor problem..help appreciated

What is "desired"?  You need to verify angles between all voltage and current phasors preferably with and without the cap banks and make sure the current is changing in the manner it should be.  

Can the meter be configured for either positive or negative phase sequence?  

RE: capacitor problem..help appreciated

What kind of meter is this??

RE: capacitor problem..help appreciated

(OP)
there are altaris(french) meters
and they only display the magnitudes or the phase votages and currents...they also show a warning when the phase sequences are not respected.. so my guess is that it s not a sequence problem ..unfortunately :(
that was my first guess also but doesnt seem to be a wiring problem and we dont have meters available to see the angles of each phase

RE: capacitor problem..help appreciated

Do you have three PTs and three CTs?  

RE: capacitor problem..help appreciated

(OP)
just 3 CT's 75/5A in the box with the meter

RE: capacitor problem..help appreciated

(OP)
all polarities were checked on the CT's
i m new on the job ..yet none of the senior engineers seem to find the solution to this issue either

RE: capacitor problem..help appreciated

Whoa there.  You can't talk about energy if you only have CTs.  You have to have voltage also.  You need to verify how the meter is getting voltage from the three phases, or you need to ignore everything out of the meter except current.

RE: capacitor problem..help appreciated

A possibility outside of wiring/polarity issues:

Donut style CT's are known to be 'flipped over' in their packaging. There would be no way to tell unless you opened up the packaging. Without a piece of equipment to tell you this, your kind of out of luck. If this is the case, the only thing to do is start flipping them over (one at a time) or pull out three new ones and install again. I had this happen to me twice in the last ten years.
 

RE: capacitor problem..help appreciated

I think the meter must be directly connected to the system - no voltage transformers?

RE: capacitor problem..help appreciated

buzzp
Good suggestion. Flipped packeging or incorrect labelling is something we all have seen at one time or another.
As for checking. It doesn't require much equipment.
a 75:5 CT will have 15 turns on the secondary. If you hand wind 5 more turns in the proper direction you should have a 100:5 CT.
If the CT is reversed internally, the new turns will subtract and the effective ratio will be 50:5. You can check the ratio with a clamp-on ammeter.
yours

RE: capacitor problem..help appreciated

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.

RE: capacitor problem..help appreciated

(OP)
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

RE: capacitor problem..help appreciated

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?  

RE: capacitor problem..help appreciated

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

RE: capacitor problem..help appreciated

(OP)
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

RE: capacitor problem..help appreciated

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?

RE: capacitor problem..help appreciated

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?

RE: capacitor problem..help appreciated

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

RE: capacitor problem..help appreciated

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?

RE: capacitor problem..help appreciated

(OP)
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

RE: capacitor problem..help appreciated

(OP)
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

RE: capacitor problem..help appreciated

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

RE: capacitor problem..help appreciated

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.

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