CT Problem or Bad Wiring

[ndwade]

Hello,

I'm working on a project where we have installed permanent Power Quality meters on 4160V switchgear for a Turbine Generator.

The voltages and currents for each phase of the generator are essentially the same except for the current passing through the C-phase which is substantially less in magnitude (80% less). From other metering we are able to see that the turbine generator is no unbalanced.

The visual waveform for this C-phase current shows a non-sinusoidal wave with a rapid rise and an exponential decline which corresponds to the peaks of the expected sine wave during each cycle.

The generator is not unbalanced so my question is whether this non-sinusoidal wave for this C phase current is evidence of a faulty CT or perhaps incorrect wiring?

Please let me know what you think and what you've experienced.

Thanks

Nadir Wade

 

[Laplacian]

Shorting meachanism (bar at the CT terminals, or screw in terminal strip) left in place?

 

[PHovnanian]

Its too bad that no waveform traces were attached. But from the description, I'd be looking for something with a nonlinear characteristic. Given a setup consisting of a CT, wiring and one set of metering equipment, I'd look at the C phase of the metering equipment first.

CT metering most often involves a CT and a burden internal to the metering equipment. The metering device actually looks at the voltage developed across that burden. If the burden is open circuited, a CT will
generate a very high voltage as it attempts to drive its secondary current somewhere. Once that voltage reaches the breakdown voltage of some component, that high voltage collapses. That's what your waveform description sounds like to me. Since your power quality instrumentation is seeing this voltage, I suspect the CT and wiring are OK up to this point.

 

[dpc]

What else in in the CT circuit? It sounds like a high-impedance path for C-Phase CT secondary for reasons unknown.

I'd short out the CTs at the source and check current with a small clamp-on. If that looks OK, start working your way toward the meter in question.

These meters have another little CT inside the box for each CT input. They can have their own problems, although this is fairly unusual.

 

[ndwade]

All,
Thanks for the responses to date.

The only additional information that I can give since our Generators can not be taken down at this point is that I do not believe shorting blocks exist for this particular gear.

Also, there is double-metering going on here where we've identified that the wiring was placed in parallel rather than in series; thus splitting the current which is seen by our PQ metering device.

While we know that the parallel connection is not correct, we do not believe this to be the issue as all the other phases have sine waves and are of similar magnitudes (and this one shouldn't be that different).

The waveform which is being generated is our main concern as it may be showing evidence of something more significant.

Previously, the PQ meter wiring was traced for errors and loose connections so we believe that while in parallel, it should not result in the C-phase waveform we are seeing.

The devices are new so internal CT problems are our last guess.

We'll have to invstigate further by tracing the wiring, meter and perhaps get someone to take a look at the actual CT's

Anymore opions on this are much appreciated.
Thanks
Nadir

 

[PQ101]

Wonder what type of power quality monitor you utilize? and how the internal pprogram is setup. I use Dranetz-BMI, Hioki, FLuke, and able to help you with any problems:

1) Make sure that the arrow on all 3-phase CTs either face the load or the line (depending on your setup)

2) Verify the CTs (assuming they came with the power monitor-manufacturer) are of the same rating. Under program setup (monitoring device) make sure CT ratio are clicked the same on all 3-phase

3) Next, look at your phasor diagram, the voltage and current should match indicating proper setup and phase angle.

4) With respect to PTs, typically there are no power monitors that come with 4160V rating-unless they are provided with 4200:120V PTs. Setup on all CTs and PTs should be program the same inside the monitor. If setup properly your phase rotation, phase angle, voltage and current reading (all related) should be good to go.

 

[PQ101]

Not sure of you're using clamp on CTs, or permanent ring type CTs, with leads departing or going to your monitoring device.

If panel type monitors are used (let's say PQ meters on switchgear door) the polariy on all 3-phase CTs must be checked-veified (for no Reverse polarity).

Regarding the spikes on captured waveforms, check for lose connections somewhere along the path. Another check that you can perform is: swap the CT leads with another phase to see if the problem still exists. This way you check for bad CT as well. Hope this helps.

 

[JJHorak]

Check if any device in CT string is self powered, using power from the C phase CT as its source of power. Devices that do this creates a non-linear load; they are higher burden during part of the current wave, lower burden during other parts, and might be shunting current to the parallel CT path during part of the current waveform.

 

[PQ101]

Good point Timesabroom.

Next time check and make sure the clamp-on CT, or flex CT (whichever you are utilizing) is plugged in properly all the way, go back open up the program on your power monitoring/analyzer equipment (under CT ratio setup), and make sure CT ratio are all identical on all 3-phase. 1St thing is to verify your CT is good though. They usually done go bacd, but always good to have one spare.

What model-type of PQ monitor you got out there?

Power quality solutions that empower you!!!

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[ndwade]

Thanks to all for the help with this one.

I believe that the CT's are permanent ring type. The only way to check the problem out at this point will be to check the CT's and see whats going on.

Our PQ monitors are permanent and are connected to terminal blocks.

When we go in to fix the connection problems, we'll also have them look at the CT's and see whats going on.

Thanks again for all the help.