ct failed excitation test - why? 2

[electricpete]

We had a motor that was running fine. Took it out for maintenance. Relay calibrations were performed ONLY AFTER the motor was shutdown. All relays are GE style electromechanical relays.

Upon startup of the motor, it tripped on unbalanced current approx 1 second after start. Motor meggered/bridged fine. Changed unbalance relay and started again - tripped approx 1 second after start.

We meggered CT and relay circuit (with neutral ground lifted) - everything fine.

Checked resistive balance on CT's. Very close. Something like 0.56 ohms, 0.56 ohms, 0.57 ohms.

We performed secondary excitation (saturation) test on the CT's (200:5 ratio). Phases C and B had a knee around 90 volts and required 115-120 volts to produce 2A secondary. Looks normal.

Phase A produced 2A secondary at 4 volts. Looks bad. Took it up to around ~10 volts and 5A, then slowly back to zero, then raised voltage again and same results.

To check for possible influences outside the CT on secondary or primary side we checked the following:
For secondary - Determ'ed B phase and A phase CT secondaries (to ensure there was no problem in the wiring) and repeated saturation test and same results.
For primary Verified that bus grounds were hung only on the line side of the CT's, so no apparent loop path on the primary side (except possibly through motor itself... but that should affect all phases equally...presumably a high impedance path?).

We conclude there is a problem with A phase CT that failed it's saturation test.

We were unable to perform ratio test at the time I left site because the primary side was still taped up and we couldn't access it for testing. The plan was to replace the CT.

So now my questions:
Do you think the failure is shorted turn or residual magnetism on the CT?
If shorted turn, wouldn't it show on the resistance test? Maybe not.
If residual magnetism, why didn't we clear it when we increased to 5A excitation current and back down?

In either case, how can the failure possibly be related to the work done which was relay calibration? Test paddles were installed for testing.

I think that saturation could occur if the CT circuit were interrupted while carrying current, but there is no reason to think that happened... testing didn't begin until after the motor was shutdown.

We will have an opportunity to ratio test the removed CT. Will that tell us enough to distinguish between shorted turn, residual magnetism, or some other problem? Is there anything else we should test on the removed CT?

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[ScottyUK]

Hi Pete,

My first guess would be a shorted turn, or maybe a couple of adjacent turns, is the cause of your problems. If the CT is a high ratio with many turns then the percentage resistance change due to one or two turns being shorted will be difficult to measure without lab standard instruments. Even then I doubt you have a reference measurement of sufficient accuracy class from that CT before it went bad to make the measurement meaningful.

Do you have any thermography equipment? A shorted turn passing 5A through it should show up as a localised hotspot on the core if the IR camera is sufficiently sensitive. A sensitive search coil might also pick up leakage flux near the problem area.


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[electricpete]

Thanks Scotty. Does a magnetized CT (due to interrupting secondary current) have any different symptoms?

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[electricpete]

The other question - is there any way shorted CT could be related to relay calibration? We're having a hard time with the coincidence.... apparently the CT was working fine when the motor was running a week ago. Then took it out for relay cal and now CT is failed? Must have been related or a big coincidence?

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[ScottyUK]

I'm not entirely sure how it would behave. My gut feeling is that it would draw a large current because the coil will have a much lower impedance if the core is magnetically saturated, which I guess is where you are coming from. Whether the current would show some assymmetry might depend on how heavily magnetised the core is: there should be some even-order harmonics if the core is coming out of saturation in one or other half cycle. You'd probably see that on a scope - second harmonic is hard to mistake for anything else.

Dumb question - how does a compass behave near the de-energised core?


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[electricpete]

I haven't tried the compass. Will try that next week.

One thing we noticed. We pulled the nameplate off of the A CT before we removed it so we could determine the type of CT for purposes of ordering an identical new CT. There are two screws that came out to take the nameplate out. The screws had something resembling iron filings which was clinging to the screws as if magnetic. I don't know where the iron filing material came from. I didn't have time to try pulling label off the sister CT to see if it had similar stuff (wish I had).

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[alehman]

I wonder if somehow the CT secondary wiring was inadvertently opened with load. Was the motor ever operated with relays withdrawn (perhaps a bad shorting switch in the relay case?).

 

[electricpete]

alehman - that's the scenario I was imagining too, but our guys say nothing was done until after the motor was secured.

Now here's one thing I'm wondering. What if somehow when relays were restored A-phase ct secondary was somehow left open (maybe bad connection at the relay).

Then when motor started the first time, immediate unbalance and also magnetized the CT. (Would starting a motor with CT open magnetize the CT same as opening CT during motor operation? I think so).

Then when replaced the relay, we may have cured the open (which we never found during later troubleshoting), but we still had the CT problem.

This seems to be the only scenario that makes sense.. consistent with what the guys told me (relays not pulled until motor shutdown) and explains the coincidence of problem upon startup after relay cal. Does this scenario make sense to you guys?

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