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#19538 - 12/29/02 09:28 AM Help me shoot this transformer.
watthead Offline
Member

Registered: 10/13/01
Posts: 182
Loc: South Carolina
Went on a service call where the 100amp breaker feeding a 75KVA dry type was tripping two or three times about 10 to 15 minutes apart. The breaker was hot but could be reset. I found one of the load lugs was loose and I don't mean a little, as it was backed out 3/8 to 1/2 inch further than the others. The insulation on this conductor was toast for about 18 inches. I taped this section of the conductor, torqued the lug on the breaker and set up a return trip at night to replace the conductor. As the transformer feeds two 100 amp panels that supply the kitchen of a retirement/nursing home. Problem solved right?? Wrong!! They called back the next day with breaker tripping again two or three times in thirty minutes again. Returned and put a amprobe recorder on the transformer feeder, at which time it was running at about 40 to 50 amps and was fine. About 17 hours later it tripped again twice about 10 to 15 minutes apart. The recorder showed maximun amps as 632 on A phase, 305 on B phase and 674 on C phase. At no time have the 100 amp mains or any branch breakers in the two panels that are feed by the transformer tripped. How do I meg this transformer?? Can I just turn off the feeder breaker to the transformer and the mains on the two panels that are fed by the transformer or do I need to unhook the transformer at the taps to isolate it for megging? Also what readings (ballpark) should I expect to find on a good or bad transformer when it is megged ? Help please!

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#19539 - 12/29/02 01:28 PM Re: Help me shoot this transformer.
Bjarney Offline
Moderator

Registered: 04/10/02
Posts: 2561
Loc: West-Southern Inner-Northeast ...
Is this 480∆ to 208Y/120V transformer? If so, primary current should be about 90 amps with a secondary rating of around 208 amperes. At 480V, a 100-amp breaker on the high side is a little small—125A+ may be better, but that’s not necessarily the complete cause of the problem. [There’s nothing like being there; compared to trying to explain things by ”long distance”.]

The recorder readings in your posting don’t make much sense based on the 90-/208-amp ratings for 75kVA. Does your recorder also display long-term average current, besides faster peaks? Motor starting may be causing short-term overcurrents, which would be acceptable. An infrared thermometer may be useful for some readings after a breaker trip and reset.

You may want to hire a specialty electrical testing firm to root out the problem. He’s north of Raleigh, but lacking anyone closer you may want to contact Gerald Gentle at 919..556.6106. Or, possibly someone like Instel at 864..963.4105 may be able to help. {These testing outfits are generally in this type if work…I do not have any financial or business interest in either—it’s just a long-shot suggestion if you cannot resolve the problem on your own.}

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#19540 - 12/29/02 04:02 PM Re: Help me shoot this transformer.
watthead Offline
Member

Registered: 10/13/01
Posts: 182
Loc: South Carolina
Bjarney
Thanks for your input as you were correct on the 480X208/120 and the average current was @ 60 amps per phase. The transformer seems to be about 60% loaded and has been there since 1999. I am perplexed as to how the primary current can shoot up without tripping anything on the secondary side. Looking at the numbers for the current would suggest to me locked rotor current or a fault in the transformer or its feeder, and I am leaning toward the latter and is why I need information on the procedure for megging said 75 KVA dry type. Any info on this or any other input would be greatly appreciated.
Thanks,
Watthead

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#19541 - 12/29/02 04:52 PM Re: Help me shoot this transformer.
Bjarney Offline
Moderator

Registered: 04/10/02
Posts: 2561
Loc: West-Southern Inner-Northeast ...
To test insulation with a megger, disconnect all “H” high- and “X” low-side cables including secondary-side X0-bonding jumper. At 1kV, test three times—high-side to ground, then low-side to ground, and high-side to low-side. Roughly one megohm minimum on each.

I doubt it’s an insulation problem. Carefully check primary current on each high-side phase with the secondary cables disconnected and meter on a low range. Offhand, it should be less than amp on all three phases. If the drytype seems noisy or overly warm with no secondary load, there may be a shorted turn in the transformer. A shorted turn won’t automatically fail an insulation-resistance test.

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#19542 - 12/29/02 04:54 PM Re: Help me shoot this transformer.
Scott35 Offline

Broom Pusher and
Member

Registered: 10/19/00
Posts: 2724
Loc: Anaheim, CA. USA
Watthead,

Adding to Bjarney's posted information, I have a few Q's / comments / etc.



    [*]As to the high load amperes recorded, how long was the duration? Does the current stay steady thruout the complete surge time duration?

    [*]Is there a corresponding load amperes increase on the secondary side, or at the subpanels, during this surge period?

    [*]Do you have an equal 100/3 or 125/3 frame to swap with the existing breaker, while you shoot trouble on this situation?

    [*]Have you examined the Transformer Coils for signs of arc faults? Have you checked the Branch Circuit and Conduit feeding the Transformer completely for other barbequed insulation?

    [*]Can you smell fried Varnish or essence of barbequed thermoplastic insulation?


I am leaning towards an Arc fault possibility - either in the transformer enclosure (Coil/s to Core or Coil to Coil, also the branch circuit conductors terminating at the lugs), or somewhere inside the conduit feeding the transformer.

If the surges are corresponding to draws on the secondary side, then ace the arc fault ideas!

Maybe you should Megger the transformer's coils, to see if you find leaks. Also Megger the branch circuit feeding the transformer.

Really sounds like an Arc Fault, but maybe the 100/3 breaker is toasted from the excessive heat concentration of the loose termination lug - that barbequed conductor may have also been a result of an arc fault, but more likely the loose connection under a steady high load heated the heck out of everything available, and the copper wire did it's normal heat sinking action! (and the breaker became the slave sink!!!)

Let us know the outcome!

Scott s.e.t.
_________________________
Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!

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#19543 - 12/29/02 06:34 PM Re: Help me shoot this transformer.
watthead Offline
Member

Registered: 10/13/01
Posts: 182
Loc: South Carolina
Some observations and about the situation. There was no detectable odor of burned electrical components, which I did think was unusual after finding the toasted insulation. I don't know how long the terminal had been loose on the breaker, but from the looks of it my guess was that it had been that way since original installation in 1999. The fellow that reset the breaker several times said that one time the transformer was very noisy, this was before finding the loose connection on breaker. I am monitoring the secondary side at present but no trips yet. I don't know if my recorder shows duration or not it is a amprobe DM-1 Digital recorder and if it does, I didn't check. I have switched the breaker on and off and it dosen't feel gritty. I know this isn't very scientific but many other times such a high resistance connection on a breaker has left the ones I have seen with a noticable feel in the switch operation. I do not have a breaker to swap and as much as I hate to admit it I may become a parts swapper to solve this one. I really appreciate all of your help.
Thanks,
Watthead

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#19544 - 12/29/02 06:52 PM Re: Help me shoot this transformer.
Bjarney Offline
Moderator

Registered: 04/10/02
Posts: 2561
Loc: West-Southern Inner-Northeast ...
An AC millivolt-drop test might be done on the breaker. See http://ecmweb.com/ar/electric_testing_contact_quality/index.htm
The article talks about a magnetic starter, but with appropriate caution and the correct meter, the same principles apply.

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