Transformer Core de-magnetization
Transformer Core de-magnetization
(OP)
I have a GSU trasformer that I have someone testing and they did the testing out of order and did a winding resistance test first. Now the power factor readings a all messed up because the core is now magnetized. I know to de-magnetize it you need to use either DC current or voltage. We have a 400A DC source available but you cannot get much voltage across the leads of this piece of equipment. Would a DC welder be better and just bump it to give it a "DC shot"?






RE: Transformer Core de-magnetization
I do not think that the resistance measurement caused any problem at all. Your transformer core most likely had a DC field in one or more of the legs when the test started. The reason is that you cannot disconnect from the mains with zero flux in all three legs. Usually you will have more or less DC remanent field in all the legs.
If you really need to de-flux the core's all legs you will need to apply a three-phase voltage that is high enough to saturate the core and then reduce the voltage slowly down to zero volts. Most GSUs that I know about are in the 10+ MVA range so it will not be very easy to do this. And I do not think that you should do so either.
You should find your problems elsewhere.
RE: Transformer Core de-magnetization
What are the symptoms of the power factor readings being messed up? Do all your instruments agree with each other? Cross-check MW, MVAr, PF against each other. Cross-check MW against V, I, and PF. until you find something that doesn't tally.
If the instruments all correlate each other, please post some more information about how the problem manifests itself.
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RE: Transformer Core de-magnetization
RE: Transformer Core de-magnetization
I think the procedure for demagnetizing would involve applying your current source in alternative polarity and decreasing magnitude. Exactly what jbartos said. I have heard of it done with a car battery so I suspect your current source will be fine if you have the ability to vary (decrease) the current.
The scenario of residual magnetism associated with sudden deenergization, I believe this is more commonly associated with ct's. (I'm not positive).
I believe in rare/extreme situations it is possible for residual magnetism to lead to tripping of the transformer upon energization.
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RE: Transformer Core de-magnetization
RE: Transformer Core de-magnetization
RE: Transformer Core de-magnetization
RE: Transformer Core de-magnetization
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RE: Transformer Core de-magnetization
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RE: Transformer Core de-magnetization
OK. I think I get it.
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RE: Transformer Core de-magnetization
I could be wrong and maybe this is not a consideration, but seems such a rapid discharge of a lead acid battery could result in bad things happening. I remember hearing this somewhere along the way. However, batteries do get warm when asked to produce currents for starting a vehicle (typical battery is ~700-800A max). Not sure the current draw when attempting to demagnetized a transformer.
RE: Transformer Core de-magnetization
buzz, the hazard you describe sounds diametrically opposed to that in this electrical “accident”… first entry at http://electrical-contractor.net/ubb/Forum1/HTML/004077.html
RE: Transformer Core de-magnetization