Power Transformer restart problem - how to avoid

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#1
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Hello freaks....

 

First... I wish a happy Christmas...

 

There is a problem that intrigues me.

There is a lot power transformers (12,5 to 25 kV <---> 380 V, from 50 to 300 kVA) that works like that: 4 to 5 months working and the rest of the year is stopped. 

When the power transformers will be starting to working again, after the stooped time, some of transformers got broken in the time that they are energized. In two situation that I was there, I could hear the oil inside of it bubbling and after it the fuse of high voltage side opened.  In other situation it work for 10 to 15 minutes and it broken. 

 

What could be done to avoid it?

Experience guys over here, said that this happen because the oil is stopped in the lower part of the transformer then it lower the isolation and the problem happen. It make sense to me. If it´s the reason then before the restart of the transformer, the oil should be moved inside of it. Comments? Experience?

Regards,

Bruno Muswieck

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Bruno, that's a specialised question you ask - i doubt many of us here deal with power distribution. You want to find a forum where people deal with this type of equipment on a daily basis and they should be able to give you guidance. Or talk to the manufacturers.

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Yes the oil should be checked before energizing. Also it should be chemically tested at intervals,  and replaced if contaminated. You might think that an idle transformer would just sit there unchanged, but an idle transformer is more susceptible to moisture infiltration than one which is running continously. At a factory I worked in tests were done yearly. 

 

 

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A very specialized question that is not really about transformers that are common in electronics (i.e. laminated iron core) but high-voltage power systems.  I'm sure there are plenty of papers and documentation on these things, their failure modes, and how to re-energize them properly after a long-term shutdown but you're going to want to look at ELECTRIC POWER journals and such.  I'm an Electrical Engineer and did take some power systems courses but it was not my area of specialty.  

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Thanks guys...

I know that subject is not your main area but i put the question over here that is the forum that I participate more and there a lot of experience freaks over here and maybe already had seen something like that....

Thanks again...

Regards,

Bruno Muswieck

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From what I recall from my school days....

 

The transformer is filled with oil for cooling, and to prevent arcing when powered on or off.

During operation the oil will get warm that will ensure that no contamination (water is the worst enemy in this, but also air in this case is a contamination) will be present. This also causes a slight over pressure as the warm oil has a slight higher volume.

 

When you power the system OFF, the oil will cool, and you get under pressure, causing air to enter the system and also thus water.

 

Before restarting, as said, you need to check the oil consistency to make sure it is not polluted to much. I know that sometimes also with real big transformers the oil has to be pre heated. when I was in school doing internship in a company that was located close to a big transformer factory we were evacuated once after the preheating of a 380KV transformer went bad and the oil caught fire, they were afraid that the massive oil reservoir might explode.

it was one that looked like this:

 

What you will have heard is the primary side arcing through the oil.

Make also sure that the transformer is really topped with oil, I asume there is some sort of tank (perhaps post a few pictures of the transofrmers).

You need to check this on a regular basis, specially right after power on. Contamination that is present will vaporize causing the level to drop during the first couple of minutes/hours after power on.

Once a stable situation is reached you could reduce the number of checks.

 

Also IIRC the oil used is a special type, can not recall the name.

 

 

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Ross McKenzie ValuSoft Melbourne Australia