GSU Vaccum Test

[Lenz81]

Ladies and Gentlemen

Has any body used Helium to do vacuum test on GSUs ? Can you please advise .

Thanks

 

[rcw retired EE]

We used helium during a pressure leak test to locate a small leak that was preventing the transformer from passing the vaccum test. Unit did not hold vacuum well enough to pass the test. Filling the tank with helium and using a detector enabled us to locate and repair the small leak.

 

[prc]

Sometimes a pressure test with dry air or nitrogen ( at 3-5 psi ) will help in locating the leaking point.

 

[Lenz81]

rcwilson ,was that weight of helium which did let transformer to keep the pressure ? was manufacturer ok with using helium ?

 

[rcw retired EE]

It's been a few years so I don't remember the details. We were providing services to the plant after some maintenance/repairs were performed. The units were well past their warranty, I doubt if the owner contacted the manufacturer.

 

[Lenz81]

Thanks I appreciate , I did some research it seems that helium does not stand long enough to pass the vacuum test .

 

[electricpete]

When you say vacuum test, I assume you mean pressure test (with a purpose is to help the transformer is tight subsequent vacuum processing).

As PRC said, we have used dry air or nitrogen for this purpose. Along with that, we used soap bottle solution (or "snoop") sprayed on suspect areas to look for bubbles. Also we tried ulstrasonic gun to listen for leaks but that didn't help... the soap bubbles worked but just had to do the work.

It makes sense that if you could put a tracer gas in there detectable by a very sensitive detector to help you zoom in on the area, you could make the job faster, at least it might help narrow down your search, then confirm it with bubble solution when you get close.

Helium is inert, so I can't imagine the transformer manufacturer would have any objection.... it won't interact with any seal materials or insulation materials if that's what you're worried about. Subsequent vacuum processing will clearly remove all traces before you put the oil back in an progress towards energization. There may be of course a human suffocation hazard, just like with nitrogen to be cognizant of, but that can be managed.

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(2B)+(2B)' ?

 

[electricpete]

And Helium has the advantage that you can talk like Donald Duck!


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(2B)+(2B)' ?

 

[Lenz81]

Thanks everybody .electricpete i have tried that that was funny .

 

[prc]

Probably one of the reasons of transformer industry not using helium for tank testing is cost( large transformers will require 50-100 KL of gas at atmospheric pressure) Some of the On-load tap changer makers are using helium for checking the sealing of diverter switch chamber(oil in that chamber should not cmmunicate with main tank oil and quantity of gas required is only a few litres)

Transformer tank sealing is checked in two ways at site.One is pressure test with nitrogen or dry air at 3-5 psiand checking leak with soap solution.Then during vacuuming, pump is stopped at about 0.3kPa(2.3 torr).First vacuum reading is taken after one hour,say P1 torr and second reading after 30 minutes there after,say P2. Then leak rate of tank is (P1-P2)x oil volume in litres/time elapsed in seconds ie 30X60. Tank leak rate shall be less than 20 millibar litres/sec.