Megger under Vacuum for Transformer 3

[NCTHAI]

X'mer ratings: 220/15.75KV // 300 MVA // OFAF // YNd1 // 10.5%

Had 220KV transformer under maintenance and vacuum pulled on tank for several days (At least 4-5). When tank under vacuum, IR at 1000 V DC was measured. Values are very good.

It is said in the manual that do not apply potential when tank under vacuum.

1) Please clarify why potential should not be applied under vacuum? What are the effects?

2) Is the megger test considered as application of potential.

Thanks,

 

[waross]

Vacuum is a good insulator.
Partial Vacuum is a poor insulator.
High potential testing under vacuum conditions may generate X-rays.
High potential testing with less than a perfect vacuum may lead to damage of the transformer and of the worker.
Bad practice.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[edison123]

Transformers are often dried out under vacuum to reduce the dew point and water evaporation temperature. During this drying process, megger readings are taken hourly to determine the point of ending the drying out and this is done without breaking the vacuum. Compared to the rated voltages of the primary and secondary, the megger voltage is very low and hence do not do any damages.

I have personally dried out large power trafos up to 125 MVA under vacuum and never had any problems due to meggering under vacuum.

 

[electricpete]

Here was a thread on the subject:
thread238-236809

One thing you can be sure, this precaution appears in several references and was not put there for no reason.

I spent a few years working on transformers full time at two different utilities. Both utilities had a policy of no electrical testing with the transformer under vaccum. So to monitor progress of the dryout, we had two options:
1 - Stop the vacuum processing for 24 hours to do a dew point test.
or
2 - If a cold trap is installed, look for stabilization of rate of moisture removal at some low level.

Several times we had no cold trap and so we stopped the test for 24 hours to check dryness. If lucky you're done. At least once we had to go back and apply vacuum some more. That is quite a bit of extra work to have done if there was no reason.

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[electricpete]

Actually come to think of it there some limits on magnitude of voltage applied while dry and then prohibition against testing under vaccum.

So we could have broke vacuum and done a test. But we still did dew point. I think the reason we did the dew point was because it was a better indicator of moisture.

Nevertheless, it is a very widespread caution and must be there for some reason as described in linked thread above

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[waross]

Another method of testing for moisture content is watching the vacuum gauge for "boil off".
When the vacuum pump is stopped on a transformer that is still moist, the remaining moisture will continue to vaporize resulting in a rise in pressure. A skilled and experienced engineer or operator can evaluate the degree of moisture remaining in a transformer by reference to a plot of internal pressure rise for a few hours after the vacuum pump has been stopped.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[electricpete]

Actually as I remember there were reduced voltage limits when tesitng normally-oil filled transformers which had oil drained but dry air blanket, and then no testing under vacuum.

So another option (besides 24 hour dewpoint test) would have been for us to break vaccum and test under air. But we still did dew point test because we thought that was a better indicator of moisture.

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[NCTHAI]

Thnaks to you for sharing the experience. Great deal of information.

regards,

 

[electricpete]

waross: When I stated "Actually..." I was referring back to my own post. I didn't even see your post. That is a good alternative method for evaluating dryness.

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[prc]

There are some cardinal rules that we follow in trf factories.
-Never lift a trf when under vacuum.Never apply voltage when under vacuum.

As waross explained at some critical values of partial vaccum,electric break down value of air comes down drastically and can cause a electrical breakdown.

Why edison escaped any embarassment was he might have used a low voltage megger (say 250 or 500 v and not 5 Kv megger normally used)and during meggering the clearances stressed are pretty large (line to earth).At field, sometime staff used to take one hourly readings with megger during hot oil circulatiuon with vacuum in conservator and the intial IR values will come down and later become a steady state level.
Nowadays these measures are no longer relied on.

Peter how can you measure dew point under vacuum esp at fag ends when moisture release will be negligible?How to insert probe without breaking vacuum?Normally I used to fill trf with dry air or N2 after vacuum and then check dew point of gas after 24 hours.

 

[edison123]

Wow, thanks guys. Learnt something today. I always use a 2 KV megger for meggering under near-full vacuum and somehow never faced any problem.

But wouldn't breaking the vacuum will let in the moisture again thereby prolonging the dry-out ?

 

[oldfieldguy]

edison123-

When you "break" the vacuum, you DON'T use ambient air. We always used bottled nitrogen. I have heard of people using "dry" bottled air. Your local purveyor of industrial gases will be able to supply either.


old field guy