Gas in power transformer

[PeterJ007]

I need some help understanding what is happening to one of the 2.5MVA power transformers at one of our solar project sites.

We have two, identical. hermetically sealed, mineral oil filled, 2.5MVA transformers, each fitted with a DMC protection relay. One transformer is functioning perfectly, the other is exhibiting the generation(?) of gas. The gas pressure builds up in the DMCR, forces the oil level in the DMCR below the oil-level trip threshold, and the transformer trips. Upon manually releasing the gas pressure in the DMCR, the oil level returns to its original level and the transformer can be operated again. This appears to happen about every four hours of operation.

We have had the mineral oil tested and no hydrocarbons or other contaminants have been found – which would indicate that there are no internal arcs causing the gas – so we are at something of a loss as to understand the source of the gas.

I just wondered if you had come across such a phenomenon on your travels and if so what the cause/solution might be?

 

[Hv&Lv]

Any oil on the ground to indicate air is entering the transformer?
I know you looked when re-energizing… just had to ask.

Oil testing has me scratching my head also.. was it just outside air?

What temperatures are recorded while in operation? Overheating due to over voltage? Possible with a solar farm.

You may simply need to top off the oil in the unit.
If the DGA shows no anomalies I would look at oil levels at the 25c mark.(or whatever the dataplate suggested)
There may be a measurement on the dataplate for the oil level from the top.

 

[PeterJ007]

Thanks Hv&Lv - Not seen any oil on the ground. Not sure of the source of the air - if it came from outside or maybe from an internal air cushion. Both transformers exhibit the same oil temperature - about 30 deg C in the morning rising to about 60 deg C at the end of the day. T1 and T2 temperatures are within 1 or 2 degrees of each other.
We just received a drum of oil to try topping off - will report back!

 

[PeterJ007]

One more observation Hv&Lv - if we leave the high voltage connected over night the oil has depleted in the morning and we have to release the gas to get the oil level back. If we disconnect the high voltage at night, the oil level remains high and no gas has to be released in the morning!

 

[Hv&Lv]

Thanks Hv&Lv - Not seen any oil on the ground. Not sure of the source of the air - if it came from outside or maybe from an internal air cushion. Both transformers exhibit the same oil temperature - about 30 deg C in the morning rising to about 60 deg C at the end of the day. T1 and T2 temperatures are within 1 or 2 degrees of each other.
We just received a drum of oil to try topping off - will report back!

Please do!
we have so many posts on here that we never hear the end result.
I would love to hear what the issue is in case some others run into this problem.

 

[zbang]

This is out of my wheelhouse, but how new are these transformers? If new enough, is the manufacturer involved? I'd hope they were interesting in the issue.

 

[winnie]

Is there any way to sample the gas being released and get it analyzed?

-Jon

 

[PeterJ007]

Hi - latest observation. The gas does not build up gradually in the DMCR over the four hours or so of operation but enters in a 5 second burp causing the DMCR oil level to drop and the transformer to trip.

This is out of my wheelhouse, but how new are these transformers? If new enough, is the manufacturer involved? I'd hope they were interesting in the issue.

Hi zbang - transformer is five years old and out of warranty but has only just (2 months) been placed into service.

Is there any way to sample the gas being released and get it analyzed?

-Jon

Hi Winnie (Jon) oil has been tested zero contaminants or hydrocarbons.
We're looking at ways to collect the gas and have it tested - colorless, odorless - we assume (!!!!) it's air.

 

[Clutch57]

Does temperature cause the enclosure to change size? Like a transition from a moderately concave wall to slightly convex sorta like the old style oil cans?

 

[PeterJ007]

Does temperature cause the enclosure to change size? Like a transition from a moderately concave wall to slightly convex sorta like the old style oil cans?

Could be Clutch57 - however, at some point excess gas would no longer be expelled (maybe) This has been going on for a month now. The DMCR oil-level trip will occur when 170mL of oil is displaced. Two or three times a day would sum up to 15liters of gas/oil.

 

[Barbqranch]

Has the frequency of burping changed over the month, or is it pretty much constant?

 

[PeterJ007]

Has the frequency of burping changed over the month, or is it pretty much constant?

No it hasn't - approximately four hours of operation between burps. We don't know how long it takes to trip at night - no operation but still connected to the 33kV line - we're trying to get our security folks to watch out for that tonight. Will report back.

 

[zbang]

Just throwing pasta at the wall, here-
Have you done IR scans of both transformers? Not sure what I'd expect, but maybe there's an interesting difference.

 

[PeterJ007]

Just throwing pasta at the wall, here-
Have you done IR scans of both transformers? Not sure what I'd expect, but maybe there's an interesting difference.

Pasta is good!! IR scans - not yet - getting details of a thermal camera we could obtain and use - not just for the transformers but other electrical equipment to look for errant hot-spots.

 

[paulengr]

You don’t bother with the atmosphere on top. You grab an oil sample and test the dissolved gases. It’s cheap. It requires a special glass syringe to do it. This will tell you what is going on inside.

There should be pressure or vacuum on a transformer. During the day and as they warm up from operation you get a little pressure. If there is no pressure then the transformer is open to air. This allows moisture in which attacks the transformer insulation and structure. As that breaks down it creates acids that attack and destroy the oil. Moisture and oxygen need to be avoided at all costs with oil filled transformers.

With corn oil (FR3, bio temp, or whatever they call it), it gasses a lot more than traditional mineral oil. Also the varnish system on larger transformers will gas a lot as it runs for a couple weeks.

The bushings can sometimes be a problem. It’s just a draw lead nut pulling in the oil side with a rubber seal under the bushing/terminal interface. There is a certain amount of torque you need but not too much so sometimes these get bumped in shipping or are too loose abc leak.

 

[PeterJ007]

You don’t bother with the atmosphere on top. You grab an oil sample and test the dissolved gases. It’s cheap. It requires a special glass syringe to do it. This will tell you what is going on inside.

There should be pressure or vacuum on a transformer. During the day and as they warm up from operation you get a little pressure. If there is no pressure then the transformer is open to air. This allows moisture in which attacks the transformer insulation and structure. As that breaks down it creates acids that attack and destroy the oil. Moisture and oxygen need to be avoided at all costs with oil filled transformers.

With corn oil (FR3, bio temp, or whatever they call it), it gasses a lot more than traditional mineral oil. Also the varnish system on larger transformers will gas a lot as it runs for a couple weeks.

The bushings can sometimes be a problem. It’s just a draw lead nut pulling in the oil side with a rubber seal under the bushing/terminal interface. There is a certain amount of torque you need but not too much so sometimes these get bumped in shipping or are too loose abc leak.

OK - gotcha - yes, that has been done. I've attached the DGA results. Nothing of any consequence was found.

 

[PeterJ007]

Something a little different this morning. DMCR had tripped before 10:00pm last night - going to have security do earlier checks tonight - but when DMCR cap was removed this morning no air was observed to exit and yet the floats came back up to their operating level Going to try to get some oil into the DMCR tomorrow.

 

[Hv&Lv]

What other protection relays do you have on this unit, if any?
one float should be an indicator only, the big float should be the trip float.
I’m also going to assume both units are set to the same settings..

Don't these things have a dip tube on them? Could it have fallen off inside the tank??

Im almost tempted to think this is nuisance tripping.. just need to figure out why.

 

[PeterJ007]

What other protection relays do you have on this unit, if any?
one float should be an indicator only, the big float should be the trip float.
I’m also going to assume both units are set to the same settings..

Don't these things have a dip tube on them? Could it have fallen off inside the tank??

Im almost tempted to think this is nuisance tripping.. just need to figure out why.

There are supposed to be three protection items on the DMCR - over pressure, low-oil and over temperature. The large float operates a magnetic reed switch. Not sure about the drip tube I'm afraid. Both transformers have the same model DMCR and all settings are the same. I've attached the DMCR data sheet FYI.

 

[PeterJ007]

Something a little different this morning. DMCR had tripped before 10:00pm last night - going to have security do earlier checks tonight - but when DMCR cap was removed this morning no air was observed to exit and yet the floats came back up to their operating level Going to try to get some oil into the DMCR tomorrow.

This really makes no sense to me - we've asked for verification tomorrow morning