When does a transformer become a bomb? 5

[miketech59]

Could anyone please advise me ? Currently we are having problems with a Parsons 1963 generation transformer rated at 59MVA with a conservator . The oil is water cooled .The problem seems to be the load Tap changer which for the last ten years has be only changed of-line .Dissolvee gas analysis found high levels of ethylene in the oil. The unit was inspected and no slack connections found .The oil was then filtered . On its return to service the buchoz alarm was triggered after two days .A DGA was done every day on the unit . The results showed Hydrogen 784 at 89.48 ppm per day ,Methane 126.53ppm per day, ethane50.67 ppm per day,ethylene 262.18 ppm per day, acetylene 2.34 ppm per day. The unit cannot be replaced before 2007. What can be done ?

 

[alehman]

Trends are more meaningful that a single reading. Is there a noticable upward trend for any of the gases?

Elevated hydrogen typically indicates an arcing condition. Increasing ethylene would tend to indicate cellulose at high temperature.

 

[itsmoked]

If its overheating a possible stop gap "might" be an external fans.

 

[wfowfo]

Clarification. Was the DGA sample from the LTC tank or the main tank?

 

[ScottyUK]

LTC which has never operated on line will possibly be suffering from pryolytic carbon buildup and consequent heating. If it has degraded sufficiently to be evolving hydrogen then it is indicative of a serious problem as alehman has suggested.

When was the tapchanger and diverter last overhauled rather than just a re-torque on the terminals? I'd be proposing to the management that the unit is removed from service until the fault is found. If you don't find it, keep looking until you do. Easier to say than to do I wholeheartedly agree, but a transformer blew up at our site - in different circumstances - which took the lives of three friends and knocked out a large portion of the station for several months. I have a healthy degree of paranoia about transformer faults which appear suddenly and have the potential to turn critical.


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One day my ship will come in.
But with my luck, I'll be at the airport!

 

[subtech]

Hello miketech

First, let's slow down a bit and get some more info. Based on your first post, you are not near "bomb" state yet.

1. This unit has an on-load tap changer, but it has only been operated while the unit was out of service?

2. Is the TC a free breather, or sealed and under nitrogen?

3. Is this transformer loaded beyond its nameplate rating on a regular basis, only occasionally, or almost never?

4. If you did a dga prior to the tightening of bolts and filtering, what were the original amounts in ppm of the following?
a. Hydrogen
b. Oxygen
c. Nitrogen
d. Methane (CH4)
e. Ethane (C2H6)
f. Ethylene (C2H4)
g. Acetylene (C2H2)

An acid number would also be very helpful in determining the overall oil condition before your work began. Was there any sludge buildup in the TC or any particulate?
Describe the color of the oil in your sample prior to your filtering and other work.

Please also state whether you simply filtered the oil,(with cartridge type filters in a typical press) or did you process with heat and degas?

Keeping in mind I'm referring to tap changers here, the presence of Ethelene in large amounts in insulating oil is indicative of over heating of components in the tap changer compartment without any arcing. There are several areas where this heating could originate from. Obviously, bolted conections would be a great place to start. However, don't overlook "contact fingers" or other contact areas which rely on spring tension to maintain contact. The compartment should be evacuated, flushed to remove any particulate that may have accumulated, and a very comprehensive inspection made of every single current carrying part of the tap changer. Elevated levels of ethylene can come about by surprisingly small over heated areas or "hot spots". Don't just look at leads, look at everything with a very critical eye. Use bright lights and inspection mirrors to look at ALL parts of the tap changer.

 

[miketech59]

The unit is free breathing and the elevated gases were found mainly in the LTC the main tank when tested showed only slightly higher levels of Total Combustible gases possibly a carry over from the LTC. When filtered the unit was taken off line and the oil was heated and degassed .The key gases after degassing were Hydrogen 12ppm,Methane 18 Ethane 4ppm, Acetylene 2 ,Carbon Monoxide 30 and carbon dioxide 847 ten days ago . The acidity is 0.08 mgKOH/g and has run high moistures of 43 ppm.
The current genration rate of ethylene is 262.18ppm per day.We are currently sampling once per day however some workers are refusing to go that close to the transformer .

 

[Sargardani]

Here is a link that I found thru a web search:

http://grouper.ieee.org/groups/tran...ower/C57.143-EPRIDCXIII-ESKOMpresentation.pdf

It states in one of the case studies that the rate of rise for Methane and Ethylene is 2PPM / day.

But the over all message is TO REMOVE THE TRANSFORMER FROM SERVICE for thorough inspections.

You never know when that one small bubble may trigger a huge explosion.


Sarg

 

[subtech]

Hi miketech

Okay, I think I understand much better now. If the unit is generating ethylene at ever increasing numbers such as you stated, It's time for it to come off line no matter what you have to do. It is quite evident that something is getting very hot in the TC. I cannot quote you an exact number as to when insulating oil becomes flammable, but with generation of ethylene at the rate you describe, I'd be jumping through hoops to get this thing off. One safety note to keep in mind is that IF combustible gas content gets high enough, the oil becomes flammable. However, there is a relatively easy way to reduce that hazard. AFTER you get the unit off line, and BEFORE you connect any oil handling equipment, open the vent at the top of TC compartment. Connect a cylinder of nitrogen gas(the same stuff you should use to keep a nitrogen blanket on top of the oil in the main tank) to the bottom of the TC compartment. Using a pressure regulator on the cylinder, allow nitrogen to bubble up at a slow steady rate through the oil and out the top of the vent.
Let a full cylinder of nitrogen bubble up through the TC . This will "agitate" the oil and allow the ethylene levels to lower somewhat. It will also saturate the oil completely with inert nitrogen and make the ratio of combustible to inert look much better than it does now. This purging process will take several hours. Keeping things properly gounded and observing ALL of the safety rules for oil handling is a must with this stuff even after the nitrogen purge. Make sure everybody knows and understands that. Good luck with your situation. If you are so inclined, I would be very interested to hear just what is getting hot when you find out.
Regards
Mike

 

[electricpete]

I remember there are two types of LTC's referred to as arcing and non-arcing. Newer styles are almost all non-arcing but with the vintage you are talking about it could be an arcing style LTC.

In the arcing style LTC there is arcing in the oil as a natural consequence of any energized tap shift and huge quantities of combustible gases are expected . If you have an arcing style and have only recently begun to cycle taps while energized, that could be one possible explanation for what you are seeing. (Or it could certainly be a real deteriorating condition).

Another tool for rough analysis of LTC's is infrared. Throw the typical electrical infrared rules out the window because you are not looking directly at the contacts. However, if you see a 5C rise from bottom of LTC tank to top or from main tank to LTC tank, that is a very good indication of a problem. If you see anything less than that, you really have to study to see what the infrared is telling you (it is much easier for infrared to provide conclusive evidence of a problem than conclusive evidence of absence of a problem).



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[miketech59]

How valuable will an Acoustic Emission test be in pinpointing location of fault, Considering background noise , cooling pumps etc ?.
There has been literature on new ratios for LTC the "rose ratios" does anyone have experience using it and how effective is the "Load tap changer signiture Analysis "programme in detecting faults before the combustible gases reach high levels ( eg 10,000 ppm Ethylene).

 

[alehman]

These documents may be useful

http://www.usbr.gov/power/data/fist/fist3_30/fist3_30.pdf

http://www.usbr.gov/power/data/fist/fist3_31/fist3-31.pdf

 

[RRaghunath]

I have a feeling that the transformer oil in the diverter switch compartment of LTC is communicating with that in the main tank, Meaning that the sealing that is supposed to be there between the two compartments is not working.

The problem can be at the tap terminals in the diverter switch wall or the drain plug at bottom of the diverter switch chamber. It can also be the conservator, if the design of your transformer provides for a common conservator (with a metallic separator) for main tank and the LTC diverter switch chamber oils.

The gases, especially the acetylene is acceptable / expected in the LTC and are of serious concern in case of main tank oil.

 

[electricpete]

I have heard people in this thread claim the gas results are acceptable for LTC and other people claim the gas results are unacceptable for LTC. To the best of my knowledge nothing in this thread has told us what type of LTC this is, so that no-one here has any basis for drawing conclusions that these results are normal or abnormal. I feel pretty strongly the results listed would be normal for older arcing LTC's assuming it had just begun cycling while energized; and abnormal for newer non-arcing LTC's.

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[ScottyUK]

Excellent observation ePete. I'd kinda forgotten that there is still a large installed base of arcing LTCs out there. LPS for you.



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One day my ship will come in.
But with my luck, I'll be at the airport!

 

[RRaghunath]

electricpete,

"Newer" - do you refer the LTCs where the switching takes place inside vacuum chamber, such as Vacutap of MR, Germany! If so, you are right in saying that the gases in oil are not acceptable.

Thanks for bringing it up.

I am still in the world of oil filled diverter switch chambers with separate / segregated conservator to prevent contamination of main tank oil.

 

[miketech59]

The LTC has not been used online for the past 10 years .I would assume if it was high energy arching that the acethylene would be increasing rapidly . However the ethylene is increasing at a faster rate which to me indicates a 500 to 700c fault probly a loose ,carbonized or sludged connection ( we have had this discussion before). I thank alehmanu for his guidance ( plenty to read and digest ) and I really would like to get my hands on a commercial Load Tap Changer analysis programm to integrate with my DGA.
Right now nobody is willing to sample the transformer while energised, there is no online combustible gas monitor and transformer doing 90 % of rated capacity, and CEO says the machine cannot be removed from service . I think it is time to retire ?

 

[stevenal]

I was just speaking to the Cooper rep hawking the latest regulator control. This one can be programmed to sense infrequent operation, and respond with a few maintenance tap changes. Seems infrequent operation can cause coking of the contacts. Same thing occurs in LTCs.

Questions for the CEO. Which is worse, the planned or the unplanned outage? Which is worse, the outage that might possibly injure or kill, or the one that doesn't?

 

[rcw retired EE]

Three years ago, the electrical engineer at a large plant in the Southwest USA asked for a shutdown because the main transformer DGA tests showed definite overheating and possible arcing. His request was refused. The EE continued to pull samples, tried to keep the trasnformer cool and continued to ask for a shutdown. This went on for two years.

The plant ran continuously 24/7 except for a major plant wide shutdown each summer. But the manager would not allow a main transformer shutdown because power was needed for other shutdown work.

When the transformer finally blew up, the plant had an unexpected shutdown and was down for a few weeks with many $$$ in losses from the transformer fire and lost production. Fortunately, no injuries.

Within hours of the blow up, the manager fired the EE for not preventing the failure. In his defense, the EE pointed to his several memos requesting a shutdown to investigate the bad DGA results. He was still fired. The reason given was that he did a poor job of communicating the problem to management and convincing everyone to take an outage or buy a replacement.

We engineers may understand the risks, but we need to communicate them clearly and forcefully to the business types who only understand production and profits. Otherwise, some damage to life, limb, property and profits may occur.

I had similar high DGA readings on a LTC autotransformer except the acetylene was "the highest I have ever seen without an explosion" according to the transformer oil test lab. We took it out of service that afternoon. When untanked, we found a 0.5 meter ball of carbon around the no-load tap changer. Only two of its 8 contact fingers were carrying current. The others had warped away and the last two were ready to fail also.

Thank goodness I had managers who listened to us EE's.

 

[subtech]

I would like to address epete's comment made on the 28th.
Epete, it is not that the current gas levels in the TC are, or are not at an acceptable level.(any of the gases) It is the fact that miketech has stated that he is doing dga daily and the average GAIN in ppm of ethylene is in excess of 200. It is simply this fact that alarmed me and led to my second post that recommended removing the trans from service. Ethylene content in oil only makes advances such as these when there is significant OVERHEATING of component(s) in the TC. Ethylene DOES NOT make such advances during normal tap changing operations and it matters not that the tapchanger is the older "arcing type" or the more modern type which suppresses arcing in vacuum bottles. Arcing tapchangers will increase most notably Acetylene, not ethylene. I believe miketech also stated that the TC has not been in normal service for years. This would again re-enforce my posistion that this sudden and drastic daily elevation of ethylene is resulting from something very abnormal.

Now, to miketech I will again say, if it is at all possible, I would remove the unit in question from service and take care of the TC.
If you absolutely cannot remove the unit from service, I would do the following:
1. Document everything you have done in effort to assess and solve this problem.
2. If your supervisor, foreman, or manager is forbidding you to remove the unit from service, do anything you can to get his signature on a document that will prove his posistion.
3. MOST IMPORTANTLY, stay the hell away from this transfomer and do your best to see that others do the same, it IS just a matter of time and the unit will come off line.............. ALL BY ITSELF!

Best wishes and stay safe
Mike