750 kva xfmr question

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steveng

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Texas
i am throwing this out for any comments from those who have experience
with transformers like this one,

we have a 12,470 pri d , 480/277 y vantran xfmr with a recent oil analysis that indicated an excessive acytelene readings and hydrogen readings, i dont have the readings in front of me or i would list them,
the xfrmr is 30 yrs old, the testing co recommends a replacement and suggest asap, said it could possibly blow, is this accurate?
i am not versed in the oil analysis results, who can give some meaningful insight into conditions like these.

thanks
 
The last time I read about an xfmer explosion, it had caused an earthquake at the college campus I was attending. After living through several of these in my region, I would have estimated the shaking at about 4 on the Richter scale. The local newspaper described the xfmer as several blocks away from campus, when station logs recorded a fault. At least one utility worker was killed and an underground-concrete vault had evaporated.

Without experience in xfmer oil analyiss, you only need be concerned with validation of those measurements.

After 30 years of cooking in a transformer, the break down of complex hydrocarbons (oil) into acetylene & hydrogen gas does not seem unreasonable. What seems unlikely, is containing the hi-pressure gas in a sealed xfmer vessel (cooking for 30 years) without exploding.

Think of the famous footage of the Hindenburg zeppelin exploding over New Jersey in 1945; both explosion and live narrative were extraordinary performances caused by hydrogen. Acetylene is a gas used with cutting torches. This can't be much safer than hydrogen.

When flammable gases and vehicles using them are repaired or stored indoors, NEC Art. 511.1, requires explosion proof equipment within 0.5 meters of fumes accumulation on ceilings or floors, unless ventilation systems provide 4 atmosphere exchanges per hour. If that xfmer is outgassing while workers cut, weld, or light cigarettes nearby, ventilation may be the only thing that prevents a fire.
 
What I would do


1. Do you have a good relation with your testing company? If so do you trust them? If so you have to put weight in their opinion that's why you hire them.
2. Contact the manufacture.
3. Do you have an engineer at the university?
4. Do a web search?
5. Call another testing company for a second sample (though most likely they both would use the same lab).
6. Either case I would get a second sample.
7. Locate a rental ASAP and have cables ready.
8. Can you re-feed the system from a second source?
9. Can you de-energize over the holidays as you proceed with investigation of what to do.
 
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First, I have zero experience in this area, so if my questions is a dumb one, please excuse me.

If the oil is breaking down and the transformer is still functional, why can't you do a shut down and change out the oil? Seems like this could be completed in a day (providing the transformer is engineered to allow for oil change).
 
hardworkingstiff said:
First, I have zero experience in this area, so if my questions is a dumb one, please excuse me.

If the oil is breaking down and the transformer is still functional, why can't you do a shut down and change out the oil? Seems like this could be completed in a day (providing the transformer is engineered to allow for oil change).
Click to expand...

Yes, it is possible to change the oil in a transformer. However, you may still need to replace the unit. Not only does the oil break down but so does the insulation on the conductors. Transformer winding insulation is usually paper based so it eventually decomposes leading to internal short circuits. Some transformers also have wooden internal support structures which decompose and plug the cooling ducts built into the coils causing overheating which cause further decomposition.

You should get a second opinion on the oil sample reports. If things are bad you may want to start doing oil samples every few months. Some liquid filled transformer are taking more than 6 months for delivery.

But in any case do not delay, transformer fires are extremely destructive.
 
I second everthing Jim said. But it depends on the oil sample results, what is extremely high? Can you post the results?

From what you stated (OP) you have internal arcing, the arcing produces hydrogen and aceteleyne, H2 is explosive with an ignition source. An explosion is possible, but not probable.

You can have the unit degassed and filtered but like Jim said if the insulation is damaged that is pointless. Did you have this transformer tested or just the oil sample?, if so post the test results for IR, DAR, and PI.
 
we have a second testing scheduled this week, they will shut down and do an oil sample and other tests , insulation tests .
the results of the 1st oil sample:
Dissolved gas analysis
oil temp 35c
hydrogen 988ppm h2
methane 949 ch4
ethylene 1251ppm c2h4
acetylene 1590ppm c2h2
carbon dioxide 480ppm c02
tdcg 5432 ppm

moisture in oil 20ppm
dielectric breakdown 44

dga key gas/interpretive method ieee (c57.104)
dga rogers ratio method / rogers ratios suggest high energy discharge
dga cellulose insulation / co2/co ratio not applicable- neither gas exceeds its limit
moisture in oil / exceeds its limit for in service oil (15%) max




we have 3 like xfmrs side by side in this service, they are 2 feet apart,
the hv comes into s/c switchgear inside the bldg. this xfmr feeds 1600 amp
main switchboard/ mcc.

thanks for the suggestions brian and jim, i have one available in 2 weeks.
vantran is 40 weeks lead time, like jim said.
zog, i dont have those results yet. but will post when i receive it.

i sincerely appreciate the comments and suggestions.
 
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We were involved in a oil filled (rental) to cast coil replacement took over a year to get this transformer, job was finally scheduled for this week but was delayed due to on going activities at the university, now they want Christmas week...UGH
 
steveng said:
we have a second testing scheduled this week, they will shut down and do an oil sample and other tests , insulation tests .
the results of the 1st oil sample:
hydrogen 988ppm
ethylene 1251ppm
acetylene 1590ppm
carbon dioxide 480ppm
tdcg 5432 ppm
Click to expand...

Don't forget to ask for testing for "suspended solids" and not just gasses. It has been awhile, but I believe you need to look at the amount of celulose.
 
no experience here, but found an interesting article
http://www.energypulse.net/centers/article/article_display.cfm?a_id=1563

note if you scroll down that the graph indicated that high levels of H2 and C2H2 are often found in arcing faults, but the article primarily is indicating that there are many faulty paradigms currently in use for diagnosing the faults based on the gasses found, and that all the factors must be taken into consideration in order to properly diagnose the fault or state of the unit.
 
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750 KVA small potatoes $15,000 plus mark up labor material crane...Possible blow up, sounds like big potatoes to me.
 
You probably should look at going to a 1000KVA while you are at it. Cost per KVA is cheaper and will give you additional capaciity.
 
Yes and if the load is satisfied by the 750 KVA there are additional losses from utilizing an over sized transformer over the next 30 years. On a 1 or 2 transformer site this may be insignificant but for a large university this could lead to tuition dollars.
 
Brian, the testing co took another oil sample today but, were not prepared to remove the hv boots, fearing they could break them, to do the resistance tests, so now we have to reschedule so they can get their stuff in one sock.:confused:

why don't they come prepared to the Job?
 
"why don't they come prepared to the Job?"

Good question, are they a certified testing company? That is a pretty routine job, I would question thier qualifications.
 
zog

zog

yes, they are a neta certified co, but, they did not come prepared to do the job, because if they had, the job would be over now and we would not be rescheduling for another day.

do you go prepared to do these jobs? if so, come on down to texas, and bring some new boots and your testing gear. when can you be here?:grin:

zog, when you go out to test a xfmr, is it routine to take new boots and a splice kit for the hv side? since the average life of power xfmrs out there is >42 yrs.
and probably over half of them have not been serviced, maintained or even tested.
 
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update on xfmr

update on xfmr

we shut it down today, the 2nd sample showed elevated readings on the dga,
and confirmed we have a bad xfmr, getting quotes for replacement.

acetylene readings showed significant level incresaes in 2 weeks interval,
better safe than sorry, ;)
 
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