Power transformer oil analysis

[mjeshtri]

Our AT3 -400MVA autotransformer is mounted at 400/220kV SS and it is in servise with two another autotransformers AT1(1983) and AT2 (1987) with same rated Power

Two years in service we did not take any oil samples for gas analyzing, but at 24 march 2006 we did send the oil sample to the laboratory and from them we have this results:

Dissolved Gas in Oil Analysis of transformer oil

AutoTransformer: AT3
Manufacturer: Siemens-Koncar
Date of production: 2003
Power: 400MVA
Voltage: 400/220kV
No load Tap changer 3 pos.
Cooling system: OFAF
First group ventilators aktivation at 53C degree
Second group At 63 C degree

Temperature of oil (top) when the samples taken was T=34C degree

The results of Oil analysis:

Hidrogen H2 303 ppm
Methane CH4 795 ppm
Acetylene C2H2 17 ppm
Ethylene C2H4 1139 ppm
Ethane C2H6 151
Carbon monoxide CO 176
Carbon Dioxide CO2 345 ppm

I am asking to you: It is our transformer in dangers, wat can we do to save Autotransformer. It is problem indicated because of cooling system , because the first group ventilators activate when the temperature oil is 53 C degree. that mean the pump will activated with ventilators.
The firs group has 9 ventilators and second group 9

With respect
Gazi

 

[Zogzog]

Let me start off by saying I am not a chemist or even a specialist in analysing oil samples, I just see alot of test results from our lab.

According to ASTM D-3612:

Max Methane - 100 - you have 795
Max Ethane - 50 - you have 151
Max Acetylene - 1 you have 17
Max Ethylene - 50 you have 1139

H2,CO2, and CO look OK.

I dont think you have arcing, H2 is too low for that.

It appears you have corona (PD) occuring in your transformer, in which case you may need to rebuild or replace your unit.

It may be due to an overheating problem, usually you would see higher H2 from that but you are a little above normal. What was the max oil temp?

Again I am not an expert but it appears you have a serious problem. I recommend you contact a firm that specilizes in this (Like Doble Engineering) to give you a detailed analysis before you replace this unit.

 

[Gauss2k]

The high concentration of ethane, ethylene and methane points toward overheating oil.

But we must be extremely careful when we have only a single analysis like this one.

What is really indicating a trouble or a potential dangerous situation is not the gas itself but the variation of those concentrations.

If those gasses are stable, they can be the result of old faults or problems that would not represent a problem anymore. But if you notice a fast increase in the concentration, then you may have to worry and think about further inspection. IEEE Std C57.104-1991 is very interesting and may help you understand better the results of those DGA.

I would suggest that you take a new sample in one week or one month and see the evolution.

 

[eglaude]

Mjeshtri,

You do need to monitor this transformer closely as the DGA clearly indicates that there appears to be a high temp oil pyrolysis condition occuring. There apprears to be significant overheating as well based on the indicated CO2/CO ratio as well. Also, C2H2 is high indicating that there is possibly some arcing going on. Acetylene is the key gas in an arcing condition. It is present no matter what the concentration is. Again this might be an old wound so trending this is important.

Increase your sampling and analysis frequency to monthly and determine if the results get worse and then you can draw a better conclusions as to what the next plan of action will be on the transformer. More advanced testing from Doble testing is the next step.

 

[unclebob]

I suppose with the power of that unit (400 MVA) that your transformer have a conservator thus allowing air to go in and out of the unit. If that is the case, it is important to sample the oil on a regular basis to establish a trend. Stable
values form DGA don't necessary indicate that a possible fault is now gone (true with sealed transformers), it still can be present. Only a reduction in the gasses values would tell us that the fault is no more.

One other point. The high value of C2H4 Ethylene represent more a tap changer problem than overheated oil. From what I know, overheating produces H2, CH4 Methane and C2H6 Ethane.
Hot spots produces C2H4 Ethylene and arcing C2H2 Acethylene.

If possible, de-energize the unit and rotate the tap changer a couple of times. This will clean all the contacts and may resolve your problem. Continue monitoring the C2H4 by DGA.
If the value is not droping, it may be necessary to by-pass the tap-changer.

Hope it helps...

 

[oramar]

Try an online gas monitor (Morgan Schaffer, GE, etc).
That will help you a lot, as you can get alarms based on trends. Usually, just following the H2 is enough to tell you when the fault develops.

 

[mar888]

we have similar trnasformers with similar DGa values for more than 10 years and everything is OK
please remember that:
A fault is not a failure.

 

[prc]

It looks like a case of transformer oil heating due to poor contact or circulating current through metal parts from leakage flux.This current path is meeting with some loose contacts where arcing will occur and hence the reason for acetylene.Some typical points are oil connection pipe/OLTC support inside tank etc.Most probably you will be able to locate the arcing /heating point by an internal body inspection along with OEM engineer .Verify and ensure that there is no oil leakage from OLTC compart(this can be checked by draining oil from diverter switch chamber and look for oil entry from main transformewr tank )