Oxygen in Transformer oil

[power2020]

Dear Folks,
Pl. explain me the significance of oxygen in transformer oil for both Bagged type conservator/ open type of conservator.

Advanced Thanks...

 

[Zogzog]

Oxidation

 

[Marmite]

Oxygen is available both from the air in contact with the oil surface and to a lesser extent from air dissolved in the oil. The only exceptions to this are transformers where the oil surface is blanketed with an inert gas, such as nitrogen. Temperature is a very important factor. The oxidation of a modern transformer oil at normal ambient temperature is negligible. However, the rate at which oxidation takes place almost doubles for each 8-10 deg C temperature rise.
regards
Marmite

 

[oldfieldguy]

If your transformer is sealed, gas-blanketed and vacuum filled, then the presence of oxygen in the first test after the filling indicates improper procedures in filling. If it is a number that has risen over subsequent tests, then your transformer is "breathing" usually due to a leak.

If yours is the second case, you're also adding moisture to your transformer as the moisture in the outside air gets into the transformer.


old field guy

 

[subtech]

Agree with oldfieldguy.
Additionally, O2 occurs as a result of the aging and degradation of paper insulation.
If the transformer is of relatively recent manufacture (6-10 yrs.) your oil may contain an O2 inhibitor which in effect "absorbs" oxygen and keeps the numbers relatively low. As the inhibitor saturates the O2 numbers will elevate. It is wise to have the inhibitor level checked.
I'm not sure of the exact numbers, but it seems that a level of .08% is considered to be the level at which the inhibitor is completely ineffective. (A reputable lab can verify that). If the inhibitor is forgotten, the added O2 in the oil will help the paper insulation degrade even faster.

 

[electricpete]

Good point about O2 as a byproduct.

It is well known that three factors contribute to lots of oxidation reactions such as insulation aging: temperature, water and oxygen.

An interesting illustration:

We had generator stepup transformers that were very regularly disassembled and reassembled almost every 18 months for a variety of reasons (low voltage bushing leaks for example). Don't ask me why. At our nuke plant that's the way we did things being very conservative to address even small problems. The frequent disassembly seemed to lead to more leaks and things and was self-perpetuating since we had a low toelrance for leaks.

After several drains and refills, the CO and CO2 usually grew high (in the range 1000ppm CO and 10,000ppm CO2) and came back quickly after oil drain and vacuum processing (indicates insulation aging). We chalked it up to the sustained high temperatures. Also we had quite high oxygen routinely 10,000 ppm - 20,000 ppm. Moisture not so bad 15 - 30 ppm at typical transformer oil temperatures around 70C-90C.

We had a strange event where the transformer started burping oil out of a relief. Investigation revealed the dresser couplings at the suction of the oil pumps were sucking air in like a seive and this condition existed on all those transformers. We implemented a change to a different accordion style flange that was bolted at each end and a much better seal.

The unexpected benefit: O2 went down towards much lower levels perhaps 500ppm - 2000ppm. Moisture may have went down a little. The CO and CO2 pattern went down dramatically CO2 now doesn't get much above 1,000 (even though temperatures are the same). Seems like pretty clear proof that the moisture and the oxygen contribute to that oxidation

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[electricpete]

I messed up the punchlines. Let me try again the last paragraph:

The expected benefit: O2 went down towards much lower levels perhaps 500ppm - 2000ppm. Moisture may have went down a little.

The unexpected benefit:The CO and CO2 pattern went down dramatically CO2 now doesn't get much above 1,000 (even though temperatures are the same). Seems like pretty clear proof that the moisture and the oxygen contribute to that oxidation

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[power2020]

Thank you for all replies.
Mr.Subtech :
Additionally, O2 occurs as a result of the aging and degradation of paper insulation: In this condition, if active oxidisation inhibitor is available, O2 will be absorbed. Then how can aging and degradation of paper insulation be noticed?

 

[prc]

Some times, esp if the oil is highly refined type,oxidation inhibitors are added(not o2 inhibitors as subtech mentions) to oil to the extent of 0.3-0.4 % by weight to get better oxidation stability( ie ageing).Normal inhibitor added is DPBC (Ditertiary butyl crysol) which gets depleted in service. I dont thing this is due to reaction or absorbtion with oxygen.When DPBC level reaches about 0.1 % or less, oxidation accelerates to a high level and inhibitor is to be added.

Once it was thought that O2 is the major factor in ageing of oil and transformer insulation.Today it is understood that its effect is much less than that of water and temperature in paper ageing.

It should me remembered that bag or bladder will not prevent 100 % sealing as it is permeable to some extent.Definetely it will reduce O2 and water ingress.But it will also prevent breathing out of water from paper oxidation.

Peter's experience is interesting. I believe the excess ageing would be due to the high level of moisture sucked in rather than the high level of O2 entered in to system.Is this transformer N2 sealed type or free breathing type? O2 level may have come down as no more air is dissolving in to oil.Water level in oil may not have gone up.But definetely water level in paper must have goneup.Remember, in transformers 99 % of total water is in paper and only 1 % by weight is remaining in oil.

One disturbing phenomenon noticed during last five years is the transformer failures due to corrosive sulphur from oil.This is noticed more where o2 level (ie bagged type consrvator)is low.Reason for this is not yet known.

 

[power2020]

In all our GSU transformers have this corrosive sulphur in oil problem however all are free breathing type and Oxygen levels are approx. from 7000 to 10000 ppm. Waiting for OEM advise.
How to find moisture in Insulation paper? Is there any co-relation with the moisture quantity in oil?

 

[Yuma]

The following document can be helpful:
IEEE Std C57.104 guide for interpretation of gases generated in oil-inmersed transformers.

 

[electricpete]

"How to find moisture in Insulation paper? Is there any co-relation with the moisture quantity in oil?"

That is a tricky one. The bulk of the moisture is stored in the paper. After a period of time the oil comes into equilibrium with the moisture content of paper surface. Higher oil temperature causes higher equilibrium moisture in oil for same moisture in paper. There are charts you can use to estimate water in paper from water in oil but assumes equilibrium.

For off-line tests, as a rough rule the % power factor during insulation test is equal to percent moisture of the paper. Also dew point test with transformer drained can be used to estimate surface moisture of the paper.


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[power2020]

Thank you Mr.Electricpete for your reply.
"dew point test with transformer drained can be used to estimate surface moisture of the paper" - Shall we need to carry dew point test of residue oil from the tank after draining? Pl. eloborate it.

Thanks...

 

[electricpete]

Going from memory (I haven't worked on transformers in 7 years), the procedure for dew point test is to put dry air into the drained transformer and allow it to come to equilibrium (something like 24 hours). Then draw a sample of the air and analyse it for moisture content which can be characterized by temperature and dewpoint (we used a device called an Alnor). Then use charts to convert that into an estimate of moisture in the paper insulation. I'll see if I can find some more info.

In the meantime take a look at figure 22 on page 65/87 and figure 23 on page 66/87 - one means for estimating paper moisture from oil moisture (again dependent on equilibrium)

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

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[electricpete]

on the dew point test - it may have been N2 instead of dry air. Let me check on that...

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[electricpete]

Dew Point Procedure from EPRI 1002913

8.4 Taking a Dew Point Measurement of the Main Tank
After the leak test is complete and at the end of vacuum hold time of four to six hours, the next
steps should be followed:
• Break the vacuum with dry air or nitrogen and apply a positive pressure of two to three
pounds. The dry air or nitrogen should have a dew point of -50ºC (-70ºF) or lower.
• Allow the transformer to sit idle for 24 hours in order to allow the gas and the surface
moisture of the insulation to approach equilibrium.
• Measure the dew point after a 24-hour hold time. Determine if the dew point measurement is
acceptable. The graph in Figure 8-1 may be used as a guide.
In order to use the graph that follows, the insulation temperature must be estimated. The best
estimate would be to use the temperature of the oil from the oil temperature gauge. The reading
should be made when the oil temperature is steady and should be taken early in the day before
the sun has had an effect on the oil and tank temperatures.
If the dew point is found acceptable, the transformer can be vacuum oil filled. If the dew point
reading is found unacceptable, a dry-out procedure must be followed before proceeding with oil
filling.

Figure 8-1 provides a linear graph:
Insulation temp 10C => max dewpoint is -40C
Insulation temp 50C => max dewpoint is -15C



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[electricpete]

There is a note that the chart applies to a pressure range of 1 - 3 psi. I interpret that to be psig.

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[slavag]

Hi.
Please see attached links, maybe it will be intresting for you

http://library.abb.com/GLOBAL/SCOT/...0661879/$File/1LAB000057_EternlYthApril05.pdf

http://www.abb.com/cawp/seitp202/4346b9c80edf4200c125727b002cd92e.aspx

 

[power2020]

Thanks for all your replies.
For BDV check of transformer oil, shall we need to consider the oil temp? As mentioned in the above posting "Higher oil temperature causes higher equilibrium moisture in oil for same moisture in paper" and if temp drops, again insulation will absorb the moisture from transformer oil.
Advanced thanks...

 

[prc]

electricpete-Thank you for putting up the EPRI instruction.But I believe some clarifictaion is required.How oil temperature can be taken when tank is filled with dry air ?Probably it meant that OTI reading may be taken as it reflects the temp of the gas inside the trf. Am I right?

Values by EPRI shows a surface moisture of 1.25 %.Normally in new transformers manufacturers try to limit to less than 0.8 %.ABB transformer installtion manuals give a graph connecting moisture content vs insulation temp vs dew point.
Dew point measurement will only give surface moisture content and may not reflect the actual moisture inside the insulation structure.
USBR graphs have been modified several times and the latest that one can refer to is fig A3 as given in IEC TS 60076-14 Technical Specifications for Design &Application of liquid immersed trfs using high temp insulation materials.Limitations of these graphs are given in IEEE draft std PC 57.106 D6.
A generation of enginners used Alnor, but today much better meters are available (eg Vaishala)for dew point measurement.
To the question how moisture content in paper of a working trf can be found- answer,very difficult,may be impossible.But many solutins are tried but with its own limitations.Latest in the field is PDC measurement ie measurement of polarisation and depolarisation current under a small DC voltage and then use computer models to estimate water content in paper.
electric pete- do you remember the nuclear trf you mentioned was with membrane (bag)inside the conservator or not?or N2 sealed.USBR maintenance manual (page 52 &53) gives some indication of CO2 values in oil and how leaks can afffect it.

BDV of oil will not change with temperature.It is more affected by particle content and to some extent by moisture content.Moisture content in working trf will vary with temperature,but it can vary only say 5-40 ppm.