ECN Forum Forums General Topics of the Trade Non-US Electrical Systems & Trades 11kv Transformer failed test.

Hi,

At work we own a 1mw 11kv - 400v transformer, it is about 3 years old.

We have had a test done on it and they came back with urgent action required, red crosses etc.

It has failed the oil analysis due to "considerable fibre content" and the resistivity is 1100.10 giga ohm.m (not sure what an ohm.m is) They said it might explode! I always like to know what I am on about, does anyone have experience of such things? it seems a bit young for this. I am concerned about it but also concerned that they might be exaggerating it somewhat.

Any insight would be appriciated.

Thanks

Please be a bit clearer with your 1100.10 giga ohm.

If you mean 1100 Giga ohms or 1.1 Tera ohms, that is an excellent reading.

A transformer can be tested for its insulation resistance at the HT side, LT side and between HT and LT windings.

Generally these values depend upon the age, overloading, poorly ventilated transformer room.
Most 11 kV distribution transformers in New Zealand are tested at 2.5 kV dc at the primary and values of 500 Mega ohms to high 100's of Giga ohms can be measured. (100 kVA to 2 MVA)

You didn't say how heavily the transformer is overloaded in the last 2 or 3 years.

Has their any load survey been done on it?
Overload WILL destroy a transformer in a reasonable short time.

Transformer oil will gradually deteriorate and parts of varnish from the windings will discolour the oil and the acidity level will go up.
Deterioration of the paper between winding layers will occur too and add to the process.
If an arc occurs between windings and no Buchholtz* protection is in place the transformer will burn out.
The best practice is to get an oil sample taken from the bottom tap and analysed by an other oil lab and await their results.

I have done a fair amount of oil samples on 10 and 20 MVA 33/11kV substation transformers.
When taking oil samples it is very important to work clean !! First drain about 1/2 litre of oil from TX before taking sample. It should be done via a clean plastic hose into a clean type of glass seringe with T connector to only allowe oil and NO air in the sample. The air will add to possible poor readings. Also a 1/2 litre bottle sample is taken for analysis.

* Buchholtz protection is generally used in substation transformers and will pick up gasses caused by arcing in the TX. If these are small amounts they will be collected in a small reservoir up to a certain level 500 mL, then a trip will occur. If a large arc or explosion happens the Buchholtz relay will act immidiately and trip the primary and secondary of the TX to avoid backfeeding.

Hi,

I wasnt particularly clear. These results are from an oil sample from the bottom tap. The resistivity line is:

Resistivity (giga ohm.m @20 degrees C) 1100.10 - acceptable range 4-60.

and it just sais considerable fibre content. The transformer is loaded about 1000-1200A per phase 12hours /day and 800A remainder.

There were no insulation resistance readings taken at the transformer. It is sited outside in a cage, it does get nice and warm, my guess would be about 50C when fully loaded ambient 19C. We dont have Buchholtz protection. When he took the sample, he left with 2 bottles and replaced the breather.

Thanks

Ok i get it now, that reading was from the oil.
I thought it was from the windings.

I check at work what it exactly means. I take the occasional oil samples but these get sent off to the lab and rarely get to see the results. Usually whe a TX passes, it's fine, no further enquiries needed.

The 1200 Amps loading is well within it's 1000 kVA capacity, no problem there.

I would suggest to do some insulation tests on the HV winding especially, with cables disconnected.

It doesn't sound good with considerable fibre content in the oil. Something is defenitely deteriorating there, or the tank was dirty prior to assembly of the core.

Any signs of carbon traces in the oil ?

To throw this into the pot.
The unit for resistivity is the Ohm-metre, based upon a cubic metre of the conductive or insulator material.
As you mentioned Ray, this sounds like a pretty good reading.
However, the fibre content of the test does sound a tad worrying.
What the fibre content refers to is the amount of things in the oil sediment that should not be there in a healthy transformer.
This could be things like water (moisture), bits of insulating enamel varnish from the windings, bits of insulating materials from between the HV and LV windings.
Now, most transformers will handle short term overloads, but having a continuous overload on any given transformer is both stupid and unsafe.
Bear in mind that resistivity is temperature based, as the temperature goes up, the resistivity of the oil will drop, opening up the idea that a winding to case flash-over could occur internally, especially from the HV winding to Earth (the case of the transformer).

Hi,

The report states that there is no sediment As far as carbon the only reference is for:
CO: 14 microlitres/litre
CO2: 1219 microlitre/litre.
It then states that oxides of carbon gases are consistent with normal ageing.

The breather was discoloured and replaced but I dont know why. The summary said that all requirements are met apart from the heavy fibre content it fails the electric strength test. Smaller at the bottom it states that it is satisfactory for continued service but requires an oil change.

Johnno,
A blocked breather in a HV transformer is like having a blocked exhaust in a truck while it is running.