2MVA dry transformer moisture issue - future reliability?

[bam55]

I have a 30 year old 2 MVA dry type transformer that was sitting for 4 years resulting in moisture in the insulation.

It was tested and failed the megger test indicating moisture. I would like to dry out the windings and retest but is it worth it? What is the prediction of reliability of the machine under say 1/2 load over the next 5 years?

Anyone have direct experience in such an issue?

Thanks,

 

[RRaghunath]

I think it should be good.

You can keep the transformer charged, if possible at lower voltage than rated for a few days. This helps drive the moisture out (that would have gone deep in to the insulation) and improve the IR value.

 

[edison123]

A short circuit is a good way to dry out without involving high voltages

 

[alehman]

What edison said... If you energize with rated voltage when the insulation is moist, it may fail. Short the windings on one side and apply only enough voltage on the other to get a signifcant percentage of rated current. Let it sit that way for several days and re-test.

 

[RRaghunath]

I agree. I missed elaborating that in my mail.

 

[BJC]

You can connect a low voltage load through the windings and let it set until it drys out ( if it's ever going to).
By low voltage I mean a load such as a few incandescent light bulbs. The exact configuration will depend on what windings you can get at.
On the low voltage side wire L1 from a 120 volt source to X1 then from X0 to three 150 watt incandescent bulbs and then back to the neutral of the 120 volt source.
You can vary the connections such that you put a moderate amount of heat on it for a longer period of time.
Also arrange the bulbs so they heat the transformer.
After all that let it "cook" a couple of weeks.
I would find a panel with a three phase 40 amp breaker and run a branch circuit to the transformer where I would connect the drying circuits. The last time I did this GFIs were not available but I would use them now.

 

[electricpete]

I'm not sure I understand the configuration described by BJC. Perhaps the HV bushings need to be shorted together to reduce the impedance presented at the lv windings? Also beware of stepped up voltages on hv side when applying voltage to lv side.

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[electricpete]

I have never been involved in field-drying a dry transformer.

For induction motor dryouts we have found a dc welding machine does a better job than an ac welding machine due to relatively high inductance of the motor. May not be necessary for transformer where you can short out or load the opposite winding. Also keep any eye on your temperature indicators. Allow air flow preferably dry/heated to assist moving the moisture out of the transformer. If the moisture is deeply embedded I don't know if you will be able to effectively dry it out. (Others have experience?)

For oil transformers I'm sure you know vacuum dryout is typically condcuted but I don't think that's an option in-place because you don't have an air-tight transformer cubicle. Surely if you remove the transformer to a repair shop they can do a complete dryout with vacuum and heating.

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[edison123]

Look up the nameplate for the trafo impedance. (probably around 6% for 2 MVA). That is the percentage of rated voltage to be applied to one side (normally HV) with the other side shorted (normally LV) to pass the rated current in both the windings.

Continous temperature vs IR is to be noted. A bathtub curve is used to determine the point of final dry out.

 

[bam55]

Thanks for all the fairly detailed information. I checked on the inspection report and it says there are cracks in the varnish on the insulation. I know that the history has not been severe, only one secondary breaker failing due to a short/surge. So we plan to send the transformer to a shop where it can be cleaned, dried at low heat, retested, tightened and revarnished.

Regards,

 

[BJC]

ElectricPete
I wasn't trying to give a detailed connection diagram. It depends on the transfromer. The ideal was just to hook up some load through the transfromer. A bank of 120 V or 240 light fixtures, some resistance heaters etc.
The ideal is to have some low level of current through the windings and some heat outside the transfromer.
I forgot to mention a cover. Use some kind of flameproof cover to "tent" the transfromer and let the heat exit at the top. This may take a couple of weeks ( after all it took 10 years to get in it's present condition).
I don't know if anyone told Bam55 but rodent damage is a real posiblility. Drying out won't help if the pesky things have been gnawing on the insulation.