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Pole Transformer Tank Rust.

Zoomup
I noticed a grey painted pole mounted transformer today. The cooling oil tank and one side of the transformer was very rusty. Just what is the metal thickness of the cases of these transformers? When will it rust through and spill its cooling oil?


Z.
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    They are quite thick - few mm, they have to be for the weight of the oil and the transformer.


    This ABB spec suggests a minimum of at least 2 or 2.5mm for stainless or mild steel respectively.
    DNO specs may add additional things, like a sight glass window that allows oil to be checked with binoculars.


    so how fast will it corrode. Well it all depends on the environment. 

    If we said C3 medium, clean air periodic light dew condensation then 25-50um per year

    1mm = 1000um ,so 1mm every 20 -40 years.

    I suspect the DNO inspect it by then.

    But if it was  in a very high corrosion environment, like by the seaside or in the chlorinated atmosphere of a swimming pool vent, then it could go ten times faster..

    You can also see why galvanizing with zinc helps, and how corrosion of the steels really takes off once the protective coat of zinc is depleted. (typical galvanising is 85um nominal - UK map typical corrosion estimator by postcode)

    2b848ab5a0f7827ba4803bb795cb6a91-original-corrosion.png


    Mike
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    I had a similar conversation with the MOT tester about the thickness of the LPG tank on my pickup, the rust became an advisory and I don’t run it on gas anymore.
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    So I suppose that the transformer normally runs quite cool up on its pole. The cooing oil may not be essential if it leaks out and the transformer is lightly loaded. Or is it essential for insulation as well as cooling?


    Z.
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    It is very rare for them to rust enough to leak. If it does leak, it might go up in flames inside, but not much change outside except popping the HV protection. Modern oil is not too environmentally dangerous. The thickness is at least 3mm probably more.
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    Zoomup:

    So I suppose that the transformer normally runs quite cool up on its pole. The cooing oil may not be essential if it leaks out and the transformer is lightly loaded. Or is it essential for insulation as well as cooling?


    Z.


    It may well work dry for a while, but yes it will overheat, a lot, with what should be normal loads, the oil is a large part of the cooling ability. The other problem is if oil can get out then damp air and then condensation is getting in,  even new transformers are enamelled wire (think varnish)  and oily paper, and water is not your friend.

    Mike


    PS Dry type transformers do exist, usually epoxy filled  but for the same power are larger heavier units,more prone to overheating if they experience overload. As a result, they have higher electrical losses, and it is more expensive to run over time. Oil units are smaller and more efficient. They require less copper and create a smaller environmental footprint .


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    My uncle used to work for Shellmex and delivered transformer oil to a few power stations  he told be that delivering a tank of oil used to take 3 days due to not being able to allow air in as would normally happen was a very slow process then when the TX was full the engineers would switch on occasionally there would be a bang and my uncle would have to take the oil out again returning it to Shellmex for drying and cleaning. One of the power stations he would deliver to was Hastings   he ended up living in Ore village near Hastings years later

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