Sometimes, just occasionally I am asked ‘as an engineer why did you become involved in transformers, after all they really are very boring – just sitting there doing nothing apart from a constant hum supplying power’. Well I can agree, most of the time it is or seems that way. However sometimes exciting things do happen and you are NEVER prepared for the inconvenience – particularly when a transformer fails mechanically due to short circuit.This results in scrambled windings (very quickly) and very occasionally with an oil filled unit if the tank is weak and splits (at a weld) releasing insulating oil a fire may be the result. In this case much of the evidence which is combustible disappears and the fire also consumes whatever is in the locality of the transformer. Definitely a scenario to be avoided. The Coy has just examined a transformer, rated at 800kva, recently returned from site due to possible mechanical failure under short circuit conditions. How the transformer was short circuited is an open question still to be considered but the results are interesting – see the reproduced photos. 
As can be seen A phase coils are somewhat of a mess - the outer half (4 layers) of the HV coil has compressed against the top yoke of the transformer, likewise the inner lv (not really seen in photos) has also moved in an upwards direction. Meanwhile the inner part of the HV coil has compressed itself against the bottom core yoke and clamp frames. The end packing blocks, made from natural hard wood have come away completely – splitting & breaking up thereby eliminating the physical support essential to maintaining the integrity of the transformer ( interesting question, how can you predict and control the properties of natural wood ? unless subject to a rigorous selection programme. What happened to industrially produced laminated wood which has guaranteed minimum mechanical characteristics? Simple - probably more expensive to purchase, but where was protection for the client, the user ?A visit to B phase is also interesting because things are happening here – the top set of blocks has again gone, the lv coil has moved up to the underside of the top yoke BUT the HV coil has not yet moved. Also the bottom packing /support blocks are still in position although one has already split thro’ its overall thickness ! 
Meanwhile at C phase nothing has really happened, the coil combination has not moved and the top and bottom pack blocks are entire, all still in place.So in 3 phases we progress from the apparently normal state with HV/lv in proper order (C phase) to complete disaster (A phase). Other items of interest are: movement within the core structure, possible opening of mitred core joint at bottom of A limb . Current arc activity at OCS moving contacts for A & B phase some movement and distortion of clamp frames particularly adjacent bottom yoke. This particular transformer is now fit only for scrap; the cost of recovery/repair would be too high and so will be replaced by a new transformer of similar rating. Fortunately no one was injured, there was no fire – the user only suffered the inconvenience of supply loss for a day or two. Sometimes the outcome can be different, somebody gets hurt or worse, there is a fire, a business is not properly insured so may disappear and people are put out of work. So be warned! That grey box which looks so dull and uninteresting may one day briefly develop a life of its own and cause trouble with an exciting failure. If it is a short circuit you now have some idea what to expect and what needs to be done to get you going again, so its as well to be prepared. If you have been reading, please make comment if you feel so inclined – we all learn in the process.
This entry was posted
on Thursday, May 15th, 2008 at 9:55 am and is filed under Misc, Transformer Technology.
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