Wilson Power Solutions

Transformer Rating - this can cause some confusion particularly to anyone who has to be an occasional player - occasional player being the person who got the job of ordering that electrical power equipment which is needed for the new site location or modification and who is not, by their own admission, an expert in these matters.

The voltage of the supply, incoming, is usually known, what exactly is to be supplied may not be as clear so a round up of required loading is made. This can be a little difficult: is that machine running all the time ? How does the load vary from idle to operational mode? Are several machines being loaded at any one time? A survey can provide a load utilization factor. Now it is possible to play for safe and add everything together, add a safety factor or two and go with the end result OR you could decide that in some circumstances it is possible to overload the transformer (exceed the stated nameplate rating) for short periods (say).

Unfortunately it is not possible to overload on a permanent basis; or you can ignore the facts of life and do it anyway and the transformer will run over temperature and may be permanently damaged. To apply the latter approach requires some intelligence and skill to apply but can be rewarding IF practical possibilities allow. For example it will allow some money to be saved – the initial buying cost of a lower rated transformer and the general running costs (no load loss) if standing idle but energized for lengthy periods.

IMPORTANT : this approach may not work with load losses as the load loss of a smaller unit running near to nameplate rating may exceed that of a larger more highly rated transformer operating at the same loading.

Some help/guidance can be got by looking at the following std publications IEC 60076 Pt 8 ‘Application Guide’ or more useful IEC 354, BS 7735 ‘Loading of Oil Immersed Power Transformers’ (if your transformers are OIL IMMERSED, look elsewhere if not) or you may obtain some suitable software to do all the hard work for you.

Whatever way you choose it would be good to accept the challenge and save your Coy some money. Just be SURE you understand what the loading is to be, does it have any harmonic content? If so what is the extent of the frequency spectrum and the overall effect on transformer rating. Are there relatively large motors to be started? Are same unloaded or loaded at time of start ? Is there an unsatisfactory p f (low) to be considered ? which may cause a larger than required regulation (volt drop).

So why all this concern about loading and NOT overloading a given transformer?

It’s all to do with the temperature capability of the insulating materials and preserving their longevity – overloading causing high temperature rise/high temperature which can rapidly shorten the life of any equipment and in the worst of circumstances terminate the insulation system almost immediately. SO IT IS IMPORTANT to know what you are about.

How then do you monitor the maximum temperature(s) that are likely to occur ? Ideally place a very small temperature sensing device in a position where the maximum temperature is likely to be found. Unfortunately dealing with oil filled transformers, particularly if working at high voltage, this is not always convenient so that other means of measurement (indirect) have to be employed. Traditionally this would be a Winding Temperature Indicator set up to give a reasonable read out of the average winding or coil temperature. It is normally provided with means to ensure it provides alarm & trip facilities. It can be purely analogue or electronic. The latter is a step forward as it is possible to log temperature variation with load and we begin to have a means of controlling load & time of load by means of temperature. More sophisticated means can be employed IF NEEDED, the principle being to monitor temperature and control the loading (a model can be established) accordingly with the benefit of saving energy and hence costs. It would be interesting to hear of some real life operational experiences where the principle of controlling overload without exceeding given temperature parameters has been used.

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.

The department specifically chosen to introduce 5S into the business has been the previously untouched “Trip-unit” assembly, test and dispatch areas.5S is a Japanese business improvement tool with many variations upon the same theme. Based upon,

1. Set
2. Sort
3. Shine
4. Standardise
5.  Sustain

“Set” is to clearly define the area for improvement, with an agreed work-flow allowing adequate storage and presentation of necessary materials, tools and information required to complete the pre-determined tasks.“Sort” is to remove all unnecessary materials, tools, equipment from the workplace.“Shine” is generally to clean the workplace to an agreed standard.“Standardise” is to agree how materials, tools and work instructions are to be presented and this should be repeated wherever possible at other work-stations within the business.“Sustain” is to create a system of making sure that the agreed standards are maintained, usually with the aid of visual aids and a routine daily checksheet. The team so far consists of Terry Martin and Nicky Winter. They are working extremely hard making sure of “business as usual” while still making dramatic changes to their working environment. Currently they are approximately 50% through the Sort and Shine stages and should soon be able to move onto “Standardising “the first work station.

It’s been a busy time of late in the Sales/engineering offices of Wilson Power Solutions in the last few weeks. But that doesnt stop when here when the phones stop ringing. When the Office doors shut, Anthony replaces his Shirt and Trousers with his Mucky Overalls and moves into the Wilson Power Solutions Workshops to work on the Company F1 Stock Car.

This weeks main project has been to get some paint on the car, to allow it to become available for the Sign Writer to apply the Wilson Power Solutions Decals ASAP.