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.

A query often asked by buyers (or users) goes reasonably as follows:

‘Why use refurbished or partially rebuilt equipment, thereby continuing old technology?

Surely newly designed & manufactured equipment would be more suitable?’ Fair questions & for many new prestige projects the answer would be affirmative. However there are still many situations, taking fitness for purpose & reliability as paramount considerations, and where particular oil filled equipment is already installed,risk assessment(s) in place and maintenance routines well understood & established that the older equipment will perform the required function and the client can save money.

Why should this be?

Historically, in the UK, much equipment purchased and installed by the Public Utilities in the past – C E G B, Scottish Electricity & Area Boards – was fully maintained and underutilized.

Hence when de commissioned at original site, whether oil filled transformer or switchgear, the condition is usually excellent and much operational life remains.

An example : a standard pattern distribution transformer recently acquired, 1000kva, produced 40 years ago by a well known manufacturer located in the east midlands on examination is in pristine condition, virtually no insulation deterioration or loss of life – ready to work for another 20 – 30 years if needed.

Yes the no load loss may have increased slightly but losses are still comparable with modern ‘standard loss’ units & the physical design, particularly HV disc coils (NOT a usual feature with modern units of same rating due to cost) make it inherently mechanically strong. This coupled with an attractive price saving compared to a new unit fulfills a market need particularly if req’d for relatively short time usage. Such units carry a 2 year warranty demonstrating our company’s confidence in these selected items.

Likewise with Switchgear – users with existing sets or suites of oil filled equipment having a long & reliable pedigree of satisfactory operation find the availability of similar/identical equipment attractive. Said equipment(s) are factory checked to original manufacture specifications and items replaced with original manufacturer approved parts if needed (eg current breaking contacts). Back in time it was assumed that ALL refurbishment of oil switchgear would have ceased by the year 2000. In 2008 quantity may have reduced but definite demand still exists & will probably continue unless finally ended by specific dictate from Europe.

Wilson’s ensure that both refurbished transformers & switchgear are routine factory tested to original BS and/or IEC Std. requirements prior to shipment to site. Some American Std requirements can also be accommodated.

Still concerned ? Yes – the Coy does also regularly scrap transformers and switchgear where existing condition does not meet the defined criteria set by the Coy for reliable re use and recovery is un economic. So the Coy advocates the continuing use of equipment where it can be demonstrated it is STILL fit for purpose and norms arising from current legislation and safety can be satisfied.

Why simply destroy every item of existing resource simply to feed the expanding need for scrap metal in China ?

bob bennett.

Since the earliest attempts at utilizing the technology of a c (as opposed to d c), the transforming of voltage from one level to another has been in use.

The economics of moving power from one point (say generation) to another (usage) at higher voltage and lower current have been well understood – smaller x section of conductor etc. So having moved your power to the point of usage it is usually necessary to ensure it is available at the required or a convenient voltage. So devices to transform voltage were required at either end and a manufacturing technology was brought into being.

With production of transformers came the requirement for some insulating medium, not least between turns and particularly to earth, to ensure NO breakdown caused by voltage with consequent equipment failure occurring The original transformers were small, low kva, air cooled insulated with a variety of tapes and shellac or varnish to bind together. Demand for larger thro’put kva soon led to other thoughts and a system using forms of paper and refined oil came into being which in developed, modified form still exists today.

The perceived advantages were

• Various grades of paper were available. Paper can be an absorbent medium and by asorbing, say oil, can assume a di electric factor not unlike oil itself.

• Various fluids were available, crude oil being one such and even then being fractionally distilled to produce a variety of by products.

Specific disadvantages

• Water absorbent, reducing or eliminating insulant properties.

• General deterioration in properties of cellulose (irreversible) and oil (partly treatable).

The establishment of paper and oil as a suitable system of insulation came about by experiment (pure water might have been used, but its properties as an insulating medium are quickly compromised by a low degree of contamination). The other useful property provided by oil is as a cooling medium – oil possessing a reasonable thermal capacity and so usable to assist the cooling of any working transformer by acting as a heat transfer vehicle between the coils of the transformer and the means of dissipating such waste heat from the transformer.

So the paper/oil system came into being as the major technology supporting the manufacture of transformers of most voltages (except the very lowest) and power thro’put (again except the lowest) in use world wide. Since the early days much research has been conducted to improve the long term performance of materials used but the basic concept remains the same even in the 21^st Century. 

Bob Bennett