Transformer Rating - Its the loading that confuses.
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.