Loading of power transformers

For many years, the maximum temperature power-transformer insulation would withstand was regarded as fixed, and was used as the basis of standard kVA ratings and of overload calculations. Later, the life of the insulation was found to be a thermal-aging process; so the maximum permissible temperature now became a variable with time. The derived aging formulas and the parameters in present use, together with the resulting standard overload values, are reviewed. Practical life studies to set bounds to the permissible temperatures have not been successful; so only relative conclusions can be drawn, and the absolute limit is still unknown. It is, however, clear that quite large margins are available over present-day practice, and some part of these have been incorporated in the more recent loading guides. The desirability of increasing present ratings to utilise these margins is considered. Analysis suggests that any extra rating thus derived cannot be economically realised and is better reserved for emergencies. It is concluded that loading capacity is not the only, or even the major, factor in determining the expectation of life of a transformer. There are other, more exigent, limits. Nevertheless, for practical-operation purposes and to permit systematic planning, a standard loading practice, however arbitrary, must be established. The national loading guides fulfil this function in a relatively simple and practical manner. In particular operating conditions where the loading requirements can be more closely prescribed, it is possible to work directly on the hottest-spot temperature and dispense with some of the orthodox restrictions. The `integrated system transformer¿, to which brief reference is made, is the first practical application of this principle.