EVALUATION OF TRANSFORMER PERFORMANCE ABOVE NAMEPLATE RATING

Abstract

The rating of a transformer depends on the maximum temperature which its insulation can withstand without damage. Thermal aging of insulation materials is due to changes in the mechanical, electrical and chemical properties of the materials as a result of chemical reaction induced by high temperature. The life of the transformer insulation system depends mostly on its mechanical strength. If the changes proceed to the extent that the insulating materials can no longer w:thstand the stresses encountered in an emergency situation, the transformer will fail. The temperature of importance is that at the hot-spot in the insulation. The temperature of transformer insulation under varying load does not change instantaneously owing to thermal capacity of the transformer, and its relation to the life of the transformer can be derived from Arrhenius Law of chemical reaction. Computation on the hot-spot temperature and the associated rate of use of transformer life are discussed in this dissertation. The theory is then applied to an investigation on the design of transformer installation policy in distribution substations used by some utilities. A value of 200 kVA has been specified by some utilities, including Singapore's, as a cutoff' value below which no transformer needs to be installed in a substation. Based on the prevailing costs in Singapore, it is found that a value of 400 kVA would be more economical with a saving in cost of some $130 000 per annum.