SPECIFYING AND LOADING CAST-RESIN TRANSFORMERS

The cast-resin dry-type transformer began to be widely used in the Uni ted States in the 1980's. This transformer design was incorporated int o the first IEEE standard in 1989. The thermal considerations in speci fying and loading cast-resin transformers are presented in this paper. These considerations are based on conclusions reached from performing thermal tests on a 200-kVA cast-resin transformer with imbedded therm ocouples. Specifications should require that the correct insulation te mperature class is applied to individual windings and that hottest spo t temperatures do not exceed this insulation temperature class at rate d self-cooled and forced-air kilovoltamps. Specifications should requi re calculation of hottest-spot temperature rises using mathematical mo dels verified by thermal tests on a prototype transformer representati ve of the design family. The loading back-test method gives higher hot test-spot temperature rises than expected from the short-circuit test method. A method to correct the data to determine the hottest-spot tem perature rise using the short-circuit method is presented. A specifica tion form is suggested, and loading equations are presented to allow p redictions of loading capability and comparisons of competitive design s.