Specifying and loading cast-resin transformers

The thermal considerations in specifying and loading cast-resin transformers are presented. These considerations are based on conclusions reached from performing thermal tests on a 200 kVA cast-resin transformer with imbedded thermocouples. Specifications should require that the correct insulation temperature class be applied to individual windings and that hottest spot temperatures do not exceed this insulation temperature class at rated self-cooled and forced-air kilovoltamps. Specifications should also require calculation of hottest-spot temperature rises using mathematical models verified by thermal tests on a prototype transformer representative of the design family. The loading back-test method gives higher hottest-spot temperature rises than expected from the short-circuit test method. A method to correct the data to determine the hottest-spot temperature rise using the short-circuit method is presented. A specification form is suggested, and loading equations are presented to allow predictions of loading capability and comparisons of competitive designs