Evaluation of Transformer Components for High Temperature Transformers

The use of high temperature transformers, previously limited to special applications, is now being considered for distribution networks. Besides the evaluation of the solid and liquid insulation, it is essential to verify all components of the transformer exposed to temperatures higher than conventional. Sealing gaskets, de-energized tap changers, polymeric bushings and pressure relief valves are some of the items considered critical to be tested. Two standard transformer sizes, of 37.5 kVA and 50 kVA, filled with natural ester liquid, were subjected to extreme overload conditions. The transformers were loaded at 170% of nominal rating during three separate cycles, lasting for 4h, 8h and, finally, 24h. An additional cycle was applied, targeting to reach a hotspot temperature of 180°C, which required 240% of nameplate rating for the 50 kVA and 223% for the 37.5 kVA. Temperatures above 125°C and 160°C were reached respectively for top liquid and average winding. A complete teardown of the transformers was performed, followed by the evaluation of the components. Despite the high temperatures reached there was no evidence of any damage internally or externally except for the wrinkling of external stick-on tank labels.