Case study: What is melting here? Time for an SFRA test

This case study has been generously supplied by the Hitachi Energy Service group in Sweden. A 22 MVA, 10.5 kV/276 V (voltage regulation from 336 to 216 V), Diii furnace transformer, operating in heavy use for many years at a steel melting plant in Europe was subjected to a complete series of tests after a sudden failure during operation. The service contractor evaluated the electrical and dielectric condition of this transformer. During service, this type of transformer has its secondary winding connected externally in a Delta configuration through a high current bridge.