An experience with transformer connections and relay settings for temporary generator step up unit

As the world becomes more globalized, incidences occur where devices designed within different standards systems must be used together. This case study illustrates the problem, and concerns a replacement transformer. A 67 MVA, 90 kV GSU transformer failed, with no protective relay operation and with an unknown cause. The cause of failure was investigated by means of a teardown on site. The replacement transformer had different winding configurations and MVA ratings. The purpose of the study was to determine appropriate transformer connections and new relay settings to protect the replacement transformer.