On the significance of recent EHV transformer failures involving winding resonance

The recent failures of four EHV auto-transformer no load tap changers is interpreted. The failures were a direct and immediate consequence of transmission line faults as far as 340 miles away. The related system transient behavior is analyzed, and the transformer response to these transients is presented on a quantitative basis. It is demonstrated for these transformers that the ANSI standard dielectric surges offer an inadequate test of winding insulation remote from the line terminals where significant winding resonance is known to occur. The extent of these test limitations to other transformers is discussed.