Medium voltage reduced voltage autotransformer starter failures -explaining the unexplained

For the past century the autotransformer, or "Komdorfer" starter has been a standard in the electrical industry. However for more than the past thirty years the autotransformer starter has been experiencing unexplained "high voltage stress" failures in typical applications. These failures have been reported on 2,400-volt starters from South America to 11,000-volt starters in the North Sea. In most cases the zero tap or the turns close to the zero tap are involved in the failure. This work discusses the failure mode and the corrective action that prevents "high voltage stress" failures.