Medium-voltage reduced-voltage autotransformer starter failures-explaining the unexplained

For the past century the autotransformer, or "Korndorfer" starter, has been a standard in the electrical industry. However, for more than the past 30 years the autotransformer starter has been experiencing unexplained "high-voltage stress" failures in typical applications. These failures have been reported on 2400-V starters from South America to 11000-V 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 paper discusses the failure mode and the corrective action that prevents "high-voltage stress" failures.