Medium voltage switching transient induced potential transformer failures; prediction, measurement and practical solutions

During commissioning of a large data center, while switching medium voltage circuit breakers without any appreciable load, several potential transformers failed catastrophically. A detailed investigation, including computer simulation was performed. Ferroresonance produced by switching transients associated with opening and closing the vacuum breakers, was determined to be the cause. The analysis also determined that the close-coupled power transformers were also in jeopardy. Field inspections involving grounding improvements coupled with solution simulations were made. High speed switching transient measurements were performed to verify the analysis and the surge protective devices solution (arresters and snubbers). This paper walks the reader through problem recognition, simulation, field measurements and solution implementation. Special focus will be made on the field measurements verification.