Failure investigation of vacuum contactor switched medium voltage adjustable speed drives at a pipeline PUMP station

A pipe line pump station experienced failure of the dry-type, 2.5MVA, medium voltage, Adjustable Speed Drive (ASD) Input transformers called ASD-2 and ASD-5, on three different occasions. The authors conducted a failure investigation involving on-site measurements and transformer tests, as well as transient modeling of the system and recommended solution. The key to understanding the failures were high-speed measurements using precision voltage dividers during energization and de-energization staged tests of the ASD transformers. These measurements quantified voltage transients that the transformer experienced during vacuum contactor switching, revealing evidence of prestrike and reigntion previously thought to be associated with vacuum breakers. Supplemental testing included a SFRA (Sweep Frequency Response Analysis) test that determined the natural resonate frequency of the transformer windings which was excited under some switching conditions. Power factor testing quantified the capacitance the transformer windings that was used to build an accurate model of the transformer. Subsequent to the field testing, the Alternative Transients Program (ATP) model was developed and refined to simulate the transient response obtained in the field measurements. The simulations gave excellent correlation to the measured transient response. ATP simulations proved transient mitigation in the form of custom designed RC snubbers at the input of each ASD reduces and/or dampens the risks associated with the transients.