Turn Insulation Capability of Large AC Motors Part 1 - Surge Monitoring

Failure of many large ac motors in generating stations is caused by failure of the turn insulation in the stator winding. Fast risetime surges resulting from circuit breaker switching operations can cause severe voltage stress on the turn insulation due to the non-uniform distribution of voltage across the winding. In order to characterize the surge environment as seen by typical utility motors in normal service, an extensive program of surge monitoring has been undertaken. This work was done as one part of an EPRI funded project to investigate the turn insulation capability of large ac machines. The results of monitoring switching surges for 26 motors controlled by air magnetic circuit breakers and 7 motors controlled by vacuum switchgear in 11 utilities are presented. No significant surges were ever detected during circuit breaker opening operations. Surges with magnitudes as high as 4.6 pu were measured during normal motor starting operations. Tests on a motor switched by an air magnetic circuit breaker, and then a vacuum contactor, showed no significant differences in surge risetimes and magnitudes as recorded at the motor terminals. However vacuum switchgear does give rise to a large number of steep fronted transients for each motor closing operation. Some correlation was found between motor feeder cable types and switching surge characteristics.