Steep-fronted switching voltage transients and their distribution in motor windings. Part 2: Distribution of steep-fronted switching voltage transients in motor windings

Severe interturn insulation stresses may be produced in motor windings by steep-fronted switching voltage transients. To evaluate the severity of such stresses, transient-voltage distribution tests must be carried out under conditions closely simulating those obtained in service. In a complementary paper, system tests were described which were carried out to determine the characteristics of the transients produced by circuit-breaker closure operations. The steepness and magnitude of the transients were determined and the system parameters defining them evaluated. Based on this information, a series of interturn voltage distribution tests were carried out on a motor winding under conditions closely simulating practical system conditions. From these tests, the magnitude of the interturn voltages in the winding, produced by normal switching operations, were estimated and shown to be severe. The effect that system parameters, external to the motor winding, and also the use of series inductors or shunt capacitors used as surge-protection devices, have on the voltage distribution are also studied. Impulse voltage tests to breakdown were undertaken on a number of the motor coils and the breakdown levels shown to be only marginally above the transient levels produced by normal switching operations. The impulse voltage wave used for these tests was closely correlated to that which might be produced by steep-fronted switching transients.