Steep-fronted switching voltage transients and their distribution in motor windings. Part 1: System measurements of steep-fronted switching voltage transients

Insulation failure in motor windings can be caused by interturn breakdown resulting from stresses generated by steep-fronted transient voltages. The production and severity of such transients and degree of voltage stress on the motor-winding insulation is governed by the characteristics of the overall system to which the motor is connected. The investigations described in this paper cover extensive tests carried out at two large offshore oil production/water injection installations on a number of high-voltage motor circuits where motor failures had occurred. In particular, the tests were made to determine not only the nature and severity of the switching transients but also to determine the system characteristics governing their steepness and amplitude. For this purpose, the test programme included circuit-breaker timing tests, recurrent surge tests and synthetic tests in addition to full energisation tests. The results obtained and presented indicate that although, in all the circuits studied, steep-fronted transients are produced - in certain circuits, the amplitude of the transient would be sufficient to cause severe interturn insulation stresses in the motor winding. The system parameters that define the steepness of the transients and the combination of parameters that give rise to high-amplitude transients are evaluated. Tests with capacitors connected to the motor-winding terminals, to extend the wavefront duration of the transient voltages reaching the motor winding, are also described and discussed.