New IEC standards for qualifying stator insulation systems for PWM converter drives

Premature failure is sometimes suffered by stator insulation systems of variable speed motors driven by modern voltage source pulse width modulated (PWM) converters. The failure mechanism in random wound stators is generally thermal. Form wound stators fail through dielectric loss-induced heating, rapid deterioration of stress control coatings and partial discharge (PD). The International Electrotechnical Commission (IEC) has developed two new documents to provide guidance on qualification of stator insulation systems subject to voltage surges from PWM drives. IEC 60034-18-41 discusses the qualification and acceptance tests for random windings not expected to see PD during normal operation; it was published in 2006. IEC 60034-8-42 covers form-wound stators, whose insulation design is expected to withstand PD in operation. The latter document is still under development and near completion. This paper describes the main features of one of the most important new machine insulation standards to be issued in many years.