Requirements for insulation system of motors fed by modern voltage source converters

Insulation system of motors fed by modern voltage source converters (VSC) is subjected to higher dielectric stress. High voltage gradients and transient overvoltages that occur in each converter switching point dramatically decrease motor lifetime. This paper investigates motor overvoltages, their impact on motor insulation and also explores relationship between converter control strategy (converter output voltage pattern) and occurring surges. Obtained results are compared with existing standards (NEMA MG1-1998 (Rev. 3: 2002) and IEC/TS 60034-17 (3rd Edition)) in order to review capability of employed motor insulations. Moreover, the paper discusses breakdown locations within the stator winding insulation and describes principles of these failures. The aim of this paper is to present general theory of motor transient overvoltages, because general theory of these effects is still missing in the literature, and to provide useful approach enabling evaluation of the reliability of the drive in practice. A lot of experiments have been performed on industrial and especially traction drives. Presented results and theories are confirmed by tests performed on the main drive of light rail vehicle recently developed in Skoda Electric Company.