End Winding Protections Improvements Helping To Downsize dV/dt Filter For Medium Voltage Machine Fed By NPP Multi-Levels VSI Drive

In recent years, a growth trend is evident in power electronics from Load Commutated Inverter to Voltage Source Inverter drives based on Neutral Point Piloted 5-levels topology up to 15kV using fast switching devices for better efficiency and compactness. However, a dV/dt filter is required to avoid fast insulation degradation of the motor when exposed to rapid rise repetitive impulses voltage from the Drive. Especially in the ending winding region where preeminent electrical stress is localized toward the slot exit with fast rise time of pulsed voltage. For dV/dt above 3kV/us, traditional corona protection method using conventional stress grading tape is no longer sufficient to prevent the surface discharge and excessive heating at the slot exit. Numerical simulation established in multi-physics finite element analysis indicates that electrical stress distribution and power dissipation at the end winding is closely related to the rise time of drive pulses and stress grading materials properties. New corona protection method is developed based on modeling results and validated on real stator bars under fast pulses. This paper will describe the typical dV/dt patterns generated by those multilevels drives impacting the dV/dt filter and the system of insulation of medium voltage machine. It will also discuss the effectiveness of end winding surface discharge prevention by proper stress grading design and materials selection under high stresses of dV/dt.