Investigating the Effect of Multilevel Inverters on Motor Stator Insulation Stress

This paper investigates the dielectric stress and leakage current density as seen by medium-voltage motor stator ground-wall insulation when the stator winding is excited by 3-, 5-, and 7-level multilevel inverter output waveforms. Effect of firing angle on insulation stress is also studied for a 3-level multilevel inverter. Results show that in addition to the number of output voltage levels, PWM wave shape is also important. Both these factors work together to aid or inhibit the effect on the insulation health. Strong correlation is shown between the increase in the voltage RMS value and increase in dielectric stress when ignoring the dv/dt impact for a fixed DC input voltage and operating frequency of the inverter. Similarly, the dielectric stress in the stator insulation increased with an increase in firing angle, since larger firing angles led to higher RMS voltage values.