A voltage threshold in operating condition of PWM inverters and its impact on reliability of insulation systems in electrified transport applications

There is a trend in industrial applications and electrified transport towards rising supply voltage and switching modulation frequencies, in order to reach better efficiency and higher specific power of rotating machines. However, the aspect of insulation system design and reliability for the whole chain of the adjustable speed drive, including inverter, cable and rotating machine, is often neglected. The major electric aging mechanism that can bring insulation to failure in short times is partial discharges, which can incept in insulation defects, being triggered by local electric stress. Understanding PD phenomenology in insulation fed by PWM is, therefore, a must to ensure adequate life to the insulation chain. This paper shows the existence of a peculiar PD behavior, associated with a threshold voltage below and above which the PD regime may change significantly in terms of amplitude and repetition rate. Inverter behavior under PD is modelled, as a function of the number of inverter levels, and the validity of the model is tested through PD and accelerated life testing. Good fitting to experimental results is obtained using twisted pairs test objects, with striking evidence about the effects on life of operating below or above the threshold on insulation reliability.