The influence of PWM stresses on degradation processes in electrical insulation systems

A special group of exploitation stresses of electrical insulation systems occurs at PWM (Pulse Width Modulated) inverter supply. The fast switching in PWM drive applications, which has fundamental components 50/60 Hz, produces complex transients that stresses the cable and motors insulation. Such conditions have essential influence on inception and development of partial discharges (PD) in insulating systems subjected to non-sinusoidal stimuli. The intensity and dynamics of these processes is a consequence of local working electric field strength, which accelerates the aging processes in the polymer insulation. This paper presents experimental results of application of semi-square impulse voltage on a polymer surface, development of ionization processes and impact of a space charge. The experiments performed in wide frequency range from 0.1 Hz up to 2 kHz, reveal the occurrence of discharges not only when |dV/dt|> 0, but also when the voltage level is constant. PD patterns were acquired during the aging of cable and machine polymer insulation. The main impact in PD mechanism in PWM-fed insulation systems, has a local electric field amplification due to interaction of a space charge with an electrical structure of polymer.