Partial discharge phase and amplitude distribution and life of insulation systems fed with multilevel inverters

This paper presents an investigation on the effect of the number of levels and voltage magnitude of a multilevel power electronic supply on reliability of insulation systems. Particular attention is given to partial discharge, PD, behavior, referring to organic (Type I) insulating materials used for rotating machines fed by variable source inverters, VSI. As described in standard IEC 60034-18-41 and 42, repetitive voltage impulses, as those generated by pulse-width modulation, PWM, supply, can incept PD due to uneven voltage distribution in windings and transient overvoltage, which would not occur with an AC power supply. The presence of PD will affect drastically reliability, and thus life, of electrical insulation, to an extent which depends on their amplitude and repetition rate. The effect of number of inverter levels and peak voltage on PD repetition rate and amplitude is modelled, validated by experimental data, and correlated to insulation aging rate, showing that increasing the number of levels is beneficial for insulation reliability and providing an analytical support to PD measurements under PWM