Analysis of some application aspects in modern (PWM) adjustable speed drive systems and new solutions

In this dissertation, the effects of application aspects such as high common mode dv/dt, high differential mode dv/dt, neutral shift, and capacitor heating in modern PWM adjustable speed AC motor drives are thoroughly analyzed. These aspects have been described as the cause for premature failures in many modern PWM adjustable speed AC motor drive (ASD) system installations. First, an in depth analysis of the effect of high common mode and high differential mode dv/dt on two popular ASD systems is presented. Then, analysis of the effect of neutral shift on two medium-voltage (4160V) ASD systems is presented. Finally, a new method to evaluate dc-link capacitor heating in ASD systems is proposed. The evaluation is based on the level of ripple currents dc-link capacitors can endure that lead to self-heating and reduction of capacitors' operating life. Three popular utility interface options for ASD systems are considered for evaluation.; To mitigate the effect, this dissertation also proposes some new solutions. First, two new inverter output filters with interconnection to the dc-link mid-point is proposed to mitigate the effect of high differential mode and high common mode dv/dt in diode rectifier/PWM and PWM rectifier/PWM inverter type ASD systems. The proposed filters are shown to effectively reduce both differential mode and common mode dv/dt at motor terminals even in the presence of long motor leads. Both filters permit use of long motor leads and the components can be installed within inverter enclosure. Next, a new multi-level PWM strategy is introduced which results in zero common mode voltage for medium-voltage cascaded multi-level (CML) type inverters. A zero sequence blocking transformer (ZSBT) is also proposed to reduce common mode voltage in 3-level NPC inverters. Finally, a new term capacitor heating factor (CHF ) based on the dependence of ESR on frequency is introduced to evaluate various utility interface options. A relative comparison of capacitor heating as a function of utility interface options is presented and it is shown from analysis and experiments that 12-pulse rectifier results in lower heating of the capacitor and hence promotes higher reliability.