DC-link low-frequency current and voltage ripple analysis in multiphase voltage source inverters with unbalanced load

Abstract

Inverter's performance and operating mode may be negatively affected by inverter input (dc-link) current and voltage ripple. It is a common experience that even theoretically balanced loads with perfectly balanced supply voltages, such as multiphase ac motors supplied by pulse-width modulation (PWM) inverters, in practice show a certain degree of current unbalance, in the range of a few percent, which introduces a low-frequency instantaneous power oscillation. This reflects in current and voltage low-frequency ripple on the dc-link inverter side (i.e. at the double-fundamental frequency). A possible method to analyse this matter is through the symmetric sequence components. In particular, based on the first negative current sequence component and by considering the equivalent dc-link impedance calculated at the dominant double-fundamental frequency, the amplitude of the corresponding dc-link voltage ripple component is calculated in this work for a general multiphase load. Finally, the design of the dc-link capacitor in multiphase inverters is proposed considering requirements referred to the double-fundamental dc voltage ripple. The feasibility of the proposed developments has been verified for three-, five- and seven-phase inverters by both numerical simulations and comprehensive experimental tests, always showing a good matching.