Dc-link current and voltage ripple analysis in single-phase and multiphase voltage source inverters

Dc-link switching and low-frequency current and voltage ripple components in two-level single-phase and multiphase voltage source inverters have been analyzed in this thesis. Voltage ripple components are calculated based on the corresponding dc-link current components and by considering a non-ideal dc voltage source, representing an input filter (such as a dc reactor) or a resistive dc supply (such as a PV system). However, switching ripple analyses are also valid in most of the dc power supplies since the dc source impedance usually becomes much higher than the dc-link capacitor reactance at the inverter switching frequency. The peak-to-peak voltage switching ripple amplitude is derived over a fundamental period as a function of operational conditions: modulation index, the amplitude of phase current and power phase angle. The initial investigation of the dc-link variables has been focused on single-phase H-bridge inverter, considering both, switching and low-frequency dc ripple components. The analysis has been further extended to the case of two-level multiphase inverters with balanced load. The approach has been specifically developed for three-, five- and seven-phase inverters. The cases with different phase numbers are compared and the effect of increasing the number of phases on the dc-link capacitor sizing has been discussed. The final analysis has also included the impact of a slight load unbalance on the dc-link variables in multiphase inverters. Considered unbalance is present in practice in every multiphase ac motor, it is in order of a few percent, and introduces a low frequency instantaneous power oscillation. This reflects in low frequency (i.e. double-fundamental) current and voltage ripple on the dc-link inverter side. Therefore, the amplitudes of both ripple components are calculated and the importance of taking them into consideration for the dc-link capacitor design has been discussed. All analytical developments are validated with detailed simulations and experimental results.