DC bus voltage balancing and control in multilevel inverters

摘要

A multilevel inverter system having four or more levels of output voltage on each inverter output line. The voltage across each of three or more series connected DC bus capacitors is provided to an inverter which is formed to have redundant switching states, such as a plurality of controlled switching devices and diodes connected in a diode- clamped multilevel inverter configuration. The switching of the switching devices is controlled to provide a selected output voltage waveform utilizing redundant switching states, when such states are available on each voltage transition, by selecting a one of the available switching states that serves to provide the desired phase-to-phase voltages and to tend to equalize the voltages across the DC bus capacitors. Active or passive rectifiers may be used to provide the DC voltage across the DC bus lines and across the DC bus capacitors. Separate rectifiers may be provided for each capacitor which are supplied from secondaries of transformers connected to the three phase input lines to provide isolation and voltage level adjustment. The multilevel inverter system provides output voltage waveforms with lower total harmonic distortion that conventional two or three level inverters, while maintaining voltage balance across the several DC bus capacitors.