DC Bus Regulation in Cascaded Three Phase AC Power Converters with only Decoupling Capacitors

Cascaded power conversion systems with a dc link are used in electric traction applications, industrial motor drives, renewable energy systems, uninterruptible power supplies, etc. The voltage across the intermediate dc bus is typically maintained at a nearly constant level and is a critical prerequisite for classical pulse width modulation (PWM) techniques used to synthesize the desired power conversion with adequate fidelity and dynamic performance. This is typically realized using a bulky dc link capacitance with a large amount of energy storage across the dc bus. The inspiration behind this work is to maintain a stiff dc bus, but without a large amount of energy storage, and still realize high quality waveform synthesis. The proposed approach determines switching intervals of the various solid state switching devices in an integrated fashion and places them within each switching period in a particular sequence determined by the desired voltage and current waveforms such that the stiffness and the charge of the dc link can be maintained without large amounts of energy storage during steady state and transient conditions. This paper presents the analytical and the design development of the proposed approach along with circuit simulation and experimental results of a prototype dc to three phase ac converter.