Regenerative medium-voltage AC drive based on a multi-cell arrangement with minimum energy storage requirements

Abstract

Multi-cell power circuit topologies have probed to be an effective alternative to medium-voltage AC drives. Commercial units are built up based on modular units, namely cells, which are composed of a three-phase diode-based front-end rectifier, a DC link electrolytic capacitor, and a single-phase fullwave voltage-source inverter. The main advantage is the improved power quality at both AC system and motor sides. However, two drawbacks are present in these configurations. These are: (a) the single-phase operation of the inverter in each cell generates a large second current harmonic that is injected back into the DC link capacitor, thus, a large electrolytic capacitor has to be used in order to guarantee a reduced voltage ripple across it; and (b) the three-phase diode-based front-end rectifier does not provide the regenerative operating mode as required, for instance, by down-hill belt conveyors in mining applications, where this operating mode is the normal one. This paper proposes to replace the diode-based front-end rectifier by an active front-end rectifier and a novel control strategy in order to overcome the aforementioned drawbacks. In fact, the active front-end rectifier allows the topology to regenerate and the control strategy removes the large second current harmonic from the DC link capacitor, thus reducing its size. Both features are achieved without any penalties in the quality of the waveforms. Simulated results confirm the theoretical considerations.