A Generalized Voltage Balancing Algorithm for Modular Multilevel Cascaded Converters

Abstract

The use of an inter-cell voltage balancing scheme to evenly distribute the total dc-link voltage among the multiple floating capacitors is crucial in modular multilevel cascaded converters (MMCs). This paper presents an intuitively simple and computationally efficient sorting-based modulation algorithm for capacitor voltage balancing applicable to any MMC. The proposed modulation stage is inherently decoupled from the upper-level control scheme and it is able to both generate the required converter output voltage and balance the capacitor voltages suitably. Furthermore, it maintains a constant harmonic performance even as the number of switching transitions is varied to achieve voltage balance under different loading conditions with minimum incurred switching losses. The effectiveness of the proposed algorithm is validated on an experimental seven-level 350-VA single-phase cascaded H-bridge working as a static compensator. In addition, a simulated 350-VA system is used to obtain complimentary results.