Extending the application range of MMCs in high-performance drives: limiting issues and new proposals

Modular Multilevel Converters (MMCs) are becoming more and more popular in medium-voltage applications thanks to several interesting features. The major limitation for applications with variable speed drives (VSDs) is the submodule capacitors’ voltage ripple, which depends directly on the output current/torque and inversely on the output frequency/speed. Among all the hybrid MMC topologies addressed in recent literature, the Flying-Capacitor Passive Cross-Connected arms (FC-PCC) MMC seems one of the most promising to cope with this issue. This article provides a comprehensive overview of its features and capabilities. The main open issues limiting safe and reliable converter operation in VSD applications are addressed, and novel solutions are presented, such as the optimization of the converter digital control strategy, the distributed implementation of the architecture, the L‑C oscillations damping during the pre-charge/start-up of the converter, and the optimized arm inductor design based on magnetic coupling concepts. The effectiveness of all the proposals is demonstrated through an accurate simulation of a 22 kW Permanent Magnet Synchronous Machine (PMSM) drive, including a complete model of the effects of the real-time communication and the processing latency. A prototype of the system has been developed employing some known and novel architectural solutions, as discussed in the last section of the paper.