Integrated Control and Performance Analysis of High-Speed Medium-Voltage Drive Using Modular Multilevel Converter

The modular multilevel converter (MMC) is highly attractive to develop high-speed medium-voltage variable frequency drives (MV VFDs) due to its cascaded design, high-quality output, and strong reliability. However, high-speed MV VFDs have demanding requirements in capacitor voltage stabilization, current regulation, dynamic response, etc. The instability and distorted voltage/current could easily occur if MMCs are used as high-speed MV VFDs. This article presents a high-speed VFD with silicon carbide (SiC)-based MMC feeding a permanent magnet synchronous machine. A new integrated control method is specifically proposed to address the aforementioned issues covering the low-/middle-/high-speed range. Performance analysis is conducted in terms of efficiency, power density, output current quality, and speed range. Steady-state and dynamic conditions are evaluated by a full-scale simulation model and a high-power SiC-based MMC testbed. Results show that the proposed method effectively removes the large capacitor voltage fluctuations, resonant circulating current, and saturated output voltage from zero to the highest speed. Compared with existing MMC VFDs of limited speed (<4000 r/min), the SiC-based MMC VFD has three times higher speed (15 000 r/min), improved efficiency (99.43%), high power density, and low current harmonic distortions.