4.8 Article

Control Optimization of Modular Multilevel Resonant DC Converters for Wide-Input-Range MVdc to LVdc Applications

Journal

IEEE TRANSACTIONS ON POWER ELECTRONICS
Volume 37, Issue 5, Pages 5284-5298

Publisher

IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC
DOI: 10.1109/TPEL.2021.3133174

Keywords

Switches; Switching frequency; Power transformer insulation; Voltage control; Regulation; Zero voltage switching; Zero current switching; Medium-voltage dc (MVdc); modulation index regulation; modular multilevel resonant dc converter (MMRDC); switching frequency regulation; wide input range

Funding

  1. National Nature Science Foundations of China [51807176, 52107214]
  2. NationalNature Science Foundation of China for China-U.K. Joint Project [52061635101]

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This article studies the output voltage control strategy of modular multilevel resonant dc converters (MMRDC) for medium-voltage wide-input-range applications. The feasibility and influence of switching frequency regulation and modulation index regulation are investigated, and a design and control optimization method based on modulation index regulation and switching frequency regulation is proposed. Experimental validation shows that the optimized control methods are effective, with an efficiency over 96.4% under a wide input range.
In this article, the output voltage control strategy of modular multilevel resonant dc converters (MMRDC) for medium-voltage wide-input-range applications is studied. First, the feasibility and influence of the switching frequency regulation and modulation index regulation are investigated in detail. It is found that the switching frequency choice has a significant effect on the voltage stress of arm inductors and the transformer. Then, based on these considerations, the modulation index regulation plus switching frequency regulation based design and control optimization is proposed. With the proposed design and control method, the switching frequency range for adapting to a wide input range is further reduced, especially under light load operating conditions. Besides, the high voltage stress on arm inductors is avoided and the high voltage dv/dt on the transformer is also eliminated. Finally, a laboratory prototype with 8-16 kV input and 375 V/60 kW output has been built and the experimental validation under a wide input range has been done. The efficiency over 96.4% under the wide input range proved the effectiveness of the optimized control methods.

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