An Asymmetric Hybrid Phase-Leg Modular Multilevel Converter With Small Volume, Low Cost, and DC Fault-Blocking Capability

Abstract

Hybrid modular multilevel converters (MMCs) have exhibited superior power density and cost-effectiveness than the conventional half-bridge submodule-based MMC (HB-MMC), while still maintaining the hassle of using high-voltage dc supporting capacitors or nonstandard transformers. To alleviate the hassle, an asymmetric hybrid phase leg MMC (AHPL-MMC) is proposed in this article. The presented AHPL-MMC consists of a half-bridge submodule-based phase leg and two full-bridge submodule-based hybrid phase legs. This asymmetric configuration reduces bridge arm inductance, the number of submodules, and total capacitance, thereby improving the converter's power density and cost-effectiveness. In addition, the proposed converter can operate across the full range of modulation indices and power factors with the features of dc fault blocking and low dc current ripples. The operating principles, parameter design, and performance comparison of the AHPL-MMC are provided in this article. Close-loop energy balancing control methods are further proposed for the hybrid phase legs and the conventional phase leg, respectively. The effectiveness of the proposed AHPL-MMC is validated by both simulation and experimental results.