Arm Phase-shift Modulation and Pre-charge Strategy for an Enhanced Alternate Arm Converter

Modular multilevel converter (MMC) is one of the most promising topologies in high-voltage applications. Kinds of modular topologies have been widely studied and discussed, but the dc fault ride-through capability and operating efficiency are still considered as a trade-off. In this paper, an enhanced alternate arm multilevel converter (AAMC) is proposed with the arm phase-shift modulation. By adjusting the phase-shift angle, the energy balance is achieved to allow a wide operating range. Combining the merits of low-cost half-bridge submodules and bipolar full-bridge submodules, the proposed T-type AAMC requires less power devices than traditional MMCs and AAMCs while keeps the dc fault ride-through capability. Since an additional ac arm stack is equipped in the T-AAMC, the dc-side based pre-charge methods is not applicable. The ac-dc coordinated pre-charge strategy is proposed to charge both the full-bridge stacks and half-bridge stacks to the reference voltage. Simulation cases are presented to validate the effectiveness of the proposed topology, modulation and pre-charge strategy.