Full-Bridge MMC Converter Optimal Design to HVDC Operational Requirements

The design and operation of full-bridge (FB) MMCs that meet HVDC specifications are studied in this paper. Three new design parameters: 1) the overmodulation index (kmmc); 2) the dc modulation index (Mdc); and 3) the minimal dc voltage (V-minpu) are introduced to specify the operation of an FB MMC. Power increase and semiconductor count increase with the increase of kmmc are analyzed to understand the benefits of overmodulation. The required number of submodules and the number of more costly FB submodules for specified rated dc voltage V-minpu and kmmc are calculated. The relationship of the submodule inserting logic and dynamics of an arm is analyzed. Submodule voltage balancing is studied, and the constraints on the required number of FB submodules are deduced. The capability of overmodulation and the operation under low dc voltage with optimal submodule counts are verified using Electromagnetic Transients Program simulation.