Asymmetrical modular mutilevel converter (A-MMC) based HVDC system

Over the past years, modular multilevel converters (MMC) have been widely researched as a preferred option among voltage source converters for HVDC transmission system. The converter presents modularity to generate high number of output voltage levels by simple series connection of modules, resulting in higher reliability and easy failure management/maintenance. Synthesis of near sinusoidal output voltage waveforms by multilevel operation requires pulse-width modulation at relatively lower switching frequencies, resulting in reduced switching losses. In this paper, asymmetrical submodules in the MMC based HVDC system have been investigated. Use of asymmetrical submodules with different capacitor voltages results in lesser number of semiconductor switches for the same number of output voltage levels, thus reducing the overall size of the converter system. The operating principle using different carrier based modulation schemes is discussed in detail. Modified voltage balancing algorithms based on well known sorting algorithm and PI controller have been investigated for their feasibility to asymmetrical-MMC operation. Simulations with preliminary experimental results are presented to validate the proposed converter operation.