A Switched Mid-Point Modular Multilevel Converter for HVDC Applications

Abstract

The modular multilevel converter (MMC) has emerged as the industry standard for voltage source converters (VSC) based high voltage direct-current (HVDC) applications since its benefits of scalability, modularity, high efficiency and better output quality outweigh its drawback of large energy storage. However, with a growing number of offshore HVDC installations, the volume and footprint of the converter, which are largely influenced by its energy storage, are becoming increasingly critical. This has led to a rising interest in alternative converter topologies that reduce the energy storage while maintaining the benefits of the MMC. This paper introduces a switched mid-point modular multilevel converter (SMPC), that is able to generate the desired AC-side waveforms with significantly less energy storage (around 50%) than the MMC. The DC-side is free from any distortion that requires additional filtering. Reduced energy storage and the associated reduction in volume is achieved at the expense of requiring more switches and slightly lower efficiency when compared to the MMC. Like many other MMC-style circuits, except when operating at a 'sweet-spot', active control of the energy balance is needed, for which a technique based on second harmonic injection is proposed. The converter operation and the proposed control scheme are verified by simulations for a medium-voltage setup and experimentally on a low-voltage labscale prototype.