Hybrid cascaded modular multilevel converters for HVDC transmission

Abstract

This thesis presents a comprehensive study of a class of modular multilevel converters (MMCs) namely hybrid cascaded MMCs. These converters have topological dc-fault blocking capability and are suitable for large-scale, long-distance high-voltage direct current (HVDC) transmission. This thesis investigates an existing hybrid cascaded MMC (HC-MMC) and novel variations thereof (mixed-SM HC-MMC) with a multi-pronged research approach based upon mathematical analyses, detailed computer simulations, and where possible experimental verifications. Several methods are proposed for control and operation of the converter under normal and faulted conditions with a view to (i) enable regulation of submodule capacitor voltages in the phase limb with reduced harmonics and the ability of extending linear modulation range, (ii) ride through balanced and unbalanced ac faults with balanced phase currents and efficient ac-fault recovery, and (iii) successfully ride through dc faults with prompt isolation of the ac and dc sides and rapid decay of the dc fault current. These methods are extensively analyzed using detailed electromagnetic transient simulation and experimental work where possible. Converter losses and efficiency maps are also quantified using detailed computer modeling methods to evaluate the benefits of the existing and proposed HC-MMCs. Compared with the original HC-MMC, the proposed mixed-SM HC-MMC has superior performance in terms of extended linear modulation range, system efficiency, and dc-fault clearing performance. The thesis also formulates the design guidelines of SM capacitor sizing considering submodule redundancy and different control modes. Extensive analytical, simulation-based, and experimental measurements are provided to confirm the validity and efficacy of the developed guidelines.