Enhanced modular multilevel converter for HVdc applications: assessments of dynamic and transient responses to ac and dc faults

This paper describes the operating principles and theoretical relationships that underpin the modelling and control system of the enhanced modular multilevel converter (EMMC). A full-scale model of a point-to-point HVdc link that employs EMMCs is used to examine its performance during normal operation in all four quadrants, and resiliency to symmetrical and asymmetrical ac and dc faults. Results of exhaustive simulation studies reveal that the improved ac and dc power qualities, which are achieved by incorporation of a few full-bridge cells into the arms of conventional half-bridge modular multilevel converter (HB-MMC) with medium-voltage cells to create the EMMC do not affect its ac and dc fault ride-through capability nor its dynamics during normal operation as active and reactive power set-points being varied. In addition, a variant of the EMMC is proposed, in which the number of full-bridge cells to be added into the arms of HB-MMC could be increased to offer bespoke features beyond that explicitly defined in original vision of the EMMC, such as reduced dc voltage operation during pole-to-ground dc fault, and potential extension of fault clearance times in multi-terminal HVdc grids. Moreover, the validity of the new variant has been confirmed using results obtained from highfidelity HVdc link models developed in EMPT-RV platform, in which the EMMCs are replaced by the proposed variant.