Research On Control And Protection Strategy Of Modular Multilevel Converter-HVDC Systems

Modular multilevel converter (MMC) is proposed in recent years, and recognized as a major breakthrough on the type of voltage source converter topology. Compared to the traditional two-level or three-level topology, the MMC-based HVDC system has low requirements for power electronic devices, good scalability, can be easily extended to high voltage and power levels, and has a better steady-state and transient performance. This thesis makes researches on control and protection strategy of MMC-HVDC and the main works are organized as follows:(1) The research background and significance is Introduced. An overview of the traditional multilevel topology and MMC research situation is made, and then the main work of this thesis is described.(2) The modular multi-level converter topology, the specific sub-module functions and working principle, MMC-HVDC system operating characteristics are introduced in details.(3) The nearest level modulation strategy is presented for MMC-HVDC system. A mathematical model in PSCAD/EMTDC environment is made to build a 21-level double-end MMC-HVDC simulation platform. The control strategy based on instantaneous power theory is validated on such a platform.(4) Researches on the system's fault characteristics is made, and on this basis, adding the negative sequence component of the control loop protection and sub-module redundancy protection strategy are proposed.