Research On The Control Of VSC-Mutiterminal HVDC System

There is not any VSC-MTDC projects in the world, and the research about it is in the stage of theoretical reserch and experimental simulation.Although this paper aims at the VSC-MTDC system, the VSC controller of muititerminal system is the same with two-teiminal system. So the two-terminal VSC-HVDC system is analysised and researched firstly. It includes the following contents.(1) The technical advantages of VSC-HVDC compared to traditional HVDC are summarized, and then the feasibility and broad application prospects of VSC-MTDC are illustrated.(2) The working principle, topological structure and operating characteristic were analysed separately.(3)The VSC-HVDC mathematical model based on its steady-state operation condition is established in the abc coordinate. Then, the mathematical model in dq rotating coordinate was derived through the Park transformation. According to the mathematical model and the requirements of steady-state operation, the control quantities, controlled quantities and their correlations were determined.(4) The controllers based on series PI regulator, the theory of the feedback linearization of nonlinear system and the inverse system theory were designed separately. According to the role of the main circuit components of VSC-HVDC system, the constraints of component parameters were summarized. Then, a two-terminal active electric network model and a model supplying power to passive network are established through the electromagnetic transient simulation software PSCAD/EMTDC, and the simulation results verify the feasibility of the controller.(5)The basic control strategies of VSC-MTDC system are introduced, and their advantages and disadvantages were analysed based on their operating characteristics. Then, a three-terminal VSC-MTDC system model is established with PSCAD/EMTDC, and the master-salve single-point DC voltage control strategy is verified. But this strategy has its limitation, and aiming at this limitation, a multi-point DC voltage control strategy based on mode switching is proposed. Then, the improved control strategy is verified through simulation.