Research On Control Strategy Of Hybrid Three-terminal HVDC System

Result from the situation of Chinese energy supply and demand-uneven distribution and long distance,the UHVDC project is implemented,which makes steady progress in sustainable construction by the State Grid Corporation of China,and actively promotes the transformation of world energy and strengthen the Global Energy Interconnection.With the improvement of power electronics,Hybrid HVDC,which is the combination of line commutated converter(LCC)and voltage source converter(VSC),is seen as an effective solution to solve the problem of energy integration and multiterminal topology constitution.Now some HVDC projects have the embryonic form and programme of hybrid multiterminal topology,so it is valuable to conduct the research on Hybrid HVDC technology.Hybrid three-terminal HVDC system is the main research object of this dissertation,and the main contents are listed:(1)Based on the basic structure of Hybrid HVDC,the principle analysis and mathematical modeling of conventional HVDC technology and flexible HVDC technology are carried out,thus the basic control mode of conventional converter and flexible converter is defined.This dissertation studies the topology and control strategy of multiterminal systems,and the control strategy is verified by simulation,which lays a theoretical foundation for the follow-up research.(2)The structure of topology,and basic principles of hybrid HVDC transmission systems,especially control strategies,are studied and verified by PSCAD model.Then this dissertation extends the two-terminal topology to a three-terminal topology,and performs further analyze on the research of coordinated control strategies.According to the principle of at least one voltage control side in multiterminal system,two control strategies are designed.With software simulation,both positive and negative effects of the two control strategies could be compared and analyzed.(3)This dissertation proposes a hybrid cascade three-terminal UHVDC system and its coordinated control strategy.Firstly,based on the background of the Global Energy Interconnection,a transition scheme of conventional HVDC retrofitting is put forward,which is for flexible HVDC.Secondly,a mathematical model and equivalent circuit of hybrid cascade three-terminal UHVDC are established,thus the control variables and basic control strategies of the system are defined.Thirdly,a coordinated control strategy which achieves an equilibrium between high-end converters and low-end converters is studied.The coordinated control strategy includes steady-state and transient control methods,and then the designing concepts of the voltage balance controller and the additional voltage controller are proposed.Finally,simulations of various scenarios verify the effectiveness of the proposed control strategy.The purpose of this dissertation is to provide references for control theory and engineering application of hybrid HVDC technology.