Research On Control Parameter And Control Strategy Of Hybrid Multi-terminal HVDC System

With the increase in transmission corridor and operating cost requirements,hybrid multiterminal HVDC(Hybrid-MTDC)systems have become a highly sophisticated technology in the field of scientific research,due to combining the advantages of both line commutation converters(LCCs)and multi-level modular converters(MMCs).However,due to the large number of converter stations and controllers in the Hybrid-MTDC system,the analysis of the control parameters and the configuration of control strategies become more complicated,which makes it difficult to ensure the stable and reliable operation with good dynamic performance.Therefore,this thesis focuses on control parameters analysis and control strategy design for hybrid multi-terminal HVDC systems.The main work is as follows.(1)The system-layer control parameter optimization model of Hybrid-MTDC systems is proposed,and the converter-layer state-space model is derived to analyze the influence mechanism of the control parameters.On the system layer,an optimal power flow control parameter model that takes into account line loss and voltage offset is constructed and solved by the interior point method.On the converter layer,the general mathematical expressions of LCC and MMC stations are derived based on the average switching function method,and the parallel DC network model is derived to construct a Hybrid-MTDC model described by dynamic phasors.The comparison of the step response of the simulation model verifies the accuracy of the proposed detailed mathematical model.(2)Combining Lyapunov’s indirect method and hybrid potential function theory,a global stability analysis method of Hybrid-MTDC system is proposed,and a dynamic performance evaluation function based on eigenvalue distribution is constructed to optimize the control parameters in the stability domain.By constructing a three-terminal hybrid HVDC transmission system simulation model,the accuracy of the proposed stability analysis method and the effectiveness of the optimization of control parameters are verified.(3)Based on the principle of differential geometry,an improved DC droop coordinated control strategy combining adaptive outer-loop control and state feedback linearization sliding mode inner-loop control is proposed to realize the reasonable distribution of unbalanced power among stations and improve the robustness of the control system.The simulation results show that the proposed DC droop coordinated control strategy can effectively avoid reaching the power and voltage limit of the stations compared with the traditional droop control strategy,and has better anti-disturbance performance.(4)Developed a Hybrid-MTDC control parameters optimization software,and realized the functions of rapid modeling with multiple control structures,analysis of the main influencing parameters,calculation of the feasible region for control parameters,and optimization of the control parameters.The proposed software does not require complicated code modification,which reduces engineering debugging costs.The tests were carried out with the "KUNLIULONG" Hybrid-MTDC project data to verify the accuracy and effectiveness of the control parameter optimization software for the hybrid multi-terminal HVDC systems.