Research On Steady-state Operation Region Of MMC-HVDC

As a large country with a vast territory,China’s power supply center and electricity demand center are extremely mismatched geographically,which leads to the adoption of long-distance and large-scale transmission in China.Under this circumstance,HVDC has a wide application prospect because of its technical and economic advantages.Modular Multilevel Converter(MMC),as the most advanced flexible HVDC converter,has attracted wide attention from academia.However,the current research on MMC-HVDC mainly focuses on the fundamental principles,control and protection,and the steady-state operation region of MMC-HVDC is less studied.In this paper.the steady-state operation region of MMC-HVDC is studied.The steady-state operation region of MMC-HVDC in three-phase balanced and unbalanced of AC system is drawn on PQ plane,and the control strategy is studied.Firstly,the mathematical model of single-terminal MMC-HVDC system is established,and the symmetrical component control strategy is designed to control the positive and negative sequence components in the system respectively.Then,in the case of three-phase balanced of AC system,the constraints which affecting the steady-state operation region of MMC-HVDC are analyzed from two aspects:the influence of AC system and the characteristics of converter itself.According to the constraints.the steady-state operation region of MMC-HVDC are drawn on the PQ plane by point-by-point scanning method and analytical method respectively.On this basis,the influence of three-phase unbalanced of AC system on the steady-state operation region of MMC-HVDC is analyzed,and the constraints are corrected.The steady-state operation region of three-phase unbalanced is also drawn by analytic method and point-by-point scanning method on PQ plane.In consideration of the truth that the steady-state operation region of MMC-HVDC is narrowed in three-phase unbalanced AC system.the corresponding power over-limit control strategy is designed,so that when asymmetric fault occurs in AC system,the over-limit power operation point can return to the boundary of operation region.Finally,the electromagnetic transient models of MMC-HVDC and related control systems are built in PSCAD/EMTDC simulation softw are,and the results of theoretical analysis are verified by simulation experiments.The simulation results show that the analysis results of steady-state operation region of MMC-HVDC in this paper are correct.and the proposed control strategy is effective.