| The advantages of large capacity,low loss and flexible control of HVDC system make it more and more widely used in the field of large-capacity and long-distance transmission.However,the large-capacity weak inertia HVDC transmission converter connected to the grid will reduce the inertia coefficient of the grid,thereby reducing the voltage and frequency stability of the grid.To solve this problem,it is necessary to study a control method that can improve the inertia of the power grid with large-scale HVDC systems.VSG control method can simulate the inertia and damping characteristics of synchronous generator,which is an effective method to solve this problem.In order to improve the inertia of the grid system,this paper study the control strategy of MMC-HVDC converter based on VSG algorithm.And further study the fault response characteristics of the DC transmission system in the case of power grid faults,and propose an LVRT control algorithm that can achieve grid fault ride-through.The specific work is as follows:(1)Introduce the background and current research situation of MMC-HVDC at home and abroad in detail.The advantages and disadvantages are analyzed.The key technologies of the latest development are pointed out,including HVDC system VSG control strategy and LVRT method.(2)Based on the MMC-HVDC system topology,a mathematical model of the MMC converter station is established,the bridge arm inductance and the sub-module capacitor are designed,and according to the characteristics of the MMC structure,an MMC converter station control system including loop current suppression and sub-module voltage equalization based on the voltage and current double-loop control is established.(3)The control system of MMC-HVDC converter station based on VSG algorithm is established,the small signal control model of converter station and AC system is analyzed,and the selection principles of VSG inertia coefficient J and damping coefficient D are given.(4)LVRT control method for double-ended MMC-HVDC converter is proposed.The failure mechanism of the MMC-HVDC system is analyzed according to the two conditions of the voltage drop at the receiving end and the sending end,and a LVRT control strategy for MMC-HVDC system without communication between converter stations is proposed.(5)Under the environment of MATLAB/Simulink,an MMC-HVDC system simulation platform based on VSG control was built,and all research results were simulated and verified. |
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