Research On Control Strategy Of Cascade Multilevel SVG In Wind Farm

With the increasing proportion of wind power installed capacity in the power grid,the impact on the grid voltage stability also increases.Especially in the area of high wind power penetration,the impact should not be ignored.With the development of power electronics technology,the cascade multilevel SVG has gradually replaced the traditional reactive power compensation device such as fixed capacitor group and static var compensator(SVC)with its significant advantages such as large compensation capacity and fast response speed,and is paid more and more attention.In this paper,the control strategy of cascade multilevel SVG and the improvement of output voltage and current harmonics are all in-depth studied both in the grid voltage balanced and unbalanced operating conditions.In this paper,the cascade multilevel SVG is used as the research object,and the working principle of the cascade multilevel SVG is analyzed.The mathematical model of the cascade multilevel SVG and the decoupling control method of AC side are studied.Based on the in-depth study of the DC side capacitor voltage imbalance mechanism,a new active power superposition algorithm is proposed to balance the dc side voltage between three-phase chain,and the capacitor voltage of DC side is controled from the global,between three-phase chain,and between the H-bridge module in each phase chain.At the same time,segmentation control with variable parameters is presented to solve the contradiction between the voltage balance speed and the harmonic content of the output voltage.The quality of the output voltage is improved while ensuring a fast equalization of the DC side voltage between the three-phase chain.The simulation model of cascade multilevel SVG based on Matlab/Simulink environment is built,and the simulation results verify the feasibility of the proposed DC side voltage balance control method and the performance of segmentation control with variable parameters.The phase-locked technology and the positive and negative sequence separation method under unbalanced grid voltage condition is studied,and a control strategy to achieve full compensation for reactive power and negative sequence current in single rotating coordinate system without the separation and extraction of the positive and negative sequence control current was proposed for the control of cascade multilevel SVG based on the study of the positive and negative sequence model of the cascade multilevel SVG under the unbalanced operating conditions,which improves the control precision of the current loop and the harmonic content of the output current.It is proposed to add the resonant part to PI controller of the AC side to further reducing the harmonic current of the output current by improving the follow-up accuracy of the control current by the infinite gain of the resonant point.The simulation model of cascade multilevel SVG control strategy under grid voltage unbalanced operating condition is established,and the correctness of the proposed control strategy within the single rotation coordinate system and the effect of harmonic improvement are verified by simulation compared with the traditional control strategy within twocoordinate system.The control effect of the PI controller and the PIR controller within the single rotation coordinate system is compared,and the results show that the PIR controller has better current tracking performance and can effectively reduce the lower harmonic content of the output current.