Research On Control Strategies Of Cascaded H-bridge SVG Considering Unbalanced Grid

With more nonlinear loads connected to the grid,the proplem of the power quality has become more serious.The reactive power compensation technology is an effective way to improve the power quality and the stability of the grid.As an effective technical means of reactive power compensation,SVG is well studied due to fast dynamic response,remarkable compensation ability and high power quality.Because of its modularity and easy expension,this thesis takes the cascade H-bridge SVG as the research object and studies its control strategies,especially under unbalanced grid.Firstly,this thesis reviews the research background of reactive power compensation and the development and application of its devices,as well as the research status of cascaded H-bridge SVG control strategies.The basic operation mechanism of the SVG and the CPS-PWM modulation strategy is analysed.Based on the circuit principle of the SVG,the thesis establishes mathematical models respectively under the banlanced and unbalanced grid,which provides a theoretical basis for the subsequent study of SVG current decoupling control and parameter design.Secondly,under the unbalanced grid,the adaptive phase-locked loop based on SRF-PLL is studied,and all the parameters of the PLL are given.The current decoupling control under dq coordinates is analyzed,and the design process of its PI controller is provided.Then,the thesis discusses the causes and effects of DC voltage imbalance,and the DC voltage balancing hierarchical control is analysed,including the overall voltage balancing,clustered voltage balancing as well as individual voltage balancing.The design of the balancing controller is presented as well.When the grid imbalance gets larger,the traditional clutered voltage balancing is no more suitable.Therefore,this thesis analyses the zero-sequence voltage injection clustered balancing.Based on the power flow analysis,the mechanism of zero-sequence clustered balancing and its controller design is studied.In the frequency domain,the influence of zero-sequence voltage on the negative sequence component of output current is analyzed,and a grid voltage proportional feedforward method is used to suppress the negative sequence current.The design of the proportional coefficient is also presented.Finally,MATLAB/Simulink simulation model and experimental prototype is established.All the control strategies researched in this thesis are verified by both simulation and experimental results.