Research On Off-grid Operation Control Strategy Of Two-stage Non-isolated Inverter

With the development and application of new energy and energy storage technologies,the demand for higher voltage levels has increased while improving the power density and efficiency of the system.However,the output voltage range of a single-stage inverter is limited,which fails to meet its needs.Therefore,a DC/DC converter is added on the basis of a single-stage inverter to form a two-stage inverter,which not only increases the output voltage of the system,but also enables more flexible power control.This thesis focuses on the two-stage inverter,concentrating on the control strategy and modulation strategy of the pre-stage DC/DC converter and the post-stage DC/AC converter to achieve bus voltage stability,constant voltage and constant frequency of output voltage.The Buck/Boost converter is selected as the pre-stage DC/DC converter,and the two-level inverter is chosen as the post-stage DC/AC converter.As the same time,the electrolytic capacitor is used to maintain the intermediate bus voltage stability,which enables the decoupling of the front and rear converters.Therefore,the controller parameters of the pre-stage and post-stage converters can be separately designed.Firstly,the small signal analysis method is used to establish the mathematical model of the Buck/Boost converter working in Buck mode and Boost mode,and the mathematical model of the inverter under the stationary coordinate system and rotation coordinate is established by using the equal amplitude coordinate transformation.Based on the established model,the design of the controller is completed.Secondly,for Buck/Boost converter,dual closed-loop control is used to maintain stable output voltage.To facilitate mode switching,the switch tubes in the Buck/Boost converter are complementary on.The controlled object of the current inner loop is the same,which combines the current loops in both Buck mode and Boost mode.The dual closed-loop PI control consists of two voltage outer loops and one current inner loop.In order to avoid saturation of the PI controller,the integral limiting algorithm is adopted.For two-level inverter,the control strategy of off-grid operation is mainly studied.When the inverter has a three-phase balanced load,the feed-forward decoupling control strategy is used to achieve the control goal of constant voltage and constant frequency of the output voltage.In order to suppress the negative sequence component of the output voltage when the inverter has unbalanced load,the repetitive controller is used in this thesis.The basic principle and structure of the repetitive controller are introduced,and the performance of the repetitive controller is analyzed from four aspects: stability,anti-interference performance,error convergence speed,and steady-state error.Due to the existence of the delay link,the dynamic response speed of the repetitive controller is slow.Therefore,the PI controller,the quasi-PR controller and the repetitive controller are connected in parallel to form a composite control to improve the dynamic response ability of the system.At the same time,the detailed design process of the repetitive controller parameters is given.Additionally,aiming at the sizeable computational workload of the traditional SVPWM algorithm,a two-level simplified PWM modulation algorithm is proposed.By deriving the traditional seven-segment SVPWM volt-second balance formula,the constraint conditions are strengthened,the calculation process is simplified,and the three-phase action time is directly obtained.By injecting zero-order components,the three-phase action time is modified to realize the expansion of the modulation system.The theoretical proof shows that the maximum modulation system is 1.15,which is the same as the traditional SVPWM algorithm.Simulation and experimental results verify the effectiveness of the simplified modulation strategy.Finally,Simulink is used to build a simulation model,and a two-stage inverter experimental platform with DSP+FPGA as the control center is built.Hardware circuit parameters and software design flow are described in detail.The effectiveness of the modulation strategy proposed in this thesis and the control strategy adopted is verified by simulation and experiments.