Research On Control Technology Of Grid-connected And Off-grid And Smooth Switching Of Cascaded Three-phase Bridge Inverter

In recent years,China has actively promoted green and low-carbon development,committed to striving to achieve carbon peak by 2030 and carbon neutrality by 2060,which means that it is necessary to build a clean,low-carbon,safe and efficient energy system.Therefore,photovoltaic power generation has attracted more and more attention as the most promising new energy technology in the future.In particular,cascaded photovoltaic inverters have been studied by many scholars.Aiming at the disadvantage that the cascaded H-bridge inverter needs to solve the problem of threephase power balance,a cascade three-phase bridge inverter with natural self-balancing power and higher output voltage is studied in this paper.Aiming at the problems such as the impact on the power grid caused by the grid-connected photovoltaic cascaded three-phase bridge inverter,based on the virtual synchronous machine technology the control research is carried out.The pre-synchronization control without phase-locked virtual synchronous machine and the super-twisting sliding mode control strategy based on virtual synchronous machine technology are proposed.The main research contents include :(1)Aiming at the problem of large common-mode voltage in cascaded inverter,the circuit of cascaded three-phase bridge inverter is studied.On this basis,a carrier phase shift distribution modulation strategy based on vector screening is proposed to suppress the common-mode voltage of the cascaded three-phase bridge.According to the size of the reference vector,the vector synthesis area is divided into high pressure area and low pressure area.According to different regions,zero vectors and other adjacent vectors are selectively added to suppress the common-mode voltage of the cascaded three-phase bridge system.Firstly,the equivalent model of cascade threephase bridge inverter circuit is established to simplify the circuit structure and control process of inverter.At the same time,a carrier phase shift allocation modulation strategy based on vector screening is proposed,and the correctness of the modulation strategy is verified by simulation.(2)A pre-synchronization control strategy of phase-locked virtual synchronous machine is proposed to solve the problem that the lack of inertia and damping of distributed power grid brings impact on the power grid and the problem of nonlinearity and complex parameter calculation of traditional grid phase-locked loop.By directly using the voltage component in the static coordinate system to avoid PI control,the voltage phase Angle of the grid is directly obtained.At the same time,the frequency and voltage regulator are studied and designed to realize the rapid phase-locking of the system during grid-connection and reduce the impact of the system on the grid during grid-connection.Firstly,the virtual synchronous machine model and control principle are studied,and the small signal model is derived to analyze the influence of related parameters on the control.Thirdly,the pre-synchronization technology of virtual synchronous machine without phase-locked loop is proposed.Through mathematical derivation and model,it is deduced that the strategy can achieve fast and smooth grid connection,and the correctness and reliability of the strategy are verified by simulation.(3)A sliding mode control strategy based on virtual synchronizer is presented to overcome the poor robustness of the traditional dual-loop voltage and current control algorithm.Based on the mathematical model of inverters,a sliding surface for sliding mode control is established,and a new control law for second-order superspiral sliding mode is obtained,which eliminates high frequency jitter with high-order derivatives.In order to enhance the response speed and stability of the system,and improve the operation effect and waveform quality under grid connection and islands.First,the high-order sliding mode control is studied,and the deduction and design of cascade three-phase bridge system are carried out.Secondly,the super-twisting control strategy is presented based on virtual synchronizer,which can effectively enhance the system stability and response speed compared with the traditional double-loop control strategy.Finally,the correctness of the proposed control strategy is verified by simulation.(4)An experimental platform for two-stage three-phase bridge inverters is built.The validity of the proposed theory is verified by the experimental results under islands and grid-connected conditions.Firstly,for the control characteristics and requirements of cascade three-phase bridge inverters,a dual-chip control hardware circuit with DSP and GA is used.At the same time,the related software algorithms are designed.The validity of the pre-synchronization control strategy and the hyperhelix control strategy based on the virtual synchronizer for the unlocked virtual synchronizer is verified by experiments.