Research On Control Strategy And Switching Technology Of Single-Phase Grid-Connected/Off-Grid Inverter

Due to our country's strong advocacy and support for renewable energy generation,the new energy industry has developed rapidly.Renewable power system requires the inverters with grid-connected / off-grid functions.In this paper,the single-phase grid-connected / off-grid inverter is taken as the research object.The control strategy of grid-connected and off-grid inverter and the switching technology between the two operation modes are also studied.Firstly,the mathematical models of the inverter under two operating conditions are established,and two independent controllers are adopted for the two different operating modes of the inverter.Double closed-loop control structure which uses the inductor current for the inner loop and output voltage for the outer loop is used when the inverter works in off-grid mode.Besides,the output voltage and load current feedforward are used to realize the decoupling control of the inverter.In order to eliminate the influence of non-linear load on the output voltage waveform of the inverter,the outer loop of the output voltage adopts the compound controller structure of a proportional controller and an odd harmonic repetitive controller in parallel,furthermore,the frequency adaptability of the repetitive controller is also studied.In order to reduce the number of sensors required for inverter waveform control,a newly control method based on state observer is proposed,which only needs to measure the output voltage of the inverter and the inductor current and load current can be estimated.When the inverter works in grid-connected mode,the control strategy with the grid voltage feedforward is proposed to eliminate its own disturbance for the grid current,and a linear proportional-integral controller is also used for the grid-connected current loop.Secondly,the switching process of grid-connected mode and off-grid mode is also studied in this paper.Firstly,the single-phase-locked loop technology was studied.In order to solve the problem of dc bias in analog sampling circuit,a single synchronous coordinate phase-locked loop structure based on an improved second-order generalized integrator is adopted.The principle of the second-order generalized integrator is analyzed in detail,and the parameter design flow of the single-synchronous coordinate phase-locked loop is arranged to ensure the accuracy of the phase-locked loop.Meanwhile,the switching logic of the grid-connected and off-grid process of the inverter is designed.Finally,a simulation model of the grid-connected / off-grid inverter was built by Matlab / Simulink simulation software,and the control strategy was verified by simulation results.The software and hardware of the inverter system were designed based on the theoretical analysis and simulation results.A 2k W inverter experimental platform was built and the experimental results verified the correctness and effectiveness of the control strategy.