Research On Current Control Strategy Of Single-Phase Grid-Connected Inverter Based On Repetitive Control

Traditional fossil energy sources have being steadily supplanted as new energy technologies have emerged.Because of their renewable nature and limitless development,new energy generation technologies such as solar and wind power are widely utilized.Gridconnected inverters are an essential component of distributed power generation systems and play a vital role in converting DC energy sources such as photovoltaic and wind power into AC power and connecting them to the grid.The harmonic content of the output current on the inverter side is one of the most important indicators of the power quality of the grid,and repetitive control is a common method of achieving output current control due to its excellent harmonic suppression performance.However,renewable energy technologies such as photovoltaics and wind power are affected by weather,which can cause fluctuations in the fundamental frequency of the grid,resulting in a reduction in the harmonic suppression capability of the repetitive controller.Therefore,this paper takes a single-phase grid-connected inverter as an application target and develops a fractional-order compound repetitive control strategy to suppress harmonics in the output current of the grid-connected inverter and improve the quality of the incoming current when the fundamental frequency of the grid fluctuates.The main work of this dissertation is as follows.(1)Analysis of the working principle of single-pole frequency doubling SPWM,modeling of a single-phase LCL-type grid-connected inverter,and damping strategy for LCL filter resonant peaks.Firstly,a single-phase full-bridge structure inverter is chosen,and the power switching devices are driven using a single-pole frequency doubling SPWM technique.Secondly,a single-phase LCL-type grid-connected inverter model is created,the LCL filter parameters are designed,and various damping strategies are investigated to eliminate the filter’s resonant peaks.(2)Improved compound repetitive controller performance is provided.Since the PI controller is not able to make the output current track the command current without error,and the repetitive controller has the ability to accurately track the periodic signal but has the problem of poor dynamic response,a combination of PI controller and repetitive controller is used.A Plug-in Compound Repetitive Control(PCRC)structure is selected,and stability analysis and parameter design are carried out.(3)The Fractional Order Plug-in Compound Repetitive Control(FOPCRC)strategy is investigated.In order to enable the repetitive controller to accurately track the command current despite fluctuations in grid frequency,an IIR filter is proposed to approximate the fractional delay caused by internal modes.Firstly,the performance of the FIR filter and the IIR filter are compared.Secondly,the stability analysis and the harmonic rejection capability of the IIR filter-based FOPCRC are analyzed.Finally,the simulation results demonstrate the effectiveness of the proposed FOPCRC.(4)The Double-fractional Multi-rate Plug-in Compound Repetitive Control(DFMPCRC)approach is studied.Multi-rate repetitive control(MRC)is introduced to reduce the amount of digital resources required by the control algorithm and to reduce the computing effort required by traditional repetitive control.A DF-MPCRC strategy is proposed to address the problems of imprecise MRC phase compensation and the same poor adaptation to grid frequency fluctuations.A Farrow structure filter based on Taylor series expansion is used to achieve fractional overrun and fractional delay,and the design steps for the required parameters are given according to the stability conditions of DFMPCRC.Finally,the feasibility of the proposed DF-MPCRC is verified by comparing the simulation results of MPCRC,MPCRC with fractional phase lead compensation,and DFMPCRC at different grid frequencies.