Study On Digital Control Scheme Of Grid-connected Inverter Based On LCL Filter

Energy crisis and environmental pollution are now seriously restricting the sustainable development of the society,and the distributed generation technology based on renewable energy has been paid more and more attention and research.The grid-connected inverters are universal interfacing device for connecting distributed generation systems with the utility grid.The LCL Filters are applied widely in the distributed generation system because they show a good performance on suppression of high-frequency harmonics.However,LCL filter has a resonant peak which may raise instability if the controller is not suitably designed.This thesis focuses on the design and optimization of the LCL filter parameters,the control strategy of the grid connected inverter system and the design of the closed-loop parameters under the digital control.Firstly,the equivalent model of LCL filter is built and its operating characteristics is analyzed,revealing the relationship between the filter parameters.Based on this,an optimization design program of LCL filter parameters is proposed.In the program,the basis and idea of parameter calculation are demonstrated,and then are the criteria and steps of the filter parameter selection procedure,at last,the filter parameters are further optimized regarding to the harmonic suppression performance,current ripple and THD of the grid connected current,etc.Secondly,for the control strategy of grid interfaced invertion system with LCL filter,the impact of digital control system are analyzed,and the stability of the single loop control of the inductor current in the network side and in the inverter side under the digital control are revealed,and under the control of digital control delay,the capacitance current feedback active damping is equivalent to the virtual impedance,and the virtual impedance at different frequencies show different characteristics,and according to the relationship between f_r and f_sw/6,the constraint conditions in three cases are derived.The closed loop control parameters are designed for meeting the requirement of phase margin and stability margin.Besides,in order to solve the influence of the digital control delay on the stability of the system,a digital control capacitor current feedback delay compensation strategy is proposed.Thirdly,the basic characteristics of sliding mode control are introduced.The sliding mode controller is designed for the LCL grid connected inverter,and a sliding mode control strategy is proposed,and the influence of filter parameters,load changes and power grid harmonic changes on the system are analyzed.Finally,the simulation results are given to verify the effectiveness of the proposed control strategy.Finally,the prototype of a LCL-type single-phase grid connected inverter has been constructed in the laboratory,and the three kinds of control methods,which are the single-loop control of the inductor current on the network side,the single-loop control of the inductor current on the inverter side,and the control method of the outer loop of the inductor current in the network,are carried out.In addition,the time delay compensation strategy of the inner-loop inductance current and outer-loop capacitor current is verified.The experimental results show that the control strategy can effectively reduce the THD of the grid current.