Research On Active Damping Control Of Single-phase LCL Grid-connected Inverter In Weak Grid

In order to improve the efficiency of renewable energy,distributed generation technology has been widely applied.The inverter plays the role of connecting the distributed power supply and the grid.Its safe and stable operation is of great significance for injecting high-quality current into the grid.At the same time,in order to make full and efficient use of renewable energy,distributed power stations are usually built on remote areas.Therefore,it is necessary to realize the cross regional transmission through long wire,so that the influence of line impedance must be considered,that is,the grid-connected inverter system shows the characteristics of weak grid.The change of the line impedance in the weak grid will cause the resonant frequency to shift,and the control performance of the existing strategy will be affected or even fail,which will affect the quality of the grid-connected current.Therefore,this paper taking the single-phase LCL gridconnected inverter as the research object,and discusses the improvement of the existing active damping strategy under weak grid,so as to enhance the quality of grid-connected current and improves the grid-connected performance.Firstly,this paper establishes the LCL grid connected inverter and its control model,explains the main functions that can be realized by each part,and makes appropriate selection according to the required functions.In connection with the traditionaly capacitor current feedback active damping,the influence of delay and the change of grid inductance on control accuracy and stability is mainly considered and verified by simulation.Secondly,the traditional active damping needs to detect the system state and use it for feedforward/feedback control,but the control accuracy will be affected by the sensor,and the sensors are generally more expensive.Therefore,a capacitor current feedback active damping strategy based on state observer is proposed,and the optimal proportional feedback coefficient of the control strategy is analyzed based on passivity theory.Considering the influence of filter parameter offset on the optimal scale coefficient,an all-pass filter with phase compensation is added to reshape the impedance,improve the passivity and ensure the system stability.The feasibility of theoretical analysis is verified by simulation.Then,to solve the problem of narrow valid damping region in the traditional capacitive current feedback active damping control strategy,this paper proposes an improved capacitive current feedback active damping strategy based on negative first-order low-pass filter.Aiming at the shortcomings of traditional capacitive current feedback active damping,the advanced capacitive current feedback active damping strategy can compensate the phase lag caused by delay and expand the system stability range by introducing the negative first-order low-pass filter into the capacitor-current feedback branch.At the same time,the relationship between negative firstorder low-pass filter coefficient and grid inductance is given.The effectiveness of the proposed strategy is verified by simulation.Finally,the hardware in the loop simulation platform RT Box is used to build 2.5k W singlephase LCL grid connected inverter experimental prototype.The practical feasibility of the impedance reshaping passive improvement strategy based on state observer and improved active damping strategy based on negative first-order low-pass filter is verified.