Research On Active Harmonic Compensation Methods In Power-Electronized Distribution Grids

With the rapid development of renewable generation,harmonic compensation and other applications,power-electronized device has continuously increased the penetration level of the distribution grids,and there is a clear trend that the distribution grid is being power-electronized.On the other hand,the flexible characteristics of power-electronized devices cause the structural characteristics of the distribution grids to continuously change,thereby increasing the risk of failure of the active harmonic compensation method and seriously threatening the safe and stable operation of the distribution grids.Therefore,it is of great significance to improve the adaptability of active harmonic compensation methods in power-electroniczed distribution grids and improve the effect of harmonic compensation.This article takes the three-phase three-wire two-level LCL grid-connected inverter as the research object,and the goal is to improve the power quality in the power-electronized distribution grids safely and effectively.The main objectives of this paper are highlighted as follows:Firstly,the mathematical model of the grid-connected inverter in various reference frame is given,which lays the foundation for the establishment of the impedance model of the grid-connected inverter later.Because the grid-connected inverter controls the current more frequently in the grid-connected state,the paper then establishes its impedance model in the current control mode.Considering the complexity of the power-electronized distribution grids,this paper establishes the impedance model of the grid-connected inverter in voltage control mode.Then,the impedance stability criterion is given and used to determine the external stability of the grid-connected inverter in the current control mode and the voltage control mode,which lays the foundation for the analysis of active harmonic compensation in the power-electronized distribution grids.Secondly,the basic working principle of active harmonic compensation is briefly analyzed,and the impedance model of current harmonic compensation is established.Then the impedance model of the grid-connected inverter in the current control mode and the voltage control mode is used to analyze the passivity and stability of the current harmonic compensation in the power-electronized distribution grids,and it is pointed out that with the number of grid-connected inverters increase,the passiveness of equivalent admittance weakens and has different passiveness under different combination schemes.In order to improve the passivity of equivalent admittance under multi-grid inverters,on the premise of ensuring the stability of the system,the parameters of low-pass filterK_f and a should be reduced as much as possible,and the feedback gain of the capacitor currentK_ad and the proportional gainK_p should be increased.The integral gianK_ihh has little effect on the passiveness of equivalent admittance.Finally,considering the limitations of current harmonic compensation,the impedance model of voltage harmonic compensation is established,and its admittance is analyzed.It shows that the reduction of parameter k,although it can improve the compensation effect,does not contribute to the stability of the system.Then this paper analyzes the passivity and stability of voltage harmonic compensation in power-electronized distribution grids,and points out that its passive change law is consistent with current harmonic compensation.In order to improve the passivity of equivalent admittance under multi-grid inverters,on the premise of ensuring system stability and response speed,the parameters of low-pass filterK_f and the proportional gainK_p should be reduced as much as possible,and the feedback gain of the capacitor current K_ad should be increased.The parameters of low-pass filter a and the integral gianK_ihh have little effect on the passiveness of the system.