Research On Modeling Of Harmonic Current Source At Point Of Common Coupling For Grid-Following Inverter

New-type power system is entering the direction of high proportion of new energy and high proportion of power electronic equipment("double high ").Large scale new energy generation equipment such as solar and wind energy are integrated into the power system.Grid-connected inverter is the key equipment of new energy grid-connected.However,its nonlinear characteristics and control characteristics lead to abundant harmonic components in the grid-connected output current,which not only causes harmonic pollution of the grid,but also may cause the system resonance problem.Grid-connected inverters are mainly divided into grid-forming inverter and grid-following inverter,among which gridfollowing inverter occupies the main market.So this paper takes grid-following inverter as the research object to study the harmonic current source model of grid-following inverter in strong and weak current grid,in order to accurately obtain the harmonic current amplitude and phase output of following inverter,and provides a theoretical basis for harmonic control and resonance analysis of power grid.Firstly,this paper analyzes the wide-band harmonic spectrum generated by grid-following inverter.This paper analyzes the primary harmonic spectrum caused by pulse width modulation(PWM)of grid following inverter,and analyzes the secondary harmonic spectrum caused by PWM under background disturbance and frequency coupling effect under control asymmetry,which lays a theoretical foundation for the harmonic current source model of grid following inverter considering the frequency coupling effect.Then,the harmonic current source model of grid-following inverter in strong grid is established in this paper.The transfer process of the disturbance voltage in the outer loop,inner loop,phase-locked loop,signal modulation module and filter of the inverter is analyzed.The harmonic current source model of the inverter is established by using the direct transformation method and the forward and reverse rotation method,and the advantages and disadvantages of the two methods are compared.A simulation model of grid-following inverter is built,and the accuracy of the harmonic current source model of grid-following inverter in strong grid is verified by comparing the simulation value and the calculated value of harmonic current under 5th to 13th background disturbance harmonic.Finally,the harmonic current source model of grid-following inverter in weak grid is established.Large grid impedance will affect the stability of gridconnected inverter.Therefore,this paper designs a control strategy combining the voltage proportional feedforward control and capacitive current feedback active damping to achieve good stability of grid-connected inverter in weak grid.The transfer process of the disturbance voltage in each link of the inverter is analyzed,and the harmonic current source model of the inverter is established by the forward and reverse rotation method.The similarities and differences of harmonic current source models in strong grid and weak grid are compared.In weak grid,two response harmonic currents interfere with each other through the grid impedance,while in strong grid two response harmonic currents are relatively small in the output channel.A simulation model of grid-following inverter in weak grid is built.The accuracy of the harmonic current source model of gridfollowing inverter in weak grid is verified by comparing the simulation value and the calculated value of harmonic current under 5th to 13th background disturbance harmonic.