A Grid Impedance Shaping Method To Improve Robustness Of Grid-connected Inverter Under Weak Grid Condition

As an interface for energy exchange between renewable energy generation unit and power grid,the grid-connected inverter usually encounters the safety and stability issues.Especially,when it is connected to a weak grid,where the grid impedance commonly varies in a wide range,it will lead severe robustness challenges to the grid-connected inverter.Based on the impedance-based stability criterion,this paper reveals the stability issue of the gridconnected system,and further studies an adaptive impedance matching method for the gridconnected system,which aims to improve the robustness of the grid-connected inverter against grid impedance variation.Firstly,the paper builds the mathematical model of digitally-controlled LCL-type gridconnected inverter with capacitor-current-feedback active damping.According to the control block diagram,the corresponding Norton equivalent circuit which consists of a current source in parallel with an inverter output impedance is deduced.The interaction between the inverter output impedance and the grid impedance is analyzed according to the impedance-based stability criterion,and the fundamental cause which leads to the poor robustness of the digitally-control grid-connected inverter is revealed.In order to improve the inverter robustness,it is essential to keep the inverter output impedance and the grid impedance always matching.To achieve this objective,this paper adopts an impedance matching method,where an active capacitor converter(ACC)is installed at point of common coupling(PCC).The terminal characteristic of the ACC at AC side behaves like a virtual capacitor,and its capacitance can be adaptively adjusted in terms of the oscillation at PCC caused by impedance mismatching.From perspective of grid,the adjusted grid impedance and the inverter output impedance meet the impedance-based stability criterion.With the help of the mathematical model of the ACC,the adverse effect of switching frequency on its output impedance characteristic is revealed,and a phase lead compensator is proposed for reasonable phase compensation to the ACC,so that its AC side terminal characteristic can be closer to an ideal capacitor.To accurately extract the oscillating signal of PCC voltage,a bandpass filter is adopted,and the related parameters design is presented.The output impedance adaptive tuning algorithm of the ACC is presented,and the phase lead compensator,the outer voltage loop and the inner current loop of the ACC are properly designed so as to ensure the good performance of the ACC.Finally,a 6-kW experimental prototype of LCL-type grid-connected inverter and an ACC are built,and the theoretical analysis and the design of the ACC will be verified by the experiments.