Research On Drive And Robust Control Of High-frequency Grid-connected PV Inverter Under Weak Grid

Photovoltaic power generation system play an important role in the situation of primary energy’s shortage.Grid-connected inverters are the key link of the whole PV system,and the power semiconductors,the core of inverters,determine the overall performance of the system.In recent years,the "third-generation semiconductor device" SiC MOSFET has attracted much attention due to its superior physical properties such as high saturation electron drift rate and high thermal conductivity,which has remarkable effect on improving efficiency and power density of the inverters.Therefore,it is of great practical significance to study the grid-connected inverters based on SiC MOSFET.However,the high-speed switching characteristics of SiC inverters make it more prone to voltage and current oscillation during switching process,while the stability of SiC grid-connected inverters is greatly affected under complex grid conditions such as weak grid.So how to design a high-stability SiC grid-connected inverter is an important research direction.Aiming at the switching oscillation in high-speed switching state,this paper first analyzes the distribution and influence of parasitic parameters in SiC application circuit.By establishing switching oscillation models,the main factors causing oscillation and the damping absorption circuit are studied.Combined with the estimated loop parasitic parameters and the designed absorption circuit parameters,the double-pulse test is carried out to verify the suppression effect of the proposed method on the oscillation,and to provide hardware guarantee for the stable operation of the high-frequency grid-connected inverters.Aiming at the problem of low stability of LCL high-frequency grid-connected inverters under weak grid,an impedance model based on sensorless resonant damping method of negative HPF(High-pass Filter)is established in this paper.The GMPM(Gain Margin and Phase Margin)criteria is adopted as the stability analysis method.The limitations of high-frequency inverters based on traditional voltage feedforward method under the condition of grid impedance and background harmonics are analyzed,and then a composite robust control method is proposed.By improving the feedforward function and the current controller,this method can not only suppress the harmonics of output current and improve the quality of grid-connected power,but also ensure the system’s stability under weak grid.The simulation model is built in PLECS software to verify the correctness of the method.On the basis of the above research,the experimental platform of LCL high-frequency grid-connected inverter is built to verify the composite robust control method under weak grid proposed in this paper.The experimental results show that this method can not only suppress the resonance of LCL filter and improve the stability margin of inverters under weak grid,but also minimize the impact of grid background harmonics on grid-connected power quality.