Research On Current Harmonics Suppression And Robust Optimization Strategy Of LCL Type Three-phase Gridconnected Inverter

With the continuous increase of global energy consumption,the research and application of new energy such as wind energy and solar energy are developing rapidly,especially the proportion and growth of photovoltaic power generation.The performance of photovoltaic inverter is related to the quality,efficiency and safety of photovoltaic power generation.Therefore,LCL type three-phase grid-connected inverter is selected as the research object in this paper to explore the optimization strategy to improve the grid-connected harmonics and robust.The research on damping technology,current control technology,harmonic suppression strategy and robustness of LCL type PV inverter is summarized firstly.The working principle and peak damping control strategy of LCL type PV inverter are analyzed.The stability criterion based on impedance in weak grid is deduced.The LCL type three-phase grid-connected inverter and its output impedance model in weak grid are constructed.On this basis,the current harmonic caused by the background harmonic of the grid voltage and its suppression method are studied in this paper.The grid voltage proportional-feedforward control strategy and the grid voltage full-feedforward control strategy are compared and analyzed.Using the concept of equivalent grid-connected voltage,an improved grid voltage proportional-feedforward control strategy is proposed.By reasonably selecting the value of filter capacitor,the value of inductance and the gain of inverter bridge in LCL type three-phase grid-connected inverter,the harmonic content of equivalent grid-connected voltage can be reduced and the quality of grid-connected current can be improved.In the weak grid,the output impedance phase angle of grid-connected inverter will be reduced by using feedforward control strategy of grid voltage,which leads to the decrease of phase margin and robustness of the system.Therefore,based on the stability criterion of LCL type three-phase grid-connected inverter and the output impedance model,the relationship between output impedance and harmonic suppression ability and robustness is deduced.The parallel virtual impedance method of grid voltage proportional-feedforward control strategy and grid voltage full-feedforward control strategy is compared and analyzed.A series and parallel virtual impedance correction method based on proportional-feedforward control strategy of grid voltage is proposed.By changing the nature of series virtual impedance and compensating the steady-state error of current source,the harmonic of grid-connected current is reduced and the robustness of the system is improved.Finally,the LCL type three-phase grid-connected inverter experimental prototype is built to realize the improved proportional-feedforward control strategy of grid voltage.In the theoretical range of filter capacitance,the low harmonic in the gridconnected current is effectively suppressed by reducing the value of filter capacitance,and the total harmonic distortion rate is reduced.The virtual impedance correction method based on proportional-feedforward control strategy of grid voltage is also realized.Under different grid impedance conditions,the total harmonic distortion rate of grid-connected current is always less than 3%.The experimental results fully verify the effectiveness of the optimization strategy.