| With the rapid development of the distributed generation system, the application of grid-connected inverter has been widely used, and the stability of inverter has been paid more and more attention. Especially when Distributed Generation may be located far away from load centers, because of the existence of long distance transmission lines and lots of transforms, the equivalent impedance is introduced to the grid and cannot be ignored, which would show a larger impedance characteristic. The interactive system is composed between the weak grid and the grid-connected inverters, and the control stability of grid-connected inverter would be also affected by the grid impedance.The effect of grid impedance to the stability of grid-connected inverter is analyzed in the thesis. Firstly, the small signal model of grid-connected inverter based LCL filter is derived, and the operation principle and design method of PR(Proportional Resonant) controller are discussed. Based on the weak grid modeling with the inductance and resistance in series, the stability of grid-connected inverter caused by the grid impedance is analyzed in quantity. Secondly, some impedance measurement methods are summarized, and the small signal injection method is used to measure grid impedance online. Furthermore, the passive damping and active damping control strategy are compared, and a novel hybrid-damping control strategy is proposed. Combined with the grid impedance online measurement, the adaptive control scheme of the grid-connected inverter based on hybrid-damping is introduced, which ensure the stable performance of grid-connected inverter with different grid impedance. At last, a 1.5kW grid-connected inverter prototype is built, and the design process of hardware and software are depicted, and the effect of grid impedance to the stability of grid-connected inverter is verified, and the feasibility of the adaptive hybrid-damping control strategy of grid-connected inverter is evaluated by the experiments. |
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