Research On The Islanding Detection Of Grid-connected Inverter System Based On Droop Loop Control

In recent years, the inverter based droop loop control used more and more widely in the distributed generation system because of its "plug and play", and there is no need for communication and other advantages.The key technology of reasonable and effective use of the Distributed Generation is to connect it to the grid. One of necessary functions of distributed grid connected system is island detection. But these methods are pointed for direct current of the inverter. At present, domestic and foreign scholars on droop control studies focused on its stability analysis, seamless conversion between grid connected operation mode and islanding mode, and the improved droop characteristic etc.. There’s almost no research on islanding detection on grid connected inverter droop control, which is the significance of this study.This paper firstly introduces the related knowledge of the droop loop control grid connected inverter and islanding effect. It analyzed the Non Detected Zone of the passive detection method for droop loop control grid connected inverter. Secondly, it classified the existed islanding detection methods which is used for direct current of the inverter to analysize their applicability for the grid connected inverter of droop loop control. Then through the reason of phase shifting method, frequency shifting method, and negative sequence component perturbation method can’t applied in the grid connected inverter of droop loop control, this paper proposed three modified islanding detection methods which suit to the grid connected inverter of droop loop control. Respectively, "phase shift detection method based on the change rate of phase difference", "improved frequency shift detection method with positive feedback" and "improved negative sequence voltage perturbation method". The detection principle is analyzed, and the important parameters are given in this paper.Thirdly, the proposed islanding detection methods are tested and verified by the simulation software MATLAB. And a three-phase grid-connected inverter system with the power 6k W is designed to the further verification. Then the tests of three-phase inverter in the cases of island, active synchronization control and connected to the grid are successfully completed. At last the two proposed islanding detection methods, "phase shift detection method based on the change rate of phase difference" and "improved frequency shift detection method with positive feedback", are verified on the experimental platform. It verifed the correctness of the theoretical analysis previously described.