Research on an improved active frequency drift method for the grid-connected anti-islanding detection

In general, the active frequency drift (AFD) method is used to detect the islanding phenomenon for the distributed grid-connected power system. However, when the angle of the current disturbance matches with the load angle in the range of the non-detecting zone (NDZ), AFD method cannot identify the islanding. In order to solve the above problem, an improved AFD method based on the alternate current disturbances between the odd and even cycles is proposed. The current frequency drifts up in the even voltage cycle, and in the odd voltage cycle the current frequency drifts down. The islanding is detected by judging whether the voltage frequency at the point of common coupling drifts up or down between the even and odd cycles alternately. Simulation and experimental results verify that the proposed method can not only identify the islanding very well, but also solve the NDZ problem.