Composite islanding detection method based on the active frequency drift and voltage amplitude variation

Efficient and reliable islanding detection method is essential for the grid-connected photovoltaic systems. In spite of a high detection efficiency, the active frequency drift (AFD) method cannot identify the islanding in the case of power matching and is limited by the amount of total harmonic distortion (THD) injected into the grid. To cope with the above problems, a novel composite islanding detection method based on the change of both voltage frequency and amplitude is proposed, which is divided into dominant and auxiliary detection in accordance with active frequency drift and amplitude variation respectively. Simultaneously the assignment way of chopping fraction is improved to further reduce current harmonic distortion. The results derived from Matlab simulation indicate that it not only can largely narrow the non-detection zone of AFD method, especially the most difficult condition of islanding detection, but also significantly diminish the harmonic impact on the grid.