Analysis and performance assessment of the active frequency drift method of islanding prevention

Islanding of photovoltaic (PV) systems can cause a variety of problems and must be prevented. However, if the real and reactive powers of the load and PV system are closely matched, islanding detection by passive methods beco mes difficult. Also, most active methods lose effectiveness when there are several PV systems feeding the same island. The active frequency drift meth od (AFD), also called the frequency bias method, enables islanding detectio n by forcing the frequency of the voltage in the island to drift up or down . In this paper, AFD is studied analytically, using the describing function analysis technique, and by simulation, using MATLAB. It is shown that AFD has a nondetection zone (NDZ) in which it fails to detect islanding, and th at this NDZ includes a range of unity-power-factor loads. Finally, the pape r describes a novel method using positive feedback which significantly shri nks the size of the AFD NDZ.