Islet operation detecting method for photovoltaic parallel network generating system

The invention discloses an islet operation detecting method for a photovoltaic parallel network generating system, which comprises a passive detection part and an active detection part, wherein the passive detection comprises the following steps of: detecting a voltage value of a parallel network point and a frequency value of a voltage in real time, judging whether the voltage value of the parallel network point and the frequency value thereof reach a protected threshold value, and if one value or two values reach the protected threshold value, outputting islet protection information; otherwise, performing active detection which comprises the following steps of: acquiring the voltage frequency at the parallel network point, shifting the inverter output current frequency to cause fluctuation of voltage frequency of a load end, if the voltage of a power grid disappears, showing that the point voltage frequency of the parallel network point deviates from the original value due to the influence of the current frequency, and when exceeding a normal value, sending information that an islet is detected and sending an off-line actuating signal. The method can efficiently detect the islet phenomenon, and has the characteristics of wider use, higher efficiency and stronger reliability.