According to the voltage magnitude tendency toward islanding and the pulse width modulation control principle of grid-connected inverter

an active islanding detection method based on modulation ratio directional shift control is proposed. The modulation ratio

as the voltage control signal of pulse width modulation

is able to change the voltage magnitude in the islanding mode. The modulation ratio directional shift control method can rapidly decrease the voltage magnitude to detection threshold to eliminate the non detection zone without negative influence on power quality. Compared with the traditional islanding detection by voltage magnitude

this method has improved the stability of detection speed in different active power conditions while doing away with the ineffectiveness of the traditional method in the maximum active power mode. Simulation results verify the effectiveness of the method.