An Islanding Detection Method by Using Frequency Positive Feedback Based on FLL for Single-Phase Microgrid

An active islanding detection method based on Frequency-Locked Loop (FLL) for constant power controlled inverter in single-phase microgrid is proposed. This method generates a phase shift comparing the instantaneous frequency obtained from FLL unit with the nominal frequency to modify the reference phase angle. An initial low frequency variable triangular disturbance is added to the phase shift in order to reduce NDZ and accelerate the detection process especially in the case of power matching. With the modified phase angle, the frequency at PCC will be drifted away from the nominal frequency until exceeding the threshold because of the frequency positive feedback after islanding. Besides, FLL is introduced to this method in order to lock frequency quickly considering that the frequency is time-varying during the islanding detection process. Simulation and experiment have been done to evaluate this method.