The function of islanding detection is required of the grid-connected fuel cell system to protect safety of equipment and humans and to maintain the quality of power supply. A novel active islanding detection method with short detecting time and small non-detected zone is proposed. This method is a combination of the adaptive slide mode frequency shift method that adds acceleration gain when frequency is drifted out of grid's operational frequency and the q-axis reactive current disturbance method. As the grid is disconnected

the islanding phenomenon is detected by making the frequency disturbance faster and drifting the frequency out of the allowed normal range to result in under/over frequency protection. Moreover

very small disturbance is added to the control signal in this method so that there is no negative impact on power quality when the grid works in the normal state. In the coordinates of Qf-f0 and Qf0-Cnorm

the non-detected zone of the proposed method and two traditional methods are analyzed. Simulation and experimental results show that the method proposed is applicable to loads with high quality factor

shortens the time of islanding detection and is suitable for smaller non-detected zones in comparison to previous methods.