Anti-islanding performance of grid-connected inverters based on frequency droop PLL

Unity power factor can be obtained if phase lock loop (PLL) with droop characteristic is used for grid-connected inverter. However, the non-detection zones (NDZ) of the droop PLL is too wide to satisfy some standard. In order to decrease the NDZ, an improved PLL is proposed, which introduces phase disturbance into the control strategy. The operating principle is analyzed and the selecting rule for phase disturbance parameter is given. The anti-islanding capacity of the droop PLL is obtained and the NDZ of the improved PLL is significant decreased. Moreover, the biggest advantage of the improved PLL is that the quality of grid current is not affected by the phase disturbance. The simulation and experimental results verify that the proposed method is effective in islanding detection.