An Islanding Detection based on Virtual Impedance and Phase-Locked Loop

Synchronous reference frame Phase-locked loops (PLL) are widely used in three-phase DG (Distributed Generation) systems, and the PLL behaviors always affected by the grid and local load. When DG is at the grid-connected condition, the output of PLL always stays at power frequency, while islanding occurs, the output of PLL would jump to the resonant frequency of local load. However, we cannot identify whether islanding happens if the resonant frequency is almost equal to power frequency. To overcome this problem, we introduce virtual impedance in the control channel of the DG system to periodically change the output impedance so as to change the resonant frequency. We can use it to eliminate None Detected Zone (NDZ) of the passive method to detect islanding.