Research on 2nd harmonic impedance measurement based active islanding detection method

A new active islanding detection method based on 2^nd harmonic impedance measurement is studied. With this method, the inverter current reference is slightly modified by an injected signal which affecting the in-grid current in a negligible way. When the grid becomes unavailable, the value of grid impedance moves outside of a preset threshold value, and the 2^nd harmonic voltage of the inverter becomes bigger enough to identify the islanding. Influence on the in-grid current and Non-Detection-Zones (NDZ) by the islanding detection method is analyzed in detail. This method is promised smaller disturbance to the grid and almost no NDZ. In addition, the method is highly robust to different grid disturbances and stiffness, and effective for multiple inverters running in parallel. A NDZ map which illustrates the NDZ by quality factor of the loads and 2^nd harmonic component of the grid voltage is proposed. The new islanding detection method has been tested on a prototype inverter successfully, and it is verified that the new method meets the standard requirements IEEE 929-2000 well.