| Summary: | For safety and maintenance problems, the grid-connected inverters have been required to detect whether the utility voltage source is connected or disconnected to the grid. If the inverter continues to energize local loads while the utility voltage source is disconnected, it is called islanding operation. This paper proposes a new islanding detection method using phase shifted feed-forward voltage. In the grid- connected inverter system, the PCC voltage is simultaneously measured, and those values are used as the feed-forward values in the current controller. In this paper, the phase of feed-forward voltage is modified to detect the islanding condition. As the measured PCC frequency changes, the proposed method modifies the phase of feed-forward voltage. Because the phase angle of feed-forward voltage is determined according to the variation of measured frequency of PCC voltage, the proposed method makes positive feedback to the frequency. If the frequency variation exceeds the allowable range, it is judged to be islanding operation. Since the frequency response of the proposed method is fast, the islanding operation could be detected in 90 ms despite high quality-factor ($Q_{f}$ ), 10. Moreover, when the grid is normal condition but grid frequency changes in allowable range, the proposed method does not produce continuous reactive power which reduces the quality of inverter output power. Lastly, proposed method has very small NDZ because there is no limitation on reactive power injection. The performance of the proposed islanding detection method was verified by a computer simulation and experiment with 600 W single/three phase grid-connected inverters.
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