Islanding Detection With Positive Feedback of Selected Frequency for DC Microgrid Systems

The oscillation frequency of the point of common coupling (PCC) voltage is proposed as a new detecting freedom for dc-based islanding events in this article. In the presented method, the voltage positive feedback with a frequency selection is applied to a distributed generator to make the PCC voltage oscillate at the selected frequency when islanding events occur. With the frequency information, islanding events can be identified by the small-amplitude oscillation superimposed on the normal dc voltage without the necessity of voltage amplitude to be shifted out of the normal range. Thus, the islanding events can be effectively distinguished from other abnormal conditions that lead to large voltage fluctuations, and so, the accuracy of islanding detection is improved. Also, the power quality at islanding events is enhanced, which provides important benefits for seamless transfer from the unintentional islanding conditions to the controlled islanding conditions. Moreover, the frequency selection is designed to only transfer the voltage signal at the sensitive frequency band of the islanding condition. As a result, the deterioration of stability caused by the voltage positive feedback can be relieved under the grid-connected condition compared to the existing methods. Finally, the effectiveness of the proposed method has been verified by both time-domain nonlinear simulation and experimental results.