Do Thanh Hieu, Nguyen Viet Ngu, and Le Thi Minh Tam. “AN IMPROVED SANDIA FREQUENCY SHIFT METHOD FOR ISLANDING DETECTION OF GRID CONNECTED PHOTOVOLTAIC INVERTERS”. UTEHY Journal of Science and Technology, Vol. 28, Jan. 2021, pp. 36-42, http://tapchi.utehy.edu.vn/index.php/jst/article/view/405.