Anti-islanding detector based on a robust PLL

International Standards impose to Distributed Energy Resources, connected to the grid by an inverter, to detect an islanding condition within a suitable time interval. In this paper a Phase Locked Loop (PLL), based on a third-order prediction-correction filter, is proposed to implement an islanding detector with reduced detection time. Such feature is obtained using the estimation of the grid angular frequency and acceleration provided by the PLL with a negligible time delay. The proposed approach is implemented on a DSP and validated through an experimental comparison among different detection methods, such as Rate of Change of Frequency (ROCOF) and Sleep Mode frequency Shift (SMS). A combined use of ROCOF and SMS is also illustrated and discussed.