An Impedance-Based Islanding Detection Method for DC Grids

Islanding detection in both AC and DC grids is a critical issue since failure to detect it could lead to unstable operating points and danger to the user and equipment. While significant research has been done on AC grid islanding, DC grid detection techniques are in their infancy. A popular hybrid DC grid topology uses a line regulating converter that interfaces with the main grid to regulate the voltage by exchanging power with it, while distributed generators, loads, and storage focus on optimizing their energy production/consumption profile. Most modern techniques for islanding detection in DC grids use over/under voltage ranges (which fail if the load closely matches the source during the event) or the injection of increasingly larger perturbations (which take time and disturb the operating point). In this manuscript, a novel impedance-based method for islanding detection is introduced. The method is implemented using a digital Lock-In Amplifier along with sensors normally included in PV systems. By using the impedance, this method is capable of quickly identifying the islanding event and acting on it. This proposed method offers: 1) low amplitude signal injection; 2) high speed detection; and 3) high sensitivity. The behaviour of the proposed technique under different kinds of loads (constant resistance, constant power, constant current) is studied and simulations are shown using different loads. Finally, experimental validation of the impedance detection technique is presented implemented in a standard microcontroller.