Local anti-islanding protection for distributed generators based on impedance measurements

Distributed generators (DG), while working in conjunction with the main electricity grid, must have antiislanding protection. This paper presents a local detection scheme for distributed generators by using an SCR based device to draw a current pulse around the zero- crossing of the voltage waveform and analyzing the pulse to determine whether islanding has occurred. This method is based on the fact that the system impedance will change when islanding occurs, and therefore the current pulsepsilas characteristics will change. The peak of the current pulse is used to determine the impedance change and the decision to trip the DG. Theoretical analysis along with extensive computer simulations have shown that this method is largely independent on the system parameters and the proposed method was found capable of islanding detection very reliably. The proposed method has a negligible impact on the voltage waveform, thus reducing the power quality impact the device introduces.