Accuracy evaluation of a grid islanding impedance detection system

The incomplete knowledge of the power distribution grid status and in particular the impossibility to constantly monitor the number of subjects (feeder) able to inject current autonomously at a certain time can represent a hazardous condition for the provider operators safety. Whenever an unexpected islanding condition is induced on the utility grid, every subject injecting power on the grid should stop such injection switching to a safety shut down condition. Due to the presence of multi injecting sources on the utility grid it is possible that, one or more feeders do not recognize such islanding event and continue to supply energy to a portion of the utility grid, keeping the main provider operators unaware of such condition. The present work aims at describing an anti-islanding DSP based device able to satisfy the measurement time constraints (2 seconds) introduced by the reference standards, and to allow the identification of masked islanding conditions by means of both traditional techniques (frequency shift, over/under voltage frequency jumps etc.) and interharmonic voltage measurements. Such device will constitute a low cost, stand alone, automatic detection system for grid disconnection detection, to be supplied in conjunction with devices like the photovoltaic inverters (PV) in order to minimize their islanding non detection zone (NDZ).