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Distributed power generation systems (DPGSs) based on inverters require reliable islanding detection algorithms (passive or active) in order to determine the electrical grid status and operate the grid-connected inverter properly. These methods are based on the analysis of the DPGS voltage, current, and power in time or frequency domain. This paper proposes a time-frequency detection algorithm based on monitoring the DPGS output power considering the influence of the pulsewidth modulation, the output LCL filter, and the employed current controller. Wavelet analysis is applied to obtain time localization of the islanding condition. Simulation and experimental results show the performance of the proposed detection algorithm in comparison with existing methods.

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Abstract-Grid-connected photovoltaic (PV) inverters employ an islanding detection functionality in order to determine the status of the electrical grid. In fact the inverter must be stopped once the islanding operation mode is detected according to the standards and grid-code limits.Diverse islanding detection algorithms have been proposed in literature to cope with this safety requirement. Among them active methods, based on the deliberate perturbation of the inverter behavior, can minimize the so called non detection zone (NDZ) that is the range of conditions in which the inverter does not recognize that is operating in undesired island. In most cases, the performances of these methods have been analyzed considering an highly dispersed generation scheme, where only one distributed generation power system (DGPS) is connected to the local electrical power system (EPS). However in some studies it has been highlighted that if two or more PV inverters are connected to the same local EPS, their anti-islanding algorithms do not behave ideally and can fail in detecting the islanding condition. However there is no systematic study that has investigated the overall capability of different anti-islanding methods employed on several inverters connected to the same EPS to detect islanding condition. This paper is a first attempt to carry out a systematic study of the performances of the most common active detection methods in case of two inverters connected to the same EPS. In order to evaluate the global capability of the two systems to detect islanding condition a new performance index is introduced and applied also to the case when the two inverters employ different anti-islanding algorithms.

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