Investigation of Frequency drift methods of Islanding Detection with multiple PV inverters

Islanding Detection is considered one necessary functionality for grid-connected photovoltaic (PV) inverters according to standards and grid-code limits. Diverse islanding detection methods, passive and active, have been proposed in literature to cope with this safety requirement. Among them, active methods are used to reduce and eliminate the nondetection zone (NDZ) when power is almost balanced between source, grid and load. However, it is believed that if two or more PV inverters are connected to the same point of common coupling (PCC), their performance may degrade and can fail in detecting islanding. Few works focus on this field. In this paper, the commonly used active methods of active frequency drift (AFD), Sandia Frequency Shift (SFS) and slip-mode frequency shift (SMS) are comprehensively investigated for multiple inverters system. One phase criterion is theoretically derived to analyze the principle and nature of mutual interference among the aforementioned methods. Simulation is subsequently designed and carried out in the Matlab/Simulink environment to validate this phase criterion. Results of simulation suggest that, for these frequency drift methods, the perturbation type - unidirectional or bidirectional - is the key characteristic resulting in mutual interference in multiple inverters system.