Because unplanned islanding of distributed generation system seriously affect the maintenance security

damage network equipment and have other damages

therefore grid-connected inverter requires islanding detection.Active frequency drift（ AFD） islanding detection method is commonly used for detecting island

but many antiislanding active frequency shift methods active in single inverter while deteriorate or even fail in a multiple inverter parallel system. Based on resonant frequency coordinate and non-detection zones（ NDZ） theory of load qualityfactor

failure mechanisms are analyzed under multi-parallel system. Simulation and experimental results are presented to validate the correctness of theoretical analysis

provide theoretical guidance for multi-inverter islanding detection design and multi-machine parallel operation.