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Positive feedback anti-islanding methods have been widely applied to inverter-based distributed generators recently due to their high cost-performance ratio. The effectiveness of these anti-islanding methods is usually demonstrated by means of steady-state non-detection zones (NDZs) represented in a load parameter space (LPS). However, these NDZs cannot accurately describe the impact of the inverter interface controls on the anti-islanding methods' detection performance, especially in multi-inverter systems. In addition, the intrinsic destabilization characteristic of the positive feedback anti-islanding scheme cannot be exhibited by these NDZs. This paper proposes an improved dynamic NDZ in the LPS to evaluate the islanding detection effectiveness of the positive feedback anti-islanding methods. The modal analysis approach is employed to determine the critical RLC load quality factor which is defined as the small-signal stability limit index of the islanded distributed generation (DG) systems for the dynamic NDZs. The sensitivity analysis of the dynamic NDZs for different DG system parameters is conducted in this paper. The applications of the proposed dynamic NDZs in multi-DG systems are also presented.