Aiming at the non-detection zone problem of traditional passive islanding detection for grid-connected PV（PhotoVoltaic） system

a detection method based on bi-direction voltage harmonic variation is proposed. With the analysis of voltage harmonic variation before and after islanding and failure mechanism of traditional voltage harmonic detection method

an islanding detection strategy based on bi-direction voltage harmonic is proposed. The windowed Fourier transform is used to calculate the harmonics of outgoing line current and bus voltage in centralized grid-connected PV system

and thus the harmonic power is obtained. The direction of each harmonic source is determined by the positive or negative polarity of harmonic power in grid-connected operation

and the characteristic orders of harmonic at system side and PV side are selected according to the maximum positive and negative harmonic power values. The sliding windowed Fourier transform is adopted to analyze bus voltage harmonic variation of the two orders

and the islanding detection criteria is established. If the reduction of voltage harmonic at system side and the increment at PV side both meet the criteria

there is an island. Simulative results verify the effectiveness of the proposed method. The method does not affect the output power quality of grid-connected inverters

and can effectively detect islands under serious background harmonics with a small non-detection zone.