Resonant frequency detecting method of the grid-connected inverter in accordance with one embodiment of the present invention, the harmonic having an integral multiple frequency of the fundamental frequency in a harmonic current extraction step, the harmonic current to extract the harmonic current filters the grid current of the grid-connected inverter a first coordinate conversion step, up to harmonic components extraction process, harmonic components having the maximum value of extracting a harmonic component having a maximum value by comparing the size of the DC component of the harmonic wave component to extract the DC component and converts the coordinates of the component a second coordinate conversion step, and a direct current component of the harmonic component having the non wanted frequency in the predetermined frequency range centered on the frequency transform coordinates of the harmonic component (Inter-harmonic) having a non wanted frequency for extracting a direct current component and comparing the magnitude of the DC component of the harmonic component having the maximum value W and a further resonance frequency detection stage for detecting the frequency of the harmonic components with a value of the resonance frequency.;
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