Discrete Fourier transform/fast Fourier transform (DFT/FFT) spectrum leakage will occur during nonsynchronous sampling in fundamental frequency

with harmonics and inter-harmonics spectrum mutual interference affecting the measurement results

while the traditional interpolating windowed FFT method ignores the influence of long-range leakage from other components on the component of concern. Firstly

the spectrum of Hanning weighted signal in IEC framework is transformed

to change side lobes adjacent spectral phase difference to 180 degrees in other components. The vector sum of adjacent lines is used to eliminate the influence of long-range leakage on the component of concern

thereby proposing the group measurement method and accurate measurement method of harmonics and inter-harmonics. The measurement of harmonics and inter-harmonics specific parameters is realized. Based on this

the harmonics and inter-harmonics separating method is presented

and furthermore

by successive removal of the leakage value of known components on the spectral lines of the component of concern the measuring accuracy is improved. During nonsynchronous sampling without increasing the length of the sampling data

an effective solution to the harmonics and inter-harmonics inability to be accurately measured when closely spaced is found. Simulation and engineering data have validated real-time and effective characteristics of the proposed methods in the IEC framework.