Due to the increasing number of wind turbines directly connected to the electric utility grid, harmonics and inter-harmonics of variable speed wind turbines are capturing the attention of electrical power engineers. This paper develops a framework of doubly fed induction generator (DFIG)-based wind turbines for analyzing the harmonics and inter-harmonics at low frequencies due to wind turbines operated in different conditions. Two methods have been proposed to analyze harmonics and inter-harmonics related to the quantities such as excitation current, DC voltage, and AC. In order to detect the precise frequencies and amplitudes of the harmonics at low frequency, a modified S-transform measurement method with adjustable Gaussian window regulator is proposed. The time-frequency resolution of the S-transform can be changed when measurement accuracy enhancement is needed. In contrast to the conventional fast Fourier transform, the improved S-transform method considers the variations of frequencies and amplitudes of excitation current harmonics. Thus, the overall measurement performance is considerably enhanced with higher frequency resolution. Results from both simulation and experiment are presented to illustrate the merits of the modified S-transform measuring method in spectral analysis of harmonics and inter-harmonics. The experimental results from a practical 1.25 MW DFIG-based wind turbine under different operation conditions reveal that the characteristic frequencies with maximum harmonic currents change smoothly with the increase of the output active power.