Abstract: The dual-pulse width modulation (PWM) adjustable speed drives (ASDs) couple the power system and the mechanical system. Through its modulation, the energy of the mechanical oscillation is injected into the grid. This article first analyzed the mechanism of common-modulation of harmonics and fundamental.。Oon this basis, a 2-D Fourier series switching function derived from the Bessel function was obtained. After that, the switching function was used to derive the harmonic impedance models of DC link and grid side of the dual PWM ASD, including the motor and the inverter. In particular, the model considereds the change of the steady-state impedance of the dual PWM ASDs due to the motor torque oscillation, and quantitatively expresseds the mechanical oscillation as the harmonic nonlinear impedance. The accuracy of the harmonic impedance model was verified by the developed dual PWM ASD, motor and mechanical load. Finally, the application of the model and developed theoretical analysis, combined with the measured data in the distribution grid of the Shanghai Yangshan deep-water port area, analyzed the characteristics and mechanism of the low-frequency oscillation events, and proved the accuracy and validity of the model and the conclusions.