MIMO sequence impedance modeling and stability assessment for grid-connected dual-PWM adjustable speed drives

• Grid-side converter exhibits frequency invariance during multiple modulations. • Derivation of the dual-PWM ASDs grid-side MIMO sequence impedance model. • IMDs interharmonic injection and bilateral coupling on dc link were incorporated. • Utilizes the impedance model to analyze grid interaction stability. • Impedance characteristics were analyzed and compared under different operating conditions. The dc link of dual pulse width modulation (PWM) adjustable speed drives (ASDs) is coupled with inverter-motor-drive systems (IMDs) and grid-side converters, forming a multi-coupled system that complicates grid stability analysis beyond that of traditional inverters. By identifying a previously unexplored frequency-invariance characteristic of the grid-side converter, this article clarifies the mechanism and characteristics of multiple modulations of perturbations from both sides of the converter. Building on this foundation, a novel dual-PWM ASDs grid-side MIMO (Multiple-Input Multiple-Output) sequence impedance model is developed. This model addresses the overlooked effects of IMDs’ interharmonic injection and bilateral coupling on the dc link, providing a comprehensive framework for analyzing grid interaction stability and offering new insights into grid-side dynamics. Validated through a laboratory prototype, this model uniquely characterizes the impedance behavior under varying motor operating conditions and dc-link capacitance values. Using Bode plots and the Nyquist stability criterion, the analysis and experimental results reveal new insights into how negative-resistive-inductive (NRI) impedance characteristics, particularly when the motor operates in the IV quadrant or when the dc-link resistance is low, contribute to poorer grid stability, leading to interharmonic oscillations at specific frequencies. The models and conclusions developed in this article provide a new perspective on refining grid-connected stability assessments for dual-PWM ASDs and similar AC/DC/AC converters.