Sequence-Impedance-Based Interactions Analysis of a VSG With Dynamic and Constant-Power Loads at Varying Grid Strength Conditions

Induction motor loads (IMLs) and constant-power loads (CPLs) are common in commercial and industrial load environments, underscoring the importance of understanding their impact on modern converter-based power systems. This paper characterizes the dynamic interactions of a grid-forming converter controlled as a virtual synchronous generator (VSG) with local IML and CPL under varying grid strength conditions. Analytical sequence impedance models for the VSG, IML, and CPL are developed to represent their dynamic behavior and to examine VSG-load-grid interactions using frequency response and Nyquist stability criterion analyses. The study reveals that an IML degrades the dynamic performance of the VSG, introducing low frequency oscillations under weak-grid conditions and potentially causing instability as the short-circuit ratio increases. The CPL, in contrast, enhances the VSG dynamics at low and high short-circuit ratios. Moreover, when coexisting with an IML in a VSG-IML CPL grid system, a CPL can enhance oscillation damping, broaden the range of stable control parameters, improve system dynamics during grid disturbances, and restore system stability at higher short-circuit ratios. The theoretical findings are validated through detailed nonlinear-model time-domain simulations.