Modeling and Stability Analysis of Converter-Dominated Grids with Dynamic Loads

In recent years, continuous efforts have been made on the modeling and stability analysis of converter-dominated grids (CDGs) to guarantee efficient, stable and resilient operations. The literature has tried to reveal the mechanism behind abnormal instability and resonances caused by the interaction between multiple time-scale control loops within a single converter, different types of converters and power networks. It is commonly assumed that CDGs are three-phase balanced systems and supply only passive loads. In reality, CDGs can experience imbalance caused by asymmetric networks, loads or faults. Moreover, induction motor (IM) loads, which exhibit highly nonlinear couplings between dynamics of power, voltage and frequency, typically account for a large portion of electric loads. Ignoring the impact of imbalance and dynamic loads in the modeling and stability analysis of CDGs can lead to unrealistic stability assessment results. To fill the gap, this paper presents a general small-signal modeling framework for CDGs in the presence of IM loads. Linear time-periodic (LTP) eigenvalue analysis is performed to investigate the impact of the interaction between IM loads, grid-following (GFL) converters and virtual synchronous generator (VSG) converters on the system stability. The time-domain simulation and experimental results validate the theoretical analysis.