MIMO and SISO Impedance-based Stability Assessment of a Distributed Power Source Dominated System

In modern power grids, many distributed generators (DGs) are connected via grid-connected inverters. Generally, the possibility of resonances among inverters and other equipment placed in proximity and the network itself must be analyzed in advance. However, it is often difficult to approach the problem for complex systems by using model-based techniques, as they require obtaining a linear approximate analytical model. The impedance-based stability analysis method provides a practical alternative to the linear model approximation method. It directly measures the impedances of the source and load subsystems and evaluates its stability using the Nyquist criteria. In this study, we assume a scenario in which grid-connected inverters with advanced inverter control, such as the virtual synchronous generator (VSG) control, will become the primary power source in the near future and focuses on the possibility of instability phenomena caused by (i) a large-scale system with long-distance transmission lines using 7r-sections and (ii) the output filter of DGs with the natural frequencies in high frequency band. Accordingly, the impact of the dominant gridconnected inverter on stability in a large, complex system is evaluated based on the impedance in both the dq (MIMO) and equivalent sequence (SISO) domains. The results suggest that grid-forming controls, such as VSG controls, may resonate with their output filters in the kHz band and that capacitance in long transmission lines cannot be ignored as they can cause system instability. Moreover, it is found that the unstable frequencies identified in the two eigenvalue loci of the dq impedance are also identified in the sequence domain impedance within a single Nyquist plot, further validating the presence of unstable frequencies.