Stability analysis of two types of grid-forming converters for weak grids

Grid-forming converters are designed to be capable of operating without a main grid. It is known that grid-following converters operating in weak grid conditions may experience dynamic voltage stability issues. Low-frequency oscillations have been observed in the real world. The objective of this paper is to examine grid-forming converters' weak grid operating characteristics. To this end, two types of grid-forming converters are examined for weak grid operation. Firstly, the steady-state operation limits are identified relying on optimization problem formulation and solving. It is found that both grid-forming converters may reduce the steady-state operation limit, compared to a grid-following converter. Secondly, dynamic stability limits are identified through electromagnetic transient (EMT) simulations. Furthermore, each converter's frequency-domain admittance and impedance characteristics are characterized using a data-driven system identification method. s-domain admittance-base eigenvalue analysis confirms the dynamic stability limit for each converter. It is found that low-frequency oscillations appear in one type of converters while do not appear in another type of converters. The two grid-forming converters can enhance dynamic stability comparing to the specific grid-following converter.