Global analysis of frequency stability and inertia in AC systems interconnected through an HVDC

In this paper, global frequency stability and inertia analysis is performed on a power system consisting of two asynchronous grids connected through a High Voltage Direct Current (HVDC) transmission. Frequency and inertia support providing controllers in the HVDC converters are analyzed with both linear/modal analysis and non-linear dynamic simulations. The analyses focus on dynamics that are in the range of seconds to several minutes in the ac grids. Electromechanical and governor (frequency) modes are studied in detail and interactions between the ac grids are identified. The results show that even though the auxiliary frequency controllers in the HVDC converters improve the frequency stability of the grid that is receiving frequency support, they also reduced the damping of the inter-area mode in the grid: especially when frequency controller with inertia is used. DIgSILENT PowerFactory simulation tool was used in the study.