Investigation of Frequency Stability in Hybrid Grids During HVDCBased Curative Congestion Management

Inhalt:
Over long distances, high-voltage direct current (HVDC) systems offer greater transmission capacity and lower losses than high-voltage alternating current (HVAC) systems. In addition to their transmission task, HVDC systems can be used to provide ancillary services. Previous studies have shown that HVDC underground cables can contribute to congestion management in HVAC and HVDC systems via leveraging their inherent thermal reserves. This study extends these findings by investigating the impact of higher-loaded HVDC systems on frequency stability in the underlying HVAC system. A case study is conducted with a particular focus on the influence of grid-forming and grid-following control modes of HVDC converters on the AC frequency in a combined HVAC/HVDC system with a meshed HVDC overlay grid. The results indicate that curative congestion management using HVDC thermal reserves does not affect frequency stability. The quasi-steady-state frequency deviation exhibits only a marginal increase of 0.0034 Hz during additional generator outages in proximity to converter nodes. These findings support the feasibility of leveraging thermal reserves of HVDC cables while maintaining system stability.