Abstract
Converter-dominated renewable power systems are increasingly vulnerable to rapid frequency excursions because the displacement of synchronous generation reduces effective system inertia. This study evaluates a grid-forming (GFM) control strategy based on a Virtual Synchronous Machine (VSM) supported by a direct-current (DC)-coupled Battery Energy Storage System (BESS) in a representative three-terminal voltage-source converter high-voltage DC renewable power system. The main contribution is a compact, five-state, reduced-order model that simultaneously captures the alternating current (AC) frequency, common DC bus voltage, governor response, BESS active power, and state-of-charge dynamics while enabling a reproducible assessment of coupled AC/DC stability under multiple contingency conditions. The model was independently cross-verified in the software using adaptive BDF, fixed-step fourth-order Runge–Kutta, and Radau solvers, with the existing Simulink implementation retained as an additional check. Three disturbances were considered: a 280 MW load increase, 400 MW wind-generation loss, and 300 MW solar-power ramp over 100 ms. Compared with the grid-following baseline, the GFM-BESS improved the frequency nadir by 1.615–1.616 Hz across all the tested scenarios and reduced the 200 ms Rate of Change of Frequency (RoCoF) in the severe wind-trip case from 2.186 to 1.244 Hz/s. The common DC-bus voltage remained within the ±5% study benchmark. The BESS reached 99% of its 200 MW rating within 0.117–0.166 s and delivered only 0.141–0.144 MWh up to the nadir, demonstrating that short-term performance is constrained primarily by power capability rather than energy capacity. These results highlight the effectiveness and limitations of the DC-coupled storage for coordinated frequency and DC voltage support in low-inertia multiterminal DC systems.