Frequency margin evaluation for hybrid alternating-current/direct-current grids under high renewable penetration: a scalable online framework

• Frequency assessment method couples nonlinear control with multi-dimensional indices. • Scalable online framework features tri-state coordination for power grid assessment. • Adaptive stability control parsing embeds control strategies in fast assessment. • Second-level computation achieved for systems with over 20,000 nodes. • Multi-level validation shows below 5% error in complex scenarios, proving robustness. With the rapid expansion of renewable energy and the development of cross-regional hybrid alternating-current/direct-current power grids, frequency stability is facing new challenges characterized by low inertia and multiple disturbances. To address the balancing difficulties among model accuracy, computational efficiency, and engineering applicability in traditional assessment methods, this paper proposes a scalable and dispatch-oriented online assessment framework for transient frequency security margins in hybrid alternating-current/direct-current power grids with high renewable penetration. First, a unified adaptive frequency response model is established, which couples nonlinear control characteristics with multi-dimensional frequency security margin indices using a binary table of unit ramp rate and frequency deviation. Second, a tri-state collaborative assessment system including real-time state, future state and study state is constructed, incorporating adaptive parsing of stability control strategies and a standardized parallel computing architecture. Finally, a multi-level verification system is developed, ranging from benchmark testing to actual fault replay. Validation results demonstrate that the proposed method has a computation time of 3.24 s in a 22,300-bus system, achieving second-level computation for power grid with over 10,000 buses and maintains errors below 5% in complex scenarios such as single faults, multi-point disturbances, and cascading failures. The study indicates that the proposed framework significantly enhances the engineering applicability in energy management systems and dynamic security assessment platforms, providing theoretical and practical support for the secure and flexible operation of next-generation low-inertia power systems with large-scale renewable energy integration.