SCR Independent Polytopic Lyapunov Control for MMC HVDC Networks

The growing High-Voltage Direct Current transmission networks require modern control strategies in converter stations to ensure reliable operation and uninterrupted energy supply, particularly under unstable and low Short-Circuit ratio conditions. Conventional Grid Following converters become unstable in low Short-Circuit ratio scenarios, while modern Grid Forming converters, though more robust, exhibit slower dynamic response in high Short-Circuit ratio scenarios. This paper presents a hybrid control switching strategy based on polytopic Lyapunov functions that combines the strengths of Grid Following Control and Grid Forming Control strategies. Switching between these control strategies occurs at defined hyperplanes of polytopes derived from the state-space equations, enabling the system to maintain fast and stable performance under changing Short-Circuit ratio conditions of the grid. Because the method is grounded in polytopic Lyapunov function theory, it demonstrates inherent large-signal stability. This proposed Hybrid Control Strategy is validated using real-time simulations, showing robust performance during Short-Circuit ratio variations, highlighting its potential for future High-Voltage Direct Current systems.