A novel frequency control strategy based on UFLS and demand response coordination considering uncertain time delays and weakly controllable DC infeed

The combination of multiterminal diode rectifier units (DRUs) for DC power collection and Line Commutated Converter (LCC)-based transmission is a highly competitive solution for provincial-level renewable energy accommodation. However, due to the uncontrollable nature of diode devices, such systems depend heavily on grid-forming energy storage (ES) for stable operation and fault ride-through. This dependence exacerbates the vulnerability of the receiving-end grid to power deficits. As system inertia declines, traditional Under Frequency Load Shedding (UFLS) struggles to mitigate rapid frequency drops due to its delayed activation and limited precision. While integrating Demand Response (DR) can enhance UFLS, its unpredictable response delays present a significant challenge. To overcome this obstacle, this paper proposes an optimal coordination strategy for UFLS and DR. First, a cumulative distribution function (CDF) model is developed to characterize the uncertain delays of DR. Then, a comprehensive system frequency response model is formulated to capture the joint effects of UFLS and DR. Ultimately, this approach enables the proposed strategy to dynamically determine the optimal DR shedding amount in real time under varying communication conditions. Numerical simulations demonstrate that the proposed strategy achieves secure and precise load shedding at lower cost.