Short-circuit ratio-constrained unit commitment for HVDC sending-end grids: A combinatorial encoding-based MILP formulation

Maintaining an adequate short-circuit ratio (SCR) is essential for the secure operation of high-voltage direct-current (HVDC) sending-end grids, particularly for mitigating transient overvoltages following DC-blocking faults. However, incorporating SCR requirements into unit commitment (UC) is computationally challenging because rigorous SCR evaluation requires the inversion of a network admittance matrix that depends on the commitment configuration of thermal power units (TPUs), creating an implicit coupling between UC decisions and grid strength. To address this challenge, this paper proposes a novel SCR-constrained UC framework based on a decoupled offline-mapping-to-online-optimization architecture that explicitly incorporates grid strength requirements into system scheduling. Specifically, a combinatorial encoding technique uniquely represents each TPU commitment configuration by a scalar code value, enabling an offline lookup mapping between code values and the corresponding short-circuit capacities (SCCs). This mapping is then used to reformulate the implicit nonlinear SCR constraints as mixed-integer linear constraints for integration into the UC formulation. Unlike existing approaches based on fitted or analytical SCC approximations, the resulting formulation preserves the rigorously calculated SCC values stored in the offline mapping while retaining the standard MILP structure of the UC problem. Numerical results demonstrate the accuracy and computational tractability of the proposed framework, as well as its ability to produce UC schedules that satisfy the prescribed SCR requirements.