Unified Fault-Disturbance Modeling for Transient Multi-Infeed Short-Circuit Ratio Assessment in LCC-HVDC Systems

The multi-infeed short-circuit ratio (MSCR) is widely used to characterize the steady-state strength of AC receiving systems with multiple line-commutated converter high-voltage direct-current (LCC-HVDC) infeeds. Its rated-power denominator, however, does not describe the disturbance actually imposed during converter blocking, commutation failure, AC-fault clearing, or concurrent disturbances at neighboring infeeds. This paper introduces a unified disturbance-driven transient multi-infeed short-circuit ratio (UDTMSCR). Six quantities obtained from the fault trajectory—reactive-power deviation, active-power reduction, reactive-power ramp, disturbance energy, recovery duration, and a fault-class correction—are normalized and combined into an infeed disturbance term. Substitution of this term for rated power retains the original impedance-coupling structure of MSCR, weights each neighboring disturbance by a bounded participation coefficient, and thereby accounts for the severity and timing of local and neighboring disturbances. The denominator is further separated into local and mutual contributions, and percentile thresholds may be used for severity classification. Numerical evaluation comprises eight representative two-infeed cases and a 48-case parametric study of mutual-path impedance, neighboring-event delay, and neighboring-disturbance amplitude. For the eight cases, the Pearson and Spearman correlations between inverse UDTMSCR and the fault-side voltage peak are 0.979 and 0.976, compared with 0.193 and 0.246 for inverse MSCR. The corresponding values in the parametric study are 0.973 and 0.983 for inverse UDTMSCR and −0.294 and −0.310 for inverse MSCR. A peak-only transient index and a transient voltage severity index are included as additional references, and the sensitivity of the results to the disturbance weights, coupling weights, grading thresholds, event window, and record imperfections is quantified. These results show that the proposed index retains the engineering interpretation of MSCR while better reflecting fault severity, inter-infeed coupling, and recovery. Because the trajectories are generated by a reduced-order model, verification with detailed electromagnetic-transient models and field records remains necessary.