Early Multi-Bus Transient-Voltage Stability Margin Assessment and Warning in Hybrid AC/DC Grids

The secure operation of a hybrid AC/DC grid requires not only a stable/unstable decision but also the voltage recovery margins of vulnerable buses. This paper presents a cascaded dual-criterion assessment and severity warning framework using a 0.75 p.u./1 s low-voltage-duration check and a 0.8 p.u./10 s recovery check. For trajectories labeled stable by the offline criterion, a multi-output ConvLSTM estimates the normalized margins of 22 monitored 500 kV buses from approximately 0.25 s of post-disturbance measurements comprising 18 electrical variables. These margins are mapped to four stable-state warning levels; the first-stage stability decision assigns unstable cases to the fifth emergency level. Evaluation uses 11,305 simulated CEPRI-VC scenarios and a source-reported random in-distribution split. On this split, ConvLSTM achieves an RMSE of 8.949 × 10−3, an MAE of 5.531 × 10−3, and an R2 of 99.684%, reducing RMSE and MAE by 18.7% and 22.2% relative to ConvGRU. These values quantify stable-state margin regression within the represented operating domain; a separate reference evaluation of the first-stage classifier gives 98.28% accuracy and a 3.05% missed-instability rate. Reference grouped hold-outs, repeated runs, and disturbance tests further characterize the dependence on data grouping and input quality; complete five-level end-to-end accuracy remains to be established. The framework provides bus-wise margin estimates and candidate weakest-bus information without demonstrating generalization to unseen topologies or converter control settings.