Seam‑Level Structural Coherence Measurement for Bulk Power Systems: Identifying Elevated‑Risk Hours Within the Operational Horizon

Bulk power systems are not becoming fragile; they are becoming more tightly coupled, more timing‑sensitive, and more difficult to interpret at the boundaries where jurisdictions meet. This manuscript isolates a specific structural problem: in large interconnections operating under sustained strain, the interval between measurable degradation and proximity to collapse is compressing. Collapse appears sudden, but the degradation that precedes it is not. It accumulates within the existing operational horizon through reserve erosion, voltage‑margin compression, and the progressive loss of operator degrees of freedom. The proposed contribution is a seam‑level structural measurement layer: a time‑aligned observability framework spanning the boundary interfaces between operational control areas. It does not extend the physical intervention window of the grid, nor does it alter protection systems, dispatch authority, or compliance obligations. Its function is to identify periods within the existing 0–4 hour operational horizon where structural coherence is degraded and the risk of mis‑handling an event is elevated. These hours already exist; SCADA and EMS alone cannot make them visible, prioritizable, or governable. A structural layer can. The duration of the degradation window is not uniform across systems. In large, meshed interconnections with deep reserve portfolios and wide geographic spread, hours of structural degradation commonly precede collapse proximity. In tighter or more isolated systems—including island‑mode and peninsula configurations such as those managed by AEMO in Australia—the interval may compress to minutes or fall outside dispatch‑cycle alignment entirely. The framework does not assume a universal window. Topology determines the window, and the window determines the value. Where the window is structurally absent or too compressed for dispatch‑cycle intervention, the domain may fall outside the framework’s current scope. For operations executives, the value is prioritization: identifying which hours carry elevated structural risk changes how existing tools are sequenced and deployed. For risk officers and fiduciary actors, degradation interpretable as a trajectory rather than a binary shock is more governable; variance compression follows visibility improvement. For actuarial interests, the manuscript addresses actuarial brittleness—the condition in which models remain coherent but lose predictive stability under regime shifts. For governance institutions, the argument is historical: measurement evolution follows demonstrated operational necessity, not mandate. The manuscript is explicit about limits. It makes no deterministic predictions, asserts no universal tipping constants, and prescribes no regulatory actions. All claims are bounded, domain‑calibrated, and explicitly falsifiable. In tightly coupled systems, minutes matter. So do the hours that precede them.