Engineering Specification — Normative · Version 1.0 · July 8, 2026

The Minimum-Inertia Condition

Frequency-Response Absorption Specification for Behaviorally-Coupled Computational Load

1. Scope and the shifted contingency

Reserve adequacy and frequency-response planning have historically been sized against an enumerated largest credible contingency — typically the loss of the single largest generating unit — treated as physical and independent. The interconnection of large computational load introduces a contingency of a different kind: a set of geographically dispersed loads that, through a shared orchestration or control plane, reduce demand in near-simultaneity — a behaviorally-coupled block. This block is invisible to topology-based N-1 analysis, which scores dispersed loads as independent low-probability events.

Normative. Where the coordinated-drop magnitude of a coupled block exceeds the loss-of-largest-unit value, the interconnection study SHALL treat the coupled block as the largest credible contingency and SHALL NOT rely on the loss-of-largest-generator assumption as the binding case.

2. Governing physics — the absorption threshold

A load drop pushes system frequency upward. The immediate rate of change of frequency, before primary frequency response engages, is governed by the swing equation:

where ΔP is the net imbalance from the coupled-block drop, H is the system inertia constant, and S_base is the system base. The coupled-block magnitude is:

Normative. Two conditions MUST hold simultaneously for a coupled block to be deemed absorbable: (A) the initial RoCoF MUST remain below the protection threshold at which relays trip generation; and (B) the frequency excursion before PFR arrests it MUST remain within the trip band. Both conditions depend on H and MUST be evaluated at the minimum-inertia condition of Section 3.

3. The minimum-inertia condition (H_min)

System inertia is not constant. As synchronous generation retires and is displaced by inverter-based resources, H declines, reaching its lowest values under high non-synchronous output and low synchronous commitment — conditions that recur on a dispatch-driven schedule. Available inertia (H) shrinks while the coordinated-drop magnitude (ΔP) grows; both drive RoCoF upward. Evaluation at average or representative inertia understates the risk, because the binding failure occurs at the minimum, not the mean.

Normative. The absorption test SHALL be evaluated at a credible minimum-inertia condition H_min. The study SHALL compute the inertia contribution from each committed synchronous resource across all hours of the dispatch stack; SHALL construct the inertia duration curve and define H_min at a stated low-exceedance percentile, adjusted for scheduled retirements and forecast resource mix; and MAY credit grid-forming inverter and synchronous-condenser contribution, but MUST count only firmly committed, dispatchable capacity and MUST NOT count planned or contingent capacity.

4. Normative pass/fail interconnection requirement

For any load cluster whose behavioral coupling exceeds the coupling-coefficient threshold defined in the Coupling-Discrimination Specification, interconnection at the studied magnitude is permissible only if, evaluated at H_min, both hold:

Normative. If either condition fails at H_min, the cluster MUST NOT be interconnected at the studied magnitude without one of the following committed mitigations: (i) a binding limit on the coordinated-drop fraction; (ii) committed fast frequency response or grid-forming capability sufficient to satisfy Condition B at H_min; or (iii) committed synchronous inertia sufficient to satisfy Condition A at H_min.

Where none is committed, a coupled-load penetration exists beyond which further interconnection is not physically feasible — a limit set by the swing equation at H_min, not by policy.

Appendix A — Invariant reference