Unified-State-Variable-Based Multi-Region Shared Energy Storage Coordination for Long-Horizon Power System Production Simulation

High-penetration renewable power systems increasingly rely on shared energy storage to coordinate local balancing, inter-regional support, and boundary-state inheritance across long-horizon production simulation. However, many engineering-oriented simulation frameworks still maintain local storage trajectories, inter-regional support variables, boundary correction states, and monthly carry-over states in separate model layers. This separation can lead to duplicated flexibility allocation, inconsistent state-of-charge accounting, and discontinuities between rolling monthly subproblems. To address this issue, this paper develops a unified-state-variable-based multi-region shared energy storage coordination framework for production simulation. The proposed method defines a single physical energy state for each regional storage type, reconstructs it from local charge/discharge trajectories, maps it into the inter-regional coordination layer, and updates it jointly with local charging, local discharging, shared-support, shared absorption, boundary correction, and monthly inheritance. A layered optimization structure is then constructed, including regional monthly production simulation, inter-regional shared support allocation, unified-state reconstruction, boundary reset checking, and audit-oriented output. The method is demonstrated on a three-region test system with heterogeneous storage reset rules and interconnection limits. Compared with a separated-state baseline, the proposed framework removes duplicated use of the same storage flexibility, keeps cross-month state inheritance explicit, and provides auditable indicators for shortage, curtailment, support energy, boundary correction, and state continuity. The results indicate that unified-state accounting is essential for embedding shared storage coordination into long-horizon production simulation without breaking local model autonomy. From a sustainability perspective, this unified ledger enables renewable accommodation, shortage reduction, and flexibility-sharing benefits to be evaluated against a physically credible storage state, thereby supporting a more reliable assessment of shared storage contributions to low-carbon power system operation. A focused three-month partial simulation further reports zero cross-month continuity error, boundary corrections of 13.9–21.3 MWh, and reductions in average shortage and curtailment rates from 1.76% to 1.27% and from 7.45% to 6.18%, respectively.