The transition to high-renewable electricity systems increases the importance of balancing actions that remain physically deliverable within transmission-network limits. Variable renewable energy sources reduce marginal energy costs when available, while weather-dependent output increases the need for balancing resources. Reserve capacity may be sufficient at national level while activation remains constrained by the electrical location of providers, demand patterns and internal transmission capacity. Portugal’s National Energy and Climate Plan 2030 (NECP 2030) and the Man0EUvRE NECP Essentials (NECP-E) pathway provide relevant 2030 capacity trajectories for assessing these conditions.

This study examines the upward manual Frequency Restoration Reserve (mFRR) deliverability in four case studies combining the NECP 2030 and NECP-E pathways with no-additional-flexibility and fully deployed flexibility states. The Portuguese Regulation Reserve Band (BmFRR) is represented using observed 2025 contracted upward balancing-capacity requirements as a common operational sensitivity. The transmission system is represented by a reduced equivalent model and its operation assessed through linear optimal power flow (LOPF). The model includes the principal transmission network corridors, conventional and renewable generation, pumped hydro storage (PHS), battery energy storage systems (BESS), electrolyser demand and nodal electrical demand. Separate alternating-current power-flow calculations assess convergence and branch loading for the resulting energy-only dispatch.

Zonal least-cost selection of available upward-BmFRR providers and their activation volumes is compared with network-constrained selection using direct-current power-transfer distribution factors. The formulation considers provider headroom, incremental branch flows and branch ratings. Zonal selection delivers the complete BmFRR requirement in every case but produces maximum projected internal branch loading up to 1.92 per unit (p.u.). Network-constrained selection limits maximum projected loading to 1.04–1.08 p.u. and delivers 99.92–100% of the requirement. Residual non-deliverable energy reaches 1.28 GWh in the NECP 2030 fully deployed case and 2.28 GWh in the NECP-E fully deployed case. A synthetic 100 MW, one-hour BESS at Recarei reduces the NECP 2030 fully deployed shortfall by 0.50 GWh and makes four limiting hours fully deliverable. The results show that national reserve sufficiency and zonal merit-order activation alone provide an incomplete representation of balancing performance in a transmission-constrained system.