The transition to a high-renewable energy system is increasing the operational complexity of electricity networks across Europe. In this context, the growing penetration of variable renewable energy sources (vRES), particularly from wind and solar photovoltaic generation, is intensifying challenges related to forecast uncertainty, grid congestion, and the need for more frequent balancing actions. In Portugal, these challenges are especially relevant in what concerns reserve activation since balancing is operated on a zonal basis and the deliverability of activated reserves is limited by internal physical transmission constraints. This creates a potential mismatch between economically efficient reserve activation and technically feasible system operation.

This work examines reserve activation in the transmission network for a 2030 scenario with a high penetration of vRES in the generation mix. Developed within the scope of the CETP Man0EUvRE project, the scenario is based on the 2030 Portuguese national energy and climate plan (NECP). The study compares two activation approaches: a conventional zonal approach and a network-constrained approach based on linear optimal power flow (LOPF). In addition, it assesses how network constraints affect the locational value of new renewable generation and flexibility resources, such as battery storage, foreseen for 2030. The two activation approaches are compared using metrics such as reserve-activation costs, congestion, and reserve deliverability.

An equivalent of the Portuguese transmission grid was implemented in PyPSA - Python for Power System Analysis, including explicit the grid technical characteristics and transformer constraints. Transmission system operator (TSO) network data from 2023 were used, together with market and operational information relevant to the Portuguese context. A scenario projected for 2030 and reflecting expected changes in electricity demand, renewable generation capacity, and flexibility needs was applied.

Results show that incorporating network constraints into reserve activation provides a more realistic representation of system operation therefore improving the physical feasibility of balancing actions, as expected. By contrast, least-cost zonal activation may select the cheapest reserves in market terms, but their activation may not always be possible without violating internal network constraints, thereby increasing the need for corrective redispatch. The analysis also indicates that flexibility resources, located at critical nodes, provide significant operational value by relieving congestion and supporting power system operation in a cost-effective and robust manner.

This research was funded by CETPartnership, the Clean Energy Transition Partnership under the 2022 joint call for research proposals, co-funded by the European Commission (GA N°101069750) and with the funding organizations detailed on https://cetpartnership.eu/funding-agencies-and-call-modules, in specific, the FCT - Fundação para a Ciência e a Tecnologia (CETP/0001/2022).