The five critical challenges facing transmission outage planning today 

Why does it need to be optimized now?

Critical Challenges in Outage Planning

What is transmission outage planning?

It is the process of scheduling and coordinating the temporary unavailability of grid assets for maintenance, upgrade, repairs, or new infrastructure projects. As such, it is a core process of transmission owners and system operators, who are responsible for ensuring a reliable, secure, and efficient operation of the high-voltage electricity grids. 

As the energy landscape undergoes a fundamental transformation, this outage planning process is becoming more complex and critical to optimize, mainly due to the following challenges:

 1) Significant increase in outage works, due to:

• Aging of assets: Over 25% of Europe’s transmission lines will need major renewal or replacement by 2030 due to aging [1]. Moreover, around 40% of Europe’s electricity distribution networks are over 40 years old [2]. In this context of aging assets faced by European TSOs & DSOs, corrective or preventive maintenance is frequently required, as well as full asset replacements. Older assets are also more prone to unplanned outages, which cascade into the planned schedule and further reduce the available coordination space. The trade-off between keeping aging assets in service (to avoid reducing grid capacity) and taking them offline for essential maintenance creates a permanent scheduling dilemma.
• Ambitious CAPEX plans: European transmission owners and system operators must execute ambitious CAPEX and maintenance plans, representing an estimated €477 billion in transmission grid (and €730 billion in distribution grid) investment for the EU transmission network alone by 2040 [3] to avoid degradation-related failures, connect new renewable capacity and modernize the grid for the energy transition, which significantly increases the number of planned works and outages per year. 

2) Increased Complexity of Coordination and Regulatory Requirements:

• This surge in outage volumes, combined with growing grid complexity, puts significant pressure on coordination. Each outage requires coordination and arbitration across many involved stakeholders (such as planning engineers, field operators, planning managers, distribution system operators, neighboring transmission owners and operators, and coordination centers) [4], which makes coordination more difficult and optimization hard to anticipate.

• Outage planning has evolved from an internal exercise to a multi-stakeholder, regulation-driven process that requires coordination among system operators, transmission owners, coordination centers, and market participants. Under the System Operation Guideline (SOGL) [5], TSOs are required to:
  • Coordinate outage plans at pan-European and regional levels over different timeframes (year-ahead, month-ahead, week-ahead). 
  • Make sure planned outages are considered in cross-zonal capacity calculations.
  • Perform coordinated security analyses with RCCs and propose remedial actions if outages threaten grid security.
• These regulatory requirements add layers of validation to the approval process, which extend lead times and increase the cost of late changes. 

3) Energy transition Impacts on outage feasibility:

• The increasing penetration of renewables (wind and solar) and electricity demands introduces significant uncertainty into grid operations [6]. Generation patterns are more and more weather-dependent and less predictable. This affects the feasibility of planned outages: maintenance that is secure under calm weather may become infeasible if solar or wind generation surges unexpectedly. This requires transmission owners and system operators to consider: 
  • Conditional outages: this type of outage is approved only if specific conditions are met (e.g., sufficient generation margin, favorable weather forecasts) 
  • Restituable outages: assets are out of service but kept ready for rapid re-energization if system conditions deteriorate. 
• Due to this increased complexity, outage windows become scarcer: more assets compete with fewer available slots that satisfy N-1 security criteria. This inevitably leads to more re-planning, rescheduling and cancellations, with the risk of raising operational costs and reducing efficiency. For example, Elia has reported last-minute outage cancellations driven by the growing uncertainty of renewable generation, resulting in mobilized field crews standing idle and wasted operational resources [7].

4) Climate Resilience & Extreme Weather:

Extreme weather events are becoming more frequent and severe, directly threatening grid infrastructure and forcing emergency outages. The consequence is that these unplanned outages disrupt the planned maintenance schedule. System operators must now consider climate risk into outage planning. The launch of the TSO Innovation Alliance (Elia, TenneT, Swissgrid, Terna, Amprion, and others) in 2025 designated “Weather and Grid Resilience” as a top priority, underscoring the urgency of this challenge [8].

5) Loss of Expert knowledge:

Outage planning is a discipline that relies heavily on accumulated expertise: anticipating grid security constraints, understanding asset behavior and sensitivity, and building efficient coordination practices. As this generation of experts retire, TSOs face the risk of losing this knowledge faster than it can be transferred. Grid-related professions are already experiencing retirements outnumbering new entrants by 1.4 to 1 [9]. This makes the case for better tooling and decision-support systems even stronger: a tool that can flag constraint violations, support in rescheduling, indicate valid  placement options and help less experienced planners manage an increasingly demanding scheduling environment.

The Cost of Inaction

The main cost is operational. Poorly coordinated outages increase the risk of grid congestion, forcing system operators into costly remedial actions such as redispatching, countertrading, and curtailment. Cancelled or rescheduled outages mean mobilized field operators ending up inactive, extended asset unavailability, and infrastructure projects delayed by months (in the best scenarios!). Delaying essential maintenance to preserve grid capacity in the short term only raises the probability of unplanned failures later on. This creates a problematic cycle where reactive firefighting replaces proactive planning. More broadly, temporary network unavailability can reduce cross-border exchange capacity, limit injections or withdrawals, and leave the grid less robust to further incidents. 

There is also an efficiency cost tied to poor outage bundling. Outage bundling (grouping works on the same asset or related assets into coordinated outage windows) is a key lever for maximizing the number of planned works while minimizing network unavailability. When bundling is suboptimal, assets are taken out of service more often than required, each time reducing available grid capacity and triggering additional rounds of security checks, stakeholder coordination, and approval workflows. In practice, poor bundling means more outage windows consumed for the same volume of work, which further reduces an already constrained scheduling space. This is precisely what makes outage planning a combinatorial optimization problem: the planner must explore how to group works together to maximize execution while minimizing the footprint on the network.

Finally, there is a resilience cost. On a grid with more variable renewables and more extreme weather exposure, outage plans need to be continuously re-validated and adjusted on a rolling-window basis (month-ahead, week-ahead, day-ahead) and impactful outages to be condition-based (approved only if predefined system conditions are met). If they are not, system operators lose room to absorb shocks: a weather event, or an unexpected asset failure is more likely to trigger emergency cancellations, forced outages, congestion, or wider national/regional security issues. Recent ENTSO-E incident reporting [10] and the launch of the TSO Innovation Alliance [8] both point in that direction.

How might outage planning evolve in the future?

The scale of today’s outage planning challenge, thousands of outage requests, a major part of them being subject to security constraints, dependency constraints, weather uncertainty, resource availability, and cross-border coordination, is a combinatorial optimization problem that traditional, experience-driven methods can no longer reliably handle. 

This raises a fundamental question: how can transmission owners and system operators move from experienced-based schedule management to proactive, optimized outage planning?

The answer is likely to be found at the intersection of advanced optimization, better use of data, and closer collaboration between stakeholders. Transmission owners and system operators who find ways to regularly explore more scheduling alternatives, anticipate constraints earlier, and adapt faster to changing conditions will be better positioned to execute their ambitious investment and maintenance plans while keeping the grid secure and costs under control.

References

[1] European Commission Infrastructure Study: INVESTMENT NEEDS IN TRANS-EUROPEAN ENERGY INFRASTRUCTURE UP TO 2030 AND BEYOND: https://publications.europa.eu/resource/cellar/431bc842-437c-11e8-a9f4-01aa75ed71a1.0001.01/DOC_1

[2] Power Barometer 2023 report by Eurelectric :  https://powerbarometer.eurelectric.org/power-barometer-2023/

[3] European Commission, Guidance on anticipatory investments for developing forward-looking electricity networks, June 2025 https://energy.ec.europa.eu/news/eu-guidance-ensuring-electricity-grids-are-fit-future-2025-06-02_en

[4] Regional outage planning coordination: https://eur-lex.europa.eu/legal-content/EN/TXT/?uri=CELEX%3A32019R0943&qid=1770678622207

[5] Commission Regulation (EU) 2017/1485. https://eur-lex.europa.eu/legal-content/EN/TXT/PDF/?uri=CELEX:02017R1485-20210315

[6] ENTSO-E, Report on Flexibility from Renewable Energy Sources, November 2025. https://eepublicdownloads.blob.core.windows.net/public-cdn-container/clean-documents/Reports/2025/251118_entso-e_flexibility_from_RES_Report.pdf

[7] Optimizing offshore outage planning via renewable forecasts — N-SIDE. https://www.n-side.com/en/insights/optimizing-offshore-outage-planning-via-renewable-forecasts/

[8] TSO Innovation Alliance, European TSOs Launch Alliance to Drive Innovative Weather-Resilient Grid Solutions, July 2025. https://news.europawire.eu/european-tsos-launch-alliance-to-drive-innovative-weather-resilient-grid-solutions/eu-press-release/2025/07/02/16/36/19/158025/

[9]  IEA, World Energy Employment 2025, December 2025. https://www.iea.org/reports/world-energy-employment-2025

[10]  ENTSO-E, Grid Incident in South-East Europe on 21 June 2024 — ICS Expert Panel Final Report, February 2025. https://eepublicdownloads.blob.core.windows.net/public-cdn-container/clean-documents/Publications/2024/entso-e_incident_report_240621_250225_02.pdf