1. BACKGROUND AND PROGRESS REQUIRED BEYOND THE STATE OF THE ART
Transmission planning studies have so far been proposing discrete, long-lived modifications to more and more complex networks which do face a more and more uncertain future . They take into account a large number of multidimensional choices, uncertainty, huge investments, and long periods of time over which such investments must be assessed. Yet, the current state of the art in transmission planning is only able to address moderate levels of uncertainty on an energy scenario basis. Current methods have not coped so far, at least in Europe and the USA, with planning over large geographic areas and with increased uncertainty as well as renewable energy integration.
Today, any national transmission system in Europe is planned using expert judgment with the help of technical simulations:
demand is forecasted 10 years into the future,
system performance is then assessed for chosen snapshots, using detailed network description to pinpoint reliability issues and potential economic improvements,
system reinforcements or remedies are then studied, so that numerical simulations are re-run to ensure that the reinforced system meets the prescribed reliability requirements and that the delivered energy costs are minimized.
Since any national transmission system is a complex meshed network, reinforcement options, which can resolve system concerns, are many. Expert planners tend to consider one investment at a time to solve local problems rather than focus on the overall system outcomes. As a matter of fact, only a few optimization techniques appear able to deliver system plans without this limitation, although showing intrinsic weaknesses of their own2.
The forward-looking studies often consider only the design of networks for a static year and single energy scenario3. These analyses yield suboptimal expansion paths to the eventual desired network, without addressing their robustness to situations in which the envisioned scenario does not unfold.
Thus, overall, each TSO member of ENTSO-E has so far planned the expansion of its transmission network based on the same functional needs: to transport electrical energy from generation centres to the distribution areas, where consumption is located, and to use interconnections between transmission systems in order to ensure the security of supply, provide mutual support, and, overall, reinforce the pan-European transmission network.
The development of the single European electricity market indeed emphasizes the critical role of interconnections between transmission network operators. The European Commission has highlighted the need to increase cross-border flows and consequently expand interconnection capacity by 10% around 2020, with probably further integration beyond. Moreover, the expansion of renewable electricity generation, as implied by the long-term decarbonisation goals of European energy and climate policies policy will require that renewable (intermittent) generation units grow while somtimes located far away from consumption sites.
Electricity must be transported over longer distances, across borders, and delivered where consumption needs are sited. The pan-European network requires therefore further integration between ENTSO-E members and beyond, to use, for instance, offshore wind energy from the North Sea, large-scale PV power from North-Africa, and biomass from Russia.
The development of such novel network infrastructures requires a new top-down approach for planning, over longer time frames: such approaches involve the development of energy scenarios enabling to address the most constraining uncertainties linked to the evolution of generation, demand and exchanges with neighbouring regions, while accounting for the advent of new power system technologies.
Even though ENTSO-E already provides energy regulators with a Ten Year Network Development Plan (TYNDP), thus offering visibility over a 10-year horizon, longer term approaches, taking into account decarbonisation scenarios, are needed to account for many more uncertainties in order to analyse the different routes to reach the 2050 decarbonisation routes, also called the Modular Network Development Plan towards 2050.
Recent methodology attempts have been made to address these development issues. The European Climate Foundation (ECF) study4 has performed a fact-based study in support of the EU decarbonisation 2050 goals. It impacts the European industry, particularly in the electricity sector, and emphasizes the role of the transmission network.
The transmission network planning will have a crucial role in the EU market integration, i.e. the realization of the European single electricity market, and the decarbonisation process for the next 40 years. It has highlighted a first appraisal5 of the pan-European transmission grid by 2030, in coherence with their 2050 vision, using a back-casting approach, but without an explicit modelling of an offshore HVDC grid.
This 48-node study has the advantage of providing a coherent framework to point out some of the major challenges to be faced by transmission operators for 2030, while allowing a first comparison of the investments requirements as foreseen for instance by the SUSPLAN EC-supported study6. Both the ECF and the SUSPLAN studies7 conclude on the need for a massive grid expansion requirement within the 2020-2030 decade (approx. +100% of additional transmission capacity). For 2050, another recent EC communication stresses the key elements that which will shape the EU's climate actions by 2050, emphasizing energy efficiency policies and environmental impact concerns.
In the meantime, transmission networks will have to cope with more uncertainties at all levels (macro-economic growth, generation and consumption patterns, new power system technologies), while helping energy players to meet the three pillars of the EU energy policy (viz. competitiveness, sustainability and the security of supply). This requires Transmission System Operators (TSOs) to simultaneously support the efficient use and optimization of existing transmission infrastructures and the implementation of new efficient infrastructure investments.
However, the above attempts are not addressing the paradigm shift underlying the development of new planning methodologies to address such very long-term horizon 2050, over such wide areas. New top-down approaches require R&D in order to be able of accounting for several irreversible trends listed hereunder.
i. Independent planning for generation and transmission
Before the 1980’s, considerable research activities were focused on the study of integrated resource or composite expansion planning. Generation and transmission planning were generally managed by large integrated utilities following long-term, detailed national energy policies and promoting large centralized power plants. In the 1990’s, a market-based approach to operate and plan the electric system was introduced in the USA and then in Europe. Unbundling of the whole electricity value chain implies that generation and transmission planning are now performed independently: transmission planning uses assumed generation planning, since competing generators are no longer willing to disclose their long-term strategic plans.
ii. Massive development of renewable generation
Since the late 90’s, European energy policies have been progressively promoting renewable energy sources. This has produced small or spread electricity generation, which is usually variable. Moreover, many of the generation sites are away from consumption ones, as, for instance, for on-shore or off-shore wind parks, thus requiring network expansion to bring secure electric power to where it is needed.
iii. A growing reluctance to implement new infrastructures
The transmission development takes more time than new generation units to be installed, because of public acceptance. This can provoke unacceptable system behaviors, like drops of voltages or sudden disturbances coming either from generation or consumption sides. Hence, in their task of upgrading or replacing the existing AC overhead transmission lines, TSOs will need to implement new technological solutions, which, in turn, makes the pan-European system increasingly more complex to design and to operate.
iv. New and evolving technology background
Finally, power electronics will be more and more deployed at generation level (DFIG8 for wind turbine, full electronic inverters for PV) and within the grid (FACTS devices, point-to-point DC links, and meshed HVDC networks) to allow an increasing real-time power-flow control. This would lower today’s pan-European system inertia making the system even more sensitive to any type of disturbances. The expected dynamic behavior of such power system will have to be considered very early in the planning methodology and the planning process, although this is an area still relatively unexplored.
At the same time, novel technology solutions down streaming the transmission network open routes for improved network design and operations. Let us mention for the sake of illustration:
Overall, the planning for 2050 requires implementing a new planning methodology considering both the size of system and the very long-term horizon. In order to cope with these challenges, very different possible future energy scenarios must be taken into account. Such a study dealing with a very high level of combinatorial aspects has never been done before.
2. OUTCOMES PROVIDED BY THE PROJECTS THAT ADDRESS THE CHALLENGES OF THE CLUSTER
A. The e-Highway2050 project
The eHighway2050 projet develops and validates novel planning methodologies which will bring support to planning of the pan-European electricity transmission network from 2020 to 2050. Energy scenarios dealing with generation mix and demand mix scenarios are set on the basis of macro-economic data, the energy adequacy between generation and consumption being ensured whatever the scenario studied. They are supposed to be the ones which are the most constraining for the pan-European transmission power system going from 2020 to 2050. Next, power localization over the whole interconnected system are proposed with assumptions on the generation mix and demand mix for each control zone covered by the TSOs. Power flow simulations are then performed to detect the weak points in the current grid architecture when implementing each of the scenarios by 2050. Reinforcement options are proposed to alleviate all the simulated issues by 2050 (overloads but also voltage and/or stability problems) while ensuring an acceptable level of system reliability.
B. The GARPUR project
The GARPUR project designs and evaluates new power system reliability criteria to be used within the key activities of TSOs at different time scales: system development, asset management and power system operation. If successful, these criteria could be progressively implemented at the pan-European level, optimally balancing reliability and costs. Indeed, the increasing uncertainty caused by (among others) the massive renewable energy integration calls for the use of probabilistic reliability criteria to supplement and enhance the pure preventive N-1 criterion.
 “The Future of The Electric Grid: an interdisciplinary MIT study” (2011), see also http://web.mit.edu/mitei/research/studies/the-electric-grid-2011.shtml
 G. Latorre, R. D. Cruz, J. M. Areiza, and A.Villegas, “Classification of Publications and Models on Transmission Expansion Planning,” IEEE Transactions on Power Systems 18, no. 2 (2003): 938–946
 See for instance EnerNex Corporation, Eastern Wind Integration and Transmission Study (Golden, CO: NationaRenewable Energy Laboratory, U.S. Department of Energy, 2010)
 “ROADMAP 2050: practical guide to a prosperous, low-carbon Europe” , European Climate Foundation, 2011
 G.Strbac, C.Hewicker “Vision on Long Term Electricity Grid Development: ECF Roadmap/Power Perspectives”, Brussels 29 February 2012
 B. H Bakken et al. SUSPLAN Final Report, Deliverable D7.2, www.susplan.eu
 Project "Linking Global and Regional Energy Strategies", SINTEF 2012
 DFIG: double-fed induction generator.