The European electricity system has been evolving with a very fast pace in the last years, mostly as a combined effect of two main drivers: the increasing penetration of renewable energy sources (RES), especially wind and solar power plants, and the development of cross-border trade of energy. The former driver causes constantly increasing flows on the North-South European axis (see Fig. 1), connecting zones with high RES potentials (especially North Sea wind offshore and South European solar) with the big loads located in Central Europe, where there are lower RES potentials. However, in consequence of the typical variable behaviour of wind and solar generation, one could expect a sequence of hours with a production surplus and some others in which there is a shortage. In the former case, the excess of production has to be convoyed to the few storage facilities (being nowadays limited to the hydro pumped power stations), whereas in the latter the energy stored in the pumping facilities has to be released in order to compensate the generation “gap”. Reserve availability is to be steadily present to assist wind generation in a situation where, due to the scarcity of storage resources, load and generation have to match rigorously in real time.
On the other side, the increasing development of cross-border trade exploiting arbitraging opportunities between the different national markets is stressing the existing cross-border backbones, originally planned for providing mutual support rather than for hosting significant commercial flows. Ideally, in these cases, all bottlenecks limiting the power transfers should be removed from the network so as to approach the behaviour of a busbar system. By contrast, a significant amount of European transmission infrastructure assets is ageing and has to be replaced. However, it is becoming increasingly difficult to build new lines, due to the strong opposition of the public opinion and to the complex and un-harmonised authorization procedures in force across and within the single nations. This is one of the main reasons explaining the large gap between the time needed for realising a new generator (a couple of years) and the one necessary for building a new electricity link (up to 10 years).
Besides the above-mentioned evolution in the transmission system, critical changes are also taking place in the distribution systems, traditionally characterised by the usage of less advanced technologies with respect to transmission. In this regard, a close interaction between TSOs (transmission system operators) and DSOs (distribution system operators) is essential to ensure an optimal and costeffective grid expansion. Also, in order to address all the above described issues, a key role will be played by the utilisation of innovative technologies for transmission, with the goal to make the existing system “smarter”, i.e. more flexible and responsive to sudden conditions changes, able to handle large amounts of variable generation.
All these trends and issues motivated the European research project REALISEGRID (http://realisegrid.rse-web.it), aimed at developing a set of criteria, metrics, methods and tools to assess how the transmission infrastructure should be optimally developed to support the achievement of a reliable, competitive and sustainable electricity supply in the European Union (EU). The most important themes of the project REALISEGRID, co-funded by the European Commission’s
DG ENERGY, were:
• Assessment of the usage of new transmission technologies as a support to the planning
• Investigation of new planning methodologies able to cope with systems characterised by large variability
• Set-up and validation of a comprehensive analysis able to establish priority expansion paths for the transmission backbones
• Analysis of the regulatory provisions that could help reaching a swifter and more effective decision process tackling the problem of public consensus
• Set-up of an efficient incentivizing remuneration mechanism for the transmission system operators (TSOs)
During the 3 years in which I coordinated the project REALISEGRID, I realised that, notwithstanding the great amount of books and papers already published on the theme of the new technologies supporting the power system, there was a significant gap between the expectance of the system stakeholders and what is already available. Actually, while there is no need for an extra contribution on the technological details of a single technology (be it a FACTS, HVDC or WAMS device), there is practically nothing available in the scientific literature able:
• To compare different technologies highlighting pros and cons of each
• To explain the different roles each technology could optimally play within the system
• To outline where each technology could be helpful to assist the planning phase of the electrical system
• To provide a qualitative and (whenever possible) a quantitative basis for the appraisal of costs and benefits so as to help building up a business case and compare the different investment alternatives (new overhead line vs. cable vs. new technologies for boosting a better usage of the existing infrastructure)
These considerations have motivated me and all the authors of the different chapters to pick up the results of REALISEGRID, completing them with further data and a more in-depth analysis, also in order to enlarge the geographical scope from Europe focused to worldwide.