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Dr. Peter Hoffmann, Director Energy System Planning, TenneTThe extreme weather events of last summer clearly brought the consequences of climate change to everyone’s attention. At the same time, Europe adopted the Green Deal for climate neutrality by 2050. This is already ushering in a fundamental change to our energy system – as can be seen in the political objectives of the new government for 2030. Net energy savings are only possible by switching to electricity-based processes – such as electromobility and heat pumps – while also covering this increased electricity demand with renewable power generation. Wind potential – especially offshore – is being developed on a large scale, thereby increasing the need to transport this energy to the metropolitan areas and industrial regions. Faster grid expansion is the order of the day at all voltage levels. A further 380 kV AC grid expansion (alternating current) by the transmission system operators (TSOs) does not by itself live up to this challenge either on a technical level or in terms of public acceptance. Intelligent new concepts for integrating enormous quantities of renewable energy into the grid are therefore urgently needed. The following innovations in the transmission system grid are particularly promising:
1. 525 kV DC cable technology
Until recently, 320 kV direct current cables were still the state of the art. As a result, all offshore wind farms of the most recent generation in Germany are connected to the AC grid via corresponding converters and cables. Now the new German government intends to speed up the expansion of offshore wind farms in the North and Baltic Seas from the current 8,000 MW to 30,000 MW by 2030. The new 525 kV technology enables a transmission performance of 2 GW, which has made it a fixed part of the plans of German TSOs for all new offshore connection systems and onshore direct current power lines to be installed as of the year 2029. This will allow a significant reduction in the number of required offshore grid connection systems compared with use of the established 320 kV technology. This means less space is required, while reducing the impact on natural landscapes and lowering costs.
2. Multi-terminal hub concepts
So far, all direct current lines in the world have been designed as pure point-to-point connections. An initial pilot installation in Asia has now shown that reliable operation with more than two converters is also possible. At least on a small scale, this opens up the possibility of linked DC systems (direct current) as long as the consequences of a cable fault do not exceed the limits in terms of balancing capacity in Europe (3000 MW max. permissible power shortfall) and the power load supportable by the AC grid (2000 MW in Germany). In the current grid development plan, a DC multi-terminal hub concept was confirmed for the first time on the coast of Schleswig-Holstein. This will link two offshore connection systems to a direct current line to Brandenburg and a converter on the coast. Compared with pure point-to-point connections, this eliminates two converters in the Heide coastal region. At the European level, research projects for standardising the converters have been announced so that systems from various providers can be used together.
3. Hybrid offshore interconnectors.
Currently, TenneT transports offshore wind power via long DC cable connections to the coast, from where it is exported at low energy prices back north to Norway via the NordLink cable, which was put into operation in 2021.hub concepts offshore as well, meaning that a cable from Norway would only have to be run to the offshore wind farms. When the wind is calm and power prices are high, hydropower could be exported to Germany via the offshore connection systems, and when the wind is blowing – making for excess electricity in Germany – the wind power could be exported directly to Norway. However, implementation of this concept still faces European legal obstacles that assign priority to wind power over international energy trading. Questions concerning an offshore bidding zone must also still be answered.
4. DC circuit breakers for meshed DC overlay grids
As already described, linked DC systems are only possible on a small scale – since 525 kV DC circuit breakers do not exist yet that could safely divert a fault in a cable section. ABB in particular has made tremendous progress on the development of 320 kV DC circuit breakers, demonstrating what is possible. The full potential for cost savings and targeted shifting of the load flow across Europe via a DC overlay grid can only be realised once such circuit breakers are available. To resolve this chicken-and-egg dilemma (in other words, the industry will only finalise the development of these circuit breakers once a demand for them is expected), the German TSOs are planning to directly include circuit breakers in the invitation to tender for the planned 525 kV multi-terminal structures – meaning such will be designed as overlay-ready.
5. Innovations in system management – InnoSys2030
Alongside the efforts to establish a DC overlay grid, research projects are currently in progress to increase the capacity utilisation of the existing AC grids. Such grid operations equipment will be installed in the German grid in the coming years – though the corresponding optimisation methods for control by system management are still under development. Furthermore, the batteries that will be available in the future thanks to electromobility and photovoltaic systems for storage will offer great potential for rapidly available flexibility. This will permit a switch from the current practice of preventive redispatching (i.e. the grid is only loaded up to about 80percent as a precaution in case of a potential fault) to a corrective redispatching approach. In the future, it will be possible to take advantage of the temporary overload capability of the grid operations equipment and to initiate the redispatching only after the fault has occurred. This means that utilisation levels near the 100percent limit are conceivable during normal operation and the redispatch costs can be lowered significantly. In order to develop secure methods for control and grid analysis in the control centres, TenneT will install large batteries (so-called grid boosters with 100 MW capacity) in the transmission system grid as an initial step starting in 2023. The processes for SMART control of the small-scale flexibility options should then be available by 2030.
“Intelligent new concepts for integrating enormous quantities of renewable energy into the grid are therefore urgently needed”
Safe and reliable operation of the critical electricity infrastructure remains the goal of the grid operator in order to safeguard Germany’s economic activity. In the past, the TSOs have therefore relied primarily on established technical solutions. All the stops must now be pulled to create and improve innovative technology that is capable of keeping up with the transport demand. Such innovations are essential to overcoming the challenges of the energy transition.
