Over the last few decades the energy landscape has been changing drastically in Europe. An increasing amount of electricity is now generated from renewable sources, such as solar and wind energy.
Last year, during a historical event, the whole of Germany ran for a few hours on renewable energy only. The country is experiencing the “Energiewende revolution”, where all nuclear power will have to be phased out by 2022. France and Belgium will also have to close down aging nuclear plants.
These developments now require new and efficient power infrastructure that spans the whole of Europe. Its long-distance backbone will consist of high-voltage direct-current transmission (HVDC) lines connected to a large number of voltage source converters (VSC). These will themselves be connected to alternating-current (AC) transmission lines, which are part of the local distribution systems. Power stations, solar and wind farms, and energy storage facilities will also feed into the main HVDC grid.
“HVDC is a key technology for transmitting large amounts of power over very long distances and allows the decarbonisation of the energy system. And climate change goals will only be achieved if we are able to carry power from clean sources to the locations where it is actually used,” says Vicente Gonzáles López, from Red Eléctricita de Espana, a company that has recently completed a 65 km long DC connection between France and Spain
In this context, the European project Best Paths is investigating novel grid technologies that would allow a better integration of the energy supply in Europe. Five large demonstration programmes are exploring how a new European grid could be optimised for an efficient distribution of energy from the energy supplier to the individual user. Key to the new distribution infrastructure will be the change to using direct current for energy transport over long distances.
Alternating currents mainly flow at the surface of a conductor, with less current in the middle. Known as the skin effect, this phenomenon significantly increases loss due to the wire’s resistance. With direct current, there is no skin effect and so the whole cross-section of the conductor is used by the current. This can reduce loss by 30 to 40% compared to AC lines.
Another advantage is that the power can be transmitted via two wires, while AC lines require three conductors to carry the three different phases of alternating current. Furthermore, plans are being developed for superconducting high-voltage DC transmission lines that reduce power loss even more.
The project’s Demo 2 will deal with the new technologies that will accompany these changes and their interoperability. “We’ve got three main European voltage source converter manufacturers - ABB, Alstom and Siemens - to put together a programme for off-line, and real-time simulations based on DC converters. This becomes a critical issue when you move towards a future world with DC networks,” says Stephen Finney of the University of Strathclyde in Glasgow, Scotland.
In these networks the function of transformers in AC networks will be taken over by solid-state voltage source converters. In fact, these converters are comparable to the small power supplies that connect your laptop to the mains, explains Finney. “They are just bigger versions, based exactly on the same kind of power-conversion principle, except that they are designed for very high voltages and currents.”
Interoperability is the watchword here: the ability to seamlessly share data and information across the various components of the HVDC grid, such as voltage converters and circuit breakers, is a major goal of the future pan-European energy network.
It will take some time, says Gonzáles López: “We at Best Paths appreciate we won’t solve all the interoperability issues, but we are on the way to providing some solutions and, perhaps more importantly, we are gaining much expertise in this area.” Beyond the new technology developed, the project will deliver a set of new guidelines for operating HVDC-VSC networks, standardising VSC stations, and better integrating renewable wind power to avoid simply halting wind turbines when their power is not required.
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