Creating Europe's new backbone for efficient power distribution

For a few hours in 2016, during what the press called a historic event, almost all of Germany’s power came from renewable sources. The country was in the middle of its Energiewende, under which all nuclear power was to be phased out by 2022. France and Belgium also faced having to close ageing nuclear plants.

Changes like these call for a new power infrastructure across Europe. Its long-distance backbone would be made of high-voltage direct current (HVDC) lines linked to a large number of voltage source converters (VSC). These in turn would connect to alternating current (AC) lines in the regional networks, while power stations, solar and wind farms and storage sites would also feed 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,” said Vicente González López of Red Eléctrica de España, the company that had recently completed a 65 km DC connection between France and Spain.

Five demonstrations, one direction

The European project BEST PATHS was investigating new grid technologies to make Europe’s energy supply work better as a whole. Its five large demonstration programmes looked at how a new European grid could be optimised to move energy efficiently from supplier to user. Central to the new infrastructure would be the switch to direct current for moving energy over long distances.

Alternating current flows mainly near the surface of a conductor, with less current in the middle. This skin effect raises the losses caused by the wire’s resistance. Direct current has no skin effect, so the whole cross-section of the conductor carries current, which can cut losses by 30 to 40% compared with AC lines.

DC also needs only two wires, where AC lines need three conductors for the three phases. Plans were also being drawn up for superconducting high-voltage DC lines that would reduce losses further.

Converters from rival makers

The project’s Demonstration 2 dealt with the technology that would come with these changes and whether its pieces could work together. “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,” said Stephen Finney of the University of Strathclyde in Glasgow.

In DC networks, solid-state voltage source converters would take over the job that transformers do in AC networks. Finney compared them to the small power supply that connects a laptop to the mains. “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 was the watchword: the ability of the parts of an HVDC grid, such as converters and circuit breakers, to exchange data and information without friction was a major goal for the future pan-European network.

It would take time, González López said: “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 technology, the project was to deliver guidelines for operating HVDC-VSC networks, for standardising VSC stations, and for integrating wind power so that turbines would not simply be halted whenever their output was not needed.

The France–Spain link González López mentions is the INELFE interconnection, a 2,000 MW VSC link at 320 kV between Baixas and Santa Llogaia, in service since 2015.