Into the future of electricity with DC voltage

When electricity and electric lighting began to be used in the late 19th century, the American industrial patron George Westinghouse favoured alternating current as the standard for the power grid to be constructed. A dispute as fierce as it was ludicrous erupted with his rival Thomas Alva Edison, who championed direct current, eventually going into the history books as the ¡°War of Currents¡± (see box). Alternating current emerged victorious and with it Westinghouse, who has been regarded as a pioneer of the technology that forms the basis for today¡¯s electricity supply ever since. 100 years after his death, the question as to which will be the technical current type of the future has resurfaced.

Direct current in the ascendant

Alternating current changes direction periodically. Compared to direct current, which always flows in the same direction, it has the disadvantage that it generates considerably higher losses during transmission, which increase with distance. Conventional power grids are thus designed to only transport electrical energy 100 to 200 kilometres from a few central power stations to consumers in the vicinity.

As part of the energy transition, Europe is now looking to supply consumer centres with sustainable electricity over large distances, for instance from offshore wind farms in the North and Baltic seas or solar power stations in the south. ¡°Existing alternating current networks are unsuitable,¡± says Christian Franck, a professor of high-voltage technology at ETH Zurich. Together with his team, he is developing technologies for an efficient power grid of the future. ¡°This will be based upon direct current,¡± Franck is convinced. The focus is on so-called high-voltage direct-current transmission (HVDC), which is able to transport energy with losses of merely around three per cent per thousand kilometres. HVDC will form the basis for the European ¡°supergrid¡±, which is to be constructed parallel to the existing alternating current grid.

HVDC is not a new technology in itself. It is already used to connect the Scandinavian grid with mainland Europe through the North and Baltic seas, for instance. So far, however, HVDC lines have only existed as point-to-point connections, at the beginning and end of which stand complex converter stations that transform the alternating current into direct current and back again with heavy losses. In order to be able to exploit the advantages of direct current technology, a stable network is needed. Not only does this mean that the producers of direct current technology have to agree on one standard; there are also major technical gaps that need to be plugged.

Fuses for the supergrid

One problem is that there are still no protection devices for HVDC. If a tree falls on a line and causes a short circuit, for example, an efficient breaker is needed to quickly interrupt the faulty line. Much like with fuses at home, speed is of the essence here. In the case of high-voltage direct current networks, fault currents of several thousand amperes would need to be interrupted within thousandths of a second. ¡°So far, there are only a handful of prototypes for such circuit breakers,¡± says Franck. While they are able to switch off the current quickly, they incur high losses and are very expensive. In an industrial collaboration with ABB, Franck¡¯s group is working on a breaker concept that uses normal metallic contacts to compensate for these drawbacks. As they have to be opened mechanically to interrupt the current, however, the disconnection times are still somewhat longer.

Using existing overhead power lines

At ETH Zurich¡¯s high-voltage lab, the research team headed by Franck is also contemplating how an HVDC grid could be constructed in practice. The supergrid concept is based on new power lines, as Germany is currently planning for the north-south axis. However, experience has shown that the population reacts strongly against such power highways. ¡°It is still not technically or economically possible to realise a fully subterranean network with cables as their transmission capacity is still too low,¡± says Franck. He sees modifying high-voltage pylons in such a way that individual conductors also transport direct current as one solution. Ideally, this would almost triple the usable transmission capacity. In a project with Swissgrid, the scientists are studying how Swiss pylons would have to be converted for this to work.

Environmentally friendly insulating gases

Franck¡®s team is also researching gases that can be used to insulate high voltages in a small space, such as in breaker systems in electricity plants or on platforms at sea. Until now, the insulating gas sulphur hexafluoride (SF6) has most commonly been used. However, SF6 is one of the strongest greenhouse gases of all: one kilogram of it warms the climate as much as 22,800 kilograms of CO2. Consequently, the team is on the lookout for climate-neutral gas mixtures to replace SF6. If they succeed, more direct current cables could be insulated with this substitute gas instead of with solid polyethylene, as has mostly been the case thus far, because gas-insulated lines have a much higher transmission capacity. ¡°Sustainable and more efficient cables could become an alternative for new overhead power lines one day,¡± hopes Franck. If the ETH Zurich researchers are successful with all their projects, Thomas Alva Edison could eventually win the War of Currents after all.