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The world’s first grid-integrated superconducting power cable has been connected to Germany’s grid for six months. Tildy Bayar reviews its progress

For over half a year, the world’s first grid-integrated superconducting power cable has been transmitting electricity in Essen, Germany as part of the AmpaCity grid modernization pilot project.

Touted as “the power supply of the future”, superconducting cables have the potential to significantly reduce transmission losses, assist in the grid integration of large amounts of renewable energy and save valuable urban space.

On the 180-day anniversary of the cable’s connection to the grid, I travelled to Essen to see the installation and hear reports from operator RWE and cable manufacturer Nexans on the results of the project to date.

Situated at the heart of Germany’s Rhine-Ruhr metropolitan region, Essen has been called the ‘powerhouse of Europe’ and features the headquarters of a number of global energy sector players. One of these companies is utility RWE, which said it chose to host the AmpaCity demonstration project as “another important step towards the Energiewende”.

In this light, Nobel Prize-winning physicist Dr Johannes Bednorz (who discovered the superconducting properties of the materials used in the cable in the 1980s) said efficiency in power generation and consumption has not been sufficiently stressed in setting Europe’s energy efficiency targets. As well as significantly decreasing power losses, he said, superconducting cables could help make renewable power more efficient, for example by offering around a 35 per cent power enhancement for hydroelectric plants. Bednorz is excited about the “huge potential” for the cables’ use in hydro refurbishment in Germany alone.

AmpaCity is not just a demonstration project but is intended to eventually modernize Essen’s existing grid, which consists of 110 kV underground cables and 10 kV substations built on the model of a central power plant supplying the city’s industrial customers. Given that many of the city’s existing power assets are nearing the end of their useful lifetimes, and that the industry and power infrastructure in the Ruhr region has evolved, including the addition of significant amounts of renewable energy, RWE took the long view in its grid modernization plans.

Partnering with RWE in the project are cable manufacturer Nexans, the Karlsruhe Institute for Technology (KIT) and project management consultancy Jülich. In addition, support in the amount of €6.3 million ($7.9 million) was provided by Germany’s Federal Ministry of Economics and Technology (BMWi). In total, the project cost was around €13.5 million.

The demonstration is based around the superconducting cable, which adds another 1 km to Essen’s 6500 km power distribution grid and is the world’s longest such cable. At 10,000 V, the cable connects two transformer stations in the city centre, replacing a conventional 110,000 V cable and transmitting up to 40 MW. The superconducting cable transports around five times as much electricity as a conventional copper cable of the same diameter, with almost zero loss.

In addition, installing superconducting cables can reduce the need for transformer stations, enabling utilities to move them to the outskirts of cities and freeing up much-needed urban space. In the case of AmpaCity, two out of five substations were able to be eliminated.

Finally, superconducting cables can offer environmental benefits. Their heavy insulation means there is zero heat loss and no surrounding magnetic field – so they can be used in places where traditional cables cannot.

The AmpaCity project has drawn worldwide interest, RWE said, with delegations visiting from Korea, China, India and the US. Indeed, Nexans foresees a demand anywhere there are conurbations and industrial networks.

How it works

The cable’s superconducting ceramic tape is cooled to around -200°C inside a thick cooling jacket filled with 2.5 m3 of liquid nitrogen which evaporates in air, serving as a heat exchanger. According to Oliver Sauerbach of Westnetz GmbH, AmpaCity’s plant manager, this cryotechnology is “very new”.

In the system, the cable is combined with a superconducting fault current limiter (SCFL) which protects the grid and the cable from overloading caused by short circuit currents, and stops fault currents from propagating throughout the network by limiting the current to less than 13,000 A.

The SCFL can be integrated into a conventional grid, and in fact RWE has already done so. According to Jean-Maxime Saugrain, Nexans’ technical vice-president, the SCFL is fast, reacting before the first peak; it is self-triggering, and has no grid impact during normal operation. It is also able to protect integrated renewable energy sources.

Operational issues

Of the 8760 hours in a year, the project’s official target is to be non-operational for four hours or less. Dr Frank Merschel, project manager for new technologies at RWE, said his company expects “99.99 per cent reliability based on operation so far”, with only two or three hours per year when the system is not in operation.

According to Sauerbach, problems with the system have so far been minor, with no challenges arising in connecting to the conventional network.

In May a hurricane caused a power supply interruption across the grid, but Sauerbach’s team had switched off the cable in anticipation due to the sensitivity of the control system. The system is now protected against weather events and power failures, he said, with conventional 10 kV cables connected between the two substations as power supply backup. He added that the team does not yet have specific plans for backup power when the system is eventually expanded to cover more of Essen’s grid.

The biggest “inconvenience” so far, Sauerbach said, has been the installation time, which is longer than required for a traditional cable. So far, he said, the team has “played it safe: we do it very slowly, with no risk”. According to Frank Schmidt, head of Nexans’ superconductor division, in future the installation procedure will be optimized to be easier and faster, with “a bit more prefabrication” of components that will then be assembled on-site. As a first step in this simplification, Nexans is developing an installation joint to connect two cables together, “to be ready in the near future” according to Schmidt.

When asked whether any problems are likely to arise in future, Sauerbach said he expects the main problem to be maintaining the cooling system, with the reliability of pumps being the most important issue. The pumps are the “most critical point”, he said, because vacuum pumps, for example, have moving parts. At the moment “we have redundancy if one pump fails,” he added.

The cooling system is another “new challenge” that isn’t there with traditional cables, said Dr Joachim Schneider, RWE Technology Board member, and given this there have been “minor problems”, mostly “short interruptions” due to the sensitivity of the cooling equipment protection systems.

In case of damage to the cable, repairs are more complex than for a traditional cable. The system must be warmed up before any repairs take place, and cooled down again afterwards. The cool-down time for AmpaCity’s cable is about five days from ambient temperature to operating condition. However, Dr Mark Stemmle, Nexans’ project manager for superconducting cables, said the repair time for a medium-voltage superconducting cable is “comparable” to that for a conventional high-voltage cable.

Depending on the damage, the repair process can range from easy – repair of the cable cryostat from the outside; re-pumping the vacuum – to very complex if part of the cable is completely destroyed. In the latter case, that cable section could be replaced, or two joints and a short cable section could be introduced, Stemmle explained.

Sauerbach anticipates that hot weather will have “very little” effect on the cable’s efficiency, although “maybe a little bit” on the efficiency of the cooling system. According to Schmidt, a longer cable – say 100 km – would need to have intermediate cooling systems.

While the system’s energy balance is still being measured, the first estimate is that its total power consumption is around 6.7 kW or 160 kWh per day.

Current status and future plans

Saugrain perhaps put it best in saying: “We could be considered the market leader – the only problem is that we have no market yet”. Nexans’ plans for this technology include large-scale commercialization, and according to Dr Mathias Noe, head of the KIT Institute of Technical Physics, the firm is now “well-positioned to roll it out into the world”.

However, according to Bednorz, investors will still “need to be risk-friendly to support” such a “pioneering, door-opening” project. In practical terms, the cost of the superconducting cable is still a factor of two more than copper cable. Additional costs arise when the cooling system, installation and civil works are added – but Schmidt emphasized that the money recouped from the lack of power losses must also be considered, and that the maintenance costs for conventional 10 kV technology are higher. And, he said, “if you put everything in the picture for a full view, then the real estate value makes the superconducting cable’s advantage bigger”.

For long-distance power transmission, though, superconducting cable technology does not offer the same cost benefit. Schmidt noted that there is a benefit to having underground rather than overhead power lines, “but maybe 10 km rather than 100.”

Uwe Beckemeyer, Germany’s secretary of state, said Essen chose this project at this time because the city “wanted to support something new”. The importance of energy efficiency “cannot be overemphasized,” he added, and is “where we can make the most gains in the future” toward a low-carbon economy”. And, once the technology is implemented on a large scale, it can facilitate the transport and sale of electricity, for example from Germany’s northern region to the power-hungry south.

Essen plans to eventually cover its entire 23 km inner-city ring with superconducting cables, elminating more substations. But, a city official said, “Let us have more experience first to make sure the system works in summer, in storms”. The testing phase of the project will last until 2016.

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