CABLING & CABLE MANAGEMENT r
Superconductors ready to rethread urban power networks
Jean-Maxime Saugrain, Nexans Machines, Cryogenics & Superconductors vice president, explains how superconductor power cables could enable urban power networks to adapt to the demands of the decarbonised future
infrastructure with superconductors could address bottlenecks to meet the demands of power-hungry city dwellers.
MOVING FROM HV TO MV
An even smarter way to use superconductor cables is to carry the same power as a conventional system, but with the operating voltage reduced from high-voltage (HV) to medium-voltage (MV). This can enable DNOs to develop networks with smaller space requirements. It also allows for smaller, less obtrusive substations while studies have shown that in a typical urban network the number of transformers might be reduced by 40 percent. These factors all start to tip the financial balance in favour of HTS. The MV concept was adopted by RWE, the German
D
istribution network operators (DNOs) across the world need to upgrade and expand their urban networks to support the transition to
electrification and decarbonisation. They have to handle increased loads and the integration of new distributed and often volatile generation resources while maintaining the highest possible reliability and continuity of service. Yet while the urgent need for grid renewal is well recognised, there is often significant resistance from planning authorities, and the public they serve, to grant the essential rights of way for new overhead lines or underground cables. This has become a key driver for innovations that can increase the capacity and flexibility of power infrastructure.
HTS CABLES
A technology that shows particular promise is high- capacity, underground superconductor cables based on HTS (High Temperature Superconductor) materials. Discovered in the late 1980s, these materials acquire their superconducting properties at a much higher ‘critical temperature' than conventional superconductors. That means they need only to be cooled to around -180°C, which can be achieved using liquid nitrogen, a cheap, abundant and environmentally friendly cooling liquid. Because HTS cables offer almost no resistance they can carry enormous currents with minimal losses, so they offer a much greater power density than copper wires. In addition, they are actively cooled and thermally independent of the surrounding environment, so they can fit into more compact installations than conventional
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infrastructure, without any concerns for spacing or special backfill materials to ensure dissipation of heat. They also eliminate EMF concerns. HTS cables are often seen as a potential replacement for conventional high voltage cables as the backbone of long-distance transmission grids. However, where they can really make the greatest impact is in short lengths, from a few hundred metres up to a few kilometres, deployed in strategic locations. This could be useful for example when it comes to crossing obstacles like rivers. So instead of directional drilling, or constructing a dedicated cable tunnel, the power cable could be carried by an existing structure, such as a bridge or service tunnel.
Another interesting possibility is in built-up urban areas where it is becoming increasingly difficult for DNOs to obtain the rights of way to install the new overhead lines or underground cables. When operating at the same voltage, HTS cables can carry between five to 10 times as much power as conventional cables. So simply rethreading existing
utility, for the ‘AmpaCity’ project in the Ruhr city of Essen. Nexans installed a 1 km underground HTS cable operating at 10 kV, about one tenth of RWE’s usual 110 kV transmission voltage. The system, which has an operating current of 2.3 kA and a capacity of 40 MW, has been in operation of over six years. It is now regarded as an integral part of Essen’s distribution network and is treated in the same way as any other cable.
FAST FAULT CURRENT PROTECTION
The AmpaCity project was also the first to combine a superconductor cable with a resistive superconducting fault current limiter (SFCL) for overload protection. In normal operation, an SFCL allows current to flow easily and with no losses. But should a fault current start to flow the superconductor heats up above its critical temperature and it transitions from a perfect conductor to having a very high resistance. This transition takes place in less than 2 milliseconds (ms) – about 50 times faster than the blink of an eye. The fault current is limited immediately, protecting equipment such as transformers, switchgears and busbars on the same circuit. A key advantage of the SFCL, unlike some devices that rely on explosive devices to break the circuit, is that it does not need to be replaced or reset after use. As soon as the fault current is cleared and the superconductor is cooled back down, the SFCL will return to operation. While it might seem exotic, HTS cable technology has emerged from the laboratory. It is ready to rethread congested energy-intensive urban power grids where superconductors offer an important, financially attractive alternative to existing cable solutions.
NEXANS
nexans.com
ELECTRICAL ENGINEERING • SUPPLEMENT • FEBRUARY 2021 S5
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