Equipping Europe’s future energy grid with superconductors
Europe’s “Best Paths” project was the hotbed where the first 3-gigawatt-class superconducting cable system was designed, optimized, fabricated, and tested. Frédéric Lesur, Senior Engineer High Voltage Cable Systems and Power Grids, and Jean-Maxime Saugrain, VP Machines, Cryogenics & Superconductors at Nexans, take us behind the scenes.
When carrying high currents, the resistance of conventional high-voltage power cables with copper or aluminum conductors causes them to produce heat. This heat translates to lost energy, which can be nearly 10 percent over long-distance transmission projects. The waste represents the equivalent output of several of Europe’s largest power plants.
This sparked interest in superconducting cables since they offer electrical transmission with zero resistance. To put it in perspective – within a compact footprint, a single superconducting cable could carry several nuclear reactors’ joint output over long distances with no losses.
When cooled below a critical temperature, superconductors have no electrical resistance. This temperature varies from nearly absolute zero (-273°C) to -135 °C, depending on what material is being used.
Although superconductors were discovered in 1911, the technology’s potential for perfect power transmission remained untapped for decades, mainly because of the commercially available superconductors’ extremely low operating temperatures. In 1987, however, the discovery of high-temperature superconductors (HTS) created new possibilities for superconducting power applications. This turned the focus to energy storage, cables for power grids, and fault current limiters.
In recent years, the high efficiency, compact size, and reduced environmental footprint of superconductors have sparked renewed interest. Grid operators are taking note of these advantages as they prepare for the transition to renewable energy.
HVDC superconducting cable systems
The future need for multi-gigawatt links was the key motivation to study HVDC superconducting cables in the Best Paths project. The cable system is designed to operate at high voltage with a range of possible currents exceeding 3 kA. Conventional resistive cable technology is more capable of meeting grid requirements below this current level.
Figure 1 indicates that the HVDC cable system is bipolar, allowing electricity to flow in both directions. The system comprises five key elements:
- Cryostat (cryogenic envelope), which stores the cooling fluid required to maintain the superconductor temperature
- Cryogenic terminations and joints
- High-voltage insulation
- Sufficient cooling devices attached to associated power and fluid supplies for auxiliary equipment
Along with the cable, the system comprises two terminations to connect to the grid and joints. The cable length determines the number of cooling systems needed to maintain the operating temperature along the link.
European-funded Best Paths project
The four-year project (2014-18) featured a full-size cable system demonstrator operating at 320 kV and 10 kA. The 320 kV operating voltage was selected to facilitate insertion into the transmission grid, while the 10 kA-current was the maximum amount the AC/DC converters could carry.
An extensive preparatory phase – involving the specification, development, and optimization of the main system components – preceded Best Paths. Selecting MgB2 as the superconductor material had a significant impact on the cable design and remaining system components’ choice.
A cryogenic envelope houses the superconducting cable to maintain the operating temperature along the link’s length. Flexible cryogenic lines – with proven reliability obtained over more than 50 years – were used to simplify the cable laying. Figure 2 shows the final design.
Although operating at 320 kV, the HVDC superconductor cable system was tested at nearly 600 kV to meet Cigré recommendations. Future development could include reducing the operating voltage to keep the cable’s overall diameter to a minimum while minimizing its footprint. The current would then be increased correspondingly to maintain the GW power levels.
A viable option for bulk power transmission
Successfully designing, manufacturing, and testing a 3-gigawatt-class HVDC superconducting cable system in the Best Paths project has widened the field of applications for superconductors. It proved that they represent a realistic solution for bulk power transmission. They also contribute to global decarbonization by reinforcing and increasing electricity grids’ efficiency, with minimal environmental impact.
Robust and reliable superconducting cables also open the promising possibility of the simultaneous transportation of two energy carriers – hydrogen and electricity – if hydrogen was to replace helium gas as the cooling medium.
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