Superconducting cables are based on special superconducting materials that are cooled down to very low temperatures (e.g. -180°C) using liquid nitrogen (or liquid helium for MgB2) to activate the superconductivity phenomenon (very low resistance). These conductors are placed in a pipe with vacuum (cryogen) which thermally isolates the superconductor from the remaining environment. They carry five times the current of a conventional cable system with the same outer dimensions and they do not emit any heat to the environment. When comparing the cost benefit of HTS vs. conventional conductors, the losses of the superconducting cables are equivalent to the energy needed to keep low nitrogen temperatures and its circulation. The technology requires special cable joints and specific cable termination for extreme temperature differences and permanent cooling for keeping cryostat. This factsheet focuses on voltage levels above 110 kV.
Three types of superconductors are commercially available for AC or DC power cables:
- Bi2Sr2Ca2Cu3O10 (BSCCO) with a critical temperature of -160°C
- YBa2Cu3O7 (YBCO) with a critical temperature of -180°C
- MgB2 with a critical temperature of -235°C
Components & enablers
The main components are:
- HTS tapes or wires
- High voltage insulating material (dielectric)
- Cryostat wall
- Liquid nitrogen (for BSCCO and YBCO)
- Copper and hollow former
- Polyethylene sheath
- Cooling system
- Cable joints and terminations.
Two main types of superconducting power cables according to the type of dielectric used are existing:
- The “warm dielectric design” is based on a conductor, cooled by the flow of liquid nitrogen surrounded by a cryogenic envelope using two concentric flexible stainless steel tubes with vacuum and superinsulation in between, and the outer dielectric insulation, the cable screen and the outer cable sheath are at room temperature.
- In the “cold dielectric” design, the liquid nitrogen is used as a part of the dielectric system. While more ambitious to manufacture, the cold configuration presents the advantage to contain the electromagnetic field inside the superconducting screen, which significantly reduces the cable inductance.
The design of superconducting HVDC power cables is very similar to the design of superconducting HVAC power cables. The inner HTS layers are separated through the dielectric from a screen, consisting of copper wires only.
Advantages & field of application
HTS cables offer several advantages compared to conventional cables depending on the case study:
- Easier and shorter installation time. Grid operators can benefit from shorter installation time as HTS cables are compact and can be routed underground through existing gas, oil, water or electric corridors or along highway or railway rights of way. In addition, HTS cables are actively cooled and thermally independent of the surrounding environment making them easier to install. These aspects pave the way to higher capacity transmission corridors. However, a careful analysis of the magnetic field impact shall be carried out to the infrastructures sharing the same corridors.
- Low impact on the environment. Reaching much higher levels of current density enabling compactness and higher capacity power transmission over the cables than conventional cables constitutes a key advantage for the operators and the environment.
Due to the low electromagnetic fields generated by HTS cables, the effect on the surrounding area is significantly reduced.
Despite the superconducting properties with electric resistance close to zero at temperatures below the critical temperature, HTS cables are still subject to energy losses taking mainly the form of thermal leaks (induced current in metallic part remaining low). Energy losses in an HTS cable depend on the load and thereby on where the cable is placed in the power transmission grid. For an HTS cable at 2 kA losses could be at a level of about 25% of the losses in the conventional cable system (but at no-load the losses in the HTS cable are larger due to the thermal leak).
The significant reduction of transmission losses is thus counter-balanced by the necessary cooling requirements. The no-load losses due to a non-ideal thermal insulation would thus push for deploying HTS cable in connections with high load current in a large part of the time.
HTS DC Cable are well suited for long-distance high power transmission, bulk energy transfer.
As of today, it is observed that a high number of HTS cables are being installed in urban areas requiring high current capacity at medium voltage levels.
Technology Readiness Level
- TRL 5-6 for HTS in DC transmission systems. Literature provides a of 5, since testing on integrated systems is still limited, but the recent demonstration nb.5 of FP7 funded Best Paths project with range of 5– 10 kA at 200–320 kV could pretend to a TRL of at least 6.
- TRL 7 for high power HTS AC transmission systems or HTS distribution systems for congested urban areas, since integrated pilot systems have been already demonstrated. In particular several projects could be mentioned in Jeju (South Korea) with 154 kV, Long Island (USA) with 138 kV.
Research & Development
Current fields of research: Magnetic design and stress analysis on new superconductor material e.g, Bi2 or MgB2, cable stability analysis against internal flux jumps and external thermal perturbations, electromagnetic field analysis for HVDC superconducting cables, applications in HV and EHV.
Innovation priority: Optimization of the cryogenic cooling system, optimization of manufacturing process and material physical footprint.
Best practice performance
- HVAC: 350 kV, 700 kVA, 4 kA
- HVDC: 650 kV, 10 kA
- Impulse voltage: 1,200 kV, 60 kJ
- Cooling temperature: -208.15°C to -193.15°C
- Pressure: up to 1.5 MPa
Best practice application
The AmpaCity project is a 1 km 10 kV HTS cable installed in 2014 to replace a 110 kV underground cable system connecting two 10 kV substations in Essen Germany.
The three-phase, concentric cable replaces conventional 110 kV copper line connecting two substations in central Essen and eliminates the need for a high-voltage transformer at one of the substations.
The cost of the energy needed to cool the cable down to eliminate its resistance over its lifecycle is found to be 15% lower than equivalent to the cost of compensating losses in conventional 110 kV cables. HTS are mentioned as the best technical and economically viable solution to avoid the necessary extension of the 110 kV grid in urban areas.
Germany, Hungary, Norway, Belgium, Sweden, Spain, Denmark, Switzerland, France, United Kingdom and Italy
BEST PATHS was a collaborative project of 40 leading European organizations from science and industry, supported by the EC FP7 (2014-2018). The project investigated the feasibility of technological innovations that could advance high-capacity transmission links. This included a demonstrator project dedicated to superconducting electric lines, to validate the novel MgB2 technology for GW-level HVDC power transmission.
Through Insulated cross-arms, long-term tests with HTLS as well as dynamic line rating, existing lines are to be optimized to maximize power transmission.
The operation of a full-scale 320 kV MgB2 monopole cable system that can transfer up to 3.2 GW was demonstrated (demonstration nb. 5 of the project).
National project in which, a 500-m cable connected an Internet data center (iDC) to a large scale array of photovoltaic cells to supply DC power.
Construction of two DC superconducting power cables of respectively 500 m (Line 1) and 1000 m length (Line 2). The cable of the Line 1 is installed into the underground, and is composed of two cables.
The heat leak of the cryogenic pipe is ~1.4W/m including the cable pipe’s and the return pipe’s. The heat leak of the current lead is ~30W/kA in the test bench. Finally the current of 6kA/3sec and the current of 5 kA/15 min were achieved in Line 1.
Saint Petersburg, Russia
Cables between two substations in downtown St. Petersburg spanning a distance of 2.5 km. Connecting the 330 kV “Tserntralnaya” and 220 kV “RP-9” substations will provide reserve power network capacity, allowing new consumers to connect to the system and improve system reliability and limit fault currents for existing end users.
50 MW, 20 kV HTS DC cable on 2.5 km
Tests were conducted on two 30-meter cable samples, two 430-meter cables, three pairs of current leads, and three joints. Critical current (IC) tests were carried out at 68 K to 78 K; resistance remained stable and the cable performed as expected. The cables also passed high-voltage testing.
Oak Ridge National Laboratory (Phase 1), Yonkers, NY (Phase 2), USA
The HYDRA project Phase 1 was a 25-meter prototype that was successfully tested at Oak Ridge National Laboratory. The Phase 2 consists in connecting two substations in Westchester County. This HTS FCL Cable installation will allow asset sharing of 13.8 kV transformer combined with fault current protection to equipment.
Phase 1 using 25 meters HTS FCL Cable, phase 2 aims at connecting 2 Con Edison 13.8 kV substations with a cable length of 170 m.
The phase 1 of the projects helped the qualification of HTS FCL cable for Power Network.
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