High Temperature Superconductor (HTS) Cables

Superconducting cables are based on special superconducting materials that are cooled down to extremely 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 required 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, although the operational properties of HTS allows operating voltages at lower levels.

Technology Types

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; 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. Although more ambitious to manufacture, the cold configuration has the advantage of containing 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 for higher capacity transmission corridors. How-ever, a careful analysis of the magnetic field impact will be carried out on 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. A superconductor system also has a smaller footprint in an underground installation as a result of not requiring large separation between cables.
• High Power carrying capacity. Achieving higher levels of current density means that operational voltages can be reduced while still facilitating bulk power transfer at high capacities. Lower operating voltages reduces the size and volume of the electrical equipment required at both ends of the cable.

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, mainly taking the form of thermal leaks (the induced current in the metallic part remaining low). Energy losses in an HTS cable depend on the load and, therefore, 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 approximately 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 will therefore result in 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 and 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 to 6 for HTS in DC transmission systems. The literature provides a TRL of 5 as testing on integrated systems is still limited, but the recent demonstration nb.5 of the FP7 funded Best Paths project, with a range of 5 – 10 kA at 200 – 320 kV, indicates a TRL of at least 6.
• TRL 7 to 8 for HTS in AC transmission systems. TRL 7 for high power HTS AC transmission systems or HTS distribution systems for congested urban areas, as integrated pilot systems have already been demonstrated. The Shingal Project in South Korea is the first-of-a-kind commercial project and has achieved a TRL of 8. In particular, several projects have been discussed in Jeju (South Korea) with 154 kV and Long Island (US) 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: Optimisation of the cryogenic cooling system, optimisation 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

References

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[ 6 ] BEST PATHS project website [Link]

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