HTS Cables
High-temperature superconducting (HTS) cables use superconducting materials that, when cooled to cryogenic temperatures (below -160°C), exhibit near-zero DC electrical resistance. In practical power applications, overall system losses still occur due to AC effects in the superconductor and the energy required for cryogenic cooling. HTS cables can therefore be attractive where the cooling energy and system losses are significantly lower than the losses of conventional cable alternatives for the same power transfer.
HTS Cables are based on special superconducting materials that are cooled down to extremely low temperatures (< 77 ° K or - 213 °C) using liquid nitrogen (or ~20 ° K for liquid helium cooled MgB2 conductor) to activate the superconductivity phenomenon (very low resistance). The superconducting phases are installed inside a vacuum-insulated cryostat to thermally isolate them from the environment. The systems require specialised joints, terminations, and warm-cold transition equipment to accommodate extreme temperature gradients, mechanical tolerances, and continuous cooling for stable operation.
In the case of DSOs, there are a limited number of use cases for implementation of HTS cables. Today, they are targeted towards dense urban areas with space constraints and for specific applications requiring high power density. Therefore, for DSOs this technology is at the demonstration stage.
HTS cables offer several advantages compared to conventional cables, depending on the case study:
- Compact, high power-density: Very high current densities enable bulk power transfer at lower operating voltages, reducing the size and footprint of associated substation equipment while achieving high transfer capacity in constrained corridors.
- Easier routing in dense urban environments: Small outer diameters, negligible heat emission to the surrounding soil, and coaxial designs enable installation in narrow ducts or existing utility corridors with reduced clearance requirements and minimal thermal backfill constraints.
- Low external electromagnetic fields: Coaxial or concentric return configurations allow strong cancellation of external electric and magnetic fields, reducing EMF exposure and electromagnetic interference.
- Reduced environmental and urban impact: Limited thermal impact on soil, low visual footprint, and shorter, narrower trenches can reduce construction disruption and ease permitting.
- Case specific loss reduction: For selected applications, total lifecycle energy for cooling and operation can be lower than the compensation of losses in conventional high voltage cables, improving overall efficiency
Challenges faced for HTS Cables are:
- Cryogenic cooling required
HTS cables must be cooled to temperatures of around -196 °C (77 K) using liquid nitrogen or even lower using liquid helium. A consequence of this is high energy and maintenance costs for cooling technology, risk of coolant loss.
- Complex system integration
Integration into existing networks requires special transition stations (cable terminations, transitions to conventional cables). Specialised terminations, joints, warm-cold transitions, and interfaces to conventional cable systems increase engineering complexity and cost; network compatibility and protection coordination require careful design.
- Cost-intensive materials
HTS materials such as YBCO (yttrium barium copper oxide) or BSCCO (bismuth strontium calcium copper oxide) are expensive to manufacture and process. HTS cable systems are significantly more expensive than conventional copper or aluminium cables. In total calculation including electrical losses for cooling, the superconducting cables are frequently more expensive as standard solutions.
- Mechanical sensitivity
Superconducting materials are often brittle and sensitive to mechanical stress (e.g. bending, vibration). Protective measures increase the complexity of cable design.
- Lack of long-term experience
There are only a few long-term demonstration projects. Uncertainty exists about service life, ageing and reliability under real operating conditions. In addition, the low technology readiness level is also a barrier for application within extra HV grids.
- Susceptibility to faults during quenches
A quench is the transition from the superconducting to the normal state can lead to sudden resistance and heat generation, which can ultimately lead to risks of cable damage.
- Limited standardisation
Very limited international standardisation for HTS cable systems exist (in contrast to conventional high-voltage cables according to IEC 62067 etc.).
- Limited purchasing power
Superconducting materials are not widely available on the market and are a special product. Moreover, the cost of these materials are high and could be lowered only by heightened demand.
The price of superconducting materials is high and can be lowered only by an increased demand. In total calculation including electrical losses for cooling, the superconducting cables are frequently more expensive as standard solutions.
Moreover, the superconducting materials are not widely available on the market and are a special product.
The low technology readiness level is also a barrier for application in extra high voltage grids.
- Higher voltage and longer length demonstrators: AC above 145 kV and DC up to 525 kV at utility-relevant lengths, including full qualification of long-distance joints, terminations, and intermediate cooling stations under realistic load cycles.
- Standardisation and testing: Development of IEC/EN standards for HTS cable systems (type tests, routine tests, prequalification, accessories).
- Fault and quench management: Faster, more selective quench detection, coordinated protection schemes for high short-circuit duties (e.g., 40-60 kA, 1 s), and validated fault current limiter integration.
- Reliability and lifetime: Ageing models and long-term data for superconducting tapes, cryostats, and accessories under thermal cycling, mechanical loads, and transient stresses
- Cryogenic systems: Higher efficiency, modularity, and redundancy of cooling plants; safer coolant management (leak detection, venting).
- System integration: Grid modelling, protection and control for HTS links (AC and DC), interaction with converters, harmonics and reactive power (AC), space-charge and polarity reversal behaviour (DC), and earthing/transfer current studies.
- Lifecycle economics and sustainability: Transparent CAPEX/OPEX and loss accounting including cooling energy, maintainability, and end-of-life strategies; environmental and safety guidelines for urban deployment.
- Compact multi-circuit concepts: Modular, multi-phase/multi-circuit cryostats to maximise corridor utilisation in urban applications.
Demonstrators of superconducting cables for AC voltages above 145 kV and for DC voltage 525 kV are needed, especially for longer sections where the application of conventional cables is highly restricted for example routing through industrial areas or for temporary applications to increase the flexibility of system extension or bridge the faulty components.
The technology is in line with milestone “Development of high power innovative transmission components” under Mission 1 of the ENTSO-E RDI Roadmap 2024-2034.
M. Yazdani-Asrami et al., “High temperature superconducting cables and their performance against short circuit faults: current development, challenges, solutions, and future trends,” Supercond. Sci. Technol, vol. 35, p. 083002, Jun. 2022.
The following TRL is observed for high temperature super conducting cables:
TRL 6 for high AC voltage.
TRL 5 for extra high AC voltages.
TRL 6 for DC voltage at 320 kV.
TRL 5 for DC voltages at 380 kV and 525 kV.
To find more information on TRL definition for the Technopedia, read here.