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ADVANCING SUPERCONDUCTING LINKS
FOR VERY HIGH POWER TRANSMISSION
What are the prerequisites for employing superconducting links
in the power grid of the future? This document assesses the main
elements of a new 3-gigawatt-class superconducting cable. In
addition to discussing the technical details of the cable conductor,
electrical insulation, and grid connections, it outlines the environ-
mental benets and future implementation challenges of this new
technology. The concluding remarks include recommendations
for industry and policymakers.
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Ad vanc in g s up erc on du ctin g li nk s for v er y hi gh powe r tr an sm is sion
Best Paths focused on validating high-voltage
direct-current (HVDC) superconducting links
capable of transporting large amounts of electricity –
on the gigawatt scale [3].
This is the rst time a high-voltage superconducting
cable system has been designed that is capable of
operating in direct current. Other projects deal with
alternating current only and use high-temperature
superconducting materials that are manufactured
in a low-yield and complex process. By contrast,
the Best Paths cable employs the superconducting
material magnesium diboride (MgB2), which is very
economical to produce.
What do the main cable components look like?
What can be improved in terms of costs and ef-
ciency? Apart from testing the suitability of the MgB2
superconductor for high-power electricity transfer,
the remaining cable components – including the
insulation and terminations – were also examined.
Particular care was taken to employ real-grid
conditions and assess the economic viability and
environmental impact of the cable system.
INTRODUCTION
Thirty per cent of the electricity in Europe is currently
generated by renewable energy sources. At the
present rate of growth, the proportion of renewables
could reach 50 per cent by 2030 [1]. What will
our future grids look like and what role can super-
conducting links play in them?
Recent studies have shown that additional transmis-
sion corridors extending over several hundred kilo-
metres with capacities of 5 to 20 gigawatts (GW)
are needed in the future European grid [2]. As solar
and wind farms are often located far away from the
consumption centres, long-distance transport lines
are required, with direct-current transmission having
a clear advantage in terms of efciency.
Beyond purely technological challenges, the interplay
of ecological, social and economic dimensions adds to
the complexity of the system.
In this context, the EU-funded project Best Paths
aimed to develop novel grid technologies to increase
the European transmission capacity and electricity
system exibility. A demonstration area within
The most important specications of the Best Paths
3-GW-class HVDC superconducting cable system
are summarised below. The upper part of the table
lists the main nominal parameters of the cable, and
the lower part shows the requirements imposed
by transmission system operators for successful
integration into the electricity grid. The fullment of
these requirements was a key consideration in the
design of the cable [4].
The gure on the right-hand side illustrates the
basic cable conguration, with the key components
labelled accordingly. Due to their large size, the
electrical terminations used to provide the grid
connection are not represented here.
KEY COMPONENTS AND CHARACTERISTICS OF THE CABLE
10 kA MgB2
conductor
in helium gas
1
High-voltage lapped
insulation in
liquid nitrogen
3
Inner cryogenic
envelope
2
Outer cryogenic
envelope
4
Copper (Cu) Magnesium diboride (MgB2)
Vacuum
*
Helium gas
3
2
4
Liquid nitrogen
Liquid nitrogen
*
*
*
*
1
Structure
Power
Voltage
Current
Cooling media
Fault current
AC ripples on
10 kA DC current
Change of power
ow direction
Monopole
3.2 GW
320 kV
10 kA
Liquid N
2
for the electrical
insulation
He gas for the MgB
2
conductor
35 kA during 100 ms
< 1% amplitude 50 Hz
100 MW/s up to 10 GW/s
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Ad vanc in g s up erc on du ctin g li nk s for v er y hi gh powe r tr an sm is sion
Impressions from the industrial manufacturing process
• Wire and cable conductor: The project con-
rmed that
MgB2
superconductors can carry large
amounts of electricity, up to 500 times more than
copper. Furthermore, the superconducting wires
are manufactured in a robust and reproducible
industrial process. Using these wires, 10 kiloam-
pere (kA) cable conductors have been designed
and assembled on standard cabling machines.
The performance of the cable conductors has been
tested and conrmed at different temperatures
and magnetic elds. No degradation has been
found after mechanical stress tests such as bend-
ing and pulling.
• High-voltage insulation: A novel HVDC insu-
lation operating at cryogenic temperatures has
been designed and successfully tested within the
project. The insulation consists of multiple layers
of paper immersed in liquid nitrogen. In case of
an electrical breakdown, the nitrogen automati-
cally lls any gap in the paper and the insulation
properties are thus recovered. Tests on the nitro-
gen-impregnated insulation proved its very high
electrical performance and reliability, conrming
its suitability for future use in the electricity grid.
The results were shared with the international
electrical insulation community [5].
• Terminations: Managing the connection between
the superconducting cable and the existing grid
is one of the most challenging technical aspects
due to the high current and voltage levels in-
volved. Hence, the innovative design of the termi-
nations aims to separate the current and voltage
functionalities. The terminations are therefore split
into two independent parts: In the upper part,
the current is injected through special current
leads connected to the cable conductor, while the
high-voltage gradient is managed in the lower
part. With this design, the performance of the
superconducting cable system can be easily
adapted to the grid voltage and current without
the need for any new development work.
The high-voltage testing was carried out at a dedi-
cated test platform on a 30-meter superconducting
loop connected to two terminations. It was conducted
at up to 592 kilovolts (kV), which is the testing volt-
age required to qualify 320-kV-class systems. These
pioneering tests of superconducting cables have set
benchmarks for future HVDC standards.
WHAT ARE THE MAIN RESULTS?
Superconducting cables (MgB
2
)
2 cables (320 kV/10000 A)
Resistive cables (XLPE)
8 cables (320 kV/2500 mm
2
Cu)
Footprint ≈ 10 m Footprint ≈ 1 m
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Ad vanc in g s up erc on du ctin g li nk s for v er y hi gh powe r tr an sm is sion
Superconducting cables can transmit high currents at
exible voltage levels that can be tailored for optimal
performance. This makes high power transmission
possible even at moderate voltages (up to 320 kV)
and holds great promise for the next generation of
electricity grids.
That said, the further development of superconduct-
ing lines faces a number of challenges. In particular,
the need to combine two technologies – electricity
transmission and cryogenics – introduces a new
complexity. This is why within Best Paths substantial
work was dedicated to elaborating a viable concept
for very long superconducting links. The different
options still need to be thoroughly evaluated. Some
of the remaining challenges include:
• Setting up production lines on a scale required to
manufacture the cryogenic envelopes needed for
link lengths of several hundreds of kilometres;
• Qualifying eld joints for both the cable conductor
and the high-voltage insulation;
• Examining appropriate coolants for long-distance
links, in particular in areas with steep inclines.
One issue that is often mentioned in conjunction with
very high-current links is the absence of converters
with a rating above 2 kA. However, a current rating of
5 kA is expected to become available within the next
ve years, and converters operating in parallel are
expected to overcome this barrier in the future.
Finally, the system’s reliability and availability still
need to be accepted by grid operators. Even though
superconducting cables are based on proven and safe
technologies and have been successfully tested for
more than ve years with 100 per cent availability,
gaining the acceptance of transmission system opera-
tors remains a challenge.
IMPLEMENTATION CHALLENGES
CAPITAL COSTS FOR A 6.4 GW
LINK OF 500 KM LENGTH
XLPE
29 %
31 %
40 %
MgB
2
4 %
16 % 32 %
48 %
Converters Cable Cooling stations Engineering and
right-of-way
Footprint < 1 m
The advantages of superconducting cables over
conventional HVDC cables are:
• No heat leakage into the surrounding soil
• Signicantly smaller overall size – one pair of
high-power superconducting cables has the
same transmission capacity as eight conven-
tional cables (see footprint gure to the left),
which translates into:
• Lower impact on the soil during installation
• The possibility of using narrow or existing
corridors
• Reduced impact on nature, especially in
forested or pristine areas.
KEY ENVIRONMENTAL BENEFITS
In terms of their overall costs, resistive and superconducting links
are very similar. As seen above, the cost of the converters that de-
liver 320 kV and 10 kA is comparable for both solutions. Due to the
small footprint of the superconducting link, expenditure on engineer-
ing and right-of-way can be reduced by a factor of 2.5. Surprisingly,
the cost of the cooling stations is not that signicant. It is, in fact,
the cryogenic envelope that accounts for the main cost share. This
gure is, however, based on existing production lines. More efcient
production lines will be needed to install links that are several hun-
dreds of kilometres long. And the costs of the cryogenic envelope are
expected to decrease by at least 30 per cent as a consequence of
this industrialisation.
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Ad vanc in g s up erc on du ctin g li nk s for v er y hi gh powe r tr an sm is sion
In Best Paths, gigawatt-scale superconducting cables were investigated and shown to be technologically
mature and cost-competitive for the transmission of large amounts of electricity. Thanks to their high
efciency, compact size, and reduced environmental impact, superconducting cables are likely to nd
higher public acceptance than overhead lines and conventional cables. In order to deploy this new
technology, appropriate de-risking instruments should be put in place within the framework
of European energy-climate policies.
In the long term, superconducting links are expected to transport large amounts of electricity over long
distances. In the short term, the most suitable applications are areas where civil work is expensive, but
also urban areas where space is limited. Here, a short superconducting cable could serve as a ‘bridge’
connected to resistive cables or overhead lines. For feasibility studies and tenders of new transmis-
sion projects, it is recommended to take the superconducting option into due consideration.
PROJECT RECOMMENDATIONS
Furthermore, a set of new standards and availability
is needed for equipment operating at high current
and moderate voltage in substations. This includes
not only the converters, but also circuit breakers to
protect the grid and switchgear to operate it.
Ultimately, the insertion and operation of a short
MgB2-based link in the electricity grid will demon-
strate the potential of this technology in a denitive
way. Particular attention and specic case studies
should be devoted to the implementation constraints
identied in the socio-economic evaluation related to
load rate, link length, and repair time. Demonstrable
success in real-life operating conditions will help to
convince grid operators.
WHAT HAPPENS NEXT?
Within Best Paths, the operation of an HVDC cable
system was demonstrated on test platforms. Signi-
cant efforts were made to integrate the knowledge
gained in this project into the Cigré Working Group
WG D1.64 (Cryogenic electrical insulation) and vari-
ous Standardization Technical Committees such as
TC 90 (Superconductivity) & TC 20 (Electric cables).
This will ensure that the Best Paths results contribute
to setting the HVDC standards of the future.
The next step will be to develop testing guidelines
for high-voltage direct-current superconducting
cables to guarantee safety and quality standards.
A consortium of manufacturers and transmission
system operators would need to be formed and
further develop the testing procedures.
REFERENCES
[1] Agora Energiewende and Sandbag (2018), “The European Power Sector in 2017. State of Affairs and
Review of Current Developments”.
Available: https://sandbag.org.uk/project/european-energy-transition-power-sector-2017.
[2] e-HighWay2050 Project results (2015), “Europe’s future secure and sustainable electricity infrastructure”.
Available: http://www.ehighway2050.eu/e-highway2050.
[3] A. Ballarino et al. (2016), “The BEST PATHS Project on MgB
2
Superconducting Cables for Very High
Power Transmission”, IEEE Transactions on Applied Superconductivity vol. 26, 5401705.
[4] C. E. Bruzek et al. (2017), “Cable Conductor Design for the High-Power MgB2 DC Superconducting
Cable Project within BEST PATHS”, IEEE Transactions on Applied Superconductivity vol. 27, 4801405.
[5] A. Marian et al. (2018), “Validation of the superconducting and insulating components of a high-power
HVDC cable”, IEEE Electrical Insulation Magazine, vol. 34, pp. 26–36.
Ad vanc in g s up erc on du ctin g li nk s for v er y hi gh powe r tr an sm is sion
BEST PATHS stands for ‘BEyond State-of-the-art
Technologies for rePowering Ac corridors and
multi-Terminal HVDC Systems’ and involves 38
partners from 11 European countries. The project
was funded by the European Commission within
the 7
th
Framework Programme for Research, Techno-
logical Development and Demonstration under
grant agreement no. 612748.
The project united experts around ve large-scale
demonstrations to validate the technical feasibility,
costs, impacts, and benets of the tested grid tech-
nologies. They have found solutions for the transi-
tion from HVDC lines to HVDC grids, to upgrade and
repower alternating-current parts of the network, and
to integrate superconducting high-power DC links.
ABOUT BEST PATHS
Authors: Adela Marian, IASS; Christian-Eric Bruzek, Nexans France
Author contacts: adela.marian@iass-potsdam.de; christian_eric.bruzek@nexans.com
Editing: Nina Schwab, IASS
Credits: Cover image and images 1,2,4,5: Nexans France, image 3: ASG/Columbus Superconductors
Publisher: Institute for Advanced Sustainability Studies Potsdam (IASS) e. V.
Publication date: September 2018
DOI: 10.2312/iass.2018.017
Web: www.iass-potsdam.de/en
www.bestpaths-project.eu/en/demonstration/demo-5
The superconducting demonstration encompassed expertise from transmission system
operators as well as industry and research organisations from the elds of material
sciences, cryogenics, energy systems, and electrical engineering:
Nexans France (Leader) • CERN • Columbus Superconductors • ESPCI Paris
IASS Potsdam • Karlsruhe Institute of Technology • Nexans Germany
Nexans Switzerland • Ricerca sul Sistema Energetico • Réseau de Transport d’Électricité
Technische Universität Dresden • Universidad Politécnica de Madrid
