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The BEST PATHS Project on MgB
2
Superconducting
Cables for Very High Power Transmission
Amalia Ballarino, Christian E. Bruzek, Nico Dittmar, Sebastiano Giannelli, Wilfried Goldacker, Giovanni Grasso,
Francesco Grilli, Christoph Haberstroh, Stéphane Holé, Frédéric Lesur, Adela Marian, José M. Martínez-Val,
Luciano Martini, Carlo Rubbia, Delia Salmieri, Frank Schmidt, and Matteo Tropeano
Abstract—BEST PATHS (acronym for “BEyond State-of-the-
art Technologies for rePowering Ac corridors and multi-
Terminal HVDC Systems”) is a collaborative project within the
FP7 framework of the European Commission that includes an
MgB
2
-based power transmission line among its five constituent
demonstrators. Led by Nexans and bringing together
transmission operators, industry and research organizations, this
demonstrator aims at validating the novel MgB
2
technology for
very high power transfer (gigawatt range). The project foresees
the development of a monopole cable system operating in helium
gas in the range 5–10 kA/200–320 kV, corresponding to a
transmitted power from 1 to 3.2 GW. The main research and
demonstration activities that will be pursued over the four-year
project duration are: 1) development and manufacturing of
MgB
2
wires and of the cable conductor; 2) design and
manufacturing of the HVDC electrical insulation of the cable; 3)
optimization of the required cryogenic system; 4) electromagnetic
field analysis; 5) design and construction of a prototype electrical
feeding system including terminations and connectors; 6) testing
of the demonstrator; 7) study of grid connection procedures and
integration of a superconducting link into a transmission grid;
and finally, 8) a socio-economic analysis of the MgB
2
power
transmission system. CIGRÉ recommendations will be used to
take into account the established international practices, and
guidance will be given on newly addressed technical aspects. An
overview of the project is presented in the paper, including the
main tasks and challenges ahead, as well as the partners and
their roles.
Index Terms—BEST PATHS, high-power transmission lines,
HVDC, MgB
2
cables, superconducting links
BEST PATHS is supported in part by the European Commission within the
7th Research Framework Programme, Grant Agreement 612748.
(Corresponding author: Adela Marian.)
A. Ballarino, S. Giannelli, C. Rubbia, and D. Salmieri are with CERN-
European Laboratory for Nuclear Research, 1211 Geneva, Switzerland.
C. E. Bruzek is with Nexans France, 92587 Clichy, France.
N. Dittmar and C. Haberstroh are with Technische Universität Dresden,
01062 Dresden, Germany.
W. Goldacker and F. Grilli are with the Karlsruhe Institute of Technology
(KIT), 76021 Karlsruhe, Germany.
G. Grasso and M. Tropeano are with Columbus Superconductors S.p.A.,
16133 Genoa, Italy.
S. Holé is with ESPCI ParisTech, 75005 Paris, France.
F. Lesur is with Réseau de transport d'électricité (RTE), 92400
Courbevoie, France.
A. Marian is with the Institute for Advanced Sustainability Studies (IASS),
14467 Potsdam, Germany (e-mail: adela.marian@iass-potsdam.de).
J. M. Martínez-Val is with the Technical University of Madrid, 28006
Madrid, Spain.
L. Martini is with RSE S.p.A., 20134 Milan, Italy.
F. Schmidt is with Nexans Deutschland GmbH, 30179 Hannover,
Germany.
I. INTRODUCTION
RANSMISSION SYSTEM OPERATORS (TSO) are
responsible for the balance between electricity generation
and consumption, at any time. While the power demand has
been increasing for decades, the present situation in Europe
tends towards stabilization [1]. Nevertheless, the integration of
large amounts of renewable energy across the European
continent is a real challenge [2]. In addition, interconnecting
countries seek to guarantee security of supply (sharing risks as
well as capacity reserves) and to trade energy across
borderlines at the best price. All these factors require the
development of bulk power corridors. As the building of such
infrastructures necessitates many years and significant
investments, a prior assessment of the needs and constraints is
essential. For example, the recent interconnection between
France and Spain (2 x 1000 MVA, 65 km, 2015) was built
fully underground as a high-voltage direct-current (HVDC)
line, after around twenty years of strong public opposition to
the installation of 400 kV overhead lines [3]. The global
investment was 700 M€.
The importance of bulk power lines of hundreds of
kilometers length was also emphasized by the recent outcome
of the EU FP7 e-Highway2050 project [4]. The need for 5 to
20 GW pan-European transmission corridors was clearly
identified; in particular, major North-South corridors and
connections of peninsulas and islands to continental Europe
were shown to be critical.
In recent years, the necessity to limit the visual impact of
long-distance transmission lines has brought about a
remarkable evolution of the underground circuits. For
example, 97% of new high-voltage lines (63 to 90 kV) in
France were buried between 2012 and 2014. While the
number of underground lines longer than 10 km was
negligible in 2007, more than 72% of the installed circuits
between 2014 and 2016 will be longer than 10 km, and 27%
longer than 20 km.
The conventional solutions presently used to transmit power
in the range of 1 to 8 GW include overhead lines, underground
cables with extruded insulation (XLPE), and gas-insulated
lines (GIL). Most often they require large rights of way and/or
extensive civil engineering. By contrast, the compactness of
superconducting cables is an attractive feature that entails
narrow corridors and reasonable trenches. Furthermore,
existing buried links are subject to limitations in terms of
T
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transmitted power and length. For example, the longest XLPE
link has a length of 39.8 km (Shinkeiyo-Toyosu, Japan,
2.4 GW) [5], whereas the highest power for a buried GIL is
4.4 GW (Kelsterbach, Germany, but only 0.9 km long) [6].
Superconducting links do not suffer from these limitations.
In fact, superconductors have been ready for deployment in
energy-related applications for some years now, but have yet
to be utilized on a large scale and still need to be validated for
DC system operation. Within the BEST PATHS European
project, the main goal of the demonstrator called DEMO 5 is
to investigate whether superconducting HVDC links are a
viable solution for bulk power transmission in the future grids.
This demonstrator will also be a first attempt to employ MgB
2
as a superconductor for HVDC cables. Due to the low cost and
high transport current of the MgB
2
wires at the magnetic field
of interest for energy transfer applications, these cables are
expected to be competitive not only with conventional
resistive XLPE cables, but also with high-temperature
superconducting (HTS, e.g. Bi-2223 or YBCO) cables. Hence,
we aim to investigate the technological maturity of MgB
2
HVDC links for operation in the grid.
II. DESCRIPTION OF THE PROJECT
The potential of the MgB
2
superconductor for application to
high-current energy transmission (> 10 kA) was demonstrated
at CERN in 2014, when two 20 m long copper-stabilized
MgB
2
cable assemblies were connected in series and were
successfully operated in DC mode at 20 kA at 24 K. The
measurements were conducted at an ad-hoc constructed
installation that allows for the electrical characterization of
superconducting cables cooled with helium gas, at any
temperature in the range of 5 K to 70 K and at DC currents of
up to 20 kA [7]–[9].
In addition to the high-current capability demonstrated at
CERN, DEMO 5 aims to develop an HVDC monopole
superconducting cable designed to operate in the range 5–
10 kA/200–320 kV, corresponding to a transferred power of
up to 3.2 GW. The superconducting system will have a
significantly reduced environmental footprint with respect to a
conventional line, due to its compact cryogenic envelope,
which results in an overall small size.
The high-current cable conductor will be built by stranding
together MgB
2
wires, which recently became available in long
unit lengths. As part of the demonstration activity, a cryogenic
cooling system allowing for operation of the cable in helium
gas in the range of 15 K to 25 K and at 20 bar will be designed
and manufactured for the test of the superconducting line. To
keep the thermal losses at an acceptable level, a liquid N
2
thermal shield will be added to the cryostat housing the cable.
The superconducting wires will be produced by Columbus
Superconductors through the Powder in Tube ex-situ process
[10], with a layout that will be defined as the most suitable for
this kind of application. The possibility to produce round
MgB
2
wires in kilometer lengths has already been
demonstrated after many years of technical collaboration
between Columbus and CERN and with Nexans. The
preliminary wire is a monel-nickel sheathed round wire with a
diameter of around 1.3 mm, containing 36-37 MgB
2
filaments
with a filling factor of 13% to 16% of the total surface.
Several kilometers of wires will be manufactured to qualify
the production process and detailed investigations and
characterization will be carried out in collaboration with
CERN to check the homogeneity of the performance. Before
starting the cabling activities, stress tolerance of the wire
subjected to bending and tension will be extensively studied as
well.
A simple geometry for a compact cable conductor able to
transfer more than 10 kA at 20 K, and appropriate for use in
the grid, is shown is Fig. 1. The conductor contains 24 round
MgB
2
wires twisted around a flexible multi-strand copper
core, and it can be easily connected due to the
superconducting wire location in the outermost layer. The
external diameter of this conductor is about 12.5 mm.
Fig. 1. Design for a possible cable conductor, consisting of an outer layer of
24 MgB
2
wires (shown in grey) and a core of 37 copper wires (shown in red).
The cryogenic envelope will consist of multiple concentric
tubes. More specifically, corrugated tubes will be used,
because a certain degree of flexibility is needed for the cable
installation. Such tubes are routinely manufactured in
hectometer lengths, delivered on drums and can be joined on
site for multi-kilometer-long systems. Since the goal is to have
an overall thermal load lower than 1 W/m at 20 K, a rather
sophisticated insulation design will be necessary, consisting of
high-vacuum insulation combined with several layers of
multilayer insulation and the already mentioned active-shield
cooling by liquid nitrogen. The thermal heat leak from room
temperature will determine the overall required cooling power,
as the AC losses of a DC system are greatly reduced when
compared to an AC cable. In the eventuality of a fault current,
the proposed cable conductor design will result in a limited
heat generation during the ensuing quench.
Appropriate cooling machines optimized for this type of
application are already commercially available. For the
demonstrator, a refrigerator capable of delivering 120 W at
20 K and circulating gaseous He at 20 bar is under
procurement, and will be commissioned in early 2016.
The reliability and availability of the system are of key
importance for its acceptance by TSOs. During the second part
of the project, specific vision studies for very long systems
(>100 km) will be carried out to investigate the future
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technologies for the cryo-envelope and for possible cooling
systems operating with gaseous He or liquid H
2
. The level of
system availability is dependent on the cooling power and on
the redundancy of the cryogenic fluid management systems,
which can have a significant impact on the investment costs
for the system. A Turbo-Brayton cooling cycle will be the first
candidate for an efficient and reliable cooling cycle for long-
distance systems. The thermo-hydraulic design aims at
ensuring minimum values of the thermal load and of the
pressure drop along the line. Moreover, minimization of the
number of cooling stations is essential for an efficient
operation over very long distances and for decreasing the
investment costs.
These results will be used for an economic viability analysis
of the proposed superconducting HVDC cable solution, taking
into account not only the cost for the cable itself, but also
estimated costs of the substations and civil engineering. A
comparison with resistive solutions and HTS-based solutions
for relevant case studies proposed by TSOs will also be
included.
III. MAIN CHALLENGES
A. Testing the demonstrator
The high-voltage test of the superconducting system will be
conducted at the Nexans HV cryogenics platform in Hanover,
according to the CIGRÉ recommendation B1.31 [11] and to
current standards for conventional DC cables. The size of this
testing station limits the cable length to a maximum of 20 m.
The high-current tests (up to 20 kA) of the 20 m long
prototype will be carried out at CERN. Transient operation
modes as derived from grid operation requirements will be
analyzed and tested.
B. Liquid-nitrogen-impregnated lapped HVDC insulation
In the following, the cable is defined as the conductor
inserted into the inner helium-cooled cryogenic envelope
whose outer wall is lapped with high-voltage insulation. The
insulating tapes are wrapped around a flexible filling layer that
smoothens out the waves of the corrugated tube surface and
allows for bending deformations. This layer is prepared
according to proven techniques used for the AmpaCity project
[12]. In this configuration, the inner cryostat cannot slide out
of the cable dielectric for possible repair work. However, as
the cable is intended for very long power links, it will likely be
laid in an open trench. Its installation can therefore be carried
out with very limited risk to the inner cryostat. In the proposed
concept shown in Fig. 2, the cable is housed in an outer
flexible cryogenic envelope, which is cooled by liquid N
2
acting as a thermal shield. Thus, the insulating tapes will be
impregnated with liquid N
2
in the fashion currently employed
for oil-impregnated conventional cables. The material foreseen
for the electrical insulation is polypropylene laminated paper
(PPLP) tape, whose mechanical properties enable lapping. To
limit the risk of a dielectric breakdown, gas bubble formation
in the cooling liquid of the HV insulation should be avoided.
As a result, the liquid N
2
will be pressurized at up to 5 bar and
subcooled at about 70 K.
Given that the cable system operates in DC mode, a
dedicated experimental setup will be developed for testing the
HV insulation performance, with a particular focus on space-
charge distributions. Measurements will be performed using
the pressure-wave-propagation (PWP) method [13], while the
cable is electrically stressed up to 60 kV in liquid N
2
at a
pressure of 5 bar and at variable temperature. For safety
reasons the testing voltage will be lower than the operating
voltage of 320 kV. However, the thickness of the insulator
will be correspondingly reduced to maintain an electric field
that is equal or even larger than the one in operating
conditions.
In the PWP method, a short-duration pressure pulse is
transmitted to the insulator and moves the charges
encountered during its propagation. The resulting local current
due to the charge displacement is detected in the measurement
circuit connecting the insulator electrodes. The current
amplitude measured at a given time is proportional to the
charges displaced at the position of the pressure pulse at that
time. As a consequence, the signal profile is an image of the
space-charge distribution, time and position being simply
connected by the speed of sound. The pressure pulse acts then
as a probe of the charge density. Due to the cryogenic
environment, it is not possible to use lasers for generating the
pressure pulses, therefore a dedicated pressure-pulse generator
will be designed [14]. Additionally, as the layered structure of
the tested insulator could perturb the propagation of the
pressure wave and the signal generation, specific signal
treatment will be required [15].
It is the first time that measurements will be carried out
under such harsh environmental conditions. The experimental
setup developed here could not only validate the insulation
structure of the proposed BEST PATHS cable, but could also
open the door for new cable structures as well as new
insulation studies.
C. Designing the terminations
Cryogenic electrical terminations will be designed for the
demonstrator, and will be optimized both for electric-field
Fig. 2. The HVDC cable concept for the Best Paths project, schematically
illustrating the cable conductor housed in the inner cryogenic envelope at
20 K, which is wrapped with HV insulation tapes and inserted into the outer
cryogenic envelope at 70 K.
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management and for thermal management. For the former,
cryogenic bushings will be employed, based on prototypes
built for HTS cable conductors, which have already been
tested by Nexans and require only minor modifications.
Innovative solutions are needed for the thermal
management, since this will be the first instance where a flow
of cold He gas at 20 bar is injected at HV in association with
high current. The heat inleak at 20 K should be as low as
possible for a cost-competitive and robust concept.
For the current injection, the proposed concept will include
a hybrid current lead consisting of two parts: a copper upper
part making the transition from ambient temperature to an
intermediate chamber cooled at 70 K in liquid N
2
, and a lower
part extending the lead from 70 K to 20 K with a
superconducting barrel made out of HTS BSCCO tapes. Thus,
most of the heat load will be intercepted by the low-cost liquid
N
2
. This design entails managing an estimated heat load lower
than 5 Watts on the 20 K cooling system.
For the He gas flow injection, a special tube will be
installed parallel to the current lead, connecting the electrically
grounded cooling machine to the cable conductor at 320 kV
electrical potential. This injection tube will include cryogenic
thermal insulation in its radial direction and high-voltage
management in its longitudinal direction, in order to reduce
the heat load on the 20 K cooling system. The tube is currently
under study, with first experimental tests to be conducted
soon.
D. Simulations and modeling
The simulation task in the project will be carried out by KIT
and is dedicated to investigating the electromagnetic behavior
of the MgB
2
cable conductor. In particular, the power
dissipation caused by transients of the transmitted power and
by AC ripples will be studied. The latter are a common
consequence of the AC/DC rectification process. In order to
perform these investigations, a numerical model solving the
time-dependent Maxwell’s equations using the finite-element
method will be employed [16]. The model is able to reproduce
the precise geometry of the MgB
2
wire and incorporates
highly non-linear characteristics of the materials composing
the cable, such as non-linear magnetic permeability for nickel
and monel and a power-law electrical resistivity for the
superconductor. These non-linearities, as well as the geometry
of the cable, which will be discretized at the level of the
individual filaments, make the numerical simulations
challenging.
IV. DEMO 5 PARTNERS AND THEIR ROLES
DEMO 5 is coordinated by Nexans France and comprises
ten partners from five countries, bringing together industry
(Columbus, Nexans), research organizations (CERN, IASS,
KIT, RSE), universities (ESPCI ParisTech, TU Dresden, TU
Madrid) and transmission operators (RTE).
Among the industrial partners, Columbus is responsible for
the MgB
2
wire fabrication, whereas Nexans will be designing,
assembling and testing the cable system, as described in the
previous sections.
CERN will optimize the wires and the cable conductor for
the application to power transmission in close cooperation
with Columbus, for the wire, and RTE, for the cable, and will
characterize the electrical performance of the wire and of the
cable operated at currents of up to 20 kA.
IASS Potsdam is involved in the scientific coordination of
the R&D work package, together with Nexans France who is
in charge of the technical coordination. In the second half of
the project, IASS will also have a leading role in the
dissemination activities for the demonstration results.
KIT will analyze the electromagnetic behavior of the MgB
2
cable conductor, especially with respect to transient events, as
presented in Section III.
RSE will contribute to the design and development of the
10 kA hybrid current leads and will be supporting other
partners in the design, manufacturing and testing of
termination prototypes with the aim of ensuring reduced losses
in the system and lowering the impact on the cooling system.
ESPCI ParisTech will test the HV insulation of the cable
under nominal conditions, i.e., under high electric stress and
hydrostatic pressure in a cryogenic environment.
TU Dresden is responsible for the conceptual design of the
cooling system and for the proper design of the cryogenic
envelope for the superconducting cable, including insulation
and hydraulic design.
TU Madrid will study various aspects pertaining to
availability, with a particular focus on the cooling system:
fault analysis, detection mechanisms, and risk evaluation.
The French transmission operator RTE will investigate the
grid integration of the MgB
2
demonstrator and will also carry
out a socio-economic assessment of the proposed power
transmission system.
V. CONCLUSION
Within the European project BEST PATHS, DEMO 5 aims
to confirm the potential for HVDC bulk power transmission
using cables made out of MgB
2
superconducting wires. Started
less than a year ago, the project will be dedicated to R&D
activities in the first two years, followed in the final two years
by demonstration results of a full-scale cable system able to
transfer up to 3.2 GW.
In recent months, MgB
2
wire designs have been proposed
and produced for the initial cable tests. The first validation
experiments to investigate their mechanical suitability for
cabling operations have already been successfully carried out
on industrial cabling machines. The upcoming activities
include validating the cable conductor design by simulations
of fault and transient conditions, optimizing the current lead
concept, defining the He gas injection tube, as well as
commissioning the cryogenic test bench for the HV insulation.
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