Development of High-Tc Superconducting Power Cable
by Shin’ichi Mukoyama *, Noboru Ishii *, Hiroyuki Iizuka *, Masashi Yagi *,Hironobu Hirano *, Satoru Maruyama *, Yukihiro Yagi *, Masanao Mimura *2,
Osamu Sato *3 and Ayafumi Kikuchi *4

Furukawa Electric is participating in the Super-ACE Project for R&D on fundamental technologies for superconducting AC power equipment that was initiatedby METI (The Ministry of Economy, Trade and Industry) in FY2000, and is also
carrying out R&D for the High-Tc superconducting electric power cable (HTS cable), which canminiaturize cable size while supplying large electrical power. The introduction of the HTS cable
into an electrical network can be expected to produce significant effects because it can meetneeds for control of the global emission of CO2, alleviation of environmental impact, improvement
in stability levels and reduction in costs in the network.
Practical use of superconducting cable requires the development of technologies for reducing
losses in large capacity cables and cooling long cables. In developing the low loss cable,
Furukawa has achieved a technological level of 1 W/m at 3 kA by pitch adjustment and by using
twisted filament wires. In terms of cooling long cables, Furukawa has manufactured a 30-m HTS
cable model, thereby improving thermal contraction properties, heat loss properties and liquid
nitrogen flow. With the success of 30-m cable test, it is planned that field testing of a 500-m, 77kV single-core HTS cable will be carried out in 2004 at the Yokosuka Laboratory of CRIEPI(Central Research Institute of Electric Power Industry). This field testing will include a construction test, a cooling test, a long term voltage and current load test and so on.

ABSTRACT1.

INTRODUCTION

Electric power networks connecting power plants to urban
areas are composed of overhead lines and undergroundcables, with the latter used mainly in the urban areas. As
the underground cables use copper conductors, current
density is about 1 A/mm2. In contrast, the HTS cable canconduct 50 to 100 A/mm2 since the HTS wire becomes asuperconductor at the temperature of liquid nitrogen (77K). An underground cable using an HTS wire conductor istherefore considerably more compact. Existing high-voltage cables need a tunnel 3 m in diameter, but an HTS
superconducting cable can be installed in a cable ductthat is only 150 mm in diameter and can be constructed ata lower cost and in a shorter period. Operating cost is also
expected to be reduced because the loss by electric resistance can be minimized, and lower power loss can ultimately reduce CO2 emissions. The leakage magnetic fieldfrom the cable is zero and cable reactance is decreased
by a superconducting shield. The resulting increase in thelimits on transmission power means that intermediate sub-stations in the power network can be omitted, and construction costs is reduced accordingly 1). Thus the HTS
power cable is an effective technology to expand thecapacity of electrical power networks.
It is said that, from the standpoint of environmental
preservation of the earth, energy sources have to shift toelectrical power in order to maintain stabilized growth ofenergy supply. For this reason R&D on superconducting
cables is being actively conducted in advanced nations. In
Japan, the Super-ACE Project of METI, a developmental
project commissioned from NEDO (New Energy andIndustrial Technology Development Organization) to
Super-GM (Engineering Research Association for

Outer corrugated pipe
Inner corrugated pipe
Paper insulation
HTS-shield layer
HTS-conductor layerCore former*

Energy Transmission Research Dept., Ecology and Energy
Lab. R & D Div.
*2 Metal Research Center, R & D Div.
*3 Power Cable Manufacturing Dept., Power Cables Div.
*4 Super-GM

Super-insulation

PVC

Paper insulation

Figure 1

HTS power cable with single core andcryogenic dielectric.

Furukawa Review, No. 23

2003

82

Development of High-Tc Superconducting Power Cable

Superconductive Generation Equipment and Materials),started in 2000. This project involves R&D on the basic
technologies of HTS cables, HTS fault-current-limiters and
HTS magnets for reactors and transformers. Furukawa
Electric is proceeding with the development of the superconducting cable, and as part of this project is schedulingfield testing of a 500-m HTS cable at the Yokosuka
Laboratory of CRIEPI in 2003.

z
ri

r i+1

l p0
l pi
l pi+1

2.

HTS CABLE

The HTS cable is a power transmission cable that takes
advantage of superconductivity by which electrical resistance is zero. A large amount of electric power can thus
be transmitted by a compact HTS cable that can beinstalled in a duct of 150 mm diameter. Figure 1 shows
the structure of an HTS cable with a single core and cryogenic dielectric. The core of the HTS cable, consisting ofan HTS-conductor layer, an electric insulation layer andan HTS-shield layer around a flexible core former,
enclosed in a cryogenic pipe comprising double stainlesssteel pipes that provide vacuum insulation using superinsulation (SI).
The HTS conductor layer, in which the current flows,
consists of a large amount of Bi2223 silver alloy sheathtape wound spirally around the core former. The insulation
layer is a laminated structure of semi-synthetic paper. In
operation, the insulation layer is infused with the pressurized liquid nitrogen to maintain high insulation performance. The HTS shield layer, in which the shielding current flows, consists of a large amount of Bi2223 silver
alloy sheath tape wound spirally, as in the conductor
layer. By connecting the ends of three shield layers constituting three-phase AC supply, a shielding current of equalmagnitude but opposite phase is induced by the conductor
current. As a result, magnetic field leakage to the outside
is shielded and becomes zero. The current on the cryogenic pipe sheath is suppressed negligibly small, and thesheath loss is decreased effectively.

3.

REDUCING AC LOSSES IN THE HTSCABLEThe advantage of the HTS cable is the possibility of large
power transmission despite of its compact size because of
low losses. The losses of the HTS cable comprise AC
losses (hysteresis loss, coupling loss, etc.) and heat invasion loss in the cryogenic pipe. Unless these losses are
effectively reduced, the advantage of using a superconducting cable is lost. Therefore, techniques for reducing
heat invasion and AC losses are very important in applying the HTS cable in a practical power network. Furukawa
Electric is developing these aspects of the Super-ACE
Project.
In an HTS cable with identical spiral pitch in the outer andinner layers, current is concentrated in the outer layer
because its inductance is smaller than that of the inner

Figure 2

Closed circuit enclosed with an i layer tapeand an i+1 layer.

layer. This state is known as a non-uniform distribution
current state. AC current flows in the conductor with nonuniform distribution, the magnetization remaining in thetape is large and the hysteresis loss also becomes large.
The authors found this phenomenon in the current test of
the 50-m long HTS conductor 2).
We assumed that adjusting the spiral pitch would equalize the inductance. A method of adjusting spiral pitch wasdeveloped to equalize current distribution in the conductor
and reduce AC losses 3). The principle of this method is
described below 4), 5).
Figure 2 shows the closed circuit made with each tapein the i layer and the i+1 layer of the multilayer conductor
with adjusted spiral pitch. Flux (φi) in the superconducting
closed circuit cannot change with time because of zero
resistance as shown in Equation (1). This phenomenon ischaracteristic of superconductivity.
dφ i = 0
dt(1)

The permanent current mode of the superconducting
magnet makes use of the fact that the magnetic field of
the magnet does not change with time. Magnetic flux (φi)
enclosed with the closed circuit of the i layer and i+1 layer
divides into circumferential flux(φiθ) and axial flux(φiz). The
circumferential flux(φiθ) is shown by Equation (2), where
the radius and the current of the i layer are ri and Ii, theradius and the current of i+1 layer are ri+1 and Ii+1 and thelowest common multiple of the pitch of the i layer and i+1
layer is lp0.
i

φ iθ = µ0lp0 ln ri+1Ik
2π
ri k=1

Σ(2)

The axial flux(φiz) is shown by Equation (3).

(φ iθ =µ0lp0 ai+1 – ailp(i+1) lpi+µ0lp0()

iΣ πr2k ak lIpkk
k=1

( )
)Σ( )
n

ai+1 πr2
ai πr2
ak ak(i+1)–ilp(i+1)
lpi
lpk
k=i+1

Furukawa Review, No. 23

2003

(3)

83

Development of High-Tc Superconducting Power Cable

i

dφ
1
r
=0=
ln i+1dt
ri k=12π

Σ dIdtk

+

+

(
(

ai+1
a
– i
lp(i+1) lpi

( )

i

)Σ (

ai+1
a
πr2i+1 – i πr2ilp(i+1)
lpi

dIk
dt

)
( )
Σ
) ( )

πr2k ak

k=1

( )
lpk

dIk
dt

i

ak

k=i+1

lpk

(4)

N-th simultaneous equations can be made using theabove equations for all layers, such that each pitch lpi at
which the current of each layer is equal can be obtainedby solving the simultaneous equations.
At present, the superconducting wire that is the mostadvanced for the superconducting cable is a Bi2223 Ag
alloy sheath tape, the only tape that has high critical current density (Je) and a length in the 1-km class. Bi filaments are generally formed into tape by rolling and pressing in order to obtain a high critical current density. Bi2223
filaments of less than 10-µm thickness are molded in an
Ag alloy matrix with a tape shape of 3.5 mm in width 0.2
mm in thickness.
When AC current flows in the HTS wire, AC losses are
generated within the wire because of the irreversibility ofmagnetic fluxes against the external magnetic field. AC
losses reduce the efficiency of superconducting devices,and their reduction is therefore important for the HTS wire.
An effective method for reducing AC losses is the use oftwisted superconducting filaments (Figure 3), but thistwisting causes damage to the HTS filaments and degradation of its Ic. Furukawa Electric has established a technology for manufacturing twisted filamentary wire that has
almost the same Ic as the non-twisted wire 6). Moreover,
the AC losses of the manufactured twisted filamentary
wire could be reduced to 1/5 compared with the non-twisted HTS wire.
An HTS conductor was produced using the filament
twisting technology and pitch adjustment technology, and
its AC losses were measured 7). Table 1 shows the specifications of the HTS conductor.
AC currents up to 3 kA were applied to this HTS conductor, and the AC losses were measured. Figure 4
shows the results. The AC losses of the conductor were

Bi-2223 filament
AgMg sheathsmaller than the values calculated for a uniform current
distribution (UCD) model 3) or a monoblock model. AC
losses of the pitch adjusted conductor without twisted filaments, and the monoblock model well accords with those
for a conductor without adjusted pitch or twisted filaments.
From the test result, we were able to achieve AC losses of
1 W/m at 3 kA.
In studies on reducing AC losses, developments in technologies for accurate measurement of AC losses become
as important as reducing the AC losses themselves. AC
losses are represented as I×V×cosθ, where I is current, V
is voltage and θ is phase shift between current and voltage in the HTS conductor measured electrically by a lockin-amplifier. In this electrical method, however, the methodof forming terminals for measuring the voltage and errors
in measuring electrical values have not been well established. We are concerned about the accuracy with which
AC losses are measured by the electrical method, andhave therefore developed a calorimetric method for measuring AC losses 7), 8). AC losses can be measured directlyfrom the calories into which all AC losses in the HTS conductor are converted in a calorimetric device. Figure 5
shows the calorimetric device that has been developed.
AC losses can be measured accurately by reducing heatflows, such as heat conduction and heat invasion from theoutside.

Table 1

Specification
Bi2223-Ag alloy tape

HTS wire
Dimensions

0.21 mm × 3.3 mm

Filament pitch

15 mm

Ic (77.3 K, 0 T)

45 A

Core former

Outer diameter 35.0 mm

Conductor layer

4-layer structure

Pitch

1st layer

180 mm

2nd layer

480 mm

3rd layer -510 mm
4th layer
Number of tapes
Length of sample-160 mm

118
2m

10

Monoblock model
UCD model
1

0.1

0.01

0.001
100

1000

10000

Current (A)Ag sheath

Figure 3

Specifications of the HTS conductor.

Item

AC loss (W/m)

When Equations (2) and (3) are substituted into
Equation (1), we obtain Equation (4) which gives the current distribution in the i layer and i+1 layer.

Structure of twisted filament tape.

Figure 4

Results of measurement of AC losses in
HTS conductor.

Furukawa Review, No. 23

2003

84

Development of High-Tc Superconducting Power Cable

Transfer Tube
Cooling bath

Current leadHeat
exchanger

Sub-cooling bath

Calorimeter

Calorimetric container

Circulationpump

Braided wire
3m HTS conductor

Cooling Station

Figure 5

4.

Calorimetric device for measuring AC losses in HTS conductor.

TEST OF 30-M MODEL CABLEAn HTS cable model with a length of 30 m was produced
and tested to investigate the influence of the mechanicalhistory in the manufacturing, an installation and coolingprocesses 9). Table 2 shows the specifications of the 30-m
model cable. The cable core was made using a corrugated aluminum pipe, conductor layer and electrical insulation layer. The conductor layer consisted of two HTS wires
and 18 Cu wires, wound spirally around the corrugatedpipe. Photo 1 shows a cutaway view of the model cable,
and Photo 2 shows the 30-m cable installed in an oxbow
shape.
4.1 Test of Thermal Contraction
In most metals, it is known that the thermal contraction
from room temperature to 77 K is about 0.3 %. The outerpipe of the cryogenic pipe does not contract differently
from the inner pipe or the cable core. Moreover, the HTS
cable is not allowed to contract in practical use because
the cable is fixed when installed. In the 30-m cable test,
the heat stress and the heat contraction were measured instraight and oxbow configuration respectively. As a result,in the straight configuration the heat contraction length
corresponded to 0.3 % of the cable length (9 cm) and theheat stress was 8.8 kN. In the oxbow configuration, theTable 2heat contraction lengths as evaluated from the moving
lengths of both ends were 3.4 cm, and 4.5 cm respectively. The contraction length in the oxbow was approximately
1 cm shorter than that in the straight configuration,
because the cable core and inside cryogenic pipe were
pulled to the inside of the curvature in part of the oxbow.
Consequently, the heat stress in the oxbow cable configuration was decreased to 2.9 kN.
4.2 Test of Critical Current (Ic)
Figure 6 shows the critical currents (Ic) of a sample cutoff
after cable manufacture and for the 30-m cable in thestraight and oxbow configuration and under thermal stressor free of stress. Each Ic is normalized to the Ic of the wire
before cable manufacture. Ic degradation due to cablemanufacture was held within 5 %, and no Ic degradation
was caused by the cable or thermal contraction. The test
results thus indicated that the cable would withstand manufacture, installation and thermal contraction.

Specifications of 30-m HTS model cable.

Item
Core former
Outer diameter

Specification
Corrugated aluminum pipe
30 mm

Photo 1

Sample of 30m-model cable.

Conductor layer
Number of wires
Outer diameter
Insulation layer
Thickness

HTS wires

2

Cu dummy wires

18

35 mm
LN2 impregnated multilayer paper
8 mm

Cryogenic pipe
Inner diameter

82 mm

Outer diameter

123 mmPVC sheathOuter diameter

133 mmPhoto 230-m HTS cable installed in oxbow shape inFurukawa Electric’s laboratory for tests.

Furukawa Review, No. 23

2003

85

Development of High-Tc Superconducting Power Cable

200

100

180

Pressure drop (Pa/m)

120

Ic /Ico (%)

80
60
40
20

Figure 6

e
ti
ns
St
ta
ra
l
i
la
tio
w ght
ith c
n
8. onf
8
i
g
-k u
Af
r
N a
te
st tio
ro
re n
xb
ss
ow
in
w
ith O
st
a
2. xb
lla
9- ow
tio
kN c
n
si onfde ig
pr uraes tiosu nreghCritical currents of a cut-off sample after cable manufacture and installation.

140
120
100
80
60
40
20
0

st
ra
i
Af
te
r

30

-m

ca
b

le

H

TS

w

sa
m
pl

ire

0

160

0

10

20

30

40

50

60

LN2 flow (liter/min)Figure 7

Pressure drop of single-phase LN2 flowing into 30-m
long HTS cable.

4.3 Test of Heat Invasion
Heat invasion to the cable was measured by the evaporation of liquid nitrogen from the cable. The cryogenic pipe
had super-insulation (SI) in the multiple layers between
the double corrugated pipe made of stainless steel, andhad high thermal insulation performance by maintaining avacuum by SI. Heat invasion to the superconducting cable
was held to 1 W/m, half of that of previous HTS cables, by
optimizing this insulated structure. Moreover, even when
the side pressure was joined by the heat contraction in theoxbow of the cable, heat invasion was restricted to the low
value of 2 W/m.

It has been decided that Furukawa Electric will takecharge of manufacturing the cable and ancillary equipment, and of installing the cable. CRIEPI, Chubu ElectricPower, Tokyo Electric Power and Kansai Electric Powerwill be in charge of the testing and evaluation of this project. The cable line emulates the configuration of a cablethat is installed in an actual cable duct with a 10-m verticalrise, 10-degree slopes and 5-m radius curves. The 500-m
cable will have the functions of electrical insulation, andthe cable installation and short circuit characteristics
required in an actual transmission cable.

4.4 Test of Pressure Drop in the Cable
In designing a cooling system and determining the installation interval for the cooling system for a practical HTS
cable, it is important to investigate the pressure drop of
the liquid nitrogen that flows to the cable. Subcooled liquid
nitrogen was circulated at 50 l/min in the 30-m cable, andthe pressure drop was measured. Figure 7 shows resultsof measuring the pressure drop per meter.

6.

5.

FIELD TEST OF 500-M LONG HTS
CABLEField testing of a 500-m long HTS cable for the SuperACE Project will be carried out at the Yokosuka Laboratory of CRIEPI from March to December of 2004. Various
techniques for cooling the cable to extremely low temperature will be examined with the aim of achieving practicaluse of a 5-km class HTS cable. The test items scheduledare:• Fundamental test: to investigate properties of initialcooling and endurance of the conductor against cableinstallation, and to measure AC losses.
• Operation test: to investigate the stability of long-term
voltage and current loading, and the influence of thermo cycles.
• Load change test: to investigating the influence ofload change and overcurrent.
• Extreme conditions test: to investigate the limits ofcurrent, voltage and overload in case of interruption ofliquid nitrogen circulation.

CONCLUSION

It is necessary to understand the cooling characteristics oflong-length superconducting cables and the circulation ofliquid nitrogen to realize the HTS cable. Therefore, fieldtesting of the 500-m HTS cable longest in the world isplanned in the Super-ACE Project 10). The 30-m HTS cablemodel was tested to obtain design data and improve theproduction technologies for the manufacture and installation of the 500-m cable, and this was accomplished successfully. Based on this success with the 30-m cable tests
we have started preparing substantive tests and manufacture of the 500-m long cable.

REFERENCES
1) Achievements Report for Commissioned Tasks for FY2001,
issued by NEDO (in Japanese)
2) S. Mukoyama, K. Miyoshi, H. Tsubouti, N. Ichiyanagi, M.
Mimura, N. Uno, Y. Tanaka, H. Ishii, S. Honjo, Y. Sato. Y. Iwata
and T. Hara: “50m- LONG HTS CONDUCTOR FOR POWER
CABLE”, IEEE Trans. Appl. Superconductivity IX, Vol. 7, No. 2.
p1069 (1997)3) Mukoyama et al.: On AC losses in HTS cables, Brochure forSuperconductive Power Equipment Joint Research Group, IEEJ,
ASC-00-12/LD-00-12, (2000) (in Japanese)
4) Hamashima et al.: Analysis of Electric Current Distributions inSuperconductive Conductors, Cryogenics, Vol.35, No. 4, p176
(2000) (in Japanese)5) Achievements Report for Commissioned Tasks for FY2000 issued
by NEDO (in Japanese)
6) M. Mimura, A. Takagi, K. Kosugi, M. Sugimoto, A. Kimura, T.

Furukawa Review, No. 23

2003

86

Development of High-Tc Superconducting Power Cable

Tanigawa, S. Honjo, T. Mimura and Y. Takahashi: “Developmentof Twisted Multifilamentary Tape Using AgMg Alloy”, Advancesin Superconductivity XII, Springer-Verlag, p661 (1999)
7) M. Yagi, S. Tanaka, S. Mukoyama, M. Mimura, H. Kimura, S.
Torii, S. Akita and A. Kikuchi: “Measurement of AC Losses ofSuperconducting Cable by Calorimetric Method andDevelopment of HTS Conductor with Low AC Losses”, ASC2002,
Houston, USA, 2002, 1LG05
8) Yagi et al.: Development of a Calorimetric Apparatus for AC Loss
Measurement in Superconductive Cables, Conference
Proceedings for Electric Power and Energy Sector (B), IEEJ,
2002, p358 (in Japanese)
9) S. Mukoyama, H. Hirano, M. Yagi, H. Kimura and A. Kikuchi:“Test results of 30m High-Tc Superconducting power cable”,
ASC2002, Houston, USA, 2002, 1LH03
10) K. Ueda, O. Tsukamoto, S. Nagaya, H. Kimura and S. Akita: “R &D of 500m Super-conductive Cable in Japan”, ASC2002, Houston,
USA, 2002, 1LL01

Furukawa Review, No. 23

200387