Physica C 310 Ž1998. 345–350
Transient stability of AC multi-strand superconducting cables
A. Ishiyama
a
a,)
, M. Sasaki a , T. Susa a , S.B. Kim a , M. Tsuda a , H. Yumura b,
K. Ohmatsu b, K. Sato b
Department of Electrical, Electronics and Computer Engineering, Waseda UniÕersity, 3-4-1 Ohkubo, Shinjuku-ku, Tokyo, 169-8555,
Japan
b
Sumitomo Electric Industries, Japan
Abstract
AC application, it is necessary to estimate the stability of multi-strand superconducting cable. Therefore, we have been
studying the transient stability of non-insulated multi-strand cable when one strand in a cable turns into the normal state
locally. In the quench process, local temperature rise produced by current redistribution among strands is not desirable for
stability. In a previous work, we discussed the effect of Cu matrix allocated to each strand on the transient stability and
showed that the Cu matrix allocation can improve the stability of non-insulated multi-strand cable through mainly numerical
simulations. In this paper, we carried out experiments on three kinds of non-insulated three-strand cables; one consists of
NbTirCuNi strands and the others consist of NbTirCurCuNi strands having different cross-sectional arrangement. These
sample strands have almost the same diameter, the same matrix to superconductor ratio and the same B–J characteristics to
evaluate the effect of Cu allocation quantitatively. We choose to define the transient stability in terms of the minimum
quench energy ŽMQE. at each DC transport current. We also investigated the transient stability of sample cables when
quench is initiated in two or three Žall. strands simultaneously. q 1998 Elsevier Science B.V. All rights reserved.
Keywords: Stability; Superconducting cable; Quench; AC use
1. Introduction
Quench of superconducting multi-strand cable for
AC use is thought to be triggered by sudden local
releases of energy within the cables. The main source
of this energy is believed to be mechanical and
occurs locally at one or two strands in the cable. We
have discussed the electromagnetic and thermal behaviors in a quench process when one strand in a
cable turns into the normal state and have evaluated
)
Corresponding author. Tel.: q81-3-5286-3376; Fax: q81-33208-9337; E-mail: atsushi@mn.waseda.ac.jp
the transient stability of non-insulated three-strand
cables, through mainly numerical approach w1–6x.
Quench process in non-insulated cable with only
CuNi matrix depends significantly on the Joule heating produced by current flowing across the strands
during current redistribution after one strand in a
cable turns into the normal state. Although such
excessive Joule heating is not desirable for stability,
it can be improved by Cu matrix allocation which
has the advantage of: Ž1. lower electrical resistance
in the longitudinal direction of the strand, which can
make the length of the current redistribution much
longer, resulting in lower Joule heating density; and
0921-4534r98r$ - see front matter q 1998 Elsevier Science B.V. All rights reserved.
PII: S 0 9 2 1 - 4 5 3 4 Ž 9 8 . 0 0 4 9 0 - 0
346
A. Ishiyama et al.r Physica C 310 (1998) 345–350
Ž2. large thermal conductivity in both longitudinal
and azimuthal directions of the strand which can
bring about more effective heat diffusion and heat
transfer to the coolant. To evaluate these desirable
effects of Cu matrix allocation quantitatively, we
prepared one NbTirCuNi Žno Cu matrix. three-strand
cable and two NbTirCurCuNi three-strand cables
with Cu-sheath and with Cu matrix mainly allocated
to center of the strand. In this paper, the comparison
of the experimental results in the three types of
sample cables are shown. We also investigate the
transient stability in the case when a quench is
initiated in two or three Žall. strands simultaneously
and the external magnetic field dependence on the
stability. We adopt a relationship between the DC
transport current and the minimum quench energy
ŽMQE. of the cable, i.e., the heater input, of short
duration Ž; ms. applied to the selected strand in the
cable, which is just sufficient to quench the whole
cable.
NbTirCuNi Žno Cu matrix. strand; ŽCable B.
NbTirCuNi strand with Cu-sheath; and ŽCable C.
NbTirCuNi strand with Cu matrix mainly allocated
to center of strand and also Cu filaments to the
outside of NbTi filaments region. Sum of cross-sectional area of the Cu filaments is very small. To
evaluate the effect of Cu allocation on the transient
stability quantitatively, the strand of sample cables
have almost the same diameter and the same matrix
to superconductor ratio, and fortunately, the B–J
short sample data of sample strands lie on a single
line. Each sample of 1200 mm in length is wound in
a groove of FRP cylinder with a diameter of 30 mm.
To initiate a quench, Manganin wire is wound to
each sample as a heater and contacts to only the
selected strand of cable as shown in Fig. 1. Voltage
taps are attached around the heater with 300 mm
separation to detect the quench of whole cable. In
each DC transport current, the minimum heater input
to make whole cable quench was measured as a
function of external DC magnetic field, B0 , in the
range of 0 to 2 T.
2. Experiments
We prepared three types of non-insulated threestrand cables. Table 1 summarizes the specifications
of sample cables used in experiments: ŽCable A.
Table 1
Specifications of single strand in sample cables
3. Quench property line
We choose a ‘quench property line’ which shows
a relationship between the DC transport current and
A. Ishiyama et al.r Physica C 310 (1998) 345–350
347
Fig. 1. Cross-sectional geometry of heaters and cables in experiments: Ža. Heater input to one strand; Žb. Heater input to two strands; Žc.
Heater input to whole strands.
the minimum quench energy, as a criterion for the
quantitative evaluation of transient stability in noninsulated multi-strand cables. Fig. 2 shows a schematic drawing of quench property lines; solid and
dashed lines are the lines before and after the improvement of transient stability, respectively. The
left side of the ‘quench property line’ represents the
case when the normal zone in the cable recovers to
the superconducting state before the entire cable
quenches and the right side is the condition for
propagating normal zone. Moving the property line
towards the upper-right side in Fig. 2 means an
improvement of transient stability. The inflection
point, shown in Fig. 2, is also an important parameter for discussing the stability of multi-strand cables.
This point separates regions where quench is initiated by a single strand quenching or by all strands
quenching w6,7x. The detail of these phenomena were
described in the previous paper w6x.
4. Results and discussion
In quantitative evaluation of the transient stability
using the quench property lines, accurate estimation
of heat input is required to compare the results in
samples with different Cu allocation. In practice,
however, it is difficult to make the same contact
condition between strand surface and heater wire.
Therefore, we prepared some samples of each Cables
A, B and C, and confirmed the reappearance of the
quench property. Fig. 3 shows one of the results in
NbTirCuNi strand cable wCable ŽA.x. Agreement
between three cases is good in terms of inflection
points, while the minimum heater inputs at each
transport current, which were estimated from the
data measured by the four-terminal method, have
some difference. From these results, we think that
the net and the effective amount of heater input
which triggers a quench were the almost same, but
the difference caused by the experimental condition
should be considered in stability evaluation.
4.1. Effect of Cu allocation on quench property line
Fig. 2. Schematic drawing of quench property lines indicating
improvement of transient stability. The arrow means the direction
of improvement of stability.
In the previous paper w6x, we reported how Cu
matrix allocated to NbTirCuNi strand cable con-
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A. Ishiyama et al.r Physica C 310 (1998) 345–350
Fig. 3. Reappearance of quench properties in three samples ŽCable
A, B0 s1 T..
tributes to the transient stability in non-insulated
three-strand cables by the numerical simulation based
on the 3-D finite element analyses. From these results, we concluded that the transient stability can be
improved by Cu matrix allocation because: Ž1. the
length of current redistribution around the normal
spot becomes longer in the longitudinal direction;
then, Ž2. the Joule heating density caused by the
current flowing across the strands during current
redistribution decreases, so that the local temperature
rise in the neighboring strands is reduced; and Ž3. Cu
allocation to the outside of superconducting filament
region makes the thermal diffusion in both longitudinal and azimuthal directions more effective against a
disturbance that occurs at the strand surface. We
Fig. 4. Quench property lines of three sample cables Ž B0 s1 T..
measured and compared the quench properties of
three kinds of sample cables to verify the analytical
results mentioned above. In these experiments, Manganin heater wire was set to contact to one strand
within the length of twist pitch of the cable as shown
in Fig. 1Ža. and the duration of the heater input pulse
is 5 ms which is constrained by the geometry of the
contact condition and the capacity of power supply
for heater. Fig. 4 shows typical quench property lines
obtained at 1 y T external field in Cables A, B and
C; note that a logarithmic scale is adopted in vertical
axis. Quench property lines of Cables B and C,
which have Cu matrix, shift to the upper-right side
which is more desirable than that of Cable A without
Cu matrix and, especially, the quench margin of
Cable B with Cu-sheath increases by ten or more
times in comparison to the cables without Cu matrix
Fig. 5. External magnetic field dependence on quench property
lines: Ža. Cable A; Žb. Cable B.
A. Ishiyama et al.r Physica C 310 (1998) 345–350
349
the cable, especially in the neighboring strands, and
higher temperature margin in the neighboring strands
makes the larger difference.
4.2. Quench property line in different initial condition
In real multi-strand cables, the initial quench occurs at one or two strands in a cable. Therefore, we
carried out experiments in which the Manganin heater
was wound to contact to one strand, two strands and
three Žall. strands as shown in Fig. 2Ža., Žb. and Žc.,
respectively. Fig. 6Ža. and Žb. show the comparison
of quench property lines in Cables A and B. It is
clear that the smaller number of initial-quench strands
is more stable. Fig. 7 shows the schematic drawings
for the current redistribution among strands in the
different initial conditions. When an initial quench
occurs in two strands simultaneously, which is the
case of Fig. 7Žb., the current which redistributes to
the neighboring strand is four times larger than that
in the case of Fig. 7Ža.. The Joule heating generated
by the current redistribution also affects the difference at the inflection point. In the case when the all
Žthree strands. cables quench simultaneously ŽFig.
7Žc.., there is no current redistribution and only one
inflection point exists at lower current, which shows
the recovery current of the cable.
Fig. 6. Quench property lines in different initial quench condition:
Ža. Quench property lines in Cable A; Žb. Quench property lines
in Cable B Ž B0 s1 T..
ŽCable A.. The magnitude of this improvement is
much larger than the difference which is caused by
the heater contact condition described in Section 3.
Fig. 5Ža. and Žb. are the quench property lines of
Cables A and B measured as a function of external
magnetic field. As the external magnetic field increases: Ž1. the quench property line shifts to lowerleft side; and Ž2. the difference of the minimum
heater inputs at both sides separated by the inflection
point decreases. This means that more heater input is
required to quench the whole cable in lower external
magnetic field at the same transport current due to
the higher temperature margin. The difference at the
inflection point depends on the thermal property of
Fig. 7. Situation of each initial quench condition: Ža. One strand
initially quenched; Žb. Two strands initially quenched; Žc. Three
Žwhole. strands initially quenched.
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A. Ishiyama et al.r Physica C 310 (1998) 345–350
5. Conclusion
The effect of Cu allocation on the transient stability of non-insulated three-strand cables has been
evaluated quantitatively by experiments. By allocation of Cu to the surface ŽCu-sheath. or to the
outside of superconducting filament region,
1. the quench property line, which was chosen as a
stability criterion, shifts to the upper-right side
which is desirable, and
2. the minimum quench energy ŽMQE. increases by
an order of magnitude in comparison with a cable
without a Cu matrix.
We also investigated and clarified the influence of
the number of initial quench strands on the quench
property of the cables.
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