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Electric Measurement of the Critical Current,
AC Loss, and Current Distribution of a
Prototype HTS Cable
Jiahui Zhu, Zhenyu Zhang, Huiming Zhang, Min Zhang, Ming Qiu, and Weijia Yuan
Abstract—HTS cable is an emerging technology for electricity
power transmission. It is very important to set up an accurate and
easy-to-build measurement system to characterize HTS cables. In
this paper, a novel measurement system has been developed to
measure the ac losses for a prototype HTS cable using cancelation
coil technique. This system is based on the National Instrument
Data Acquisition card. It can also be used to measure the critical
current and current distribution in the prototype cable. The prototype cable has been innovatively designed using an improved particle swarm optimization algorithm which is more computationally
efficient than conventional methods. The measurement system will
be improved further by introducing an elaborate pre-amplifier
prior to data collection process and the result will be compared
with theoretical calculations for validation.
Index Terms—AC loss, critical current, current distribution,
National Instruments Data Acquisition (NI DAQ), prototype HTS
UPERCONDUCTING cables are an emerging and attractive technology for electrical power transmission and distribution due to significant developments of high-temperature
superconductors in the past decade. What makes superconducting cables highly desirable for electricity transportation
are their high current density and low energy loss. This leads
to four major benefits over conventional counterparts [1], [2]:
i) they are significantly safer and cheaper to operate as the same
amount of power can be transported at a much lower voltage,
ii) their power transmission capacity in the same volume is
up to ten times larger (including the cooling system) than
conventional underground cables and overhead cables, thus
the right-of-way requirement is substantially reduced, iii) their
resistive losses are only one third of conventional cables even
after including the cooling power, iv) magnetic field leakage,
with its associated environmental impact, can be eliminated by
Manuscript received July 16, 2013; accepted September 27, 2013. Date
of publication October 2, 2013; date of current version October 14, 2013.
This work was supported in part by NSFC Grant 51207146, RAEng Research
Exchange scheme of UK and EPSRC EP/K01496X/1.
J. Zhu is with the China Electric Power Research Institute, Beijing 100192,
China. He is also with the Department of Electronic and Electrical Engineering,
the University of Bath, Bath BA2 7AY, U.K.
M. Qiu is with the China Electric Power Research Institute, Beijing 100192,
Z. Zhang, H. Zhang, and W. Yuan are with the Department of Electronic and
Electrical Engineering, the University of Bath, Bath BA2 7AY, U.K. (e-mail:
M. Zhang is with the Department of Engineering, the University of
Cambridge, Cambridge CB2 1PZ, U.K.
Color versions of one or more of the figures in this paper are available online
at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/TASC.2013.2284295
inclusion of a superconducting shielding layer. As a result of
these characteristics, superconductors offer an attractive alternative to conventional energy transmission in supporting energy
transportation from distant renewable generation as well as in
relieving electricity transportation congestion in metropolitan
A novel characterization system has been developed to characterize a prototype HTS cable in the University of Bath, U.K.
Using Labview and NI data acquisition modules, this system
has an integrated design. It can characterize the current currents,
ac losses, and current distributions of HTS samples using one
control interface. In particular, the ac losses measurement is
using an electric method—cancellation coil tecnique, which is
much faster than a calorimetric method. It can also be used
to characterize different samples such as tapes and coils. A
132 kV/1.2 kA cable structure has been optimally designed to
minimize the current difference in each conductor layer using
an improved particle swarm optimization (PSO) algorithm. A
short prototype has been constructed based on this design. Its
critical current, ac loss, and current distribution have been characterized using the developed measurement system to validate
the cable’s design work.
A. Critical Current Measurement System
In this paper, the critical current values of HTS samples are
measured using 1 uV/cm criterion at self-field condition, 77 K
operating temperature, with no bending. A control program
based on Labview and NI data acquisition module was developed. A dc power supply is controlled by a Labview interface
panel. The ramp rate of the applied current can be set at various
values (1 A/s, 2 A/s, and 4 A/s). The protection of HTS samples
is essential. A thermal resistor (PT100) was used to monitor the
operation temperature. The upper limit voltage across the HTS
sample, beyond which the ramping current will be stopped, was
set at 10 times of the critical voltage.
The general critical current measurement system panel is
shown in Fig. 1 and the flow chart in Fig. 2 presents the working
principle of the measurement system.
The measured voltage contains an inductive component of
the HTS sample (a coil or a cable), as in (1). This voltage
Ls (di/dt) is induced when the current is increasing. It should
be removed from the measured voltage Vm for accurate critical
current values.
1051-8223 © 2013 IEEE
V m = Ls
+ Vs .
Fig. 4.
Fig. 1. Control panel of the critical current measurement system.
Control panel of the ac loss measurement system.
the hysteresis loss and an inductive voltage due to the large
self-inductance and mutual inductance between the conductor
Vs = RI + jωLI.
Only the resistive component in (2) represents the ac loss
voltage [5]. The mutual inductance of the compensation coil
M can be adjusted by shifting the secondary coil, while the
inductive component Vs is minimized by subtracting the voltage
of the secondary coil voltage Vc = jωMI from Vs . When M is
equal to L, the resulting voltage Vr is purely resistive, as shown
in (3):
Vr = Vs − Vc = RI + jω(L − M )I = RI.
The resulting voltage Vr is in phase with the current I flowing
through the HTS cable, and the multiplication of in-phase
resulting voltage and transport current gives the ac loss, as
shown in (4):
Fig. 2. Flow chart of general critical current measurement procedure for HTS
Vr · I
Where: l is the length of the HTS conducting layer of the
cable and f is the frequency of transport current.
An alternative method is using a lock-in amplifier to measure
the phase difference θ between Vs and I, then the in-phase
voltage VR can be obtained from VR = Vs · cos θ. And the ac
loss can be calculated using (5):
VR · I
C. Current Distribution Measurement of the Prototype Cable
Fig. 3. Circuit diagram of ac loss measurement.
B. AC Loss Measurement of HTS Cable
The ac losses of HTS cables are essential information for the
design of cryogenic systems. Fig. 3 shows the overall ac loss
testing circuit diagram. The ac loss measurement system panel
is shown in Fig. 4. Since the cable produces a large inductive
voltage, a cancellation coil has been developed to cancel the
inductive voltage so as to leave only resistive voltage to measure
[3], [4].
The measured voltage Vs (shown in Fig. 3) of the HTS
cable includes two components: a resistive voltage RI due to
The current sharing issue is an important design aspect for
an ac HTS cable for power transmission. Due to the selfand mutual inductance, there might be different transport currents flowing in different layers. To minimize the ac loss, the
transport current needs to be equally shared between different
layers. An improved particle swarm optimization method has
been used for optimization. The optimization procedure can be
found in [6].
A circuit model can represent a multilayer structure of HTS
cable, as shown in Fig. 5 [7]. Due to the relatively large
inductance, the resistance of each layer can be ignored in
current sharing calculation. The inductance values of each layer
are different due to various radii and pitch angles. The current
Fig. 5.
Circuit model of a multilayer HTS cable.
flowing through each layer is affected by the self- and mutual
inductances and therefore can be changed by adjusting the pitch
angle and radius. The design variables are presented in (6). The
objective function for minimization is presented by (7), which
represents the current difference in different conductor layers.
Fig. 6. 0.2 m, 132 kV/1.2 kA prototype HTS cable.
Fig. 7. Setup of the HTS cable measurement system.
X = [α1 , α2 , . . . , αn , r1 , r2 , . . . , rn ]
n−1 n
G(X) = min F (X) = min ⎝
|Ii (X)−Ij (X)|⎠ . (7)
i=1 j=i+1
Where: X is a vector containing the pitch angles α and radii
r of each HTS conductor layer, n is the number of total layers,
i, j = 1, 2, . . . n, G(X) is the objective function.
A. Prototype HTS Cable and the Test System
The structure parameters of a 132 kV/1.2 kA prototype HTS
cable after current sharing optimization are shown in Table I.
Two layers of BSCCO tapes are helically wound on a copper
former to form a prototype HTS cable, as shown in Fig. 6.
An insulation layer was wrapped around the outer conducting
layer and a BSCCO HTS shielding layer was wound on top of
the insulation layer. A bundle of copper strings is used as the
current lead of this HTS cable.
Fig. 7 presents the picture of the experimental system. The
system includes a dc/ac power supplier, a power amplifier, a NI
data acquisition, a Labview interface panel and LN2 tank. The
shunt and the compensation coil are used for the testing of the
critical current and ac loss of HTS cable.
B. Measurement Results and Analysis
First, the measurement system has been used to measure the
critical currents of HTS thin tapes, BSCCO coils, and HTS
Fig. 8. Summary of the critical current test results of YBCO tape, BSCCO
tape, and BSCCO coil.
cables. The critical currents of the HTS tapes and coils are
measured to validate the function of the measurement system.
The results are plotted and summarized in Fig. 8. The “n” values
were calculated based on the equation: E/Ec = (I/Ic )n , where
Ec is 1 μV/cm and Ic is the relevant critical current value. The
critical current of the BSCCO coil is 83 A which is 68% of
that of the BSCCO tape due to the interaction between Ic and
self-magnetic field.
The critical current measurement of the prototype cable is
presented in Fig. 9. We can find that from I = 1500 A there is
a clear sharp rise in the cable voltage. Therefore the critical
current can be determined as 1500 A. There is about 20%
Fig. 11.
Current distribution testing results of HTS cable at 60 Hz.
Fig. 9. Critical current testing result of prototype HTS cable.
Fig. 10. AC loss testing results of the prototype HTS cable at various
safety margin if the cable operates at 1200 A as shown in our
According to the design (Table I), 17 BSCCO tapes are required to construct this two-layer cable. So the critical current in
theory is 17 × 122A = 2074 A. And the actual critical current
1500 A is 72.3% of the theoretical current. This degradation is
due to the interaction between Ic and self-magnetic field when
the BSCCO tapes are wound into a cable.
Second, the results of ac loss under various testing frequencies based on the lock-in amplifier method are summarized in
Fig. 10 using a power supplier with 0–200 Hz, 0–3000 A. It can
be seen that the ac loss per cycle is approximately proportional
to the I 3 [8] and is independent of the frequencies.
Finally, the ac transport currents flowing through each superconducting layer in a HTS cable are measured by applying
Rogowski coils wrapped around the current lead of each HTS
layer. The data are recorded by a high accuracy NI acquisition
system which is similar with the critical current measurement
system. An ac current flows through this HTS cable, such as
800 A which is less than the rated current 1200 A, for the safety
operation. The measurement result is presented in Fig. 11. The
testing current of each layer is not exactly evenly distributed
due to the inaccurately controlled pitch angle during the cable
constructing process.
A novel HTS characterization system with a visual interface panel has been developed based on National Instruments
data acquisition cards and Labview program. This system has
an integrated design and can measure critical currents, ac
losses and current distribution of HTS cables. A 0.2-m-long,
132 kV/1.2 kA prototype two-layer HTS cable has been optimally designed and constructed. The cable design has used
an improved PSO algorithm to minimize current difference in
conductor layers. The characteristics of the cable including ac
losses and current sharing have been experimentally studied.
The measurement result validates the design which can share
the same ac transport current in each conductor layer. The
measurement system will be improved further by introducing
an elaborate pre-amplifier prior to data collection process and
the result will be compared with theoretical calculations for
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