120913308

Available online at www.sciencedirect.com

Physics Procedia 36 (2012) 908 – 913

Superconductivity Centennial Conference

Application of superconducting power cables to DC electric

railway systems
Hiroyuki Ohsakia*, Zhen Lva, Masaki Sekinob, Masaru Tomitac
a
Graduate School of Frontier Sciences, The University of Tokyo, Kashiwa 277-8561, Japan
b
Graduate School of Engineering, The University of Tokyo, Kashiwa 277-8561, Japan
c
Railway Technical Research Institute, Kokubunji 185-8540, Japan

Abstract

For novel design and efficient operation of next-generation DC electric railway systems, especially for their
substantial energy saving, we have studied the feasibility of applying superconducting power cables to them. In this
paper it is assumed that a superconducting power cable is applied to connect substations supplying electric power to
trains. An analysis model line was described by an electric circuit, which was analyzed with MATLAB-Simulink.
From the calculated voltages and currents of the circuit, the regenerative brake and the energy losses were estimated.
In addition, assuming the heat loads of superconducting power cables and the cryogenic efficiency, the energy saving
of the total system was evaluated. The results show that the introduction of superconducting power cables could
achieve the improved use of regenerative brake, the loss reduction, the decreased number of substations, the reduced
maintenance, etc.

©2012
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and/or peer-review
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Peter Kes.
Keywords: superconducting cable; DC electric railway; regenerative brake; feeder

1. Introduction

Although a DC electric railway system is widely used in Japan including metropolitan areas, it has
some problems such as relatively low voltage, regeneration cancelation, energy losses, etc. A regenerative
brake is an electric brake, and it transforms kinetic energy to electric energy using propulsion motors in a
regenerative mode. Advantages of the regenerative brake are energy saving, reduced wheel wear,

* Corresponding author.
E-mail address: ohsaki@k.u-tokyo.ac.jp.

1875-3892 © 2012 Published by Elsevier B.V. Selection and/or peer-review under responsibility of the Guest Editors.
doi:10.1016/j.phpro.2012.06.228
 Hiroyuki Ohsaki et al. / Physics Procedia 36 (2012) 908 – 913 909

improved riding comfort, reduced temperature rise in tunnels, etc. However, when a powering train does
not exist near a braking train or the operation conditions are not met, the regeneration is canceled [1,2].
Superconducting power cables have been widely developed in Japan, U.S.A., Korea, China, and
Europe [3]. Grid-connected demonstration and system reliability evaluation tests of superconducting
power cables of several hundred meters have been also carried out. Most of the superconducting cable
projects involved AC cables. Recently, DC superconducting cables have attracted attention and now there
are some projects for DC superconducting cables [4,5].
To solve the above-mentioned problems of DC electric railway systems, we have studied the feasibility
of applying superconducting power cables to DC electric feeder systems, aiming the essential energy
saving of next-generation electric railway systems. In this paper, the improved use of regenerative brake,
the loss reduction, the decreased number of substations, the reduced maintenance, etc. are discussed.

2. Analysis Model and Method

Fig. 1 shows a model line for numerical analysis of power feeding systems using a superconducting
cable. The total length of the line is 26.5 km. There are 24 stations and 5 substations (SS) along the line.
Electric current flows from the substations through the feeder to a train and returns through the rail to the
substations. A superconducting cable is placed parallel to the feeder and connected to all the substations
as shown in Fig. 1. The length of the superconducting cable is 22.2 km.
Fig. 2 shows a basic part of an approximated electric circuit for analyzing the model. It is an example
of the part having two trains between two adjacent substations. A substation is simply modeled as a set of
a no-load voltage V0 and an output resistance rS, and the train as a set of a current source IT and a
resistance rT. A resistance rLR includes both a feeder resistance and a rail resistance. MATLAB-Simulink
was used to analyze the model.
Fig. 3 shows train operation curves used in the analysis. The vertical axis is the distance from the first
station and the horizontal axis is the time. Each curve shows the trajectory of a train on the distance –
time plane. Locations of the five substations and 24 stations are also indicated by the arrows. A periodic
operation of trains with a period of five min. was assumed in the analysis, and therefore the analysis for
five min. is enough for evaluation of the system. Horizontal portions of a curve indicate that the train is
standing in the station. In this analysis it was assumed that the route was a double track but their electric
circuits were independent of each other.

Substations
5.7 8.9 3.5 4.1
SS1 SS2 SS3 SS4 SS5
Superconducting cable

22.2
Feeder

Rail
Train
24 Stations
26.5
[km]

Fig. 1. Analysis model line

910 Hiroyuki Ohsaki et al. / Physics Procedia 36 (2012) 908 – 913

Fig. 2. A basic part of electric circuit for analyzing the analysis model

Fig. 3. Train operation curves on the distance – time plane

Fig. 4 shows the acceleration and the velocity of the train that starts from the first station at 0 s among
several trains shown in Fig. 3. The acceleration and the deceleration of 0.83 m/s2 (3.0 km/h/s) and the
maximum velocity of 25 m/s (90 km/h) were assumed. If the distance between adjacent stations is so
short, the velocity does not reach 25 m/s.

Fig. 4. Acceleration and velocity curves of a train

 Hiroyuki Ohsaki et al. / Physics Procedia 36 (2012) 908 – 913 911

3. Analysis Results

3.1. Voltage distribution and regeneration improvement

Fig. 5 shows voltage distributions along the feeder at 170 s. The voltages at the substations and trains
are indicated with symbols. There are two braking trains between the substations SS1 and SS2, and two
powering trains between SS2 and SS3 and between SS3 and SS4. The trains are operated to make most
use of regenerative brake, and if the regeneration is not enough, then a mechanical brake is used. Without
a superconducting cable the voltages of the braking trains increases much and the voltage of the leftmost
train reaches the upper-limit of the train voltage. It causes the regeneration cancelation. When a
superconducting cable is installed, the change in the voltage is reduced and the regenerative brake is more
effectively used.
Fig. 6 shows the time variation of train power for 300 s of the train shown in Fig. 4. The positive
power indicates the train in the powering phase and the negative power in the braking phase. In the
braking phase, a blue curve indicates the total braking power and a green curve the regenerative brake
(Fig. 6 (a)). The difference between the two curves indicates a mechanical brake contribution. By
introducing the superconducting power cable (Fig. 6 (b)), the regenerative brake is more effectively used
and less mechanical brake is used. It contributes the energy saving of the system. In the first braking
phase between about 80 s and 100 s, the introduction of a superconducting cable brings no improvement.
It is because of the missing of the powering trains that can receive the regeneration power.

Fig. 5. Voltage distribution along the feeder line

(a) Without superconducting cable (b) With superconducting cable

Fig. 6. Train power curves (+: powering; - : braking)
912 Hiroyuki Ohsaki et al. / Physics Procedia 36 (2012) 908 – 913

3.2. Energy saving evaluation

Fig. 7 shows the energy flow in the system. Electric power is supplied from the substations to the
feeding system, and then to the trains. Some of kinetic energy of the train is recovered by regenerative
brake to the power feeding system, but the rest wastes into the heat by the mechanical brake. There is also
Joule loss in the components of the system. Introduction of superconducting cables would bring the
reduced Joule loss and improved regenerative brake, and also reduced maintenance of mechanical brake.
In addition, the reduction of the number of substations or the improvement of redundancy and reliability
of the substations could be expected. On the other hand, cooling power is needed for a cryogenic system
with heat load along the superconducting cable and at the cable terminals.
Table 1 summarizes the analysis results of energy saving and the capacity of substations. Since the 5
min. periodic operation of trains was assumed, energy flow for five minutes and the maximum current
and the maximum power of substations were evaluated for that period. For a superconducting cable
system, the heat load along the cable of 1 kW/km, the heat load at the cable terminals of 0.25
kW/terminal, COP of the cooling system of 0.1 were assumed. The number of cable terminals is ten for
the case shown in Fig. 1.

Power
Input
Kinetic
energy
Regenerative brake
Substation Feeders, Rails, etc. Trains
Mechanical brake
Loss Loss Loss
Travel
Cooling power resistance
- cable losses
- terminal losses

Thermal energy, etc.

Fig. 7. Energy flow in an electric railway system with superconducting cables

Table 1. Comparison of energy (evaluated for the period of five minutes)

Superconducting
Conventional Difference
cable
Total energy needed for train
acceleration Wtotal (MJ) 1318 1318 0
Input power to substations Win (MJ) 631 570 -61
Regenerated energy Wrg (MJ) 809 912 +103
Joule loss Wloss (MJ) 122 90 -32
Cooling energy Wcooling (MJ) 0 74 +74
Max. substation current (kA) 2.1 1.3 -0.8
Max. substation power (MW) 3.2 2.0 -1.2
Max. feeder current (kA) 1.6 1.4 -0.2
Wtotal = Win + Wrg – Wloss – Wcooling
 Hiroyuki Ohsaki et al. / Physics Procedia 36 (2012) 908 – 913 913

By introducing the superconducting cable, the regeneration power increases by 103 MJ and the Joule
loss decreases by 32 MJ, although the cooling power of 74 MJ is needed. As a result, the total input
power of the substations decreases by 61 MJ for five minutes. In addition, the maximum current and
power of substations decrease drastically. They become about 2/3 of those for the conventional system
without a superconducting cable.

3.3. Modified analysis models and their results

A four-substation case (SS1, SS2, SS3 and SS5) and a three-substation case (SS1, SS3 and SS5) were
analyzed. The train operation conditions are the same as the above. Energy saving by introducing a
superconducting cable is not so much influenced by the number of substations. The maximum current and
power of substations becomes a little higher but still much lower than those of conventional system
without a superconducting cable. In the three-substation case, the maximum current was 1.6 kA and the
maximum power was 2.5 MW. The introduction of superconducting cables would make it possible to
reduce the number of substations or to improve the redundancy of the substations.

4. Conclusions

We studied the feasibility of applying superconducting power cables to DC electric feeder systems,
aiming the essential energy saving of next-generation electric railway systems. From the calculated
voltages and currents of the circuit, the regenerative brake rate and energy losses were estimated. In
addition, assuming the heat loads of superconducting power cables and their terminals and the cryogenic
efficiency, the energy saving of the total system was evaluated. The results show that the introduction of
superconducting power cables can improve the use of regenerative brake and the energy saving, reduce
the substation capacity, and/or improve the redundancy of the substations.
For further study, improvement of the analysis models, conditions and methods are being carried out.
Introduction of energy storage facilities are also investigated.

Acknowledgements

This research was supported by Japan Science and Technology Agency, JST, under Strategic
Promotion of Innovative Research and Development Program.

References

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