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bikkuri bahn

Superconducting cable system for electric railways

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bikkuri bahn

I came across this project in the Nikkei newspaper a couple days ago.  It's still in the testing stages, but shows some promise.  Apparently they are aiming for a more large-scale test application by 2020.



1. Introduction
In conventional electric railways, a DC feeding system is widely used, especially in railway lines with
dense train traffic. However, the DC feeding system has some essential problems due to its low
feeding voltage. The problems are a relatively lower efficiency of power transmission, a larger voltage
drop along the feeder, the regenerative braking cancellation, a large number of substations along the
line, etc. We consider that the introduction of superconducting cables could be a solution to them [1-3].
By laying superconducting cables along the feeder in parallel and connecting them to the railway
substations and, in some cases, to several points of the feeder, a voltage change along the feeder can
be significantly reduced. This allows the trains to use more a regenerative braking system, thus
improving energy efficiency of the whole system. Furthermore, the use of superconducting cables
would enable effective load sharing among the substations. The substations have less peak power and
can be designed to have a smaller capacity. It leads to a less expensive design of the substations and,
as a result, a less expensive DC electric feeding system.




First test application, at the RTRI facility in Kokubunji:



Second test application, on the Izu Hakone Rlwy (first success on an operational rail line):


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I guess the logical solution of using the local AC eletric grid has never happened to them. And i mean the normal household grid, that can be connected to the railway traction system. In practice this means simply connecting the local 15kV or 25kV AC grid to the overhead and that's all. No need for dedicated railway substations or anything just a few connection wires. Power fed back by the trains can be used by the normal grid. This system is in use in Hungary since 1932. The early 1.6 megawatt high speed locomotives sometimes caused noticable light intensity changes in houses around the tracks as they went by, but this effect disappeared as the generating and transmission capacity of the unified network increased.


For local low catenary commuter systems (like trams and trolleybuses), where high voltage AC can't be used, the common solution is to connect the substations to the local AC grid and to use a switched rectifier/inverter system, where the DC is generated by switching the polarity of the AC voltage at the line frequency behind the main step down/up transformer. In the past this was done with a rotary converter, a synchronous motor driven commutator (polarity reversal was done during zero crossing with zero voltage and current), while nowdays handled by solid state switches. This requires only late 19th, early 20th century equipment or its modern equivalent. The trick is to use the local grid, which avoids the costs of a separate traction system. Of course, a local grid outage can take down the traction line running next to it, in this case there is usually a swichover point to temorarly feed a traction section from its two neighbours with decreased efficiency, resulting in a backup setup that matches the one used by default on the lines described in the article. A typical substation nowdays is nothing more than a gray equipment box along the tracks, usually placed at track level or mounted directly on the poles. The distance between the substation and the catenary is a few meters at most.

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