IEEE Electrification Magazine - September 2014 - 41

as depicted in Figure 1. Third rails are interrupted at crossovers and level crossings. Ramps are provided at the section
ends to provide a smooth transition to the contact shoe
pads. The position of the contact pad between the rail and
the train could vary: some of earliest models made contacts
with the top of the third rail. However, later developments
used side or bottom contacts (see Figure 2) with the rail,
which allowed the third rail to be covered, protecting track
workers from accidental contact and protecting the third rail
from falling leaves and adverse weather conditions.
In practice, third-rail voltages are lower than 1,500 V to
prevent electric shock hazards close to the ground. The list
of standard third-rail voltages include: the 750-V Metro
line 13 of Beijing in China; the 1,200-V rapid mass transit
railway of Hamburg S-Bahn, in Germany; the 1,200-V Manchester-Bury railway in England; the 1,500-V Culoz-
Modane railway in France; and the 1,500-V Metro line 4
and line 5 of Guangzhou in China.

Overhead Wire System
Alternatively, trains are energized by an overhead wire system. In 1881, Werner von Siemens presented the first tram
with an overhead wire system at the International Exposition of Electricity in Paris. In 1882, Siemens tested a similar
overhead wire system on his early precursor of the trolleybus. The overhead wire system shown in Figure 3 is
equipped with a pantograph that supplies electricity to locomotives, tramcars, and light-rail vehicles. The overhead wire
system could carry either ac or dc. For instance, a 1,500-V dc
system is used in the Netherlands and Japan; 3,000-V dc is
used in Belgium, Italy, Spain, Poland, and Slovenia; 25-kV ac
is used in the United States, China, India, and France; dc voltage between 600 and 1,500 V are used by tramcars, trolleybus
networks, and subway systems.

regenerative energy braking
for storage at train stations
Energy storage devices use the regenerative energy efficiently, improve the stability of dc supply voltage, lower
the peak load at train stations, and reduce the heat and
dust in subway tunnels. To make the most out of the energy storage devices in railroad systems, they absorb the
regenerative energy released by trains and feed the stored

energy back for other applications in traction systems.
Such energy storage devices must possess a compact size,
require low manufacturing and maintenance costs, and
be able to be fixed quickly.
Regenerative energy absorption devices would be
installed in traction systems to reduce energy consumptions by braking resistors, lowering the rise in temperature
due to excessive energy losses, and cutting the extra
weight carried by the train due to the installation of railway-mounted braking resistors. Four main regenerative
energy storage devices include battery storage, superconducting magnetic energy storage (SMES), ultracapacitor
storage, and flywheel energy storage.
The battery storage system has been used since the 1990s
in the Berlin metro in Germany. However, battery storage has
not been used widely in regenerative braking for traction systems. In practice, a battery storage system could pose various
shortcomings such as lower power density, lower energy efficiency, and limitations on charge-discharge cycles. Figure 4
shows the use of a battery storage system in a traction system. The value of R in Figure 4 is relatively small, while R1
located in parallel with thyristors is large. When the thyristor
is not triggered, the battery is disconnected from the third rail.
When regenerative braking is engaged, the third-rail voltage
is boosted up, which would trigger the thyristors. If the thirdrail voltage exceeds its upper limit, the power flow through R
would charge the battery. When the third-rail voltage is below
its lower limit, the thyristor will be triggered, which will result
in discharging the batteries through R to supply the third rail.

Superconducting Magnetic Energy Storage System
An SMES system includes a superconducting coil, a cryogenic
container, refrigerating equipment, a converter, and a measuring and controlling unit. Figure 5 shows the SMES system used
in the commuter train regenerative energy absorption. The
SMES system stores the regenerative energy in electromagnetic energy by superconducting coils, which could be fed back to
the third rail for supplying train station loads. The energy storage coil, produced by superconducting lines, is kept at a critical
temperature for superconductivity. Theoretically, the lossless
energy would be stored in the coil until it is released. Compared with other energy storage devices, such as lithium-ion
battery storage, SMES offers several advantages:

Third Rail
Running Rails

+
-

Figure 2. A contact shoe for the top-contact third rail.

Figure 3. An overhead wire system for a locomotive.
IEEE Elec trific ation Magazine / s ep t em be r 2 0 1 4

Table of Contents for the Digital Edition of IEEE Electrification Magazine - September 2014