IEEE Electrification Magazine - September 2014 - 40

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Figure 1. The tracks with a conductor (third) rail.

The working principles and topological structures of the
two methods are introduced. In addition, main circuits
and corresponding control systems are simulated, and
analyses are conducted to validate the proposed methods
for train stations in China.
There are two types of braking in traction systems:
dynamic (electric) braking and the more traditional friction
braking. In dynamic braking, as a traction motor is switched
to a generator, the generated current is employed for rheostatic or regenerative braking. In rheostatic braking, the current is dissipated in the banks of resistors, which would
slow down the train. This type of braking is valuable on
heavy-haul diesel locomotives running on routes with
extensive down grades. When regenerative braking is
employed, the current in the electric motors is reversed for
slowing down the train. During braking, the traction motor
connections are altered to turn them into electrical generators. The motor fields are connected across the main traction generator, and the motor armatures are connected
across the load. In dynamic braking, for a given direction of
travel, the current flow through the motor armatures will
be reversed and the motor exerts torque in the opposite
direction to that of rolling.
Electric trains and hybrid diesel locomotives can be
equipped with regenerative braking. The more frequently a
train stops, the more it can benefit from regenerative braking.
Therefore, the technique is especially valuable for commuter
trains and intercity subways, which both stop frequently.
Another form of regenerative braking, which is used in places
such as the London Underground, uses small slopes leading
up to and down from stations. The train is slowed by the
climb and then leaves down a slope, so kinetic energy is converted to gravitational potential energy in the station.
In regenerative braking, the energy saving is dependent
on the ability of other trains on the same route to accept the
energy (i.e., receptivity). On a busy commuter train at peak
hours, receptivity can be as high as 15%. One method of
improving receptivity is to provide storage at train stations,
which will be discussed later. Dynamic braking would maintain a constant speed, reduce the need for friction brakes,
and offer substantial savings in brake maintenance. Employing regenerative braking in trains can lead to substantial

I E E E E l e c t r i f i c ati o n M agaz ine / september 2014

CO2 emission reductions, especially when applied to intercity
trains (15%) and very dense suburban commuter trains (30%).
The harvested energy during regenerative braking is proportional to the product of the magnetic strength of the field
windings, multiplied by that of the armature windings. If
regenerative energy exceeds the energy required by potential
loads, the additional regenerative energy would increase the
distribution grid voltage at the station or that of filter capacitors located on trains. In such cases, regenerative braking
would fail as a supply and friction braking would slow the
train down at train stations.
In principle, braking can be all dynamic with the friction
braking used only for emergency stops and for bringing the
train to a halt. The friction brakes on high-speed trains would
generally require an extra (triple) disk per axle for stopping the
train, which adds both unsprung mass and rotating inertia. The
heavier axles would require increased track forces for accelerating this mass. The additional calipers and brake rigging would
also add complexity and cost. Environmental concerns further
promote the use of dynamic braking. However, dynamic braking alone would often be insufficient to stop a locomotive, as its
braking effect rapidly diminishes below about 10-12 mi/h (16-
19 km/h). Therefore, it is always used in conjunction with friction braking.
The energy harvested using regenerative braking is
important because it can essentially convert a municipal
train station to a microgrid. Federal Energy Regulatory Commission guidelines in the United States stipulate that energy
providers who can supply energy quickly (higher flexibility)
can charge a higher premium. While it is only designed to
provide power for minutes at a time, regenerative braking
can offer extremely versatile harvested energy, which would
have otherwise been wasted as additional heat in the atmosphere. Regenerative braking, which can earn several hundreds of dollars annually in a typical municipal train station,
is a rare bright spot in economically constrained cities, which
can also save fuel usage and reduce the adverse effects on
our environment.
In the following sections, we focus the discussions on
several models and options for implementing dynamic
braking in high-speed electric trains and offer examples
for the implementation in the railway system in China.

supply of electric power to electric trains
The power supplied to electric trains is through either the
third rail or overhead wires. In this section, the two
options are briefly introduced.

Third (Conductor) Rail System
Siemens, a German engineering firm, exhibited a series of
prototype trains with the third trail at the Berlin Industrial
Exposition in 1879. Later, many railway trains began to use
the third trail traction for power supply. In 1890, the electric
subway of London started using the third trail. The third rail
made of highly conductive steel which is placed alongside or
between the running rails, would provide dc power to trains

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