IEEE Electrification Magazine - September 2014 - 33

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Infrastructure: line layout and electrification design,

which includes the electrification system, line voltage,
connection of isolated electrical sectors, conductor
section, energy recovery, and sideway energy storage.
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Railway line operation: scheduling design, ecodriving,
and centralized operation.
A significant amount of research resources has been
devoted to this topic. One of the main conclusions is the
important consumption reduction associated with the use
of the energy from regenerative braking. However, in dcelectrified metropolitan railways, this energy cannot
always be efficiently used by the system.
Today, most of the rolling stock in railway systems is
equipped with regenerative braking, meaning that the
kinetic energy of the train is partially converted back into
electric energy when braking. This energy can be either 1)
sent to onboard resistor banks called rheostats, thus
wasting it, or 2) sent to the system to be reused. In electrified railway systems, the interconnection among different vehicles in the line makes it possible for other
trains that are motoring in the same instant to effectively
use the regenerated energy, thus reducing the global
energy consumption in the system.
Figure 1 shows the potential of regenerative braking to
reduce energy consumption in electrified railway systems. It
includes the performance of a metropolitan train running
between two passenger stations separated by 1.4 km without
any grade and using the minimum time, the so-called flat-out
speed profile. Figure 1(a) depicts the power in the pantograph,
while Figure 1(b) shows the speed profile. Three different sections can be distinguished: 1) the first, in which the power
consumed is used to accelerate the vehicle with a certain efficiency; 2) the second, in which the energy is used to cruise;
and 3) the third, in which the train brakes and a fraction of the
kinetic energy stored in it can be sent back to the system.
The energy consumption during the three phases in
this example is 23.7, 4, and −10 kWh, respectively. The
energy recovered during braking represents 44% of the
energy required to accelerate the train and 38% of the total
energy consumed. Furthermore, the energy recovered during braking can be used by another train accelerating
along the same electrical section with an obvious savings
in the energy taken from the electric substations. In fact,
energy consumption can be up to 38% less than without
regenerative braking if losses are neglected. These results
show the enormous savings potential of regenerative
braking in the total energy consumption of electrified railway systems.
However, the theoretical reduction in energy consumption is limited in practice because of several factors, such as
joule losses in the conductors and conversions efficiency.
Furthermore, power regeneration provokes an increase in
pantograph voltage, and railway system regulation (UIC-600)
limits the voltage at any point of the electric network to a
maximum of 20% above the nominal voltage [except in
750-V systems, where a maximum of 1,000 V (33%) is

allowed]. Overvoltage limitations make it necessary for trains
equipped with regenerative braking to include rheostats to
dissipate the surplus of regenerated power.
Events of energy sent to rheostats are frequent in
dc-electrified railway lines, making it difficult, under some
circumstances, to take full advantage of regenerative braking. This article spells out several alternatives to improve
energy efficiency in metropolitan dc-electrified railway
systems, focusing on those related to improvements of the
electrical infrastructure. The main characteristics affecting
the performance of this kind of system are mentioned as
well as the main techniques used for analysis.

receptivity to regenerated energy
Railway systems may be either ac or dc electrified. In ac systems, the train voltage supply is usually as high as 25 kV,
while in dc systems, the maximum standard voltage is 3 kV.
Therefore, transmission losses will be much higher in dc
systems for the same rated power. In addition, ac systems
are connected to the utility grid by means of transformers,
which allow energy to flow in both directions, while dc systems are connected through ac-to-dc converters-rectifiers.
In most cases, these converters are made up of diodes,
allowing the energy to flow only from the utility grid to the
railway network. As a consequence, periods of time in
which the amount of global regenerated power is higher
than consumption are problematic because of the surplus
energy. In general, this surplus energy will be sent to rheostats and wasted.
The receptivity concept can be defined as the capacity of
a given railway system to accept the braking regenerated
energy. Unfortunately, there are a number of circumstances
that tend to reduce the receptivity of a railway system:
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Regenerated power exceeds consumption: The network
voltage tends to increase when the power supplied by
braking trains is more than the power taken by motoring trains. In ac systems, the surplus energy is easily
sent back to the utility grid and the voltage stays within
limits. In dc systems, on the contrary, if electric substations are unidirectional, which is usually the case, the
surplus energy will probably provoke the voltage in
braking trains to reach the maximum accepted value,
making it necessary to use onboard rheostats. Situations with more regeneration than consumption are
not likely to happen during peak hours, but they are
frequent when the headway increases. Therefore, dcelectrified railway systems may show receptivity problems, especially in off-peak hours (long headways).
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Distance between regeneration and consumption: Power
transmitted from one train to another requires a voltage
drop along the conductor between them. The longer the
distance, the bigger the voltage drop. This increases the
probability of reaching the maximum allowed voltage at
the sending end.
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High no-load voltage: In weak railway systems, it is a
common practice to intentionally increase the no-load
IEEE Electrific 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