IEEE Electrification Magazine - September 2014 - 51
In some countries, some transmission and distribution
(T&D) grids and power plants, owned by railway companies,
are used specifically for traction purposes. Because of their
usage, these grids have particular characteristics (e.g., twowire, single-phase lines using a low frequency of 16.6 Hz).
Regarding the railways side, when it is not vertically
integrated, the infrastructure manager [referred to as the
railway system operator (RSO) as an analogy with ESOs in
power systems] plays a dual role: 1) regulating the traffic
to ensure safety and an adequate flow of trains and
2) controlling the railway power system. The train operators are independent companies whose activity is to
transport loads or passengers with trains. As users of the
railway infrastructure, they must pay for the services they
use, including power supply, to the RSO.
As shown in Figure 1, RPSs are connected to transmission or distribution grids by means of traction substations
(TSSs). It should be noted that not all TSSs are fed directly
from the T&D grid: sometimes railway-side electrical lines
connect several TSS. In the case of dc-fed railways, substations include transformers and rectifiers. In the case of acfed railways, substations include mainly transformers
and, when the frequency of the T&D grid and railways is
different, frequency converters (static or rotary).
Although they are not widely used, energy storage systems (ESSs) allow the temporal excesses of power to be
stored and used later in a deferred way.
Finally, the railway power plants (referred to as railwayside distributed generation (RDG) in Figure 1) are generators
(typically distributed sources of energy) controlled directly
by the RSO, which allows for railway-oriented operation,
and connected directly to the railway grid.
The Operation of Electrified Railways
Control of the System
The operation of an electrified railway includes two different facets that have to be controlled in a compatible way:
1) the traffic flow operation (which refers to the way the
trains move) and 2) the electrification operation (which
refers to the way the power is supplied). Therefore, the
control centers of the railway are charged with supervising and operating both the traffic and the electrification.
To ensure the safe and efficient operation of the railway,
a signaling system is typically in place to manage the traffic flow. The traffic control is typically structured in layers.
First, a protection layer is responsible for the safety of the
train movements and is in charge of giving the orders to
ensure that no train leaves its safe operation conditions
(for instance, by getting too close to another train or by
exceeding its maximum speed in a specific section).
Depending on the specific technology used in the signaling
system, the degree of automation of the control may be
very different: from manual control (based on visual signals and relying on a person taking the right actions) to
fully automated control (based on communications and
relying on a control unit to ensure that the system is always
in a safe state). Additional layers, always subordinated to the
protection layer, are commonly used to improve the quality of the traffic flow according to different criteria (such as
punctuality and regularity). The control related to energy
consumption optimization would correspond to these
operational layers.
The electrification has a similar architecture. A first
layer, in charge of protecting the electrical equipment and
infrastructure, continuously checks if all of the electrical
quantities (voltages, currents, etc.) are within the allowable
range and, otherwise, isolates the failure to avoid further
damages. Additional layers are responsible for optimizing
the operation of the railway grid by reconfiguring its topology, operating the tap changers of the transformers, etc.
Unfortunately, these control actions are quite limited and
are often too slow for launching them frequently. Thus, the
grid is normally designed to supply power in the worst-case
scenario with very few control actions, which leads to quite
oversized infrastructures.
Although the traffic and electrification are two facets
physically coupled in railways (the electrical loads depend
on the way each train is driven and the way a train is driven
depends on the voltages and, therefore, on the electrical
loads), even the upper layers of these two control systems
are usually completely uncoupled.
The Operation of Train Services
An important concept for understanding RPS operations is the
interrelation between the train movement and its power consumption, which can be used to accelerate the train, to compensate the losses due to running resistance forces-red curves
in Figure 2-and/or to feed the onboard equipment (air conditioning, pumps, compressors, lighting, etc.). Similarly, when a
train equipped with electrical braking systems brakes, the
kinetic energy is converted into electrical power and used to
feed onboard equipment, to feed other electrical loads by injecting this power back into the catenary (regenerative brake), or, if
none of the previous options is possible, to heat up the onboard
resistors installed for that purpose (rheostatic brake).
The power usage related to train movement depends
essentially on how the train is driven. The driver, which can be
a person or an automatic driving system, decides which force
is required to move the train as wanted within the operating
limits of the train (see Figure 2)-this depends on the voltage
and the speed at which it is operating. Four types of driving
actions are normally performed: 1) accelerating, where the
traction equipment exerts a force to increase the speed, 2)
braking, by exerting a force to reduce the speed, 3) cruising, by
exerting only the force required to compensate the running
resistance (which maintains the speed), and 4) coasting, when
the train does not exert any force at all.
For a given journey duration, a train can be driven in
many different ways: accelerating, braking, and coasting differently (in different locations and with different intensities).
One driving strategy commonly used for analysis is the
IEEE Elec trific ation Magazine / s ep t em be r 2 0 1 4
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Table of Contents for the Digital Edition of IEEE Electrification Magazine - September 2014
IEEE Electrification Magazine - September 2014 - Cover1
IEEE Electrification Magazine - September 2014 - Cover2
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IEEE Electrification Magazine - September 2014 - Cover3
IEEE Electrification Magazine - September 2014 - Cover4
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https://www.nxtbook.com/nxtbooks/pes/electrification_december2017
https://www.nxtbook.com/nxtbooks/pes/electrification_september2017
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