IEEE Electrification Magazine - September 2014 - 35

substations or ESSs may reduce them substantially, as
will be shown in the "Energy-Saving Techniques" section.
Figure 2 shows that rheostat losses affect directly the
amount of energy actually supplied by substations (rectified
energy). This may partially waste the large saving potential of
regenerative braking, which is sometimes estimated to be of
above 30% of the energy given by substations in the absence of
this braking system. It is then important to improve the system's receptivity to regenerated energy as much as possible.

energy-saving techniques

Energy-storage technologies are characterized by their
energy- and power-density characteristics. The former
determines the amount of energy that can be stored per
each unit of mass, while the latter refers to the rate at which
this energy can be stored or released. The main technologies
used in railway systems are electrical double-layer capacitors or supercapacitors, flywheels, and batteries.
The control strategies for these devices are designed to
charge the ESS when the voltage is high at the connection point
and to discharge it when the voltage is low. If the ESS is tailored
for energy saving by increasing the system receptivity, the control strategy should make the ESS itself as receptive as possible
at every moment. This implies releasing the stored energy as
soon as possible to make room for the next charge, thus avoiding situations where the ESS could not accept the regenerated
energy coming from the next braking train because it is full.

Figure 3 shows some technologies currently used in dc-electrified railway systems to improve energy efficiency by increasing system receptivity. The two main technologies are:
xx
Reversible substations: They make it possible to give
occasional energy surpluses back to the utility grid. In
practice, this is only useful if the country regulation
Application example
permits to discount this energy from the total energy
In this section, the energy savings in a metro line derived
bill paid by the railway operator. Indeed, this kind of
from the application of reversible substations and ESSs is
regulation can be used to promote energy efficiency
illustrated.
by using this technique.
xx
ESSs: They store energy from braking when the system
Trains are moving loads, meaning that the topology of
is not receptive and will release it once the network
the system changes. In addition, they behave like constantagain becomes a power consumer. ESSs may be
power sinks or sources, which yield nonlinear equations.
deployed in fixed locations or on board. The latter
As a result, electric railway systems are usually analyzed
makes it possible for the train to run along nonelectriby means of multitrain simulators. The results shown in
fied sections of the line, e.g., in city downtowns.
this study were obtained from an electric railway simulator
Recent studies are devoted to analyzing the feasibility of
developed in the Institute for Research in Technology (ICAI
novel technologies such as superconductive feeding lines.
School of Engineering, Comillas Pontifical University). This
Reversible substations may be built using a single
simulator is oriented to energy efficiency studies and
bidirectional converter or by
adding another converter in parallel with the original one. The
Substation
Power Chain
Losses
first solution may be preferable
Ohmic
Electrical
Losses
when designing the substation
Losses "Go"
for a new line. The second solution is preferred when trying to
Mechanical
improve receptivity in alreadyLosses
in-use lines. When an extra conRectified
Flowing
Traction
verter is required, this can be of a
Through Absorbed
from
Catenary
lower power rating than the
Catenary
main one according to the
expected amount of energy to be
Regenerated
Back
sent back to system. In both
Auxiliary
to Catenary
Services
cases, this technology is mature
Regenerated to Auxiliary
and is already being used in several metro systems.
ESSs are developing rapidly
Useful
as energy storage devices and
Regenerated
Regenerated
for the optimal charge-discharge control of the device.
ESSs can be used for two main
Rheostat
Power Chain
Losses
purposes: voltage-level support
Electrical
and energy savings. Device conLosses "Return"
trollers may, consequently, have
Figure 2. A dc-electrified railway system Sankey diagram.
two different operating modes.
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