IEEE Electrification Magazine - September 2016 - 31

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with these kinds of technologies can
reach up to 99%. However, it is not
possible to implement reversible
systems in combination with catenary-free technology. Another proposal is the use of portable
reversible substations like the one
that Construcciones y Auxiliar de
Ferrocarriles (CAF) installed in the
Eusko Trenbide Sarea line. This substation can work in direct (ac to dc)
and in reverse mode (dc to ac) and
can be operated in parallel with the
traction substation.
An alternative to the substation's
reversibility is the off-board accumulation at substation level. Commercial solutions can be found using
nickel-metal hydride batteries like
the Kawasaki (Gigacell) and supercapacitor-based energy storage like
Bombardier (EnerGstor) or Maxwell
modules. Flywheel-based technologies are proposed, for instance, by
Kinetic Traction Systems or VYCON.
In all cases, the cost comparison of the off-board energy
storage devices with respect to the reversible substation
reveals that reversibility is much more cost-effective. Neither reversibility nor off-board accumulation allows the
use of catenary-free systems. In addition, even when they
are able to recover part of the energy that would be
burned in the rheostatic system, they do not guaranty a
loss reduction in the dc system or an improvement of the
trains efficiency or performance.
For all these reasons, this article will concentrate on
the study of the on-board accumulation systems.
According to Table 1, on-board accumulation is the best
choice from the point of view of safety, availability, energy recovery, and overall efficiency of the dc traction system since it eliminates undesirable power flows while
reducing the losses. As discussed in the following, onboard accumulation allows the catenary-free working
mode in a safe way without high-energy provider
dependence. Regarding the different accumulation
technologies, the f lywheel's energy density is still
quite poor when compared with other technologies,
and the hydrogen-based fuel cells are not yet a mature
technology. That is why most of the train manufacturers have decided to install on-board accumulation

diode rectifier. When the dc voltage increases at substation level due to the effect of the regenerative braking,
the controlled rectifier is activated, and the energy is
transferred from the dc to the ac system through it. This
is the case of the solution proposed, for instance, by
INGETEAM already implemented in dc systems like Lines
2-6 of Metro Brussels (750 dc voltage), Metro Bilbao (1,500
dc voltage), or Suburban Malaga-Fuengirola (3,300 dc
voltage). ALSTOM proposed an alternative based on the
use of controlled thyristor-based rectifier in parallel with
an IGBT converter. This solution has been already implemented in the London Metro and Paris Tramway (750 dc
voltage) as well as Milan Metro (1,500 dc voltage). The
combination of IGBT-controlled converters with the thyristor-based rectifiers can optimize not only the reverse
working mode (dc to ac) but also the forward mode of
operation (ac to dc). It allows, for instance, an increase in
the distance between substations. On the other hand, the
installation of IGBT-based converters in parallel with the
conventional diode rectifier does not require any modification of the existing system, and it has no effect over its
availability; high cost elements like transformers or rectifiers can be reused, and its operation is transparent to
the existing infrastructure. The braking energy recovery

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Table of Contents for the Digital Edition of IEEE Electrification Magazine - September 2016