18-04-2012, 11:52 AM
Simplified Power Converter for Integrated Traction Energy Storage
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INTRODUCTION
WITH GROWING importance being placed on decarbonizing
the world economy and achieving energy
security, electrified public transportation is playing a progressively
greater role in society. Compared with personal transportation,
a substantial energy saving is achieved with public
transportation, particularly at peak commuter times. Further
carbon savings may be made since the electrical network would
allow renewable and low-carbon energy to provide motive
power.
The energy consumed in an electrified transit system can
further be reduced by installing energy storage systems (ESSs)
onboard vehicles. Energy storage devices can be used to regenerate
energy during braking, energy which would otherwise be
dissipated in either mechanical brakes or braking resistors. This
energy can then be reused.
ENERGY STORAGE VIABILITY
Such techniques have a long history. The Central London
Railway was built with stations that were raised, meaning that
Manuscript received February 13, 2010; revised July 16, 2010 and
November 26, 2010; accepted January 22, 2011. Date of publication
February 17, 2011; date of current version May 16, 2011. This work was
supported in part by the U.K. Engineering and Physical Sciences Research
Council and in part by HILTech Developments Ltd. The review of this paper
was coordinated by Prof. M. E. Benbouzid.
Choice of Energy Storage
For traction systems, robustness is paramount: an energy
storage device in a traction application will be discharged every
time a vehicle sets off and stops, and since this can equate to
hundreds of times a day, it is essential that the degradation of
an energy storage device due to the charge–discharge cycle is
minimal. Maintenance requirements should be kept as low as
possible. Additionally, a high power density is required, energy
density is important but is less critical. Ultracapacitor energy
storage devices meet these requirements. They also have no
moving parts, further increasing reliability and robustness in
mobile applications.
ENERGY MANAGEMENT
During braking, as much of the braking energy as possible
should be stored in the ultracapacitor. The braking energy not
stored by the energy storage device is dissipated in braking
resistors. However, the storage power is limited by the converter
power, and the energy storage device must not be allowed to
exceed its maximum voltage level. The converter systems are
typically operated with a specific input voltage range for several
reasons: to maintain operability, the ultracapacitor should not
be allowed to fall below a minimum voltage level; little extra
energy is available at low voltages since three quarters of the
energy is available from the top half of the voltage range; and
to extract power at the same rate requires more current at lower
voltages.
Advantages
The novel power electronic circuit removes the need to use
bidirectional dc–dc converters, which contain bulky components.
The novel converter is potentially more efficient than
an arrangement containing a dc–dc converter by using fewer
energy conversion stages.
Other power electronic circuit configurations can be used
to improve the efficiency by not employing dc–dc converters.
However, the circuits proposed either use transformer windings
[19] to inject currents or require access to both ends of the traction
motor windings [22] and therefore require modifications to
traction motors.
CONCLUSION
This paper has presented a new converter topology for light
rail traction. The Blackpool tram system in the U.K. has been
taken as a study case. It has been shown that energy storage
onboard each tram can substantially reduce energy use per
kilometer. A new converter circuit has been presented. It has
been shown that further energy savings per kilometer can be
achieved with the novel converter as opposed to a conventional
power electronics topology.