15-01-2014, 04:29 PM
Simplified Power Converter for Integrated Traction Energy Storage
Simplified Power Converter.pdf (Size: 988.02 KB / Downloads: 73)
Abstract
Electrical energy storage has a significant role to
play in improving the performance of future electric traction
systems. This paper proposes a new power electronics topology
that integrates the energy storage power electronics with those of
the inverter drive system. This topology reduces weight and com-
ponent count compared with previous topologies but still allows
the use of standard machines. Practical results from a laboratory
test system are shown, and indicative energy savings for a full-sized
system are presented. A study on a City Class Tram on the public
transportation system in Blackpool, U.K., shows that clear energy
savings may be made by employing ultracapacitor energy storage
with the proposed topology.
INTRODUCTION
WITH GROWING importance being placed on decar-
bonizing the world economy and achieving energy
security, electrified public transportation is playing a progres-
sively greater role in society. Compared with personal trans-
portation, 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 regen-
erate energy during braking, energy which would otherwise be
dissipated in either mechanical brakes or braking resistors. This
energy can then be reused.
CONVERTER S ELECTION —PRIOR A RT
While both track-side and onboard solutions are possible,
having energy storage units on the tram has the advantage of
putting the solution next to the problem. When multiple trams
run on the same section of the network, the local energy storage
capability is increased if each tram carries its own storage
system (see Fig. 5).
Installing energy storage devices onboard vehicles is not a
new idea. The development of the Yverdon Gyrobus began in
1945 [7]. The bus was powered by a flywheel that drove an in-
duction generator. The flywheel charged at three-phase charge
points periodically. The Gyrobus required a complex propul-
sion system, consisting of three motors connected together
through reduction gears. Each motor could be driven in two
different pole configurations, effectively giving six different
motor speed–gear configurations, covering the required speed
range. Such a mechanically complex system is undesirable and
perhaps resulted in the limited adaptation of the Gyrobus.
D RIVE S WITCHING C ONTROL FOR
D UAL -M ODE O PERATION
Even with appropriate design of the traction supply and
energy storage unit, the two dc sources will frequently be at sig-
nificantly different voltages, particularly as the ultracapacitor
voltage can vary depending on its state of charge. This requires
different modulation constants to achieve the same applied volt-
seconds. Two switch control signals can be generated: one
for the dc supply operation of the converter and one for the
energy storage operation of the converter. Each phase has three
switches (counting the bidirectional switch as a single switch).
However, only one of these switches can be turned on at any
instance to prevent short circuits between the dc sources or
between one of the dc sources and ground.
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.