review

1 RhB provides a year-round service.
© 2016 Eduard Kiener
The railway network of RhB
(Rhaetische Bahn) extends over
384 km in the Alps of southeastern
Switzerland. The railway
serves such popular tourist resorts
as Davos, Klosters and Sankt Moritz, and
carries around 10 million passengers per
year. Parts of the system, featuring
breathtaking sequences of twisting tunnels
and elegant viaducts, are classified
as UNESCO world heritage sites ➔ title
picture. Besides RhB’s function serving
tourism, the system assures local transportation
and carries freight all year round.
In some locations, where roads are regularly
closed due to winter snowfall, the
railway provides the only viable alternative
transportation.
Commencing in 2010, RhB began modernizing
its fleet by introducing a new
family of multiple unit trains branded
Allegra. These 20 trains are supplied by
Stadler with compact electrical equipment
by ABB including traction transformers
and converters ➔ 2. The Allegra
units were designed to fulfill highly demanding
requirements imposed by the
tight curves, steep gradients and challenging
climatic conditions ➔ 1 of RhB’s
network as well as the long and heavy
trains they are required to pull.
As is often the case in engineering, the
power requirements were dimensioned
to meet the toughest possible conditions
under which they were required to
operate. The greatest traction power is
required, for example, when moving a
heavy train up a steep slope. For much
of the time the units work under less
demanding conditions (lighter loads, level
track). Efficiency under such conditions
may thus be suboptimal. A project was
launched to investigate and implement
ways of improving overall energy efficiency.
Motors
The power delivered by a traction motors
is the product of the magnetic flux and
the torque-forming current in the stator.
Both of these factors contribute to losses.
As the currentdependent
losses
dominate at high
power, the motor
is typically operated
at the maximum
flux with the
current being used
to control output.
However, at smaller
power levels it can actually be more efficient
to operate at a lower flux. Every
speed / torque value pair has an optimum
dependent on the motor parameters.
DC-link
Normally, when a traction converter varies
its power output, this is achieved by
maintaining the DC-link voltage as constant
as possible and varying the output
current. As the full DC-link voltage is only
actually required at full power, it is viable
to permit this voltage to sink to lower
values when operating at lower power.
Optimums were identified for different
scenarios including power, tractive effort
and variations in catenary voltage. These
calculations did not consider the converter
in isolation, but included losses in
the transformer and motors resulting
from converter switching patterns.
Disconnecting traction motors
When the train is required to operate at
a high power output, all traction motors
are required. For lower power output
however, it is more efficient to selectively
A project was launched to
investigate and implement
ways of improving overall
energy efficiency.
use a reduced number of motors (and
the associated inverters) and disconnect
the others.
In view of the abundance of curves,
considerations over maintaining a good
dynamic behavior of the unit meant that
both axles on a bogie should always exert
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