IEEE Electrification Magazine - September 2016 - 34

Basically, all
on-board
accumulation
systems use the
energy regenerated
during the braking
process to charge
the on-board
accumulation device.

only available during the dwell
time in the stations. In these
cases, most of the accumulated
energy is used between two substations, and the system must be
recharged again in the next substation, a fast-charging OESS
based on ultracapacitors is
required. Additionally, to deal
with unexpected situations, a
backup battery is added.
For example, the vehicle developed
for Kaohsiung City will be described.
It is a high catenary-free operation
(100% without catenary), and the
OESS, shown in Figure 3, is based on
ultracapacitors for nominal operation
with a backup battery. The configuration of this vehicle
has five modules, and the accumulation system is
mounted on the cabin cars. The bogie in the middle is a
trailer bogie, and it is equipped with the pantograph for
connecting the train with the catenary or the overhead
charging system. The tare of the five modules vehicle is
around 49 tons, and the maximum mass is around
69 tons. Other version with three, seven, and nine modules are available. The five-module version has 48 seats
and a total capacity for 228 people. The operation is 100%
without catenary, and the OESS is based on ultracapacitors for nominal operation with a backup battery (high
catenary-free operation).
When the train is in a catenary-free area, the pantograph is down and the OESS dc/dc module controls the
train bus voltage feeding the traction and auxiliary converters. In this particular case of study, the rated operational voltage is 750 V, and the highest and lowest permanent
voltages are, respectively, 900 V and 550 V. The highest nonpermanent voltage (the maximum value of the voltage
likely to be present for maximum 10 min as defined in
EN50163) is 1,000 V. The rated power of the OESS systems is
226 kW per bogie (452 kW per train), and the maximum
power is 400 kW per bogie (800 kW per train). The converter efficiency is 95%.
When the pantograph is connecting the train bus with
the catenary, the voltage is fixed by the catenary, and the
accumulation system works in a coordinated manner
with the traction system. Even when multiple control philosophies can be implemented, the most usual is the one
that assigns priority to the accumulation system consumption and charging. That means that in traction mode,
when the traction converter demands energy, the accumulation system will provide this energy as far as possible. If not, the rest will be extracted from the catenary.
When the traction equipment is working in regenerative
braking mode, the accumulation system tries to absorb all
the regenerated power as far as possible. The surplus is
injected in the network if the catenary voltage is lower

I E E E E l e c t r i f i c ati o n M agaz ine / SEPTEMBER 2016

than the highest permanent voltage.
Above the highest permanent voltage,
part of the energy surplus will be
burned into the rheostatic system. If
the voltage reaches the highest nonpermanent voltage, the entire surplus
will be burned in the rheostatic system and no power will be injected in
the catenary.
In Figure 5, all possible working
modes are summarized. For Figure 5(a)-( j), the left bar represents the
energy generation and the right bar
represents energy consumption. For
instance, when the train is in traction
mode, the train energy is in the right
bar, but in regenerative braking mode, it
is in the left bar. When the accumulation system is providing power to the train, it is in the left bar, but when it is in
charging mode, it is in the right one. The same principle can
be applied to the catenary power. The auxiliary energy and
the rheostatic braking energy are always in the right bar
since they are always power consumptions. Modes (a)-(c)
correspond with traction modes, and (d)-( j) are braking
modes. The possible traction modes are:
xx
Mode (a). All the power requested by the traction converters and the auxiliary equipment is provided by the
accumulator. This is the only possible working mode
when the train is working in a catenary-free path, but
it is also the preferred situation when the train is connected to the catenary. In case there is energy available in the accumulation system and the power
requested by the train is within the limits of the
power that the accumulation system can provide for
its specific level of charge, all the traction plus the
auxiliary power will be provided by the OESS. The
maximum power provided by each device depends on
its level of charge for avoiding deep discharges and
extending the life of the device.
xx
Mode ( b). This mode is only possible when the train is
connected to the feeding system (catenary or third
rail). Both the OESS system and the catenary provide
the sum of the traction and auxiliary power. In this
case, the maximum power that the accumulation system can provide is lower than requested, so part of
the power is absorbed from the feeding system (catenary or third rail).
xx
Mode (c). In this mode, the accumulation system is
empty, and all the power requested by the traction
and auxiliary equipment is absorbed from the feeding system. This mode is only possible when the
train is connected to the catenary, and it is also the
conventional mode when the train does not have onboard accumulation.
In case the train is braking, there are seven possible
combinations.

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