IEEE Electrification Magazine - September 2016 - 36

TABLE 3. The energy storage ratings in the specific case of study with three ultracapacitor

modules and one hybrid ultracapacitor-battery module.
New

End of Life Cycle (70%) According
to IEC61881-3

Total energy in the ultracapacitors

4.1 kWh (8,2 kWh/train)

2.8 kWh (5.6 kWh/train)

Available energy in the ultracapacitors

3.75 kWh (7.5 kWh/train)

2.6 kWh (5.2 kWh/train)

Total energy in the batteries

15 kWh (30 kWh/train)

10.5 kWh (21 kWh/train)

Available energy in the batteries

6.75 kWh (13.5 kWh/train)

4.7 kWh (9.4 kWh/train)

system. This can be also the case of a train in a catenary-free path when the accumulation device is full.
In Table 3, the energy storage ratings of this specific configuration are shown. The energy
capacity is not too large, but it has to
be delivered very quickly. One single
string of batteries (15k Wh) has more
than three times the energy of seven
strings of ultracapacitors (4.1 kWh).
When the train is working in catenary-connected mode, the requested
traction power is always fulfilled
because the feeding system acts as a
backup; in some special occasions,
when the catenary voltage is abnormally low, the overcurrent protection
of the traction equipment can reduce
the requested traction power for safety purposes.
When the train is working in a catenary-free area, the accumulation
system is always calculated to fulfill
the nominal operation conditions. So,
in normal conditions, the train should
be able to reach all the charging
points loosely. For each section between two substations, a
set of curves, as the ones represented in Figure 6, are
implemented in the train control system. The dotted
curve represents the real ultracapacitor's energy in

nominal operation conditions. In most of the operation
cases, the real curve should not differ too much from this
dotted nominal curve. In this particular configuration, the
batteries are not considered as primary sources for traction. As shown, in
this particular example, the energy
consumption is high during the first
stage when the train is accelerating at
the highest rate. In the final meters, in
braking mode, the ultracapacitors are
partially charged again. The ultracapacitor charging system at the substation is extremely fast, and it
guarantees that the ultracapacitors
will be full charged in less than 20 s.
When the ultracapacitor's real
energy curve drops below the red
line, the control system starts applying preventive actions, which will not
affect the performance of the train. If
the real ultracapacitor's curve drops
below the reserve model line, the
system will take corrective actions
that will affect the train performance, such as the limitation of the
traction power. Both actions (preventive and corrective)
are taken automatically by the train control system without the need of any driver input. There are three preventive actions, depending on the real energy curve. If it
drops below the red curve, the train interior cooling will
be reduced to 50% of its capacity until the next charging
station is reached. Below the green line, the train interior
air conditioning system will be disconnected, keeping
the ventilation system on. Below the blue line, the hybrid
traction mode will be activated. That means the batteries
will be used for traction purposes. For this hybrid mode,
the battery energy reserved will be 2 kWh. If the train
already consumed this battery energy in the current section or in a previous one, the corrective measures are
activated. There are two kinds of corrective measures;
the first one is adopted when real ultracapacitor's energy drops below the reserve model line. In such case, the
train performance starts to decrease gradually. The
reserve mode line is fixed in the 20% of the ultracapacitor's total energy. Below the pink line, the train will

It must be
considered that the
capacity of the
battery string is
extremely high when
compared with the
ultracapacitors, but
the maximum
charging and
discharging power
is much lower.

Energy in
UC-s

Real Energy Curve
HVAC 50%
HVAC 0%
Hybrid Mode
Reserve Mode
Backup Mode

Figure 6. OESS energy versus distance curves.

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

Distance

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