IEEE Electrification Magazine - December 2017 - 83

6

Power (kW)

4
2
0
-2

Load Power

Ultracapacitor Power

Fuel Cell Power

Voltage (V)

-4
130
120
110
100
90
80
70
60
50

Ultracapacitor Voltage

0

20

40

60

80

100

120

140

Time (s)

Figure 16. A stored energy charging loop enters into difficulty.

xx
Case 1: The load disconnects (step of nominal one to
Voltage (V)

100
80
60
40
20
0

0

5

10
15
Time (min)

20

Storage Voltage
Removal of the Charging Loop
Figure 17. A stored energy charging loop suppression.

Power (kW)

zero); all of the current supplied by the fuel cell is
therefore absorbed by the ultracapacitors; they
charge but must above all not exceed their maximum voltage.
xx
Case 2: The load reconnects (step of zero to nominal
one); the ultracapacitors empty to supply the power
required for the load, but they must not discharge too
substantially so the system can continue to operate.
This is shown in Figure 16, where we guarantee that
the state of charge for the stored energy does not endanger the system when faced with these two situations.
The high and low limits for the stored energy are 150 V
and 0 V.
Figure 17 shows the purpose of the stored energy
charging loop described in the "Impact of System Losses"
section. Indeed, when this loop is not present, the stored
energy starts to completely discharge, and the more its
state of charge decreases, the higher the requested currents, which makes the voltage decrease even further at
its limitations.
The issue of starting for hybridization mentioned in
the "Problem Starting the Frequency Sharing" section
must be experimentally validated to check the stability
of the filter switching with the stored energy charging
loop. This is because hybridization, when starting, is
launched with all of the power consumed by the loads
supplied by the stored energy; its state of charge
decreases very quickly, and its control loop will tend to
increase the power supplied by the fuel cell to recharge
it. Figure 18 shows this startup for a test with an emulator. The fuel cell quickly reaches its nominal power, and
we can easily distinguish between the moments where
the frequency-sharing filter switches thanks to the

8
7
6
5
4
3
2
1
0
-1 0
-2

10

20

30

40

50

Time (s)
Load Power
Ultracapacitor Power

Fuel Cell Power
Switching Filter

Figure 18. Hybridization startup with an emulator.

harmonics present on the power supplied by the fuel
cell. The experiment on the fuel cell power increase is in
line with the theoretical switching of the filters shown
in Figure 12.
IEEE Elec trific ation Magazine / D EC EM BE R 2 0 1 7

83



Table of Contents for the Digital Edition of IEEE Electrification Magazine - December 2017

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