IEEE Electrification Magazine - December 2017 - 78

The high-frequency
power components
required by the
application must
be supplied by the
ultracapacitors and
the low-frequency
components by the
fuel cell.

Fuel cell production was noted to
be higher than the mean load power.
This would have also been valid in
nonhybrid cases, taking into account
the converter losses and the auxiliary
consumption. However, the fuel cell
supplies few high-frequency components, which should enable its compressor to correctly control the air
supply flow rate and not speed up its
aging. Compared to the ideal case
presented in Figure 4, apart from the
mean value for the power produced
by the fuel cell, which differs for the
reasons given above, slightly larger
fluctuations can be seen for the supplied power. These fluctuations are
due to the stored energy charging loop, which itself is
dependent on its capacitance.

Implementation of the Selected
Energy Management
DC Bus Voltage Control Strategy

As explained in the "Impact of System Losses" section,
losses in the converters and stored energy have a slow
effect (low frequencies) on the state of charge of the stored
energy and, therefore, on the system sizing. Ultimately, the
fuel cell will have to supply power for these losses to maintain the state of charge for the stored energy.
However, for the strategy shown in Figure 11(a), these
losses by the stored energy will consequently have only a

85

8

80

7

75

6

70

5

65

4

60
55

1
0

40

-1
50

100 150

200 250
Time (s)

300

350

400

Load Power
Ultracapacitor Power
Fuel Cell Power

2

45

0

× 104

3

50

35

78

Dynamic Compensation of Losses

Power (W)

Voltage (V)

As the architecture has two static converters, there is a
DoF for controlling the bus voltage because there are three
components connected to the dc bus: the loads, the stored
energy and its converter, and the fuel cell and its converter. Only one of these elements can manage the bus voltage. Clearly, the loads cannot do this. It is therefore
possible to control the bus voltage via the fuel cell converter or the stored energy converter.
Both of these solutions are shown in Figure 11. In some
cases, it is of particular interest to assign the bus voltage
management to the stored energy. This is true, e.g., when
the fuel cell is replaced by a RAT, as maximum power

point tracking can be associated with
it (Rafal 2010, Langlois 2006). However,
there currently seems to be minimal industry interest in a fuel-cellbased solution.
Regardless of the choice of dc bus
control, frequency sharing must be
performed as shown in Figure 3, i.e.,
the high-frequency power components required by the application
must be supplied by the ultracapacitors and the low-frequency
components by the fuel cell. This is
achieved by a filter applied to the
measured load current (Figure 11).
The difference between both possible
bus control strategies is that, when it
is the fuel cell that commands the dc bus voltage [i.e.,
strategy one, Figure  11(a)], the converter associated with
the stored energy must be more dynamically effective
than the converter associated with the fuel cell. If it is less
effective, the fuel cell will also inevitably supply the highfrequency components to control the bus. For strategy
two [Figure 11(b)], the fuel cell will supply the low-frequency components only in the event its converter directly receives this instruction.

-2

0

50

100 150

200 250
Time (s)

300

350 400

Figure 9. The change in stored energy voltage in resizing for the

Figure 10. The power sharing obtained via simulation for the weight-

case in question (Table 1).

and consumption-optimized system.

I E E E E l e c t r i f i cati o n M a gaz ine / DECEMBER 2017



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

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