IEEE Electrification Magazine - December 2019 - 96

Power (kW)
Power (kW)
Power (kW)
Voltage (Vdc)

Load

90
60
30
0

100

200

300

400

500

600

PDG

30
20
10
0

100

200

300
Pbatt.

400

500

600

100

200

300

400

500

600

300
Time (s)

400

500

600

40
20
0
590
Voltage Droop

560
530

100

200

better fuel efficiency based on DG's SFC map. Figure 6
demonstrates the example of unequal load sharing
between DGs and the battery. The left side, which is denoted as "-ve," represents the charging region of the battery.

Fuel Consumption
Fuel consumption is estimated based on the engine's
SFC map, which is provided by the engine manufacturers. Figure 7 shows the curve fitting of a Perkins
2506C with a rated capacity of 460 kW, taken from the
experimental data. Unlike an ac system, the variable
speed of DG in the dc system is proven to have better
fuel consumption based on the SFC curve. Due to the
absence of frequency in the dc system, the engine
speed can be varied. Therefore, the minimum SFC can
be achieved by varying the engine speed to the optimal point. Moreover, DGs are built in such a way to
have high efficiency when connected to a 70-90%
load factor.
Lower fuel consumption can be achieved by using strategic loading to the DG. For example, during low-load conditions, the battery can be discharged while the DG is off.
When the load is high, the battery can share the load with
the DG to avoid another DG to be dispatched with a lowload factor. Furthermore, during low-load conditions and
when the battery has a low state of charge (SOC), DG can
serve the load and charge the battery. To shift the DG's
operating point to the lowest SFC, excess power from the
generator can charge the battery.

I E E E E l e c t r i f i cati o n M agaz ine / DECEMBER 2019

Power and energy management in Figure 8 demonstrates the classification of the control strategy of the HPS
lab. Due to the similarity of a shipboard power system and
a stand-alone microgrid, a similar hierarchical control
scheme can be implemented. The lowest control level,
called primary control, will perform a local control, i.e. voltage deviation or load power-sharing (such as droop or isochronous mode). The next level, called secondary control
(power management system), performs the voltage restoration introduced by primary control and performs control
action for ensuring the security of the system in meeting
the load demand. The highest control level, called tertiary
control, has more responsibilities to provide the power reference based on optimal strategy or power scheduling,
including the rule-based, equivalent consumption minimization strategy (ECMS), optimization algorithm and so
on. At this level, the control strategy can be done by
including specific optimization objectives (e.g., minimizing fuel costs or reducing emissions) and by considering
several technical constraints (battery SOC limit, generator
ramp-rate limit, startup shut-down limit, and so on). All
individual control levels are connected to the secondary
and tertiary control by the communication signal, which
carries all of the measurements from the sensors.

Experimental Verification
Figure 9 shows a sample of experimental results conducted in the HPS lab for the testing of the ship power
management, with a predefined rule-based strategy in

IEEE Electrification Magazine - December 2019

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