IEEE Electrification - September 2020 - 60

Shore charging is a
great opportunity to
use a land-based
grid supported by
renewable energy
systems for powering
the propulsion of
marine vessels.

voltage and descending current. The
controller for such a dc-dc converter
is comprised of a power control
block, which ensures tracking the
reference power and includes an
inner current control loop, as well as
a PWM block. Similarly, for the
onboard battery pack, converter C32
controls the charging power. Thus, it
regulates the charging power when
the vessel is at berth and discharging power when the vessel is operating. The control mechanism can
be the same as for converter C36.

System-Level Control
To discuss the high-level control, it is better to study the
onboard and onshore controllers separately since they
can be highly dependent on the type of application. An
example of a high-level control scheme is depicted in
Figure 16. As mentioned in the previous sections,
onshore battery packs are recharged from the grid and
discharged into the onboard batteries, so there are four
modes of operation for a charging station: 1) charging
onshore batteries when no vessel is docked, 2) when a
vessel is docked; charging based only on power from
the grid, 3) transferring power to the ship only from the
onshore batteries, and 4) transferring power to the ship
from both the grid and the onshore batteries. The
onshore batteries are usually charged overnight with
low power or between ferry dockings with higher power.
Thus, in the charging station, an EMS and a power management system (PMS) are needed for generating the
references for the total charging power, the charging
and discharging power of the onshore battery bank, and
the power from the grid. Apart from the power flow,
controlling the voltage of the shore bus is another
objective for the onshore controllers. Furthermore, a

battery management system (BMS)
is typically used to perform battery
monitoring and battery cell balancing. The BMS communicates with
the EMS to operate the battery in a
safe and optimal manner.
When a ferry is at berth, onboard
charging control would send the
amount of required charging power,
so the onshore EMS should decide
the share of the grid power and
the onshore battery bank. Utilizing
the onshore battery reduces the
stress of handling high charging
power on the local grid and can
allow for reducing the total electricity costs by charging during off-peak hours. On the
other hand, drawing the charging power from the
onshore battery bank is less energy efficient than
using the grid because of the energy loss generated by
the additional power electronics converters used to
interface with the onshore battery and the battery
itself. In other words, using energy buffers such as
onshore battery packs generates additional energy loss
in the process of charging and discharging the onshore
batteries. Hence, the onshore EMS should choose the
optimal share of sources in terms of energy transfer
efficiency, power quality issues, and/or cost of energy
from the grid. In a smart charging station, the information from the port substation is considered for making
the decision of load sharing between onshore batteries
and the grid.
For instance, assume the required charging power to
be 800 kW for 10 min and that the ferry will come back at
berth for the next charging after 30 min. If the charging
power from the grid and the onshore battery pack are
200 kW and 600 kW, respectively, the onshore battery
pack can be recharged by the constant charging power of

Ship

Shore

Onboard Energy Management System

Onboard SoC

Onshore Energy Management System

Grid Integration

Power Demand
Onboard Power Management System

Onboard
Battery-Connected
Converter Control

Onboard
Battery
Management
System

Onboard
Bus Voltage
Start/Stop
Grant

Onshore Power Management System

Onshore
Battery-Connected
Converter Control

Figure 16. An example of the system-level control structure for a shore-to-ship charging system.

I E E E E l e c t r i f i cati o n M agaz ine / SEPTEMBER 2020

Onshore Battery
Management
System

Shore
Bus Control

IEEE Electrification - September 2020

Table of Contents for the Digital Edition of IEEE Electrification - September 2020