IEEE Electrification - September 2020 - 53

scheduled ferries, in which charging time is critical, wireless charging also eliminates the need for connecting and
disconnecting plugs and receptacles, making the best use
of docking time to charge the batteries. In fact, the charging can be started as soon as the receiver side on the ship
is close enough to the sender side on shore. A simplified
model of an inductive charging for a ship with a dc main
bus is shown in Figure 3.
In inductive power transfer, transmitter and receiver
coils act like a transformer with a low mutual inductance.
The relatively low magnetic coupling results in a high
magnetizing current, so capacitive compensation networks (P51 and P52 for the example of series-series compensation in Figure 3) are used for generating the reactive
power consumed by the coils. Converter C56 generates a
high-frequency (several kilohertz) square-wave voltage for
the transmitter coil and C55 rectifies the high-frequency
output of the receiver coil. It is worth mentioning that, for
C56 and C55, a two-level voltage source converter and a
diode rectifier, respectively, can be used. Thus, similar converter designs like conventional ac-dc or dc-dc conversion
can be utilized, although the control strategy differs from
the other topologies.
As can be seen, transmitter and receiver coils provide
galvanic isolation, obviating the need for a dedicated
onboard transformer. All in all, the inductive charging system offers unparalleled advantages in terms of utilization of
the docking time for charging, especially in situations where
vessels are frequently berthed for short periods. Further,
because of enhanced available charging time in wireless

charging, the required power level for charging would
decrease, which will also help to limit infrastructure costs.
Although inductive charging offers unique benefits, it poses
a few challenges, for instance with respect to cost and
onboard weight. Furthermore, the achievable transfer efficiency is sensitive to the transmission distance and the
requirements for maintaining power transfer capability
under misalignment. Increasing the transmission frequency
and/or coil dimensions can improve the efficiency of the
power transfer. However, increased coil dimensions will
increase the weight and volume, while challenges with losses and thermal management limit the potential for increasing power density by increasing the operating frequency.

Battery-Swapping Methods
Replacing batteries has been considered as a rapid battery
refueling method, especially for electric heavy-duty trucks
and electric buses. It can be a suitable solution for shortdistance ferries, which have a critical docking time. In this
method, discharged onboard batteries are exchanged
with fully charged batteries while the vessel is at berth.
Regarding the power grid, the battery-swapping solution
can reduce the adverse impacts of charging stations on
the local power grid since onshore battery packs are not
being charged in a short time, rather they can be charged
at off-peak times with cheaper electricity or transferred to
a central station that may incorporate renewable energy
resources (e.g., solar, wind, and hydropower energies). In
other words, by using such a method, a peak load caused
by charging for a short time can be distributed into a

T51
C51
Main dc Bus
C53

C54

Aux
Load

C52

B51
+

Shore Bus
Grid
C57

C55

Coils

P52

P51

T52
-
+

C56
C58

Ship

B52

Shore

Figure 3. Inductive shore-to-ship charging for a dc-based propulsion system.

IEEE Elec trific ation Magazine / S EP T EM BE R 2 0 2 0

IEEE Electrification - September 2020

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