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Page 6 of 15 Li et al. Complex Eng Syst 2023;3:1 https://dx.doi.org/10.20517/ces.2022.46

3.1. Harbor AC microgrid
The offshore wind power is connected to the onshore power grid. When charging, the ship is connected to
shore power through AC/DC converter, and all the energy is transmitted to the ship through AC
connection. The AC/DC converter, which charges the battery, is placed on the boat in a similar way to an
on-board electric car charger. For small battery-powered fishing and recreational boats, shore-charging with
a standard three-phase 400 V AC plug is the most common solution, which is also common in industrial
environments. However, dedicated infrastructure should be built for passenger or car ferries that need more
electricity to recharge the batteries on board. The power rating required for port infrastructure may vary
depending on the number of ships calling at the port and the capacity of the batteries on board.

[26]
Karimi et al. described a charging system based on AC shore power . Figure 1A shows a DC marine
propulsion system based on AC shore power. In addition to the grid interface, there is a fixed battery
storage system, which is usually slowly charged from the grid. Charging at night or off-peak hours not only
reduces the strain on the local grid but also takes advantage of cheaper electricity. Transformer T12 is a
50 Hz transformer that reduces the grid voltage to the shore bus voltage and isolates the shore bus from the
grid. The converter C15 acts as a charger and is responsible for rectifying the energy received from the
shore. The C12 converter is directly connected to the onboard battery B11 to control the transfer of power
during charge and discharge. Similarly, the converter C17 controls the energy of the onshore battery B12. In
addition, the converter C16 acts as a rectifier when the onshore battery is charged and an inverter when the
shipboard battery is charged. To reduce costs, some onboard propulsion systems have battery packs
connected directly to the main bus, eliminating the need for a DC/DC converter. At this point, C15 controls
the charging power. However, for multi-bus propulsion systems, each bus should have a dedicated charging
converter, such as the C15, which controls the charging power balance of each onboard battery pack.
Figure 1B shows the AC ship propulsion system based on AC shore power. The shore charging system is the
same as that in (a), but it is connected to the AC propulsion system. An AC charging solution for an AC
propulsion system requires synchronizing the voltage, phase, and frequency of the onboard power system to
the onshore grid prior to connection. To avoid the synchronization process and to shorten the time, a
dedicated active or passive rectifier, C27, can be used instead of a converter, C21. For the system in
Figure 1B, a shipboard transformer can be added to provide current isolation or to adjust the voltage
between the AC grid and the shore bus, but adding on-board transformers would result in higher costs and
lower energy efficiency.

3.2. Harbor DC microgrid
At present, the mainstream ships use low-voltage AC power supply, and its shore power system also uses
low-voltage AC systems. However, with the continuous enhancement of the functions of various types of
ships and the increase of electrical equipment, medium pressure ships will become the main trend of ships
in the future. The ship integrated power system represents the future development direction of the ship
power system, and its power transmission and distribution subsystem adopts the medium voltage DC
distribution network.

Compared to AC microgrids, DC grids have greater reliability and efficiency, simpler controls, and natural
interfaces with renewable energy sources, as well as electronic load and energy storage systems.
Dragicevic et al. studied typical power supply hardware topologies and their applicability to different
emerging smart grid applications, and outlined the current state of development of DC MG protection and
grounding technologies, but did not solve the problem of arc not being naturally extinguished due to DC
current breakdown .
[27]