IEEE Electrification - September 2020 - 29

and SOx are converted into fine particles of sulfate and
nitrate aerosols.
To tackle emission issues, international organizations
and regional government started introducing regulations
and schemes. For reducing CO2, the International Maritime Organization adopted the so-called greenhouse gas
(GHG) initial strategy in 2018, envisaging a reduction in
total GHG emissions from global shipping by at least 50%
by 2050 compared to 2008. The European Union (EU) has
introduced the shipping monitoring, reporting, and verification regulation, which is designed to gather data on CO2
emissions based on ships' fuel consumption. Furthermore, the EU introduced the European Green Deal to put
Europe on track of zero GHG emission by 2050. In terms of
reducing SOx and NOx, the 0.5% global sulfur limit of maritime transport came into effect on 1 January 2020. The
Emission Control Areas, namely, the
Baltic Sea, North Sea, North America,
and the U.S. Caribbean, have also
deployed additional control of SOx
and NOx emissions. China has also
established the ship-generated air
pollutant emission control areas and
issued regulations to reduce the
emission of ship-generated air pollutants such as SOx, NOx, particulate
matters, and volatile organic compounds. The U.K. government has
introduced the Clean Maritime Plan,
requiring that all new and commercial vessels in U.K. waters achieve
zero emissions by 2025.
Restrictions imposed on ship
owners force them to invest in technologies on cutting emission. Removal
of SOx and of NOx from exhaust requires significant alterations onboard, including additional tanks, pipes, pumps,
water treatment systems, and storage of chemicals and special waste to be disposed at dedicated facilities. Scrubbers
removing sulfur from the engine exhaust increase the
power consumption, thereby increasing the emission of CO2.
Integrated electric propulsion (IEP) systems have
become a more welcome and beneficial approach to
reduce emissions due to their clear economic benefits in
fuel saving. IEP systems have successful records, including
use on a large cruise liner (Queen Elizabeth), smaller ferries,
large navy ships (the Type 45 destroyers from the Royal
Navy), and smaller oil-platform supply vessels.
The propulsion thruster in the IEP system is provided by electric motors, which are more efficient than
internal combustion engines (IEC) particularly at lowspeed operations. In addition, the bulky and heavy
gearbox and clutch used in IEC propulsion are eliminated. Continuous development and advancement of
power electronics-based motor drives have enabled
high dynamic performance and controllability of

propulsion thrusters. The mechanical transmission of
energy is replaced by electrical transmission, offering
more flexibilities in ship compartment design because
the rigid propeller shafts are replaced by flexible electric cables. The average fuel savings of the full IEP system is more than 30% of the IEC counterpart for the
same ship and mission profile. This incentive of fuel
savings, in conjunction with smaller footprint and
weight of the entire system, is convincing for ship owners to electrify their fleets. As a consequence, emissions
have been reduced, too.
To date, IEP systems for ships are dominated by the
ac bus system where the common connecting point is at
a fixed frequency (60 or 50 Hz) and a fixed voltage. The
ac bus is formed by ferromagnetic components such as
transformers, line reactors, and circuit breakers in the ac
switchboard, and only the conventional analog control can be applied.
The switch-mode power conversion
(digital control) only controls the
propulsion drives, and the ac bus is
controlled by analog means due to
its ferromagnetic nature. All diesel
generators (DGs) are directly connected to the ac bus and have to be
synchronized, which limits the controllability and adaptability of the
entire IEP system.
In contrast, the dc bus system is
naturally simpler than its ac counterpart thanks to the absence of frequency, phase angle, and reactive power.
The entire dc bus is fed by power electronics-based converters, requiring all
onboard loads and supplies, from the
DGs to propulsion to hotel loads, to be independently controlled by these converters. Therefore, the dc bus system
is a high semiconductor power system, which can be
entirely controlled by digital means.
In this article, the typical dc bus system for full IEP of
ships is introduced. Modular power electronics-based
power converters used in dc bus systems are described,
including the converter for the hotel loads and its special
fault ride-through ability. The advantages of the dc bus
system are then assessed using a real test case, the mission of the Shen Kuo research vessel (2,100-ton deadweight) from Hainan, China, to the Mariana Trench.

Integrated electric
propulsion systems
have become a more
welcome and
beneficial approach
to reduce emissions
due to their clear
economic benefits
in fuel saving.

Conventional ac Power Systems for Ships
A conventional ac bus system is detailed in Figure 1. The
shipboard electricity is generated by DGs, each consisting
of an ac generator coupled with a diesel engine as the
prime mover. All DGs are directly connected to the ac bus,
thus all generators need to be strictly synchronized. The
hotel loads are also synchronized with the DGs due to
direct connection.

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

IEEE Electrification - September 2020

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