IEEE Electrification Magazine - December 2019 - 9

buck converter is applied for stepping
down the voltage, whereas a boost
steps it up. A bidirectional converter
(buck-boost) provides the flexibility
to step down and step up the output
voltage. Shipboard energy-storage
devices (for example, batteries and
supercapacitors) supply power to the
grid during the discharging phase
and consume power during the charg-
ing cycle. A bidirectional buck-boost
converter controls the power flow to
the storage system during charging
and discharging.

Conventional Onboard ac
Power Systems
To date, the conventional ac system
has dominated the power system
topology, including on ships. Most of
the major electric-energy converters,
electric generators, and motors use it.
The ac system has some complexi-
ties that are due to reactive power,
synchronization between multiple
sources and loads, and so on. It is
most commonly used in marine elec-
trification because of its sophisticat-
ed safety and robust control systems,
and it prevails in high-voltage sys-
tems. For the most part, the voltage
and frequency in the ac power grid
do not fluctuate much with the load;
however, that synchronicity requires
proper coordination between the
control systems. Highly nonlinear
loads are troublesome, as they create
harmonic distortions.
Various converters are used in ac
systems based on different power
grid topologies. In general, the ac sys-
tem has major sources producing its
power. However, it can incorporate
hybrid energy sources, including bat-
tery banks and fuel cells, which pro-
duce dc power. In that case, inverters
are employed to convert the dc
power to ac. VSDs are frequently
used in modern electric propulsion,
which often consists of a rectifier,
inverter, and dc link. Diode- and
thyristor-based rectifiers are com-
mon. Voltage levels can be man-
aged in thyristor-based rectifiers by

controlling the firing angle. Diode-
A high switching frequency is re-
rectifier topologies with higher pulse
quired to generate a sinusoidal cur-
rent. Square-wave VSIs operate in the
orders (12, 18, and 24) reduce har-
monics. Active front-end PWM con-
square-wave mode, and thus, phase-
controlled thyristors are employed
verters are incorporated into the
four-quadrant operation. They in-
to control the output voltage. Phase-
clude self-commuted IGBTs (the
controlled thyristors possess low-
active line module) and a clean power
order harmonics that result in signifi-
cant harmonic currents, a high torque,
filter (the active interface module).
and speed ripples at
The clean power fil-
lower velocities.
ter, which is installed
The development of
The CSI feeds cur-
between the active
the dc power system
rent that is created
line module and sup-
by a large inductor
ply systems, removes
has, once again,
at the output of the
the harmonics from
reached a high point,
phase-controlled thy-
the PWM voltage to
since ships use many
ristor rectifier. A dc
produce a sinusoidal
renewable energy
link inductor smooths
current that has min-
the current so that
imal harmonic effects
sources to meet
the line-side convert-
on the supply system.
stricter emissions
er acts as a source
Machines with a
regulations.
for the inverter mod-
wide range of power
ule. The motor speed
levels (spanning from
changes according to
watts to megawatts)
the inverter output-current's fre-
are fed through VSDs, whether in
high-performance or high-precision
quency, and the motor flux and
applications. The power electronic
torque are changed in relation to the
converter is the key drive component
dc link current's amplitude. The CSI
that enables the speed and position
finds its shipboard application in
control, decreases the load-generated
high-power propulsion, such as in
instabilities, and eventually saves a
cruise ships. Simplicity, reliability,
tremendous amount of energy. The
possibility to attain higher output fre-
basic requirements for VSDs include
quency than the supply frequency,
frequency adjustments according to
and quiet propulsion are some of its
the desired output speed, output
benefits, whereas high torque pulsa-
voltage adjustments to maintain a
tions and the need for harmonic fil-
constant air-gap flux in the constant-
ters are its limitations. Controlling the
torque region, and supply-rated
speed of synchronous and induction
current at any frequency. Commonly
machines can be done by using a
cycloconverter, which utilizes line-
used VSD converters can be classi-
frequency-commuted converters.
fied into
xx
VSC-based VSDs, which are PWM
Each phase consists of two back-to-
VS inverters (VSIs) with a diode
back, connected thyristors that con-
rectifier or square-wave VSIs with
vert the ac to ac directly, without
a thyristor rectifier
rectifying the voltage. The thyristors'
xx
CSC-based VSDs, CS inverters
firing angle is managed to produce a
(CSIs) with a thyristor rectifier
low-frequency sinusoidal output.
xx
cycloconverter-based VSDs.
The maximum output frequency is
A VSI with a PWM controls the
roughly one-third of the input ac fre-
magnitude and frequency of the out-
quency (approximately 20 Hz) to
put voltage. Hence, an uncontrolled
maintain a waveform with a low har-
rectifier can also be used for generat-
monic content, which enhances the
ing dc VS, although it imposes funda-
thyristors' application in gearless,
mental and higher-order harmonics.
direct-shaft drives.
	

IEEE Elec trific ation Magazine / D EC EM BE R 2 0 1 9

9
IEEE Electrification Magazine - December 2019

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