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AC- or DC-distribution for installations aboard ships
Increase in the use of
frequency converters
About 15 years ago, no one questioned the type
of power distribution. Most ships were equipped with
AC power distribution. The majority of the rotating
equipment aboard was driven by asynchronous
motors, which needed an alternating current to be able
to rotate.
DC distribution existed only in special cases, such as
in submarines. Now there is a tendency to choose DC
power distribution. Why should DC-distribution be
used
The answer to that question can be found in the increased
amount of frequency-controlled electrical equipment. 15
years ago the amount of frequency-controlled electrical
equipment was very limited. This was due to the fact that
the inverter was very new, expensive and not so reliable.
Today, inverters are commonly available and are reliable
components at a reasonable price. Frequency-controlled
electrical equipment is therefore now available to many
consumers. This has led to a strong increase in the use of
frequency converters on board, especially for ships that
have a full or partial frequency-controlled electric drive.
What is a frequency converter
The function of the frequency converter must be explained
to understand the reason for DC distribution.
Almost all frequency converters consist of a rectifier and
an inverter. The rectifier recovers the fixed AC and
converts it into DC. The inverter uses DC and creates a
variable alternating current by switching the elements of
the inverter in a fixed order.
There are two types of rectifiers: the diode front-end
rectifier and the active front-end rectifier. The diode
front-end rectifier is a static rectifier that simply rectifies
the AC voltage supplied. The diode front-end rectifier is
considered to be static because no active switching takes
place within the controller.
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The most basic diode front-end rectifier is a 6-pulse
rectifier. Diode front-end rectifiers create harmonic
distortion on the AC grid. As long as the harmonic distortion
remains below certain limits, there is no problem in
using this type of rectifiers. AC-filters can be used in front
of the diode front-end rectifier to reduce the harmonic
distortion. When the harmonic distortion level remains
too high, the number of pulses of the diode front-end
rectifier can be increased. This leads to more complex,
more expensive and larger installations. An active frontend
rectifier can also be used.
The diode front-end rectifier only allows energy flow from
the AC grid to the controlled device. This means that if the
device produces back power, such as in cranes or fixedshaft
drive lines, a brake chopper must be installed to
prevent damage to the inverter. The brake chopper converts
the back power into heat.
The active front-end rectifier uses switching elements in
such a way that almost a sine-wave current is obtained
from the AC grid. The active front-end rectifier needs an
LCL filter to obtain this.
It is clear that the active front-end rectifier is more expensive
than the 6-pulse diode front-end rectifier. The losses
are also slightly higher.
The active front-end rectifier can control the flow of
energy in both directions. This means that if back power
is expected, and if the auxiliary power consumption of
the vessel is large enough to consume this back power,
the rectifier does not require a brake chopper. The back
power is reused in the ship.
Since 12-pulse or higher pulse rectifiers lead to more
complex systems with transformers that have more
volume, weight and cost compared with active front-end
rectifiers, they are not considered as attractive alternatives.
The inverter is in principle the same as an active front-end
rectifier without the LCL-filter. The inverter creates a variable
frequency and voltage so that the electric motor will
rotate at the desired speed.
When do you apply DC-distribution
DC-distribution makes the use of standard inverters
possible without the large filters. Furthermore, DCdistribution
makes back power exchange possible and
thereby reduces the need for brake choppers. Other positive
effects of DC-distribution are: no problems with
harmonic current and achieving almost unity power
factor.
Can we conclude that DC-distribution is always the best
solution for a full or partial frequency regulated electric
drive The answer to that question is no.
The DC power has to be created. Standard auxiliary ship’s
generators produce alternating current, so a rectifier is
required between the generator and the DC-distribution
in order to produce DC power. As with the frequency
converter, this rectifier can be a diode front-end rectifier
or active front-end rectifier. If the frequency-regulated
electrical equipment draws about the same amount of
current as the current generated, there will be no advantage
in installation costs or volume. However, if the ship
contains frequency-regulated electrical equipment that is
not in simultaneous use, a gain in volume and investment
can be expected. In that case, the same active front-end
rectifier is used for several inverters.
Another reason to use DC-distribution is when variablespeed
auxiliary generators are used. Variable-speed
generators deliver alternating current, but not at a fixed
frequency. As a result, the auxiliary diesel engine can run
at its optimum working speed, which leads to a better
efficiency. Depending on the brand and type of diesel
engine, an improvement of up to 6% can be achieved. The
auxiliary diesel engine can often supply more power from
the same engine, since most engines are not designed for
generator operation, requiring a fixed speed (usually 1500
or 1800 rpm), but as a drive (for example, a speed range
between 600 and 2200 rpm).
If DC distribution is used, the AC generators do not require
synchronization. The converter between the AC generator
and DC distribution will adapt to the DC power supply.
If the voltage is at the correct level, the generator can be
connected to the distribution system. Since the converter
is very fast, this will happen even before the auxiliary
diesel engine is running at normal speed.
The frequency freedom for the auxiliary generators is
more than welcome for LNG powered auxiliary engines.
The reason is that LNG powered auxiliary engines have
poor dynamic response to load steps and can reduce in
speed faster than the allowable frequency decrease of the
AC net.
DC-distribution should be considered when the ship’s
distribution system needs a battery for energy storage.
Battery systems can either be connected directly to the
ship’s distribution system, or via a converter. When
connected directly to the ship’s batteries, the ship’s
distribution system must be DC. In other cases, both
direct and alternating current can be considered.
Technical problems with DC distribution
As described above, DC power distribution has a number
of advantages. However, it also has some disadvantages.
Although most electrical power is consumed as DC, the
rest of the ship is still needs AC power, for which converters
are needed. The required AC supply is produced
electronically by the static converter. Normally, two are
required for redundancy. This is necessary to ensure the
energy supply to vital equipment.
Both static converters operate simultaneously and can
work in parallel. However, this is not recommended. If a
short circuit occurs on the AC network, the di/dt can
increase to the point where both static converters are
disabled, even if the maximum converter current has
not been reached yet. Therefore the converters should
be connected to isolated AC nets. This does not apply if
rotating-converters are applied.
Another problem is the calculation of the short-circuit
levels of the DC distribution.
At present, the classification bodies offer no directive
on the calculation of short circuit levels for DC systems.
Lloyds is currently working on guidelines.
Further calculation of the DC short-circuit current is not
as easy as for AC distribution. This is mainly due to the
capacitors in the converters connected to the DC distribution
system and the very low resistance of the DC
distribution system. The DC voltage in combination with
the very low-resistance leads to high short-circuit
currents.
Alewijnse uses a simulation program for the correct calculation
of short circuit. The calculation of a short circuit
is used to prove that the DC distribution system is strong
enough to withstand short circuits.
Conclusion
As an alternative to AC distribution, DC distribution can
in certain circumstances offer significant advantages with
regard to lower investment and higher efficiency. This is
especially interesting for ships with frequency-controlled
electrical equipment. The design of a reliable and efficient
DC distribution system requires extensive calculation and
an understanding of the electrical system. Based on experience
with AC and DC distribution systems, Alewijnse can
help you to find the best electrical solution to meet your
specific needs and wishes.
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