Transcript Slide 1

Electric Propulsion
and High Voltage
Practice
(Adapted from:D.T. Hall:Practical Marine Electrical Knowledge)
The earliest electric propulsion for ships
was demonstrated in Russia in 1832 with
a d.c. motor powered from a battery. In
1886 an electrically propelled vessel called
the Volta crossed the English Channel. By
1888 the improvements to batteries and
motors led to the first commercial
applications in passenger launches on the
River Thames in London.
 As with road transport, electric river boats
were soon eclipsed by the arrival of the
internal combustion engine.
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Electric propulsion for many new ships is
now re-established as the popular choice
where the motor thrust is governed by
electronic switching under computer
control.
 The high power required for electric
propulsion usually demands a high
voltage (HV) power plant with its
associated safety and testing procedures.
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Electric Propulsion Scheme
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Electric propulsion of ships has a long O ''
vbut somewhat chequered history. There
have been periods when it has enjoyed
popularity, with a significant number of
installations being undertaken, whilst at
other times it has been virtually ignored
as a drive system.
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Passenger ships have always been the largest
commercial vessels with electric propulsion
and, by their nature, the most glamorous.
This should not, however, obscure the fact
that a very wide variety of vessels have been,
and are, built with electric propulsion.
Early large passenger vessels employed the
turboelectric system which involves the use of
variable speed, and therefore variable
frequency, turbo-generator sets for the
supply of electric power to the propulsion
motors directly coupled to the propeller
shafts. Hence, the generator/motor system
was acting as a speed reducing transmission
system. Electric power for auxiliary ship
services required the use of separate
constant frequency generator sets.
A system that has generating sets which
can be used to provide power to both the
propulsion system and ship
 services has obvious advantages, but '
this would have to be a fixed voltage and
frequency system to satisfy the
requirements of the ship service loads.
The provision of high power variable
speed drives from a fixed voltage and
frequency supply has always presented
problems. Also, when the required
propulsion power was beyond the capacity
of a single d.c. motor there was the
complication of multiple motors per shaft.
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Developments in high power static converter
equipment have presented a very convenient
means of providing variable speed a.c. and d.c.
drives at the largest ratings likely to be required
in a / marine propulsion system.
The electric propulsion of ships requires electric
motors to drive the propellers and generator sets
to supply the electric power. It may seem rather
illogical to use electric generators, switchgear
and motors between the prime-movers (e.g.
diesel engines) and propeller when a gearbox or
length of shaft could be all that is required.
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There are obviously sound reasons why,
for some installations, it is possible to
justify the complication of electric
propulsion:
Flexibility of layout
Load diversity between ship service load
and propulsion
Economical part-load running
Ease of control
Low noise and vibration characteristics
Flexibility of layout
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The advantage of an electric transmission is that the
prime-movers, and their generators, are not
constrained to have any particular relationship with the
load as a cable run is a very versatile transmission
medium. In a ship propulsion system it is possible to
mount the diesel engines, gas turbines etc., in
locations best suited for them and their associated
services, so they can be remote from the propeller
shaft. Diesel generator sets in containers located on
the vessel main deck have been used to provide
propulsion power and some other vessels have had a
10 MW generator for ship propulsion duty mounted in a
block at the stern of the vessel above the ro-ro deck.
Another example of the flexibility provided by an electric
propulsion system is in a semi-submersible, with the
generators on the main deck and the propulsion motors in
the pontoons at the bottom of the support legs.
Load diversity
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Certain types of vessels have a requirement
for substantial amounts of electric power for
ship services when the demands of the
propulsion system are low. Tankers are one
instance of this situation and any vessel with
a substantial cargo discharging load also
qualifies. Passenger vessels have a
substantial electrical load which, although
relatively constant, does involve a significant
size of generator plant. There are advantages
in having a single central power generation
facility which can service the propulsion and
all other ship loads as required.
Economical part-load running
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Again this is a concept that is best achieved when there
is a central power generation system feeding propulsion
and ship services, with passenger vessels being a good
example.
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It is likely that a typical installation would have between
4-8 diesel generator sets and with parallel operation of
all the sets it becomes very easy to match the available
generating capacity to the load demand. In a four engine
installation for example, increasing the number of sets in
operation from two that are fully loaded to three
partially loaded will result in the three sets operating at
a 67% load factor which is not ideal but also not a
serious operating condition, It is not necessary to
operate generating sets at part-load to provide the spare
capacity to be able to cater for the sudden loss of a set,
because propulsion load reduction may be available
instantaneously, and in most vessels a short time
reduction in propulsion power does not constitute a
hazard.
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The propulsion regulator will continuously
monitor the present generator capability
and any generator overload will
immediately result in controlled power
limitation to the propulsion motors. During
manoeuvring, propulsion power
requirements are below system capacity
and failure of one generator is not likely to
present a hazardous situation.
Ease of control
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The widespread use of controllable pitch
propellers (cpp) has meant that the
control facilities that were so readily
available with electric drives are no longer
able to command the same premium.
Electric drives are capable of the most
exacting demands with regard to dynamic
performance which, in general, exceed by
a very wide margin anything that is
required of a ship propulsion system.
Low noise
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An electric motor is able to provide a drive
with very low vibration characteristics and
this is of importance in warships,
oceanographic survey vessels and cruise
ships where,/-for different reasons, a low
noise signature is required. With warships
and survey vessels it is noise into the
water which is the critical factor whilst
with cruise ships it is structure borne
noise and vibration to the passenger
spaces that has to be minimised.
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For very high power, the most favoured
option is to use a pair of high efficiency, high
voltage a.c. synchronous motors with fixed
pitch propellers (FPP) driven at variable
speed by frequency control from electronic
converters. A few installations have the
combination of controllable pitch propellers
(CPP) and a variable speed motor.
Low/medium power propulsion (1-5 MW)
may be delivered by a.c. induction motors
with variable frequency converters or by d.c.
motors with variable voltage converters.
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The prime-movers are conventionally constant
speed diesel engines driving a.c. generators to
give a fixed output frequency. Gas turbine driven
prime- movers for the generators are likely to
challenge the diesel option in the future.
Conventionally, the propeller drive shaft is
directly driven from the propulsion electric motor
(PEM) from inside the ship. From experience
obtained from smaller external drives, notably
from ice-breakers, some very large propulsion
motors are being fitted within rotating pods
mounted outside of the ship’s hull. These are
generally referred to as azipods , as the whole
pod unit can be rotated through 360° to apply
the thrust in any horizontal direction, i.e. in
azimuth. This means that a conventional steering
plate and stern side-thrusters are not required.
Ship manoeuvrability is significantly
enhanced by using azipods and the
external propulsion unit releases some
internal space for more cargo/passengers
while further reducing hull vibration.
 Gradual progress in the science and
application of superconductivity suggests
that future generators and motors could
be super-cooled to extremely low
temperatures to cause electrical resistance
to become zero.
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