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POWER ELECTRONIC
CONVERTERS FOR SHIP
PROPULSION ELECTRIC MOTORS
Damir Radan
Marine Cybernetics-Energy
Management Systems
Part of the NTNU project All
Electric Ship
Department of Marine Technology,
NTNU
1.
Introduction
The predominant type of prime movers
for DP propulsion plants is the electric drive.
Practically every DP propulsion
device installed in newly constructed vessels as well as
in
most of the conversions is driven by
an electric motor.
In the beginning of DP technology
(which coincided with the advent of the DC/SCR
technology),
either AC motors were utilized
driving CP propellers at constant RPM or SCR controlled DC
motors
were utilized driving fixed-pitch
propellers at variable RPM.
In recent years, variable speed AC
drives have become available and have been used in some
applications for DP
propulsion.
The most commonly used motor drives
are:
• DC converters, or SCR
(Silicon Controlled Rectifier) for DC motors
• Cycloconverters (Cyclo) for
AC motors, normally for synchronous motors
• Current source inverter type
(CSI) converters for AC motors (synchronous motors)
• Voltage source inverter (VSI)
type converters for AC motors, i.e. asynchronous,
synchronous
and permanent magnet synchronous
motors.
Fig. 1. Variable speed drive,
showing a frequency converter with DC Link,
typically for VSI and CSI type
converters (Cyclos do not have DC link) [1]
2. AC Systems
with Controllable Pitch Propeller
• Several generators which feed
AC of constant frequency and voltage (4160 - 6000
VAC)
into a common bus.
• Constant-speed motor drive
motors drive the propeller
• Usually cage-type
induction motors and may be designed with pole-changing switches
to
allow for two operating
speeds
• Electrical
simplicity
• Highest efficiency at
design point (maximum load) of the electric drive
systems
• CP propeller is less
efficient than a fixed pitch propeller in partial load conditions -
power
drawn by a CPP at zero thrust is
approx. 20% of the rated power
• Electrical part of an
AC-CP system is a simple and reliable arrangement,
• But CP propeller is
considerably more complex than a fixedpitch propeller (FPP)
– also
unaccessible for routine maintenance
and requires drydocking of the vessel to gain access to
or removal of the
thruster
Fig. 2. Controllable Pitch
Propeller
•
CP propeller is considerably more
complex than a
fixedpitch propeller (FPP) –
also unaccessible for routine
maintenance and requires drydocking
of the vessel to gain
access to or removal of the
thruster
When started direct-on-line (DOL),
the induction motor has a large starting current
transient,
typically 5-7 times the nominal
current, with significant shaft torque transients and voltage
drops in
the network
minimum running generator capacity
often must be defined to be able to start a large
motor.
Stardelta switching is often used to
provide higher starting torque with reduced transients, but is
not
the best solution. Soft-starting
devices such as auto-transformers have been shown to give
better
results. Solid-state soft
starters are not commonly used for high power levels.
Fig. 3. Load characteristics for
a direct on line asynchronous
motor with load curves for a CPP
propeller [1]
Fig. 4. Load characteristics for
a
direct on line asynchronous
motor
Siemens
downloads
3. SCR
Controlled DC Drive and Fixed Pitch Propeller
• AC current produced by Diesel
generators at constant voltage (600 VDC max.) and
frequency.
• A fixed pitch propeller is
driven by the DC propulsion motors. Varying the propeller RPM
and
reversing the rotation direction of
the propeller shaft allow the thrust to be controlled in
magnitude and
direction.
• AC/DC propulsion combines the
highly efficient and reliable generation of AC current
with
the DC motor capability of producing
high torque at low speed, the feasibility of varying
the
characteristic by adjusting the
excitation, and easy reversing of the direction of
rotation.
• Full-bridge thyristor
rectifier (Silicon Controlled Rectifier = SCR) feeds
the DC motor with a
controlled armature (rotor winding)
current. The field winding (stator) is excited with a
regulated field
current.
Fig. 5. Full bridge thyristor DC
drive (SCR) [1],
[2]
Fig. 6. The (average) DC voltage
to motor is controlled by delaying
conduction of the thyristors with
the gate firing angle
<
?
(here,
?
=30 degrees), [1]
• Speed of the DC motor is
proportional to the DC armature voltage.
• The DC voltage on the
motor armature windings (rotor) is controlled by phase shifting
the
thyristors’ conduction
interval by the gate firing angle ?. The gate firing angle ? can
be
controlled from 0 to 180 degrees,
and the voltage on the armature windings can hence be
regulated from +1.35 to –1.35
V
s
(line voltage).
• The torque is controlled
accurately and with low ripple (if the armature inductance is high,
L),
but this, on the other hand, reduces
the dynamic performance since the time constant of the
armature increases.
• In practice, ? > 15
degrees, in order to ensure controllability of the motor drive
also with
voltage drops in the network, and ?
< 150 degrees to have a commutation margin.
• Power
factor
Since the armature current is
controlled by use of the firing angle of the thyristor devices,
the
AC currents will be phase-shifted
with respect to fundamental voltage (v
s
). In a DC motor
drive, where the speed is varying
from 0 to 100%, the power factor (PF) will also vary from
0
to 0.96. Theoreticaly, power factor
can be calculated by folowing equation [2]:
PF = (3 / ?) ·
cos
?
? 0.955 · cos
?
.
From equation follows that highest
value of power factor is obtained for zero speed (
?
= 180
0
or full speed
?
= 0
0
). Taking into consideration that ?
> 15 degrees we can not even
theoreticaly obtain PF higher than
0.92, i.e. PF < 0.92.
Fig. 7. Representation of power
factor (PF = cos
?
) [3]
Fig. 8. DC side voltage waveforms
assuming
zero AC side inductance
(L
s
=0) [3]
Fig. 9. Input line-current
waveforms assuming
zero AC side inductance
(L
s
=0) [3]
- PF=cos ? is the function of the
gate firing
angle ?
Disadvantages:
• The limitation of
voltage (maximum voltages are 600 VAC/750 VDC)
leads to heavy, expensive motors and
cable runs
• Commutator
wear
higher maintenance requirements of
the DC motors
• Practical limit for DC
motor drives is 2-3 MW
• The power factor will vary
from 0 to 0.92 (?=15degrees) – corespondant to 0 to
100%
propeller RPM.
• Power electronic equipment
requiring a clean and cool environment (important for
smaller
vessels)
• The presence of
electromagnetic interference (EMI)
• Undesirable response in
electronic equipment (such as engine and generator
controls,
instrumentation, navigation
equipment, engine room automation, computers, etc.)
Applications:
• Introduction of the high
current SCR in the late 1960s, the AC/DC electric propulsion drive
has
become quite popular.
• Vessels with this type of
propulsion system include fishing trawlers and factory
vessels,
research vessels,
icebreakers, offshore supply vessels, conventionally moored
and
dynamically positioned drill
vessels, and semisubmersibles.
Efficiency:
AC generators 97%
x
SCRs 98%
x
Propulsion DC motors
94%
(x reduction gear 98%)
-----------------------------------------
Total system efficiency 89%
(87%)
4. Ship
electrical propulsion
PODDED
PROPULSION
• Freely rotateable (azimuthing
through 360°) and may produce thrust in any
direction.
• Incorporates an electrical AC
motor mounted directly on the short propeller shaft, inside
a
sealed pod unit that is submerged
under the vessel hull.
• The motor drives a
fixed-pitch propeller (FPP).
• Controlled by a frequency
converter that converts a three-phase AC voltage of
constant
frequency into a variable
three-phase AC voltage with variable frequency.
• Torque is available in either
direction over the entire speed range (tipicaly from 0 to
300
RPM).
System
description:
• Variable speed drives has
been in industrial use since in many decades, but first at the end
of
the 1960’s by use of power
semiconductors. At the beginning, DC motors where the
most
feasible alternative for propulsion
control.
• During the 1980’s, AC
motor drives became industrially available, and
commercially
competitive.
• Since then, almost all new
deliveries of electric propulsion are based on one of the AC
drive
topologies.
• AC system generates medium
voltage AC (3.3 kVAC, 6.6 or 11 kVAC) at constant
frequency
and voltage.
• Controls the RPM of the drive
motor (induction or synchronous motors) by varying the
frequency of the
system.
a)
b) Radiance of the
Seas, 2 x 19,5 MW
Azipod® propulsion –
ABB
Marine
c) Application of podded
propulsion (higly improved
manouverability) ABB Marine
Fig. 10. Azipod power range and
application
a)
b)
Fig. 11. SIEMENS/SCHOTTEL PODED
PROPULSOR
Siemens
Marine
Fig. 12. Shaft line drive
configurations (in respect to redundancy) [4]
AZIMUTH
THRUSTERS
• Azimuthing thruster, is
powered from an in-board, typically a horizontal electro motor,
and
the mechanical power is transferred
to the propeller via a Z-shaped gear. The
underwater
shape is optimized for low
hydrodynamic resistance at higher ship velocity, for
higher
propulsion efficiency.
Z-type gear
transmission
vessels with limitation of in-board
height of the
thruster room, the electric motor
will normally be horizontal.
L-type gear transmition
– will normally be selected when the height in the
thruster
room allows for it simpler
construction with less power transmission losses,
vertically
mounted motors.
• Thrusters that can be rotated
in order to produce thrust in any direction.
• The thrust is controlled
either by constant speed and CPP design, variable speed
FPP
design, or in rare cases with
a combination of speed and pitch control. Variable -speed
FPP
designs has a significantly simpler
mechanical underwater construction with reduced low-
thrust losses compared to constant
speed, CPP propellers.
• Conventional azimuth
thrusters are at present (2002) in use with power ratings up to 6-7
MW.
ABB
Marine [1],
[4]
Contra rotating
propellers
Aquamaster CRP
Rolls-Royce
Twin-propellers
(rotating at the same
side)
Schottel
propulsors
Fig. 13. AZIMUTH
THRUSTERS
Fig. 14. Power distribution and
control networks onboard ship, ABB Marine
Fig. 15. Power distribution and
control networks
onboard FPSO vessel, ABB Marine
5. FREQUENCY
CONVERTERS
Three basic system configurations
are available for the variable frequency control:
Fig. 16. AC motor drives used in
marine applications, [5], ALSTOM
Power Conversion Ltd
315
2’000
P
(kW)
U
(kV)
27’000
5’000
6’000
40’000
30’000
Motor
Voltage
1.0
2.4
1.5
4.5
6.0
3.3
9’000
10’000
16’000
6.9
1.8
Cyclo-
converters
CSI
690
VSI
IGBT
!
VSI: Voltage
Source Inverters
with PWM or
DTC
CSI: Current
Source Inverters
with
Thyristors
Cyclo: Direct
Converter
with
Thyristors
!!
VSI
IGCT
or
IGBT
LV
MV
?
Fig. 17. AC motor drive
technology review, [4],
ABB
Marine
Fig. 18. Power flow and
efficiency of electric installation, [1], [4]
5.1.
CYCLOCONVERTER VARIABLE SPEED DRIVES
• The cycloconverter is an SCR
Converter System which converts a fixed frequency, fixed
voltage input into a variable
frequency, variable voltage output in a single stage without
the
need for a DC link and may be
used to power either synchronous or asynchronous motor.
• In marine applications, only
synchronous motors (AC motor with DC excitation) have
been
used with cycloconvertors.
Synchronous machines are preferred to cage induction
motors
(asynchronous machines) due to their
large air gap giving them a higher degree of robustness.
• Motor nominal voltage 1500V
or 1800V
• The Cycloconverter and
Current Source drives (Synchro, CSI, LSI) are direct descendants
of
DC drive technology and use the same
basic naturally commutated thyristor converters
(same 6 arm Graetz
bridges)
• Its major advantage is
high torque at low speeds with low torque pulsations and
excellent
dynamic response
performance:
applied as direct propeller drives
on modern icebreakers (possible to free a
propeller frozen in ice or to cut a
block of ice without stalling the motor)
in dynamic possitioning and
passenger vessel applications (not necessary) where
low speed / maneuvering performance
is essential
• Can inherently reverse and
regenerate
• Can easily provide large
overloads (e.g. 250% and field weakening)
• Multiple bridges give high
power ratings
• Ratings typically up to 30MW
pre drive motor, 500 RPM
Cycloconver
tutorial
Limitations:
• Output frequency is
limited to 30 to 40% of AC supply frequency (aprox. 20
Hz)
• Complex AC supply
effects
• Because of the phase control
modulation, the cycloconverter will always draw
lagging
reactive current, even if the
motor operates at unity power factor.
• The supply power factor
(PF) is motor voltage-dependent and is about 0.76
The installed kVA capacity would be
approximately 25 to 30% more than that
required for AC/DC
alternative.
A
B
C
Single phase
cycloconverter
P Converter N
Converter
Load
Fig. 21. 3-pulse half-wave
cycloconverter (not used in marine applications)
Fig. 22. 6-pulse cycloconverter -
ACS6000c – ABB
•
Direct AC to AC
converter
•
SCR Thyristor
•
Synchronous motor (AC motor with DC
excitation)
•
High power at low
speed
Fig. 19. Cycloconverter
with
6-pulse
configuration:
• 6 pulse circuit uses
three
anti-parallel
thyristor
bridges (green rectangle).
• Each bridge is the same as
used for a DC drive and
uses
6 forward (detail on
the
bottom of the figure) and
6
reverse
thyristors.
ALSTOM Power
Conversion Ltd
Fig. 20. Cycloconverter with
6-pulse configuration, [1],
[2], [4], ABB Marine
• Cycloconverter bridge
configuration is constructed of two 6 arm Graetz bridges connected
in
anti-parallel and supplying each
phase of a three phase machine
• The cycloconvertor
“constructs” the output voltage wave-form from
sampled portions of the
supply wave-form, in effect the
process is one of modulated phase control in which
the
supply side current harmonics
(I
sh
) depend upon the supply to load
frequency ratio (f
s
/ f
d
).
Fig. 23. 5.6 MW Water Cooled
Cycloconverter Construction, [5]
Cycloconverter drive
technology was ideally
suited to the extreme
requirements
(large powers at low speeds and high
dynamic
performance) of the
Icebreaker
Icebreaker example:
• twin shafts each rated at
11.2 MW,
• each shaft being powered by
two 5.6
MW Cycloconverters capable
of
providing 175% full load
torque
(FLT) for 30 seconds at zero
speed.
Fig. 24. Cycloconverter drive
applied on icebreaker, [5]
Fig. 24. Shuttle tanker
equiped with
• cyclo converter propeller
drives and
• silicon controlled rectifier
(SCR) drives for DC motor cargo & ballast
pumps
ABB
Marine
Special power electronics
animation-educational links [8]:
• Basic Thyristor Converter
• Single-Phase Full-Bridge
• Three-Phase Full-Bridge
5.2. SYNCHRO CONVERTER - CURRENT SOURCE
VARIABLE SPEED DRIVES
(Load Commutated
Inverter – LCI, Current Source Inverter - CSI)
• The line side
converter (naturally commutated AC/DC thyristor input
converter) takes power
from a constant frequency (60 Hz)
bus and produces a controlled DC voltage on so called DC
link, on the same way as SCR - DC
drive converter.
• Current flow in the line side
converter is controlled by adjusting the firing angle of the
input
bridge thyristors (line side
converter) and by natural commutation of the AC supply
line.
• The DC link inductor L
is used to smoothe the DC current I
d
, see figure bellow. It
effectively
turns the line side converter into a
current source converter (with I
d
? const., constant
output
current of line-side converter), as
seen by the machine side converter. As a result of the
action
of the link inductor L, such
an inverter is frequently termed a naturally commutated
current
source inverter
(CSI).
• The machine side
converter (output side converter) normally operates in the
inversion mode.
Inverter thyristors are commutated
by the synchronous motor induced voltage (emf).
• Pulse width modulation is not
possible to apply in this type of converter (because thyristors
are
only on-controllable), so the
inverter output current is composed by quasi square
wave,
generating a large amount of low
frequency current harmonics into the motor (5th and
7th),
increasing the losses and the
heating inside the machine [8].
• The motor speed is controlled
by changing the inverter output current frequency,
f
o
, while the
motor flux and torque are adjusted
by controlling the amplitude of the DC link current
I
dc
• At low motor speed a minimum
level of machine emf is required to ensure correct
commutation of the thyristors.
Hence, for operating speed lower than 10 % of the rated
value,
the method of “dc link
pulsing” is used to commutate inverter thyristors. This
method consists
of reducing the dc link current to
zero by temporarily operating the rectifier in the
inversion
mode. During this zero-current
interval, the previously conducting thyristors regain
their
blocking capability and the motor
current can be transfered from one inverter leg to the
other.
• In order to assure the
appropriate induced voltage at the motor terminals, which is necessary
to
turn off the inverter thyristors,
the synchronous motor must operate in the capacitive
mode,
that is with leading power
factor.
•
Regeneration
The drive power circuit is
inherently regenerative to the main supply system thus enabling
the
vessel or the thruster to be stopped
and reversed quickly. A dynamic braking resistors may be
required to execute dynamic braking
and they will convert the regenerated energy into the
heat.
The dynamic braking resistors are
water cooled.
Fig. 25. 6 Pulse Synchronous LCI
[1], [2]
• The thyristors of the
input bridge (line converter) are fired using natural
commutation and
are controlled to keep the current
at the required level in the DC link reactor.
• The thyristors of the
output bridge (load converter) are fired in step with
the rotation of the
motor and act as an
electronic commutator. This works by using the back emf of the
motor to
also give natural load
commutation of these thyristors.
• CSIs, also called current-fed
inverters, behave like a constant current generator,
producing
an almost square-wave of current.
This gives 6 steps of stator current per motor cycle,
see
figure for six-step
waveform.
Fig. 26. 12 Pulse controlled
rectifier with 6
pulse LCI) [9]
In marine applications LCI drives
with 12
pulse on both converters has been
used
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