Vector-Controlled Cycloconverter Drive for Icebreaker

IEEE TRANSACTIONS
ON INDUSTRY
APPLICATIONS,
VOL.
IA-23,
NO.
6,
NOVEMBER/DECEMBER
1987
A
Vector-Controlled
Cycloconverter
Drive
for
an
Icebreaker
WALTER
A.
HILL,
RICHARD
A.
TURTON,
ROBERT
J.
DUNGAN,
AND
C. LOUIS
SCHWALM
Abstract-A
high-power
high-performance
variable-speed
drive
system
using a
cycloconverter-fed
synchronous
motor
is described.
The
control
uses the
field-orientation
principle
to
give
the drive
system
good
steady-
state and dynamic
performance
criteria.
The
accuracy
of the
field
orientation
is
improved
with the
use of current
regulators
operating
in
the
airgap
flux
reference
frame.
The
system
was
manufactured
to
deliver
16
000
hp to
the
twin propeller
shafts
of a Canadian
Coast
Guard
icebreaker.
The
electric
components
of
the
system
were tested
as a
unit
in
the factory
and
the desired
system
performance
was achieved.
I. INTRODUCTION
THE
use
of
cycloconverter-fed
synchronous
motor
drive
systems
is
increasing
in
high-power
low-speed
applications
such as grinding
mills,
rolling
mills,
and
ship
drives.
These drives
have
become attractive
because
of the
robust
construction
of
the
synchronous
machine
and
the
excellent dynamic
response
that
can be obtained
when
these
machines
are fed
by
cycloconverters.
Since
these
drives
are
expected
to offer
the same
fast
dynamic
response
as
variable-
speed
dc drives, provision
must
be made to
control
the
electrical
torque
quickly
in
proportion
to
a
torque
reference
generated
by
the
speed
controller.
The same
applies
to
magnetization.
Both
control
operations
must be
decoupled
so
that one
must
influence
only
the
torque
and active
power
while
the other
must
influence
only
the
magnetization
and
the
reactive power.
The
vector
control,
which
operates
on the
principle
of
field
orientation,
attempts
to
decouple
the
nonlinear
characteristics
of the
synchronous
motor and
give
the
ac
drive characteristics
normally
associated
with variable-voltage
dc drives.
Since
the
torque
of the
synchronous
machine
is
proportional
to
the
cross
product
of the
air
gap
flux and
the stator
current,
the
field-
oriented
vector
control system
tries
to
impress
the
stator
current
in such
a
manner
so that
it is
in
quadrature
with
the air
gap
flux.
Fig.
I
shows
the vector
diagram
of the current
and
flux
components
in the
various
reference
systems.
The
three-phase
stator
currents
can either
be
represented
by
three
vectors
in the
abc
reference frame
or can
be transformed
into
the
two-phase
alpha-beta
reference
frame.
Both
of these
reference
frames
are
called stationary
reference
frames because they
are related
to
Paper
IPCSD 87-13, approved
by the
Inidustrial
Drives
Committee
of the
IEEE Industry Applications
Society
for presentation
at the
1986
Industry
Applications
Society
Annual Meeting,
Denver, CO,
September
28-October
3.
Manuscript
released
for
publication
April
1.
1987.
The authors are
with the Motor and
Drives
Department,
Canadi;
n General
Electric
Company.
107
Park
Street
North,
Building
2/3,
Peterborough,
ON,
Canada
K9J
7B5.
IEEE
Log
Number 8715780.
TORQUE
T
M
C.FLUX
phnse
cx'
AI
phose
e
Fig.
1
Vector
diagram
of current
and
flux component.
phase
a
of
the stator
and the
stator
currents appear
as
sinusoidally
varying
quantities.
The two-phase
alpha-beta
stator
currents can
either
be resolved
into
two
components
lying
along
the
direct
(d)
and
quadrature
(q)
axis of
the rotor
or into two components
lying
along
the
magnetizing
(M)
and
torque
(T)
axis of
the
synchronous
machine.
The last two
reference
frames
are
known as
synchronous
or
rotating
reference
frames and
allow
us
to
view the stator
currents
and
fluxes as
dc quantities.
The angle
alpha
(a)
is the
angular
displacement
between
the
rotor
and
phase
a of
the stator,
while
the
angle
gamma
(y)
is
the
angle
between
the
flux
vector
and
phase
a.
The difference
between
the last two
angles
is
proportional
to
the load
of
the
machine
and
is
known
as
the load
angle
delta
(6).
The difficulties
with applying
the
field-orientation
strategy
occur
in
detecting
the
magnitude
and
angle
of the
air
gap
flux
vector
at
the
low end
of
the speed
range
and
enforcing
the
stator
current
near
the
top speed of
the
machine
so that
it
remains
in
quadrature
with
the
air gap
flux.
II. POWER SYSTEM
Fig.
2 shows a one-line diagram
of
the ship
propulsion
system,
which is
powered
by
three
marine
diesel
engines
driving
three 6250 kVA
synchronous
generators.
Depending
on the
load,
one,
two, or three
of
these
synchronous
generators
are connected
to the
4160-V
bus,
which
feeds
the
cycloconverter
transformers.
The
ship is
driven by two
8000-
hp
145-180
r/min
14.5-18
Hz
synchronous
motors,
which
feature brushless
excitation.
Each
of
the
two
cycloconverters
feeding
the
synchronous
motors
consists
of three
antiparallel
six-pulse
thyristor
bridges,
which feature
optically
coupled
firing
and
operate
without
circulating
current.
The use
of
two
77-mm 4.5-kV
thyristor
cells
in
parallel
for
each
leg
of the
0093-9994/87/
1100-1036$01.00
©
1987
IEEE
1036

HILL el
al.:
VECTOR-CONTROLLED CYCLOCONVERTER
FOR
ICEBREAKER
Fig.
2 Power
system.
Fig.
3 Vector
control
system
block
diagram.
bridge
allows
a
fuseless
thyristor
bridge
design.
Converter
faults are
cleared
by
suppression
of
thyristor
gating
with
subsequent
automatic
restart of
gating
after
the fault is cleared.
Three
consecutive
faults within a
preset
time
period
causes the
removal of all
gating
pulses and
forces the
operator to
manually
reset
the
drive.
A
log of all
faults is
kept and
displayed
on a
terminal
attached to
the
drive
control.
III.
CONTROL
SYSTEM
STRUCTURE
A
functional
block
diagram
of
the
field-oriented
vector
control for
the
synchronous
motor is
shown in
Fig.
3. The
basic
structure
of
the
speed
control
loop
is similar to
that
of
a
dc drive with
the
output
of
the
speed
regulator
becoming
the
torque
reference.
Note
that the
speed
reference and
torque
limit,
as
well
as the
enabling
signal,
come from
two
programmable controllers that
interface each drive to
the
ship's
load
management
system.
The vector
rotator block
in
cascade with the
speed
regulator
converts the
stationary
flux-oriented current references
into
sinusoidal
stator-oriented current
references. The
instantane-
ous stator flux
angle
must be
acquired
and
shifted 900 forward
so that
flux-oriented current
references are transformed into
1037

IEEE
TRANSACTIONS ON
INDUSTRY APPLICATIONS. VOL.
IA-23.
NO. 6.
NOVEMBER/DECEMBER
1987
sinusoidal stator current
references that are in phase
with the
stator voltage. Since the
frequency of these sinusoidal
current
references is proportional
to rotor
speed,
the synchronous
machine is said to be self-clocked
and not dependent
on the
supply frequency.
A. Current Control Loops
The three stator current
regulators are proportional
control-
lers with a bandwidth of
200-300 rad/s. Since this bandwidth
is not sufficient to ensure
tracking accuracy for
current
reference frequencies above
12 Hz, two current regulators
operating
in
the magnetizing
and torque axes reference
frame
are used
to
augment the
stator current references so
that the
combined system is capable
of tracking frequencies
above the
design frequency
of
18 Hz.
The function of the
two current regulators ahead
of the
current reference rotator
is to control the magnetizing
and
torque-producing components
of stator current
(Im
and It) to
be
proportional to the
magnetizing reference and
torque
reference,
respectively.
Since it is most advantageous
to
operate
the
cycloconverter
at unity power factor
[1],
the
magnetizing
current
reference Imr
is set
to
zero
and the
magnetizing (Im) current
regulator controls the phase
of the
stator current
reference
vector
so as to
regulate
unity power
factor.
The
torque reference
coming
from the
output
of
the
speed
regulator
is
used
as a reference
to
the torque (It)
current
regulator.
In
addition,
the torque reference
is
fed forward
to be
summed
with the
output
of
the torque
current
regulator.
This
allows
the
stator current
regulators
to react
quickly
to
any
change
in
torque
reference
while the
slower
torque
current
regulator provides
the accuracy
that is inherent
in
any
proportional-integral
controller.
Note
that
the voltage
command feeds the calculated
three-
phase voltage of the synchronous
machine directly to
the
input
of
each
cycloconverter,
where
it
is summed with
the
output
of
its
respective
stator
current
regulator.
The
net
result
of
this
action is
to
reduce
the interaction between
the
magnetizing-
and
torque-current regulators
[2].
IV.
FLUX SENSING
The
basic task
of the
vector control
is to
determine the
magnitude
of the stator
flux and
the
angle between
stator flux
and stator
current,
which
is
the
field-orientation angle.
Two
indirect
flux-sensing
methods
are used in this control
system:
the
current-based method and
the
voltage-based
method.
The
current-based
method
relies
on
the
dq
model of the
synchro-
nous
machine and derives the
flux
and the associated
field-
oriented
angle
from a
knowledge
of the
stator
currents, the
rotor
current, and
the machine parameters as well as
the rotor
position. Although
this
method is
parameter-sensitive
[3], it
does work
throughout
the
complete speed range
including
zero
speed.
The
voltage-based
method
requires
a
knowledge
of
the
stator
voltages
and currents to
calculate the stator
airgap
voltages of the machine.
The integrated airgap voltages
are
proportional
to
the
product
of the flux and the sine and
cosine
of the
field-orientation
angle.
The
magnitude
of the flux and
the field-orientation
angle then can be calculated by the
vector
analyzer using
basic trigonometric [41 formulae.
The limitation
of the
voltage-based
method
is that the stator
voltages disappear
near zero
speed,
causing
large errors of
flux and making
this method unusable below
some
minimun
speed.
The control strategy employed is to
use the current-
based model to
calculate flux and the
field-orientation
angle in
the
speed
ranige
from 0-10 percent (0-2 Hz),
and to use the
voltage-based method
for the rest of the speed
range.
V.
FL(UX REGULATOR
Brushless excitation
eliminates the
need
for
slip rings
on all
the
synchronous
machines.
Even
though
this
reduces the
bandwidth
of the
synchronous
motor excitation
system,
the
relatively
slow acceleration
of
less than
2.5 Hz/s inherent in
this
application
allows
the use of a brushless excitation
system.
The magnitude
of
the
flux,
which was
indirectly
measured
by
the flux-sensing
system,
is
subtracted
from the flux reference
and
forms the error
signal
for the
proportional-integral
flux
regulator. The
output
of the flux
regulator
becomes a
reference
to
the
stator current
regulator
of
the
ac
brushless
exciter.
The rotor
current of the
brushless exciter
(which
operates
as
an
induction
generator)
is rectified
to
provide
excitation
to
the
field
of
the
synchronous
motor,
even at zero
speed.
VI. VECTOR CONTROL
IMPLEMENTATION
The
vector
control
function,
which
requires
extensive
mathematical
calculations,
is
implemented
by
a 16-bit
micro-
computer.
The
vector control
microcomputer
sends
three
stator
current
and
three stator
voltage
references
to
the three
8-
bit
microcomputers,
which
regulate
the three stator
currents
and
provide
the
phase
control
to
three
reversing six-pulse
thyristor bridges,
which make
up
the
cycloconverter.
Each of
the
three
stator current
regulator microprocessors
has its
own
interface card,
which
allows
it to
monitor the stator current
and
output
firing pulses
to the
thyristor
bridge
via an
optical
link.
The 16-bit
microcomputer
has its
own
digital
and
analog
input
and
output
cards,
which allow it to communicate with
the drive
position,
current,
and
voltage
sensors,
as well as
to
provide
an interface
to
the
programmable
controller,
which in
turn
provides
the
speed
and
torque
limit references
to the
drive.
The
programmable
controller
also sends
digital signals
to
enable
the
drive
as
well
receiving digital
signals indicating
drive
status.
The
four
microprocessors
and their interface
cards
are housed
in
a
breadbox-size
card
cage
at one end of the
cycloconverter
lineup.
VII. DISCUSSION
OF
RESULTS
Prior
to
implementation
of the full-scale
system,
the
system
was tested
on
a
15-kW
synchronous
motor in the
laboratory.
The
load
for
the
laboratory
machine consisted
of
a
dc
motor
fed from a
reversing
dc
thyristor
converter.
In
addition,
a
digital
simulation
of the
cycloconverter system
was carried out
to find out the basic characteristics
of the vector
control.
Although
the
simulation
helped
visualize
the drive
responses,
not
all of
the
converter
control
could
be
simulated
with
1038

HILL
et
al.:
VECTOR-CONTROLLED CYCLOCONVERTER
FOR ICEBREAKER
sufficient
accuracy for
implementation.
The
laboratory
setup
was
useful
in
pointing
out
the
requirements that the vector
control made on
the stator current
regulators
and
the
power
conversion
equipment.
The crucial
link
in
the
vector
control
is
the
response of
the
stator current
regulators to sinusoidal
0-20
Hz
references. If
the stator current
regulators
can track a
20-Hz
signal
without
any
significant
phase
and
magnitude
error,
then the
elegant
vector control
strategy
works
very
well. Because there are
three
stator
current regulators and
only
two
independent
stator
current
paths,
the
system
is
interactive
and the bandwidth
of
the stator
current
regulators
is
limited
to below 300
rad/s.
Although
fine-tuning
of the
voltage
commands
can
minimize
the
tracking
error of the
stator current
regulators
in
the
upper
part
of
the
speed
range,
the inaccuracies of
the stator current
regulators
are such
that
they require
the
assistance
of
two
current
regulators, which
operate
in the
synchronous
reference
frame. The
magnetizing
(Im) current
regulator
provides
sufficient
phaseshift
to the current
command
signal
to
enable
the
proportional
stator
current
regulators to enforce the
phase
of the stator
current. The
torque (It) current
regulator
appears
to
change
the
magnitude
of the
current reference
signal
to
enforce the
magnitude
of
the
stator
current.
To
improve
the
system
power
factor,
the
cycloconverter
drive
operates
in
trapezoidal
mode
when
the
frequency
is
above the base
value
of
14.5 Hz. At
this
point
the
output
voltage
of the
cycloconverter
is no
longer
sinusoidal but
exhibits flat
tops
that resemble
trapezoids.
In
thyristor
converters
the
advance limit
is
always higher
than the retard
limit
of
the
converter,
which causes
more severe
trapezoidal
operation
in
regenerating
than
in
motoring.
Although
trapezoi-
dal
operation
is
attractive
from a
power
systems
point
of
view,
it
does
lower the
gain and
the
effectiveness
of
the
stator
current
regulators, which in
turn
makes
it more
difficult for the
vector
control to enforce
the desired stator
current.
The
soft nature
of the
ship's
power
system
makes it
impossible
to utilize
all the
fast
dynamic characteristics
of
the
vector control.
Since the
ship's diesel
engines have
only
limited
regenerating
capabilities,
the
crash reversal rate
is
limited to
2
Hz/s,
which
gives
the
ship
a
top-to-top speed
reversal time
of
approximately 18
s.
Fig. 4
shows
chart
recorder traces
of a
crash
reversal done
during system
tests
in
the
factory,
where
the
complete system
was
tested.
Only
the
prime
movers
were
missing;
their
place
was
taken
by a
3000-
hp
variable-speed
dc
drive,
which
supplied
the
losses
of
the
back-to-back
coupled
cycloconverter
drives.
Fig.
5
is
a
photograph
of the
systems
test
setup in
the
factory.
Extensive
customer
acceptance tests were
conducted
at
the
factory,
which
included
24 h of
heat
runs at
1
per-unit
load,
2 h
at 1.1
per-unit
load,
and 1.5
per-unit load for
1
min.
The
equipment
included two out of
three
synchronous
genera-
tors,
two
sets of
cycloconverter
transformers, the
two cyclo-
converters,
and the
two
coupled
synchronous
motors.
To
demonstrate
the fast
torque
response of
the
vector
control,
a
torque
reference
step
was
applied to the
cyclocon-
verter, and the
resulting
response in
torque-producing
current
is
shown in
Fig. 6.
Note
that some
of the
apparent
time
delay
between
the
change
in
torque reference
(It*)
and the
change
in
Fig.
4
Recordings
of crash reversal
tests.
torque
current
(It)
is
caused
by
delays
in
AID
and
D/A
conversions that
are
necessary
to
display
signals
that
exist
in
the
microprocessor.
The
stator
current
Ia,
which
is
an
analog
signal,
shows
a
response
time of
less
than 10
ms,
while
the
torque
current
feedback
signal
(It) shows a
dead time of 10 ms
followed
by
a rise
time of 10
ms.
VIII.
CONCLUSION
The
dynamic
and
steady-state
performance of the
cyclocon-
verter
drive is
equivalent to
that of
a dc
drive of
similar
size.
The
maintainability
of the
synchronous
machines should
be
better than the dc
machines that
they replace.
Although
each
cycloconverter
consists
of three
antiparallel
thyristor
con-
verters,
the total number of
thyristor
cells and
gating
circuits
is
similar to
a
dc drive
converter
of the
same
rating.
Extensive
use
of
microcomputers
reduces
the
hardware
required
for
the
vector
and converter control
to
a
fraction
of what would
have
been
required
in
an
analog
implementation.
The
diagnostics
provided
in
the
digital
drive
make
it
easier
to tune
up
and
maintain.
1039

IEEE
TRANSACTIONS
ON
INDUSTRY
APPLICATIONS,
VOL.
IA-23,
NO.
6, NOVEMBER/DECEMBER
1987
-
-_
t
Fig.
5 Systems
test
site.
The
power
factor
of
the
cycloconverter
drive
should
be
equivalent
to
the
dc
drive
when
operating
in
trapezoidal
mode
above
base
speed.
V.
NOMENCLATURE
I.
Flux
1.
Time
msec.
Fig.
6
Current
regulator
response
to
torque
relerence
steps
at
13.6
Hz.
Va
Phase-a
stator
voltage.
Vo
Phase-:
stator
voltage.
V,
Magnetizing
voltage
command.
V*
Torque
voltage
command.
Vb*
Phase-b
stator
voltage
command.
I,
Phase-a
stator
current.
Id
Direct-axis
stator
current.
Iq
Quadrature-axis
stator
current.
Ie
Exciter
stator
current.
IT
Stator
current
resolved
onto
torque
axis.
IM
Stator
current
resolved
onto
flux
axis.
IMr
Magnetizing
current
reference.
I,
Phase-a
stator
current.
{d
Direct-axis
flux
linkage.
{q
Quadrature-axis
flux
linkage.
ika
Airgap
flux
*
cos -y.
{:
Airgap
flux
*
sin
-y.
|;|
Airgap
flux
magnitude.
a
Rotor
position.
-y
Field
orientation
angle,
or
angle
between
flux
vector
and
phase
a
of
stator.
Wr
Rotor
angular
velocity.
K,
Synchronous
machine
voltage
constant.
1
040