Note: Descriptions are shown in the official language in which they were submitted.
43~7
The invention relates to a procedure for starting and reversing a
gas-turbine electric power-plant for an icebreaker.
Icebreakers have hitherto been equipped with direct-current drives.
However, in the case of ships designed for the future and having propeller-
shaft powers of 50,000 HP per shaft, the necessary direct-current machines
would be much too large, too heavy, and too expensive.
In the case of three-phase drives of conventional design for normal
ships, using a synchronous or asynchronous propeller motor with slipping
drive, starting torques of only up to about ôO% of the rated torque are
available form stop to the lowest possible frequency, this being determined
by the minimal r.p.m. of the gas-turbine which is between about 33 and 40%
of the rated r.p.m. Torques of about 130 to 160%, or more, of the rated
torque, required for icebreakers, cauld not be obtained without oversize
machines, but this again would make the machines too large and too expensive.
Icebreaking would be quite impossible without considerable oversizing since,
in normal designs, only a small number of starts per hour are admissible
because of high slip losses which cause thermal overloading of the machines.
However, sudden overloads, constant changing from operation in open water to
operation in ice, and a large number of starts and reverses are essential for
an icebreaker.
The following publications are mentioned in connection with the
state of the art:
- "Schiff und Hafen" 1967, Vol. 3, pages 155 et seq.
- "Investigation of gas-turbine-electric drives",
(STG Annual, 67. Bd 1973)
- German Patent 2 353 97~
- "Arctic Tanker Marine Technology", July 1971, pages 361 et seq..
For instance, a total drive power of 150,000 HP, distributed to
three shafts, is to be used for a power-plant for a polar gas-tanker ice-
breaker. The primary dri~e is to be in the form of light ~split~shaftl' gas
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turbines, coupled mechanically and directly to synchronous generators, con-
nected in turn electrically to synchronous and asynchronous propeller motors.
rt is the purpose of the invention to provide an installation with
a drive which is not oversized, but which allows the propeller to be started
as often as may be desired ~icebreaking by starting and stopping)without
thermal overloading of the engine. Since gas-turbine r.p.m. can be reduced
only~to a~out 30 to 40% of rated r.p.m. (nT in)' whereas in the lower r.p.m.
range ~nT~nTmin) a high torque must be available, a special starting and
reversing procedure must be devised.
This purpose has been achieved by the procedure according to the
present invention. The procedure according to the invention allows the
propeller to be started and reversed as often as may be desired. In spite of
the high torques required, the power plant need not be oversized and it is
therefore economical and suitably priced.
The present invention provides a method for starting and reversing a
gas-turbine-electric power-plant for icebreakers, comprising, as the primary
drive, a light gas-turbine directly driving a synchronous generator coupled
electrically to a propeller motor mechanically driving a propeller through a
shaft, two switches being arranged between the synchronous generator and the
propeller motor, in such a manner that when one of said switches is actuated, the
propeller-motor has a field which rotates to the left, while when the other of
said switches is actuated, the propeller-motor field rotates to the right, said
generator and motor having three-phase windings characterized in that the
propeller is started against a large counter- rotating torque, and when the
propeller is being braked from forward-running for reversing, a minimal output
of the gas-turbine is preset, windings of the synchronous generator and propeller
motor are short-circulted by closing both switches, and the excitation of the
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synchronous generator is adjusted to cause the generator frequency and the gas-
turbine r.p.m. to drop to low values; and in that the three-phase windings of
the synchronous generator and the propeller motor are subsequently connected by
eliminating the short-circuit by opening one of said switches, an increase in
gas-turbine torque is produced by increasing the fuel supply, and the generator
voltage is adjusted to a value U - UN ff where UN is nominal voltage and fN is
nominal frequency.
Examples of the installation according to the invention are illus-
trated in the drawing attached hereto, wherein each of Figures 1 to 6 illus-
trates a drive unit for a propellor shaft, but having different switching
technology.
In Figure 1, a light gas-turbine 1 is coupled rigidly, through a
drive-shaft 2, to a synchronous generator 3. The latter is connected to a
synchronous or asynchronous propeller motor 6 through leads R/S/T in which
switches 4,5 are arranged. These are used to reverse the drive, in known
fashion, by phase-change. Switch 4 is for ahead, switch 5 for astern. Pro-
peller motor 6 drives propeller 8 through a drive shaft 7. The number of
drive units is governed by the size of the power-plant.
Light gas-turbine 1 is started by a starter-motor, not shown and
when generator 3 is unexcited, it adjusts itself to an idle r.p.m. of about
30 to 40% of its rated r.p.m., corresponding to its minimal-power curve.
Switches 4,5 are now actuated to short-circuit the three-phase windings of
generator 3 and motor 6. The fuel supply to the gas-turbine is fixed by
adjusting it to the minimal value.
The generator 3 is now excited, so that a short-circuit current
flows in the three-phase winding thereof. A limit controller (not shown)
governs the excitation of the generator, to ensure that a predetermined cur-
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rent value is not exceeded. Losses in the short-circuited three-phase wind-
ing of the genera~or cause torque to increase, and the turbine is slowed
down, to about 3% of its rated r.p.m., for example. Depending upon the r.p.m.
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and, if necessary, upon the position of the rotating field of the generator
in relation to the motor, the short-circuit is eliminated by opening switch
5. It is also possible to eliminate the short-circuit as a function of the
intens;ty and phase position of the generator and motor currents. The open-
ing of switch 5 produces abrupt excitation of the synchronous generator, and
the gas-turbine torque is increased by increasing the fuel supply.
It is now possible to design the starting procedure for a power-
plant of this kind in such a manner as to ensure reliable starting even in
extreme cases, for example when the propeller is frozen into the ice, i.e. in
the case of unusually high counter-torques.
Since at the predetermined minimal fuel setting, the gas-turbine
delivers only between 5 and 10% of its rated torque, and since when the fuel
rate is increased, the torque increases only slowly, because the compressors
have to be accelerated, a high torque may be achieved more quickly, for
starting and reversing, by increasing the turbine fuel consumption after the
turbine has been slowed down and with the three-phase windings short-cir-
cuited. If the r.p.m. are reduced, for example, to 3% of the rated r.p.m.,
and if the generator current is ad~usted to a limiting value in the vicinity
of the rated current, this produces the full, current-dependent generator
losses which are, for example 2% of the rated power. Generator torque is the
quotient of power and r.p.m.; in the example, therefore, 2/3 of the rated
torque (67% T i 1) Thus the gas-turbine torque may be increased to this
value without increasing the r.p.m. After the short-circuit has been elimi-
nated by opening switch 5, this value is immediately available for accelerat-
ing the propeller motor 6,
Further increase in propeller r.p.m. is achieved by changing to
turbine-r.p.m. control. In this case, the generator voltage i9 ad~usted to
a value of U _ Unom., f If necessary, the excitation of the synchro-
nom.
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nous propeller motor is adJusted in such a manner that the power factor, or
the rotating-field angle, of the motor is constant.
When the ship is halted and reversed, the procedure is carried out
in reverse. When a stop co~mand is issued, generator 3 and, if necessary,
motor 6 are de-energized or the excitation is set to a low value. The tur- .
bine-r.p.m. control is eliminated by reducing the fuel supply to the min-
imum. Switches 4,5 short-circuit the three-phase windings of the generator
and motor. The generator is excited and slows down the turbine to the pre-
determined r.p.m. close to zero. Turbine torque is then increased by in-
creasing the fuel consumption. In the meanwhile, the propeller has slowed
to windmilling r.p.m. Since switch 5 is for astern, switch 4 must be opened
in order to eliminate the short-circuit which is carried out, as in starting,
as a function of generator r.p.m. and, if necessary, of the position of the
rotating field of the generator in relation to the motor, or as a function of
the magnitude and phase position of the generator and motor currents. Fur-
ther increase in r.p.m. in reverse is effected as when the motor is started.
Before the short-circuit i8 applied, the generator and, if nec-
essary, the synchronous motor are de-energized, or their excitation is set to
a low value, in order to prevent sudden short-circuit currents and to ensure
that the switches and machine parts are not overloaded.
If an asynchronous motor is used, it should be noted that althoueh
this motor can be overloaded, the load torque must not be higher than the
stalling torque of the motor. In order to achieve a higher starting torque,
the starting control is designed to ensure that the stalling slip sslip of the
propeller motor is not exceeded~ After switchine on the motor and generator
short-circuit, and providing a minimal fuel flow to the turbine, generator
excitation is also adJusted to reduce the eenerPtor frequency to close a
Sslip nom- - 5 -
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After the short-circuit and minimal fuel supply have been elimi-
nated and turbine torque has been increased by increasing the fuel supply,
shock excitation is provided for the genera-tor, in order to reduce slipping
of the asynchronous motor as quickly as possible and thus to prevent un-
necessary heating of the rotor.
In an asynchronous machine in which rotor resistance is dependent
upon frequency and U~f is constant, stalling slip sslip varies as a function
of frequency f. ~he slip produced by the counter-torque must in no case
reach or exceed the stalling slip. ~o this end, a limit-value controller is
provided to ensure a specific safety margin from the stalling slip. Since
stalling slip is frequency-dependent, the limit slip slimit is set to equal
sslip (f) - ~s, wherein As is a constant safety margin. When this
limiting value is applied, gas-turbine torque increases, thus ensuring that
stalling slip is never reached.
It is also possible, however, if the demands upon the dynamics of
the drive are restricted, to achieve a limiting value by adjusting the cur-
rent to ensure that a current value of Ilimit ISlip
When the ship iB changed from ahead to astern, the generator is
first of all de-energized, or the excitation is set to a very low value, and
the generator three-phase winding is short-circuited. The fuèl supply to the
gas turbine is set to minimal. The short-circuit current in the three-phase
winding of the generator is now ad~usted to produce the lowest possible
generator r.p.m. This i8 determined by the induced voltage required to
maintain the short-circuit current, the said voltage being proportional to
the r~p.m. at constant maximal flux.
Now when the propeller has reached its windmilling r.p.m., and the
short-circuit has been eliminated by opening switch 5, whereas switch 4
remains closed, a high torque becomes available for counter-current braking
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the propeller motor to zero and accelerating it in the opposite direction.
By increasing the fuel supply, the gas-turbine torque is increased to bring
the generator r.p.m. to a minimal operating value and to hold it there until
the motor slip has reached its stationary value. Only after a reverse start
has been made is the control procedure carried out as for travel ahead. The
minimal operating value of generator r.p.m. is determined by the induced
voltage required to maintain the starting current, the said induced current
being proportional to r.p.m. at constant maximal flux.
Since at the minimal fuel setting, and with the r.p.m. braked
down to values close to zero, the turbine has an extremely low torque of
about 6.o% Mdnom and again the amount of fuel may be increased before the
short-circuit is eliminated, in order to obtain a torque high enough for
starting.
The use of a synchronous propeller motor makes it possible, when
reversing, to slow down the motor and the generator, by short-circuit
losses, to very low r.p.m.'s, although the water flowing over the propeller
exerts a torque. To this end, the motor is excited during the short-circuit,
so that a short-circuit current flows in its three-phase winding. The limit
control ensures that this does not produce any inadmissibly heavy currents.
During this procedure, the short-circuit currents in the generator and motor
flow ~ointly through the short-circuit in the three-phase winding formed
by switches 4 and 5.
The torque is dependent upon the rotating-field position in rela-
tion to the motor, reaching a minimum at rotating-field angles of 90 and
then declining until, at angles of between 180 and 360 , it becomes negative.
Provision is therefore made for eliminating the short-circuit in the three-
phase windings as a function of the rotating-field position in relation to
the motor, so that at this moment the motor torque accelerates the propeller
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in the desired direction of rotation.
The magnitude and phase position of the generator and motor cur-
rents, when the three-phase windings are short-circuited, are functions of
the rotating-field positions. It is therefore possible, according to another
configuration of the invention, to eliminate the short-circuit in the three-
phase windings as a function of the magnitude and phase position of the
generator and motor currents, in such a manner that, at this moment, the
motor torque accelerates the propeller in the desired direction of rotation.
If optimal starting procedure for the propeller motor is to be
achieved, it is desirable that, after electrical coupling of the generator
and motor three-phase windings, by eliminating the short-circuits in these
three-phase windings, the motor shall immediately fall "into step", i.e.
that the rotating-field angle of the generator in relation to the motor shall
not exceed 180 . This is accomplished by the provision of abrupt excitation
for the generator and motor, and this increases the torque to an extent such
that this falling into step is assured.
After the starting procedure has been completed by the synchronous
propeller motor falling into step, the generator and propeller-motor r.p.m.'s
are further raised by increasing the fuel supply to the turbine. The torque
developed by the gas-turbine is used to accelerate the generator, motor and
propeller and to overcome propeller counter-torque. This propeller torgue
may vary sharply in icebreakers, since ice-floes may reach the propeller.
In order to be sure of preventing the synchronous propeller motor from fall-
ing out of step at this time, provision is made to restrict the increase in
fuel supply to the turbine in such a manner that predetermined rotating-field
angles of the generator in relation to the motor are not exceeded.
Switches 4 and 5 are provided for reversing. Switch 4 is closed
for moving ahead and switch 5 for moving astern. Switch 5 thus changes over
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two leads R and S in order to change the direction of the rotating field in
the motor 6. Simultaneous closing of these two switches now produces the
short-circuit in the three-phase windings of generator 3 and motor 6. In
order to obtain a three-pole short-circuit, the switches are of three-pole
design; the third poles are connected in series between lead T and one of
leads R or S (Figure 1).
In installations with reduced requirement for the dynamics of the
starting and reversing procedures, and for available propeller torque, the
procedure according to the invention may also be carried out with a two-lead
short-circuit of the three-phase windings in generator 3 and motor 6. In
this case, allowance may be made for minor electric coupling between the
generator and the motor while the three-phase windings are short-circuited by
switches 16 and 17 (Figure 2).
Braking of generator 3 and of motor 6, if necessary, is carried
out by the losses produced by the short-circuit currents in the effective
resistances in the windings. Any increase in these losses, as a result of
an increase in effective resistances in the short-circuit, improves the
effectiveness of the procedure. To this end, provision is made for a resis-
tor 9 to be wired in series with the third pole of first switch 4 and the
third pole of second switch 5. However, this simple arrangement causes a
certain amount of asymmetry ~Figure 3).
It is better, as shown in Figure 4, to provide a third switch 10
which has two poles, with resistors 11 and 12 in series therewith. The third
poles of switches 4,5 are now connected in parallel, while a resistor 9 is
arranged in series with them. When ahead is selected, and the three-phase
windings are short-circuited, switches 4 and 10 are closedJ and the resistors
are connected into three leads in order to absorb the braking energy. When
astern is selected, and the three-phase windings are short-circuited,
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switches 5 and 10 are closed.
Another possibility is shown in Figure 5, in which switches 16, 17
and 10 are double-pole switches. When ahead is selected, switches 16 and 10
close both poles, and when astern is selected, switches 17 and 10 close both
poles. Arranged :in series with the two poles of switch 10 are resistors 11,
12. During the short-circuit, these are connected between the first and second
and the second and third leads. However, this two-lead short-circuit also
results in some, although slight, asymmetry.
Although when operating in reverse, only double-pole switches are
generally used for phase-changing, it is desirable for switches 16 and 17 to
be double-poled and switch 18 triple-poled, as shown in Figure 6. Resistors 13
to 15 are in series with the three poles of switch 18 so that the installation
can be triple-pole short-circuited by the resistances both for starting and
reversing.
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