U.S. patent application number 12/634454 was filed with the patent office on 2010-04-08 for stator controlled induction generators with short-circuited rotor.
This patent application is currently assigned to Wind to Power System, S.L.. Invention is credited to Santiago Arnaltes Gomez, Jose M. Corcelles Pereira, Jose L. Rodriguez Amenedo.
Application Number | 20100084865 12/634454 |
Document ID | / |
Family ID | 39582836 |
Filed Date | 2010-04-08 |
United States Patent
Application |
20100084865 |
Kind Code |
A1 |
Corcelles Pereira; Jose M. ;
et al. |
April 8, 2010 |
STATOR CONTROLLED INDUCTION GENERATORS WITH SHORT-CIRCUITED
ROTOR
Abstract
A generator is connectable to a turbine for generating electric
power or a motor. An electric power generator system or a motor
comprises an asynchronous short-circuited rotor generator or motor
comprising a stator, a rotor, and a transformer having a first
winding and a second winding, the first winding having a first end
and a second end. The stator and the transformer are connectable in
series with an electric power distribution grid.
Inventors: |
Corcelles Pereira; Jose M.;
(Madrid, ES) ; Rodriguez Amenedo; Jose L.;
(Madrid, ES) ; Arnaltes Gomez; Santiago; (Madrid,
ES) |
Correspondence
Address: |
PATTON BOGGS LLP
1801 CALFORNIA STREET, SUITE 4900
DENVER
CO
80202
US
|
Assignee: |
Wind to Power System, S.L.
Madrid
ES
|
Family ID: |
39582836 |
Appl. No.: |
12/634454 |
Filed: |
December 9, 2009 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
11716438 |
Mar 9, 2007 |
7652387 |
|
|
12634454 |
|
|
|
|
Current U.S.
Class: |
290/44 |
Current CPC
Class: |
H02P 9/105 20130101;
H02J 3/18 20130101; H02P 2101/15 20150115; H02J 3/381 20130101;
Y02E 40/30 20130101 |
Class at
Publication: |
290/44 |
International
Class: |
H02P 9/04 20060101
H02P009/04 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 28, 2006 |
ES |
PCT/ES2006/000721 |
Claims
1. An asynchronous short-circuited rotor generator for generating
electric power wherein a stator is connected in series to a first
end of a first winding of a transformer, and an electric power
distribution grid is connected to the second end of the first
winding of the transformer.
2. An electric power generator system or motor comprising: a
short-circuited rotor induction generator or motor comprising a
stator and a rotor; a transformer having a first winding and a
second winding, said first winding having a first end and a second
end; and wherein said stator and said transformer are connectable
in series with an electric power distribution grid.
3-12. (canceled)
13. A method of generating electrical power, said method
comprising: generating electrical power using a short-circuited
rotor induction generator comprising a stator and a rotor;
connecting a first winding of a transformer in series between said
stator and an electric power distribution grid; and flowing power
between said stator and said electric power distribution grid via
said transformer.
14. A method as in claim 13, and further comprising: connecting a
first converter, a direct current link, and a second converter in
series between said stator and a second winding of said
transformer; and regulating one of the current between said first
converter and said stator, and the sum of the reactive power of
said stator and the reactive power of said converter.
15-19. (canceled)
20. A system for controlling: a short-circuited rotor induction
generator providing electrical power to an electric distribution
grid, or a motor connected to said grid, said generator or motor
having a stator and a rotor, said system comprising a media
readable by a processing unit, said media containing instructions
for directing said processing unit to regulate one or more of the
voltage of said stator independent of the voltage of said electric
distribution grid, and the reactive power applied to said grid by
said generator or motor.
21. A system as in claim 20 wherein said instructions further
comprise instructions for comparing a voltage on a DC bus between
said stator and said grid with a DC bus reference voltage to
regulate the transfer of active power to said grid via said DC
bus.
22. A system as in claim 20 wherein said instructions further
comprise instructions for comparing the voltage on said stator with
a stator reference voltage to determine a voltage component in the
direction aligned with the current of said grid.
23. A system as in claim 20 wherein said instructions further
comprise instructions for comparing the total reactive power
delivered to said grid with a total reactive power reference to
determine a voltage component in the direction orthogonal with the
current of said grid.
24. A method of controlling a wind turbine generator connected to a
grid, said turbine including a short-circuited rotor induction
generator system having a stator and a rotor, said method
comprising: generating a voltage; and injecting said voltage in
series between said stator and said grid.
25. A method as in claim 24 wherein said voltage has a component
oriented in the direction of the grid current, which component
injects active power into said grid.
26. A method as in clam 24 wherein said voltage has a component in
quadrature with the grid current, which component injects reactive
power into said grid.
27. A method as in claim 24 wherein said generating comprises
comparing the total reactive power applied to said grid to a
reference reactive power to produce a reactive power error signal,
and using said reactive power error signal to determine a value of
said injected voltage orthogonal to the current of said grid.
28. A method as in claim 24 wherein said generating comprises
comparing the voltage applied to said stator to a reference stator
voltage to produce a stator voltage error signal.
29. A method as in claim 28 wherein said generating further
comprises using said stator voltage error signal to determine the
value of said injected voltage in a direction aligned with the grid
current.
30. A method as in claim 24 wherein said generator system further
includes a first converter, a DC link, and a second converter
connected between said stator and said grid, and wherein said
generating comprises comparing a measured DC voltage of said DC
link to a reference DC voltage to provide a DC error signal.
31. A method as in claim 29 wherein said generating further
comprises using said DC error signal to determine a component of
the current between said first converter and said stator in a
direction aligned with the stator voltage.
32. A method of operating a short-circuited rotor induction power
generator or motor connected to a power distribution grid, said
generator or motor comprising a rotor and a stator, said method
comprising controlling the voltage applied to said stator
independent of the voltage of said grid.
33. A method as in claim 32 wherein said stator voltage is held
constant.
34. A method as in claim 32 wherein said controlling the voltage
applied to said stator comprises injecting a voltage between said
stator and said grid.
35. A method as in claim 34 wherein said injecting is performed by
applying said voltage via a transformer connected between said
stator and said grid.
36. A method as in claim 34 wherein said controlling comprises
comparing the voltage applied to said stator to a reference stator
voltage to produce a stator voltage error signal.
37. A method as in claim 36 wherein said generating further
comprises using said stator voltage error signal to determine the
value of said injected voltage in a direction aligned with the grid
current.
38. A method as in claim 34 wherein said generating further
comprises comparing the total reactive power applied to said grid
to a reference reactive power to produce a reactive power error
signal.
39. A method as in claim 38 wherein said generating further
comprises using said reactive power error signal to determine a
value of said injected voltage orthogonal to the current of said
grid.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This Application is a divisional application of U.S. patent
application Ser. No. 11/716,438 filed Mar. 9, 2007, which claims
priority to the application filed in Spain under PCT Application
No. PCT/ES2006/000721 on Dec. 28, 2006. The foregoing applications
are hereby incorporated by reference to the same extent as though
fully disclosed herein.
FIELD OF THE INVENTION
[0002] The present invention refers, in general, to an asynchronous
short-circuited rotor generator, or induction generator,
connectable to a turbine, such as a wind turbine, to generate
electric power that is delivered to an electric power distribution
grid. The system also applies to a motor.
BACKGROUND OF THE INVENTION
[0003] It is known in the state of the art that currently there are
many asynchronous short-circuited rotor generators, such as the
so-called squirrel cage rotor, coupled to turbines, such as wind
turbines, and connected directly to a three-phase electric power
distribution grid by voltage step-up transformers. Consequently,
said configuration of turbine connected to a generator is used to
produce electric power that reaches end users through the
three-phase electric power distribution grid.
[0004] Asynchronous short-circuited rotor generators, i.e.,
squirrel cage, are widely used because they are simple, robust, and
relatively inexpensive. However, such squirrel cage generators also
have disadvantages, such as high current demand during startup
requiring a soft start function, a minimal ability to vary the
rotational speed of the turbine because of a stiff characteristic
torque versus rotational speed in the stable operation region, with
resulting significant oscillations of the electromagnetic torque
and of the active power transmitted to the electrical system, the
inability to meet a requirement for dynamic reactive power exchange
from the distribution grid for proper operation, the inability of
starting up and operating as a stand-alone system, the inability to
be insulated from the external power oscillations from the
distribution grid, and the inability to damp such power
oscillations.
[0005] It would be highly desirable to have a squirrel-cage
generator which retained the features of simplicity, robustness.
and relative low cost without the disadvantages discussed
above.
BRIEF SUMMARY OF THE INVENTION
[0006] The present invention resolves or reduces one or more of the
disadvantages explained above by providing a short-circuited rotor
generator in which the stator of the generator is connected in
series with the electric power distribution grid through a first
winding of a transformer. Preferably, the generator is connectable
to a turbine, such as a wind turbine. Those skilled in the art will
also recognize that the principles of the invention also apply to a
squirrel cage motor.
[0007] An object of the invention is to connect the stator of a
squirrel cage generator or motor in series with an electric power
distribution grid through a first winding of a transformer.
Preferably, the voltage applied to a second winding of the
transformer is controlled through a transformer side electric power
converter; consequently, the voltage level of the generator's
stator is controlled.
[0008] Preferably, the stator of the generator or motor is also
connected to the same distribution grid through a stator side
electric power converter connected by a direct current link to the
transformer side electric power converter.
[0009] The above-described dual connection makes it possible to
increase the overall performance of the electric generator or motor
by reducing the losses in the iron of the generator.
[0010] A further object of the invention is to provide a
short-circuited rotor generator or motor with many different modes
of operation allowing it to continue to operate when one or more of
the converters fail.
[0011] Another object of the invention is to allow a smooth
connection of the generator or motor to the electric power
distribution grid, increasing the quality of the electrical
production during this period.
[0012] Another object of the invention is to permit the
short-circuited rotor machine to continue the supply of electric
power or operate as a motor when voltage variations occur on the
electric power distribution grid, both in balanced as well as
unbalanced operating conditions of the generator. A further object
of the invention is to contribute to the stability of the
distribution grid by providing reactive power to the grid.
[0013] Yet another object of the invention is that the generator or
motor be capable of dynamically swapping reactive power with the
distribution grid, regardless of the amount of load on the
generator.
[0014] Another object of the invention is that the generator be
capable of generating a voltage of nominal value at its output when
the electric power distribution grid is not available.
[0015] Still another object of the invention is that the generator
coupled to a wind generator is capable of being connected to the
electric distribution grid when wind speed is low. Consequently,
sites with low wind resources can be used with the short-circuited
rotor generator according to the invention.
[0016] Yet another object of the invention is to be able to operate
with at least a small amount of speed variation to permit the
recovery of the torque oscillations reducing stresses and loads and
increasing the mechanical performance.
[0017] Another object of the invention is to retain ruggedness and
reliability of asynchronous short-circuited rotor generators and
motors as well as a large capacity for transitory overloads.
[0018] Another object of the invention is to provide an apparatus
and method that can effectively retrofit already installed
short-circuited rotor generators or motors to make them compliant
with new regulations.
[0019] Numerous other features, objects, and advantages of the
invention will become apparent from the following description when
read in conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0020] A more detailed explanation of the invention is given in the
following description, based on the attached figures in which:
[0021] FIG. 1 shows a block diagram of a wind generator according
to the invention;
[0022] FIG. 2 shows a vector diagram illustrating the various
vectors, vector components, and angles relevant to the invention
and showing how the stator voltage may be controlled while
injecting a desired reactive power;
[0023] FIG. 3 is a vector diagram illustrating how the reactive
power can be dynamically varied for two different working points of
the generator according to the invention;
[0024] FIG. 4 is a vector diagram illustrating the soft start
function of a generator or motor according to the invention;
[0025] FIG. 5 is a vector diagram illustrating how the generator
according to the invention applies the predetermined value of
|V.sub.s| corresponding to |V*.sub.s| to the stator during voltage
dips of the grid voltage;
[0026] FIG. 6 is a block diagram illustrating a preferred
embodiment of controller for the grid side inverter; and
[0027] FIG. 7 is a block diagram illustrating a preferred
embodiment of controller for the stator side inverter.
DETAILED DESCRIPTION OF THE INVENTION
[0028] FIG. 1 is a block diagram illustrating a preferred
embodiment of a generator or motor system 27 according to the
invention. In this embodiment, the generator system 27 is
incorporated into a wind turbine generator 10, which includes a
turbine 12 and generator system 27. Generator system 27 includes a
generator 11, often referred to as an induction generator, and a
controller 30. Generator 11 includes a rotor 13 and a stator 14.
System 10 preferably is a wind turbine system, and generator 11
preferably is a shorted-rotor generator 11. Turbine 12 is
connectable to generator 11 in such a way that the turbine is
coupled to the rotor 13 that turns inside stator 14 of the
generator 11. Power produced by system 10 is fed to a power grid
22. As will be seen in detail below, the generator system 27
according to the invention controls the voltage V.sub.s applied to
the stator by injecting a voltage V.sub.i into the stator/grid
connection via transformer 15 using a novel control system. This
and other features of the invention described below results in a
generator or motor system that is much more flexible than prior art
systems.
[0029] Generator controller 30 includes transformer 15, a first
electric power converter 16, a second electrical power converter
17, a direct current link 33, a filter 18, a first controller
module 20, a second controller module 21, a generator/transformer
switch 34, a transformer/grid switch 35, a stator/converter switch
31, and an inductance 23. Transformer 15 includes a first winding
15-1 and a second winding 15-2. Direct current link 33 includes a
capacitor 19, a resistor 24, and a switch 25. Filter 18 includes an
inductor 36 and a capacitor 37. First controller module 20 includes
a microprocessor 40 and memory 41, and second controller module 21
includes a microprocessor 42 and memory 43.
[0030] Stator 14 is connected in series to a first end 44 of first
winding 15-1 of transformer 15, and electric power distribution
grid 22 is connected to the second end 45 of the first winding 15-1
of the transformer 15.
[0031] Stator 14 is connected to an input 47 of a first electric
power converter 16, the output 50 of which is connected in cascade,
using a direct current connection, to an input 52 of second
electric power converter 17, which has an output 48 connected to
second winding 15-2 of the transformer 15 through filter 18.
[0032] Capacitor 19 is connected across direct current link nodes
56 and 57. Capacitor 19 stores electric energy in accordance with
the active power swapped between first converter 16 and second
converter 17.
[0033] Furthermore, resistance 24 is connected through switch 25
across direct current link nodes 56 and 57. Resistor 24 and switch
25 are used to ensure that the maximum voltage levels of the direct
current link are not exceeded in the different modes of
operation.
[0034] First electric power converter 16 transforms an essentially
fixed frequency alternating current deviated from the stator/grid
electrical path 32 into direct current; subsequently, the second
converter 17 transforms the direct current from the DC link to
alternating current at the frequency of the grid. In this way, a
portion of the total power delivered by the generator 11 is
transferred between the generator's stator and the distribution
grid 22.
[0035] In another mode of operation of generator 11, the
distribution grid 22 can supply electric power to the generator
stator via electrical path 32 and also through the second power
converter 17 and first electric power converter 16. That is,
electric power can flow bi-directionally through the connections
32, 60 between the stator and the distribution grid 22. Thus, it is
evident to those skilled in the art that the system of the
invention is applicable not only to a generator, but is also
applicable to a motor.
[0036] The total electric power output from the generator 11 is
obtained at the grid 22 by adding the partial electric power
transfers via the path 32 and 60, i.e., power converter 16, DC
link, and power converter 17, to the rest of electric power
generated by the generator 11, i.e., power injected at the point 44
via transformer 15.
[0037] The first converter 16 includes a set of switching elements,
symbolized by switch 62 and diode 63, each of which has a control
terminal 65 through which an on and/or off signal is applied.
[0038] With reference now to FIGS. 1 and 2, first controller module
20 generates and supplies the switching signals to first converter
16 via line 67, and to achieve this, the first controller 20
calculates and/or receives a signal V.sub.dc proportional to the
voltage in the DC link 33, a signal i.sub.p proportional to the
current at node 47, a signal i.sub.g proportional to the current
output, and a signal V.sub.s proportional to the voltage at the
stator. A DC link voltage reference signal V*.sub.dc and a reactive
power reference signal Q*.sub.39 are also applied to controller 20.
These reference signals provide the set points for the DC link
voltage and the reactive power at node 39, respectively. As is
known in the art, the DC link voltage V.sub.dc is determined by
external parameters. For example, in the example of the wind
turbine, it is determined by the grid voltage. In the preferred
embodiment, the desired value of Q*.sub.39 is zero so as to
minimize the current at node 39. The manner in which the system
according to the invention uses the set points to control V.sub.dc
and Q.sub.39 will be described below.
[0039] The first controller 20 includes a memory 41 that stores a
control algorithm utilized by microprocessor 40, which algorithm
may be a vector control algorithm, a direct power control
algorithm, or any other suitable control algorithm, with which the
voltage of the DC link, V.sub.dc, is regulated to permit
instantaneous transfer of active power through electrical path 60,
and the reactive power at node 39, Q.sub.39, is regulated to
guarantee that V.sub.s and i.sub.g are aligned which naturally
decouples the effects of V.sub.id and V.sub.iq. That is, V.sub.id
affects only the value of |V.sub.s|, and V.sub.iq affects only the
value of Q.sub.g, the total reactive power applied to the grid.
[0040] Similarly, second converter 17 includes a set of switching
elements, symbolized by switch 72 and diode 73, each of which has a
control terminal 75 through which an on and/or off signal is
applied.
[0041] Second controller module 21 generates and supplies the on or
off signals to second converter 17 via line 77. To achieve this,
second controller 21 calculates and/or receives a signal V.sub.s
proportional to the voltage of the stator 14, a signal i.sub.s
proportional to the current of the stator 14, a signal V.sub.g
proportional to the distribution grid 22 voltage, and a signal
i.sub.g proportional to generator current applied grid at output
22. An absolute value, also referred to as the modulus, of the
stator voltage reference signal |V*.sub.s| and a total reactive
power reference signal Q*.sub.g also are applied to grid side
inverter controller 21. These reference signals provide the set
points for the absolute value of the stator voltage |V.sub.s| and
the total reactive power Q.sub.g. As is known in the art, the set
point |V*.sub.s| is determined by a higher level control loop as
known in the art. The set point Q*.sub.g is a reactive power value
desired to be output as determined by the operating conditions of
the grid.
[0042] Second controller 21 stores an algorithm in memory 43
utilized by microprocessor 42 to regulate the total reactive power
Q.sub.g following a control strategy that utilizes reference value
Q*.sub.g. Memory 43 of second controller 21 also stores an
algorithm utilized by microprocessor 42 to regulate the modulus of
the voltage resulting from or applied to the generator's stator 14,
following a control strategy that utilizes reference value
|V*.sub.s|. These algorithms may be a vector control algorithm,
such as a voltage oriented control algorithm, a direct power
control algorithm, or any other suitable control algorithm.
[0043] Consequently, first controller 20 and second controller 21
govern the first 16 and second 17 converters, respectively, in such
a way that they directly control the absolute voltage applied to
the generator's stator 14 and the total reactive power applied to
the grid, therefore making the system according to the invention
much more stable under grid variations and better able to
strengthen the grid as required by grid code requirements.
[0044] Turning to FIG. 2, there is shown a vector diagram
illustrating the various vectors, vector components, and angles
relevant to the invention and showing how the stator voltage may be
controlled while injecting a desired reactive power. In this
example, for ease of understanding, the stator voltage V.sub.s and
the grid current i.sub.g are aligned, which is the preferred
operating condition of the system. The vectors, vector components,
and angles are illustrated in a stationary coordinate system along
the directions .alpha. and .beta.. The coordinate system d and q is
a synchronous coordinate system with d in the direction of the
stator voltage V.sub.s and the grid current i.sub.g, and q in a
direction orthogonal to the direction the stator voltage V.sub.s
and the grid current i.sub.g.
[0045] In FIG. 2, V.sub.s is the stator voltage, V.sub.g is the
grid voltage, and V.sub.i is the voltage injected via transformer
15. Also, i.sub.s is the stator current, i.sub.g is the grid
current, and i.sub.p is the current flowing at node 47.
.theta..sub.ig is the angle between .alpha. and the grid current
i.sub.g, which angle, in this operating mode, is the same as
.theta..sub.vs, the angle between .alpha., and the stator voltage
V.sub.s. V.sub.id is the component of V.sub.i in the direction of
the grid current and the stator voltage, and V.sub.iq is the
component of V.sub.i in the direction orthogonal to the grid
current and the stator voltage, while i.sub.pd is the component of
i.sub.p in the direction of the grid current and stator voltage,
and i.sub.pq is the component of i.sub.p in the direction of the
grid current and stator voltage. As will be shown below, controller
20 determines the reference values of i*.sub.pd and i*.sub.pq to
force the system to the set points V*.sub.dc and Q*.sub.39, and
controller 21 determines the values of V.sub.id and V.sub.iq to
force the system to the set points |V*.sub.s| and Q*.sub.g.
[0046] FIG. 3 is a vector diagram illustrating how the reactive
power can be dynamically varied for two different working points of
the generator according to the invention. Since the purpose of this
figure is to illustrate how the reactive power is adjusted, the
absolute value of the stator voltage V.sub.s and the grid voltage
V.sub.g are assumed to be the same so as not to unduly complicate
the figure. However, those skilled in the art will recognize that
all of these variables can change simultaneously. At working point
(1), the power factor is unity and V.sub.g.sup.(1)=V.sub.s. For a
shorted-rotor induction machine, the stator current i.sub.s must be
leading in relation to the stator voltage V.sub.g. The requirement
that the active power at node 47, i.e., P.sub.p=V.sub.s,i.sub.pd
must be the same as the active power at node 48, i.e.,
P.sub.i=V.sub.id, i.sub.g requires that i.sub.pd is zero because
V.sub.id is zero. At working point (2), it is decided to apply a
reactive power at the node 22 which is indicated by the fact that
the grid voltage V.sub.g is now out of phase with the grid current
i.sub.g by an angle .phi..sub.g. The generator is set to this
reactive power by applying the shown V.sub.iq.sup.(2). To keep
|V.sub.s| constant, the system adjusts V.sub.id to V.sub.id.sup.(2)
as shown. The additional reactive power added to the generator
output causes i.sub.g to increase to i.sub.g.sup.(2) as shown. This
requires controller 20 to add an i.sub.pd.sup.(2) and an
i.sub.pq.sup.(2) as shown to hold V.sub.dc to V.sub.dc* and
Q.sub.39 to Q*.sub.39.
[0047] FIG. 4 is a vector diagram illustrating the soft start
function of a generator or motor according to the invention. Three
working points of the start process are shown. For all three
working points, V.sub.g is the same. At the first working point
(1), the stator voltage V.sub.s is made very small by inserting a
V.sub.i (not shown so as not to complicate the figure) equal to
V.sub.i.sup.(1)=V.sub.s.sup.(1)-V.sub.g via transformer 15. Due to
the required lag between the stator current and stator voltage,
i.sub.s.sup.(1) is nearly 90 degrees leading to the stator voltage
indicating the system is absorbing significant reactive power. At
the second working point (2), V.sub.s.sup.(2) has a smaller
negative value to yield a larger V.sub.s.sup.(2) and an
i.sub.s.sup.(2) that is leading less, resulting in a small reactive
power being absorbed. At the third working point (3),
V.sub.s.sup.(3) is zero to yield V.sub.s.sup.(3)=V.sub.g resulting
in i.sub.s.sup.(3) lagging only by the nominal amount required by
the short-circuited rotor inductive system.
[0048] FIG. 5 is a vector diagram illustrating how the generator
according to the invention applies the predetermined value of
|V.sub.s| corresponding to |V*.sub.s| to the stator during voltage
dips of the grid voltage. For simplicity in this figure, V.sub.g is
in the same direction as i.sub.g. Two working points (1) and (2)
are shown. At the first working point (1), V.sub.g.sup.(1)=Vs,
which is equal to the set point value of the stator voltage, and
i.sub.s has the required lead determined by the machine parameters.
Controller 20 applies an appropriate i.sub.p.sup.(1) due to the
fact that the grid current and grid voltage are aligned. At working
point (2), the grid voltage V.sub.g suddenly drops. To keep the
stator voltage at the set point, the system applies, via controller
21 and converter 17, a V.sub.i.sup.(2) as shown via the transformer
15. This causes i.sub.g.sup.(2) to rise, which requires controller
20 to apply a larger i.sub.p.sup.(2) as shown to keep the grid
current and grid voltage aligned.
[0049] FIG. 6 is a block diagram illustrating a preferred
embodiment of controller for the stator side inverter 20. This
example assumes a control algorithm using voltage vector oriented
vector control, though other control systems and algorithms may be
used. Stator side controller 20 comprises comparators 502 and 506,
PI controllers 504 and 508, rotational transformation 510,
switching pattern generator 512, which preferably is a pulse width
modulator, and argument calculator 516. The V.sub.dc* reference
signal and the measured V.sub.dc signal are input into comparator
502, which outputs a signal representative of their difference to
PI controller 504. The Q.sub.39* reference signal and measured
Q.sub.39 signal are input into comparator 506 which outputs a
signal representative of their difference to PI controller 508. PI
controller 504 is designed to guarantee that the set point
V.sub.dc* is reached with the specific dynamics of the generator 27
and outputs the required value reference of i*.sub.pd to reach this
set point. PI controller 508 is designed to guarantee that the set
point Q.sub.39* is reached with the specific dynamics of the
generator 27 and outputs the required value reference of i*.sub.pq
to reach this set point. Current controller 509 provides an inner
control loop that compares the measured value i.sub.p to i*.sub.pd
and i*.sub.pq and outputs V.sub.pd and V.sub.pq to the rotational
transformation 510. Argument calculator 516 calculates the angle of
V.sub.s and outputs this angle .theta..sub.vs to the rotational
transformer 510. Using the angle, rotational transformer 510
rotates the coordinates of V.sub.pd and V.sub.pq from the
synchronous coordinates to the stationary coordinates .alpha. and
.beta.. The resulting current components Vi.sub.p.alpha. and
V.sub.p.beta. are applied to switching pattern generator 512 which
applies an appropriate duty cycle generator, such as pulse width
modulation, to the voltages to determine the drive signals 67 to be
applied to the converter 16.
[0050] FIG. 7 is a block diagram illustrating a preferred
embodiment of controller for the grid side inverter. This example
assumes a control algorithm using current oriented vector control,
though other control systems and algorithms may be used. Grid side
controller 21 comprises comparators 702 and 706, PI controllers 704
and 708, rotational transformation 710, switching pattern generator
712, which preferably is a pulse width modulator, and argument
calculator 716. The V.sub.s* reference signal and the measured
V.sub.s signal are input into comparator 702, which outputs a
signal representative of their difference to PI controller 704. The
Q.sub.g* reference signal and measured Q.sub.g signal are input
into comparator 706, which outputs a signal representative of their
difference to PI controller 708. PI controller 704 is designed to
guarantee that the set point V.sub.s* is reached with the specific
dynamics of the generator 27 and outputs the required value of
V.sub.id to reach this set point. PI controller 708 is designed to
guarantee that the set point Q.sub.g* is reached with the specific
dynamics of the generator 27 and outputs the required value of to
reach this set point. Argument calculator 716 calculates the angle
of i.sub.g and outputs this angle .theta..sub.ig to the rotational
transformer 710. Using the angle .theta..sub.ig, rotational
transformer 710 rotates the coordinates of V.sub.id and from the
synchronous coordinates to the stationary coordinates .alpha. and
.beta.. The resulting voltages V.sub.i.alpha. and V.sub.i.beta. are
applied to switching pattern generator 712, which applies an
appropriate duty cycle generator, such as pulse width modulation,
to the voltages to determine the drive signals 77 to be applied to
the converter 17.
[0051] It should be observed that both the first and second
controllers 20, 21 can work in coordinated mode or either of them
can work with the other one disconnected, or even neither of the
two activated, the generating capacities being reduced in each
case.
[0052] The way the voltage resulting from and/or applied to the
stator 14 is governed based on controlling the voltage delivered in
series from the second converter 17 to the electric distribution
grid 22 through the transformer 15. The voltage V.sub.i of the
second converter 17 is vectorally added to the voltage V.sub.g of
the distribution grid 22.
[0053] Moreover, it should be observed that the present invention
can be implemented in a variety of computers that include
microprocessors, a computer-readable storage means that includes
volatile and non-volatile memory elements, and/or storage elements.
The logic of the computer hardware that cooperates with various
sets of instructions is applied to the data in order to carry out
the previously described functions and to generate output
information. The programs used for the computer hardware, by way of
example, preferably can be implemented in various programming
languages, including a high-level-process- or object-oriented
programming language for communicating with a computer system. Each
computer program preferably is stored in a storage means or device
(e.g., ROM or magnetic disc) that can be read by a general use or
special use programmable computer for configuring and operating the
computer when the storage means or device is read by the computer
in order to execute the procedures described above. Moreover, the
first and second controller can be considered as being implemented
as a computer-readable storage medium, configured with a computer
program, where the storage medium thus configured makes the
computer operate in a specific, predefined way.
[0054] The two microprocessors of the first and second controller
can be in communication or encapsulated in a single component.
[0055] There has been described a novel short-circuited rotor
(squirrel cage) generator or motor. Now that the apparatus and
processes of the invention have been described, those skilled in
the art may make many variations. It should be understood that the
particular embodiments shown in the drawings and described within
this specification are for purposes of example and should not be
construed to limit the invention, which will be described in the
claims below. The description, as it has been explained, is not
intended to be exhaustive of the invention or to limit the
invention to the specific form described. Many modifications and
variations are possible in light of the foregoing examples, without
going beyond the spirit and scope of the following claims. For
example, many different controllers other than PI controllers may
be used. It is also evident that those skilled in the art may now
make numerous uses and modifications of the specific embodiments
described, without departing from the inventive concepts. It is
further evident that the methods recited may, in many instances, be
performed in a different order; or equivalent components may be
used and/or equivalent processes may be substituted for the various
processes described. Consequently, the invention is to be construed
as embracing each and every novel feature and novel combination of
features present in and/or possessed by the invention herein
described.
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