U.S. patent application number 13/871309 was filed with the patent office on 2014-10-30 for switching-based control for a power converter.
This patent application is currently assigned to General Electric Company. The applicant listed for this patent is GENERAL ELECTRIC COMPANY. Invention is credited to Cornelius Edward Holliday, Robert Gregory Wagoner.
Application Number | 20140319838 13/871309 |
Document ID | / |
Family ID | 51626940 |
Filed Date | 2014-10-30 |
United States Patent
Application |
20140319838 |
Kind Code |
A1 |
Wagoner; Robert Gregory ; et
al. |
October 30, 2014 |
SWITCHING-BASED CONTROL FOR A POWER CONVERTER
Abstract
A method for operating a power generation system that supplies
power for application to a load is disclosed. The method may
generally include receiving, at a power converter, an alternating
current power generated by a generator operating at a speed that is
substantially equal to its synchronous speed and converting, with
the power converter, the alternating current power to an output
power, wherein the power converter includes at least one switching
element. In addition, the method may include receiving a control
command to control a switching frequency of the at least one
switching element and adjusting the switching frequency to an
adjusted switching frequency that is substantially equal to a
fundamental frequency of the load.
Inventors: |
Wagoner; Robert Gregory;
(Roanoke, VA) ; Holliday; Cornelius Edward;
(Forest, VA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
GENERAL ELECTRIC COMPANY |
Schenectady |
NY |
US |
|
|
Assignee: |
General Electric Company
Schenectady
NY
|
Family ID: |
51626940 |
Appl. No.: |
13/871309 |
Filed: |
April 26, 2013 |
Current U.S.
Class: |
290/44 |
Current CPC
Class: |
F03D 7/028 20130101;
Y02E 10/763 20130101; F05B 2270/337 20130101; H02P 2207/073
20130101; Y02E 10/76 20130101; H02J 2300/28 20200101; H02P 9/007
20130101; Y02E 10/72 20130101; H02J 3/381 20130101; H02J 3/386
20130101; F03D 7/0272 20130101; F03D 7/0276 20130101; Y02E 10/723
20130101; H02P 2201/01 20130101 |
Class at
Publication: |
290/44 |
International
Class: |
F03D 7/02 20060101
F03D007/02 |
Claims
1. A method for operating a power generation system that supplies
power for application to a load, the method comprising: receiving,
at a power converter, an alternating current power generated by a
generator operating at a rotor speed that is substantially equal to
its synchronous speed; converting, with the power converter, the
alternating current power to an output power, the power convertor
including at least one switching element; receiving a control
command to control a switching frequency of the at least one
switching element; and adjusting the switching frequency to a
frequency that is substantially equal to a fundamental frequency of
the load.
2. The method of claim 1, wherein the frequency is within +/-5% of
the fundamental frequency.
3. The method of claim 1, wherein the rotor speed is within about
+/-5% of the synchronous speed.
4. The method of claim 1, wherein the load comprises an electrical
grid and wherein the fundamental frequency is equal to about 50 Hz
or about 60 Hz.
5. The method of claim 1, wherein the power converter comprises a
rotor side converter and a line side converter, the at least one
switching element forming part of the rotor side converter.
6. The method of claim 1, wherein the generator is a wind-driven
doubly fed induction generator.
7. The method of claim 1, wherein the at least one switching
element comprises an insulated gate bipolar transistor (IGBT).
8. A power generation system for supplying power for application to
a load, the power generation system comprising: a generator a power
converter coupled to the generator, the power converter including
at least one switching element; and a controller configured to
provide control commands to adjust a switching frequency of the at
least one switching element, wherein the controller is configured
to adjust the switching frequency to a frequency that is
substantially equal to a fundamental frequency of the load when the
generator is operating at a speed that is substantially equal to
its synchronous speed.
9. The system of claim 8, wherein the frequency is within +/-5% of
the fundamental frequency.
10. The system of claim 8, wherein the rotor speed is within about
+/-5% of the synchronous speed.
11. The system of claim 8, wherein the load comprises an electrical
grid and wherein the fundamental frequency is equal to about 50 Hz
or about 60 Hz.
12. The system of claim 8, wherein the power converter comprises a
rotor side converter and a line side converter, the at least one
switching element forming part of the rotor side converter.
13. The system of claim 8, wherein the generator is a wind-driven
doubly fed induction generator.
14. The system of claim 8, wherein the at least one switching
element comprises an insulated gate bipolar transistor (IGBT).
15. A wind turbine system for supplying power for application to a
load, the wind turbine system comprising: a wind turbine rotor, the
wind turbine rotor including a hub and a plurality of rotor blades
coupled to the hub; a doubly fed induction generator coupled to the
wind turbine rotor; a power converter coupled to the doubly fed
induction generator, the power converter including a rotor side
converter and a line side converter, the rotor side converter
including at least one switching element; and a controller
configured to provide control commands to adjust a switching
frequency of the at least one switching element, wherein the
controller is configured to adjust the switching frequency to a
frequency that is substantially equal to a fundamental frequency of
the load when the doubly fed induction generator is operating at a
speed that is substantially equal to its synchronous speed.
16. The system of claim 15, wherein the frequency is within +/-5%
of the fundamental frequency.
17. The system of claim 15, wherein the rotor speed is within about
+/-5% of the synchronous speed.
18. The system of claim 15, wherein the load comprises an
electrical grid and wherein the fundamental frequency is equal to
about 50 Hz or about 60 Hz.
19. The system of claim 15, wherein the at least one switching
element comprises an insulated gate bipolar transistor (IGBT).
Description
FIELD OF THE INVENTION
[0001] The present subject matter relates generally to power
convertors for power generation systems and, more particularly, to
a system and method for operating a power converter used in a DFIG
wind turbine system when the generator is operated at speeds at or
close to its synchronous speed.
BACKGROUND OF THE INVENTION
[0002] Power generation systems often include a power converter
that is configured to convert an input power into a suitable power
for application to a load, such as a generator, motor, electrical
grid, or other suitable load. For instance, a power generation
system, such as a wind turbine system, may include a power
converter for converting variable frequency alternating current
power generated at the generator into alternating current power at
a grid frequency (e.g. 50 Hz or 60 Hz) for application to a utility
grid. An exemplary power generation system may generate AC power
using a wind-driven doubly fed induction generator (DFIG). A power
converter can regulate the flow of electrical power between the
DFIG and the grid.
[0003] In general, the output parameters of a DFIG generator
typically vary as its rotor speed is adjusted across the
generator's operating speed range. For example, FIG. 1 illustrates
a graphical representation of the relationship between rotor
voltage and rotor speed for a typical 60 Hz DFIG (e.g., a DFIG
having a turns ratio of 3, a synchronous rotor speed of 1200 RPM
and an operating speed range from 800 RPM to 1600 RPM). As shown,
at the extremes of its operating speed range, the DFIG has a rotor
slip of +/-0.33, a rotor frequency of +/-20 Hz. In addition, the
rotor emf magnitude is generally equal to the stator emf magnitude.
However, as the rotor speed is increased or decreased towards the
synchronous speed, such output parameters generally approach zero.
For example, as shown in FIG. 1, the rotor frequency crosses
through DC at the synchronous speed.
[0004] Additionally, FIG. 2 illustrates a graphical representation
of the relationship between power and rotor speed for the same 60
Hz DFIG (assuming that a constant power is delivered from the
DFIG's stator). The total power (line 202) flowing from the DFIG to
the grid may be expressed as the summation of the stator power
(line 204) and the rotor power (line 206), with the rotor power 206
being a function of the rotor speed. As shown in FIG. 2, for rotor
speeds above the synchronous speed (i.e., super-synchronous
speeds), the rotor power 206 is positive and flows from the rotor
into the grid. In contrast, for rotor speeds below the synchronous
speed (i.e., sub-synchronous speeds), the rotor power 206 is
negative and flows from the grid into the rotor. However, when the
rotor speed is equal to the synchronous speed (e.g., 1200 RPM), the
rotor power 206 is equal to zero.
[0005] At or near the synchronous speed of a DFIG system,
conventional power converters typically operate at relatively
constant current, and consequently the average power loss in an
IGBT remains relatively constant. However, as shown in FIG. 1, as
the generator speed approaches the synchronous speed, the rotor
fundamental frequency approaches DC. Because the transient thermal
resistance of the IGBT increases at low frequency, the peak
temperature of the rotor side IGBT increases at or near the
synchronous speed, resulting in a reduction of the total output
current capability of the rotor side of the converter. In addition,
operation of a DFIG generator at or near synchronous speed is even
more complicated because current harmonics feed through the
generator from the rotor side to the stator side and then directly
to the transmission utility grid. These harmonics must be
controlled to levels dictated by utility grid harmonic
requirements. As the speed of the generator approaches the
synchronous speed of a DFIG system, the thermal cycling of the IGBT
junction increases, again based on the transient thermal resistance
of the IGBT, which leads to the switching elements wearing out
prematurely.
[0006] Accordingly, a system and method that operates a power
converter in a way to reduce the power loss of the convertor's
switching elements when a generator is operating at or near its
synchronous speed would be welcomed in the technology. Ideally the
power loss reduction at or near the synchronous speed of a DFIG
system would allow a converter to operate without reducing the
total output current capability of the rotor side of the
converter.
BRIEF DESCRIPTION OF THE INVENTION
[0007] Aspects and advantages of the invention will be set forth in
part in the following description, or may be obvious from the
description, or may be learned through practice of the
invention.
[0008] In one aspect, the present subject matter is directed to a
method for operating a power generation system that supplies power
for application to a load. The method may generally include
receiving, at a power converter, an alternating current power
generated by a generator operating at a speed that is substantially
equal to its synchronous speed and converting, with the power
converter, the alternating current power to an output power,
wherein the power converter includes at least one switching
element. In addition, the method may include receiving a control
command to control a switching frequency of the at least one
switching element and adjusting the switching frequency to an
adjusted switching frequency that is substantially equal to a
fundamental frequency of the load.
[0009] In another aspect, the present subject matter is directed to
a power generation system for supplying power for application to a
load. The power generation system may generally include a generator
and a power converter coupled to the generator. The power converter
may include at least one switching element. In addition, the power
generation system may include a controller configured to provide
control commands to adjust a switching frequency of the at least
one switching element. The controller may be configured to adjust
the switching frequency to an adjusted switching frequency that is
substantially equal to a fundamental frequency of the load when the
generator is operating at a speed that is substantially equal to
its synchronous speed.
[0010] In a further aspect, the present subject matter is directed
to a wind turbine system for supplying power for application to a
load. The system may generally include a wind turbine rotor and a
doubly fed induction generator coupled to the wind turbine rotor.
The system may also include a power converter coupled to the doubly
fed induction generator. The power converter may include a rotor
side converter and a line side converter. The rotor side converter
may include at least one switching element. In addition, the system
may include a controller configured to provide control commands to
adjust a switching frequency of the at least one switching element.
The controller may be configured to adjust the switching frequency
to an adjusted switching frequency that is substantially equal to a
fundamental frequency of the load when the doubly fed induction
generator is operating at a speed that is substantially equal to
its synchronous speed.
[0011] These and other features, aspects and advantages of the
present invention will become better understood with reference to
the following description and appended claims. The accompanying
drawings, which are incorporated in and constitute a part of this
specification, illustrate embodiments of the invention and,
together with the description, serve to explain the principles of
the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] A full and enabling disclosure of the present invention,
including the best mode thereof, directed to one of ordinary skill
in the art, is set forth in the specification, which makes
reference to the appended figures, in which:
[0013] FIG. 1 illustrates a graphical representation of the
relationship between rotor voltage and rotor speed for a typical
DFIG generator;
[0014] FIG. 2 illustrates a graphical representation of the
relationship between power and rotor speed for a typical DFIG
generator;
[0015] FIG. 3 perspective view of one embodiment of a wind
turbine;
[0016] FIG. 4 illustrates a schematic diagram of one embodiment of
a DFIG wind turbine system in accordance with aspects of the
present subject matter;
[0017] FIG. 5 illustrates a schematic diagram of one embodiment of
a power converter suitable for use with the DFIG wind turbine
system shown in FIG. 4;
[0018] FIG. 6 illustrates a flow diagram of one embodiment of a
method for operating a power generation system (such as the DFIG
wind turbine system shown in FIG. 4) in accordance with aspects of
the present subject matter; and
[0019] FIG. 7 illustrates a data table showing the difference in
peak junction temperatures achieved at synchronous speed using
conventional control methodologies and using the method disclosed
herein.
DETAILED DESCRIPTION OF THE INVENTION
[0020] Reference now will be made in detail to embodiments of the
invention, one or more examples of which are illustrated in the
drawings. Each example is provided by way of explanation of the
invention, not limitation of the invention. In fact, it will be
apparent to those skilled in the art that various modifications and
variations can be made in the present invention without departing
from the scope or spirit of the invention. For instance, features
illustrated or described as part of one embodiment can be used with
another embodiment to yield a still further embodiment. Thus, it is
intended that the present invention covers such modifications and
variations as come within the scope of the appended claims and
their equivalents.
[0021] In general, the present subject matter is directed to a
system and method for operating a power generation system, such as
doubly fed induction generator (DFIG) wind turbine system. In
particular, the present subject matter is directed to a system and
method for operating a power converter of a wind-driven DFIG in a
manner that reduces the power loss of the convertor's switching
elements when the generator is operating at or near its synchronous
speed. For example, a DFIG is typically operated at
super-synchronous speeds. However, during specific operating modes
(e.g., during a noise-reduced operating mode of the wind turbine
system), the generator may be operated at speeds at or near its
synchronous speed. In such instances, it has been determined by the
inventors of the present subject matter that the performance of the
power convertor may be enhanced by reducing the switching frequency
of the switching elements to a frequency at or close to the
fundamental frequency of the grid (e.g., 50 Hz or 60 Hz).
[0022] Typically, the switching frequency on the rotor side of a
DFIG power convertor is maintained at an elevated frequency (e.g.,
about 2000 or 3000 Hz) for all rotor speeds within the generator's
operating speed range. However, while such an elevated switching
frequency is desirable for most operating speeds, it can present
problems when the generator is operated at or near it synchronous
speed. For example, at the synchronous speed, the elevated
switching frequency results in a higher stress on the switching
elements (e.g., due to both an increased peak temperature and
increased thermal cycling stresses), which may cause a de-rating of
the total output current capability of the rotor side of the
converter and may also lead to the switching elements wearing out
prematurely. In addition, the elevated switching frequency may also
lead to increased harmonics at the synchronous speed.
[0023] By reducing the switching frequency with reductions in the
generator speed, the problems described above may be overcome.
Specifically, by adjusting the switching frequency to a frequency
at or near the fundamental frequency of the grid when the generator
is operated at or near to its synchronous speed, the power loss in
the switching elements may be significantly reduced. With such a
reduction in power loss, the temperature rise in the switching
elements may also be reduced, which may provide an extra margin in
the output current capability of the power convertor and may also
increase the component life of the switching elements. In addition,
by closely matching the switching frequency with the fundamental
frequency of the grid, a reduction in the amount of harmonics fed
through to the line side of the converter may also be obtained,
thereby decreasing the harmonic distortion to the grid.
[0024] Referring now to the drawings, FIG. 3 illustrates a
perspective view of one embodiment of a wind turbine 10. As shown,
the wind turbine 10 generally includes a tower 12 extending from a
support surface 14, a nacelle 16 mounted on the tower 12, and a
rotor 18 coupled to the nacelle 16. The rotor 18 includes a
rotatable hub 20 and at least one rotor blade 22 coupled to and
extending outwardly from the hub 20. For example, in the
illustrated embodiment, the rotor 18 includes three rotor blades
22. However, in an alternative embodiment, the rotor 18 may include
more or less than three rotor blades 22. Each rotor blade 22 may be
spaced about the hub 20 to facilitate rotating the rotor 18 to
enable kinetic energy to be transferred from the wind into usable
mechanical energy, and subsequently, electrical energy. For
instance, as will be described below, the rotor 18 may be rotatably
coupled to an electric generator 120 (FIG. 4) to permit electrical
energy to be produced.
[0025] Referring now to FIG. 4, a schematic diagram of one
embodiment of a DFIG wind turbine system 100 is illustrated in
accordance with aspects of the present subject matter. It should be
appreciated that the present subject matter will generally be
described herein with reference to the system 100 shown in FIG. 4.
However, those of ordinary skill in the art, using the disclosures
provided herein, should understand that aspects of the present
disclosure may also be applicable in other power generation
systems.
[0026] As shown, the rotor 18 of the wind turbine 10 (FIG. 3) may,
optionally, be coupled to a gear box 118, which is, in turn,
coupled to a generator 120. In accordance with aspects of the
present disclosure, the generator 120 is a doubly fed induction
generator (DFIG).
[0027] As shown, the DFIG 120 may be coupled to a stator bus 154
and a power converter 162 via a rotor bus 156. The stator bus 154
may provide an output multiphase power (e.g. three-phase power)
from a stator of the DFIG 120 and the rotor bus 156 may provide an
output multiphase power (e.g. three-phase power) from a rotor of
the DFIG 120. As shown in FIG. 2, the power converter 162 includes
a rotor side converter 166 and a line side converter 168. The DFIG
120 may be coupled via the rotor bus 156 to the rotor side
converter 166. Additionally, the rotor side converter 166 may
coupled to the line side converter 168 which may, in turn, be
coupled to a line side bus 188.
[0028] In several embodiments, the rotor side converter 166 and the
line side converter 168 may be configured for normal operating mode
in a three-phase, pulse width modulation (PWM) arrangement using
insulated gate bipolar transistor (IGBT) switching elements as will
be discussed in more detail with respect to FIG. 5. The rotor side
converter 166 and the line side converter 168 may be coupled via a
DC link 136 across which is a DC link capacitor 138.
[0029] In addition, the power converter 162 may be coupled to a
controller 174 in order to control the operation of the rotor side
converter 166 and the line side converter 168. It should be noted
that the controller 174 may, in several embodiments, be configured
as an interface between the power converter 162 and a control
system 176. The controller 174 may include any number of control
devices. In one embodiment, the controller 174 may include a
processing device (e.g. microprocessor, microcontroller, etc.)
executing computer-readable instructions stored in a
computer-readable medium. The instructions when executed by the
processing device may cause the processing device to perform
operations, including providing control commands (e.g. switching
frequency commands) to the switching elements of the power
converter 162.
[0030] In typical configurations, various line contactors and
circuit breakers including, for example, a grid breaker 182 may
also be included for isolating the various components as necessary
for normal operation of the DFIG 120 during connection to and
disconnection from the electrical grid 184. For example, a system
circuit breaker 178 may couple the system bus 160 to a transformer
180, which may be coupled to the electrical grid 184 via the grid
breaker 182. In alternative embodiments, fuses may replace some or
all of the circuit breakers.
[0031] In operation, alternating current power generated at the
DFIG 120 by rotating the rotor 18 is provided via a dual path to
the electrical grid 184. The dual paths are defined by the stator
bus 154 and the rotor bus 156. On the rotor bus side 156,
sinusoidal multi-phase (e.g. three-phase) alternating current (AC)
power is provided to the power converter 162. The rotor side power
converter 166 converts the AC power provided from the rotor bus 156
into direct current (DC) power and provides the DC power to the DC
link 136. As is generally understood, switching elements (e.g.
IGBTs) used in the bridge circuits of the rotor side power
converter 166 may be modulated to convert the AC power provided
from the rotor bus 156 into DC power suitable for the DC link
136.
[0032] In addition, the line side converter 168 converts the DC
power on the DC link 136 into AC output power suitable for the
electrical grid 184. In particular, switching elements (e.g. IGBTs)
used in bridge circuits of the line side power converter 168 can be
modulated to convert the DC power on the DC link 136 into AC power
on the line side bus 188. The AC power from the power converter 162
can be combined with the power from the stator of DFIG 120 to
provide multi-phase power (e.g. three-phase power) having a
frequency maintained substantially at the frequency of the
electrical grid 184 (e.g. 50 Hz or 60 Hz).
[0033] Additionally, various circuit breakers and switches, such as
grid breaker 182, system breaker 178, stator sync switch 158,
converter breaker 186, and line contactor 172 may be included in
the system 100 to connect or disconnect corresponding buses, for
example, when current flow is excessive and may damage components
of the wind turbine system 100 or for other operational
considerations. Additional protection components may also be
included in the wind turbine system 100.
[0034] Moreover, the power converter 162 may receive control
signals from, for instance, the control system 176 via the
controller 174. The control signals may be based, among other
things, on sensed conditions or operating characteristics of the
wind turbine system 100. Typically, the control signals provide for
control of the operation of the power converter 162. For example,
feedback in the form of a sensed speed of the DFIG 120 may be used
to control the conversion of the output power from the rotor bus
156 to maintain a proper and balanced multi-phase (e.g.
three-phase) power supply. In particular, as will be described
below, the sensed speed may be used as a basis for adjusting the
switching frequency of the switching elements (e.g., when the DIFG
120 is operating at its synchronous speed). Other feedback from
other sensors may also be used by the controller 174 to control the
power converter 162, including, for example, stator and rotor bus
voltages and current feedbacks. Using the various forms of feedback
information, switching control signals (e.g. gate timing commands
for IGBTs), stator synchronizing control signals, and circuit
breaker signals may be generated.
[0035] Referring now to FIG. 5, a schematic diagram of one
embodiment of the power converter shown in FIG. 4 is illustrated in
accordance with aspects of the present subject matter. As shown,
the rotor side converter 166 includes a plurality of bridge
circuits (e.g. H-bridge circuits), with each phase of the rotor bus
156 input to the rotor side converter 166 being coupled to a single
bridge circuit. In addition, the line side converter 168 may also
include a plurality of bridge circuits. Similar to the rotor side
converter 166, the line side converter 168 also includes a single
bridge circuit for each output phase of the line converter 168. In
other embodiments, the line side converter 168, the rotor side
converter 166, or both the line side converter 168 and the rotor
side converter 166 may include parallel bridge circuits without
deviating from the scope of the present disclosure.
[0036] Each bridge circuit may generally include a plurality of
switching elements (e.g. IGBTs) coupled in series with one another.
For instance, as shown in FIG. 5, each bridge circuit includes an
upper IGBT (e.g. IGBT 212) and a lower IGBT (e.g. IGBT 214). In
addition, a diode may be coupled in parallel with each of the
IGBTs. In alternative embodiments, parallel IGBTs and diodes may be
used to increase the current rating of the converter. As is
generally understood, the line side converter 168 and the rotor
side converter 166 may be controlled, for instance, by providing
control commands, using a suitable driver circuit, to the gates of
the IGBTs. For example, the controller 174 may provide suitable
gate timing commands to the gates of the IGBTs of the bridge
circuits. The control commands may control the switching frequency
of the IGBTs to provide a desired output. It should be appreciated
by those of ordinary skill in the art that, as an alternative to
IGBTs, the power convertor 162 may include any other suitable
switching elements.
[0037] Referring now to FIG. 6, a flow diagram of one embodiment of
a method 600 for operating a power generation system is illustrated
in accordance with aspects of the present subject matter. In
general, the method 600 will be described herein as being
implemented using a wind turbine system, such as the DFIG wind
turbine system 100 described above with reference to FIG. 4.
However, it should be appreciated that the disclosed method 600 may
be implemented using any other suitable power generation system
that is configured to supply power for application to a load. In
addition, although FIG. 6 depicts steps performed in a particular
order for purposes of illustration and discussion, the methods
described herein are not limited to any particular order or
arrangement. One skilled in the art, using the disclosures provided
herein, will appreciate that various steps of the methods can be
omitted, rearranged, combined and/or adapted in various ways.
[0038] At (602), the method 600 includes generating alternating
current power at a wind-driven generator. For instance, alternating
current power may be generated at a rotor of a wind-driven DFIG.
The alternating current power may be a multiphase alternating
current power, such as a three-phase alternating current power. The
generated alternating current power may be provided to a rotor bus
such that the alternating current power can be received at a power
converter (604).
[0039] At (606), the alternating current power generated at the
wind-driven generator is converted by the power converter to an
output power suitable for application to a load (e.g., to an
electrical grid). The power converter may, in several embodiments,
include a plurality of switching elements. As is generally
understood, the pulse width modulation of the switching elements
may be controlled to provide a suitable output power for
application to a load.
[0040] For instance, as described above, the power converter may be
a two-stage power converter that includes a rotor side converter
and a line side converter coupled together by a DC link. The rotor
side converter and the line side converter may each include a
plurality of bridge circuits, with each bridge circuit including a
plurality of switching elements coupled in series with one another.
The switching elements of the bridge circuits in the rotor side
converter may be controlled to convert the alternating current
power to a DC power for application to the DC link. The line side
converter may include a plurality of bridge circuits for converting
the DC power on the DC link to an output power suitable for
application to the load.
[0041] At (608), the method 600 includes receiving a control
command to control a switching frequency of the switching elements.
In general, control commands may be received by the power convertor
from a controller in order to control the switching frequency of
its switching elements. As indicated above, the switching frequency
is typically maintained at an elevated switching frequency (e.g.,
2000 Hz or 3000 Hz) regardless of the operating speed of the
wind-driven generator. However, in accordance with aspects of the
present subject matter, the switching frequency may be adjusted
when the wind-driven generator is operating at a speed that is
substantially equal to its synchronous speed. In such instances,
suitable control commands may be transmitted from the controller to
the power convertor to appropriately adjust the switching frequency
(e.g., to a frequency that is substantially equal to the
fundamental load frequency, as will be described below).
[0042] It should be appreciated that, as indicated above, the
"synchronous speed" of a generator generally refers to the speed at
which the rotor current is equal to DC current. In addition, it
should be appreciated that the operating speed of a generator may
be "substantially equal" to its synchronous speed when the
operating speed is within +/-10% of the synchronous speed, such as
by operating the generator at a speed of within +/-5% of the
synchronous speed or at a speed of within +/-2.5% of the
synchronous speed and any other subranges therebetween.
[0043] At (610), when the operating speed of the wind-driven
generator is reduced to or is otherwise at a speed that is
substantially equal to its synchronous speed, the switching
frequency of the switching elements may be adjusted to a frequency
that is substantially equal to the fundamental frequency of the
load. For example, when the load comprises an electrical grid, the
switching frequency may be adjusted to a frequency that is
substantially equal to the grid frequency (e.g., 50 Hz or 60 Hz).
As indicated above, by reducing the switching frequency in this
manner when the generator is operating at or near its synchronous
speed, the power losses, as well as the temperature-induced
stresses, associated with the switching elements may be reduced
significantly, thereby enhancing the overall performance of the
power convertor (e.g., by increasing the output current
capability). In addition, the amount of harmonic-based distortion
transmitted to the grid may also be reduced significantly.
[0044] It should be appreciated that the switching frequency may be
"substantially equal" to the fundamental frequency of the load
(e.g., the grid frequency) when the switching frequency is within
+/-10% of the fundamental frequency, such as when the switching
frequency is within +/-5% of the fundamental frequency or within
+/-2.5% of the fundamental frequency and any other subranges
therebetween. In addition, it should be appreciated that, in
several embodiments, the switching frequency may be considered to
be "substantially equal" to the fundamental frequency when the
difference between the switching frequency and the slip frequency
of the generator is within +/-10% of the fundamental frequency. For
instance, in such embodiments, assuming that the fundamental
frequency is 60 Hz and the slip frequency of the generator is 2 Hz,
the switching frequency may be "substantially equal" to the
fundamental frequency if the switching frequency, less 2 Hz, is
equal to a frequency that is within +/-10% of 60 Hz.
[0045] It should also be appreciated that, as indicated above, the
power converter may, in several embodiments, be a two-stage power
converter that includes a rotor side converter and a line side
converter coupled together by a DC link. In such embodiments, the
switching frequency of the switching elements forming part of the
rotor side convertor may be adjusted so as to be substantially
equal to the fundamental frequency when the wind-driven generator
is operating at or near its synchronous speed.
[0046] At (512), the output power is provided from the power
converter to a load. As indicated above, the load may be an
electrical grid. However, in other embodiments, the load may be a
motor, resistive load or any other load. It should be appreciated
that, while an electrical grid is traditionally a supplier of
power, the electrical grid may act as a load for the disclosed wind
turbine system 100.
[0047] Referring now to FIG. 7, an example data table is provided
that shows the difference in peak junction temperatures achieved at
various operating speeds, including synchronous speed, for a
typical 60 Hz DFIG using both a conventional control methodology
and the method 600 disclosed herein. All of the examples in FIG. 7
operate at the same rotor current. As indicated above, conventional
methodologies maintain the switching frequency on the rotor side of
the power convertor at an elevated frequency for all rotor speeds,
which leads to an increase in the IGBT/diode peak junction
temperatures at speeds at or near the synchronous speed. For
example, as shown in FIG. 7, by maintaining the switching frequency
at 2000 Hz as the rotor speed approaches synchronous speed (e.g.,
1200 RPM), the peak junction temperature rises significantly (e.g.,
to about 120.degree. C.). However, by adjusting the switching
frequency to the fundamental load frequency at synchronous speed,
the peak junction temperature may be significantly reduced (e.g.,
to about 88.degree. C.). In fact, as shown in FIG. 7, the resulting
temperature achieved using the disclosed method 600 may be
significantly lower than the temperatures achieved at other rotor
speeds, thereby providing an extra margin in the output current
capability of the power convertor.
[0048] This written description uses examples to disclose the
invention, including the best mode, and also to enable any person
skilled in the art to practice the invention, including making and
using any devices or systems and performing any incorporated
methods. The patentable scope of the invention is defined by the
claims, and may include other examples that occur to those skilled
in the art. Such other examples are intended to be within the scope
of the claims if they include structural elements that do not
differ from the literal language of the claims, or if they include
equivalent structural elements with insubstantial differences from
the literal languages of the claims.
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