System and method for reactive power control of a wind turbine

Abstract

A method and associated system for operating a power generation system to provide real power and reactive power to a load includes receiving power from a generator with a power converter having a switching element and generating reactive power within an operating range of the generator rotor speed. As the generator rotor speed changes and approaches synchronous speed, a control command is generated to reduce the switching frequency of the switching element in the power converter from a first switching frequency to a second switching frequency, wherein the reactive power output of the power converter is maintained or increased at the second switching frequency.

Description

Technical Field

[0001] The present disclosure relates generally to wind turbines, and more particularly to systems and methods for enhancing the reactive power capability of a wind turbine by controlling the switching frequency of a rotor side converter (RSC) in a doubly fed induction generator (DFIG) system. Background Art

[0002] Generally, during operation of a wind turbine, wind strikes rotor blades, and the blades convert wind energy into a mechanical torque that drives a low-speed shaft. The low-speed shaft drives a gearbox that then increases the low rotational speed of the low-speed shaft to drive a high-speed shaft at an increased rotational speed, wherein the high-speed shaft rotatably drives the generator rotor. In many conventional wind turbine configurations, the generator is electrically coupled to a bidirectional power converter that includes a rotor side converter (RSC) connected to a line side converter (LSC) via a regulated DC link. Each of the RSC and the LSC typically includes a pulse width modulated switching device group, such as an insulated gate bipolar transistor (IGBT module). The LSC converts the DC power on the DC link into an AC output power that is combined with the power from the generator stator to provide a multi-phase power having a frequency that is substantially maintained at the frequency of the grid bus (e.g., 50 Hz or 60 Hz).

[0003] The above system is generally considered to be a doubly fed induction generator (DFIG) system, the operating principle of which includes that the rotor winding is connected to the grid via slip rings and the power converter controls the rotor current and voltage. The control of the rotor voltage and current enables the generator to remain synchronized with the grid frequency when the wind turbine speed changes (for example, the rotor frequency may be different from the grid frequency). Moreover, the main sources of reactive power from the DFIG system are from the RSC (generator stator side reactive power) and from the LSC (generator line side reactive power) via the generator. The use of a power converter (particularly the RSC) to control the rotor current / voltage makes it possible to adjust the reactive power (and active power) fed from the RSC to the grid independently of the rotational speed of the generator. In addition, the generator is able to import or export reactive power, which allows the system to support the grid during extreme voltage fluctuations on the grid.

[0004] Typically, the amount of reactive power supplied by a wind farm to the grid during steady-state and transient conditions is determined by regulatory requirements dictated by a grid operator, with a wind farm controller determining reactive power requirements for each wind turbine within the wind farm. A local controller at each wind turbine receives and distributes reactive power requirements between generator sources (e.g., between generator-side reactive power and line-side reactive power).

[0005] As the generator speed approaches synchronous speed, the rotor fundamental frequency approaches DC (where thermal cycling of the IGBTs is greatest), resulting in peak temperatures of the rotor-side IGBTs at or near synchronous speed. This results in a reduction in the total output current capability of the RSC, and therefore a reduction in the reactive power capability of the RSC. Typically, the switching frequency on the rotor side of the DFIG power converter is maintained at an elevated frequency (e.g., about 2000 or 3000 Hz) for all rotor speeds. While this elevated switching frequency is desirable for most operating speeds at or near the synchronous generator rotor speed, it generates peak temperatures and thermal cycling stresses in the IGBTs and limits the reactive power capability of the DFIG system.

[0006] Additionally, operation of a DFIG generator at or near synchronous speed is more complicated because current harmonics are fed 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 the utility grid harmonic requirements.

[0007] U.S. Patent No. 8,853,876 describes a system and method for operating a DFIG power generation system in a wind turbine. When the generator is at or near synchronous speed, a control command is generated to control the switching frequency of the switching elements in the power converter to an adjusted switching frequency that is substantially equal to the load fundamental frequency. By reducing the switching frequency as the generator speed decreases, the power loss in the switching elements can be reduced. With such a reduction in power loss, the temperature rise in the switching elements can also be reduced, which can provide additional margin in the output current capability of the power converter and can also increase the component life of the switching elements. In addition, by closely matching the switching frequency to the fundamental frequency of the power grid, a reduction in the amount of harmonics fed through to the line side of the converter can also be obtained, thereby reducing harmonic distortion to the power grid.

[0008] US Patent No. 9,625,921 describes a method for regulating the temperature of an IGBT in a power converter to reduce thermal stress and extend device life. When the IGBT is not within a predetermined temperature range, the switching frequency of the device is modified to keep the IGBT within the temperature range.

[0009] It would be desirable in the industry to have systems and methods for operating power converters in a power generation system, such as a wind turbine DFIG system, in a manner that enhances the reactive power generation capability of the system in real time while keeping harmonic distortion within limits. Summary of the invention

[0010] 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.

[0011] In one aspect, the present disclosure relates to a method for operating a power generation system that supplies real power and reactive power to a load. A power converter in the system includes a switching element and receives an AC power generated by a generator, the power converter generating reactive power within an operating range of the generator rotor speed. When the generator rotor speed changes and approaches a synchronous speed, a control command is generated to reduce the switching frequency of the switching element in the power converter from a first switching frequency to a second switching frequency, wherein the reactive power output of the power converter increases at the second switching frequency. The second switching frequency is determined during the actual operating time of the generator and is based on providing a defined reactive power value generated by the power converter at a changed generator rotor speed (such as a speed at or near the synchronous speed) while keeping the switching element within a defined thermal limit.

[0012] The second switching frequency may also be based on maintaining the total harmonic distortion (THD) of the system within defined limits, which may be limits dictated by a grid operator, by minimizing harmonic distortion induced by the power converter and transferred to the load.

[0013] In a certain embodiment of the method, the power converter includes a line side converter (LSC) and a rotor side converter (RSC), wherein the RSC generates reactive power.

[0014] In certain embodiments, in a wind turbine power generation system, the generator (with LSC and RSC) may be a doubly-fed induction generator (DFIG).

[0015] In one embodiment, the switching element may be an insulated gate bipolar transistor (IGBT) element.

[0016] In a particular embodiment, the controller determines the second switching frequency from a lookup table that cross-references reactive power values ​​generated at the second switching frequency for different generator rotor speeds. The lookup table may provide different generator rotor speeds as a plurality of sub-ranges, wherein for each sub-range, the second switching frequency is provided for a particular reactive power value. For each sub-range of generator rotor speeds, the lookup table may provide a plurality of different reactive power values ​​and corresponding second switching frequencies.

[0017] In one embodiment, the required reactive power value from the power generation system at a particular generator rotor speed is predefined (eg, based on known characteristics of the power generation system and known load requirements) and input into a lookup table to determine the second switching frequency.

[0018] In an alternative embodiment, the required reactive power value from the power generation system at a particular generator rotor speed is calculated or otherwise determined in real time (e.g., by measuring the reactive power supplied by the power generation system or the load reactive power at a normal operating rotor speed of the generator (such as a normal supersynchronous speed) before the rotor generator speed is reduced to at or near synchronous speed) and input into a lookup table to determine the second switching frequency.

[0019] Furthermore, the second switching frequency may be determined such that the harmonic distortion induced by the power converter and transferred to the load is within predefined limit values.

[0020] The present disclosure also includes a wind turbine system configured to supply active power and reactive power to a load, the system including a wind turbine rotor including a hub and a plurality of blades coupled to the hub. A double-fed induction generator (DFIG) is coupled to the wind turbine rotor and is capable of operating within a range of generator rotor speeds. A power converter is operatively configured with the DFIG and includes a line side converter (LSC) and a rotor side converter (RSC), the RSC being configured to generate reactive power. A controller communicates with the power converter, and when the generator rotor speed changes and approaches synchronous speed, the controller generates a control command to reduce the switching frequency of a switching element (e.g., an insulated gate bipolar transistor (IGBT) element) in the RSC from a first switching frequency to a second switching frequency to increase the reactive power output of the RSC. The second switching frequency is determined by the controller during the actual operating time of the generator to provide a defined reactive power value generated by the RSC at a changed generator rotor speed (such as a speed at or near synchronous speed) while keeping the switching element within a defined thermal limit.

[0021] In a particular embodiment of the system, the controller determines the second switching frequency from a lookup table that cross-references reactive power values ​​generated at the second switching frequency for different generator rotor speeds. The lookup table may provide different generator rotor speeds as a plurality of sub-ranges, wherein for each sub-range, the second switching frequency is provided for a particular reactive power value. For each sub-range of generator rotor speed, the lookup table may provide a plurality of different reactive power values ​​and corresponding second switching frequencies.

[0022] In one embodiment of the system, the required reactive power value from the RSC at a particular generator rotor speed is predefined (eg, based on known characteristics of the power generation system and known load requirements) and input into a lookup table to determine the second switching frequency.

[0023] In an alternative embodiment of the system, the required reactive power value from the RSC at a particular generator rotor speed is measured in real time (e.g., by measuring the reactive power supplied by the power generation system or the load reactive power at a normal operating rotor speed of the generator (such as a normal supersynchronous speed) before the rotor generator speed is reduced to at or near synchronous speed) and input into a lookup table to determine the second switching frequency.

[0024] The present disclosure also includes a power generation system configured to supply real power and reactive power to a load, the system including a generator capable of operating at a generator rotor speed. A power converter is coupled to the generator and includes a plurality of switching elements configured to generate reactive power. A controller communicates with the power converter and generates a control command to reduce the switching frequency of the switching element from a first switching frequency to a second switching frequency to increase the reactive power output of the power converter when the generator rotor speed approaches a synchronous speed. The second switching frequency is determined by the controller in real time and is based on providing a defined reactive power value generated by the power converter at a changed generator rotor speed (such as a speed at or near the synchronous speed) while keeping the switching element within a defined thermal limit.

[0025] It should be understood that the methods and systems may also include any combination of additional features and / or steps as described herein.

[0026] 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 present invention and, together with the description, serve to explain the principles of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] A complete and enabling disclosure of the present invention, including the best mode thereof, to one of ordinary skill in the art is set forth in the specification with reference to the accompanying drawings, in which:

[0028] Figure 1 A perspective view showing an embodiment of a wind turbine according to the present disclosure;

[0029] Figure 2 Shown suitable for Figure 1 A schematic diagram of one embodiment of a wind turbine electrical power system for use with a wind turbine shown in;

[0030] Figure 3 is a schematic diagram of one embodiment of a power converter for a wind turbine according to the present disclosure;

[0031] Figure 4is a flow chart of one embodiment of a method for operating a switching device of a power converter of an electric power system connected to a power grid or a load according to the present disclosure; and

[0032] Figure 5 is an example of a lookup table according to aspects of the present disclosure. DETAILED DESCRIPTION

[0033] Reference will now be made in detail to embodiments of the present invention, one or more examples of which are shown in the drawings. Each example is provided as an explanation of the present invention, not a limitation of the present invention. In fact, it will be apparent to those skilled in the art that various modifications and variations may be made in the present invention without departing from the scope or spirit of the present invention. For example, the features shown or described as part of an embodiment may be used with another embodiment to produce further embodiments. Therefore, it is intended that the present invention covers such modifications and variations as fall within the scope of the appended claims and their equivalents.

[0034] In general, the present subject matter relates to a system and method for operating a power generation system, such as a wind turbine configured with a doubly fed induction generator (DFIG) system. As mentioned, in a specific embodiment of the system and method, the present subject matter relates to a system and method for operating a power converter of a wind-driven DFIG, so as to increase the reactive power output of the power converter when the generator is operated at or near its synchronous speed, while keeping the switching elements within defined thermal limits and controlling the harmonic distortion introduced by the converter. For example, at normal power generation speeds, the DFIG is typically operated at super synchronous speeds. However, during certain operating modes (e.g., during a noise reduction operating mode of a wind turbine system), the generator may be operated at a speed at or near its synchronous speed, wherein the reactive power capability of the power converter is limited by the thermal cycling constraints of the switching elements (e.g., IGBTs). In such cases, the present method and system propose to increase the reactive power output of the power converter by reducing the switching frequency of the switching elements to a certain frequency, which is determined by a controller in real time and based on providing a predefined reactive power value from the power converter at different generator rotor speeds. The reduced switching frequency is also based on maintaining the switching element within defined thermal limits (eg, below a defined delta temperature value during thermal cycling of the device).

[0035] Although not limited to such configurations, for purposes of explanation, the present method and system aspects of the invention are described herein with reference to a wind turbine power generation system, and more particularly with reference to a wind turbine DFIG system that supplies real power and reactive power to a grid.

[0036] Now refer to the figure, Figure 1A perspective view of one embodiment of a wind turbine 10 is shown. 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 the hub 20 and extending outwardly from the hub 20. For example, in the illustrated embodiment, the rotor 18 includes three rotor blades 22. However, in alternative embodiments, 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 converted from the wind into usable mechanical energy and subsequently into electrical energy. For example, as will be described below, the rotor 18 may be rotatably coupled to a generator 120 ( Figure 2 ), to allow the generation of electrical energy.

[0037] Wind power generation is typically provided by a wind farm having a large number (usually 100 or more) of wind turbines, wherein each individual wind turbine 10 typically experiences unique wind conditions. Thus, the output power for each individual wind turbine generator 120 may vary from one wind turbine 10 to another wind turbine 10 within the wind farm.

[0038] As generally understood, active power (P) and reactive power (Q) are provided by each wind turbine generator 120. In some embodiments, a farm-level controller provides reactive power commands to the wind turbine generators 120 based on the needs of the transmission grid (which may be indicated by a grid operator or determined based on the grid voltage). The (Q) demand may be the same for each wind turbine generator. In an alternative control method, as described in, for example, U.S. Patent Publication No. 2015 / 0295529, reactive power commands may be individually customized for wind turbine generators 120 in a wind farm based on different power generation characteristics of the respective wind turbine generators 120. It should be understood that the present invention is not limited to the manner or method in which reactive power commands are generated for individual wind turbine generators 120.

[0039] Now refer to Figure 2 , showing a schematic diagram of one embodiment of a wind turbine DFIG power system 100 according to aspects of the present subject matter. Figure 2 While the present subject matter is generally described with reference to system 100 shown in FIG. 1 , one of ordinary skill in the art (using the disclosure provided herein) will appreciate that aspects of the present disclosure may also be applicable in other power generation systems and that the invention is not limited to wind turbine systems.

[0040] exist Figure 2 In the embodiment of the present invention, the wind turbine 10 ( Figure 1)'s rotor 18 may optionally be coupled to a gearbox 118, which in turn is coupled to a generator 120, which may be a doubly-fed induction generator (DFIG).

[0041] As shown, the DFIG 120 is connected to the stator bus 154. The power converter is connected to the DFIG 120 via the rotor bus 156 and to the stator bus 154 via the line side bus 188. The stator bus 154 provides output multi-phase power (e.g., three-phase power) from the stator of the DFIG 120, and the rotor bus 156 provides output multi-phase power (e.g., three-phase power) from the rotor of the DFIG 120. The power converter 162 includes a rotor side converter (RSC) 166 and a line side converter (LSC) 168. The DFIG 120 is coupled to the rotor side converter 166 via the rotor bus 156. In addition, the RSC 166 is coupled to the LSC 168 via the DC link 136, and the DC link capacitor 138 spans the DC link 136. The LSC 168 is in turn coupled to the line side bus 188.

[0042] As about Figure 3 Discussed in greater detail, the RSC 166 and LSC 168 may be configured for a normal operating mode in a three-phase pulse width modulation (PWM) arrangement using insulated gate bipolar transistor (IGBT) switching elements.

[0043] In addition, power converter 162 is coupled to converter controller 174 to control the operation of rotor-side converter 166 and line-side converter 168. It should be noted that in several embodiments, controller 174 may be configured as an interface between power converter 162 and local controller 176 and include any number of control devices. In one embodiment, converter controller 174 may include a processing device (e.g., a microprocessor, a microcontroller, etc.) that executes 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 power converter 162.

[0044] It should be appreciated that the converter controller 174 and the local wind turbine controller 176 may each correspond to any suitable computing device and / or any combination of computing devices. For example, the controller may include one or more processors and associated memory devices configured to perform a variety of computer-implemented functions. As used herein, the term "processor" refers not only to integrated circuits that are considered to be included in a computer in the art, but also to controllers, microcontrollers, microcomputers, programmable logic controllers (PLCs), application-specific integrated circuits, and other programmable circuits. In addition, the memory device may generally include memory elements, including but not limited to: computer-readable media (e.g., random access memory (RAM)), computer-readable non-volatile media (e.g., flash memory), floppy disks, compact disks-read only memories (CD-ROMs), magneto-optical disks (MODs), digital versatile disks (DVDs), and / or other suitable memory elements. Such memory devices may generally be configured to store suitable computer-readable instructions that, when implemented by a processor, configure the controller to perform various functions, such as the steps disclosed herein.

[0045] As mentioned, for a standalone DFIG wind turbine power system 100, reactive power (Q) is primarily supplied by the RSC via the generator 120 and by the LSC.

[0046] In a typical configuration, various line contactors and circuit breakers may also be included, including, for example, a grid circuit breaker 182, for isolating various components necessary for normal operation of the DFIG 120 during connection to or disconnection from a load, such as a grid 184. For example, a system circuit breaker 178 may couple the system bus 160 to a transformer 180, which may be coupled to a grid 184 via a grid circuit breaker 182. In alternative embodiments, fuses may replace some or all of the circuit breakers.

[0047] In operation, the AC power generated at the DFIG 120 by the rotating rotor 18 is provided to the grid 184 via a dual path, which is defined by the stator bus 154 and the rotor bus 156. On the rotor bus side 156, the 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 generally understood, the switching elements (i.e., IGBTs) used in the bridge circuit of the rotor-side power converter 166 can be modulated to convert the AC power provided from the rotor bus 156 into DC power suitable for the DC link 136.

[0048] In addition, the line-side converter 168 converts the DC power on the DC link 136 into an AC output power suitable for the grid 184. In particular, the switching elements (e.g., IGBTs) used in the bridge circuit of the line-side power converter 168 may 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 may be combined with the power from the stator of the DFIG 120 to provide a multi-phase power (e.g., three-phase power) having a frequency that is substantially maintained at the frequency of the grid 184 (e.g., 50 Hz or 60 Hz).

[0049] In addition, various circuit breakers and switches (such as grid breaker 182, system breaker 178, stator synchronizing switch 158, converter breaker 186, and line contactor 172) may be included in wind turbine power system 100 to connect or disconnect corresponding buses, for example, when current is too large and may damage components of wind turbine power system 100 or for other operational considerations. Additional protection components may also be included in wind turbine power system 100.

[0050] Moreover, the power converter 162 may receive control signals from the local controller 176 via the converter controller 174. The control signals may be based, among other things, on a sensed condition or operating characteristic of the wind turbine power system 100 and provide control for 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 suitable 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 DFIG 120 is operating at or near 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 voltage and current feedback. Using various forms of feedback information, switch control signals (e.g., gate timing commands for IGBTs), stator synchronization control signals, and circuit breaker signals may be generated.

[0051] Power converter 162 also compensates or adjusts the frequency of the three-phase power from the rotor for changes in wind speed, for example, at hub 20 and blades 22. Thus, the mechanical and electrical rotor frequencies are decoupled, and electrical stator and rotor frequency matching is facilitated, substantially independent of mechanical rotor speed.

[0052] In some conditions, the bidirectional nature of power converter 162 (and in particular the bidirectional nature of LSC 168 and RSC 166) facilitates feeding at least some of the generated electrical power back into the generator rotor. More particularly, electrical power is transmitted from stator bus 154 to line side bus 188, and then through line contactor 172 and into power converter 162, in particular LSC 168, which acts as a rectifier and rectifies the sinusoidal three-phase AC power into DC power. The DC power is transmitted into DC link 136. Capacitor 138 facilitates mitigating DC link voltage amplitude variations by facilitating mitigation of DC ripple (sometimes associated with three-phase AC rectification).

[0053] The DC power is then transmitted to the RSC 166, which converts the DC electric power into three-phase sinusoidal AC electric power with a predetermined voltage, current and frequency. The conversion is monitored and controlled via the controller 174. The converted AC power is transmitted from the RSC 166 to the generator rotor via the rotor bus 156. In this way, the rotor current and voltage are controlled to facilitate generator reactive power control.

[0054] Now refer to Figure 3 , showing aspects according to the present subject Figure 2 Schematic diagram of one embodiment of a power converter shown in . As shown, the rotor side converter (RSC) 166 includes a plurality of bridge circuits (e.g., H-bridge circuits), wherein each phase of the rotor bus 156 input to the rotor side converter 166 is coupled to a single bridge circuit. In addition, the line side converter (LSC) 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, without departing from the scope of the present disclosure, 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.

[0055] Each bridge circuit may generally include a plurality of switching elements (eg, IGBTs) connected in series with each other. Figure 3As shown in , 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 connected in parallel with each of the IGBTs. In an alternative embodiment, the parallel IGBTs and diodes may be used to increase the current rating of the converter. As generally understood, the line side converter 168 and the rotor side converter 166 may be controlled, for example, by providing control commands to the gates of the IGBTs using suitable driver circuits. For example, the controller 174 may provide suitable gate timing commands to the gates of the IGBTs of the bridge circuits. The control command may control the switching frequency of the IGBT to provide a desired output. It should be understood by those of ordinary skill in the art that, as an alternative to the IGBT, the power converter 162 may include any other suitable switching element.

[0056] Reference Figure 3 , each IGBT 212 includes a power diode, wherein the IGBT and the power diode are connected to a positive or negative DC line, and the output line outputs a three-phase voltage. Changes to the output current of the IGBT 212 (used to generate a three-phase output current waveform at the output line) may result in power loss, resulting in a higher junction temperature at the IGBT 212. Such junction temperatures may cause mechanical strain and / or deformation of the bonding wires, thereby shortening the life of the converters 166, 168.

[0057] It is recognized that the junction thermal cycle temperature difference (ΔT) generated in the cycle of the IGBT from on to off significantly affects the number of cycles that the device can maintain and therefore the life of the IGBT. For example, a reduction in temperature change (ΔT) (up to about 10°C) can improve the semiconductor life by a factor of two. Changing ΔT from 10°C to 30°C can improve the life from less than 100,000 cycles to more than 400,000 cycles. By reducing the switching frequency of the RSC 166, the ΔT is reduced. Therefore, the second switching frequency can also be determined based on keeping the junction ΔT below a limited value.

[0058] Now refer to Figure 4 , a flow chart showing one embodiment of a method 300 for operating a power generation system according to aspects of the present subject matter. In general, the method 300 is described herein as using a wind turbine power system (such as described above with reference to Figure 2 However, it should be appreciated that the disclosed method 300 may be implemented using any other suitable power generation system configured to supply power (including reactive power) for application to a load (such as a power grid). Figure 4The steps performed in a particular order are depicted for the purpose of illustration and discussion, and the methods described herein are not limited to any particular order or arrangement. Those skilled in the art (using the disclosure provided herein) will appreciate that the various steps of the methods may be omitted, rearranged, combined and / or adjusted in various ways.

[0059] At (302), method 300 includes generating alternating current (AC) power with a generator of a power generation system, such as a wind turbine power system incorporating a DFIG as discussed above. The AC power may be multi-phase AC power, such as three-phase AC power.

[0060] At (304), AC power is received by a power converter via a bus and converted by the power converter into output power suitable for application to a load (e.g., to a grid). The output power includes real power (P) and reactive power (Q). The power converter includes a plurality of switching elements, such as IGBTs, wherein pulse width modulation of the switching elements can be controlled at a first switching frequency to provide output power for application to a load at a first operating speed of the generator rotor. The first switching frequency on a rotor side converter (RSC) of the DFIG power converter can be a frequency typically used for all generator rotor speeds within a normal operating speed range of the generator (e.g., approximately 2000 or 3000 Hz). However, as discussed above, although this first (increased) switching is desirable for most operating speeds, it results in peak temperatures and thermal cycling stresses in the switching elements and limits the reactive power output of the power converter (particularly the RSC) to at or near the synchronous speed of the generator rotor.

[0061] At (306), a reduced second switching frequency is determined for the switching element at a changed generator rotor speed, particularly a speed at or near synchronous speed. The power converter receives a control command from the controller to reduce the switching frequency of the switching element to the second switching frequency. The second switching frequency is determined based on the changed generator rotor speed during the actual operating time of the generator and is selected to provide a defined reactive power value from the power converter at the changed generator rotor speed at or near synchronous speed. The second switching frequency may also be defined based on keeping the IGBT below a junction ΔT value. The second switching frequency compensates for a shortfall in reactive power that would otherwise result if the first switching frequency were maintained at the changed generator rotor speed.

[0062] It will be appreciated that the second switching frequency based on the variables of rotor speed, reactive power, and ΔT limit may be determined empirically via computer modeling, testing of actual systems, and by any other suitable means.

[0063] In one embodiment, the change (deficit) in reactive power at the power converter may be measured in real time as the generator rotor speed changes, wherein the switching frequency of the IGBTs is reduced in a feedback control loop until the reactive power deficiency is compensated by the second switching frequency.

[0064] In an alternative embodiment, the second switching frequency may be predefined. For example, the controller may select a predefined and stored lookup table 216 ( Figure 5 ) determines the second switching frequency, the lookup table 216 cross-references the reactive power values ​​generated at the second switching frequency for different generator rotor speeds at or near synchronous speed (in Figure 5 In the embodiment of the lookup table 216 depicted in FIG. 1 , the synchronous speed of the generator rotor is 1200 rpm). The information in the lookup table can be presented in various ways within the range and spirit of the "pre-defined" values. For example, referring to Figure 5 , the lookup table 216 provides different generator rotor speeds as a plurality of sub-ranges (e.g., 1140-1170 rpm; 1170-1190 rpm, etc.), wherein for each sub-range, a second switching frequency is provided for a particular reactive power value, the second switching frequency also being determined to satisfy the ΔT limit defined for the switching element. For example, to provide 2250 kVar from the power converter at a generator rotor speed in the sub-range of 1190-1210 rpm, at a normal operating speed of the generator rotor (e.g., a speed outside of "at or near" the synchronous speed band), the second switching frequency is 75% of the first switching frequency (base value). As Figure 5 As shown in , for each sub-range of the generator rotor speed, the lookup table 216 can provide a plurality of different reactive power values ​​and corresponding second switching frequencies. For example, to provide 2300 kVar from the power converter at the same generator rotor speed in the sub-range of 1190-1210 rpm, the second switching frequency is 70% of the first switching frequency.

[0065] The desired reactive power generation value required to be produced by the power converter at the changed generator rotor speed (e.g., 2200 kVar or 2300 kVar) can be pre-defined based on the known characteristics of the power generation system at the first switching frequency and / or the known load (grid) reactive power requirements, and input into the lookup table to determine the second switching frequency. Alternatively, the reactive power output of the power converter can be measured at the first switching frequency and used to input into the lookup table 216 to determine the second switching frequency that will be required at the changed generator rotor speed.

[0066] In certain embodiments, it may also be desirable to limit or restrict the second switching frequency so that the harmonic distortion induced by the power converter and converted to the grid via the LSC or generator is within a predefined total harmonic distortion (THD) value set for the power generation system.

[0067] At (308), a command is generated (eg, from the controller 174) to reduce the switching frequency to a second switching frequency in order to increase reactive power output of the power converter. The command may be generated before, during, or after the generator rotor speed is changed.

[0068] At (310), the increased reactive power output from the power converter at the changed generator rotor speed is supplied to a load, which may be a grid. However, in other embodiments, the load may be a motor, a resistive load, or any other load. It should be appreciated that while the grid is traditionally a supplier of power, the grid may be used as a load for the disclosed wind turbine power system 100.

[0069] 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 the DC current. Additionally, it should be appreciated that the operating speed of the generator may be "at or near" its synchronous speed when the operating speed is within + / -10% of the synchronous speed, such as by operating the generator at a speed within + / -5% of the synchronous speed or at a speed within + / -2.5% of the synchronous speed and any other sub-range therebetween.

[0070] Various aspects and embodiments of the invention are defined by the following numbered clauses:

[0071] 1. A method for operating a power generation system for supplying real power and reactive power to a load, comprising:

[0072] A power converter having a switching element receives AC power from a generator and generates reactive power within an operating range of a generator rotor speed;

[0073] generating a control command to reduce the switching frequency of the switching element from a first switching frequency to a second switching frequency when the generator rotor speed changes and approaches synchronous speed, wherein the reactive power output of the power converter increases at the second switching frequency; and

[0074] Wherein the second switching frequency is determined during the actual operating time of the generator based on providing a defined reactive power value from the power converter at a changed generator rotor speed at or near synchronous speed and maintaining the switching elements within defined thermal limits.

[0075] 2. A method as in clause 1, wherein the power converter comprises a line side converter (LSC) and a rotor side converter (RSC), the RSC generating reactive power.

[0076] 3. A method as in any preceding clause, wherein the generator is a doubly-fed induction generator (DFIG) in a wind turbine power generation system.

[0077] 4. A method as in any preceding clause, wherein the switching element comprises an insulated gate bipolar transistor (IGBT) element.

[0078] 5. A method as in any preceding clause, wherein the controller determines the second switching frequency from a look-up table cross-referencing reactive power values ​​generated at the second switching frequency at different generator rotor speeds.

[0079] 6. A method as in any preceding clause, wherein the look-up table provides different generator rotor speeds as a plurality of sub-ranges, and for each sub-range the second switching frequency is provided for the reactive power value.

[0080] 7. A method as in any preceding clause, wherein the look-up table provides a plurality of reactive power values ​​and a corresponding second switching frequency for each sub-range at each of the reactive power values.

[0081] 8. A method as in any preceding clause, wherein a required reactive power value from the power generating system at a particular generator rotor speed is predefined and input into the look-up table to determine the second switching frequency.

[0082] 9. A method as in any preceding clause, wherein a required reactive power value from the power generating system at a particular generator rotor speed is measured in real time and input into a look-up table to determine the second switching frequency.

[0083] 10. A method as in any preceding clause, wherein the second switching frequency is determined such that harmonic distortion in the load induced by the power converter is within predefined limits.

[0084] 11. A wind turbine system configured to supply real power and reactive power to a load, comprising:

[0085] a wind turbine rotor comprising a hub and a plurality of blades coupled to the hub;

[0086] a doubly-fed induction generator (DFIG) coupled to the wind turbine rotor and operable at a generator rotor speed;

[0087] a power converter configured with a DFIG and comprising a line side converter (LSC) and a rotor side converter (RSC), the RSC comprising a plurality of switching elements configured to generate reactive power;

[0088] a controller in communication with the power converter and configured to generate a control command to reduce a switching frequency of a switching element in the RSC from a first switching frequency to a second switching frequency to increase output of reactive power from the RSC when the generator rotor speed changes and approaches synchronous speed; and

[0089] Wherein the second switching frequency is determined by the controller during actual operating time of the generator to provide a defined reactive power value generated by the RSC at a varying generator rotor speed at or near synchronous speed while keeping the switching elements within defined thermal limits.

[0090] 12. A wind turbine system as in clause 11, wherein the switching element comprises an insulated gate bipolar transistor (IGBT) element.

[0091] 13. A wind turbine system as in any of clauses 10-12, wherein the controller determines the second switching frequency from a look-up table cross-referencing reactive power values ​​generated at the second switching frequency at different generator rotor speeds.

[0092] 14. A wind turbine system as in any of clauses 10-13, wherein the look-up table comprises different generator rotor speeds presented as a plurality of sub-ranges, and for each sub-range the second switching frequency is provided for a specific reactive power value.

[0093] 15. A wind turbine system as in any of clauses 10-14, wherein the look-up table comprises a plurality of reactive power values ​​and corresponding second switching frequencies within a sub-range of the generator rotor speed.

[0094] 16. A wind turbine system as in any of clauses 10-15, wherein a required reactive power value from the RSC at a specific generator rotor speed is predefined and input into a look-up table to determine the second switching frequency.

[0095] 17. A wind turbine system as in any of clauses 10-16, wherein the required reactive power value from the RSC at a specific generator rotor speed is measured in real time and input into a look-up table to determine the second switching frequency.

[0096] 18. A power generation system configured to supply real power and reactive power to a load, comprising:

[0097] a generator operable at a generator rotor speed;

[0098] a power converter coupled to the generator and comprising a plurality of switching elements configured to generate reactive power;

[0099] a controller in communication with the power converter and configured to generate a control command to reduce a switching frequency of the switching element from a first switching frequency to a second switching frequency to increase reactive power output from the power converter when the generator rotor speed changes and approaches synchronous speed; and

[0100] Wherein the second switching frequency is determined by the controller during actual operating time of the generator to provide a defined reactive power value generated by the power converter at a changed generator rotor speed at or near synchronous speed while keeping the switching elements within defined thermal limits.

[0101] 19. A power generation system as in clause 18, wherein the controller determines the second switching frequency from a look-up table cross-referencing reactive power values ​​produced at the second switching frequency at different generator rotor speeds.

[0102] 20. A power generation system as in any of clauses 18-19, wherein the lookup table comprises different generator rotor speeds presented as a plurality of sub-ranges, and for each sub-range the second switching frequency is provided for a specific reactive power value, and wherein the lookup table comprises a plurality of reactive power values ​​and corresponding second switching frequencies within a sub-range of the generator rotor speed.

[0103] 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 device or system and performing any combined method. 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 language of the claims.

Claims

1. A method for operating a power generation system for supplying real power and reactive power to a load, comprising: a power converter having switching elements for receiving AC power from a generator and generating said reactive power within an operating range of a generator rotor speed; generating a control command to reduce the switching frequency of the switching element from a first switching frequency to a second switching frequency when the generator rotor speed changes and approaches synchronous speed, wherein the reactive power output of the power converter increases at the second switching frequency; and The second switching frequency is determined during actual operating time of the generator based on providing a defined reactive power value from the power converter at a varying generator rotor speed at or near synchronous speed and maintaining the switching elements within defined thermal limits.

2. The method according to claim 1, characterized in that: The power converter includes a line side converter (LSC) and a rotor side converter (RSC), and the rotor side converter (RSC) generates the reactive power.

3. The method according to claim 2, characterized in that The generator is a doubly-fed induction generator (DFIG) in a wind turbine power generation system.

4. The method according to claim 2, characterized in that: The switching element includes an insulated gate bipolar transistor (IGBT) element.

5. The method according to claim 1, characterized in that The controller determines the second switching frequency from a lookup table that cross-references reactive power values ​​generated at the second switching frequency at different generator rotor speeds.

6. The method according to claim 5, characterized in that The look-up table provides the different generator rotor speeds as a plurality of sub-ranges, and for each sub-range a second switching frequency is provided for the reactive power value.

7. The method according to claim 6, characterized in that The lookup table provides a plurality of reactive power values ​​and a corresponding second switching frequency for each sub-range at each of the reactive power values.

8. The method according to claim 5, characterized in that The required reactive power value from the power generation system at a specific generator rotor speed is predefined and input into the look-up table to determine the second switching frequency.

9. The method according to claim 5, characterized in that The required reactive power value from the power generation system at a specific generator rotor speed is measured in real time and input into the lookup table to determine the second switching frequency.

10. The method according to claim 1, characterized in that The second switching frequency is determined such that harmonic distortion induced in the load by the power converter is within predefined limits.

11. A wind turbine system configured to supply real power and reactive power to a load, comprising: a wind turbine rotor comprising a hub and a plurality of blades coupled to the hub; a doubly-fed induction generator (DFIG) coupled to the wind turbine rotor and operable at a generator rotor speed; a power converter configured with a doubly-fed induction generator (DFIG) and comprising a line side converter (LSC) and a rotor side converter (RSC), the rotor side converter (RSC) comprising a plurality of switching elements configured to generate the reactive power; a controller in communication with the power converter and configured to generate a control command to reduce a switching frequency of the switching element in the rotor side converter (RSC) from a first switching frequency to a second switching frequency to increase an output of the reactive power from the rotor side converter (RSC) when a generator rotor speed changes and approaches a synchronous speed; and wherein the second switching frequency is determined by the controller during actual operating time of the generator to provide a defined reactive power value generated by the rotor side converter (RSC) at a varying generator rotor speed at or near synchronous speed while keeping the switching elements within defined thermal limits.

12. The wind turbine system according to claim 11, characterized in that The switching element includes an insulated gate bipolar transistor (IGBT) element.

13. The wind turbine system according to claim 11, wherein: The controller determines the second switching frequency from a lookup table that cross-references reactive power values ​​generated at the second switching frequency at different generator rotor speeds.

14. The wind turbine system according to claim 13, wherein: The look-up table includes the different generator rotor speeds presented as a plurality of sub-ranges, and for each sub-range a second switching frequency is provided for a specific reactive power value.

15. The wind turbine system according to claim 14, wherein: The look-up table includes a plurality of reactive power values ​​and corresponding second switching frequencies within the sub-range of generator rotor speed.

16. The wind turbine system of claim 14, wherein: The required reactive power value from the rotor side converter (RSC) at a specific generator rotor speed is predefined and input into the look-up table to determine the second switching frequency.

17. The wind turbine system of claim 14, wherein: The required reactive power value from the rotor side converter (RSC) at a specific generator rotor speed is measured in real time and input into the lookup table to determine the second switching frequency.

18. A power generation system configured to supply real power and reactive power to a load, comprising: a generator operable at a generator rotor speed; a power converter coupled to the generator and comprising a plurality of switching elements configured to generate the reactive power; a controller in communication with the power converter and configured to generate a control command to reduce a switching frequency of the switching element from a first switching frequency to a second switching frequency to increase reactive power output from the power converter when the generator rotor speed changes and approaches a synchronous speed; and wherein the second switching frequency is determined by the controller during actual operating time of the generator to provide a defined reactive power value generated by the power converter at a varying generator rotor speed at or near synchronous speed while keeping the switching element within defined thermal limits.

19. The power generation system according to claim 18, characterized in that The controller determines the second switching frequency from a lookup table that cross-references reactive power values ​​produced at the second switching frequency at different generator rotor speeds.

20. The power generation system of claim 19, wherein: The look-up table comprises the different generator rotor speeds presented as a plurality of sub-ranges, and for each sub-range a second switching frequency is provided for a specific reactive power value, and wherein the look-up table comprises a plurality of reactive power values ​​and corresponding second switching frequencies within the sub-ranges of generator rotor speed.

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