DFIG converter with active filter

Abstract

A system and method for operating a power system having a doubly fed induction generator is provided. In an exemplary implementation, an electric power system (100) connected to an electric power grid (190) may include: a generator (120) including a stator and a rotor, the stator being connected to the electric power grid via a stator power path; and a power converter (130). The power converter (130) may include: a line-side converter (134) coupled to the electric power grid (190) via a converter power path; and a rotor-side converter (132) coupled to a rotor bus (124) of the rotor and coupled to the line-side converter (134) via a DC link, the rotor-side converter (132) being configured to convert DC power on the DC link into an AC signal for the rotor bus (124). The power system (100) may further include an active filter (250) including one or more actively controlled components, the active filter (250) being coupled in parallel with the rotor-side converter (132) to reduce harmonics of the electric power system (100).

Description

Technical Field

[0001] The present disclosure generally relates to electrical power systems for providing power to an electrical grid from, for example, a wind turbine. Background Art

[0002] Wind turbines are gaining more and more attention as a renewable energy source. Wind turbines use wind to generate electricity. The wind rotates a plurality of blades connected to a rotor. The spin of the blades caused by the wind spins the shaft of the rotor, which is connected to a generator that generates electricity. Some wind turbines include a doubly-fed induction generator (DFIG) to convert wind energy into electrical power suitable for output to a power grid. The DFIG is typically connected to a converter that regulates the flow of electrical power between the DFIG and the power grid. More specifically, the converter allows the wind turbine to output electrical power at the power grid frequency, regardless of the rotational speed of the wind turbine blades.

[0003] A typical DFIG system includes a wind-driven DFIG having a rotor and a stator. The stator of the DFIG is connected to the grid via a stator bus. A power converter is used to connect the rotor of the DFIG to the grid. The power converter may be a two-stage power converter, which includes both a rotor-side converter and a line-side converter. The rotor-side converter may receive alternating current (AC) power from the rotor via the rotor-side bus, and may convert the AC power into DC power. Then, the line-side converter may convert the DC power into AC power with a suitable output frequency (such as a grid frequency). The AC power is provided to the grid via the line-side bus. Summary of the invention

[0004] Aspects and advantages of embodiments of the disclosure will be set forth in part in the description which follows, or may be learned from the description, or may be learned through practice of the embodiments.

[0005] An exemplary aspect of the present disclosure relates to an electric power system. The system may include a generator having a stator and a rotor. The stator may be connected to a power grid via a stator power path. The system may include a power converter. The power converter may include a line-side converter connected to the power grid via a converter power path. The power converter may include a rotor-side converter, which is connected to a rotor bus of a rotor and is connected to a line-side converter via a DC link. The rotor-side converter may be configured to convert DC power on the DC link into an AC signal for the rotor bus. The power system may also include an active filter, which includes one or more active controlled components. The active filter may be connected in parallel with the rotor-side converter to reduce harmonics of the electric power system.

[0006] Other exemplary aspects of the present disclosure may include devices, systems, methods, control systems, and other techniques for converter modulation and / or overmodulation. BRIEF DESCRIPTION OF THE DRAWINGS

[0007] A detailed discussion of embodiments for those skilled in the art is set forth in the description with reference to the accompanying drawings, in which:

[0008] Figure 1 is a perspective view of a wind turbine according to an exemplary embodiment of the present disclosure.

[0009] Figure 2 An electric power system according to an exemplary embodiment of the present disclosure is shown.

[0010] Figure 3 A schematic diagram showing suitable components that may be included within a controller of a wind turbine and / or an electrical power system and / or a controller of a power converter according to an exemplary embodiment of the present disclosure.

[0011] Figure 4 A schematic diagram is shown of an exemplary power converter suitable for use with a wind turbine system according to an exemplary embodiment of the present disclosure.

[0012] Figure 5 An exemplary overmodulation mechanism for controlling one or more control devices according to an exemplary embodiment of the present disclosure is shown.

[0013] Figure 6 An exemplary quasi-square wave signal that may be provided to a rotor according to an exemplary embodiment of the present disclosure is shown.

[0014] Figure 7 An exemplary square wave signal that may be provided to a rotor according to an exemplary embodiment of the present disclosure is shown.

[0015] Figure 8 An exemplary active filter suitable for use with a wind turbine system is shown according to an exemplary embodiment of the present disclosure.

[0016] Fig. 9 An exemplary electric power system including an active filter according to an exemplary embodiment of the present disclosure is shown.

[0017] Fig.10 Additional exemplary electric power systems including active filters are shown according to exemplary embodiments of the present disclosure.

[0018] Fig.11 Additional exemplary electric power systems including active filters are shown according to exemplary embodiments of the present disclosure.

[0019] Fig.12 Additional exemplary electric power systems including active filters are shown according to exemplary embodiments of the present disclosure.

[0020] Fig.13is a flow chart of an exemplary method of operating an electric power converter of a doubly-fed induction generator system according to an exemplary embodiment of the present disclosure. DETAILED DESCRIPTION

[0021] Reference will now be made in detail to the embodiments, one or more examples of which are shown in the accompanying drawings. Each example is provided via explanation of the embodiments, rather than limitation of the present disclosure. In fact, it will be apparent to those skilled in the art that various modifications and variations may be made to the embodiments without departing from the scope or spirit of the present disclosure. For example, a feature shown or described as part of one embodiment may be used with another embodiment to produce yet another embodiment. Therefore, it is intended that aspects of the present disclosure encompass such modifications and variations.

[0022] Exemplary aspects of the present disclosure relate to systems and methods for operating a power converter in a doubly fed induction generator (DFIG) system. The DFIG system may include a power converter having a line side converter and a rotor side converter. A DC link may be connected between the line side converter and the rotor side converter. The power converter may use the line side converter to convert AC power from the stator of the DFIG into DC power for the DC link. The power converter may use the rotor side converter to convert the DC power on the DC link into an AC signal for the rotor of the DFIG. For example, the AC signal may be provided on a rotor bus connected between the rotor side converter and the rotor of the DFIG. The AC signal may be used, for example, to control the operating characteristics of the DFIG. The DFIG system may also include an active filter, which includes an active controlled component (e.g., a switching element, an insulated gate bipolar transistor IGBT, an insulated gate commutated thyristor, a MOSFET (e.g., a MOSFET based on silicon or silicon carbide), a bipolar transistor, a silicon controlled rectifier, or other suitable switching element). The active filter may be connected in parallel with the rotor side converter to reduce the harmonics of the DFIG system.

[0023] The rotor side converter may include one or more switching elements. In some embodiments, the switching element may be any kind of suitable switching element, such as an insulated gate bipolar transistor IGBT, an insulated gate commutated thyristor, a MOSFET (e.g., a silicon or silicon carbide based MOSFET), a bipolar transistor, a silicon controlled rectifier, or other suitable switching element. The switching element may be controlled to convert a DC signal on the DC link into an AC signal for the rotor of the DFIG using, for example, pulse width modulation.

[0024] The power converter may generate harmonics in the power system.As an example, in some embodiments, the switching elements may be controlled according to an overmodulation scheme to produce an AC signal on the rotor side converter.

[0025] For example, modulation of the switching element can be achieved by comparing the modulating wave with the carrier wave and modulating the switching element based on the comparison. For example, the switching element can be triggered at any time when the carrier wave and the modulating wave intersect. Under the overmodulation mechanism, the maximum amplitude of the modulating wave is greater than the maximum amplitude of the carrier wave. This can result in some pulses of the carrier wave not intersecting the modulating wave. In some embodiments, the modulating wave can be a periodic, constant amplitude sinusoidal signal, and the carrier wave can be a periodic triangular wave, but according to the present disclosure, other suitable waveforms for both the modulating wave and the carrier wave can be used, such as sine waves, symmetrical triangular waves, asymmetrical triangular waves including sawtooth waves, square waves, quasi-square waves, and other suitable waveforms.

[0026] In some embodiments, the rotor side converter may be operated under an overmodulation mechanism so that the output of the rotor side converter is a quasi-square wave AC signal. For example, the line-to-line voltage waveform at the rotor may be a six-step quasi-square wave having a low voltage region and a high voltage region, and having an intermediate voltage region (such as a reference voltage or zero voltage) between the low voltage region and the high voltage region.

[0027] Operating the rotor side converter under an overmodulation mechanism may have several advantages. For example, in some embodiments, the voltage gain from the DC link to the rotor may be increased relative to a non-overmodulation mechanism. In some embodiments, this may contribute to an increased RPM operating range of the generator. In addition, operating the rotor side converter under an overmodulation mechanism may result in a reduced switching frequency of the switching element. This may reduce energy losses during modulation of the switching element, and may additionally reduce wear and / or allow higher currents on the switching element. Other advantages may include extended higher limits for continuous operation of the grid voltage, improved controllability and / or reduced stress during transient grid voltages and / or high voltage ride-through (HVRT), extended overspeed limits for wind turbine systems, lower DC link regulation by the line side converter during low grid voltage conditions, and / or higher generator speeds. However, according to some embodiments, the rotor side converter may operate outside or close to the overmodulation mechanism. In these cases, some of the above advantages still apply.

[0028] Operating the rotor-side converter in an overmodulation regime may contribute to increased harmonics in the generator. In some cases, the increased harmonics may propagate to other elements in the power system, such as a connected power grid. Additionally, the harmonics with the largest increase may have a frequency similar to the fundamental frequency (i.e., the power output of the generator), such as the third, fifth, seventh, or other lower harmonics. Due to the size of these harmonics and / or their proximity to the fundamental frequency, filtering these harmonics is typically more difficult than higher harmonics (e.g., the fiftieth harmonic).

[0029] In some embodiments, filters such as active filters may be provided to counteract or reduce harmonic contributions from operating a rotor-side converter in an overmodulation regime, near an overmodulation regime, or outside an overmodulation regime. The active filter may include active controlled components and in some embodiments may be activated only at any time when harmonic contributions in the system do not meet a threshold value (such as an industry standard), for example to conserve resources within the system and / or prevent wear on the active filter. For example, it may be possible to activate the active filter when lower order harmonics exceed grid requirements. For example, the active filter may provide a current or voltage at the same frequency as the harmonics but in opposite phase to almost completely or completely eliminate the harmonics.

[0030] The active filter may be connected to the rotor busbar within the electric power system. For example, the active filter may be connected to the rotor busbar at the wires between the rotor side converter and the rotor. The active filter may also be directly connected to the terminals of the rotor side converter. The active filter may also be connected in parallel to the rotor side converter. The active filter may also be connected to a transformer that is connected to the wires between the rotor side converter and the rotor. Thus, the active filter allows filtering of harmonics before injection into the DFIG generator. Furthermore, in addition to reducing harmonics on the stator power path, the active filter also helps to reduce thermal loading on the generator caused by harmonic currents. Furthermore, placing the active filter at the rotor side converter allows closed-loop control of injected harmonic currents based on current or voltage sensing of the rotor bridge or rotor windings, further reducing thermal loading on the generator.

[0031] Referring now to the accompanying drawings, Figure 1 A perspective view of one embodiment of a wind turbine 10 is shown. As shown, the wind turbine 10 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, at least one rotor blade 22 being 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 so that kinetic energy can be converted from the wind into usable mechanical energy, and subsequently into electrical energy. For example, the hub 20 may be rotatably coupled to a rotor blade 22 positioned within the nacelle 16. Figure 2 The wind turbine 10 may also include a turbine controller 26 for controlling the yaw adjustment of the wind turbine 10, the pitch adjustment of the rotor blades 22, and / or Figure 2 The turbine controller 26 may communicate with components within the wind turbine 10 (such as Figure 2 converter controller 140).

[0032] Now refer to Figure 2 , a schematic diagram of one embodiment of a DFIG wind turbine system 100 is shown according to aspects of the present subject matter. It should be appreciated that reference will generally be made herein to Figure 2 The present subject matter is described with reference to the system 100 shown in FIG. However, those skilled in the art, using the disclosure provided herein, will appreciate that aspects of the present disclosure may also be applicable to other power generation systems.

[0033] As shown, the generator 120 (e.g., DFIG 120) may be coupled to a stator bus 122 and to a power converter 130 via a rotor-side bus 124. The stator bus 122 may provide output multiphase power (e.g., three-phase power) from the stator of the DFIG 120, and the rotor-side bus 124 may provide output multiphase power (e.g., three-phase power) of the rotor of the DFIG 120. The power converter 130 may have a rotor-side converter 132 and a line-side converter 134. The DFIG 120 may be coupled to the rotor-side converter 132 via the rotor-side bus 124. The rotor-side converter 132 may be coupled to the line-side converter 134, which in turn may be coupled to the line-side bus 138. The rotor-side converter 132 and the line-side converter 134 may be coupled via DC links 135, 137, with a DC link capacitor 136 spanning the DC links 135, 137.

[0034] In addition, the power converter 130 may be coupled to a converter controller 140 to control the operation of the rotor side converter 132 and the line side converter 134. For example, the converter controller 140 may be configured to operate the rotor side converter 132 in an overmodulation regime or a near overmodulation regime. The converter controller 140 may include any number of control devices. In one embodiment, the control device may include a processing device (e.g., a microprocessor, a microcontroller, etc.) that executes computer-readable instructions stored in a computer-readable medium. When executed by the processing device, the instructions may cause the processing device to perform operations including providing control commands (e.g., switching frequency commands) to the switching elements 142 of the power converter 130. For example, the instructions may include providing a control command (e.g., a switching frequency command) to the rotor side converter 132. Figure 4 The switching element 142 provides a control command to operate the rotor side converter 132 (eg, via the switching element 142 ) in an overmodulation regime or a near overmodulation regime.

[0035] As shown, the system 100 may include a transformer 160 that couples the wind turbine system 100 to the grid 190. The transformer 160 may be a three-winding transformer that may include, for example, a high voltage (e.g., greater than 12KVAC) primary winding 162 coupled to the grid, a medium voltage (e.g., 6KVAC) secondary winding 164 coupled to the stator bus 122, for example, and / or a low voltage (e.g., 575VAC, 690VAC, etc.) auxiliary winding 166 coupled to the line-side bus 138, for example. It should be understood that the transformer 160 may be a three-winding transformer as shown, or alternatively may be a two-winding transformer having only the primary winding 162 and the secondary winding 164; may be a four-winding transformer having the primary winding 162, the secondary winding 164, the auxiliary winding 166, and additional auxiliary windings; or may have any other suitable number of windings.

[0036] On the stator bus 122, sinusoidal multi-phase (e.g., three-phase) alternating current (AC) power may be provided from the stator of the generator 120 to the stator bus 122, and from the stator bus 122 to the transformer 160, for example to its secondary winding 164. Various circuit breakers, fuses, contactors, and other devices, such as a grid breaker 158, a stator bus breaker 156, a switch 154, and a line-side bus breaker 152 may be included in the system 100 to connect or disconnect the corresponding bus (e.g., when the current is too large 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.

[0037] Now refer to Figure 3, a block diagram of one embodiment of suitable components (e.g., one or more control devices) that may be included within the turbine controller 26 and / or converter controller 140 according to aspects of the present subject matter is shown. As shown, the controller 26 / 140 may include one or more processors 60 and (multiple) associated memory devices 62 that are configured to perform various computer-implemented functions (e.g., perform the methods, steps, calculations, etc. disclosed herein). In addition, the controller 26 / 140 may also include a communication module 64 to facilitate communication between the controller 26 / 140 and various components of the wind turbine 10. For example, the communication module 64 may be used as an interface to allow the turbine controller 26 to transmit control signals to one or more pitch adjustment mechanisms to, for example, control the pitch of the rotor blades 22. The communication module 64 may additionally and / or alternatively be used as an interface to allow the turbine controller 26 to transmit signals (e.g., control signals or status signals) to the converter controller 140. The communication module 64 may additionally and / or alternatively be used to allow the converter controller 140 to provide control signals to the power converter 130. Additionally, the communication module 64 may include a sensor interface 66 (eg, one or more analog-to-digital converters) to allow input signals transmitted from, for example, various sensors (such as voltage sensors and current sensors) to be converted into signals that can be understood and processed by the processor 60 .

[0038] Now refer to Figure 4 According to aspects of the present subject matter, it is shown Figure 2 Schematic diagram of an exemplary embodiment of a power converter 130 is shown in . As shown, the rotor side converter 132 includes a plurality of bridge circuits, wherein each phase of the rotor side bus 124 input to the rotor side converter 132 is coupled to a single bridge circuit. In addition, the line side converter 134 may also include a plurality of bridge circuits. Similar to the rotor side converter 132, the line side converter 134 also includes a single bridge circuit for each output phase of the line side converter 134. In other embodiments, without departing from the scope of the present disclosure, the line side converter 134, the rotor side converter 132, or both the line side converter 134 and the rotor side converter 132 may include parallel bridge circuits.

[0039] Each bridge circuit may generally include a plurality of switching elements (eg, IGBTs) 142 connected in series with one another. Figure 4As shown, each bridge circuit includes an upper switching element 144 and a lower switching element 146. In addition, a diode may be connected in parallel with each of the switching elements 142. In an alternative embodiment, the parallel switching elements 142 and the diodes can be used to increase the rated current of the converter. As is generally understood, the line side converter 134 and the rotor side converter 132 can be controlled, for example, by providing a control command to the gate of the switching element 142 using a suitable drive circuit. For example, the converter controller 140 may provide a suitable gate timing command to the gate of the switching element 142 of the bridge circuit. The control command can control the switching frequency of the switching element 142 to provide a desired output. Those skilled in the art will recognize that the power converter 130 may include any suitable switching element 142, such as an insulated gate bipolar transistor (IGBT), an insulated gate commutated thyristor, a MOSFET (e.g., a MOSFET based on silicon or silicon carbide), a bipolar transistor, a silicon controlled rectifier or other suitable switching element.

[0040] Figure 5 A graphical representation of an exemplary overmodulation mechanism 200 for controlling a switching device within a power converter according to an exemplary embodiment of the present disclosure is shown. Using the disclosure provided herein, those skilled in the art will appreciate that various suitable overmodulation mechanisms and / or configurations may be used without departing from the scope or spirit of the present disclosure. In addition, as described above, other modulation mechanisms including close overmodulation mechanisms may also be used.

[0041] According to the overmodulation mechanism 200, the modulated wave 202 is compared to the carrier wave 204. The modulated wave 202 is shown as a constant amplitude, constant frequency sinusoidal signal, but may be any of a variety of suitable waveforms, including a sine wave, a sine wave with harmonic additions, a square wave, a quasi-square wave, and other suitable waveforms. The carrier wave 204 is shown as a constant amplitude, constant frequency symmetrical triangle wave, but may be any of a variety of suitable waveforms, including a sine wave, a symmetrical triangle wave, an asymmetrical triangle wave including a sawtooth wave, a square wave, a quasi-square wave, and other suitable waveforms. In addition, the frequency and / or amplitude of the modulated wave 202 and / or the carrier wave 204 may change over time.

[0042] The switching element (e.g., switching element 142) may be controlled based on the overmodulation mechanism 200. For example, at any time when the modulation wave 202 and the carrier wave 204 intersect, for example, at the intersection 208, the switching element (e.g., switching element 142) may be triggered, for example, by sending a control signal from a controller (e.g., converter controller 140) to bias the voltage across the gate of the switching element 142. The modulation wave 202 may correspond to only one switching device, or may correspond to several switching elements. A plurality of modulation waves 202 and / or carrier waves 204 may be provided. For example, the plurality of modulation waves 202 may be compared to a single carrier wave 204, wherein each modulation wave 202 of the plurality of modulation waves 202 corresponds to one or more switching elements. The plurality of modulation waves 202 may be in phase or out of phase (e.g., 60 degrees, 120 degrees, 180 degrees, etc. out of phase). Alternatively, a plurality of pairs of modulation waves 202 and carrier waves 204 may be provided, wherein each pair of modulation waves 202 and carrier waves 204 corresponds to one or more switching elements. Other suitable control schemes may be used, for example, based on the configuration and / or type of switching elements.

[0043] The amplitude of the modulated wave 202 may be greater than the amplitude of the carrier wave 204, resulting in an overmodulated region 206 in which the modulated wave 202 does not intersect the carrier wave 204. In general, the greater the amplitude difference between the modulated wave 202 and the carrier wave 204, the larger the overmodulated region 206. For example, if the amplitude difference between the modulated wave 202 and the carrier wave 204 is large enough, the modulated wave 202 may intersect the carrier wave 204 only twice during one period of the modulated wave 202. Similarly, in a regime that is close to an overmodulation regime, the region 206 may be minimized so as to be close to coincide with the carrier wave 204. In these cases, the modulation regime may be referred to as a close to overmodulation regime.

[0044] The switching device (e.g., the switching device 142) can be controlled according to an overmodulation mechanism (e.g., the overmodulation mechanism 200) to generate a time-varying AC signal. The time-varying AC signal can be, for example, Figure 6 The quasi-square wave 210 shown in FIG. The quasi-square wave 210 may be a line-to-line voltage between two lines in an AC bus such as the rotor-side bus 124. The quasi-square wave 210 may also represent other configurations. Figure 6 , quasi-square wave 210 has a region 212 of intermediate voltage between region 214 of high voltage and region 216 of low voltage. Region 212 of intermediate voltage may be at zero volts, or may be at some non-zero reference voltage. Edge 218 may correspond to the triggering of a switching element. Edge 218 is shown as ideal, i.e., instantaneous, but one skilled in the art will understand using the disclosure provided herein that edge 218 may be slightly uneven or sloped.

[0045] If the power converter line is connected to a negative DC link such as ( Figure 4137), the time-varying AC signal generated from the overmodulation mechanism (e.g., overmodulation mechanism 200) may be Figure 7 Square wave 220 is shown in FIG. Square wave 220 may be a line-to-reference voltage between a line in an AC bus such as rotor-side bus 124 and a reference such as negative side 137 of DC link 135. Square wave 220 may also represent other configurations. Figure 7 , square wave 220 has edge 218 directly between region 214 of high voltage and region 216 of low voltage (i.e., without region 212 of intermediate voltage). Edge 218 may correspond to the triggering of the switching element. Edge 218 is shown as ideal, i.e., instantaneous, but one skilled in the art will understand using the disclosure provided herein that edge 218 may be slightly uneven or sloped.

[0046] As described in detail herein, an active filter may be provided in an electric power system (eg, electric power system 100), such as Figure 8 250 to reduce or eliminate harmonics caused by operating a rotor side converter (e.g., rotor side converter 132) in an overmodulation regime or near an overmodulation regime. For example, the active filter 250 can reduce or eliminate harmonics to meet one or more grid requirements for harmonics. The active filter 250 can provide current at approximately the same frequency and / or amplitude as the harmonics and in opposite phase (i.e., approximately 180 degrees out of phase). The active filter 250 can provide this power with high accuracy to eliminate harmonics, even harmonics close to the fundamental frequency, with reduced or no effect on the power at the fundamental frequency. The active filter 250 (e.g., the shunt active filter 250) can take as input at least a portion of one or more currents from the system (e.g., a portion of the fundamental current on the bus) to compensate for losses associated with the operation of the active filter 250. Other suitable active filters can be used without departing from the scope or spirit of the present disclosure.

[0047] Now refer to Figures 9 to 11 , shows an exemplary implementation of an active filter for reducing harmonics such as those caused by operating a rotor-side converter in an overmodulation regime or close to an overmodulation regime. For illustration purposes, Figure 2 A simplified version of the electrical power system is shown in FIG. Figures 9 to 11 As shown in Figure 2 The components shown in the figure together with other suitable components may still exist in the embodiments of the present disclosure.

[0048] For example, Figures 9 to 11 As shown, active filter 250 may be coupled to rotor bus 124, that is, between power converter 130 and generator 120. The active filter may be configured to convert Is (i.e., the current on stator bus 122) and / or IROTOR (i.e., the current on the rotor-side bus 124) as input. In some embodiments (such as Fig. 9 In the embodiment shown in FIG. 1 ), the active filter 250 may be directly coupled to the terminals of the rotor-side converter 132. The active filter 250 may also be directly coupled to the rotor-side bus 124. In some embodiments (such as Fig.10 In the embodiment shown in FIG. 1 , a transformer 260 , such as a dual-winding transformer 260 , may be disposed between the active filter 250 and the rotor-side bus 124 .

[0049] According to some embodiments, such as Fig.12 In the embodiment shown in FIG. 2 , transformer 260 may be implemented as a series connection of a capacitor and an inductor (shown as 261). Thus, active filter 250 may be a hybrid active filter. Hybrid active filters 250 and 261 may have additional technical advantages, such as reduced filter size for the same, similar, or equivalent filtering performance relative to other active filters.

[0050] In addition, a passive filter 270 may be provided on the line side bus 138, such as Fig.10 and Fig.11 . For example, the passive filter 270 may be disposed between the power converter 130 and the transformer 160, or between the power converter 130 and the grid 190 in other suitable configurations. The passive filter 270 may be arranged as an RC filter. The passive filter 270 may include passive components, such as inductors, capacitors, and resistors; that is, components that do not require secondary control signals or power to operate to passively filter. This is in contrast to the active components of the active filter 250.

[0051] Additionally, an active filter 250 may also be coupled to the DC links 135, 137, such as Fig.11 As shown in FIG. 1 , the active filter 250 is connected in parallel across the rotor side converter 132. The active filter 250 can convert Is (i.e., the current on the stator bus 122), I ROTORThe active filter 250 may be configured to receive at least one electrical condition from at least one of the rotor bus, the line-side converter, the rotor-side converter, the stator bus, the grid interconnection bus, and the DC link. Thus, the active filter 250 may be arranged to actively filter harmonics, including those close to the fundamental frequency, to meet grid requirements. Note that in the case where a hard connection exists between the active filter DC link and the converter DC link, a mechanism for isolation may be used to avoid circulating currents between the active filter and the bridge of the rotor converter (e.g., transformer 260 or alternatively a high frequency transformer with power switching means within 250 on the DC link side).

[0052] Now refer to Fig.12 , according to aspects of the present subject matter, a flow chart of one embodiment of a method 1300 for operating a power generation system is shown. In general, the method 1300 will be described herein as using a wind turbine system (such as the one described above with reference to Figure 2 However, it should be appreciated that the disclosed method 1300 may be implemented using any other suitable power generation system configured to supply power for application to a load. In addition, although for purposes of illustration and discussion, Fig.12 The steps are depicted as being performed in a particular order, but the methods described herein are not limited to any particular order or arrangement. Using the disclosure provided herein, one skilled in the art will recognize that the various steps of the methods may be omitted, rearranged, performed simultaneously, combined, and / or modified in various ways. Additional steps not disclosed herein may be performed without departing from the scope or spirit of the present disclosure.

[0053] At (1302), method 1300 may include converting AC power at a line-side converter to DC power for a DC link. For example, the line-side converter may be part of a power converter, such as line-side converter 134 of AC-AC power converter 130, and the DC link may be DC links 135, 137. The AC power may be three-phase AC power on an AC bus such as line-side bus 138. For example, the AC power may be converted using a plurality of bridge circuits. Other suitable systems for performing AC to DC conversion may be used in accordance with the present method.

[0054] At (1304), method 1300 may include receiving DC power from a DC link at a rotor side converter. For example, the rotor side converter may be rotor side converter 132. The DC power may include a DC link voltage such as across a DC link capacitor. The rotor side converter may include a plurality of bridge circuits.

[0055] At (1306), the method 1300 may include operating the rotor side converter in an overmodulation regime or a near overmodulation regime to convert the DC power into an AC signal. For example, the rotor side converter 132 may operate according to the overmodulation regime 200 using the converter controller 140. For example, the rotor side converter 132 may also operate according to the overmodulation regime 200 with a minimized region 206 (i.e., near unity overmodulation) using the converter controller 140. For example, the converter controller 140 may provide a control signal to a gate of a switching element 142 within the rotor side converter based on an intersection 208 of a modulation wave 202 and a carrier wave 204, wherein the amplitude of the modulation wave 202 is greater than the amplitude of the carrier wave 204 for the overmodulation regime, or is near the amplitude of the carrier wave 204 for the modulation regime near the overmodulation regime.

[0056] At (1308), the method 1300 includes providing an output from an active filter (e.g., active filter 250) coupled in parallel to the rotor side converter to reduce at least one harmonic caused by operating the rotor side converter in an overmodulation regime or near an overmodulation regime. For example, the active filter may be active filter 250 or other suitable active filters. The active filter may be disposed on the rotor side bus 124, coupled in parallel to the rotor side converter 132, directly coupled to the DC link 135, 137, or other suitable location. The active filter may provide a voltage or current at about the same frequency and in about opposite phase to the at least one harmonic to reduce or eliminate the at least one harmonic with minimal or no effect on power at the fundamental frequency. In some embodiments, a passive filter may be used in addition to the active filter.

[0057] Although the subject matter is described in detail with respect to its specific exemplary embodiments, it will be appreciated that those skilled in the art, after obtaining an understanding of the foregoing, can easily generate modifications, variations, and equivalents of such embodiments. Therefore, the scope of the present disclosure is by example rather than by limitation, and the subject disclosure does not exclude the inclusion of such modifications, variations, and / or additions to the subject matter, as will be readily apparent to those skilled in the art.

Claims

1. An electric power system connected to a power grid, comprising: a generator comprising a stator and a rotor, the stator being connected to the power grid via a stator power path; A power converter comprising: a line-side converter coupled to the power grid via a converter power path; and a rotor-side converter coupled to a rotor bus of the rotor and to the line-side converter via a DC link, the rotor-side converter configured to convert DC power on the DC link into an AC signal for the rotor bus; and An active filter, comprising one or more actively controlled components, is coupled in parallel with the rotor side converter to reduce harmonics caused by operating the rotor side converter in an overmodulation regime or close to an overmodulation regime by providing current at the same frequency and amplitude as the harmonics and in opposite phase.

2. The electric power system according to claim 1, characterized in that The electric power system also includes a control system including one or more control devices configured to operate the rotor-side converter to provide the AC signal for the rotor bus.

3. The electric power system according to claim 2, characterized in that: The rotor-side converter includes one or more switching elements, wherein the one or more control devices operate the one or more switching elements under the overmodulation regime.

4. The electric power system according to claim 1, characterized in that The actively controlled components include one or more switching elements.

5. The electric power system according to claim 1, characterized in that: The electric power system also includes a passive filter coupled to the converter power path to further reduce the harmonics.

6. The electric power system according to claim 1, characterized in that: The active filter is also coupled to the DC link.

7. The electric power system according to claim 1, characterized in that: The active filter is configured to receive at least one electrical condition from at least one of the rotor bus, the line side converter, the rotor side converter, a stator bus, a grid interconnection bus, and the DC link.

8. The electric power system according to claim 7, characterized in that The at least one electrical condition is one or more of current and voltage.

9. The electric power system according to claim 1, characterized in that: The electric power system also includes a series connection of an inductor and a capacitor coupled between the active filter and the rotor bus.

10. The electric power system according to claim 1, characterized in that The electric power system also includes at least one inductor coupled between the active filter and the rotor bus.

11. The electric power system according to claim 1, characterized in that The active filter is directly coupled to the terminals of the rotor-side converter.

12. A wind turbine system comprising: a generator comprising a stator and a rotor, the stator being connected to a power grid via a stator power path; A power converter comprising: a line-side converter coupled to the power grid via a converter power path; and a rotor-side converter coupled to a rotor bus of the rotor and to the line-side converter via a DC link, the rotor-side converter configured to convert DC power on the DC link into an AC signal for the rotor bus; and An active filter, comprising one or more active control electronic components, is coupled in parallel with the rotor side converter to reduce harmonics caused by operating the rotor side converter in an overmodulation regime or close to an overmodulation regime by providing current at the same frequency and amplitude as the harmonics and in opposite phase.

13. The wind turbine system according to claim 12, wherein: The wind turbine system further includes a control system including one or more control devices configured to operate the rotor-side converter to provide the AC signal for the rotor bus.

14. A method of operating an electric power converter for a doubly-fed induction generator system, the method comprising: Converting AC power at the line-side converter into DC power for the DC link; receiving the DC power from the DC link at a rotor-side converter; operating the rotor-side converter using one or more control devices to convert the DC power into an AC signal; and An active filter coupled in parallel to the rotor side converter is operated to reduce at least one harmonic caused by operating the rotor side converter in an overmodulation regime or near an overmodulation regime by providing current at the same frequency and amplitude and in opposite phase as at least one harmonic.

15. The method according to claim 14, characterized in that Operating the rotor side converter using the one or more control devices to convert the DC power into the AC signal includes operating one or more switching elements under the overmodulation regime.

16. The method according to claim 15, characterized in that The active filter includes one or more actively controlled components.

17. The method according to claim 14, characterized in that The method also includes reducing the at least one harmonic caused by operating the rotor side converter using a passive filter.

18. The method according to claim 14, characterized in that Providing an output from the active filter reduces the at least one harmonic caused by operating the rotor side converter in the overmodulation regime.

19. The method according to claim 14, characterized in that The doubly-fed induction generator system is a wind-driven doubly-fed induction generator system configured to generate electric power for an electric power grid.

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