Patent Description:
Wind power is considered one of the cleanest, most environmentally friendly energy sources presently available, and wind turbines have gained increased attention in this regard. A modern wind turbine typically includes a tower, a generator, a gearbox, a nacelle, and one or more rotor blades. The rotor blades capture kinetic energy of wind using known airfoil principles. For example, rotor blades typically have the cross-sectional profile of an airfoil such that, during operation, air flows over the blade producing a pressure difference between the sides. Consequently, a lift force, which is directed from a pressure side towards a suction side, acts on the blade. The lift force generates torque on the main rotor shaft, which is geared to a generator for producing electricity.

For example, <FIG> and <FIG> illustrate a wind turbine <NUM> and associated power system suitable for use with the wind turbine <NUM> according to conventional construction. As shown, the wind turbine <NUM> includes a nacelle <NUM> that typically houses a generator <NUM> (<FIG>). The nacelle <NUM> is mounted on a tower <NUM> extending from a support surface (not shown). The wind turbine <NUM> also includes a rotor <NUM> that includes a plurality of rotor blades <NUM> attached to a rotating hub <NUM>. As wind impacts the rotor blades <NUM>, the blades <NUM> transform wind energy into a mechanical rotational torque that rotatably drives a low-speed shaft <NUM>. The low-speed shaft <NUM> is configured to drive a gearbox <NUM> (where present) that subsequently steps up the low rotational speed of the low-speed shaft <NUM> to drive a high-speed shaft <NUM> at an increased rotational speed. The high-speed shaft <NUM> is generally rotatably coupled to a generator <NUM> (such as a doubly-fed induction generator or DFIG) so as to rotatably drive a generator rotor <NUM>. As such, a rotating magnetic field may be induced by the generator rotor <NUM> and a voltage may be induced within a generator stator <NUM> that is magnetically coupled to the generator rotor <NUM>. The associated electrical power can be transmitted from the generator stator <NUM> to a main three-winding transformer <NUM> that is typically connected to a power grid via a grid breaker <NUM>. Thus, the main transformer <NUM> steps up the voltage amplitude of the electrical power such that the transformed electrical power may be further transmitted to the power grid.

In addition, as shown, the generator <NUM> is typically electrically coupled to a bidirectional power converter <NUM> that includes a rotor-side converter <NUM> joined to a line-side converter <NUM> via a regulated DC link <NUM>. The rotor-side converter <NUM> converts the AC power provided from the rotor <NUM> into DC power and provides the DC power to the DC link <NUM>. The line side converter <NUM> converts the DC power on the DC link <NUM> into AC output power suitable for the power grid. Thus, the AC power from the power converter <NUM> can be combined with the power from the stator <NUM> to provide multi-phase power (e.g. three-phase power) having a frequency maintained substantially at the frequency of the power grid (e.g. <NUM>/<NUM>).

As shown in <FIG>, the illustrated three-winding transformer <NUM> typically has (<NUM>) a <NUM> kilovolt (kV) medium voltage (MV) primary winding <NUM> connected to the power grid, (<NUM>) a <NUM> to <NUM> kV MV secondary winding <NUM> connected to the generator stator <NUM>, and (<NUM>) a <NUM> to <NUM> volt (V) low-voltage (LV) tertiary winding <NUM> connected to the line-side power converter <NUM>.

Referring now to <FIG>, individual power systems of a plurality of wind turbines <NUM> may be arranged in a predetermined geological location and electrically connected together to form a wind farm <NUM>. More specifically, as shown, the wind turbines <NUM> may be arranged into a plurality of groups <NUM> with each group separately connected to a main line <NUM> via switches <NUM>, <NUM>, <NUM>, respectively. In addition, as shown, the main line <NUM> may be electrically coupled to another, larger transformer <NUM> for further stepping up the voltage amplitude of the electrical power from the groups <NUM> of wind turbines <NUM> before sending the power to the grid.

One issue with such systems, however, is that the three-winding transformers <NUM> associated with each turbine <NUM> are expensive. Particularly, the secondary winding <NUM> of the transformer <NUM> that is connected to the generator stator <NUM> can be costly. Thus, it would be advantageous to eliminate such three-winding transformers from wind turbine power systems.

Another issue that needs to be addressed in power systems is harmonics. For example, if the secondary winding is eliminated, this results in lower impedance in the system. Thus, there is an increased risk of such systems not meeting certain agency requirements for harmonics. Accordingly, power systems which include features for reducing the harmonic currents being injected into the power grid, particularly in view of agency harmonics requirements, would be advantageous.

The master thesis "WIND GENERATOR GRID INTERFACE USING PWM CONVERTER" of Magdi Mosa, DOI: <NUM>/RG. <NUM>, proposes a wind energy conversion system (WECS). The proposed WECS consists of induction generator, power electronic converters, and inductor capacitor inductor (LCL) harmonic filter. The research introduces a modified method for parameters selection of LCL harmonic filters and direct current (DC) link capacitance. Also it is provided a modified pitch angle controller and designs the control system for grid voltage supporting. In addition, two techniques for maximum power point tracking (MPPT) and the capability of the proposed WECS to operate in wind farms are studied. Further, document <CIT> refers to connection for parallel bridge circuits in a power converter is provided. A power converter can be used to provide a desired power to a load, such as a generator, motor, electrical grid, or other suitable load. The power converter can include a plurality of bridge circuits coupled in parallel. A bridge output of each of the parallel bridge circuits can be coupled together at the load instead of at the power converter. In particular, the parallel bridge circuits can be coupled together at a location that is physically proximate the physical location of the load, such as at a plurality of terminals associated with the load. By doing so, stray inductance associated with conductors used to couple the bridge outputs of the parallel bridge circuits to the load can be effectively coupled between the parallel bridge circuits.

In accordance with one embodiment, an electrical power subsystem for connection to a power grid according to claim <NUM> is provided. The electrical power subsystem includes a generator comprising a generator stator and a generator rotor, and a power converter electrically coupled to the generator. The power converter includes a plurality of rotor-side converters electrically coupled in parallel, a line-side converter, and a regulated DC link electrically coupling the plurality of rotor-side converters and the line-side converter. The electrical power subsystem further includes a stator power path for providing power from the generator stator to the power grid, a converter power path for providing power from the generator rotor through the power converter to the power grid, and a partial power transformer provided on the converter power path.

Typically, the electrical power subsystem further includes a controller coupled to the power converter, the controller configured to coordinate switching of the plurality of rotor-side converters to produce an interleaved switching pattern between the plurality of rotor-side converters.

Further, a method for operating an electrical power subsystem as claimed by claim <NUM> is provided. The electrical power subsystem includes a generator including a generator stator and a generator rotor. The electrical power subsystem further includes a power converter electrically coupled to the generator, the power converter including a plurality of rotor-side converters electrically coupled in parallel, a line-side converter, and a regulated DC link electrically coupling the plurality of rotor-side converters and the line-side converter. The electrical power subsystem further includes a stator power path for providing power from the generator stator to the power grid, a converter power path for providing power from the generator rotor through the power converter to the power grid, and a partial power transformer provided on the converter power path. The method includes switching the plurality of rotor-side converters to produce an interleaved switching pattern between the plurality of rotor-side converters.

Thus, it is intended that the present invention covers such modifications and variations as far as they fall within the scope of the appended claims.

Generally, the present subject matter is directed to electrical power systems for connection and providing power to a power grid with reduced harmonics. An electrical power system in accordance with the present disclosure may include a generator and a power converter electrically coupled to the generator. The power converter may include a plurality of rotor-side converters electrically coupled in parallel with each other. The use of multiple rotor-side converters instead of only a single rotor-side converter advantageously facilitates the reduction in harmonics. In some cases, such multiple rotor-side converters can further result in the elimination or a change in the location of a harmonic filter in the system. Further, in exemplary embodiments, switching patterns of the rotor-side converters may be coordinated to produce an interleaved switching pattern, with the switching phase of each rotor-side converter shifted from the others, resulting in significantly reduced harmonic currents being transmitted to the power grid. Further, any necessary filtering equipment can be designed for operation at the higher frequency resulting from the interleaved switching pattern, thus advantageously resulting in physically smaller and less costly equipment.

Referring now to <FIG>, a schematic diagram of one embodiment of an electrical power subsystem <NUM> according to the present disclosure is illustrated. It should be understood that the term "subsystem" is used herein to distinguish between the individual power systems (e.g. as shown in <FIG> or <FIG>) and the overall electrical power system <NUM> of <FIG> or <FIG> that includes a plurality of electrical power subsystems <NUM>. Those of ordinary skill in the art, however, will recognize that the electrical power subsystem <NUM> of <FIG> (or <FIG>) may also be referred to more generically, such as a simply a system (rather than a subsystem). Therefore, such terms may be used interchangeably and are not meant to be limiting.

Further, as shown, the electrical power subsystem <NUM> may correspond to a wind turbine power system <NUM>. More specifically, as shown, the wind turbine power system <NUM> includes a rotor <NUM> that includes a plurality of rotor blades <NUM> attached to a rotating hub <NUM>. As wind impacts the rotor blades <NUM>, the blades <NUM> transform wind energy into a mechanical rotational torque that rotatably drives a low-speed shaft <NUM>. The low-speed shaft <NUM> is configured to drive a gearbox <NUM> that subsequently steps up the low rotational speed of the low-speed shaft <NUM> to drive a high-speed shaft <NUM> at an increased rotational speed. The high-speed shaft <NUM> is generally rotatably coupled to a doubly-fed induction generator <NUM> (referred to hereinafter as DFIG <NUM>) so as to rotatably drive a generator rotor <NUM>. As such, a rotating magnetic field may be induced by the generator rotor <NUM> and a voltage may be induced within a generator stator <NUM> that is magnetically coupled to the generator rotor <NUM>. In one embodiment, for example, the generator <NUM> is configured to convert the rotational mechanical energy to a sinusoidal, three-phase alternating current (AC) electrical energy signal in the generator stator <NUM>. Thus, as shown, the associated electrical power can be transmitted from the generator stator <NUM> directly the grid.

In addition, as shown, the generator <NUM> is electrically coupled to a bidirectional power converter <NUM> that includes a rotor-side converter <NUM> joined to a line-side converter <NUM> via a regulated DC link <NUM>. Thus, the rotor-side converter <NUM> converts the AC power provided from the generator rotor <NUM> into DC power and provides the DC power to the DC link <NUM>. The line side converter <NUM> converts the DC power on the DC link <NUM> into AC output power suitable for the power grid. More specifically, as shown, the AC power from the power converter <NUM> can be combined with the power from the generator stator <NUM> via a converter power path <NUM> and a stator power path <NUM>, respectively. For example, as shown, and in contrast to conventional systems such as those illustrated in <FIG>, the converter power path <NUM> may include a partial power transformer <NUM> for stepping up the voltage amplitude of the electrical power from the power converter <NUM> such that the transformed electrical power may be further transmitted to the power grid. Thus, as shown, the illustrated system <NUM> of <FIG> does not include the conventional three-winding main transformer described above. Rather, as shown in the illustrated embodiment, the partial power transformer <NUM> may correspond to a two-winding transformer having a primary winding <NUM> connected to the power grid and a secondary winding <NUM> connected to the line side converter <NUM>. Notably, the partial power transformer may in some embodiments include a third auxiliary winding for auxiliary loads.

In addition, the electrical power subsystem <NUM> may include a controller <NUM> configured to control any of the components of the wind turbine <NUM> and/or implement the method steps as described herein. For example, as shown particularly in <FIG>, the controller <NUM> may include one or more processor(s) <NUM> and associated memory device(s) <NUM> configured to perform a variety of computer-implemented functions (e.g., performing the methods, steps, calculations and the like and storing relevant data as disclosed herein). Additionally, the controller <NUM> may also include a communications module <NUM> to facilitate communications between the controller <NUM> and the various components of the wind turbine <NUM>, e.g. any of the components of <FIG> and <FIG>. Further, the communications module <NUM> may include a sensor interface <NUM> (e.g., one or more analog-to-digital converters) to permit signals transmitted from one or more sensors <NUM>, <NUM>, <NUM> to be converted into signals that can be understood and processed by the processors <NUM>. It should be appreciated that the sensors <NUM>, <NUM>, <NUM> may be communicatively coupled to the communications module <NUM> using any suitable means. For example, as shown in <FIG>, the sensors <NUM>, <NUM>, <NUM> may be coupled to the sensor interface <NUM> via a wired connection. However, in other embodiments, the sensors <NUM>, <NUM>, <NUM> may be coupled to the sensor interface <NUM> via a wireless connection, such as by using any suitable wireless communications protocol known in the art. As such, the processor <NUM> may be configured to receive one or more signals from the sensors <NUM>, <NUM>, <NUM>.

As used herein, the term "processor" refers not only to integrated circuits referred to in the art as being included in a computer, but also refers to a controller, a microcontroller, a microcomputer, a programmable logic controller (PLC), an application specific integrated circuit, and other programmable circuits. The processor <NUM> is also configured to compute advanced control algorithms and communicate to a variety of Ethernet or serial-based protocols (Modbus, OPC, CAN, etc.). Additionally, the memory device(s) <NUM> may generally comprise memory element(s) including, but not limited to, computer readable medium (e.g., random access memory (RAM)), computer readable non-volatile medium (e.g., a flash memory), a floppy disk, a compact disc-read only memory (CD-ROM), a magnetooptical disk (MOD), a digital versatile disc (DVD) and/or other suitable memory elements. Such memory device(s) <NUM> may generally be configured to store suitable computer-readable instructions that, when implemented by the processor(s) <NUM>, configure the controller <NUM> to perform the various functions as described herein.

In operation, alternating current (AC) power generated at the generator stator <NUM> by rotation of the rotor <NUM> is provided via a dual path to the grid, i.e. via the stator power path <NUM> and the converter power path <NUM>. More specifically, the rotor side converter <NUM> converts the AC power provided from the generator rotor <NUM> into DC power and provides the DC power to the DC link <NUM>. Switching elements (e.g. IGBTs) used in bridge circuits of the rotor side converter <NUM> can be modulated to convert the AC power provided from the generator rotor <NUM> into DC power suitable for the DC link <NUM>. The line side converter <NUM> converts the DC power on the DC link <NUM> into AC output power suitable for the grid. In particular, switching elements (e.g. IGBTs) used in bridge circuits of the line side converter <NUM> can be modulated to convert the DC power on the DC link <NUM> into AC power. As such, the AC power from the power converter <NUM> can be combined with the power from the generator stator <NUM> to provide multi-phase power having a frequency maintained substantially at the frequency of the grid. It should be understood that the rotor side converter <NUM> and the line side converter <NUM> may have any configuration using any switching devices that facilitate operation of electrical power system as described herein.

Further, the power converter <NUM> may be coupled in electronic data communication with the turbine controller <NUM> and/or a separate or integral converter controller <NUM> to control the operation of the rotor side converter <NUM> and the line side converter <NUM>. For example, during operation, the controller <NUM> may be configured to receive one or more voltage and/or electric current measurement signals from the first set of voltage and electric current sensors <NUM>, <NUM>, <NUM>. Thus, the controller <NUM> may be configured to monitor and control at least some of the operational variables associated with the wind turbine <NUM> via the sensors <NUM>, <NUM>, <NUM>. In the illustrated embodiment, the sensors <NUM>, <NUM>, <NUM> may be electrically coupled to any portion of electrical power subsystem <NUM> that facilitates operation of electrical power subsystem <NUM> as described herein.

It should also be understood that any number or type of voltage and/or electric current sensors may be employed within the wind turbine <NUM> and at any location. For example, the sensors may be current transformers, shunt sensors, rogowski coils, Hall Effect current sensors, Micro Inertial Measurement Units (MIMUs), or similar, and/or any other suitable voltage or electric current sensors now known or later developed in the art. Thus, the converter controller <NUM> is configured to receive one or more voltage and/or electric current feedback signals from the sensors <NUM>, <NUM>, <NUM>. More specifically, in certain embodiments, the current or voltage feedback signals may include at least one of line feedback signals, line-side converter feedback signals, rotor-side converter feedback signals, or stator feedback signals.

Referring particularly to <FIG>, individual power systems (such as the power subsystem <NUM> illustrated in <FIG>) may be arranged in at least two clusters <NUM> to form an electrical power system <NUM>. More specifically, as shown, the wind turbine power systems <NUM> may be arranged into a plurality of clusters <NUM> so as to form a wind farm. Thus, as shown, each cluster <NUM> may be connected to a separate cluster transformer <NUM>, <NUM>, <NUM> via switches <NUM>, <NUM>, <NUM>, respectively, for stepping up the voltage amplitude of the electrical power from each cluster <NUM> such that the transformed electrical power may be further transmitted to the power grid. In addition, as shown, the transformers <NUM>, <NUM>, <NUM> are connected to a main line <NUM> that combines the power from each cluster <NUM> before sending the power to the grid. In other words, as shown, the stator power circuit of all the wind turbines <NUM> share a common ground reference provided by the neutral of the secondary winding <NUM> of the cluster transformer <NUM>, <NUM>, <NUM> or by a separate neutral grounding transformer. Each subsystem <NUM> may be connected to the cluster <NUM> via a subsystem breaker <NUM>, as shown.

Referring now to <FIG>, various embodiments of electrical power subsystems <NUM> having improved harmonic reduction features are provided. It should be noted that, while such embodiments are illustrated in the context of subsystems using partial power transformers <NUM>, such improved harmonic reduction features are equally applicable to subsystems using transformers <NUM>, and such subsystems with such features are also within the scope and spirit of the present disclosure.

As illustrated, the power converter <NUM> may include a plurality of rotor side converters <NUM> rather than only a single rotor side converter <NUM>. The rotor side converters <NUM> may be electrically coupled to each other and the DC link <NUM> in parallel, as shown. As discussed, the use of multiple rotor side converters <NUM> may facilitate the reduction in harmonics. The switching frequency components from the rotor side converter which typically contribute to the harmonic content include, for example:<MAT> where FSW is switching frequency of the rotor-side converter; Fslip is fundamental frequency of the output voltage/current of the rotor-side converter; and N is a positive integer number. Accordingly, as the power rating of the subsystem <NUM> increases, additional rotor side converters <NUM> may be added. Such additional rotor side converters <NUM> may both meet higher current requirements and facilitate reduced harmonics.

In some embodiments, as illustrated in <FIG> and <FIG>, three or more rotor-side converters <NUM> may be utilized. In other embodiments, as illustrated in <FIG>, only two rotor side-converters <NUM> may be utilized. An inductor <NUM> may be electrically coupled to each rotor-side converter <NUM>. As particularly illustrated in <FIG>, in some embodiments, the inductors <NUM> may be magnetically coupled, such as via an interface or common-mode transformer <NUM>. Such magnetic coupling may aid in harmonics filtering.

As discussed, a controller <NUM> (which may be separate from or a component of controller <NUM>) may be communicatively coupled to the power converter <NUM> for controlling operation of the power converter <NUM>. The controller <NUM> may be communicatively coupled to each of the plurality of rotor-side converters <NUM>, and may thus control modulation of the switching elements (e.g. IGBTs) used in bridge circuits of each rotor side converter <NUM>.

In exemplary embodiments, the controller <NUM> may be configured to coordinate switching of the plurality of rotor-side converters <NUM> to produce an interleaved switching pattern between the plurality of rotor-side converters <NUM>. Such interleaved switching pattern may reduce or eliminate harmonics as discussed herein. For example, the controller <NUM> may shift the switching phase of each the plurality of rotor-side converters <NUM> to be out of phase with the others of the rotor side converters <NUM>, thus resulting in an interleaved switching pattern. In some embodiments, the phase of each of the plurality of rotor-side converters <NUM> is shifted from others of the plurality of rotor-side converters <NUM> by the result of <NUM> degrees divided by the total number of rotor-side converters, plus or minus <NUM> degrees, such as plus or minus <NUM> degrees, such as plus or minus <NUM> degrees, such as plus or minus <NUM> degrees. Notably, either the phase of the switching waveform can be shifted and the fundamental reference waveform kept the same or the fundamental reference waveform shifted and the switching waveform kept the same.

For example, in embodiments having two rotor-side converters <NUM>, a first one of the converters <NUM> may switch at the switching phase with no adjustment and a second one of the converters <NUM> may have its switching phase adjusted by <NUM> degrees, such that the phase of each of the two converters <NUM> is shifted from the other converter <NUM> by <NUM> degrees. In embodiments having three rotor-side converters <NUM>, a first one of the converters <NUM> may switch at the switching phase with no adjustment and the other two converters <NUM> may have their switching phases adjusted by <NUM> and <NUM> degrees, respectively, such that the phase of each of the three converters <NUM> is shifted from others of the converters <NUM> by <NUM> degrees. In embodiments having four rotor-side converters <NUM>, a first one of the converters <NUM> may switch at the switching phase with no adjustment and the other three converters <NUM> may have their switching phases adjusted by <NUM>, <NUM>, and <NUM> degrees, respectively, such that the phase of each of the four converters <NUM> is shifted from others of the converters <NUM> by <NUM> degrees. All of these described examples may utilized exact degrees as described or approximate degrees of plus or minus <NUM>, <NUM>, <NUM>, or <NUM> degrees.

<FIG> is a graph illustrating one embodiment of coordinating switching of rotor-side converters <NUM> to produce an interleaved switching pattern. In this embodiment, three converters <NUM> are utilized, as shown. As a result of such shifting the carrier wave for each converter <NUM> is shifted to produce an interleaved switching pattern.

In some embodiments, as illustrated in <FIG> and <FIG>, the substation <NUM> may further include a harmonic filter <NUM>. The harmonic filter <NUM> may, for example, include a resistor <NUM> and a capacitor <NUM> in series, or may have another suitable configuration. For example, in some embodiments, the harmonic filter <NUM> may be on the converter power path <NUM>, such as between the generator rotor <NUM> and the power converter <NUM>, as illustrated in <FIG>. Alternatively, however the harmonic filter <NUM> may be located in other advantageous locations since harmonics on the converter power path <NUM> may be minimized. For example, as illustrated in <FIG>, the harmonic filter <NUM> may be on the stator power path <NUM>, such as between a stator power path synch switch or contactor <NUM> and the generator stator <NUM>. In still other embodiments, no harmonic filter <NUM> may be necessary in the substation <NUM>, as illustrated in <FIG>.

The present disclosure is further directed to methods for operating electrical power subsystems <NUM> as discussed herein. Such methods may, for example, be performed by a controller <NUM>. A method may include, for example, the step of switching the plurality of rotor-side converters <NUM> to produce an interleaved switching pattern between the plurality of rotor-side converters <NUM>.

Claim 1:
An electrical power subsystem (<NUM>) for connection to a power grid, the electrical power subsystem (<NUM>) comprising:
a generator (<NUM>) comprising a generator stator (<NUM>) and a generator rotor (<NUM>);
a power converter (<NUM>) electrically coupled to the generator (<NUM>), the power converter (<NUM>) comprising:
a plurality of rotor-side converters (<NUM>) electrically coupled in parallel;
a line-side converter (<NUM>); and
a regulated DC link (<NUM>) electrically coupling the plurality of rotor-side converters (<NUM>) and the line-side converter (<NUM>);
a stator power path (<NUM>) for providing power from the generator stator (<NUM>) to the power grid;
magnetically coupled inductors (<NUM>) electrically coupled between the generator rotor (<NUM>) and each of the plurality of rotor-side converters (<NUM>);
a converter power path (<NUM>) for providing power from the generator rotor (<NUM>) through the power converter (<NUM>) to the power grid; and
a partial power transformer (<NUM>) provided on the converter power path (<NUM>).