Electrical power systems and subsystems

An 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, and a converter power path for providing power from the generator rotor through the power converter to the power grid.

FIELD OF THE INVENTION

The present disclosure relates generally to electrical power systems and subsystems for providing power to a power grid from, for example, wind turbines.

BACKGROUND OF THE INVENTION

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,FIGS. 1 and 2illustrate a wind turbine10and associated power system suitable for use with the wind turbine10according to conventional construction. As shown, the wind turbine10includes a nacelle14that typically houses a generator28(FIG. 2). The nacelle14is mounted on a tower12extending from a support surface (not shown). The wind turbine10also includes a rotor16that includes a plurality of rotor blades20attached to a rotating hub18. As wind impacts the rotor blades20, the blades20transform wind energy into a mechanical rotational torque that rotatably drives a low-speed shaft22. The low-speed shaft22is configured to drive a gearbox24(where present) that subsequently steps up the low rotational speed of the low-speed shaft22to drive a high-speed shaft26at an increased rotational speed. The high-speed shaft26is generally rotatably coupled to a generator28(such as a doubly-fed induction generator or DFIG) so as to rotatably drive a generator rotor30. As such, a rotating magnetic field may be induced by the generator rotor30and a voltage may be induced within a generator stator32that is magnetically coupled to the generator rotor30. The associated electrical power can be transmitted from the generator stator32to a main three-winding transformer34that is typically connected to a power grid via a grid breaker36. Thus, the main transformer34steps 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 generator28is typically electrically coupled to a bi-directional power converter38that includes a rotor-side converter40joined to a line-side converter42via a regulated DC link44. The rotor-side converter40converts the AC power provided from the rotor30into DC power and provides the DC power to the DC link44. The line side converter42converts the DC power on the DC link44into AC output power suitable for the power grid. Thus, the AC power from the power converter38can be combined with the power from the stator32to provide multi-phase power (e.g. three-phase power) having a frequency maintained substantially at the frequency of the power grid (e.g. 50 Hz/60 Hz).

As shown inFIG. 2, the illustrated three-winding transformer34typically has (1) a 33 kilovolt (kV) medium voltage (MV) primary winding33connected to the power grid, (2) a 6 to 13.8 kV MV secondary winding35connected to the generator stator32, and (3) a 690 to 900 volt (V) low-voltage (LV) tertiary winding37connected to the line-side power converter42.

Referring now toFIG. 3, individual power systems of a plurality of wind turbines10may be arranged in a predetermined geological location and electrically connected together to form a wind farm46. More specifically, as shown, the wind turbines10may be arranged into a plurality of groups48with each group separately connected to a main line50via switches51,52,53, respectively. In addition, as shown, the main line50may be electrically coupled to another, larger transformer54for further stepping up the voltage amplitude of the electrical power from the groups48of wind turbines10before sending the power to the grid.

One issue with such systems, however, is that the three-winding transformers34associated with each turbine10are expensive. Particularly, the secondary winding35of the transformer34that is connected to the generator stator32can 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.

BRIEF DESCRIPTION OF THE INVENTION

In accordance with one embodiment, an electrical power subsystem for connection to a power grid 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.

In accordance with another embodiment, an electrical power subsystem for connection to a power grid 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 transformer connecting the subsystem to the power grid. 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.

In accordance with another embodiment, a method for operating an electrical power subsystem 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.

DETAILED DESCRIPTION OF THE INVENTION

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 toFIG. 4, a schematic diagram of one embodiment of an electrical power subsystem102according 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 inFIG. 4orFIG. 2) and the overall electrical power system105ofFIG. 5orFIG. 3that includes a plurality of electrical power subsystems102. Those of ordinary skill in the art, however, will recognize that the electrical power subsystem102ofFIG. 4(orFIG. 2) 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 subsystem102may correspond to a wind turbine power system100. More specifically, as shown, the wind turbine power system100includes a rotor104that includes a plurality of rotor blades106attached to a rotating hub108. As wind impacts the rotor blades106, the blades106transform wind energy into a mechanical rotational torque that rotatably drives a low-speed shaft110. The low-speed shaft110is configured to drive a gearbox112that subsequently steps up the low rotational speed of the low-speed shaft110to drive a high-speed shaft114at an increased rotational speed. The high-speed shaft114is generally rotatably coupled to a doubly-fed induction generator116(referred to hereinafter as DFIG116) so as to rotatably drive a generator rotor118. As such, a rotating magnetic field may be induced by the generator rotor118and a voltage may be induced within a generator stator120that is magnetically coupled to the generator rotor118. In one embodiment, for example, the generator116is configured to convert the rotational mechanical energy to a sinusoidal, three-phase alternating current (AC) electrical energy signal in the generator stator120. Thus, as shown, the associated electrical power can be transmitted from the generator stator120directly the grid.

In addition, as shown, the generator116is electrically coupled to a bi-directional power converter122that includes a rotor-side converter124joined to a line-side converter126via a regulated DC link128. Thus, the rotor-side converter124converts the AC power provided from the generator rotor118into DC power and provides the DC power to the DC link128. The line side converter126converts the DC power on the DC link128into AC output power suitable for the power grid. More specifically, as shown, the AC power from the power converter122can be combined with the power from the generator stator120via a converter power path127and a stator power path125, respectively. For example, as shown, and in contrast to conventional systems such as those illustrated inFIGS. 1-3, the converter power path127may include a partial power transformer130for stepping up the voltage amplitude of the electrical power from the power converter122such that the transformed electrical power may be further transmitted to the power grid. Thus, as shown, the illustrated system102ofFIG. 4does not include the conventional three-winding main transformer described above. Rather, as shown in the illustrated embodiment, the partial power transformer130may correspond to a two-winding transformer having a primary winding132connected to the power grid and a secondary winding134connected to the line side converter126. Notably, the partial power transformer may in some embodiments include a third auxiliary winding for auxiliary loads.

In addition, the electrical power subsystem102may include a controller136configured to control any of the components of the wind turbine100and/or implement the method steps as described herein. For example, as shown particularly inFIG. 6, the controller136may include one or more processor(s)138and associated memory device(s)140configured 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 controller136may also include a communications module142to facilitate communications between the controller136and the various components of the wind turbine100, e.g. any of the components ofFIGS. 4 and 5. Further, the communications module142may include a sensor interface144(e.g., one or more analog-to-digital converters) to permit signals transmitted from one or more sensors139,141,143to be converted into signals that can be understood and processed by the processors138. It should be appreciated that the sensors139,141,143may be communicatively coupled to the communications module142using any suitable means. For example, as shown inFIG. 6, the sensors139,141,143may be coupled to the sensor interface144via a wired connection. However, in other embodiments, the sensors139,141,143may be coupled to the sensor interface144via a wireless connection, such as by using any suitable wireless communications protocol known in the art. As such, the processor138may be configured to receive one or more signals from the sensors139,141,143.

In operation, alternating current (AC) power generated at the generator stator120by rotation of the rotor104is provided via a dual path to the grid, i.e. via the stator power path125and the converter power path127. More specifically, the rotor side converter124converts the AC power provided from the generator rotor118into DC power and provides the DC power to the DC link128. Switching elements (e.g. IGBTs) used in bridge circuits of the rotor side converter124can be modulated to convert the AC power provided from the generator rotor118into DC power suitable for the DC link128. The line side converter126converts the DC power on the DC link128into AC output power suitable for the grid. In particular, switching elements (e.g. IGBTs) used in bridge circuits of the line side converter126can be modulated to convert the DC power on the DC link128into AC power. As such, the AC power from the power converter122can be combined with the power from the generator stator120to provide multi-phase power having a frequency maintained substantially at the frequency of the grid. It should be understood that the rotor side converter124and the line side converter126may have any configuration using any switching devices that facilitate operation of electrical power system as described herein.

Further, the power converter122may be coupled in electronic data communication with the turbine controller136and/or a separate or integral converter controller154to control the operation of the rotor side converter124and the line side converter126. For example, during operation, the controller136may be configured to receive one or more voltage and/or electric current measurement signals from the first set of voltage and electric current sensors139,141,143. Thus, the controller136may be configured to monitor and control at least some of the operational variables associated with the wind turbine100via the sensors139,141,143. In the illustrated embodiment, the sensors139,141,143may be electrically coupled to any portion of electrical power subsystem102that facilitates operation of electrical power subsystem102as 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 turbine100and 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 controller154is configured to receive one or more voltage and/or electric current feedback signals from the sensors139,141,143. 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 toFIG. 5, individual power systems (such as the power subsystem102illustrated inFIG. 4) may be arranged in at least two clusters137to form an electrical power system105. More specifically, as shown, the wind turbine power systems100may be arranged into a plurality of clusters137so as to form a wind farm. Thus, as shown, each cluster137may be connected to a separate cluster transformer145,146,147via switches151,152,153, respectively, for stepping up the voltage amplitude of the electrical power from each cluster137such that the transformed electrical power may be further transmitted to the power grid. In addition, as shown, the transformers145,146,147are connected to a main line148that combines the power from each cluster137before sending the power to the grid. In other words, as shown, the stator power circuit of all the wind turbines100share a common ground reference provided by the neutral of the secondary winding124of the cluster transformer145,146,147or by a separate neutral grounding transformer. Each subsystem102may be connected to the cluster137via a subsystem breaker135, as shown.

Referring now toFIGS. 7 through 10, various embodiments of electrical power subsystems102having improved harmonic reduction features are provided. It should be noted that, while such embodiments are illustrated in the context of subsystems using partial power transformers130, such improved harmonic reduction features are equally applicable to subsystems using transformers34, and such subsystems with such features are also within the scope and spirit of the present disclosure.

As illustrated, the power converter122may include a plurality of rotor side converters124rather than only a single rotor side converter124. The rotor side converters124may be electrically coupled to each other and the DC link128in parallel, as shown. As discussed, the use of multiple rotor side converters124may facilitate the reduction in harmonics. The switching frequency components from the rotor side converter which typically contribute to the harmonic content include, for example:
FSW+/−N Fslip

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 subsystem102increases, additional rotor side converters124may be added. Such additional rotor side converters124may both meet higher current requirements and facilitate reduced harmonics.

In some embodiments, as illustrated inFIGS. 7 and 8, three or more rotor-side converters124may be utilized. In other embodiments, as illustrated inFIG. 9, only two rotor side-converters124may be utilized. An inductor160may be electrically coupled to each rotor-side converter124. As particularly illustrated inFIG. 9, in some embodiments, the inductors160may be magnetically coupled, such as via an interface or common-mode transformer162. Such magnetic coupling may aid in harmonics filtering.

As discussed, a controller154(which may be separate from or a component of controller136) may be communicatively coupled to the power converter122for controlling operation of the power converter122. The controller154may be communicatively coupled to each of the plurality of rotor-side converters124, and may thus control modulation of the switching elements (e.g. IGBTs) used in bridge circuits of each rotor side converter124.

In exemplary embodiments, the controller154may be configured to coordinate switching of the plurality of rotor-side converters124to produce an interleaved switching pattern between the plurality of rotor-side converters124. Such interleaved switching pattern may reduce or eliminate harmonics as discussed herein. For example, the controller154may shift the switching phase of each the plurality of rotor-side converters124to be out of phase with the others of the rotor side converters124, thus resulting in an interleaved switching pattern. In some embodiments, the phase of each of the plurality of rotor-side converters124is shifted from others of the plurality of rotor-side converters124by the result of 360 degrees divided by the total number of rotor-side converters, plus or minus 15 degrees, such as plus or minus 10 degrees, such as plus or minus 5 degrees, such as plus or minus 2 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 converters124, a first one of the converters124may switch at the switching phase with no adjustment and a second one of the converters124may have its switching phase adjusted by 180 degrees, such that the phase of each of the two converters124is shifted from the other converter124by 180 degrees. In embodiments having three rotor-side converters124, a first one of the converters124may switch at the switching phase with no adjustment and the other two converters124may have their switching phases adjusted by 120 and 240 degrees, respectively, such that the phase of each of the three converters124is shifted from others of the converters124by 120 degrees. In embodiments having four rotor-side converters124, a first one of the converters124may switch at the switching phase with no adjustment and the other three converters124may have their switching phases adjusted by 90, 180, and 270 degrees, respectively, such that the phase of each of the four converters124is shifted from others of the converters124by 90 degrees. All of these described examples may utilized exact degrees as described or approximate degrees of plus or minus 15, 10, 5, or 2 degrees.

FIG. 10is a graph illustrating one embodiment of coordinating switching of rotor-side converters124to produce an interleaved switching pattern. In this embodiment, three converters124are utilized, as shown. As a result of such shifting the carrier wave for each converter124is shifted to produce an interleaved switching pattern.

In some embodiments, as illustrated inFIGS. 7 and 9, the substation102may further include a harmonic filter170. The harmonic filter170may, for example, include a resistor172and a capacitor174in series, or may have another suitable configuration. For example, in some embodiments, the harmonic filter170may be on the converter power path127, such as between the generator rotor118and the power converter122, as illustrated inFIG. 7. Alternatively, however the harmonic filter170may be located in other advantageous locations since harmonics on the converter power path127may be minimized. For example, as illustrated inFIG. 9, the harmonic filter170may be on the stator power path125, such as between a stator power path synch switch or contractor176and the generator stator120. In still other embodiments, no harmonic filter170may be necessary in the substation102, as illustrated inFIG. 8.

The present disclosure is further directed to methods for operating electrical power subsystems102as discussed herein. Such methods may, for example, be performed by a controller154. A method may include, for example, the step of switching the plurality of rotor-side converters124to produce an interleaved switching pattern between the plurality of rotor-side converters124.