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> illustrates a wind turbine <NUM>. As shown, the wind turbine <NUM> includes a nacelle <NUM> that typically houses a generator. 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> (<FIG>). The low-speed shaft <NUM> is configured to drive a gearbox <NUM> (<FIG>) (where present) that subsequently steps up the low rotational speed of the low-speed shaft <NUM> to drive a high-speed shaft <NUM> (<FIG>) at an increased rotational speed.

The shaft <NUM> or <NUM> may be rotatably coupled to a generator of an electrical power system. In some embodiments, for example, the generator is a doubly-fed induction generator or DFIG. In other embodiments, the electrical power system is a full conversion system only coupled to the stator of the generator. In either case, the electrical power system provides the power generated by the wind turbine <NUM> to the power grid as electrical power in a suitable form for use in the power grid.

<CIT> describes a method and a wind turbine with a wind rotor for driving an asynchronous generator for delivering electrical power to a transformer. The transformer is provided with two primary windings and the generator is connected to one of the two primary windings via a changeover switch depending on the operating mode. This enables a shift in the operating characteristic of the wind turbine to be achieved. This means that the characteristic curve can be improved downwards to low wind conditions without the conventionally required additional effort.

One issue that needs to be addressed in power systems is harmonics. Power generation systems connected to a utility grid must meet 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.

For example, <CIT> suggests that the output of multiple wind turbines are aggregated to create a high pulse number electrical output at a point of common coupling with a utility grid network. Power quality at each individual wind turbine falls short of utility standards, but the aggregated output at the point of common coupling is within acceptable tolerances for utility power quality. The approach for aggregating low pulse number electrical output from multiple wind turbines relies upon a pad mounted transformer at each wind turbine that performs phase multiplication on the output of each wind turbine. Phase multiplication converts a modified square wave from the wind turbine into a <NUM> pulse output. Phase shifting of the <NUM> pulse output from each wind turbine allows the aggregated output of multiple wind turbines to be a <NUM> pulse approximation of a sine wave. Additional filtering and VAR control is embedded within the wind farm to take advantage of the wind farm's electrical impedance characteristics to further enhance power quality at the point of common coupling.

Document <CIT> discloses a grid-connected inverter for feeding current via a transformer into an electric power grid includes an output bridge arrangement that is actuated via a pulse width modulator, wherein a periodic auxiliary signal is used to determine switching times of the output bridge arrangement. The inverter also includes a synchronization unit for phase synchronization of the auxiliary signal with the electric power grid, wherein the synchronization unit is configured to set a predetermined phase offset of the periodic auxiliary signal with respect to a phase of the electric power grid.

Various aspects and advantages of the invention will be set forth in part in the following description, or may be clear from the description, or may be learned through practice of the invention.

In accordance with the present invention, an electrical power system is provided according to the features of independent claim <NUM>. Preferred embodiments of the invention are subject-matter of the dependent claims.

Various features, aspects and advantages of the present invention will become better understood with reference to the following description and appended claims. In the drawings:.

<FIG> illustrates an electrical power subsystem <NUM> in accordance with embodiments of the present disclosure. In these embodiments, 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> via a converter power path <NUM> and stator power path <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> to13. <NUM> kV MV secondary winding <NUM> connected to the generator stator <NUM> via stator power path <NUM>, and (<NUM>) a <NUM> to <NUM> volt (V) low-voltage (LV) tertiary winding <NUM> connected to the line-side power converter <NUM> via the converter power path <NUM>.

Referring now to <FIG>, individual power subsystems <NUM> of a plurality of wind turbines <NUM> may be arranged in a predetermined geological location and electrically connected together to form an electrical power system <NUM>. More specifically, as shown, the wind turbines <NUM> may be arranged into a plurality of clusters <NUM> with each cluster separately connected to a transformer <NUM>, such as via switches <NUM>, <NUM>, <NUM>, respectively. Transformer <NUM> may step up the voltage amplitude of the electrical power from the groups <NUM> of wind turbines <NUM> before sending the power to the grid via a main line <NUM>. An intermediate power path <NUM> may electrically connect the cluster <NUM> to the transformer <NUM>. The intermediate power path <NUM> may, for example, extend for each subsystem <NUM> from the subsystem breaker <NUM> to the transformer <NUM>, such as via main line <NUM>. It should be understood that each cluster <NUM> may include one or more subsystems <NUM>, with each intermediate power path <NUM> electrically coupling one or more subsystems <NUM> to the transformer <NUM>.

Referring now to <FIG>, a schematic diagram of another 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 <NUM> of <FIG> that includes a plurality of electrical power subsystems <NUM> or <NUM>. Those of ordinary skill in the art, however, will recognize that the electrical power subsystem <NUM> of <FIG> (or <NUM> of <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.

As shown in <FIG>, 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 in <FIG>, 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 the systems 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 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 turbine 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 turbine 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 turbine controller <NUM> may also include a communications module <NUM> to facilitate communications between the turbine 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 magneto-optical 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 turbine 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 an 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 turbine 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 turbine 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 electric current 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 electric current 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. Further, each cluster <NUM> may be connected to a transformer <NUM>, such as via switches <NUM>, 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, such as via a main line <NUM>. Each subsystem <NUM> may be connected to the cluster <NUM> via a subsystem breaker <NUM>, as shown. An intermediate power path <NUM> may electrically connect each cluster <NUM> to the transformer <NUM>. The intermediate power path <NUM> may, for example, extend for each subsystem <NUM> from the subsystem breaker <NUM> to the transformer <NUM> such as to the winding of the transformer <NUM> to which the subsystem <NUM> is connected. It should be understood that each cluster <NUM> may include one or more subsystems <NUM>, with each intermediate power path <NUM> electrically coupling one or more subsystems <NUM> to the transformer <NUM>.

Referring now to <FIG>, another embodiment of an electrical power subsystem <NUM> for an electrical power system <NUM> is illustrated. In this embodiment, the subsystem <NUM> is a full conversion subsystem rather than a DFIG subsystem, with only a single generator power path <NUM> from the stator <NUM> of a generator <NUM> (which also includes a rotor <NUM>) through a power converter <NUM> to a transformer <NUM>. The transformer <NUM> in these embodiments is a two-winding transformer which electrically couples the generator power path <NUM> to an intermediate power path <NUM>.

The power converter <NUM> may include a generator side converter <NUM>, a line side converter <NUM>, and a DC link <NUM>. Switching elements (e.g. IGBTs) used in bridge circuits of the generator side converter <NUM> can be modulated to convert the AC power provided from the generator stator <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. It should be understood that the generator side converter <NUM> and the line side converter <NUM> may have any configuration using any switching devices that facilitate operation of electrical power system <NUM> as described herein.

It should be understood that the turbine controller <NUM> and controller <NUM> as described herein may be utilized with any suitable electrical power systems, subsystems, and power converters thereof as discussed herein, such as any embodiments as discussed in <FIG>.

Referring now to <FIG> and <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. Further, each cluster <NUM> may be connected to a transformer <NUM> 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, such as via a main line <NUM>. An intermediate power path may electrically connect each cluster <NUM> to the transformer <NUM>. In some examples not covered by the invention, as illustrated in <FIG>, the intermediate power path may be power path <NUM>, as shown. In other embodiments, as illustrated in <FIG>, no transformer <NUM> may be utilized, and the intermediate power path may be the generator power path <NUM>. It should be understood that each cluster <NUM> may include one or more subsystems <NUM>, with each intermediate power path <NUM>, <NUM> electrically coupling one or more subsystems <NUM> to the transformer <NUM>.

Referring now to <FIG> as well as <FIG> and <FIG>, the transformer <NUM> in accordance with the present disclosure is advantageously a zig-zag transformer <NUM>. As is generally understood, a zig-zag winding connection is a type of star-connection in which each output is the vector sum of any two phases. The zig-zag transformer <NUM> advantageously allows each cluster <NUM>, <NUM>, <NUM> of subsystems <NUM>, <NUM>, <NUM> within a system <NUM>, <NUM>, <NUM> to operate at a different phase shift, reducing or cancelling the harmonics produced by the subsystems <NUM>, <NUM>, <NUM> and associated wind turbines. Accordingly, the harmonic current transmitted to the power grid is reduced or eliminated. Zig-zag transformers are also cost-efficient and reliable, thus reducing the overall cost and increasing the reliability of the overall system.

Zig-zag transformer <NUM> includes a primary winding <NUM> and a plurality of secondary windings <NUM>. The primary winding <NUM> is connected to the main line <NUM>, <NUM>, <NUM>. Each secondary winding <NUM> is connected to an intermediate power path <NUM>, <NUM>, <NUM>, <NUM>.

As shown, at least one secondary winding <NUM> is a zig-zag winding having two winding portions, as is generally understood. For example, the plurality of secondary windings <NUM> may include one or more zig-zag windings and a wye winding, as shown, or another suitable combination of zig-zag, wye, and/or delta windings. In exemplary embodiments, the secondary windings <NUM> are phase-staggered, thus each having a different phase. Accordingly, the phase shift between the primary <NUM> and each secondary <NUM> is different and distinct from the phase shift between the primary <NUM> and each other secondary <NUM>. Such arrangement advantageously allows harmonic currents produced by the various power converters of the various subsystems to be compensated for, thus reducing or eliminating such harmonic currents. In particular, a zig-zag transformer <NUM> in accordance with the present disclosure can be arranged to reduce or eliminate at least one or more of the <NUM>th, <NUM>th, <NUM>th, <NUM>th, <NUM>th, <NUM>th, etc. order harmonic.

The primary winding <NUM> may, in some embodiments as illustrated in <FIG>, a wye winding. In other embodiments, as illustrated in <FIG>, the primary winding <NUM> is a delta winding.

In some embodiments, as illustrated in <FIG>, each of the plurality of secondary windings <NUM> is independently grounded by a separate neutral <NUM> of such winding <NUM>. Alternatively, as illustrated in <FIG>, no such independent grounding is necessary.

Claim 1:
An electrical power system (<NUM>,<NUM>,<NUM>) connectable to a power grid, comprising:
a plurality of electrical power subsystems (<NUM>,<NUM>,<NUM>), each of the plurality of electrical power subsystems (<NUM>,<NUM>,<NUM>) comprising a power converter (<NUM>,<NUM>,<NUM>) electrically coupled to a generator (<NUM>,<NUM>,<NUM>) having a generator rotor (<NUM>,<NUM>,<NUM>) and a generator stator (<NUM>,<NUM>,<NUM>);
a plurality of intermediate power paths (<NUM>,<NUM>,<NUM>) extending from the plurality of electrical power subsystems (<NUM>,<NUM>,<NUM>) for providing power from each of the plurality of electrical power subsystems (<NUM>,<NUM>,<NUM>) to the power grid; and
a zig-zag transformer (<NUM>) electrically coupling each of the plurality of intermediate power paths (<NUM>,<NUM>,<NUM>) to the power grid, the zig-zag transformer (<NUM>) comprising a primary winding (<NUM>) and a plurality of secondary windings (<NUM>), the primary winding (<NUM>) arranged to be electrically coupled to the power grid via a main line (<NUM>), each of the plurality of secondary windings (<NUM>) connected to one of the plurality of intermediate power paths (<NUM>,<NUM>,<NUM>), wherein at least one of the plurality of secondary windings (<NUM>) is a zig-zag winding,
wherein a respective cluster (<NUM>, <NUM>) of electrical power subsystems (<NUM>) is coupled to a respective one of the plurality of intermediate power paths (<NUM>, <NUM>), and wherein each of the plurality of intermediate power paths (<NUM>,<NUM>,<NUM>) is connectable to only a respective one of the plurality of secondary windings (<NUM>).