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 a rotor. The rotor typically includes a rotatable hub having one or more rotor blades attached thereto. A pitch bearing is typically configured operably between the hub and the rotor blade to allow for rotation about a pitch axis. The rotor blades capture kinetic energy of wind using known airfoil principles. The rotor blades transmit the kinetic energy in the form of rotational energy so as to turn a shaft coupling the rotor blades to a gearbox, or if a gearbox is not used, directly to the generator. The generator then converts the mechanical energy to electrical energy that may be deployed to a utility grid.

A power output of the generator increases with wind speed until the wind speed reaches a rated wind speed for the wind turbine. At and above the rated wind speed, the generator operates at a rated power. The rated power is an output power at which the generator can operate with a level of fatigue to turbine components that is predetermined to be acceptable. At wind speeds higher than a certain speed, or at a wind turbulence level that exceeds a predetermined magnitude, typically referred to as a "trip limit" or "monitor set point limit," wind turbines may be shut down, or the loads may be reduced by regulating the pitch of the rotor blades or braking the rotor, in order to protect wind turbine components against damage.

Variable speed operation of the generator facilitates enhanced capture of energy by the generator when compared to a constant speed operation of the wind turbine generator; however, variable speed operation of the generator produces electricity having varying voltage and/or frequency. More specifically, the frequency of the electricity generated by the variable speed generator is proportional to the speed of rotation of the rotor. Thus, a power converter may be coupled between the generator and the utility grid. The power converter outputs electricity having a fixed voltage and frequency for delivery on the grid.

Wind energy generation and, particularly, reactive power control of the wind turbine power system should take an active part in the stability and quality of the electrical grid. Thus, reactive power compensation of the wind turbine power system is configured to fulfill electrical network demands and maintain a reactive power reserve in order to support grid contingencies. Such objectives may lead to giving priority to reactive power over active power production depending on network conditions. Thus, in a typical wind turbine power system, the turbine controller receives a power command from a farm-level controller that is based on various grid conditions. As such, the power command instructs each wind turbine how much reactive and active power should be generated based on the grid.

In weak grids, the response to a power command can be sluggish. The slow response can be detrimental to functions that require rapid change of power to stabilize the power system (e.g. the drivetrain damper, fast power reduction functions, etc.). Document <CIT> describes method of controlling a grid side converter of a wind turbine. An output of the grid side converter is connected or connectable via a power line to an input of a grid transformer. A converter volt-sec occurring at the output of the grid side converter is computed based on a converter voltage occurring at the output of the grid side converter. A volt-sec error between the determined converter volt-sec and a converter volt-sec reference is determined based on an active power reference, a reactive power reference, a line current and a line voltage occurring at the input of the grid transformer. Based on the determined volt-sec error, the grid side converter is controlled such that the volt-sec error is partially or fully compensated. A PLL algorithm may be used to estimate the grid angle and the angular speed of the grid voltage. The PLL algorithm may be implemented using synchronous rotating frame technique and may use moving average technique to eliminate negative sequence and harmonics components from the grid voltage.

Accordingly, the present disclosure is directed to systems and methods for reducing the delay between a power command and the actual power of wind turbine power systems using a power angle feedforward signal in a phase locked loop (PLL) of the power system.

In one aspect, the present disclosure is directed to a method for reducing a delay between a power command and actual power of a power system connected to a power grid. The method includes receiving, via a power angle estimator, a power command of the power system. The method also includes receiving, via the power angle estimator, one or more grid conditions of the power grid. Further, the method includes estimating, via the power angle estimator, a power angle signal across the power system based on the power command and/or the one or more grid conditions. The method further includes receiving, via a phase locked loop (PLL), the estimated power angle signal. Moreover, the method includes generating, via the PLL, a PLL phase angle signal based, at least in part, on the estimated power angle signal. Thus, the method further includes controlling, via a converter controller, a power conversion assembly of the power system based on the PLL phase angle signal.

In one embodiment, the method includes receiving, via the PLL, the power angle signal as a feedforward signal.

In another embodiment, the step of estimating the power angle signal across the power system may include calculating the power angle signal as a function of a transmitted power and a reactance between two electric buses at or near ends of the power system. More specifically, in one embodiment, the step of estimating the power angle signal across the power system may include calculating the power angle signal as a function of the transmitted power, voltage magnitudes at the two ends of the power system, and the reactance between the two electric buses at or near the ends of the power system.

For example, in certain embodiments, the step of calculating the power angle signal as a function of the transmitted power, the voltage magnitudes at the two ends of the power system, and the reactance between the two electric buses at or near ends of the power system may include multiplying the voltage magnitudes at the two ends of the power system to obtain a first multiplied value, dividing the reactance between the two electric buses at or near the ends of the power system by the first multiplied value to obtain a multiplier, multiplying the transmitted power by the multiplier to obtain a second multiplied value, and applying a non-linear function to the second multiplied value. In particular embodiments, the non-linear function may include, for example, polynomial, sine, cosine, or arcsine.

In further embodiments, the step of generating the PLL phase angle signal based, at least in part, on the estimated power angle signal may include receiving a terminal grid voltage feedback signal, determining a PLL error signal based on the terminal grid voltage feedback signal, determining, via a PLL regulator, a frequency signal of the PLL in response to the PLL error signal, integrating the frequency signal via an integrator of the PLL to obtain an output signal, and generating the PLL phase angle signal based on the output signal and the power angle signal. For example, in one embodiment, the step of generating the PLL phase angle signal based on the output signal and the power angle signal may include adding the output signal and the power angle signal.

In yet another embodiment, the power system may be a wind turbine power system, a solar power system, an energy storage system, and/or combinations thereof.

In another aspect, the present disclosure is directed to an electrical power system connected to a power grid. The electrical power system includes an electric generator, a power conversion assembly coupled to the electric generator, a power angle estimator, a phase locked loop (PLL), and a converter controller. The power conversion assembly is configured to receive power generated by the electric generator and convert the power received to a power suitable for transmission to the power grid. The power angle estimator is configured to estimate a power angle signal across the electrical power system based on a received power command and/or one or more grid conditions. The PLL is configured to generate a PLL phase angle signal based, at least in part, on the estimated power angle signal. Thus, the converter controller controls the power conversion assembly based on the PLL phase angle signal. It should be understood that the electrical power system may further include any of the additional features as described herein.

In yet another aspect, the present disclosure is directed to a wind turbine connected to a power grid. The wind turbine includes a tower, a nacelle mounted atop the tower, a rotor having a rotatable hub and at least one rotor blade mounted thereto, and a power generation and delivery system. The power generation and delivery system includes an electric generator connected to a power grid, a power conversion assembly coupled to the electric generator a power angle estimator, a phase locked loop (PLL), and converter controller. The power conversion assembly is configured to receive power generated by the electric generator and convert the power received to a power suitable for transmission to the power grid. The power angle estimator is configured to estimate a power angle signal across the wind turbine based on a received power command and/or one or more grid conditions. The PLL is configured to generate a PLL phase angle signal based, at least in part, on the estimated power angle signal. Thus, the converter controller controls the power conversion assembly based on the PLL phase angle signal. It should be understood that the wind turbine may further include any of the additional features as described herein.

The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate the embodiments of the invention and, together with the description, serve to explain the principles of the invention.

Generally, the present disclosure is directed to systems and methods for reducing the delay between a power command and the actual power of a wind turbine power system using a phase locked loop (PLL) feedforward signal. In weak grids, the response to a power command is sluggish, which is detrimental to functions that require rapid change of power to stabilize the power system, e.g. drive-train damper and fast power reduction functions. As such, the PLL feedforward signal is useful for reducing the delay between the power command and actual power.

More specifically, in AC transmission systems, the power transfer correlates with the power angle across the system. By estimating the power angle as a feedforward contribution to the phase-locked loop, the PLL feedforward signal establishes the reference frame for the vector control functions in the power converter of the power system. As such, the systems and methods of the present disclosure cause the reference frame to move according to the estimated angle change needed to implement the power command, thereby reducing the need for the PLL closed loop to adjust via its error path.

Referring now to the drawings, <FIG> illustrates a perspective view of one embodiment of a wind turbine power system <NUM> (also referred to herein simply as wind turbine <NUM>) according to the present disclosure. As shown, the wind turbine <NUM> described herein includes a horizontal-axis configuration, however, in some embodiments, the wind turbine <NUM> may include, in addition or alternative to the horizontal-axis configuration, a vertical-axis configuration (not shown). The wind turbine <NUM> may be coupled to an electrical load (not shown in <FIG>), such as, but not limited to, a power grid, for receiving electrical power therefrom to drive operation of the wind turbine <NUM> and/or its associated components and/or for supplying electrical power generated by the wind turbine <NUM> thereto.

The wind turbine <NUM> may include a nacelle <NUM> and a rotor (generally designated by <NUM>) coupled to nacelle <NUM> for rotation with respect to nacelle <NUM> about an axis of rotation <NUM>. In one embodiment, the nacelle <NUM> is mounted on a tower <NUM>, however, in some embodiments, in addition or alternative to the tower-mounted nacelle <NUM>, the nacelle <NUM> may be positioned adjacent the ground and/or a surface of water. The rotor <NUM> includes a hub <NUM> and a plurality of rotor blades <NUM> extending radially outwardly from the hub <NUM> for converting wind energy into rotational energy. Although the rotor <NUM> is described and illustrated herein as having three rotor blades <NUM>, the rotor <NUM> may have any number of rotor blades <NUM>. Further, the rotor blades <NUM> may each have any length that allows the wind turbine <NUM> to function as described herein.

Referring now to <FIG>, the wind turbine <NUM> also includes an electrical generator <NUM> coupled to the rotor <NUM> for generating electrical power from the rotational energy generated by the rotor <NUM>. The generator <NUM> may be any suitable type of electrical generator, such as, but not limited to, a wound rotor induction generator, a double-fed induction generator (DFIG, also known as dual-fed asynchronous generators), a permanent magnet (PM) synchronous generator, an electrically-excited synchronous generator, and a switched reluctance generator. The generator <NUM> includes a stator (not shown) and a rotor (not shown) with an air gap included therebetween. The rotor <NUM> includes a rotor shaft <NUM> coupled to the rotor hub <NUM> for rotation therewith. Further, the generator <NUM> is coupled to the rotor shaft <NUM> such that rotation of the rotor shaft <NUM> drives rotation of the generator rotor, and therefore operation of the generator <NUM>. In one embodiment, the generator rotor has a generator shaft <NUM> coupled thereto and coupled to the rotor shaft <NUM> such that rotation of the rotor shaft <NUM> drives rotation of the generator rotor. In other embodiments, the generator rotor is directly coupled to the rotor shaft <NUM>, sometimes referred to as a "direct-drive wind turbine. " In one embodiment, the generator shaft <NUM> is coupled to the rotor shaft <NUM> through a gearbox <NUM>, although in other embodiments generator shaft <NUM> is coupled directly to rotor shaft <NUM>.

The torque of the rotor <NUM> drives the generator rotor to thereby generate variable frequency AC electrical power from rotation of rotor <NUM>. The generator <NUM> has an air gap torque between the generator rotor and stator that opposes the torque of rotor <NUM>. A power conversion assembly <NUM> is coupled to the generator <NUM> for converting the variable frequency AC to a fixed frequency AC for delivery to an electrical load (not shown in <FIG>), such as, but not limited to a power grid (not shown in <FIG>), coupled to the generator <NUM>. The power conversion assembly <NUM> may include a single frequency converter or a plurality of frequency converters configured to convert electricity generated by the generator <NUM> to electricity suitable for delivery over the power grid. The power conversion assembly <NUM> may also be referred to herein as a power converter. The power conversion assembly <NUM> may be located anywhere within or remote to the wind turbine <NUM>. For example, the power conversion assembly <NUM> may be located within a base (not shown) of the tower <NUM>.

In certain embodiments, the wind turbine <NUM> may include a rotor speed limiter, for example, but not limited to a disk brake <NUM>. The disk brake <NUM> brakes rotation of the rotor <NUM> to, for example, slow rotation of the rotor <NUM>, the brake rotor <NUM> against full wind torque, and/or reduce the generation of electrical power from the generator <NUM>. Furthermore, in some embodiments, the wind turbine <NUM> may include a yaw system <NUM> for rotating the nacelle <NUM> about an axis of rotation <NUM> for changing a yaw of rotor <NUM>, and more specifically for changing a direction faced by the rotor <NUM> to, for example, adjust an angle between the direction faced by the rotor <NUM> and a direction of wind.

In one embodiment, the wind turbine <NUM> includes a variable blade pitch system <NUM> for controlling, including but not limited to changing, a pitch angle of blades <NUM> (shown in <FIG>) with respect to a wind direction. The pitch system <NUM> may be coupled to a controller <NUM> for control thereby. The pitch system <NUM> is coupled to the hub <NUM> and the rotor blades <NUM> for changing the pitch angle of blades <NUM> by rotating the rotor blades <NUM> with respect to the hub <NUM>. The pitch actuators may include any suitable structure, configuration, arrangement, means, and/or components, whether described and/or shown herein, such as, but not limited to, electrical motors, hydraulic cylinders, springs, and/or servomechanisms. Moreover, the pitch actuators may be driven by any suitable means, whether described and/or shown herein, such as, but not limited to, hydraulic fluid, electrical power, electro-chemical power, and/or mechanical power, such as, but not limited to, spring force.

Referring now to <FIG>, a block diagram of one embodiment of various electrical components of the wind turbine <NUM> according to the present disclosure is illustrated. As shown, the wind turbine <NUM> includes one or more controllers <NUM> coupled to at least one component of wind turbine <NUM> for generally controlling operation of the wind turbine <NUM> and/or controlling operation of the components thereof, regardless of whether such components are described and/or shown herein. For example, in one embodiment, the controller <NUM> is coupled to the pitch system <NUM> for generally controlling the rotor <NUM>. In addition, the controller <NUM> may be mounted within the nacelle <NUM> (as shown in <FIG>), however, additionally or alternatively, one or more controllers <NUM> may be remote from the nacelle <NUM> and/or other components of the wind turbine <NUM>. The controller(s) <NUM> may be used for overall system monitoring and control including, without limitation, pitch and speed regulation, high-speed shaft and yaw brake application, yaw and pump motor application, and/or fault monitoring. Alternative distributed or centralized control architectures may be used in some embodiments.

In one embodiment, the wind turbine <NUM> includes a plurality of sensors, for example, sensors <NUM>, <NUM>, <NUM>, <NUM>, <NUM> as shown in <FIG>, <FIG>, and <FIG>. As such, the sensors <NUM>, <NUM>, <NUM>, <NUM>, <NUM> are configured to measure a variety of parameters including, without limitation, operating conditions and atmospheric conditions. For example, as shown, the wind turbine <NUM> includes a wind sensor <NUM>, such as an anemometer or any other suitable device, configured for measuring wind speeds or any other wind parameter. The wind parameters include information regarding at least one of or a combination of the following: a wind gust, a wind speed, a wind direction, a wind acceleration, a wind turbulence, a wind shear, a wind veer, a wake, SCADA information, or similar. Further, the wind turbine <NUM> may also include one or more additional sensors for monitoring additional operational parameters of the wind turbine <NUM>. Further, each sensor <NUM>, <NUM>, <NUM>, <NUM>, <NUM> may be an individual sensor or may include a plurality of sensors. The sensors <NUM>, <NUM>, <NUM>, <NUM>, <NUM> may be any suitable sensor having any suitable location within or remote to wind turbine <NUM> that allows the wind turbine <NUM> to function as described herein. In some embodiments, the sensors <NUM>, <NUM>, <NUM>, <NUM>, <NUM> are coupled to one of the controllers <NUM>, <NUM>, <NUM> described herein for transmitting measurements to the controllers <NUM>, <NUM>, <NUM> for processing thereof.

Still referring to <FIG>, the controller <NUM> may include a bus <NUM> or other communications device to communicate information. Further, one or more processor(s) <NUM> may be coupled to the bus <NUM> to process information, including information from the sensors <NUM>, <NUM>, <NUM>, <NUM>, <NUM> and/or other sensor(s). The processor(s) <NUM> may include at least one computer. As used herein, the term computer is not limited to integrated circuits referred to in the art as a computer, but broadly refers to a processor, a microcontroller, a microcomputer, a programmable logic controller (PLC), an application specific integrated circuit, and other programmable circuits, and these terms are used interchangeably herein.

The controller <NUM> may also include one or more random access memories (RAM) <NUM> and/or other storage device(s) <NUM>. Thus, as shown, the RAM(s) <NUM> and storage device(s) <NUM> may be coupled to the bus <NUM> to store and transfer information and instructions to be executed by processor(s) <NUM>. The RAM(s) <NUM> (and/or storage device(s) <NUM>, if included) can also be used to store temporary variables or other intermediate information during execution of instructions by the processor(s) <NUM>. The controller <NUM> may also include one or more read only memories (ROM) <NUM> and/or other static storage devices coupled to the bus <NUM> to store and provide static (i.e., non-changing) information and instructions to the processor(s) <NUM>. The processor(s) <NUM> process information transmitted from a plurality of electrical and electronic devices that may include, without limitation, speed and power transducers. Instructions that are executed include, without limitation, resident conversion and/or comparator algorithms. The execution of sequences of instructions is not limited to any specific combination of hardware circuitry and software instructions.

The controller <NUM> may also include, or may be coupled to, input/output device(s) <NUM>. The input/output device(s) <NUM> may include any device known in the art to provide input data to the controller <NUM> and/or to provide outputs, such as, but not limited to, yaw control and/or pitch control outputs. Instructions may be provided to the RAM <NUM> from the storage device <NUM> including, for example, a magnetic disk, a read-only memory (ROM) integrated circuit, CD-ROM, and/or DVD, via a remote connection that is either wired or wireless providing access to one or more electronically-accessible media. In some embodiments, hard-wired circuitry can be used in place of or in combination with software instructions. Thus, execution of sequences of instructions is not limited to any specific combination of hardware circuitry and software instructions, whether described and/or shown herein. Also, in one embodiment, the input/output device(s) <NUM> may include, without limitation, computer peripherals associated with an operator interface such as a mouse and a keyboard (neither shown in <FIG>). Alternatively, other computer peripherals may also be used that may include, for example, a scanner (not shown in <FIG>). Furthermore, in one embodiment, additional output channels may include, for example, an operator interface monitor (not shown in <FIG>). The controller <NUM> may also include a sensor interface <NUM> that allows controller <NUM> to communicate with the sensors <NUM>, <NUM>, <NUM>, <NUM>, <NUM> and/or other sensor(s). The sensor interface <NUM> may include one or more analog-to-digital converters that convert analog signals into digital signals that can be used by the processor(s) <NUM>.

In another embodiment, the wind turbine <NUM> also includes a phase locked loop (PLL) <NUM>. For instance, as shown, the PLL <NUM> is coupled to sensor <NUM>. In one embodiment, as shown, the sensor <NUM> is a voltage transducer configured to measure a terminal grid voltage output by frequency converter <NUM>. Alternatively, the PLL <NUM> is configured to receive a plurality of voltage measurement signals from a plurality of voltage transducers. In an example of a three-phase generator, each of three voltage transducers is electrically coupled to each one of three phases of a grid bus. The PLL <NUM> may be configured to receive any number of voltage measurement signals from any number of voltage transducers that allow the PLL <NUM> to function as described herein.

Referring now to <FIG>, the wind turbine <NUM> described herein may be part of a wind farm <NUM> that is controlled according to the system and method of the present disclosure is illustrated. As shown, the wind farm <NUM> may include a plurality of wind turbines <NUM>, including the wind turbine <NUM> described above, and a farm-level controller <NUM>. For example, as shown in the illustrated embodiment, the wind farm <NUM> includes twelve wind turbines, including wind turbine <NUM>. However, in other embodiments, the wind farm <NUM> may include any other number of wind turbines, such as less than twelve wind turbines or greater than twelve wind turbines. In one embodiment, the controller <NUM> of the wind turbine <NUM> may be communicatively coupled to the farm-level controller <NUM> through a wired connection, such as by connecting the controller <NUM> through suitable communicative links <NUM> or networks (e.g., a suitable cable). Alternatively, the controller <NUM> may be communicatively coupled to the farm-level controller <NUM> through a wireless connection, such as by using any suitable wireless communications protocol known in the art. In addition, the farm-level controller <NUM> may be generally configured similar to the controllers <NUM> for each of the individual wind turbines <NUM> within the wind farm <NUM>.

Referring now to <FIG>, a block diagram of one embodiment of a power generation and delivery system <NUM> of the wind turbine <NUM> is illustrated. As shown, the power generation and delivery system <NUM> includes an energy source, for example, the generator <NUM>. Although described herein as the wind turbine generator <NUM>, the energy source may include any type of electrical generator that allows the system <NUM> to function as described herein. The system <NUM> also includes a power converter, such as, the power converter <NUM>. Thus, as shown, the power converter <NUM> receives variable frequency electrical power <NUM> generated by the generator <NUM> and converts electrical power <NUM> to an electrical power <NUM> (referred to herein as a terminal power <NUM>) suitable for transmission over an electric power transmission and distribution grid <NUM> (referred to herein as utility grid <NUM>). A terminal voltage (Vt) <NUM> is defined at a node between the power converter <NUM> and the utility grid <NUM>. A load <NUM> is coupled to the utility grid <NUM> where a Thevenin voltage is defined. As described above, variable speed operation of the wind turbine <NUM> facilitates enhanced capture of energy when compared to a constant speed operation of the wind turbine <NUM>, however, variable speed operation of the wind turbine <NUM> produces the electrical power <NUM> having varying voltage and/or frequency. More specifically, the frequency of the electrical power <NUM> generated by the variable speed generator <NUM> is proportional to the speed of rotation of the rotor <NUM> (shown in <FIG>). In one embodiment, the power converter <NUM> outputs the terminal power <NUM> having a substantially fixed voltage and frequency for delivery on the utility grid <NUM>.

The power converter <NUM> also controls an air gap torque of the generator <NUM>. The air gap torque is present between the generator rotor (not shown in <FIG>) and the generator stator (not shown in <FIG>) and opposes the torque applied to the generator <NUM> by the rotor <NUM>. A balance between a torque on the rotor <NUM> created by interaction of the rotor blades <NUM> and the wind and the air gap torque facilitates stable operation of the wind turbine <NUM>. Wind turbine adjustments, for example, blade pitch adjustments, or grid events, for example, low voltage transients or zero voltage transients on the utility grid <NUM>, may cause an imbalance between the torque on the rotor <NUM> caused by the wind and the air gap torque. The power converter <NUM> controls the air gap torque which facilitates controlling the power output of the generator <NUM>, however, the wind turbine <NUM> may not be able to operate through certain grid events, or may sustain wear and/or damage due to certain grid events, due to a time period required for adjustments to wind turbine operation to take effect after detecting the grid event.

Still referring to <FIG>, the system <NUM> may include a grid-dependent power limiter system <NUM>. In such embodiments, a controller, for example, but not limited to, controller <NUM> (shown in <FIG>), may be programmed to perform the functions of the grid-dependent power limiter system <NUM>. However, in alternative embodiments, the functions of the grid-dependent power limiter system <NUM> may be performed by any circuitry configured to allow the system <NUM> to function as described herein. The power limiter system <NUM> is configured to identify the occurrence of a grid contingency event, and provide the power converter <NUM> with signals that facilitate providing a stable recovery from the grid event.

The power conversion assembly <NUM> is configured to receive control signals <NUM> from a converter interface controller <NUM>. The control signals <NUM> are based on sensed operating conditions or operating characteristics of the wind turbine <NUM> as described herein and used to control the operation of the power conversion assembly <NUM>. Examples of measured operating conditions may include, but are not limited to, a terminal grid voltage, a PLL error, a stator bus voltage, a rotor bus voltage, and/or a current. For example, the sensor <NUM> measures terminal grid voltage <NUM> and transmits a terminal grid voltage feedback signal <NUM> to power limiter system <NUM>. The power limiter system <NUM> generates a power command signal <NUM> based at least partially on the feedback signal <NUM> and transmits power command signal <NUM> to the converter interface controller <NUM>. In an alternative embodiment, the converter interface controller <NUM> is included within the system controller <NUM>. Other operating condition feedback from other sensors also may be used by the controller <NUM> and/or converter interface controller <NUM> to control the power conversion assembly <NUM>.

As mentioned, the systems and methods described herein facilitate reducing the delay between a power command and the actual power of a wind turbine power system, such as wind turbine <NUM>. Thus, as shown in <FIG>, a block diagram of one embodiment of the power limiter system <NUM> of the wind turbine <NUM> according to the present disclosure is illustrated. As shown, the power limiter system <NUM> is configured to output the power command signal <NUM> (shown in <FIG>), which in one embodiment, is at least one of a real current command signal <NUM> and a reactive current command signal <NUM>. In addition, as shown, the power limiter system <NUM> includes a power limiter <NUM>, a power regulator <NUM>, and a voltage regulator <NUM>. In one instance, the power limiter <NUM> receives at least one measured operating condition of the system <NUM>. The measured operating condition(s) may include, but is not limited to, a PLL error signal <NUM> (e.g. PLLERR) from the PLL <NUM> and the terminal grid voltage feedback signal <NUM> (e.g. VT_FBK) from the sensor <NUM>.

The power limiter <NUM> also receives a stored reference power control signal <NUM> (e.g. PREF) from, for example, the controller <NUM> (<FIG>). In some embodiments, the power limiter <NUM> receives the power limit control signal <NUM> and the reference power control signal <NUM> and generates the power command signal <NUM> corresponding to the lesser of the power limit control signal <NUM> and the reference power control signal <NUM>. Thus, as shown, the power limiter <NUM> generates the power command signal <NUM> (e.g. PCMD) and transmits the power command signal <NUM> to the power regulator <NUM>. The power regulator <NUM> then generates the real current command signal <NUM> and transmits the signal <NUM> to the converter interface controller <NUM>. The real current command signal <NUM> instructs the converter interface controller <NUM> to modify a real component of current that the power conversion assembly <NUM> tries to inject onto the utility grid <NUM>.

Still referring to <FIG>, the voltage regulator <NUM> generates the reactive current command signal <NUM> (e.g. IY_CMD) and sends the command signal <NUM> to the converter interface controller <NUM>. The current command signal <NUM> instructs the converter interface controller <NUM> to modify a reactive component of current injected onto the utility grid <NUM>. As shown, the converter interface controller <NUM> may also be referred to herein as a converter firing control. As described above, the PLL <NUM> may be included within the controller <NUM>, or may be coupled to, but separate from, the controller <NUM>.

The PLL <NUM> also receives the terminal voltage feedback signal <NUM>. For example, the PLL <NUM> may receive the terminal voltage feedback signal <NUM> (shown in <FIG> as Vt) provided by the sensor <NUM> (shown in <FIG>). In addition, as shown, the PLL <NUM> receives a PLL feedforward signal <NUM> (e.g. PLL_FF), which is described in more detail in reference to <FIG> below. As described above, the PLL <NUM> also generates the PLL error signal <NUM> (e.g. PLLERR) and a PLL phase angle signal <NUM> (e.g. TH_PLL). The PLL phase angle signal <NUM> is transmitted to the converter interface controller <NUM> for control of the power conversion assembly <NUM> and for subsequent control of electrical currents injected onto the utility grid <NUM> (shown in <FIG>).

More specifically, as shown in <FIG>, a detailed schematic view of the PLL <NUM> is illustrated. As shown, the PLL <NUM> may generally include a demodulator <NUM> that receives the terminal voltage feedback signal <NUM>. As such, the demodulator <NUM> is configured to extract the phase error between the original terminal voltage feedback carrier wave (i.e. the voltage feedback signal <NUM>) and the PLL angle. The demodulator <NUM> generates the PLL error signal <NUM> that is regulated via the PLL regulator <NUM>. The PLL regulator <NUM> then determines a frequency signal <NUM> (i.e. PLL_W) of the PLL <NUM> in response to the PLL error signal <NUM>. The frequency signal <NUM> may then be integrated via integrator <NUM> of the PLL <NUM>. The output signal <NUM> of the integrator <NUM> is then sent to function block <NUM>.

Still referring to <FIG>, the PLL feedforward signal <NUM> (which may also be referred to herein as a power angle signal) may be estimated via a power angle estimator <NUM>. As used herein, the term "feedforward" encompasses its broadest interpretation, which generally describes an element or pathway within a control system that passes a controlling signal from a source in its external environment to a load elsewhere in its external environment. Thus, as shown, the power angle signal <NUM> is estimated external to the PLL <NUM> and then input into the PLL <NUM>. Accordingly, the power angle estimator <NUM> is configured to estimate or calculate a power angle across the electrical power system (e.g. the wind turbine <NUM>) based on the received power command PCMD <NUM> and/or one or more external grid conditions <NUM>. For example, in one embodiment, the power angle (PA) may be estimated using Equation (<NUM>) below (which assumes that the voltages of the power system remain constant and the angle is below about <NUM> degrees): <MAT> Where P is the transmitted power,.

In alternative embodiments, the power angle (PA) may be estimated using a non-linear function. For example, in one embodiment, the non-linear function may include a polynomial or trigonometric function, such as but not limited to sine, cosine, or arcsine. Thus, in a particular embodiment, the power angle (PA) may be estimated using Equation (<NUM>) below: <MAT> Where P is the transmitted power,.

At function block <NUM>, the PLL <NUM> is configured to generate the PLL phase angle signal <NUM> based on the estimated feedforward power angle signal <NUM> and the output signal of the integrator <NUM>. More specifically, as shown, the PLL <NUM> generates the PLL phase angle signal <NUM> by summing the estimated feedforward power angle signal <NUM> and the output signal <NUM> of the integrator <NUM>. Thus, as shown in <FIG>, the converter controller <NUM> controls the power conversion assembly <NUM> based on the PLL phase angle signal.

Referring now to <FIG>, a flow diagram of one embodiment of a method <NUM> for reducing a delay between the power command <NUM> and actual power of the wind turbine <NUM> is illustrated. As shown at <NUM>, the method <NUM> includes receiving, via the power angle estimator <NUM>, the power command <NUM> of the wind turbine power system <NUM>. As shown at <NUM>, the method <NUM> includes receiving, via the power angle estimator, one or more grid conditions of the power grid. For example, the grid condition may include, but are not limited to information that can assist in estimating the reactance X between the two electric buses at or near the ends of the power system, such as breaker status or alternatively a priori knowledge of the nature of the power grid. For example, the reactance of the grid may be known for a given project, in which case the parameter X can be set according to that a priori knowledge. Another example of such information may include knowing if a grid system may have one important transmission line that, if removed from service, will make a substantial change in the effective reactance. In such a scenario, the effective reactance would be computed a-priori knowing the characteristics of the grid with and without that important line in service. Then, during operation the selection of reactance X in the power angle estimation would be toggled between the two precomputed values of X depending upon the status of the circuit breakers on that line. As shown at <NUM>, the method <NUM> includes estimating, via the power angle estimator <NUM>, a power angle signal <NUM> across the power system based on the power command and/or the grid condition(s). As shown at <NUM>, the method <NUM> includes receiving, via the PLL <NUM>, the estimated power angle signal <NUM> as a feedforward signal. As shown at <NUM>, the method <NUM> includes generating, via the PLL <NUM>, the PLL phase angle signal <NUM> based, at least in part, on the estimated power angle signal <NUM>. As shown at <NUM>, the method <NUM> includes controlling, via the converter controller <NUM>, the power conversion assembly <NUM> based on the PLL phase angle signal <NUM>.

Thus, the feedforward function of the present disclosure provides less lag between the actual power and the power command, particularly in weak grids operating at high power. Benefits of the feedforward function of the present disclosure may depend on how close the estimated power angle is to the actual characteristic of the AC transmission system. As mentioned, Equation (<NUM>) above indicates example operating parameters needed to derive the power angle described herein.

Although specific features of various embodiments of the invention may be shown in some drawings and not in others, this is for convenience only. In accordance with the principles of the invention, any feature of a drawing may be referenced and/or claimed in combination with any feature of any other drawing.

Claim 1:
A method for reducing a delay between a power command (<NUM>) and actual power of a power system (<NUM>) connected to a power grid (<NUM>), the method comprising:
receiving, via a power angle estimator (<NUM>), a power command (<NUM>) of the power system (<NUM>);
receiving, via the power angle estimator (<NUM>), one or more grid conditions (<NUM>) of the power grid (<NUM>);
estimating, via the power angle estimator (<NUM>), a power angle signal (<NUM>) across the power system (<NUM>) based on the power command (<NUM>) and the one or more grid conditions (<NUM>);
characterised in that the method further comprises:
receiving, via a phase locked loop, PLL (<NUM>), the estimated power angle signal (<NUM>);
generating, via the PLL (<NUM>), a PLL phase angle signal (<NUM>) based, at least in part, on the estimated power angle signal (<NUM>); and,
controlling, via a converter controller (<NUM>), a power conversion assembly (<NUM>) of the power system (<NUM>) based on the PLL phase angle signal (<NUM>).