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 from wind using known foil principles and transmit the kinetic energy through rotational energy 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 often also referred to as (supply) network.

With the often-desired increasing share of renewable energy sources such as wind farms and solar farms in electric power generation, which rely on not controllable power sources ("wind" and "sun"), compliance with grid requirements becomes more important. This particularly refers to imbalances between the electrical power fed into a grid and the electrical power withdrawn from it by consumers as this may result in fluctuations of the grid frequency. For example, the grid frequency drops when the power consumption exceeds electrical power fed into the grid. For stability reasons, fluctuations of the grid frequency around a desired or target grid frequency of e.g. <NUM> for Central European grids and <NUM> for US grids are to be kept within certain limits of at most a few percent. Accordingly, grid operators typically specify primary power control requirements for the electric power generation plants in so-called grid codes which may change over time and typically also depend on the region and country, respectively.

Kinetic energy that is or may additionally be stored in rotating parts of electric power generation plants such as a rotor of a wind turbine may be used as a power reserve that can be used for (partly) compensating a deviation of the grid frequency, by de-accelerating and accelerating the rotor, however only for a short time to avoid too low and too high rotor speeds. Prior art examples can be found in <CIT> or <CIT>.

However, the current implementation of the respective grid code requirements leaves room for further developments.

Accordingly, the present disclosure provides a method for operating a wind turbine according to claim <NUM>, a power plant according to claim <NUM>, and a computer program product or a computer-readable storage medium according to claim <NUM>.

In one aspect, the present disclosure is directed to a method for operating a wind turbine. The wind turbine includes a rotor having rotor blades, and a power conversion system mechanically connected with the rotor, configured to convert input motive power into electrical output power, and electrically connected to a network for feeding the electrical output power to the network. The network is typically a utility grid. The method includes determining a current frequency of the network, and, when the current frequency is equal to or lower than a threshold frequency, operating the power conversion system at an electrical output power which is increased by an electrical output power increase in accordance with a monotonic function of the current frequency. The monotonic function is limited to a maximum value.

Accordingly, in particular large wind turbines having large rotatable masses and correspondingly large moments of inertia respectively, which can store considerably amounts of kinetic energy, and/or wind power plants or hybrid power plants having several, typically a plurality of wind turbines may quickly and efficiently contribute to stabilizing the grid frequency during an underfrequency event without damaging the wind turbine(s) due to limiting the power increase of the monotonic function. Within this specification the term "power plant" shall embrace the term "power conversion plant", "wind farm", "wind power plant" and "hybrid power plant".

Operating the power conversion system at an electrical output power which is increased by an electrical output power increase (in accordance with the monotonic function of the frequency) may also be referred to as operating the wind turbine in an overproduction operating mode in which the electrical output power of the power conversion system is increased by (providing, e.g. reducing) kinetic energy stored in the power conversion system but limited to a lower value than possible.

Typically, at least one of, more typically both of a rotational energy stored in a moment of inertia of the rotor, and a rotational energy stored in a moment of inertia of a generator rotor of a generator of the power conversion system is used to provide the electrical output power increase.

In particular, at a first time at which the current frequency of the network is determined to be equal to or lower than the threshold frequency or shortly thereafter, typically in real time, i.e. within less than about than <NUM> or even <NUM>, or in near real-time, i.e. within less than about <NUM> or even less than <NUM> and at the next possible time of the control, respectively, the electrical output power of the power conversion system is increased by the electrical output power increase (provided by / converted from the kinetic energy stored in the power conversion system).

The maximum value, also referred to as limited power increase value, may be determined as a function of a rated power of the power conversion system and the wind turbine, respectively, in accordance with a grid code of the network. According to the invention, the maximum value, also referred to as limited power increase value, is determined as a function of an initial electrical output power of the power conversion system determined at the first time.

In particular, the maximum value may be in a range from about <NUM>% of the rated power or the initial electrical output power to about <NUM>% of the rated power or the initial electrical output power, more typically about <NUM>%, for example between <NUM>% to <NUM>% of the rated power or the initial electrical output power.

Typically, the monotonic function is a function of a difference between the (current) frequency and the threshold frequency. Accordingly, controlling may be facilitated.

Alternatively or in addition, the monotonic function is a (typically predetermined) configurable function. Accordingly, adaptation to grid codes valid at a specific place of the wind turbine is facilitated.

Further, the monotonic function is typically a piecewise linear function of the (current) frequency and the difference between the (current) frequency and the threshold frequency, respectively. This may also facilitate controlling.

Furthermore, the monotonic function typically increases monotonically with decreasing (current) frequency until the maximum value is reached, and increases monotonically with increasing difference between the (current) frequency and the threshold frequency until the maximum value is reached.

Even further, the monotonic function typically corresponds, prior to reaching the maximum value, to at most about <NUM>%, more typically at most about <NUM>% of the rated power or the initial electrical output power per one Hz (Hertz) of the distance of the frequency to the threshold frequency and/or of the difference between the (current) frequency and the threshold frequency, and/or to at least about <NUM>%, more typically at least about <NUM>% of the rated power or the initial electrical output power per one Hz of the distance of the frequency to the threshold frequency and/or of the difference between the (current) frequency and the threshold frequency.

Further, the monotonic function may be discontinuous at the threshold frequency. More particular, the monotonic function may have two typically predetermined function values at the threshold frequency, typically zero and a response value larger than zero. Accordingly, the wind turbine may more strongly contribute to compensating a grid frequency drop close to the threshold frequency.

For example, the response value may be in a range from about <NUM> to about <NUM>%, more typically in a range from about <NUM> to about <NUM>% of the rated power or the initial electrical output power.

The threshold frequency is typically lower than a target frequency of the network. At least one of, typically both the threshold frequency and the target frequency are predetermined and/or configurable values, in particular in accordance with a grid code. For example, the threshold frequency may correspond to a deadband frequency of the network as specified in an applicable grid code.

The monotonic function may be constant in a range between the threshold frequency and a hysteresis frequency which is lower than the target frequency and higher than the threshold frequency. In particular, the monotonic function may have a hysteresis between the threshold frequency and the hysteresis frequency. More particular, the monotonic function may have two (predetermined) constant (frequency independent) function values in the range between the threshold frequency and the hysteresis frequency, typically zero and the response value.

Determining the current frequency and operating the power conversion system at the increased electrical output power may be repeated. Accordingly, a dynamic response to changing grid conditions may be provided.

The method may be repeated until a predetermined time interval of e.g. <NUM> is reached. Accordingly, an undesired (too strong) deceleration of the moving parts of the power conversion system can be avoided. The predetermined time interval typically depends on the specification of the power conversion system and the wind turbine, respectively.

Typically, operating the power conversion system at the increased electrical output power is stopped when the predefined time interval is reached, when the current frequency is again higher than the threshold frequency, more typically equal to or higher than the hysteresis frequency.

As already explained above, the power conversion system may be operated at the increased electrical output power if the current frequency is in a range between the threshold frequency and the hysteresis frequency.

In one aspect, the present disclosure is directed to power conversion plant, in particular a renewable power conversion plant comprising one or more wind turbines. The power conversion plant includes a rotor, and a power conversion system mechanically connected with the rotor, configured to convert input motive power (provided by the rotor) into electrical output power, and connectable to a network, in particular a utility grid. The power conversion plant further includes a sensor connectable to the network for measuring at least one signal which is at least correlated with a frequency of the network, and a controller communicatively coupled with the power conversion system and the sensor. The controller is configured to receive the at least one signal, and to determine, when the current frequency is equal to or lower than a threshold frequency, an electrical output power increase for the power conversion system in accordance with a monotonic function of the current frequency. The monotone function is limited to a maximum value. Further, the controller is typically configured to use the electrical output power increase for controlling the power conversion system.

The at least one signal may (directly) correspond to the current frequency of the network. In other words, the sensor may include or be a frequency sensor.

The controller may be provided by a turbine controller or a (wind) turbine controller and a converter controller functionally couple with the turbine controller. Further, part of the functionally may also be provided by a wind farm controller supervising the turbine controller.

The term "controller" as used herein shall embrace two or more controllers which are functionally coupled with each other.

Typically, the controller is configured to the control the power conversion system so that the power increase is provided using rotational energy stored in a moment of inertia of the rotor and/or a generator rotor of a generator of the power conversion system. For example, the controller may determine and use respective power setpoints for controlling a converter of the power conversion system.

Further, the controller may be configured to perform the methods as explained herein.

In one aspect, the present disclosure is directed to a computer program product or a computer-readable storage medium. The computer program product or the computer-readable storage medium includes instructions which, when executed by a one or more processors of a system, in particular a controller of the power conversion plant as explained herein, cause the system to carry out the methods as explained herein.

The system and the controller, respectively, can be configured to perform particular operations or processes by virtue of software, firmware, hardware, or any combination thereof.

These and other features, aspects and advantages of the present invention will be further supported and described with reference to the following description and appended claims.

A full and enabling disclosure of the present invention, including the best mode thereof, directed to one of ordinary skill in the art, is set forth in the specification, which refers to the appended figures, in which:.

Single features depicted in the figures are shown relatively with regards to each other and therefore are not necessarily to scale. Similar or same elements in the figures, even if displayed in different embodiments, are typically represented with the same reference numbers.

<FIG> is a perspective view of a portion of an exemplary wind turbine <NUM>. Wind turbine <NUM> includes a nacelle <NUM> housing a generator (not shown in <FIG>). Nacelle <NUM> is mounted on a tower <NUM> (a portion of tower <NUM> being shown in <FIG>). Tower <NUM> may have any suitable height that facilitates operation of wind turbine <NUM> as described herein. Wind turbine <NUM> also includes a rotor <NUM> that includes three blades <NUM> attached to a rotating hub <NUM>. Alternatively, wind turbine <NUM> includes any number of blades <NUM> that facilitates operation of wind turbine <NUM> as described herein. In the exemplary embodiment, wind turbine <NUM> includes a gearbox (not shown in <FIG>) operatively coupled to rotor <NUM> and a generator (not shown in <FIG>).

Referring now to <FIG>, a schematic view of one embodiment of an electrical power system <NUM> that may be used with the wind turbine <NUM> is illustrated. During operation, wind impacts the blades <NUM> and the blades <NUM> transform wind energy into a mechanical rotational torque that rotatably drives a low-speed shaft <NUM> via the hub <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 generator <NUM> so as to rotatably drive a generator rotor <NUM> having field winding (not shown).

More specifically, in one embodiment, the generator <NUM> may be a wound rotor, three-phase, doubly-fed induction (asynchronous) generator (DFIG) that includes a generator stator <NUM> magnetically coupled to 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 such embodiments, 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>. The associated electrical power can be transmitted to a main transformer <NUM> via a stator bus <NUM>, a stator synchronizing switch <NUM>, a system bus <NUM>, a main transformer circuit breaker <NUM>, and a generator-side bus <NUM>. The main transformer <NUM> steps up the voltage amplitude of the electrical power such that the transformed electrical power may be further transmitted to a grid via a grid circuit breaker <NUM>, a breaker-side bus <NUM>, and a grid bus <NUM>.

In addition, the electrical power system <NUM> may include a wind turbine controller <NUM> configured to control any of the components of the wind turbine <NUM> and/or implement any of the method steps as described herein. For example, as shown particularly in <FIG>, the controller <NUM> may include one or more processor(s) <NUM> and associated memory device(s) <NUM> configured to perform a variety of computer-implemented functions (e.g., performing the methods, steps, calculations and the like and storing relevant data as disclosed herein). Additionally, the controller <NUM> may also include a communications module to facilitate communications between the controller <NUM> and the various components of the wind turbine <NUM>, e.g. any of the components of <FIG>.

Further, as shown in <FIG>, 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 to be converted into signals that can be understood and processed by the processors <NUM>. It should be appreciated that the sensors (e.g. sensors <NUM>, <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>, <NUM> may be coupled to the sensor interface <NUM> via a wired connection. However, in other embodiments, the sensors <NUM>, <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.

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

Referring back to <FIG>, the generator stator <NUM> may be electrically coupled to a stator synchronizing switch <NUM> via a stator bus <NUM>. In an exemplary embodiment, to facilitate the DFIG configuration, generator rotor <NUM> is electrically coupled to a bi-directional power conversion assembly <NUM> via a rotor bus <NUM>. Alternatively, generator rotor <NUM> is electrically coupled to rotor bus <NUM> via any other device that facilitates operation of electrical and control system <NUM> as described herein. As a further alternative, electrical and control system <NUM> is configured as a full power conversion system (not shown) that includes a full power conversion assembly (not shown in <FIG>) similar in design and operation to power conversion assembly <NUM> and electrically coupled to generator stator <NUM>. The full power conversion assembly facilitates channeling electric power between generator stator <NUM> and an electric power transmission and distribution grid (not shown). In the exemplary embodiment, stator bus <NUM> transmits three-phase power from generator stator <NUM> to stator synchronizing switch <NUM>. Rotor bus <NUM> transmits three-phase power from generator rotor <NUM> to power conversion assembly <NUM>. In the exemplary embodiment, stator synchronizing switch <NUM> is electrically coupled to a main transformer circuit breaker <NUM> via a system bus <NUM>. In an alternative embodiment, one or more fuses (not shown) are used instead of main transformer circuit breaker <NUM>. In another embodiment, neither fuses nor main transformer circuit breaker <NUM> is used.

Power conversion assembly <NUM> includes a rotor filter <NUM> that is electrically coupled to generator rotor <NUM> via rotor bus <NUM>. A rotor filter bus <NUM> electrically couples rotor filter <NUM> to a rotor-side power converter <NUM>, and rotor-side power converter <NUM> is electrically coupled to a line-side power converter <NUM>. Rotor-side power converter <NUM> and line-side power converter <NUM> are power converter bridges including power semiconductors (not shown). In the exemplary embodiment, rotor-side power converter <NUM> and line-side power converter <NUM> are configured in a three-phase, pulse width modulation (PWM) configuration including insulated gate bipolar transistor (IGBT) switching devices (not shown in <FIG>) that operate as known in the art. Alternatively, rotor-side power converter <NUM> and line-side power converter <NUM> have any configuration using any switching devices that facilitate operation of electrical and control system <NUM> as described herein. Power conversion assembly <NUM> is coupled in electronic data communication with turbine controller <NUM> to control the operation of rotor-side power converter <NUM> and line-side power converter <NUM>.

In the exemplary embodiment, a line-side power converter bus <NUM> electrically couples line-side power converter <NUM> to a line filter <NUM>. Also, a line bus <NUM> electrically couples line filter <NUM> to a line contactor <NUM>. Moreover, line contactor <NUM> is electrically coupled to a conversion circuit breaker <NUM> via a conversion circuit breaker bus <NUM>. In addition, conversion circuit breaker <NUM> is electrically coupled to main transformer circuit breaker <NUM> via system bus <NUM> and a connection bus <NUM>. Alternatively, line filter <NUM> is electrically coupled to system bus <NUM> directly via connection bus <NUM> and includes any suitable protection scheme (not shown) configured to account for removal of line contactor <NUM> and conversion circuit breaker <NUM> from electrical and control system <NUM>. Main transformer circuit breaker <NUM> is electrically coupled to an electric power main transformer <NUM> via a generator-side bus <NUM>. Main transformer <NUM> is electrically coupled to a grid circuit breaker <NUM> via a breaker-side bus <NUM>. Grid circuit breaker <NUM> is connected to the electric power transmission and distribution grid via a grid bus <NUM>. In an alternative embodiment, main transformer <NUM> is electrically coupled to one or more fuses (not shown), rather than to grid circuit breaker <NUM>, via breaker-side bus <NUM>. In another embodiment, neither fuses nor grid circuit breaker <NUM> is used, but rather main transformer <NUM> is coupled to the electric power transmission and distribution grid via breaker-side bus <NUM> and grid bus <NUM>.

In the exemplary embodiment, rotor-side power converter <NUM> is coupled in electrical communication with line-side power converter <NUM> via a single direct current (DC) link <NUM>. Alternatively, rotor-side power converter <NUM> and line-side power converter <NUM> are electrically coupled via individual and separate DC links (not shown in <FIG>). DC link <NUM> includes a positive rail <NUM>, a negative rail <NUM>, and at least one capacitor <NUM> coupled between positive rail <NUM> and negative rail <NUM>. Alternatively, capacitor <NUM> includes one or more capacitors configured in series and/or in parallel between positive rail <NUM> and negative rail <NUM>.

Turbine controller <NUM> is configured to receive a plurality of voltage and electric current measurement signals from a first set of voltage and electric current sensors <NUM>. Moreover, turbine controller <NUM> is configured to monitor and control at least some of the operational variables associated with wind turbine <NUM>. In the exemplary embodiment, each of three voltage and electric current sensors <NUM> are electrically coupled to each one of the three phases of grid bus <NUM>. Accordingly, a current frequency of the grid may be determined by controller <NUM>. Alternatively or in addition, turbine controller <NUM> may be functionally coupled with a frequency sensor connectable with the grid. Further, it is possible that controller <NUM> receives the current frequency of the grid or at least a signal representative for the current frequency of the grid via primary plant controller such as a wind farm controller functionally coupled with a respective sensor.

As shown in <FIG>, electrical and control system <NUM> also includes a converter controller <NUM> that is configured to receive a plurality of voltage and electric current measurement signals. For example, in one embodiment, converter controller <NUM> receives voltage and electric current measurement signals from a second set of voltage and electric current sensors <NUM> coupled in electronic data communication with stator bus <NUM>. Converter controller <NUM> receives a third set of voltage and electric current measurement signals from a third set of voltage and electric current sensors <NUM> coupled in electronic data communication with rotor bus <NUM>. Converter controller <NUM> also receives a fourth set of voltage and electric current measurement signals from a fourth set of voltage and electric current sensors <NUM> coupled in electronic data communication with conversion circuit breaker bus <NUM>. Second set of voltage and electric current sensors <NUM> is substantially similar to first set of voltage and electric current sensors <NUM>, and fourth set of voltage and electric current sensors <NUM> is substantially similar to third set of voltage and electric current sensors <NUM>. Converter controller <NUM> is substantially similar to turbine controller <NUM> and is coupled in electronic data communication with turbine controller <NUM>. Moreover, in the exemplary embodiment, converter controller <NUM> is physically integrated within power conversion assembly <NUM>. Alternatively, converter controller <NUM> has any configuration that facilitates operation of electrical and control system <NUM> as described herein.

During operation, wind impacts blades <NUM> and blades <NUM> transform wind energy into a mechanical rotational torque that rotatably drives low-speed shaft <NUM> via hub <NUM>. Low-speed shaft <NUM> drives gearbox <NUM> that subsequently steps up the low rotational speed of low-speed shaft <NUM> to drive high-speed shaft <NUM> at an increased rotational speed. High speed shaft <NUM> rotatably drives generator rotor <NUM>. A rotating magnetic field is induced by generator rotor <NUM> and a voltage is induced within generator stator <NUM> that is magnetically coupled to generator rotor <NUM>. Generator <NUM> converts the rotational mechanical energy to a sinusoidal, three-phase alternating current (AC) electrical energy signal in generator stator <NUM>. In the exemplary embodiment, the associated electrical power is transmitted to main transformer <NUM> via stator bus <NUM>, stator synchronizing switch <NUM>, system bus <NUM>, main transformer circuit breaker <NUM> and generator-side bus <NUM>. Main transformer <NUM> steps up the voltage amplitude of the electrical power and the transformed electrical power is further transmitted to a grid via breaker-side bus <NUM>, grid circuit breaker <NUM> and grid bus <NUM>.

In the exemplary embodiment, a second electrical power transmission path is provided. Electrical, three-phase, sinusoidal, AC power is generated within generator rotor <NUM> and is transmitted to power conversion assembly <NUM> via rotor bus <NUM>. Within power conversion assembly <NUM>, the electrical power is transmitted to rotor filter <NUM> and the electrical power is modified for the rate of change of the PWM signals associated with rotor-side power converter <NUM>. Rotor-side power converter <NUM> acts as a rectifier and rectifies the sinusoidal, three-phase AC power to DC power. The DC power is transmitted into DC link <NUM>. Capacitor <NUM> facilitates mitigating DC link <NUM> voltage amplitude variations by facilitating mitigation of a DC ripple associated with AC rectification.

The DC power is subsequently transmitted from DC link <NUM> to line-side power converter <NUM> and line-side power converter <NUM> acts as an inverter configured to convert the DC electrical power from DC link <NUM> to three-phase, sinusoidal AC electrical power with pre-determined voltages, currents, and frequencies. This conversion is monitored and controlled via converter controller <NUM>. The converted AC power is transmitted from line-side power converter <NUM> to system bus <NUM> via line-side power converter bus <NUM> and line bus <NUM>, line contactor <NUM>, conversion circuit breaker bus <NUM>, conversion circuit breaker <NUM>, and connection bus <NUM>. Line filter <NUM> compensates or adjusts for harmonic currents in the electric power transmitted from line-side power converter <NUM>. Stator synchronizing switch <NUM> is configured to close to facilitate connecting the three-phase power from generator stator <NUM> with the three-phase power from power conversion assembly <NUM>.

Conversion circuit breaker <NUM>, main transformer circuit breaker <NUM>, and grid circuit breaker <NUM> are configured to disconnect corresponding buses, for example, when excessive current flow may damage the components of electrical and control system <NUM>. Additional protection components are also provided including line contactor <NUM>, which may be controlled to form a disconnect by opening a switch (not shown in <FIG>) corresponding to each line of line bus <NUM>.

Power conversion assembly <NUM> compensates or adjusts the frequency of the three-phase power from generator rotor <NUM> for changes, for example, in the wind speed at hub <NUM> and blades <NUM>. Therefore, in this manner, mechanical and electrical rotor frequencies are decoupled from stator frequency.

Under some conditions, the bi-directional characteristics of power conversion assembly <NUM>, and specifically, the bi-directional characteristics of rotor-side power converter <NUM> and line-side power converter <NUM>, facilitate feeding back at least some of the generated electrical power into generator rotor <NUM>. More specifically, electrical power is transmitted from system bus <NUM> to connection bus <NUM> and subsequently through conversion circuit breaker <NUM> and conversion circuit breaker bus <NUM> into power conversion assembly <NUM>. Within power conversion assembly <NUM>, the electrical power is transmitted through line contactor <NUM>, line bus <NUM>, and line-side power converter bus <NUM> into line-side power converter <NUM>. Line-side power converter <NUM> acts as a rectifier and rectifies the sinusoidal, three-phase AC power to DC power. The DC power is transmitted into DC link <NUM>. Capacitor <NUM> facilitates mitigating DC link <NUM> voltage amplitude variations by facilitating mitigation of a DC ripple sometimes associated with three-phase AC rectification.

The DC power is subsequently transmitted from DC link <NUM> to rotor-side power converter <NUM> and rotor-side power converter <NUM> acts as an inverter configured to convert the DC electrical power transmitted from DC link <NUM> to a three-phase, sinusoidal AC electrical power with pre-determined voltages, currents, and frequencies. This conversion is monitored and controlled via converter controller <NUM>. The converted AC power is transmitted from rotor-side power converter <NUM> to rotor filter <NUM> via rotor filter bus <NUM> and is subsequently transmitted to generator rotor <NUM> via rotor bus <NUM>, thereby facilitating sub-synchronous operation.

Power conversion assembly <NUM> is configured to receive control signals from turbine controller <NUM>. The control signals are based on sensed conditions or operating characteristics of wind turbine <NUM> and electrical and control system <NUM>. The control signals are received by turbine controller <NUM> and used to control operation of power conversion assembly <NUM>. Feedback from one or more sensors may be used by electrical and control system <NUM> to control power conversion assembly <NUM> via converter controller <NUM> including, for example, conversion circuit breaker bus <NUM>, stator bus and rotor bus voltages or current feedbacks via second set of voltage and electric current sensors <NUM>, third set of voltage and electric current sensors <NUM>, and fourth set of voltage and electric current sensors <NUM>. Using this feedback information, and for example, switching control signals, stator synchronizing switch control signals and system circuit breaker control (trip) signals may be generated in any known manner. For example, for a grid voltage transient with predetermined characteristics, converter controller <NUM> will at least temporarily substantially suspend the IGBTs from conducting within line-side power converter <NUM>. Such suspension of operation of line-side power converter <NUM> will substantially mitigate electric power being channeled through power conversion assembly <NUM> to approximately zero.

In the exemplary embodiment, generator <NUM>, step-up transformer <NUM> and power conversion assembly <NUM> electrically coupled to generator <NUM> and transformer <NUM> form the power conversion system of wind turbine <NUM>.

<FIG> illustrates, in the upper part, an exemplary schematic course of a grid frequency f, and, in the lower part, an output power P of a power conversion system of a wind turbine as explained above with regard to <FIG> as function of time t during operating the wind turbine. The grid frequency is desired to lie in a typically narrow band around the target frequency fnom.

At a first time t<NUM>, the grid frequency f crosses from above a given or configurable threshold frequency fthresh, in particular a deadband frequency of the grid. At this time or shortly thereafter, a dynamic response mode of the wind turbine or even a plurality of wind turbines of a wind power plant connected to the grid may be triggered.

Note that the frequency range from - fthresh is also known as lower part of the typically configurable frequency dead-band, in which no counter action against frequency fluctuations may be required.

During the dynamic response mode, the power conversion system of the wind turbine(s) is (are) operated to provide an increased electrical output power P(t) as illustrated in curve a in the lower part of <FIG>.

In the exemplary embodiment, the electrical output power P(t) of the wind turbine(s) is increased from the respective electrical output power P<NUM> at t<NUM> by an electrical output power increase PI(f(t)) as shown in the alternative curves a, b of <FIG>, i.e. in accordance with a piece-wise linear function PI(f) of the frequency f which is limited to a configurable maximum value PImax lower than a maximum allowable power increase PIavail as e.g. defined by the grid operator. For even lower grid frequencies f, the power increase PI(f(t)) is kept constant at PImax, i.e. independent of the grid frequency.

In other words, during the dynamic response mode (with grid frequency below the frequency dead-band), the power output may be increased proportionally to the dip in frequency up to maximum value PImax as indicated by curves a, c in <FIG> that may have a slope in a range from about -<NUM>%/Hz to -<NUM>%/Hz, more typically a range from about -<NUM>%/Hz to - <NUM>%/Hz, e.g. about -<NUM>%/Hz (assuming that the maximum power increase value PImax corresponds to <NUM>% of the actual electrical output power P<NUM> at t<NUM>, and PIavail corresponds to <NUM>% of the actual electrical output power P<NUM> at t<NUM> in <FIG>). For reasons of comparison, curves b in <FIG> correspond to a similar response mode, however without limiting the power increase PI(f(t)) to a value below the maximum allowable power increase PIavail. In terms of the rated power or the initial electrical output power, the slope may be equal to or larger than -<NUM>% of the rated power or the initial electrical output power per Hz, more typically equal to or larger than -<NUM>% of per one Hz and/or smaller than -<NUM>%/Hz or even -<NUM>%/Hz of the rated power or the initial electrical output power.

Nevertheless, the wind turbine(s) may effectively contribute to stabilizing the grid frequency f when limited to PImax < PIavail during the dynamic response mode as shown in the upper part of <FIG>.

The maximum value PImax may be determined as (configurable) fraction or percentage of a possible electrical output power PIavail at t<NUM>, the actual electrical output power P<NUM> at t<NUM>, or a rated power output of the wind turbine, or any combination thereof, for example as a (weighted) average of two of or all three values.

The dynamic response mode may be active until (at a second time t<NUM>) one of the following trigger events occur: (i) a typically configurable predefined time interval thold is reached, and (ii) the grid frequency f reaches the frequency dead-band again / crosses the hysteresis frequency fhyst from below.

Typically, the dynamic response mode is stopped at t<NUM> or shortly thereafter.

As further indicated by curve c and the dashed arrows in <FIG>, a hysteresis may be implemented such that the power increase PI is increased from zero to a response value PIres when the threshold frequency fthresh is reached from higher frequency values, and maintained at the response value PIres when the threshold frequency fthresh is reached or crossed from lower frequency values until the grid frequency f reaches or crosses the hysteresis frequency fhyst again.

<FIG> illustrates a flow diagram of a method <NUM> for operating a wind turbine as explained above with regard to <FIG>.

In a first block <NUM>, a frequency f of the grid is measured.

In a subsequent block <NUM>, the power conversion system of one or more wind turbines of a power plant are operated at an increased electrical output power in accordance with a monotonic function as explained herein, in particular with respect to <FIG>.

<FIG> illustrates a flow diagram of a method <NUM> for operating a wind turbine as explained above with regard to <FIG>. Method <NUM> is similar to method <NUM> explained above with regard to <FIG>.

However, steps <NUM> to <NUM> may be performed several times as indicated by the dashed-dotted arrow.

Further, prior to initiating or continuing operating the power conversion system of one or more wind turbines of a power plant with increased electrical output power in block <NUM>, it is checked in a decision block <NUM> if certain condition(s) are met.

In particular it is checked in decision block <NUM> if the grid frequency measured in block <NUM> is equal to or lower than the threshold frequency fthresh.

Depending thereon, method <NUM> is continued with block <NUM> or ended/stopped.

Further, it may be checked in block <NUM> if a predefined time interval thold has not been reached or crossed after first entering into block <NUM>. Only if this condition is met, block <NUM> is entered again.

This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. For example, while the written description refers to horizontal axis wind turbines, the embodiments may also refer to vertical axis wind turbines, in particular variable pitch vertical axis wind turbines. Accordingly, operating the rotor to move around a predefined desired angular orientation with respect to the axis of rotation of the rotor in an alternating fashion while the generator is not in a power operating mode may both applied to horizontal axis wind turbines and vertical axis wind turbines. Such other examples are intended to be within the scope of the claims if they include elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims.

The present invention is not limited to the above-described embodiments and modifications and may be embodied in various forms within the gist thereof, for example, the technical features of the embodiments referring to operating a wind turbine may be combined with the embodiments referring to designing a wind turbine may be combined, i.e. operating a wind turbine as explained herein may refer to operating a wind turbine designed as explained herein. Further, modifications corresponding to the technical features according to the aspects described in the Summary of the Invention section may be replaced or combined as appropriate to solve some or all of the above-described problems or obtain some or all of the above-described effects. The technical features may also be omitted as appropriate unless they are described as being essential in this specification.

Claim 1:
A method (<NUM>-<NUM>) for operating a wind turbine (<NUM>) comprising a rotor (<NUM>) comprising rotor blades (<NUM>) and a power conversion system (<NUM>, <NUM>, <NUM>) mechanically connected with the rotor (<NUM>), configured to convert input motive power into electrical output power, and electrically connected to a network (<NUM>) for feeding the electrical output power (P) to the network (<NUM>), in particular a utility grid, the method (<NUM>-<NUM>) comprising:
- determining (<NUM>) a current frequency (f) of the network (<NUM>); and
- when the current frequency (f) is equal to or lower than a threshold frequency (fthresh), operating (<NUM>) the power conversion system (<NUM>, <NUM>, <NUM>) at an electrical output power (P) which is increased by an electrical output power increase (PI) in accordance with a monotonic function (PI(f)) of the current frequency (f), the monotonic function (PI) being limited to a maximum value (PImax) determined as a function of an initial electrical output power (P<NUM>) of the power conversion system (<NUM>, <NUM>, <NUM>), the initial electrical output power (P<NUM>) being determined as an actual electrical output power (P<NUM>) of the power conversion system (<NUM>, <NUM>, <NUM>) at or close to a first time (t<NUM>) at which the current frequency (f) of the network is determined to be equal to or lower than the threshold frequency (fthresh).