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 modem wind turbine typically includes a tower, generator, gearbox, nacelle, and a rotor with 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.

Due to faults in the utility grid or other reasons, the wind farm may be disconnected from the utility grid. In this state, wind turbines may be required to supply power to its auxiliary loads to keep controllers, communication, yaw operation and other critical systems alive. This may be for safety reasons and/or to facilitate reconnecting the wind farm to the utility grid after grid recovery. Electric energy storage such as UPS which may be provided within the wind turbines, may only feed power to these critical systems for e.g. a few minutes during loss of grid. For longer periods, auxiliary power may be provided by diesel generators. In particular for offshore wind farms, diesel generators or the like may be too large and/or expensive to install, run, refill and/or maintain.

<CIT> discloses a method for performing island operation of two wind turbines disconnected from the main grid of a wind farm system. The method involves configuring a local power transmission system and a main grid, where the transmission system is not connected to the main grid. One of a set of deactivated wind turbines is activated, where the activated wind turbine and one of the remaining deactivated wind turbines are coupled to the transmission system. The activated wind turbine is configured to act as a power supply for the deactivated wind turbine. A set of switches in a set of power transmission lines between the turbines is controlled by the transmission system.

Accordingly, the present disclosure provides a method for operating a wind farm according to claim <NUM>, a wind farm 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 farm. The wind farm includes a string of wind turbines which are electrically connectable with each other and a grid. Each wind turbine includes a rotor including a rotor blade, a power conversion system mechanically connected with the rotor, and at least one auxiliary subsystem. The method includes operating the wind turbines of the string in an island operating mode in which the wind turbines are not connected with the grid, and the respective at least one auxiliary subsystem is supplied with electric power generated by the power conversion system of the respective wind turbine; determining that the rotor of one of the wind turbines is exposed to a wind condition at which the rotor blades of the one of the wind turbines is at risk of stalling at the currently generated electric output power (of the wind turbines power conversion system), and increasing the electric power generated by the power conversion system of the one of the wind turbines by an electric power amount which is sufficient for supplying the at least one auxiliary subsystem of at least one of the other wind turbines of the string.

In the following, this operating mode (of the one of the wind turbines) is also referred to a supply island operating mode. Further, the electric power amount which is sufficient for supplying the at least one auxiliary subsystem of at least one of the other wind turbines of the string is in the following also referred to as electric output power increase and electric surplus power (of the one of the wind turbines and its power conversion system, respectively).

Typically, the electric power generated by the power conversion system of the one of the wind turbines is increased by an electric power amount which at least substantially matches a power demand of the auxiliary subsystem(s) of at least one of the other wind turbines of the string, more typically several, a majority or even all other wind turbines of the string.

Due to increasing the electric power generated by the power conversion system of the wind turbine(s) the rotor blade(s) of which is at risk of stalling, stalling may safely be avoided at higher wind speeds. Accordingly, pitch demand and/or mechanical loads, in particular respective mechanical stress resulting in fatigue on the rotor blade(s) may be avoided at higher wind speed without affecting the energy supply of the wind turbines auxiliary subsystems.

Note that the auxiliary subsystem(s) of the wind turbines may e.g. only require auxiliary power equal to about <NUM>-<NUM>% of the rated power of the power conversion system of the wind turbine at higher wind speeds. This may require the rotor blade(s) to be pitched to extreme values which would stall its operation. The increased power output will reduce pitch demand at higher wind speeds and thus avoids stall operation and undesired effects resulting therefrom such as reducing the life time of components, in particular the rotor blade(s). Furthermore, the storage capacity and/or size of battery energy storage system(s) of the wind turbines may be reduced. Thus, materials as well as costs for equipment and maintenance may be saved. In result, even the ecological footprint of the wind turbines may be reduced.

As used herein, the term "string of wind turbines" intends to describe that the wind turbines and their power conversion systems, respectively, are electrically connectable with each other to form a series circuit, in particular via respective power connections. For example, power cables and power switches arranged between adjacent wind turbines and their power conversion systems, respectively, may be used to electrically connect the wind turbines to each other (in pairs) and disconnect the wind turbines. However, it is also possible that a string includes wind turbines of different type and/or power rating.

Determining that the rotor of the one of the wind turbines is exposed to the wind condition typically includes determining that the rotor is exposed to a wind speed that is larger than a first threshold value corresponding to a wind speed at which stalling of the rotor blade is expected at the currently generated electric output power.

This may e.g. include detecting, in particular measuring and/or estimating the wind speed at, in front of and/or behind the rotor. Further, the detected (measured) wind speed and the currently generated electric output power may be used to determine that the rotor is exposed to the wind condition at which the rotor blade(s) is(are) at risk of stalling, for example using a look-up table.

In the following, the wind condition at which the rotor blade(s) of a wind turbine is(are) at risk of stalling at the currently generated or requested electric output power of its power conversion system is also referred to as stalling wind condition.

Typically, the electric power generated by the power conversion system of the one of the wind turbines is increased by the electric power amount (output power increase, electric surplus power) upon determining that the rotor of the one of the wind turbines is exposed to the stalling wind condition.

As used herein, the wording that "the rotor blade(s) of a wind turbine is(are) at risk of stalling at the currently generated or requested electric output power" intends to describe that the risk of stalling is currently high, for example at least <NUM>%, more typically at least <NUM>% or even <NUM>%, and/or expected to be high soon, e.g. within one or a few seconds. The latter may be due to an expected increase of the wind speed.

Further, beside wind speed, an ambient temperature and/or a turbulence measure of the wind at, in front of and/or behind the rotor may be taken into account for determining the risk of stalling.

However, often it is sufficient to rely only on the wind speed to determine if the rotor of a wind turbine is exposed to a stalling wind condition. The wind speed may even be a global or averaged wind speed of the wind farm.

For example, a wind farm controller may be configured to send respective control commands (e.g. power commands) to individual controllers of the wind turbines of a string of wind turbines (of typically the same type) operating in island operating mode, if the wind speed is higher than the first threshold value for at least one of the wind turbines so that only one or at most a few of the wind turbines increase their power output while the remaining wind turbines stop power conversion and supply their auxiliary subsystem(s) with power received from the one or few of the wind turbines producing higher output power.

Accordingly, the electric surplus power may be used as a power supply for the auxiliary subsystem(s) of the at least one of the other wind turbines of the string.

In particular, the electric surplus power may be used as power supply for the other wind turbine(s) of the string that (are set to) operate in an idling operating mode or a stand still operating mode. Both in the idling operating mode and the stand still operating mode, the respective power conversion system does not output electric power.

Operating the other wind turbine(s) of the string in idling operating mode may be preferred compared to operating in the stand still operating mode. This is because reconnecting the wind turbine(s) of the string to a recovered grid and operating the wind turbine in a normal operating mode again may be facilitated when the wind turbines are already operating in the idling operating mode.

During operating the wind turbine in the normal operating mode, the power conversion system converts input motive power received from the rotor into electrical output power and provides at least a major portion of the electrical output power to the (utility) grid.

As the power demand of the other wind turbine(s) may vary over time, the electric surplus power may be updated in accordance with the power demand of the of the other wind turbine(s). For example, the power demand of the other wind turbine(s) may become lower after any rechargeable energy storage is recharged.

These steps are typically controlled by a control system of the wind farm, in particular a control system provided by a wind farm controller and wind turbine controllers. This typically applies to any method and method steps explained herein.

The electric power generated by the power conversion system of the one of the wind turbines may be increased by a factor of at least three, more typically at least <NUM> or even at least <NUM> to additionally supply a respective number of other wind turbines operated in idling operating mode or a stand still operating mode.

For example, the electric power generated by the power conversion system of the one of the wind turbines may be increased by an electric output power increase which is sufficient for supplying the auxiliary subsystem(s) of at least four of the other wind turbines of the string, typically of at least nine of the other wind turbines of the string. Further, the electric power generated by the power conversion system of the one of the wind turbines may, depending on wind speed and the wind turbines, be increased by an electric output power increase which is sufficient for supplying the auxiliary subsystem(s) of up to <NUM> or even <NUM> wind turbines.

The typically predetermined first threshold value may be in a range from about <NUM>/s to about <NUM>/s.

However, the first threshold value is typically dependent on the particular design (type) of the wind turbine.

After determining that the rotor of the one of the wind turbines is exposed to a further wind condition at which the risk of stalling is small, for example at most a few percent, typical below <NUM> percent, in particular at least substantially zero (e.g. less than <NUM> percent or even. <NUM> percent) if the electric output power of the conversion system is reduced to a value at least substantially matching a current or expected power demand of the auxiliary subsystem(s) of the wind turbines operating in supply island mode, all wind turbines of the string may return to operate / may operate again in the respective island operating mode.

Determining that the rotor of the one of the wind turbines is exposed to the further wind condition typically includes determining that the rotor is exposed to a wind speed smaller than a second a threshold value which is lower than the first threshold value, typically about <NUM>% to <NUM>% lower than the first threshold value, more typically about <NUM>% to <NUM>% lower than the first threshold value.

In one aspect, the present disclosure is directed to a method for operating a wind farm. The wind farm includes a string of wind turbines which are electrically connectable with each other and a grid. Each wind turbine includes a rotor including a rotor blade, a power conversion system mechanically connected with the rotor, and at least one auxiliary subsystem. The method includes operating the wind turbines of the string in an island operating mode in which the wind turbines are not connected with the grid, and the at least one auxiliary subsystems are supplied with electric power generated by the power conversion system of the respective wind turbine, determining that at least one of the rotors of the wind turbines is exposed to a wind condition at which the rotor blade of the at least one of the rotors is at risk of stalling at the currently generated electric output power; and increasing the electric power generated by the power conversion system of one of the wind turbines by an electric output power increase (electric surplus power) which is sufficient for supplying the at least one auxiliary subsystem of at least one of the other wind turbines of the string.

The method typically further includes operating the at least one of the other wind turbines in an idling operating mode or a stand still operating mode, and using the electric output power increase for supplying the at least one auxiliary subsystem of the at least one of the other wind turbines.

Typically, the power output of only one wind turbine is increased, in particular one of the wind turbines, of a string, for which the rotor is exposed to the stalling wind condition.

In one aspect, the present disclosure is directed to a method for operating a wind farm. The wind farm includes a string of wind turbines which are electrically connectable with each other and a grid. Each wind turbine includes a rotor including a rotor blade, a power conversion system mechanically connected with the rotor, and at least one auxiliary subsystem. The method includes operating the wind turbines of the string in an island operating mode in which the wind turbines are not connected with the grid, and the at least one auxiliary subsystems are supplied with electric power generated by the power conversion system of the respective wind turbine, determining that the rotor of (at least) one of the wind turbines is exposed to a wind having a wind speed that is larger than a first threshold value corresponding to a wind speed at which at least one of the rotor blades of the one of the wind turbines is at risk of stalling at the currently generated electric output power; and operating the other wind turbines in an idling operating mode or a standstill operating mode and supplying the at least one auxiliary subsystem of the other wind turbines with electric power additionally provided by the power conversion system of the at least one wind turbine.

In one embodiment the wind farm has several strings of wind turbines, wherein the strings are separately connectable with a point of common coupling of the wind farm connectable to an external grid. In this embodiment, the methods explained above are typically performed independently for each of the several strings.

In one aspect, the present disclosure is directed to a wind farm. The wind farm includes a string of wind turbines which are electrically connectable with each other and a grid. Each wind turbine includes a rotor including a rotor blade, a power conversion system mechanically connected with the rotor, and at least one auxiliary subsystem. The wind farm further includes a control system communicatively coupled with the power conversion system of the wind turbines and configured to operate the wind turbines of the string in an island operating mode in which the wind turbines are not connected with the grid, and the at least one auxiliary subsystem is supplied with electric power generated by the power conversion system of the respective wind turbine, to determine that the rotor of one of the wind turbines is exposed to a wind condition at which at least one of the rotor blades of the one of the wind turbines is at risk of stalling at the currently generated electric output power; and to increase the electric power generated by the power conversion system of the one of the wind turbines by an electric power amount which is sufficient for supplying the at least one auxiliary subsystem of at least one of the other wind turbines of the string.

The control system is typically communicatively coupled with a sensor for determining a wind speed at or in front of the rotor of the wind turbines. Determining the wind speed may include measuring wind speed values. Further, the wind speed may be determined as an estimated wind speed based on the measured wind speed values.

For example, the control system may be communicatively coupled with a respective sensor for determining a wind speed at, in front of and/or behind the rotor of each of the wind turbines, and to determine the risk of stalling based on the determined wind speed(s).

Typically, control system is configured to determine the risk of stalling based on the determined wind speed(s) and optionally on the current power output.

According to an embodiment, the control system includes a wind farm controller and a respective wind turbine controller for each power conversion system.

The wind farm controller is communicatively coupled with the wind turbine controller, and may be operable as a primary controller, whereas the wind turbine controllers may be operable are as secondary controllers.

The turbine controllers may be configured to control the respective wind turbine in different modes, in particular island operating mode, supply island operating mode, idling mode and/or stand still mode.

Typically, the control system is configured to control the power conversion system of the wind turbine(s) operating in supply island mode such that the generated electric output power is sufficient for additionally supplying the at least one auxiliary subsystems of at least <NUM> of the other wind turbines of the string, typically of at least <NUM> of the other wind turbines of the string.

The term "auxiliary subsystem" as used herein intends to describe systems of the wind turbine which may be desired and/or consume electrical power at least from time to time when the wind turbine is operated in an idling operating mode and/or a stand still operating mode. Accordingly, the term "auxiliary subsystem" includes operational subsystems used during idling operating mode and/or stand still operating mode.

In particular, each of the wind turbines may include one or more, typically several or even all of the following auxiliary subsystems: a pitch system, a yaw system, a heating system, a cooling system, a hydraulic system, and a rechargeable energy storage devices such as a UPS.

The wind turbines may include several respective heating system and/or cooling systems, for example for a generator, a converter and/or a transformer of the respective power conversion system.

Further, a rotor blade heating and/or rotor blade deicing subsystem may be provided as respective auxiliary subsystem for wind turbines in wind farms operating in cold regions.

Furthermore, wind turbines may include several rechargeable energy storage devices for different components.

Even further, the turbine controller, measurement devices and any communication device may also be considered as auxiliary subsystems.

The wind farm may include several strings of wind turbines. In this embodiment, the control system is typically configured to control the several strings of wind turbines independently of each other when not connected with the grid (and each other).

Further, the wind farm may be an offshore wind farm but also an onshore wind farm.

Typically, the wind farm is configured to perform the method as explained herein.

In yet another aspect, the present disclosure is directed to a computer program product or a non-transitory computer-readable storage medium comprising instructions which, when executed by one or more processors of a system, in particular a control system of the wind farm as explained herein, cause the system to carry out the method as explained herein.

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.

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 represented with the same reference numbers.

Each example is provided by way of explanation of the invention, which shall not limit the invention. In fact, it will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the scope or of the invention as defined by the appended claims.

<FIG> is a perspective view of a portion of an exemplary wind turbine <NUM>. In the exemplary embodiment, the wind turbine <NUM> is a horizontal-axis wind turbine. 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>).

In one embodiment, the rotor blades <NUM> have a length ranging from about <NUM> meters (m) to about <NUM>. Alternatively, rotor blades <NUM> may have any suitable length that enables the wind turbine <NUM> to function as described herein. For example, other non-limiting examples of blade lengths include <NUM> or less, <NUM>, <NUM>, <NUM>, <NUM> or a length that is greater than <NUM>. As wind strikes the rotor blades <NUM> from a wind direction <NUM>, the rotor <NUM> is rotated about an axis of rotation <NUM>. As the rotor blades <NUM> are rotated and subjected to centrifugal forces, the rotor blades <NUM> are also subjected to various forces and moments. As such, the rotor blades <NUM> may deflect and/or rotate from a neutral, or non-deflected, position to a deflected position.

Moreover, a pitch angle of the rotor blades <NUM>, i.e., an angle that determines a perspective of the rotor blades <NUM> with respect to the wind direction, may be changed by a pitch system <NUM> to control the load and power generated by the wind turbine <NUM> by adjusting an angular position of at least one rotor blade <NUM> relative to wind vectors. During operation of the wind turbine <NUM>, the pitch system <NUM> may change a pitch angle of the rotor blades <NUM> such that the rotor blades <NUM> are moved to a feathered position, such that the perspective of at least one rotor blade <NUM> relative to wind vectors provides a minimal surface area of the rotor blade <NUM> to be oriented towards the wind vectors, which facilitates reducing a rotational speed and/or facilitates a stall of the rotor <NUM>.

A blade pitch of each rotor blade <NUM> may be controlled individually by a wind turbine controller <NUM> or by a pitch control system.

Further, in the exemplary embodiment, as the wind direction <NUM> changes, a yaw direction of the nacelle <NUM> may be rotated, by a yaw system <NUM>, about a yaw axis <NUM> to position the rotor <NUM> with respect to wind direction <NUM>.

The yaw system <NUM> may include a yaw drive mechanism provided by nacelle <NUM>.

Further, yaw system <NUM> may also be controlled by wind turbine controller <NUM>.

For positioning nacelle <NUM> appropriately with respect to the wind direction <NUM> as well as detecting a wind speed, the nacelle <NUM> may also include at least one meteorological mast <NUM> that may include a wind vane and anemometer (neither shown in <FIG>). The mast <NUM> may provide information to the wind turbine controller <NUM> regarding ambient conditions. This may include wind direction and/or wind speed as well as ambient temperature, ambient moisture, precipitation type and/or amount (if any).

In the exemplary embodiment, the wind turbine controller <NUM> is shown as being centralized within the nacelle <NUM>, however, the wind turbine controller may also be a distributed system throughout the wind turbine <NUM>, on a support system (not shown in <FIG>), within a wind farm, and/or at a remote control center. The wind turbine controller <NUM> includes a processor configured to perform the methods and/or steps described herein.

Referring now to <FIG>, a schematic view of one embodiment of an electrical (power) and control 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>.

In the exemplary embodiment, 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>.

Alternatively, system <NUM> is configured as a full power conversion system (not shown) known in the art, wherein a full power conversion assembly (not shown) that is similar in design and operation to assembly <NUM> is electrically coupled to stator <NUM> and such full power conversion assembly facilitates channeling electrical power between stator <NUM> and an electric power transmission and distribution grid (not shown). Stator bus <NUM> transmits three-phase power from stator <NUM> and rotor bus <NUM> transmits three-phase power from rotor <NUM> to assembly <NUM>. Stator synchronizing switch <NUM> is electrically coupled to a main transformer circuit breaker <NUM> via a system bus <NUM>.

Due to the high possible power rating at given size/costs, wind turbines with full power conversion assembly are widely used in offshore wind farms.

In addition, the electrical power and control 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.

The sensors <NUM>, <NUM>, <NUM> may be sensor for currents and/or voltages desired for controlling the power conversion of wind turbine <NUM>. This is explained in more detail below.

Further, at least one additional sensor (not shown) may be provided for sensor data referring to meteorological data, e.g. sensor(s) provided by the meteorological mast <NUM> shown in <FIG>. The at least one additional sensor may in particular include a sensor for determining a wind speed at or in front of rotor <NUM> of wind turbine <NUM>.

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

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 as described herein. As a further alternative, electrical and control system 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 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>, power conversion assembly <NUM> electrically coupled to generator <NUM> and step-up transformer <NUM> form the power conversion system of wind turbine <NUM>.

Typically, the power conversion system at least includes a generator and a power conversion assembly including a power converter, in particular an indirect AC-to-AC power converter (AC/DC-AC converter) or a matrix converter, for example a respective full converter or DFIG converter depending on the generator.

<FIG> illustrates a block diagram of a wind farm <NUM>. In the exemplary embodiment, wind farm <NUM> is an offshore wind farm.

Wind farm <NUM> has several strings S1-S3 of wind turbines. For sake of clarity, only string S1 is shown in more detail in <FIG>. Each of the other strings S2, S3 may be similar or even equal to string S1. However, at least the number of wind turbines and length of string may vary between the strings S1-S3.

For sake of clarity, strings S1 includes three exemplary wind turbines 100a, 100b, 100c, for example three wind turbines as explained above with regard to <FIG>. However, string S1 may also have at least five or at least ten wind turbines.

The wind turbines 100a, 100b, 100c are electrically connected with each other via power cables Cab, Cbc (and closed power switches not shown).

Further, the wind turbines 100a, 100b, 100c are electrically connectable with a power grid bus 510a of a local wind farm grid <NUM> via a feeder (power cable) Cc and a circuit breaker <NUM>. Respective circuit breakers <NUM>, <NUM> are also provided for wind turbine strings S2, S3.

The power cables Cab, Cbc and the feeder Cc may be implemented as respective undersea cables.

<FIG> illustrates a state of wind farm <NUM> in which the circuit breakers <NUM>-<NUM> are open and the strings S1-S3 disconnected from power grid bus 510a and the (onshore) utility grid <NUM>, for example in response to a failure of utility grid <NUM> or in the electrical connection between local wind farm grid <NUM> and utility grid <NUM>.

In the exemplary embodiment, a point of common coupling (PCC) of local grid <NUM> is connectable with utility grid <NUM> via a main circuit breaker <NUM> (illustrated in open state), a grid substation <NUM> and a power link <NUM> which is typically implemented as an undersea cable.

In the exemplary embodiment, wind farm <NUM> is an offshore wind farm. However, the embodiments explained below can also be applied to onshore wind farms using a power link <NUM> to transmit power to a distant (utility) grid.

Power link <NUM> may either be a DC-link, in particular a high voltage DC-link (HVDC-link) or an AC-link, in particular a high voltage AC-link (HVAC-link).

In embodiments in which the wind farm <NUM> is electrically connectable to grid <NUM> using an AC link <NUM>, (offshore) wind farm grid substation <NUM> connectable between local AC-grid and power link <NUM> typically includes a grid transformer as indicated by the dashed electric symbol in box <NUM>.

In embodiments in which the wind farm <NUM> is electrically connectable to grid <NUM> using a DC link <NUM>, wind farm grid substation <NUM> includes an AC-DC power converter (power inverter).

Note that the main circuit breaker <NUM> and/or one or more sensors <NUM> for measuring currents and/or voltages at the lower voltage side and/or the high voltage side (not shown in <FIG>) of grid substation <NUM> may also be provided by the substation <NUM>.

As indicated by the dashed arrows in <FIG>, a wind farm controller <NUM> is communicatively coupled with the wind turbines 100a-100c, more particular its turbine controllers (not shown in <FIG>), the circuit breaker <NUM>-<NUM>, the sensors <NUM>, <NUM> and the optional substation <NUM>.

In this embodiment, wind farm controller <NUM> and wind turbine controllers <NUM> together form a control system which is communicatively coupled with the power conversion systems and the auxiliary subsystems of the wind turbines 100a-100c.

Wind farm controller <NUM> is typically directly communicatively coupled with the turbine controllers, the substation <NUM> and the main circuit breaker <NUM>. Further, wind farm controller <NUM> may be directly or via the turbine controllers communicatively coupled with current sensors <NUM>, <NUM>, meteorological data sensors provided by the wind turbines 100a-100c, and circuit breakers <NUM>-<NUM>.

The general design of the wind farm controller <NUM> may at least substantially corresponds to the design of the wind turbine controller as explained above with regard to <FIG>. However, wind farm controller <NUM> may be more complex and/or more powerful. Further, wind farm controller <NUM> typically operates as a primary controller supervising the wind turbine controllers during controlling the wind turbines in normal operating mode, idling operating mode and stand still operating mode. For example, wind farm controller <NUM> may provide SCADA (Supervisory Control And Data Acquisition) functionality for wind farm <NUM>.

In the illustrated state of wind farm <NUM>, circuit breakers <NUM>-<NUM> are open. Accordingly, strings S1-S3 of wind farm <NUM> are electrically disconnected from each other and utility grid <NUM>. This may be due to an outage or another failure of utility grid <NUM> that may e.g. be detected using data provided by sensor <NUM>.

In this state and after disconnecting wind farm <NUM> from utility grid <NUM>, respectively, the wind turbines 100a-100c of the string S1 as well as the not shown wind turbines of string S2, S3 are instructed by wind farm controller <NUM> to operate in a respective island operating mode in which each turbine controller controls the power conversion system of the respective wind turbine 100a-100c so that the power demand of the wind turbine and its auxiliary subsystems is at least substantially matched by the electric power Pa, Pb, Pc generated by the respective power conversion system.

Wind turbines 100a-100c may be operated safely in this mode for a longer period as long as wind speed is low enough.

Upon detecting that the wind speed is larger than a first threshold value corresponding to a wind speed at which stalling of rotor blades is expected for one or all of the wind turbines 100a-100c, e.g. by the wind farm controller <NUM> typically receiving metrological data such as wind speed and currently used power setpoints from the wind turbine controllers, is exposed to the stalling wind condition, the power conversion system of one of the wind turbines (wind turbine 100a in the exemplary embodiment) is commanded (and controlled, e.g. by its corresponding turbine controller) to increase the electric power from Pa by an electric surplus power ΔP=Pb+Pc not required by wind turbine 100a but sufficient to meet or even match the power demands of the auxiliary subsystems, respectively, of the other wind turbines 100b, 100c of string S1.

The electric surplus power ΔP may be transferred through power cables Cab, Cbc to wind turbines 100b, 100c operated in idling operating mode.

Typically, the wind turbines 100b, 100c are controlled in idling operating mode by their turbine controller on request of wind farm controller <NUM>.

In the following methods are explained that may be performed by wind farm <NUM> and/or controlled by the control system of wind farm <NUM>.

<FIG> illustrates a flow chart of a method <NUM> for operating a wind farm, in particular a wind farm as explained above with regard to <FIG>.

In a first block <NUM>, the wind turbines of one or more wind farm strings, which are disconnected from a utility grid and each other, are operated in an island operating mode so that the power conversion system of each wind turbine produces output electric power which at least substantially and/or on average matches the power demand of the respective wind turbine and its auxiliary subsystems, respectively.

Thereafter and while the wind turbines operate in island operating mode, it is checked in a subsequent block <NUM> if a rotor of one of the wind turbines is exposed to a wind condition at which the rotor blade(s) are at risk of stalling at the currently generated electric output power of the connected power conversion system. If so, method <NUM> is continued with block <NUM>. Otherwise, method <NUM> returns to block <NUM>.

In block <NUM>, the electric power generated by the power conversion system of one of the wind turbines is increased by an electric surplus power which is sufficient for supplying the auxiliary subsystems of one, typically all of the other wind turbines of the string which receive and use respective portions of the electric surplus power in block <NUM> for supplying their auxiliary subsystems while operating in idling mode in block <NUM>.

If it is determined that the rotors of the wind turbines, in particular the rotor of the wind turbine which outputs the electric surplus power is no longer exposed to a wind condition that results in a (sufficiently high) risk of stalling when returning to the normal island operating mode again, method <NUM> returns to block <NUM>. Otherwise, the respective current operating modes of the wind turbines are maintained.

<FIG> illustrates a flow chart of a method <NUM> for operating a wind farm, in particular a wind farm as explained above with regard to <FIG>. Method <NUM> may be similar to method <NUM> but is more specific.

In a first block <NUM>, it is checked if a loss or failure or unavailability of a utility grid the wind farm is/has been feeding output power to is detected.

If so, at least the strings of wind turbines of the wind farm are disconnected from the utility grid (and each other).

Thereafter, the wind turbines are operated in island (operating) mode in block <NUM>.

In a subsequent block <NUM>, it is checked if one or more wind turbines of each string are exposed to wind speeds exceeding a (respective) first threshold value corresponding to a wind speed at which stalling of the rotor blade is expected at the currently generated electric output power.

If so, one wind turbine per string of wind turbines operates in a supply island mode and the other wind turbines of the strings operate in idling operating mode in which their auxiliary subsystems are supplied by electric surplus power provided by the respective wind turbine operating in supply island mode, in blocks <NUM>, <NUM>.

Otherwise, the wind turbines maintain operating in (normal) island operating mode.

In a subsequent block <NUM>, it is checked if all wind turbines are exposed to wind speeds lower than a second threshold value which is lower than the first threshold value and at which stalling is not expected for the wind turbines when returning to normal island operating mode again.

If so, method <NUM> may return to block <NUM>.

Otherwise, the current operating modes of the wind turbines are maintained.

Similar as explained above for method <NUM>, <NUM>, the wind turbines of each string are, in a block <NUM>, operated in a normal island operating mode in which the wind turbines are not connected with the utility grid, and the wind turbines auxiliary subsystems supplied with electric power generated by the power conversion system of the respective wind turbine.

Upon detecting in a block <NUM> that at least one of the rotors of the wind turbines is exposed to a wind condition at which the rotor blade(s) of the at least one of the rotors is at risk of stalling at the currently generated electric output power, the electric power generated by the power conversion system of one (typically only one) of the wind turbines is increased, in block <NUM>, by an electric surplus power which is used for supplying the auxiliary subsystems of the other wind turbines of the string operated in idling operating mode, in block <NUM>.

Methods <NUM>, <NUM>, <NUM> may be performed until grid recovery is detected.

Thereafter, the wind farm may be reconnected to the utility grid.

Exemplary embodiments of wind farms and methods for operating wind farms are described above in detail. The systems and methods are not limited to the specific embodiments described herein, but rather, components of the systems and/or steps of the methods may be utilized independently and separately from other components and/or steps 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.

Embodiments of the present invention have been described above with reference to block diagrams and flowchart illustrations of methods, apparatuses (i.e., systems) and computer program products. These computer program instructions may be loaded onto a general purpose computer, special purpose computer, or other programmable data processing apparatus, such as the processor(s) <NUM> discussed above with reference to <FIG>, to produce a machine, such that the instructions which execute on the computer or other programmable data processing apparatus create a means for implementing the functions specified in the flowchart block or blocks.

These computer program instructions may also be stored in a non-transitory computer-readable memory that can direct a computer or other programmable data processing apparatus (e.g., processor(s) <NUM> of <FIG>) to function in a particular manner, such that the instructions stored in the computer-readable memory produce an article of manufacture including computer-readable instructions for implementing the function specified in the flowchart block or blocks. The computer program instructions may also be loaded onto a computer or other programmable data processing apparatus to cause a series of operational steps to be performed on the computer or other programmable apparatus to produce a computer-implemented process such that the instructions that execute on the computer or other programmable apparatus provide steps for implementing the functions specified in the flowchart block or blocks.

Accordingly, blocks of the block diagrams and flowchart illustrations support combinations of means for performing the specified functions, combinations of steps for performing the specified functions and program instruction means for performing the specified functions. It will also be understood that each block of the block diagrams and flowchart illustrations, and combinations of blocks in the block diagrams and flowchart illustrations, can be implemented by special purpose hardware-based computer systems that perform the specified functions or steps, or combinations of special purpose hardware and computer instructions.

Unless otherwise expressly or implicitly stated, it is in no way intended that any method set forth herein be construed as requiring that its steps be performed in a specific order.

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. While various specific embodiments have been disclosed in the foregoing, those skilled in the art will recognize that only the of the claims define the scope of the invention. For example, the control system of the wind farm may be provided by one centralized controller or a plurality of interconnected controllers. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims.

Claim 1:
A method (<NUM>, <NUM>, <NUM>) for operating a wind farm (<NUM>) comprising a string (S1-S3) of wind turbines (<NUM>-100c) which are electrically connectable with each other and a grid (<NUM>, <NUM>), each wind turbine comprising a rotor (<NUM>) comprising a rotor blade (<NUM>), a power conversion system (<NUM>, <NUM>, <NUM>) mechanically connected with the rotor (<NUM>), and at least one auxiliary subsystem (<NUM>, <NUM>), the method comprising:
• operating (<NUM>, <NUM>) the wind turbines (<NUM>-100c) of the string (S1-S3) in an island operating mode in which the wind turbines (<NUM>-100c) are not connected with the grid, and the respective at least one auxiliary subsystem (<NUM>, <NUM>) is supplied with electric power (Pa, Pb, Pc) generated by the power conversion system of the respective wind turbine;
• determining (<NUM>, <NUM>) that the rotor (<NUM>) of one of the wind turbines (<NUM>-100c) is exposed to a wind condition at which the rotor blade of the one of the wind turbines is at risk of stalling at the currently generated electric output power (Pi); and
• increasing (<NUM>, <NUM>) the electric power (Pa) generated by the power conversion system of the one of the wind turbines by an electric power amount (ΔP) which is sufficient for supplying the at least one auxiliary subsystem (<NUM>, <NUM>) of at least one of the other wind turbines of the string (S1-S3).