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.

With increasing rated power of wind turbines, the heat produced by the power conversion system during operation may also rise. For example, the overall produced heat may amount to about <NUM>% or even <NUM>% of produced electric power. In particular, the generator may produce comparatively large amounts of heat during converting the mechanical energy to electrical energy. Further, a gearboxes of the power conversion system optionally arranged between rotor and the generator may be required to be lubricated and cooled to function effectively. Further, a frequency converter that converts the electrical power from the speed variable generator into an electrical power that matches with grid frequency and voltage may also produce large amounts of heat during this conversion. Other components used in the electrical drivetrain of a wind turbine which may provide heat are the MV transformer and power cabling.

For cooling on or more components in the nacelle, external air may be provided to an internal heat exchanger using a fan, and the heated air may be discharged from the nacelle via an exhaust duct. Alternatively, a liquid cooling may be provided using a passive heat exchanger installed on an external surface of the nacelle.

As wind turbines are typically designed to their limits to reduce costs as well as the environmental footprint during manufacturing the wind turbine, the wind turbine provided with a cooling system as described above may not be able to deliver its rated output power during high temperature conditions and/or non-favorable grid conditions. In this situation, the turbine controller starts to curtail the turbine on active power and/or reactive power. To nevertheless be compliant with grid codes during these typically rare conditions where ambient temperatures are high, expensive VAR (reactive power) compensation devices for providing fast-acting reactive power may be added for the wind turbine and on wind farm level, respectively.

In view of the above, the present invention is defined by a method for operating a wind turbine according to claim <NUM>, a wind turbine according to claim <NUM>, and a computer program product or a computer-readable storage medium according to claim <NUM>. The following prior art documents are here identified:
Document <CIT> describes a wind turbine power system <NUM> including a cooling system <NUM> configured to deliver a cooling medium (e.g. air or liquid) to the electronic components to maintain the components within an acceptable operating temperature range. The cooling system being a closed loop path with heat exchanger. Further, document <CIT> describes a wind turbine system including a nacelle comprising a liquid-to-air heat exchanger and a liquid path configured to cool at least one of a generator, a power converter, a step-up transformer, and a gearbox or drivetrain. At last document <CIT> relates to an improved cooling system for a generator and/or a gearbox in a wind turbine.

In one aspect, the present disclosure is directed to a method for operating a wind turbine. The wind turbine includes a power conversion system configured to provide electrical output power to a grid, and an air-cooling system configured, in a cooling mode, to cool an ambient air and provide the cooled ambient air as a cooling air to the power conversion system. The ambient air is received from outside the wind turbine The method includes operating the air-cooling system in the cooling mode if at least one operating parameter of the power conversion system is equal to or greater than a respective threshold.

Accordingly, the power conversion system may efficiently be cooled even if the ambient temperature and, thus, the temperature of the ambient air received from outside the wind turbine, in particular from outside the wind turbine's nacelle is comparatively high, for example above <NUM>° C, above <NUM>° C or even above <NUM>° C.

The additional cooling of the ambient air allows for avoiding power curtailment and, thus, providing active power and/or reactive power as desired in accordance with grid code requirement at higher ambient temperature without the need for VAR compensation devices such as a STATCOM (Static Synchronous Compensator) and a capacitor bank, respectively. In particular, the grid can be supported in non-favorable grid conditions such as a weak grid.

This applies both for operating the wind turbine under normal operating conditions, i.e. within in the rated wind speed range, and at lower wind speeds or even at no windspeed. Note that using the air conditioning system will, compared to using a passive cooler e.g. on top of the turbine, allow a converter of the power conversion system to operate at higher VAR levels also in case there is no wind.

The air-cooling system is typically configured to remove heat from the ambient air with a cooling efficiency of at least <NUM>, more typically of at least <NUM>.

The air-cooling system may in particular be provided by an air conditioning system.

For example, air conditioning system may be configured to produce about <NUM> kW of cooling power per 1kW of consumed electrical energy.

As wind turbine generators also have a very high efficiency of e.g. about <NUM>%, there is a large effect on output power (produced power of generator minus consumed power of the additional air-cooling system typically received via an internal electric power distribution system) if the temperature of the coolant air is reduced by the air-cooling system (e.g. the air conditioning system).

For example, assuming a wind turbine of <NUM> MW rated power, using of a <NUM> kW air conditioning system (at <NUM> kW input power) will allow to reduce the cooling temperature by at least <NUM>, for example up to <NUM>°K which will allow at least about <NUM> kW of additional active power in case thermal limits (cooling without using the air additional conditioning system) are reached. Similar numbers apply for the reactive power.

This illustrates that using a comparatively small level of electrical power from the generator to feed an additional air-cooling system, which is typically implemented as an air conditioning system, allows the wind turbine generator to provide up to <NUM> times more active/reactive power once the system reach thermal limits without the additional air-cooling system because of high ambient temperatures.

Accordingly, investment in infrastructure as the need of VAR compensation devices is no longer desired because the wind turbine's power conversion system may still provide the necessary VAR compensation at higher ambient temperatures when provided with the additional air-cooling system.

Further, the annual energy production (AEP) may be increased. This is because curtailment at higher ambient temperature may at least be reduced.

Alternatively or in addition, other power conversion components of the power conversion system than the generator, in particular a gearbox arranged between the rotor and the generator, a power conversion assembly such as a power converter connected to the generator, and/or a transformer connected with the generator and/or the power conversion assembly are cooled using the cooling air provided by the additional air-cooling system if desired. Furthermore, an electric cabinet connected with one or more power conversion components of the power conversion system may be cooled using the cooling air provided by the additional air-cooling system if desired.

This typically result in analogous advantages, however, typically to a smaller extend compared to cooling the generator based on the cooling air provided by the additional air-cooling system.

Note that the power conversion system typically includes several power conversion components which are configured to contribute in converting input motive power into the electrical output power to be provided to the grid, in particular a utility grid, if the input motive power is received from the rotor of the wind turbine.

The air-cooling system may either be operated in the cooling mode irrespective of the actual temperature of the power conversion system and the power conversion component(s) of the power conversion system, respectively, or depending on the respective temperature(s). The latter allows for only operating the air-cooling system in the cooling mode (and thus consuming electric power) if actually desired for thermal reasons.

However, it is also possible to operate the air-cooling system in the cooling mode independent of the temperature(s) of the power conversion system, but e.g. based on the ambient temperature, more particular if a temperature of the ambient air is equal to or greater than an ambient temperature threshold. This control scheme may, compared to operating the air-cooling system in the cooling mode depending on the temperature(s) of the power conversion system, be simpler but may result in a somewhat lower AEP (still higher compared to using uncooled ambient air for cooling).

For reasons of efficiency, the air-cooling system is operated in the cooling mode only if a reactive power demand of the power conversion system is equal to or greater than a reactive power demand threshold, and/or if an active power demand of the power conversion system is equal to or greater than an active power demand threshold.

Otherwise, the (uncooled) ambient air is likely to be sufficient to remove heat from the power conversion system and cooling of the the power conversion system, respectively.

The air-cooling system may in particular (only) be operated in the cooling mode if at least one of the following conditions is met:.

The cooling mode of the air-cooling system may even depend on the respective temperature(s).

In particular, a cooling power of the air-cooling system may depend on at least one of the temperature of the ambient air, the temperature of the cooling air, and a (at least one) temperature of the power conversion system.

In one embodiment of a method for operating a wind turbine including a power conversion system and an air-cooling system, the method includes controlling the air-cooling system to cool an ambient air and to provide the cooled ambient air as a cooling air to the power conversion system depending on at least one operating parameter of the wind turbine, in particular depending on at least one operating parameter of the power conversion system.

The term "operating parameter of the power conversion system" as used herein intends any parameter that may influence and/or be used for controlling operating the power conversion system during converting input motive power into the electrical output power. The term "operating parameter of the power conversion system" typically embraces a reactive power demand, an active power demand, an active power production of the power conversion system, a reactive power production of the power conversion system, an output current of the power conversion system, an output voltage of the power conversion system, a temperature of the power conversion system and any of the components of the power conversion system, respectively, a coolant temperature of a coolant used in an inner cooling circuit of the respective component(s) of the power conversion system, but also the temperature of the ambient air and the temperature of the provided cooling air, and any combinations or functions thereof.

The air-cooling system may in particular be operated in the cooling mode depending on a least one of: the temperature of the ambient air, a coolant temperature, a temperature of a gearbox, a temperature of a power converter, a temperature of a transformer, and a temperature of the generator, in particular a temperature of a bearing of the generator and/or a temperature of a stator of the generator.

The respective temperatures are typically measured.

Different thereto, the reactive power demand and/or the active power demand are typically received, for example from a wind farm controller of a wind farm the wind turbine belongs to.

The cooling mode may be activated based on at least one of the typically measured temperature(s), the received reactive power demand, and the received active power demand.

Further, the cooling mode may (later) be deactivated based on at least one of a (later) (measured) temperature(s), a (later) received reactive power demand, and a (later) received active power demand.

As described above, the cooling air may be used to remove heat from the power conversion system.

This is typically achieved using the cooling system which receives the cooling air, and/or may include operating a cascade of three or even four cooling circuits thermally coupled to one another.

The method may further include curtailing at least one of a reactive output power of the power conversion system and a reactive output power of the power conversion system if the temperature of the power conversion system and the temperature of the at least one power conversion component of the power conversion system, respectively, is equal to or greater than a respective third temperature threshold.

The third temperature threshold is typically larger than at least one of, more typically both of the respective first temperature threshold and the respective second temperature threshold.

According to an embodiment of a method for manufacturing and/or updating (retrofitting) a wind turbine, the method includes providing a power conversion system of the wind turbine with an air-cooling system configured to cool an ambient air so that the air-cooling system can provide the cooled ambient air as a cooling air to the power conversion system of the wind turbine, in particular a cooling system for or of the power conversion system and its power converting components, respectively.

The method may in particular include thermally connecting the air-cooling system with the power conversion system for removing the heat. For example an outlet for the cooled ambient air of the air-cooling system may be connected with a cooling air inlet and/or fan of a cooling system of the power conversion system such as a heat exchanger. Further, the method typically includes electrically connecting the air-cooling system with an internal electric power distribution system of the wind turbine. Furthermore, the method may include updating a control software of a control system of the wind turbine in accordance with the control methods explained herein, in particular updating a software of a wind turbine controller.

According to an embodiment of a method, the method includes retrofitting an existing cooling system of a power conversion system of a wind turbine with an additional air-cooling system such as an air conditioning system.

The steps of the methods for operating the wind turbine as explained herein are typically performed by a control system for or even of the wind turbine. The control system is communicatively coupled with the power conversion system and the air-cooling system, and typically implemented as a controller, for example a respective turbine controller.

Note that an internal electric power distribution system of the wind turbine may be connectable with the power conversion system for receiving electric power to be distributed to the air-cooling system. In this embodiment, the power conversion system may be considered as electric power source and the air-cooling system as electric power consumer and electric load, respectively.

In 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 wind turbine having a control system providing the one or more processors as explained herein, cause the system to carry out the methods as explained herein.

In yet another aspect, the present disclosure is directed to a wind turbine including a rotor comprising rotor blades, an air-cooling system configured to receive ambient air, to cool the ambient air, and to provide the cooled ambient air as a cooling air, and a power conversion system mechanically connected with the rotor, electrically connectable to a utility grid, and configured to convert input motive power into electrical output power. A cooling system of the wind turbine is configured to receive the cooling air and to use the cooling air to remove heat from the power conversion system. In addition the ambient air is received from outside the wind turbine.

The power conversion system may be implemented as a DFIG-system.

The cooling system may be a cooling system of the power conversion system, in particular a cooling system that is, at lower ambient air temperature (lower than a second temperature threshold for the temperature of the ambient air), sufficient for reliably cooling (one or more of the components of) the power conversion system (without receiving the cooled ambient air from the air-cooling system).

The air-cooling system and the cooling system may be considered as a cascade of two thermally coupled cooling system.

As the air-cooling system may only be operated under specific conditions (high ambient temperature and high power request), the cooling system and the air-cooling system may also be considered as main cooling system of the power conversion system and supplementary cooling system of the power conversion system, respectively.

The (cascaded) air-cooling system and the cooling system typically implement a cascade of (at least) three cooling circuits thermally coupled to one another, for example a cascade of (at least) four cooling circuits thermally coupled to one another.

The cooling system may include one or more closed internal cooling circuits thermally connected with the power conversion system for removing the heat (from a respective power conversion component), an open cooling circuit thermally coupled with the respective closed internal cooling circuit and configured to receive the cooling air from the air-cooling system, a heat exchanger arranged between the open cooling circuit and the closed internal cooling circuit, and a main cooler configured to receive the cooling air and typically including a heat exchanger and/or being implemented as an air-liquid cooler, in particular an oil-air cooler. However, the latter may depend on the particular component to be cooled. For example, an outlet for the cooled ambient air of the air-cooling system may directly or via a liquid / liquid heat exchanger be connected with an oil cooler of a gearbox of the power conversion system.

Typically, the air-cooling system is provided by an air conditioning system.

The air-cooling system may be configured to remove heat from the ambient air at a rate of at least up to about <NUM> kW, more typically of at least up to about <NUM> kW, even more typically of at least up to about <NUM> kW.

Further, the air-cooling system may be configured to remove heat from the ambient air with a cooling efficiency of at least <NUM>, more typically of at least <NUM> or even at least <NUM>.

The power conversion system typically includes one or more power conversion component configured to contribute in converting input motive power received from the rotor into the electrical output power.

More particular, the power conversion system may include a gearbox, a generator, power conversion assembly typically including a power converter, for example a rotor-side power converter and a line-side power converter, and a transformer as respective power conversion components.

Typically, the cooling system is configured to remove heat from at least one of the power conversion components.

The power conversion component(s) may be arranged in a nacelle of the wind turbine.

The air-cooling system may at least partly be arranged in the nacelle or at the nacelle.

Typically, the power conversion system is connectable with an internal electric power distribution system (internal power grid) for providing electric power to the air-cooling system and the cooling system.

Accordingly, electric power may flow from the power conversion system, through the internal electric power distribution system, and to the air-cooling system as well as the cooling system.

Typically, the wind turbine includes at least one temperature sensor for measuring a respective temperature, in particular a temperature of the ambient air, a temperature of the cooling air, a temperature of the power conversion system, and a temperature of respective power conversion components of the power conversion system.

Furthermore, a controller of the wind turbine is typically communicatively coupled with the air-cooling system, the power conversion system, and the temperature sensor(s), and configured to control the wind turbine in accordance with the method 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, the invention being defined by the features of the independent claims <NUM> and <NUM> which solely limit its scope.

<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>, 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>.

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>, <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>, <NUM> may be coupled to the sensor interface <NUM> via a wired connection. However, in other embodiments, the sensors <NUM>, <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 sensor <NUM> may be provided for sensor data referring to meteorological data, e.g. sensor(s) provided by the meteorological mast <NUM> shown in <FIG>. In particular an ambient air temperature sensor <NUM> may be provided by the meteorological mast <NUM>.

Furthermore, at least one temperature sensor <NUM> may be provided for measuring temperatures within the nacelle, in particular a respective sensor for measuring a temperature of the power conversion system as explained above with regard to <FIG> and components thereof, respectively, and/or for measuring a temperature of the internal air flow and the cooling circuits explained in more detail below with regard to <FIG>.

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 (also referred to as operating parameter herein) 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>.

<FIG> illustrates a block diagram of a wind turbine <NUM>. Wind turbine <NUM> is typically similar to wind turbine <NUM> explained above with regard to <FIG> and also has a nacelle <NUM>, a power conversion system <NUM> arranged in nacelle <NUM>, mechanically connected with a rotor, and electrically connectable with a utility grid for feeding electrical output power P to the utility grid, typically via a grid circuit breaker <NUM> and optionally via a further transformer (outside nacelle <NUM>), for example a wind farm transformer.

In the exemplary embodiment, an air-cooling system <NUM>, which is typically implemented as and/or provided by an air conditioning system, is arranged on and/or at nacelle <NUM>.

In a cooling mode, air-cooling system <NUM> cools the ambient air 28a received from outside nacelle <NUM> from ambient air temperature Ta to a lower temperature Tc, and feeds or discharges the cooled ambient air as cooling air 28c into the inner of nacelle <NUM>, more particular towards or even to a cooling system <NUM> of power conversion system <NUM>, in particular via an air supply duct arranged between an outlet of air-cooling system <NUM> for the cooled ambient air 28c and a cooling air inlet of cooling system <NUM> for removing excess heat Q from power conversion system <NUM>. In this process, cooling air 28c is reheated and discharged from nacelle <NUM> as exhaust air 28d of higher temperature Td, typically via an exhaust duct.

As further illustrated in <FIG>, air-cooling system <NUM> can be provided with electric power Pi from power conversion system <NUM> via an internal electric power distribution system <NUM>.

Typically, at least a generator of power conversion system <NUM> can be cooled using cooling system <NUM> which is provided with cooled ambient air 28c by air-cooling system <NUM> if desired, in particular at higher ambient air temperature Ta, if a high waste heat Q is to be removed and/or if the power conversion system <NUM> is desired to deliver large amount of (active and/or reactive) power P to the grid.

Alternatively or in addition, a gearbox, a power converter, a transformer and/or an electric cabinet of power conversion system <NUM> may be cooled in this way to keep the respective component below a respective threshold temperature. For example, the air conditioning system may be connected to a gearbox cooler via a liquid-liquid heat exchanger.

The temperature Tc of the cooled ambient air and cooling air 28c, respectively, and/or the temperature difference Ta-Tc may even be controllable.

Typically, the temperature of one or more of the components of power conversion system <NUM> are controlled by a turbine controller communicatively coupled via a data bus and/or respective data lines with air-cooling system <NUM>, cooling system <NUM>, power conversion system <NUM>, power conversion components of power conversion system <NUM> and/or respective temperature sensors.

For cooling the power conversion system <NUM> and its power conversion components, respectively, cooling system <NUM> may have one or more closed cooling circuits for removing heat Q which are circulated with a respective coolant that can be cooled with cooling air 28c, for example one (or even more) respective closed cooling circuits for each power conversion components.

Such a closed cooling circuit is shown in <FIG> illustrating a block diagram of a wind turbine <NUM> which is typically similar to and may even correspond to wind turbine <NUM> explained above with regard to <FIG>.

In the exemplary embodiment, air-cooling system <NUM> includes a first open cooling circuit C1 for receiving ambient air <NUM>' at a first inlet and a second open cooling circuit C2 for receiving ambient air <NUM> at a second inlet. The open cooling circuits C1, C2 are thermally coupled with each other via a heat exchanger H12 of air-cooling system <NUM> so that, in the cooling mode, heat is transferred from ambient air 28a received at the second inlet to ambient air 28a' received at the first inlet. While heated air of first open cooling circuit C1 is, in the cooling mode, discharged at a first outlet as first exhaust air 28d' at higher temperature Te>Ta, cooled ambient air of the second open cooling circuit C2 is discharged as cooling air 28c of lower temperature Tc<Ta at a second outlet and transferred to an exemplary fan F of a cooling system <NUM> for pumping cooling air 28c through an open cooling circuit C3 of cooling system <NUM>. The open cooling circuit C3 is thermally coupled via a heat exchanger H34 of cooling system <NUM> with one exemplary closed cooling circuit C4 for removing heat Q from power conversion system <NUM>.

Accordingly, a cascade of four cooling circuits C1-C4 thermally coupled to one another may be used for cooling power conversion system <NUM>.

However, it is also possible that only three cooling circuits thermally coupled to one another are used for cooling power conversion system <NUM>.

For example, the first open cooling circuit C1 may be omitted, for example in an embodiment in which heat exchanger H12 implemented as thermoelectric cooler, i.e. based on thermoelectric cooling of ambient air <NUM> in open cooling circuit C2 and the transferred heat discharged via cooling fins or the like.

However, due to the higher efficiency, heat exchanger H12 is typically implemented as a vapor-compression systems (even having an additional internal closed cooling circuit).

This may also apply to heat exchanger H34.

<FIG> illustrates a flow chart of a method <NUM> of operating a wind turbine, in particular a wind turbine <NUM>, <NUM>, <NUM>' as explained above with regard to <FIG>. As such the wind turbine has a power conversion system for providing electrical output power to a grid, in particular a utility grid, and an air-cooling system for providing (in a cooling mode) cooled ambient air as a cooling air to the power conversion system.

Typically during operating the wind turbine in a normal operating mode, in which the power conversion system converts input motive power received from the rotor into electrical output power and provides a least a major portion of the electrical output power to the utility grid, method <NUM> includes a block (step) <NUM> of operating the air-cooling system in the cooling mode and providing cooled ambient air as a cooling air to the power conversion system, respectively.

According to an embodiment, block <NUM> is performed depending on at least one operating parameter of the power conversion system and/or if the at least one operating parameter is equal to or greater than a respective threshold.

Accordingly, block <NUM> is typically performed depending on determining the at least one operating parameter, e.g. including measuring one or more respective temperature, or receiving the at least one operating parameter in a preceding block <NUM>.

As indicated by the dashed arrow in <FIG>, method <NUM> may return from block <NUM> to block <NUM> at a later time to start a new control cycle.

Further, only if, despite cooling the power conversion system using the cooled ambient air as cooling air, a temperature of the power conversion system (e.g. at least one power component thereof) is equal to or greater than a respective upper temperature threshold (third temperature threshold), reactive output power of the power conversion system and/or a reactive output power of the power conversion system may be curtailed in a subsequent block <NUM>.

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

Method <NUM> is typically similar to method <NUM> explained above with regard to <FIG> and also includes a block <NUM> of operating the wind turbine's air-cooling system in the cooling mode. However, method <NUM> is more specific.

In the exemplary embodiment, the air-cooling system is operated in the cooling mode only if both the temperature Ta of the ambient air is equal to or greater than an ambient temperature threshold Th_Ta of e.g. <NUM>° or <NUM>, and at least one of the following to conditions is met: (a) a reactive power demand RPD of the power conversion system is equal to or greater than a reactive power demand threshold Th1_RPD, and (b) an active power demand APD of the power conversion system is equal to or greater than an active power demand threshold Th1_APD.

Otherwise, the active and typically also the reactive power production of the power conversion system is controlled in a block <NUM> without using the cooling mode of the air-cooling system for increasing heat removal from the power conversion system which is not desired under this conditions.

Method <NUM> may be considered as proactively increasing the heat removal from the power conversion system at high power demand and high ambient temperature which otherwise may result in to high thermal loads for components of the power conversion system.

To reduce control induced fluctuations and/or to save energy, air-cooling system may also only be operated in the cooling mode if the above conditions (Ta>= Th_Ta and (RPD>Th1_RPD or APD>=Th1_APD)) are met for a respective predetermined time period of e.g. one or several seconds.

Method <NUM> is typically also similar to method <NUM> explained above with regard to <FIG> and also includes a corresponding block <NUM> of (activating or maintaining) operating the wind turbine's air-cooling system in the cooling mode. However, method <NUM> is more specific.

In the exemplary embodiment, the cooling mode is activated in block <NUM> if a temperature Tc of the cooling air is equal to or greater than a first cooling air temperature threshold Th1_Tc, if a temperature TGS of a generator stator is equal to or greater than a first generator stator temperature threshold Th1_TGS, or if a temperature TGB of a generator bearing is equal to or greater than a first generator bearing temperature threshold Th1_TGB.

The temperatures Tc, TGS and TGB are typically monitored in a block <NUM>.

After activating the cooling mode, it may be checked if the temperatures Tc, TGS and TGB nevertheless exceed or at least reach a respective higher third temperature threshold Th3_Tc (>Th1_Tc), Th3_TGS (>Th1_TGS), Th3_TGB (>Th1_TGB).

If so, power curtailment may be activated for the power conversion system to avoid over heating in a block <NUM>.

Otherwise, it may be checked if all temperatures Tc, TGS and TGB are below a respective second temperature threshold Th2_Tc (Th2_Tc<Th1_Tc), Th2_TGS (Th2_TGS<Th1_TGS), Th2_TGB (Th2_TGS<Th1_TGS).

If so, the cooling mode may be deactivated and method <NUM> may return to block <NUM>. Otherwise, cooling mode is maintained.

Compared to method <NUM> explained above with regard to <FIG>, air-cooling system is operated in the cooling mode depending on the monitored generator temperatures. Accordingly, the cooling mode is only used when actually desired.

Alternatively or in addition, the temperatures of other power conversion components may be taken into account for controlling (activating/deactivating) the cooling mode.

Furthermore, one or more thermal properties of the wind turbine and its components, respectively, in particular the power conversion component(s), such as respective thermal time constants may be taken into account for controlling the cooling mode (operating the air-cooling system).

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 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. Accordingly, where a method claim does not actually recite an order to be followed by its steps or it is not otherwise specifically stated in the claims or descriptions that the steps are to be limited to a specific order, it is no way intended that an order be inferred, in any respect. This holds for any possible non-express basis for interpretation, including: matters of logic with respect to arrangement of steps or operational flow; plain meaning derived from grammatical organization or punctuation; the number or type of embodiments described in the specification.

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, at least one of the power conversion components such as the transformer may at least partly be located in the tower or a base instead of the nacelle. 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 turbine (<NUM>, <NUM>, <NUM>') comprising a power conversion system (<NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>) configured to provide electrical output power (P) to a grid (<NUM>), and an air-cooling system (<NUM>) configured, in a cooling mode, to cool an ambient air (28a) and provide the cooled ambient air as a cooling air (28c) to the power conversion system (<NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>), the method (<NUM>, <NUM>, <NUM>) comprising:
• operating (<NUM>, <NUM>, <NUM>) the air-cooling system (<NUM>) in the cooling mode if at least one operating parameter (APD, RPD, TGB, TBS) of the power conversion system (<NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>) is equal to or greater than a respective threshold (Th1_APD, Th1_RPD, Th1_TGB, Th1_TBS),
the method being characterised in that the ambient air is received from outside the wind turbine.