Patent Description:
Modern utility scale wind turbines typically are equipped with an active yaw control system. Yaw control systems are used to move the nacelle of the wind turbine and to ensure that the nacelle and the rotor face into the wind during power generation. This ensures efficient use of the wind turbine based upon wind conditions.

Common types of active yaw control systems involve the use of electric or hydraulic drive systems that have a plurality of active drives coupled with the nacelle of the wind turbine to move the nacelle angularly in azimuth with respect to the tower. Typically, the nacelle is mounted on the tower by a roller yaw bearing or gliding yaw bearing to allow the nacelle to yaw smoothly. A yaw braking system may also be provided to keep the nacelle at a particular azimuth position. The yaw drives may each include a yaw motor and the motors collectively drive a gear element for movement of the nacelle. A plurality of yaw drives with respective motors and output gear elements, generally mounted to the nacelle, are positioned in engagement with a large gear or toothed wheel, generally mounted to the tower. The drives may be arranged around the base of the nacelle to operate together under the direction of a yaw controller to provide the desired yaw control. Oftentimes the motors of the yaw drives may be equipped with an electromechanical brake.

Active yaw control systems are controlled by a suitable controller which, for example, may be part of the larger wind turbine control system. The controller has the task of operating the various yaw drives and moving the nacelle to a commanded yaw position, or moving the nacelle to maintain a zero heading with respect to the relative wind direction. The yaw control implemented by the controller is usually operated, based on the variable nature of the wind, to address changing wind conditions, but also to keep the activation of the yaw drives at an acceptable level that minimizes wear. Also, external conditions apart from the wind variations, are taken into account for control of a wind turbine.

For example, in order to secure a stable electrical grid, wind turbines are subjected to national grid compliance requirements. One of those requirements states that the wind turbine must stop power production under certain voltage and frequency variations on the grid. The voltage variations can be as large as -<NUM>% to +<NUM>% of nominal voltage. Generally, the more extreme variations are of a shorter duration, such as a few seconds or sometimes a few minutes.

For continuous operation in response to grid variations, the actual voltage variation can be up to +/- <NUM>%. For example, Table <NUM> is an exemplary table of grid variation compliance parameters for controlling the generation of power from a wind turbine experiencing varying grid conditions.

In addition to operational compliance of the wind turbine, variations in the electrical grid voltage and frequency can also detrimentally affect the operation of the yaw control system and particularly the operation of the yaw drives. In wind turbines having yaw drives or drive systems using electrical yaw motors, such as asynchronous induction motors, the motors are often directly fed with power that is subjected to the above-noted grid voltage and frequency variations. The yaw motors are also often equipped with an electromechanical brake. Such yaw brakes need power to disengage from their braking function. Such power for the brake is often fed directly from the motor terminal box, either as AC or DC power that is fed via a rectifier. As such, the motor brake is also subject to the issues of grid variations.

Normally electric motors and their brakes are designed to function within a voltage variation of +/- <NUM>%. Outside of that range, they may not operate properly. For example, the electric motor may be able to endure shorter burst of excessive voltage, but for an over voltage condition there is a risk of overloading the yaw system during gusts, due to higher torque from the yaw motors. In the opposite situations involving under voltage powering of the yaw system, there is a risk of having too little yaw capacity, due to the resulting lower torque delivered from the yaw motors.

Still further, an additional issue presented in the power variation scenarios is that many yaw motor brakes will not disengage properly below -<NUM>% voltage variations. As a result, in worst case scenario, most of the generated yaw motor torque will be used to overcome the brake torque from a brake that is not properly disengaged. This leads to a significantly lower yaw capacity for the system. A brake that does not disengage will suffer early wear out, when it is dragged by the yaw motor. Still further, the yaw motors and the rest of yaw system could be overloaded in such a scenario.

<CIT> discloses controlled operation of auxiliary circuits in a wind turbine system, for instance, a grid event associated with the wind turbine system can be detected and in response a control signal can be provided to an auxiliary circuit breaker of the wind turbine system. <CIT> discloses a wind turbine comprising a yaw motor with a yaw brake having a switch which is electrically connected to the yaw motor and the yaw brake and which is configured to be brought from an open state into a closed state and vice versa.

Thus, there is a need for improved systems and methods which enable the detection of variations in the grid associated with wind turbines and operation and control of the yaw systems based on the detected variations.

In one embodiment of the invention a wind turbine includes a nacelle mounted to move on a tower for providing electrical power to an electric grid. The wind turbine has a yaw system with one or more yaw motors that are operable to provide a yaw function for the wind turbine and move the nacelle with respect to the tower. A controller is coupled with the yaw system for controlling the operation of the yaw motors in the yaw function. A control signal is provided to the controller that is reflective of a grid condition of an electric grid and the controller is configured for evaluating the control signal and selectively delaying with a delay the yaw function based upon the control signal, and proceed with the yaw function after expiry of the delay. For example, a suitable grid variation range, such as for voltage or frequency, for performing the yaw function might be used by the controller. The controller compares the control signal to the grid variation range and selectively delays the yaw function based upon the comparison. If the control signal is outside of the grid variation range, the yaw function may be delayed. The delay may be a time delay.

In one embodiment, the controller, upon a determination that the control signal is outside of the grid variation range, is further configured to periodically compare the control signal to the grid variation range over time. This allows the yaw function to proceed if the control signal indicates that the grid conditions are within an acceptable range.

A time limit for the delay might be used so as to perform the yaw function if sufficient time has elapsed even if the control signal indicates that the grid condition is not within an acceptable range. To that end, the controller make use of a time limit and the controller is configured to determine if the delay of the yaw function exceeds the time limit, and will proceed with the yaw function if the delay exceeds the time limit.

In another embodiment, the yaw system includes one or more brakes for interacting with the yaw motors and a separate power supply for the brakes. A brake control signal that is reflective of the power supply for the brakes is used by the controller. The controller is configured for evaluating the brake control signal and selectively delaying the yaw function based upon the brake control signal. For example, the brake control signal may be reflective of whether the brake power supply is operational. If the brake power supply is not operational, the yaw function is selectively delayed.

The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate various embodiments of the invention and, together with the general description of the invention given above, and the detailed description of the embodiments given below, serve to explain the embodiments of the invention.

<FIG> illustrates an exemplary wind turbine <NUM> that may implement yaw control in accordance with an embodiment of the invention. The wind turbine <NUM> includes a tower <NUM>, a nacelle <NUM> disposed at the apex of the tower <NUM>, and a rotor <NUM> operatively coupled to a generator in the nacelle <NUM>. In addition to the generator, the nacelle <NUM> typically houses various components needed to convert wind energy into electrical energy and needed to operate and optimize the performance of the wind turbine <NUM>. The tower <NUM> supports the load presented by the nacelle <NUM>, rotor <NUM>, and other wind turbine components housed inside the nacelle <NUM>. The nacelle <NUM> rotates clockwise and counterclockwise with respect to the tower <NUM> under the control of a yaw system as discussed herein. The tower <NUM> of wind turbine <NUM> elevates the nacelle <NUM> and rotor <NUM> to a height above ground level that allows the rotor <NUM> to spin freely and at which the air currents have lower turbulence and higher velocity.

The rotor <NUM> includes a hub <NUM> and one or more (e.g., three) blades <NUM> attached to the hub <NUM> at locations distributed about the circumference of the hub <NUM>. The blades <NUM> project radially outwardly from the hub <NUM>, and are configured to interact with passing air currents to produce rotational forces that cause the hub <NUM> to spin about its longitudinal axis. This rotational energy is delivered to the generator housed within the nacelle <NUM> and converted into electrical power. To optimize performance of the wind turbine <NUM>, the pitch of blades <NUM> is adjusted by a pitch system in response to wind speed and other operational conditions.

<FIG> presents a perspective view in which the nacelle <NUM> is partially sectioned or broken away to expose structures housed inside. A main shaft extending from the rotor <NUM> into the nacelle <NUM> may be held in place by a main bearing support <NUM> which supports the weight of the rotor <NUM> and transfers the loads on the rotor <NUM> to the tower <NUM>. The main shaft may be operatively coupled to a gearbox <NUM> that transfers the rotation thereof to a generator <NUM>. The electrical power produced by the generator <NUM> may be supplied to a power grid (not shown) or an energy storage system (not shown) for later release to the grid as understood by a person having ordinary skill in the art. The yaw system of the invention as discussed herein moves the nacelle so the electrical power is generated efficiently based on wind conditions. In this way, the kinetic energy of the wind may be harnessed by the wind turbine <NUM> for power generation. The nacelle <NUM> may also house other equipment (not shown) used to operate the wind turbine <NUM>, such as hydraulic pumps, hydraulic accumulators, cooling systems, controllers, sensors, batteries, communication equipment, etc..

The weight of the nacelle <NUM> including the components housed therein may be carried by a load bearing structure <NUM>. The load bearing structure <NUM> may include an outer housing of the nacelle <NUM> and one or more additional structural components such as a framework or lattice, and a gear bell which through the yaw bearing <NUM> operatively couples the load of the nacelle <NUM> to the tower <NUM>. (See <FIG>) The yaw bearing <NUM> is configured to allow the nacelle <NUM> to be rotated into or out of the wind by the yaw system. The hub <NUM> may house at least a portion of a pitch system that includes one or more pitch drives that include actuators (e.g., a hydraulic cylinder, electrical actuator, mechanical actuator, etc.) configured to provide a pitch force and rotate the position or pitch of the respective blades <NUM> about pitch bearings <NUM> of rotor <NUM> as desired.

<FIG> illustrates a cross sectional view of a yaw system <NUM> for adjusting the yaw of a nacelle. With reference to <FIG>, the nacelle <NUM> is rotatably supported on a yaw gear <NUM> that is itself supported on the uppermost end of the tower <NUM>. For convenience of illustration, the nacelle <NUM> is shown as a single representative block in <FIG>, whereas in reality the nacelle is a much larger structure, as understood by a person of ordinary skill in the art. The yaw gear <NUM> comprises a yaw bearing <NUM> on which the nacelle <NUM> is mounted so that it is able to rotate with respect to the tower <NUM> and gear <NUM>. The yaw bearing <NUM> may be any type of bearing that would be suitable for such a high-load application, for example a gliding yaw bearing, or a roller yaw bearing could be used, both of which would be well-known to the skilled person.

Rotational movement of the nacelle is driven by one or more yaw drives or drive actuators <NUM>. Only a single drive <NUM> is shown in <FIG> for simplicity, but it is common for several drives to be provided in order to provide the high levels of torque needed to rotate the mass of the nacelle. For example, <FIG> illustrates an example of plurality of yaw drives <NUM> arranged in a circular fashion around a yaw gear <NUM>. Each yaw drive <NUM> may include a yaw motor component <NUM> that may be mounted to the nacelle. For example, the yaw drives <NUM> might be mounted on a main structural component such as a nacelle bed plate or base frame. The yaw drive also includes a yaw gear or pinion gear <NUM> that is engaged with gear teeth <NUM> of the yaw gear <NUM> for yaw control. Part of the gearing in the yaw gear <NUM> of the yaw drive actuator would include suitable reduction gearing for converting the high speed rotation of the yaw motor component <NUM> to a low speed rotation of the yaw gear <NUM>. Typically, the yaw motor <NUM> would be an AC motor and the reduction gearing would be such that the nacelle would take several minutes to turn through a full rotation on the tower.

The yaw drive <NUM> also includes a brake <NUM> that is an electromechanical brake used to provide a braking function for the yaw drive <NUM>. The brake <NUM> operates to provide braking for the yaw motor <NUM> or the maintenance of an inertial load when operated. In one embodiment, the brake <NUM> is a power-off brake that applies braking to the motor for creating an inertial load when power is not applied. When power is applied, the braking force or function is removed and the yaw motor <NUM> and coupled yaw gear <NUM> turn freely. The brake <NUM> may also prevent nacelle rotation by providing a torque counter to the direction in which the nacelle is being urged to rotate, for example due to rotational moments imposed by wind effects.

For braking of the nacelle an additional braking system might also be implemented as well if needed. For example, the wind turbine might utilize a yaw system that includes a mechanical yaw brake <NUM>, as is shown in <FIG>. The yaw brake <NUM> includes a braking surface provided by brake ring or disk <NUM> associated with the tower <NUM> and one or more brake calipers <NUM> associated with the nacelle <NUM>. The brake caliper <NUM> may act in the usual way to apply a braking force to the brake ring <NUM> in order to fix the nacelle <NUM> in position which removes torque from the yaw drives <NUM>. Such a mechanical yaw brake <NUM> may be hydraulically driven or electrically driven. In other embodiments the yaw system does not incorporate a yaw brake <NUM> and relies on the brakes <NUM>.

In one exemplary embodiment of the invention, the yaw system <NUM> incorporates a yaw control system <NUM> to provide monitoring and control over the activity of the yaw drives <NUM> based upon grid conditions. Referring to <FIG>, the yaw control system <NUM> may include a yaw controller <NUM> and a wind direction sensor <NUM>. Note that the yaw controller <NUM> may be implemented as part of a main wind turbine controller as shown in <FIG> which is a schematic view of an exemplary yaw control system <NUM> that illustrates a wind turbine controller <NUM> and an optional separate yaw controller <NUM>. Specifically, the yaw controller <NUM> may be implemented as a dedicated or separate controller including the required processing capability, memory and input/output functions for implementing the invention. Alternatively, the yaw control/controller and its functionality in accordance with the invention may be implemented as part of a main wind turbine controller or control unit <NUM>. As such, the invention is not limited to the physical location of the necessary yaw controller in the wind turbine and so controller <NUM>, <NUM> is referred to for addressing the various scenarios.

The wind sensor <NUM> provides a measure of the relative wind direction to the yaw controller <NUM>, <NUM> and, in turn, the yaw controller <NUM>, <NUM> provides a control signal <NUM> to each of the yaw drives <NUM>. The yaw system <NUM> may also incorporate a monitoring system for determining the yaw gear position, such as a position sensor <NUM>. Referring to <FIG>, the wind turbine controller <NUM> or separate yaw controller <NUM> are coupled to the grid <NUM> for obtaining one or more control signals <NUM> that are reflective of a grid condition of the electric grid. For example, the control signals <NUM> may include information or data regarding grid voltage or frequency conditions and variations. Drive control signals are provided by the controller <NUM>, <NUM> to the ganged drives <NUM>. Typically, the yaw motors <NUM> will be operable to run at a single speed, so the control signals <NUM> will thus activate the yaw drives and motors with an ON signal to run at a single speed and to run either clockwise (CW) or counter-clockwise (CCW), or an OFF or STOP control signal to deactivate the yaw drives. Note that although single speed yawing is common, it is also possible to have a system which uses more than one yawing speed, and also variable yawing speed depending on operational conditions.

<FIG> illustrates a schematic view of a larger exemplary control system <NUM> that may be used to control the wind turbine <NUM>. The control system <NUM> may be configured to implement an embodiment of the invention for the desired yaw control. The control system <NUM> includes a wind turbine controller <NUM> in communication with a wind sensor <NUM>, a pitch system <NUM>, a yaw system <NUM> that interfaces with the nacelle <NUM> as illustrated in <FIG>, and a supervisory controller <NUM>. The supervisory controller <NUM> may be configured to implement a system-wide control strategy for a group of wind turbines <NUM> (e.g., a wind farm) that optimizes the collective performance of the wind turbines <NUM>, e.g., to maximize power production of the group and minimize overall maintenance. The yaw system <NUM> may be controlled by the wind turbine controller <NUM> or by a separate yaw controller <NUM> to control the direction in which the nacelle <NUM> is pointed. As known for wind turbine control, the pitch system <NUM> may be configured to adjust the pitch of the blades <NUM> collectively or independently in response to a pitch command signal received from the wind turbine controller <NUM>. Data from the grid <NUM> is used by the controller <NUM>, <NUM> for yaw control according to the invention.

In accordance with the invention, the yaw system monitors a grid condition, such as a grid voltage condition or frequency condition and provides control of the yawing function based upon the determined grid voltage and/or frequency condition. More specifically, the yaw system selectively delays or prevents yawing during extreme voltage and frequency situations to ensure a more proper and accurate yawing function and to ensure proper operation of the various components, particularly the drive motors of the yaw system. Specifically, referring to <FIG> a yaw control system 40a is illustrated for implementing embodiments of the invention. The elements of control system 40a for controlling the yaw function of a wind turbine <NUM> utilizes like reference numerals, where applicable, with the yaw systems as described herein, such as controller <NUM>, <NUM>. The yaw control system 40a incorporates a power supply suitable for powering various of the components, noted as system power <NUM>. A wind turbine may also have backup power <NUM> that may be utilized for running the wind turbine and yaw system 40a in the absence of system power <NUM>.

In accordance with one feature of the invention, the system power <NUM> or backup power <NUM> may be coupled with the wind turbine controller <NUM> or yaw controller <NUM> depending on where the functionality of the invention is implemented. As noted, the invention may be implemented through operation of the wind turbine controller or through a separate yaw controller <NUM>. <FIG> illustrates control of the various yaw motors <NUM>, by a separate yaw controller <NUM>, but a person of ordinary skill in the art would understand that such yaw motors <NUM> may also be directly coupled with the wind turbine controller <NUM> for control thereby if the yaw controller functionality is implemented in the wind turbine controller.

In accordance with one feature of the invention, as illustrated in <FIG>, the yaw system 40a is coupled with the electric grid <NUM> to receive control signals and various data from the grid. Specifically, one or more grid sensors <NUM> might be implemented as shown in <FIG> for providing one or more control signals <NUM> reflective of the grid conditions, such as a grid voltage or grid frequency condition, in order to control the yawing function based upon varying grid conditions. The grid sensor <NUM> and respective control signal(s) <NUM> may be provided to the wind turbine controller. In various systems, the wind turbine controller is often coupled with the grid to receive such data and/or control signals based upon grid conditions.

In one embodiment of the invention, if the grid voltage is sensed and the sensor <NUM> indicates that the grid conditions are varying and thus the grid conditions and variations may be outside of an acceptable range, the yawing function may be selectively delayed. The sensor signals or other signals are reflective of a grid condition, such as being reflective of a grid voltage of the electric grid, and the signal or signals are provided to a controller that can then evaluate the signal. That is, the controller <NUM>, <NUM> may evaluate the grid condition signal or other signal and would selectively delay the yaw motors <NUM> and yawing would not occur while the grid conditions exist. The grid condition signal or other control signal might be compared to a grid variation range of operation for the electric grid and if the signal is outside the grid variation range, the yaw function may be delayed. Particularly if the grid voltage is outside of the acceptable range for yawing in accordance with the invention, the yawing may be selectively delayed for a few seconds or it may be delayed for up to a few minutes based upon the control signal <NUM>, such as a sensed voltage. In one embodiment of the invention, the yawing may be selectively delayed for a preset delay time. One acceptable operating range for the grid voltage in accordance with the invention might be between -<NUM>% to +<NUM>%. However, it would be understood by a person of ordinary skill in the art that such an acceptable range may vary, within reason, based upon the wind turbine, as well as the specific components of the yaw system.

In accordance with one aspect of the invention, a maximum time limit may be utilized, such as a <NUM> seconds maximum, for the delay of the yawing function. For example, upon a determination that a control signal(s) is outside of the grid variation range, the controller might further be configured to periodically compare the control signal(s) to the grid variation range over time and determine if the delay of the yaw function exceeds the time limit. If the delay exceeds the time limit or after such a maximum time limit has elapsed, the yawing function may be restored even if the sensed control signal <NUM> is still outside of the acceptable range. To delay the yawing function, the various yaw motors <NUM> would not be powered through the wind turbine controller <NUM> or yaw controller <NUM> or rather would receive a STOP control signal.

In accordance with another feature of the invention, the brake elements <NUM> of the yaw system are separated from the grid or other power system <NUM>, <NUM> used to power the yaw system. Specifically, yaw control 40a, as illustrated in <FIG>, provides a separate brake power supply <NUM> coupled with the brakes <NUM> of each of the yaw motors <NUM>. Specifically, the brakes <NUM> are powered independently of the system power or grid power which may be used to power the various yaw motors <NUM>. In one embodiment the invention, the brakes <NUM> are powered separately by a common voltage stabilized power supply <NUM>, such as, for example, a <NUM> volt DC power supply. Such a stabilized power supply <NUM> would be able to keep the voltage power signal to the brakes <NUM> within a stable range of approximately +/-<NUM>% for controlling the brakes during extreme voltage variations on the grid. In that way, the invention ensures that the brakes will disengage properly during extreme grid voltage variations. As such, the motor torque used to provide the yawing function will not have to overcome the brake torque as in current systems wherein the brake functionality is detrimentally affected by the grid variations. This leads to improved yaw capacity and also longer wear lives with respect to the brake <NUM> as well as the yaw motors <NUM>. Furthermore, the yaw motors and various other components of the yaw system are not overloaded.

It will be understood by a person of ordinary skill in the art that other stabilized power supplies might be implemented. Furthermore, the range of +/-<NUM>% might also be adjusted depending upon the operational characteristics of the brakes <NUM>. For example, a larger or smaller range of voltage stability might be used.

In accordance with another feature of the invention, the yaw control system, including yaw controller <NUM>, <NUM> is configured to control the yawing function based upon the ability to provide sufficient braking. A brake control signal <NUM> is provided to the controller <NUM>, <NUM> that is reflective of the power supply <NUM>. For example, the signal <NUM> may be reflective of whether the brake power supply is operational. To that end, the brake power supply <NUM> is coupled to the yaw controller <NUM>, <NUM> through a suitable connection for providing brake control signal <NUM>. The appropriate controller <NUM>, <NUM> monitors or evaluates the brake power supply <NUM> and/or control signal <NUM> or other data regarding the operation of the brake elements <NUM> as related to the yawing function. Based on that evaluation, such as a determination that the brake power supply <NUM> is not operational, the yaw controller <NUM>, <NUM> would selectively delay the yaw function and prevent the yaw motors <NUM> from operating.

Accordingly, the yaw control system of the invention, whether through the wind turbine controller <NUM> or a separate yaw controller <NUM>, would monitor grid conditions, such as through control signal <NUM>, and would also monitor the yaw components such as through the brake power supply <NUM> and control signal <NUM>, in order to make decisions with respect to when the yawing function would be appropriate or whether it should be delayed. The present invention selectively delays yawing in scenarios wherein extreme voltage and frequency situations may occur. Furthermore, the yawing function may be selectively delayed or otherwise prevented if the brakes <NUM> would not operate properly, such as upon the failure of brake power supply <NUM>.

While <FIG> illustrates a system 40a that incorporates both controllers sensing the grid voltage for actuation of the yawing function as well as a separate brake power supply for the yaw motor brakes, each of those features might be implemented separately. For example, if the brakes <NUM> are not operated through a separate brake power supply <NUM>, control of the yawing function through yaw controller <NUM>, <NUM> may still be implemented so as to utilize an acceptable range of grid variation that addresses the brakes. For example, if the brakes will not disengage properly below a -<NUM>% voltage and the brakes are powered based on the grid power, the acceptable range at which the yaw controller <NUM>, <NUM> operates would take that into account. For example, the acceptable limit of grid voltage variation might then be set between - <NUM>% and +<NUM>% or some other upper limit, as long as the grid voltage variation does not proceed below the operational voltage at which the brakes <NUM> are compromised.

Furthermore, an embodiment of the invention may only implement monitoring of the brake power supply <NUM> in affecting or delaying the yawing function, independently of grid variation.

In some scenarios, as illustrated in <FIG>, a backup power unit <NUM> might be coupled with various of the elements, including a wind turbine controller <NUM> or separate yaw controller <NUM> as illustrated. The backup power system <NUM> might be an external backup generator.

The wind turbine controller <NUM> or controller <NUM> as described herein will include the necessary elements, such as a processor and memory, for running a control program to effect the present invention as understood by a person of ordinary skill in the art. Such memory for example may store suitable grid variation ranges against which a control signal <NUM>,<NUM> from the grid may be compared to see if the yaw function should be delayed as described herein. For example, certain acceptable operating ranges reflective of variations from a nominal grid parameter, such as grid voltage might be stored for use in the control scheme to compare against one or more control signals. As discussed herein, one exemplary range might be -<NUM>% to +<NUM>% with respect to the nominal grid voltage for evaluating if the yaw function should be executed or delayed. Other ranges might also be used.

<FIG> illustrates one embodiment of the program flow of the yaw controller <NUM>,<NUM> in accordance with an embodiment of the invention. When a yaw command is presented by the yaw control system, a grid condition may be evaluated, and the protocol may start at block <NUM>. In the embodiment illustrated in <FIG>, the voltage is evaluated. But the grid frequency might also be evaluated based on a control signal <NUM> and compared against a suitable range or other metric.

Referring to <FIG>, a determination is made regarding whether the grid voltage is in an acceptable range at block <NUM>. The control signal <NUM> received by the controller <NUM>, <NUM> might be evaluated. For example, based on the sensed grid voltage it might be evaluated or tested by being compared against a range, such as -<NUM>% to +<NUM>% with respect to a nominal grid voltage, as noted. If the grid voltage is not within or is outside the range, there may be a delay in the yaw function at <NUM> and the controller would not operate the yaw motors to move the nacelle. The comparison of the control signal against a suitable range of conditions may be done periodically. For example, based on a loop path <NUM>, the system may check the grid voltage in relation to the acceptable range on a periodic or loop basis. Once the grid voltage is in an acceptable range, then the yaw function may follow path <NUM>, for example, and may proceed at <NUM>, as discussed herein.

In one embodiment of the invention, a time limit on the delay of the yaw function may be implemented. If a time limit function is implemented, following a determination that the grid voltage is outside of an acceptable range at <NUM>, a further determination could be made of whether the time limit has elapsed at <NUM>. If the time limit has not been reached, the yaw function delay would proceed at <NUM>. As the control loop of the yaw function delay progresses in loop <NUM>, the time limit could be consecutively checked if the grid voltage remains out of the acceptable range. If the time limit has been reached or a set amount of time has elapsed, then the yaw function might proceed at <NUM> despite the grid voltage being out of an acceptable range as shown in <FIG>.

If the grid voltage is within an acceptable range, that is, YES at <NUM>, the yaw function might proceed at <NUM>, such as through path <NUM> as noted. In an alternative embodiment of the invention, the brakes might be powered by an independent brake power supply <NUM> as shown in <FIG> and the supply might be monitored. In such a case, the controller <NUM>, <NUM> might be configured to determine the operational status of the independent brake power supply even if the grid voltage is acceptable or in an acceptable range. Referring to <FIG>, at block <NUM> the brake power supply could be checked by the controller <NUM>, <NUM>. If the brake power supply is operational, the yaw function may proceed at <NUM>. However, if the brake power supply is not operational, the yaw function may be delayed at <NUM>.

In an alternative embodiment of the invention that incorporates an independent brake power supply for the brakes <NUM>, the functionality of the controller <NUM>, <NUM> and the evaluation of the brake power supply at block <NUM> may take place independently in a separate control flow from the evaluation of the grid variation. In that scenario, before a yaw command or function would be executed, the functionality of the brakes through their supply of power could be evaluated at <NUM>, such as be evaluating the control signal <NUM> reflective of the power supply <NUM>. If the power supply is not operational, as reflected by the control signal <NUM>, the brakes would not have power. In such a scenario, the yaw function could be delayed at <NUM>. It will be understood by a person of ordinary skill in the art that the functionality of <FIG> may be implemented in the controller <NUM>,<NUM> as necessary to achieve the noted control of the yaw function.

Claim 1:
A wind turbine (<NUM>) including a nacelle (<NUM>) mounted to move on a tower (<NUM>) for providing electrical power to an electric grid (<NUM>), the wind turbine (<NUM>) comprising:
a yaw system including at least one yaw motor (<NUM>) operable to provide a yaw function for the wind turbine (<NUM>) and move the nacelle (<NUM>) with respect to the tower (<NUM>);
a controller (<NUM>, <NUM>) coupled with the yaw system for controlling the operation of the at least one yaw motor (<NUM>) in the yaw function;
a control signal (<NUM>) provided to the controller (<NUM>, <NUM>) that is reflective of a grid condition of an electric grid (<NUM>);
the controller (<NUM>, <NUM>) configured for evaluating the control signal (<NUM>) and selectively delaying the yaw function based upon the control signal (<NUM>), and proceed with the yaw function after expiry of the delay, characterized in that the controller (<NUM>, <NUM>) includes a time limit and wherein the controller (<NUM>, <NUM>) is further configured to determine if the delay of the yaw function exceeds the time limit, and to proceed with the yaw function if the delay exceeds the time limit.