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, a generator, a gearbox, a nacelle mounted on the tower, and a rotor coupled to the nacelle. The rotor typically includes a rotatable hub and a plurality of rotor blades coupled to and extending outwardly from the hub. The rotor blades capture kinetic energy of wind using known airfoil principles. More specifically, the rotor blades transmit the kinetic energy in the form of rotational energy so as to turn a shaft coupling the rotor blades to the gearbox, or if the gearbox is not used, directly to the generator. The generator then converts the mechanical energy to electrical energy that may be deployed to a power grid. <CIT> relates to a redundant electrical brake and protection system for electric generators. <CIT> relates to dynamic braking on a wind turbine during a fault. <CIT> relates to a method for controlling a wind turbine generator. <CIT> relates to a variable speed wind turbine having an exciter machine and a power converter not connected to the grid.

In a wind turbine generator, such as a doubly fed induction generator (DFIG), a stator is directly connected to the power grid, and a rotor is connected to the power grid via an AC-DC-AC power converter. When the generator is in the power generating mode, an electromagnetic (EM) torque of the generator is controlled by a controller to match a mechanical torque of the wind turbine. If a sudden grid loss event or failure of the converter occurs, the converter loses the ability to control the EM torque, and the EM torque is reduced to zero within <NUM>-<NUM> milliseconds. In contrast, it takes tens of seconds to a couple of minutes for the mechanical torque to reduce to zero when the mechanical torque is reduced only by mechanical operation of pitching out blades of the wind turbine. Due to the sudden loss of EM torque and the slow decaying of the mechanical torque, the rotor may be accelerated to exceed a rated speed even when the blades of the wind turbine are pitched out at the fastest rate feasible. The acceleration of the rotor combined with the loss of aerodynamic thrust due to fast pitching of blades results in high loading on the turbine mechanical structure, especially on the tower, blades, and hub. Therefore, the need to withstand the sudden EM torque loss event usually drives the design of most of wind turbine components.

Large rotors continue to be the most dominant trend in wind industry in recent years as they drive attractive project economics. But, larger rotors, with the heavier mass and higher inertia, lead to increased loads on the turbine mechanical and structural components. It is observed that the maximum loading on the turbine mechanical components is determined by how well the rotor over speeding is controlled during a shut-down event in response to extreme fault of sudden loss of counter torque. As such, for some systems, a mechanical brake is placed on the high-speed shaft to reduce the rotor over speeding. However, the mechanical braking system has certain drawbacks such as sub-rated brake torque (~<NUM> pu), slower kick-in time (<NUM>-<NUM> seconds), as well as wear and tear of its components.

Thus, enhanced braking capability combined with the existing mechanical brake system, could help the wind turbine better manage the loads during the extreme events. In addition, in an effort to provide a smoother turbine shutdown, some modern systems employ a <NUM>-second torque buffer; however, such systems assume converter availability. Thus, there is a need for an improved braking system that addresses the aforementioned issues.

Various aspects and advantages of the invention will be set forth in part in the following description, or may be clear from the description, or may be learned through practice of the invention.

Various features, aspects and advantages of the present invention will become better understood with reference to the following description and appended claims. The accompanying drawings illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention.

Unless defined otherwise, technical and scientific terms used herein have the same meaning as is commonly understood by one of ordinary skill in the art to which this disclosure belongs. The terms "first," "second," and the like, as used herein do not denote any order, quantity, or importance, but rather are used to distinguish one element from another. Also, the terms "a" and "an" do not denote a limitation of quantity, but rather denote the presence of at least one of the referenced items. The term "or" is meant to be inclusive and mean either any, several, or all of the listed items. The use of "including," "comprising," or "having" and variations thereof herein are meant to encompass the items listed thereafter and equivalents thereof as well as additional items. The terms "connected" and "coupled" are not restricted to physical or mechanical connections or couplings, and can include electrical connections or couplings, whether direct or indirect. The terms "circuit," "circuitry," and "controller" may include either a single component or a plurality of components, which are either active and/or passive components and may be optionally connected or otherwise coupled together to provide the described function.

Generally, the present disclosure is directed to a protection system for a wind turbine power system connected to a power grid and a method for operating same. More specifically, the protection system generally includes a main brake circuit, a battery system, and a controller. The main brake circuit has at least one brake resistive element and at least one brake switch element. Further, the brake resistive element is coupled to a DC link of the power converter of the wind turbine power system or to windings of a generator stator or a generator rotor of the wind turbine power system via the brake switch element. Moreover, the battery system is coupled to the generator via a battery switch element and includes at least one battery resistive element. As such, the controller is configured to disconnect the power converter and the generator from the power grid and connect at least one of the main brake circuit or the battery system to the generator in response to detecting an electromagnetic (EM) torque loss event so as to generate an EM torque. Thus, the controller can use the main brake circuit, the battery system, or both, depending upon converter availability, to generate EM torque.

Accordingly, the present disclosure provides many advantages not included in the prior art. For example, the system and method of the present disclosure provides a lower-cost turbine since larger components are not required to handle the EM torque events. Further, the system and method of the present disclosure improves reliability of known braking technologies to cover instances of converter unavailability.

Referring now to the drawings, <FIG> illustrates a perspective view of one embodiment of a wind turbine <NUM>. As shown, the wind turbine <NUM> generally includes a tower <NUM> extending from a support surface <NUM>, a nacelle <NUM> mounted on the tower <NUM>, and a rotor <NUM> coupled to the nacelle <NUM>. The rotor <NUM> includes a rotatable hub <NUM> and at least one rotor blade <NUM> coupled to and extending outwardly from the hub <NUM>. For example, in the illustrated embodiment, the rotor <NUM> includes three rotor blades <NUM>. However, in an alternative embodiment, the rotor <NUM> may include more or less than three rotor blades <NUM>. Each rotor blade <NUM> may be spaced about the hub <NUM> to facilitate rotating the rotor <NUM> to enable kinetic energy to be transferred from the wind into usable mechanical energy, and subsequently, electrical energy. For instance, the hub <NUM> may be rotatably coupled to an electric generator <NUM> (<FIG>) positioned within the nacelle <NUM> to permit electrical energy to be produced.

Referring now to <FIG>, a simplified, internal view of one embodiment of the nacelle <NUM> of the wind turbine <NUM> shown in <FIG> is illustrated. As shown, a generator <NUM> may be disposed within the nacelle <NUM>. In general, the generator <NUM> may be coupled to the rotor <NUM> for producing electrical power from the rotational energy generated by the rotor <NUM>. For example, as shown in the illustrated embodiment, the rotor <NUM> may include a rotor shaft <NUM> coupled to the hub <NUM> for rotation therewith. The rotor shaft <NUM> may, in turn, be rotatably coupled to a generator shaft <NUM> of the generator <NUM> through a gearbox <NUM>. As is generally understood, the rotor shaft <NUM> may provide a low speed, high torque input to the gearbox <NUM> in response to rotation of the rotor blades <NUM> and the hub <NUM>. The gearbox <NUM> may then be configured to convert the low speed, high torque input to a high speed, low torque output to drive the generator shaft <NUM> and, thus, the generator <NUM>.

Additionally, the turbine controller <NUM> may also be located within the nacelle <NUM>. As is generally understood, the turbine controller <NUM> may be communicatively coupled to any number of the components of the wind turbine <NUM> in order to control the operation of such components. For example, as indicated above, the turbine controller <NUM> may be communicatively coupled to each pitch adjustment mechanism <NUM> of the wind turbine <NUM> (one of which is shown) to facilitate rotation of each rotor blade <NUM> about its pitch axis <NUM>.

In general, each pitch adjustment mechanism <NUM> may include any suitable components and may have any suitable configuration that allows the pitch adjustment mechanism <NUM> to function as described herein. For example, in several embodiments, each pitch adjustment mechanism <NUM> may include a pitch drive motor <NUM> (e.g., any suitable electric motor), a pitch drive gearbox <NUM>, and a pitch drive pinion <NUM>. In such embodiments, the pitch drive motor <NUM> may be coupled to the pitch drive gearbox <NUM> so that the pitch drive motor <NUM> imparts mechanical force to the pitch drive gearbox <NUM>. Similarly, the pitch drive gearbox <NUM> may be coupled to the pitch drive pinion <NUM> for rotation therewith. The pitch drive pinion <NUM> may, in turn, be in rotational engagement with a pitch bearing <NUM> coupled between the hub <NUM> and a corresponding rotor blade <NUM> such that rotation of the pitch drive pinion <NUM> causes rotation of the pitch bearing <NUM>. Thus, in such embodiments, rotation of the pitch drive motor <NUM> drives the pitch drive gearbox <NUM> and the pitch drive pinion <NUM>, thereby rotating the pitch bearing <NUM> and the rotor blade <NUM> about the pitch axis <NUM>.

In alternative embodiments, it should be appreciated that each pitch adjustment mechanism <NUM> may have any other suitable configuration that facilitates rotation of a rotor blade <NUM> about its pitch axis <NUM>. For instance, pitch adjustment mechanisms <NUM> are known that include a hydraulic or pneumatic driven device (e.g., a hydraulic or pneumatic cylinder) configured to transmit rotational energy to the pitch bearing <NUM>, thereby causing the rotor blade <NUM> to rotate about its pitch axis <NUM>. Thus, in several embodiments, instead of the electric pitch drive motor <NUM> described above; each pitch adjustment mechanism <NUM> may include a hydraulic or pneumatic driven device that utilizes fluid pressure to apply torque to the pitch bearing <NUM>.

Referring still to <FIG>, the wind turbine <NUM> may also include a plurality of sensors (e.g. such as sensors <NUM>, <NUM>) for monitoring one or more operating parameters and/or wind conditions of the wind turbine <NUM>. As used herein, a parameter or condition of the wind turbine <NUM> is "monitored" when a sensor is used to determine its present value. Thus, the term "monitor" and variations thereof are used to indicate that the sensors <NUM>, <NUM> need not provide a direct measurement of the parameter and/or condition being monitored. For example, the sensors <NUM>, <NUM> may be used to generate signals relating to the parameter and/or condition being monitored, which can then be utilized by the turbine controller <NUM> or other suitable device to determine the actual parameter and/or condition. More specifically, in certain embodiments, the sensor(s) <NUM>, <NUM> may be configured to monitor the speed of the rotor <NUM> and/or the rotor shaft <NUM>, the speed of the generator <NUM> and/or the generator shaft <NUM>, the torque on the rotor shaft <NUM> and/or the generator shaft <NUM>, stator and/or rotor voltage or current, the wind speed and/or wind direction, and/or any other suitable operating parameters and/or conditions.

Referring now to <FIG>, there is illustrated a block diagram of one embodiment of suitable components that may be included within the turbine controller <NUM> (or a converter controller <NUM>) in accordance with aspects of the present disclosure. As shown, the turbine 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 disclosed herein). 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. 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 magneto-optical 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 turbine controller <NUM> to perform various functions including, but not limited to, transmitting suitable control signals to one or more of the wind turbine components, monitoring various parameters and/or conditions of the wind turbine <NUM> and various other suitable computer-implemented functions.

Additionally, the turbine controller <NUM> may also include a communications module <NUM> to facilitate communications between the controller <NUM> and the various components of the wind turbine <NUM>. For instance, the communications module <NUM> may serve as an interface to permit the turbine controller <NUM> to transmit control signals to each pitch adjustment mechanism <NUM> for controlling the pitch angle of the rotor blades <NUM>. Moreover, the communications module <NUM> may include a sensor interface <NUM> (e.g., one or more analog-to-digital converters) to permit signals transmitted from the sensors <NUM> of the wind turbine <NUM> to be converted into signals that can be understood and processed by the processors <NUM>. Further, it should be appreciated that the sensor(s) <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> may be coupled to the sensor interface <NUM> via a wired connection. However, in other embodiments, the sensors <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. It should be understood that the converter controller <NUM> as described herein may include any of the components of the turbine controller <NUM>.

Referring now to <FIG>, a schematic representation of a wind turbine power system <NUM> including a braking system <NUM> according to one embodiment of the present disclosure is illustrated. As mentioned, the wind turbine power system <NUM> includes a plurality of rotor blades <NUM> coupled to the main shaft <NUM>. Further, the generator <NUM> includes a stator <NUM> and a rotor <NUM>. Thus, the rotor <NUM> is mechanically coupled to the main shaft <NUM> via the gearbox <NUM>. Moreover, in certain embodiments, the generator <NUM> may be a doubly fed induction generator (DFIG). In addition, as shown, the wind turbine power system <NUM> also includes a power converter <NUM> having a rotor-side converter <NUM>, a DC link <NUM>, a grid-side converter <NUM>, and a converter controller <NUM>. More specifically, as shown, the DC link <NUM> of the power converter <NUM> is coupled between the rotor-side converter <NUM> and the grid-side converter <NUM>. Further, the DC link <NUM> may include one or more capacitors <NUM> for keeping the voltage variation (ripples) in the DC link voltage small. As such, the converter controller <NUM> is configured to control the various components of the power converter <NUM>, such as the rotor-side converter <NUM>, the DC link <NUM>, and/or the grid-side converter <NUM>. Further, the wind turbine power system <NUM> also includes first and second switch elements <NUM>, <NUM>. More specifically, in certain embodiments, the first switch element <NUM> may be an electric fuse or a circuit breaker and the second switch element <NUM> may be a contactor.

Windings of the stator <NUM> (hereinafter referred as "stator windings") are coupled to a power grid <NUM> via the first and second switch <NUM>, <NUM> as well as a transformer <NUM>. It should be understood that the transformer <NUM> may include a single three-winding transformer as shown as well as two transformers, with one transformer between the stator <NUM> and the grid <NUM> and one transformer between the power converter <NUM> and the grid <NUM>. In such embodiments, the transformer <NUM> is configured to provide three different voltages which may help to avoid the need for medium voltage rated resistive elements and/or a direct-inject (DI) switch. Windings of the rotor <NUM> (hereinafter referred as "rotor windings") are coupled to the power grid <NUM> via the power converter <NUM> and the first switch element <NUM>. Further, the grid-side converter <NUM> is coupled to connection points between the first and second switch elements <NUM>, <NUM>.

Still referring to <FIG>, the braking system <NUM> is provided in accordance with aspects of the present invention for protecting the generator <NUM> and the gearbox <NUM> during loss of electromagnetic (EM) torque (i.e. during an EM torque loss event) in the generator <NUM>. More specifically, in certain embodiments, the EM torque loss events as described herein may include a grid loss and/or a power system trip or fault. Thus, as shown, the braking system <NUM> generally includes a main brake circuit <NUM>, a mechanical brake <NUM> coupled to the main shaft <NUM>, and a battery system <NUM>. Further, the braking system <NUM> may be controller via a controller, such as the converter controller <NUM>. In certain embodiments, the mechanical brake <NUM> includes at least one brake pad <NUM> which may be pressed onto a brake disc <NUM> to cause a friction between the brake pad <NUM> and the brake disc <NUM>, such that the rotational speed of the main shaft <NUM> can be reduced or to inhibit a rotational movement of the main shaft <NUM>. For example, the brake pad <NUM> may be mechanically pressed against the brake disc <NUM>. Moreover, as shown, the main brake circuit <NUM> is coupled to the DC link <NUM>.

In particular embodiments, the main brake circuit <NUM> includes at least one brake resistive element and at least one brake switch element. More specifically, as shown in <FIG>, the main brake circuit <NUM> includes a rotor-side switching element <NUM>, a rotor-side resistive element <NUM>, and a storage element <NUM>. Further, as shown, the rotor-side resistive element <NUM> and the storage element <NUM> are coupled to the DC link <NUM> using the rotor-side switch element <NUM>. The rotor-side resistive element <NUM> may include a resistor, for example, or any power dumping device. The storage element <NUM> may include a battery, for example, or any energy storage device. In one embodiment, the battery may be a rechargeable battery. In another embodiment, the rotor-side switch element <NUM> may include a first DC chopper <NUM> and a second DC chopper <NUM>. In such embodiments, the rotor-side resistive element <NUM> may be coupled to the DC link <NUM> using the first DC chopper <NUM>, whereas the storage element <NUM> may be coupled to the DC link <NUM> using the second DC chopper <NUM>.

The battery system <NUM> is coupled to two phases or three phases of the rotor <NUM>.

Further, <FIG> illustrate various schematic diagrams of multiple embodiments of the battery system <NUM>. As shown, the battery system <NUM> includes a battery <NUM> coupled to the generator <NUM> via a battery switch element <NUM>. Further, as shown in basic diagram of <FIG>, the battery system <NUM> includes a battery resistive element <NUM>.

More specifically, the battery resistive element <NUM> may include a resistor or any other power dumping device. Further, the battery switch element <NUM> may include any suitable switching element such as, for example, a contactor. In further embodiments, as shown in <FIG>, the battery system <NUM> may include a plurality of battery resistive elements <NUM>, with one or more of the battery resistive elements <NUM> having a parallel contactor <NUM>. Alternatively, as shown in <FIG>, the battery system <NUM> may also include at least one inductor <NUM> as well as an additional bidirectional AC-DC or DC-DC converter <NUM>. Thus, the embodiments of <FIG> represent a battery system having controllability.

During normal operations of the wind turbine power system <NUM>, the first and second switch elements <NUM>, <NUM> are turned on. As such, if the generator <NUM> is operated in the super-synchronous mode, the windings of the stator <NUM> supply electric power to the power grid <NUM> via the first and second switch elements <NUM>, <NUM> and the windings of the rotor <NUM> supply electric power to the power grid <NUM> via the power converter <NUM> and the first switch element <NUM>. That is, the wind turbine <NUM> provides a highest output electric power. Alternatively, if the generator <NUM> is operated in the sub-synchronous mode, the windings of the stator <NUM> supply electric power to the power grid <NUM> via the first and second switch elements <NUM>, <NUM> and the windings of the rotor <NUM> draw electric power from the power grid <NUM> via the first switch element <NUM> and the power converter <NUM>. That is, the wind turbine <NUM> provides a reduced output electric power.

In the event of an EM torque loss event, however, the converter controller <NUM> is configured for controlling the first switch element <NUM> to decouple the power converter <NUM> and the generator <NUM> from the power grid <NUM>. Moreover, the converter controller <NUM> is configured for controlling the second switch element <NUM> to couple the windings of the stator <NUM> to the power converter <NUM> and the generator <NUM> in response to the EM torque loss event, such that an output of electric power of the stator windings is transmitted to the rotor side resistive element <NUM> through the second switch element <NUM> and the grid-side converter <NUM> and is consumed by the rotor side resistive element <NUM>. In addition, the converter controller <NUM> is configured to connect the main brake circuit <NUM> and/or the battery system <NUM> to the generator <NUM> in response to detecting an electromagnetic (EM) torque loss event so as to generate an EM torque.

In one embodiment, the loss of EM torque in the generator <NUM> may occur due to failure of the power converter <NUM>, failure of the generator <NUM>, opening of the second switch element <NUM>, loss of stator voltage, grid loss event, etc. As a non-limiting example, a stator voltage sensor and a stator current sensor (not shown) can detect the grid loss event and, if the detected stator voltage and/or the detected stator current is greater or smaller than a predetermined value, the converter controller <NUM> is configured to determine that the grid loss event has occurred.

In additional embodiments, the converter controller <NUM> may be configured to determine whether the power system trip or fault is critical or non-critical. As used herein, a critical power system trip or fault generally corresponds to the power converter <NUM> being unavailable to use for braking, whereas a non-critical power system trip or fault generally corresponds to the power converter <NUM> being available to use for braking. As such, in certain embodiments, if the power converter <NUM> is available, the converter controller <NUM> is configured to activate the main brake circuit <NUM> and/or the battery system <NUM> to generate EM torque. More specifically, in one embodiment, the converter controller <NUM> is configured for controlling the rotor-side switch element <NUM> of the main brake circuit <NUM> to couple at least one of the rotor-side resistive element <NUM> and/or the storage element <NUM> to the DC link <NUM> for generating the EM torque in the generator <NUM> in response to the EM torque loss event, e.g. during a super-synchronous mode of the generator <NUM>. In further embodiments, the converter controller <NUM> is configured for controlling the rotor-side switch element <NUM> to couple the storage element <NUM> to the DC link <NUM> for generating the EM torque in the generator <NUM> in response to the grid loss event during a sub-synchronous mode of the generator <NUM>. In addition, the converter controller <NUM> may be configured to control the battery switch element <NUM> so as to connect the battery resistive element <NUM> to the generator <NUM> to generate the EM torque. As such, the main brake circuit <NUM> may be used alone or in conjunction with the battery system <NUM> to generate the EM torque. In contrast, if the power converter <NUM> is unavailable, the converter controller <NUM> is configured to control the battery switch element <NUM> so as to connect the battery resistive element <NUM> of the battery system <NUM> to the generator <NUM> to generate the EM torque, rather than using the main brake circuit <NUM> (which depends on converter availability).

After activating the main brake circuit <NUM> and/or the battery system <NUM>, the converter controller <NUM> may further be configured to adjust one or more pitch angles of the rotor blades <NUM> to reduce the rotor speed. If the reduced rotor speed is greater than a threshold, the converter controller <NUM> is further configured to activate the mechanical brake <NUM> to shut down the wind turbine power system <NUM>.

Referring now to <FIG>, a schematic representation of another embodiment of the wind turbine power system <NUM> including the braking system <NUM> according to the present disclosure is illustrated. Further, as shown, the braking system <NUM> of <FIG> is configured similar to the embodiment of <FIG>; except that that braking system <NUM> of <FIG> is coupled to the windings of the stator <NUM>. In certain embodiments, it is understood by those skilled in the art that the main brake circuit <NUM> may be coupled to both the DC link <NUM> and the windings of the stator <NUM>. Similar to <FIG>, however, the battery system <NUM> may be coupled to the generator <NUM> via a battery switch element <NUM> and may include a battery resistive element <NUM>. Further like the braking system <NUM> of <FIG>, the converter controller <NUM> is configured for activating the main brake circuit <NUM> and/or the battery system <NUM> to generate the EM torque in the generator <NUM> in response to the loss of EM torque in the generator <NUM>.

More specifically, as shown, the main brake circuit <NUM> includes stator-side switch elements <NUM> and stator side resistive elements <NUM>. The stator-side resistive elements <NUM> are coupled between the stator-side switch elements <NUM> and ground. In other embodiments, the stator-side resistive elements <NUM> are coupled between the stator-side switch elements <NUM> and a predetermined potential. Each of the stator-side resistive elements <NUM> may include a resistor, for example, or any power damping device.

In such an embodiment, the converter controller <NUM> is configured to control the first switch element <NUM> to decouple the windings of the stator <NUM> and the power converter <NUM> from the power grid <NUM> in response to the EM torque loss event. In addition, the converter controller <NUM> is configured to control the stator-side switch elements <NUM> to couple the respective stator-side resistive elements <NUM> to the respective stator windings in response to the EM torque loss event, such that the windings of the stator <NUM> supplies electric power to the stator side resistive elements <NUM> via the stator side switch elements <NUM>.

The converter controller <NUM> is further configured to control the second switch element <NUM> to couple the windings of the stator <NUM> to the power converter <NUM> in response to the EM torque loss event such that the windings of the rotor <NUM> supplies electric power to the stator side resistive elements <NUM> via the power converter <NUM>, the second switch element <NUM>, and the stator side switch elements <NUM> during the super-synchronous mode of the generator <NUM>. Further, the windings of the rotor <NUM> draw current from the windings of the stator <NUM> via the power converter <NUM> and the second switch element <NUM> during the sub-synchronous mode of the generator <NUM>. Therefore, the EM torque is also generated in the generator <NUM>. In addition, the converter controller <NUM> is configured to connect the battery system <NUM> to the generator <NUM> in response to detecting an electromagnetic (EM) torque loss event so as to generate an EM torque.

In additional embodiments, the converter controller <NUM> may be configured to determine whether the power system trip or fault is critical or non-critical. As such, in certain embodiments, if the power converter <NUM> is available, the converter controller <NUM> is configured to activate the main brake circuit <NUM> and/or the battery system <NUM> to generate EM torque. More specifically, in one embodiment, the converter controller <NUM> is configured for controlling the rotor-side switch element <NUM> of the main brake circuit <NUM> to couple the resistive element(s) <NUM> to the DC link <NUM> for generating the EM torque in the generator <NUM> in response to the EM torque loss event, e.g. during a super-synchronous mode of the generator <NUM>. In addition, the converter controller <NUM> may be configured to control the battery switch element <NUM> so as to connect the battery system <NUM> to the generator <NUM> to generate the EM torque. As such, the main brake circuit <NUM> may be used alone or in conjunction with the battery system <NUM> to generate the EM torque. In contrast, if the power converter <NUM> is unavailable, the converter controller <NUM> is configured to control the battery switch element <NUM> so as to connect the battery system <NUM> to the generator <NUM> to generate the EM torque, rather than using the main brake circuit <NUM>.

As described above, the EM torque is regenerated in the generator <NUM> if the loss of EM torque in the generator <NUM> has occurred, but not reduced to zero immediately. As such, the braking system <NUM> of the present disclosure provides braking support that enables the wind turbine power system <NUM> to shut down in a much smoother manner, thereby reducing loads on the mechanical components of the wind turbine power system <NUM>.

Referring now to <FIG>, a flow diagram of one embodiment of a method <NUM> for protecting a wind turbine power system <NUM> connected to a power grid <NUM>, e.g. using the main brake circuit <NUM> and/or the battery system <NUM> as described herein, according to the present disclosure is illustrated. As shown, the method <NUM> starts at <NUM>. As shown at <NUM>, the wind turbine power system <NUM> is operated in a normal operational mode. As shown at <NUM>, the method <NUM> includes monitoring, via one or more sensors (e.g. sensors <NUM>, <NUM>), one or more operating parameters of the power grid <NUM> that can be used by the converter controller <NUM> to determine whether an electromagnetic (EM) torque loss event is occurring. Thus, as shown at <NUM>, the converter controller <NUM> (e.g. via one or more control algorithms stored therein) is configured to determine whether an EM torque loss event is occurring or has occurred based on the sensor data. If no EM torque loss event is detected, the method starts over at <NUM> (i.e. continues to monitor the power system <NUM>). If an EM torque loss event is detected, as shown at <NUM> and <NUM>, the method <NUM> includes disconnecting the power converter <NUM> and the generator <NUM> of the wind turbine power system <NUM> from the power grid <NUM> and connecting the main brake circuit <NUM> and the battery system <NUM> to the generator <NUM> of the wind turbine power system <NUM> so as to generate an EM torque. Further, as shown at <NUM>, the method <NUM> also includes generating the EM torque via at least one of the main brake circuit <NUM> or the battery system <NUM> in response to detecting the EM torque loss event.

Claim 1:
A protection system (<NUM>) for a wind turbine power system (<NUM>) connected to a power grid (<NUM>), the wind turbine power system (<NUM>) having a generator (<NUM>) with a stator (<NUM>) and a rotor (<NUM>) and a power converter (<NUM>) having a rotor-side converter (<NUM>), a grid-side converter (<NUM>), and a DC link (<NUM>) configured therebetween, the protection system (<NUM>) comprising:
a main brake circuit (<NUM>) having at least one brake resistive element and at least one brake switch element, the brake resistive element coupled to at least one of the DC link (<NUM>) of the power converter (<NUM>), windings of the rotor (<NUM>) of the generator (<NUM>), or windings of the stator (<NUM>) of the generator (<NUM>) via the brake switch element;
a battery system (<NUM>) coupled to the generator (<NUM>) via a battery switch element (<NUM>), wherein the battery system (<NUM>) is coupled to two phases or three phases of the rotor (<NUM>), the battery system (<NUM>) comprising a battery resistive element (<NUM>); and
a controller (<NUM>) configured to disconnect the power converter (<NUM>) and the generator (<NUM>) from the power grid (<NUM>) and connect at least one of the main brake circuit (<NUM>) or the battery system (<NUM>) to the generator (<NUM>) in response to detecting an electromagnetic (EM) torque loss event so as to generate an EM torque.