Patent Description:
Wind power is considered one of the cleanest, most environmentally friendly energy sources presently available, and wind turbines have gained increased attention in this regard. A modern wind turbine typically includes a tower, generator, gearbox, nacelle, and one or more rotor blades. The rotor blades capture kinetic energy of wind using known airfoil principles. For example, rotor blades typically have the cross-sectional profile of an airfoil such that, during operation, air flows over the blade producing a pressure difference between the sides. Consequently, a lift force, which is directed from a pressure side towards a suction side, acts on the blade. The lift force generates torque on the main rotor shaft, which is geared to a generator for producing electricity. <CIT> relates to fault detection based on current signature analysis for a generator. <CIT> relates to a method for detecting pitch bearing damage in a wind turbine.

In many wind turbines, the generator may be electrically coupled to a bi-directional power converter that includes a rotor-side converter joined to a line-side converter via a regulated DC link. Such wind turbine power systems are generally referred to as a doubly-fed induction generator (DFIG). DFIG operation is typically characterized in that the rotor circuit is supplied with current from a current-regulated power converter. As such, the wind turbine produces variable mechanical torque due to variable wind speeds and the power converter ensures this torque is converted into an electrical output at the same frequency of the grid.

During operation, wind impacts the rotor blades and the blades transform wind energy into a mechanical rotational torque that drives a low-speed shaft. The low-speed shaft is configured to drive the gearbox that subsequently steps up the low rotational speed of the low-speed shaft to drive a high-speed shaft at an increased rotational speed. The high-speed shaft is generally coupled to the generator so as to rotatably drive a generator rotor. As such, a rotating magnetic field may be induced by the generator rotor and a voltage may be induced within a generator stator. Rotational energy is converted into electrical energy through electromagnetic fields coupling the rotor and the stator, which is supplied to a power grid via a grid breaker. Thus, the main transformer steps up the voltage amplitude of the electrical power such that the transformed electrical power may be further transmitted to the power grid.

Capacitive coupling between the rotor and the stator, though not the main contributor to the generated power, can induce an undesirable rotor shaft voltage in the rotor. Under normal operating conditions, the current driven by the rotor shaft voltage is safely dissipated through a ground brush that is in contact with the rotor and ground. Also, an insulation is provided between the bearing housing and the frame. However, high common-mode voltage injected through the rotor-side inverter can potentially damage the bearing through Electric Discharge Machining (EDM) if the grounding path through the ground brush is degraded and the bearing insulation fails. EDM causes pitting damage that results in grooves appearing in the raceways of the bearing(s) and out-of-roundness in the rolling elements. If left unchecked, these damages can progress beyond acceptable levels, which can potentially cause failure and unplanned shutdown.

In addition, for many wind turbine, the lubricant in the generator bearings degrades differently in different regions owing to the different weather patterns and usage. Re-lubrication is completed on a predetermined schedule, therefore, at times, the lubricant may be discarded when it is still usable.

Thus, the present disclosure is directed to a system and method for detecting degradation and/or damage in the incipient stage by analyzing the rotor-side or line-side three-phase voltages and currents so as to address the aforementioned issues.

Aspects and advantages of the present disclosure will be set forth in part in the following description, or may be obvious from the description, or may be learned through practice of the present disclosure.

In one aspect, the present invention is directed to a method for preventing damage in a bearing of a generator according to independent claim <NUM>. A generator is electrically coupled to a power conversion assembly having a first converter coupled to a second converter. The method includes monitoring, via a controller, one or more electrical signals of the power conversion assembly. The method also includes converting, via the controller, the electricals signal(s) to a frequency domain. Further, the method includes extracting, via the controller, one or more spectral components in frequency bands of the frequency domain around one or more known characteristic frequencies of the bearing. Moreover, the method includes determining, via the controller, characteristics of the spectral component(s) in the frequency bands, wherein determining the characteristics comprises calculating one or more peak magnitudes of the one or more spectral components in the frequency bands, and calculating one or more entropies of the one or more spectral components in the frequency bands. In addition, the method includes comparing, via the controller, the one or more entropies of the spectral component(s) in the frequency bands to an entropy threshold. The method further includes generating, via the controller, a fault signal or a baseline signal for the bearing when the one or more entropies exceeds the entropy threshold; if the one or more entropies is less than the entropy threshold, comparing the one or more peak magnitudes to a magnitude threshold; generating the fault signal when the one or more peak magnitudes is less than the magnitude threshold; and generating the baseline signal when the one or more entropies is less than the entropy threshold and the one or more peak magnitudes is greater than the magnitude threshold. In response to the fault signal being generated, the method includes implementing, via the controller, a control action.

In an embodiment, the electrical signal(s) may include current and/or voltage collected from the first converter. More specifically, in an embodiment, the generator may be part of a wind turbine power system. In such embodiments, the first converter may be a rotor-side converter and the second converter may be a line-side converter of the wind turbine power system. Further, in such embodiments, the current may be a three-phase rotor current and the voltage may be a three-phase rotor voltage of the rotor-side converter or the line-side converter.

In another embodiment, converting the electricals signal(s) to the frequency domain may include calculating a complex rotating vector of the one or more electrical signals and calculating a complex fast Fourier transform (FFT) of the complex rotating vector.

In such embodiments, increases in the one or more entropies and decreases in the one or more peak magnitudes indicates worsening of electrical discharge machining (EDM) damage in the bearing.

In particular embodiments, determining the characteristic(s) of the spectral component(s) in the frequency bands may include calculating a location of one or more peak magnitudes of the one or more spectral components in the frequency bands and monitoring changes in friction using the one or more spectral components in the frequency bands, wherein degrading lubrication causes gradual increases in the friction and changes in the location of the one or more peak magnitudes.

In additional embodiments, comparing the characteristic(s) of the spectral component(s) in the frequency bands to the baseline value(s) may include generating a high-dimensional vector comprising a plurality of characteristics, the plurality of characteristics comprising that least one characteristic and comparing the high-dimensional vector to a baseline cluster comprising similar vectors from a baseline generator.

In further embodiments, the control action may include changing an operating set point of the generator or the power conversion assembly, shutting down the generator, generating an alarm, scheduling a repair, replacing at least one component of the generator, or any other suitable action.

In another aspect, the present disclosure is directed to a method for preventing damage in a bearing of a generator of an electrical power system. The electrical power system has a power conversion assembly with a first converter coupled to a second converter. The power conversion assembly is electrically coupled to the generator. The method includes monitoring, via a controller, one or more electrical signals of the power conversion assembly of the electrical power system. Further, the method includes calculating, via the controller, a complex rotating vector of the one or more electrical signals. Moreover, the method includes calculating, via the controller, a complex fast Fourier transform (FFT) of the complex rotating vector. The method also includes extracting, via the controller, one or more spectral components in frequency bands around one or more known characteristic frequencies of the bearing. The method further includes calculating, via the controller, one or more peak magnitudes in the frequency bands. In addition, the method includes calculating, via the controller, one or more entropies in the frequency bands. Further, the method includes comparing, via the controller, the one or more entropies to an entropy threshold. Moreover, the method includes generating, via the controller, a fault signal for the bearing when the one or more entropies exceeds the entropy threshold. If the one or more entropies does not exceed the threshold, the method also includes comparing, via the controller, the one or more peak magnitudes to a magnitude threshold. The method further includes generating, via the controller, the fault signal for the bearing when the one or more peak magnitudes is less than the magnitude threshold. In addition, the method includes generating, via the controller, a baseline signal for the bearing when the one or more entropies is less than the entropy threshold and the one or more peak magnitudes is greater than the magnitude threshold. In response to the fault signal being generated, the method includes implementing, via the controller, a control action. It should be understood that the method may further include any one or more of the features and/or steps described herein.

In yet another aspect, the present disclosure is directed to a wind turbine power system connected to a power grid according to the independent system claim. A wind turbine power system includes a generator having a rotor, a stator, and at least one bearing. The wind turbine power system also includes a power conversion assembly electrically coupled to the generator. The power conversion assembly includes a rotor-side converter coupled to a line-side converter. Further, the wind turbine power system includes a controller configured to perform a plurality of operations, including but not limited to monitoring one or more electrical signals of the power conversion assembly, converting the one or more electricals signals to a frequency domain, extracting one or more spectral components in frequency bands of the frequency domain around one or more known characteristic frequencies of the bearing, determining characteristics of the one or more spectral components in the frequency bands, wherein determining the characteristics comprises calculating one or more peak magnitudes of the one or more spectral components in the frequency bands, and calculating one or more entropies of the one or more spectral components in the frequency bands, comparing the one or more entropies of the one or more spectral components in the frequency bands to an entropy threshold. The controller is further configured for generating a fault signal or a baseline signal for the bearing when the one or more entropies exceeds the entropy threshold; if the one or more entropies is less than the entropy threshold, comparing the one or more peak magnitudes to a magnitude threshold; generating the fault signal when the one or more peak magnitudes is less than the magnitude threshold; and generating the baseline signal when the one or more entropies is less than the entropy threshold and the one or more peak magnitudes is greater than the magnitude threshold; and in response to the fault signal being generated, implementing a control action. It should be understood that the wind turbine power system may further include any one or more of the features described herein.

These and other features, aspects and advantages of the present disclosure will become better understood with reference to the following description and appended claims. The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the present disclosure and, together with the description, serve to explain the principles of the present disclosure.

Reference now will be made in detail to embodiments of the present disclosure, one or more examples of which are illustrated in the drawings. Each example is provided by way of explanation of the present disclosure, not limitation of the present disclosure. In fact, it will be apparent to those skilled in the art that various modifications and variations can be made in the present disclosure without departing from the scope of the present disclosure.

Generally, the present invention is directed to a system and method for preventing damage in a bearing of a generator (such as a DFIG, a permanent magnet generator (PMG), or any other electrical machine) of an electrical power system (such as a wind turbine power system) due to EDM and/or lubricant degradation. In particular, in an embodiment, three-phase currents and voltages injected by the rotor-side (or line-side) inverter may be analyzed to calculate FFT components around one or more known bearing characteristic frequencies. For EDM damage, the entropies and peaks of these frequency bands can be calculated and cluster changes in the high-dimensional space can be evaluated. Statistical deviation from the baseline cluster, such as gradual increases of entropy and decreases of the peak magnitude indicates worsening of the EDM damage. When lubricant degradation is present, gradual increases in friction and changes in the location of the frequency peaks can be observed in the three-phase currents and voltages. Thus, tracking the location of these peaks and/or the general deviation from a baseline in a multi-variate cluster analysis scenario provides the health metric for the bearing and a means to trend such data for estimating the remaining useful life (RUL) of the bearing. As used herein, the baseline cluster may include a collection of features corresponding to several data files captured from the generator under test, e.g., either around any particular time-period or spread over a certain extended period as necessary. Further, in an embodiment, the baseline cluster may involve features computed from different machines of similar rating/design installed at different places.

As such, the present disclosure provides many advantages not present in the prior art. For example, systems and methods of the present disclosure can achieve bearing diagnostics without the need for any separate sensor for measuring vibration or temperature. Rather, as mentioned, systems and methods of the present disclosure use inverter electrical signals for bearing diagnostics. Moreover, in an embodiment, systems and methods of the present disclosure detect EDM damage and/or lubricant degradation in an incipient stage before such damage begins to show in sensor data. As an example, systems and methods of the present disclosure can estimate ground-brush health through spectrum analysis of the current and can use entropy increases to detect damage. In addition, systems and methods of the present disclosure can use frequencies in addition to known characteristic bearing frequencies to complete high-fidelity bearing failure detection. Thus, intelligent scheduling of maintenance and/or repair activities can be employed.

Referring now to the drawings, <FIG> illustrates a perspective view of one embodiment of a portion of a wind turbine <NUM> according to the present disclosure that is configured to implement the method as described herein. The wind turbine <NUM> includes a nacelle <NUM> that typically houses a generator (not shown). The nacelle <NUM> is mounted on a tower <NUM> having any suitable height that facilitates operation of wind turbine <NUM> as described herein. The wind turbine <NUM> also includes a rotor <NUM> that includes three blades <NUM> attached to a rotating hub <NUM>. Alternatively, the wind turbine <NUM> may include any number of blades <NUM> that facilitates operation of the wind turbine <NUM> as described herein.

Referring now to <FIG>, a schematic view of one embodiment of an electrical power system <NUM> that may be used with the wind turbine <NUM> is illustrated. Furthermore, the wind turbine <NUM> and the electrical power system <NUM> may be referred to herein collectively as a wind turbine power system <NUM>. Thus, during operation of the wind turbine power system <NUM>, 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> (also referred to herein as a rotor shaft) via the hub <NUM>. The low-speed shaft <NUM> is configured to drive a gearbox <NUM> that subsequently steps up the low rotational speed of the low-speed shaft <NUM> to drive a high-speed shaft <NUM> (also referred to herein as a generator shaft) 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 <NUM> (<FIG>).

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 one embodiment, the generator <NUM> is configured to convert the rotational mechanical energy to a sinusoidal, three-phase alternating current (AC) electrical energy signal in the generator stator <NUM>. The associated electrical power can be transmitted to a main transformer <NUM> via a stator bus <NUM>, a stator synchronizing switch <NUM>, a system bus <NUM>, a main transformer circuit breaker <NUM>, and a generator-side bus <NUM>. The main transformer <NUM> steps up the voltage amplitude of the electrical power such that the transformed electrical power may be further transmitted to a grid via a grid circuit breaker <NUM>, a breaker-side bus <NUM>, and a grid bus <NUM>.

In addition, the electrical power system <NUM> may include a controller <NUM> configured to control any of the components of the wind turbine <NUM> and/or implement 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 <NUM> 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, the communications module <NUM> may include a sensor interface <NUM> (e.g., one or more analog-to-digital converters) to permit signals transmitted from one or more sensors to be converted into signals that can be understood and processed by the processors <NUM>. It should be appreciated that the sensors (e.g. sensors <NUM>, <NUM>, <NUM>, <NUM>) may be communicatively coupled to the communications module <NUM> using any suitable means. For example, as shown in <FIG>, the sensors <NUM>, <NUM>, <NUM>, <NUM> may be coupled to the sensor interface <NUM> via a wired connection. However, in other embodiments, the sensors <NUM>, <NUM>, <NUM>, <NUM> may be coupled to the sensor interface <NUM> via a wireless connection, such as by using any suitable wireless communications protocol known in the art. As such, the processor <NUM> may be configured to receive one or more signals from the sensors.

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

Referring back to <FIG>, the generator stator <NUM> may be electrically coupled to a stator synchronizing switch <NUM> via a stator bus <NUM>. In one embodiment, the generator rotor <NUM> may be electrically coupled to a bi-directional power conversion assembly <NUM> or power converter via a rotor bus <NUM>. Alternatively, the generator rotor <NUM> may be electrically coupled to the rotor bus <NUM> via any other device that facilitates operation of electrical power system <NUM> as described herein. In a further embodiment, the stator synchronizing switch <NUM> may be electrically coupled to a main transformer circuit breaker <NUM> via a system bus <NUM>.

The power conversion assembly <NUM> may include a first converter and a second converter. For example, as shown, the first converter may be a rotor-side converter <NUM> and the second converter <NUM> may be a line-side converter <NUM>. Further, as shown, the power conversion assembly <NUM> may include a rotor filter <NUM> that is electrically coupled to the generator rotor <NUM> via the rotor bus <NUM>. In addition, the rotor filter <NUM> may include a rotor-side reactor. A rotor filter bus <NUM> electrically couples the rotor filter <NUM> to the rotor-side power converter <NUM>. Further, the rotor-side power converter <NUM> may be electrically coupled to the line-side power converter <NUM> via a single direct current (DC) link <NUM>. Alternatively, the rotor-side power converter <NUM> and the line-side power converter <NUM> may be electrically coupled via individual and separate DC links. In addition, as shown, the DC link <NUM> may include a positive rail <NUM>, a negative rail <NUM>, and at least one capacitor <NUM> coupled therebetween.

In addition, a line-side power converter bus <NUM> may electrically couple the line-side power converter <NUM> to a line filter <NUM>. Also, a line bus <NUM> may electrically couple the line filter <NUM> to a line contactor <NUM>. In addition, the line filter <NUM> may include a line-side reactor. Moreover, the line contactor <NUM> may be electrically coupled to a conversion circuit breaker <NUM> via a conversion circuit breaker bus <NUM>. In addition, the conversion circuit breaker <NUM> may be electrically coupled to the main transformer circuit breaker <NUM> via system bus <NUM> and a connection bus <NUM>. The main transformer circuit breaker <NUM> may be electrically coupled to an electric power main transformer <NUM> via a generator-side bus <NUM>. The main transformer <NUM> may be electrically coupled to a grid circuit breaker <NUM> via a breaker-side bus <NUM>. The grid circuit breaker <NUM> may be connected to the electric power transmission and distribution grid via a grid bus <NUM>.

Referring particularly to <FIG> and <FIG>, alternating current (AC) power generated at the generator stator <NUM> by rotation of the rotor <NUM> is provided via a dual path to the grid bus <NUM>. The dual paths are defined by the stator bus <NUM> and the rotor bus <NUM>. On the rotor bus side <NUM>, sinusoidal multi-phase (e.g. three-phase) AC power is provided to the power conversion assembly <NUM>. The rotor-side power converter <NUM> converts the AC power provided from the rotor bus <NUM> into DC power and provides the DC power to the DC link <NUM>. Switching elements <NUM> (e.g. IGBTs) used in bridge circuits of the rotor side power converter <NUM> can be modulated to convert the AC power provided from the rotor bus <NUM> into DC power suitable for the DC link <NUM>.

The line side converter <NUM> converts the DC power on the DC link <NUM> into AC output power suitable for the electrical grid bus <NUM>. In particular, switching elements <NUM> (e.g. IGBTs) used in bridge circuits of the line side power converter <NUM> can be modulated to convert the DC power on the DC link <NUM> into AC power on the line side bus <NUM>. The AC power from the power conversion assembly <NUM> can be combined with the power from the stator <NUM> to provide multi-phase power (e.g. three-phase power) having a frequency maintained substantially at the frequency of the electrical grid bus <NUM> (e.g. <NUM>/<NUM>).

It should be understood that the rotor-side power converter <NUM> and the line-side power converter <NUM> may have any configuration using any switching devices that facilitate operation of electrical power system <NUM> as described herein. For example, <FIG> illustrates a simplified schematic diagram of one embodiment of a variable frequency drive (VFD) that maintains a constant electrical frequency output on the grid side of the generator <NUM>. As shown, the VFD configuration includes a six-switch voltage-sourced rectifier on the rotor side converter <NUM>, a DC link capacitor <NUM> to minimize DC voltage variation, and a six-switch voltage-sourced inverter utilizing pulse width modulation on the grid side. Rotor-side switching elements <NUM> are often diodes or silicon controlled rectifiers (SCR), while the grid side-switching elements <NUM> are often insulated gate bipolar transistors (IGBTs). As such, the magnitude and electrical frequency of the current supplied to the generator rotor <NUM> through the VFD may be varied to account for changes in the rotor shaft speed and to maintain a constant output on the generator stator winding.

Further, the power conversion assembly <NUM> may be coupled in electronic data communication with the controller <NUM> and/or a separate or integral converter controller <NUM> to control the operation of the rotor-side power converter <NUM> and the line-side power converter <NUM>. For example, during operation, the controller <NUM> may be configured to receive one or more voltage and/or electric current measurement signals from the first set of voltage and electric current sensors <NUM>. Thus, the controller <NUM> may be configured to monitor and control at least some of the operational variables associated with the wind turbine <NUM> via the sensors <NUM>. In the illustrated embodiment, each of the sensors <NUM> may be electrically coupled to each one of the three phases of the power grid bus <NUM>. Alternatively, the sensors <NUM> may be electrically coupled to any portion of electrical power system <NUM> that facilitates operation of electrical power system <NUM> as described herein. In addition to the sensors described above, the sensors may also include a second set of voltage and electric current sensors <NUM>, a third set of voltage and electric current sensors <NUM>, a fourth set of voltage and electric current sensors <NUM> (all shown in <FIG>), and/or any other suitable sensors.

It should also be understood that any number or type of voltage and/or electric current sensors <NUM>, <NUM>, <NUM>, <NUM> may be employed within the wind turbine <NUM> and at any location. For example, the sensors may be current transformers, shunt sensors, rogowski coils, Hall Effect current sensors, Micro Inertial Measurement Units (MIMUs), or similar, and/or any other suitable voltage or electric current sensors now known or later developed in the art.

Thus, the converter controller <NUM> is configured to receive one or more voltage and/or electric current feedback signals from the sensors <NUM>, <NUM>, <NUM>, <NUM>. More specifically, in certain embodiments, the current or voltage feedback signals may include at least one of line feedback signals, line-side converter feedback signals, rotor-side converter feedback signals, or stator feedback signals. For example, as shown in the illustrated embodiment, the converter controller <NUM> receives voltage and electric current measurement signals from the second set of voltage and electric current sensors <NUM> coupled in electronic data communication with stator bus <NUM>. The converter controller <NUM> may also receive the third and fourth set of voltage and electric current measurement signals from the third and fourth set of voltage and electric current sensors <NUM>, <NUM>. In addition, the converter controller <NUM> may be configured with any of the features described herein in regards to the main controller <NUM>. As such, the converter controller <NUM> is configured to implement the various method steps as described herein and may be configured similar to the controller <NUM>.

Referring now to <FIG> and <FIG>, various illustrations are provided to further describe the systems and methods of the present disclosure. For example, <FIG> illustrates a perspective cross-sectional view of one embodiment of the generator <NUM> of the present disclosure, particularly illustrating a plurality of brushes <NUM> configured with the rotor <NUM> thereof; and <FIG> illustrates a flow diagram of one embodiment of a method <NUM> for preventing damage in a bearing, such as bearings <NUM>, of the generator <NUM> of an electrical power system <NUM>.

As shown particularly in <FIG>, the generator <NUM> may generally include a bearing housing <NUM> for housing the generator stator <NUM> and the generator rotor <NUM>. Further, as shown, the generator rotor <NUM> includes the field winding <NUM> or coil. Moreover, as shown, the bearing housing <NUM> also generally includes bearing insulation <NUM> circumferentially arranged around the field winding <NUM>. In addition, the generator <NUM> may further include one or more bearings <NUM> rotatably mounted into the high-speed shaft <NUM>. Furthermore, the generator <NUM> may include one or more generator slip rings <NUM> with one or more brushes <NUM> secured thereto.

Accordingly, during wind turbine operation, voltage is typically induced on the high-speed shaft <NUM> due to capacitive coupling between the generator rotor <NUM> and the stator <NUM>. The capacitive coupling is directly related to the VFD operation because a change in voltage over time (dv/dt) results in capacitive coupling. In addition, non-symmetry of the generator magnetic field may also cause shaft voltage on the high-speed shaft <NUM>. Under normal operating conditions, the current driven by this induced shaft voltage is safely dissipated through the brushes <NUM>. In general, such brushes <NUM> act as sliding contacts between the rotating slip ring and the stationary bus bars.

Further, such brushes <NUM> are typically constructed from a graphite/metal composite that is pressed into rectangular bar shapes and are typically mounted on the non-drive end of the generator <NUM>. It should be understood that the generator <NUM> may include any suitable number of brushes <NUM>, including particularly at least one brush. Moreover, as shown in <FIG>, the brushes <NUM> may be mounted perpendicular to the high-speed shaft <NUM>. Alternatively, the brushes <NUM> may have any other suitable mounting configuration with respect to the generator rotor <NUM>. As such, some of the brushes <NUM> may be used to pull AC voltage off of at least one of the generator slip rings <NUM>, while the remaining brush(es) act as a controlled path to ground (i.e. a ground brush). Further, as shown, the brushes <NUM> may be positioned radially outward of the generator slip ring(s) <NUM>, with consecutive brushes in each set mounted <NUM> to <NUM> degrees apart. In addition, each brush <NUM> may be rigidly fixed in its axial position so as to minimize deflection under normal operational load. The brush holders may also use a constant pressure coil spring to maintain contact between the brush(es) <NUM> and the generator slip ring(s) <NUM>. A micro switch may also be mounted at the base of the brush holder so as to trigger a warning signal if the brush has worn down too much.

As such, the ground brush <NUM> may be sized and oriented to engage the generator rotor <NUM> to facilitate providing an electrical connection from the generator rotor <NUM> to ground (not shown). Thus, the ground brush <NUM> is configured to protect the generator bearings <NUM> as well as all other components of the generator <NUM> from harmful electrical voltages and currents. More specifically, the ground brush <NUM> is configured to provide a low-impedance path around the bearings <NUM>.

When the generator grounding system is compromised (i.e. the ground brush <NUM> loses contact with the corresponding generator slip ring <NUM> and/or the bearing insulation <NUM> becomes damaged), an excess voltage builds up on the high-speed shaft <NUM>. This shaft voltage drives a current to find the lowest impedance path to ground. In the case of a DFIG, this path is through the bearing housing <NUM>. More specifically, the high-speed shaft <NUM> will accumulate voltage first. If the grounding system is compromised (i.e. the ground brush <NUM> is lifted), all components connected or coupled to the generator rotor <NUM> will also begin to accumulate voltage. The outer race ball pass is where the majority of the discharge occurs due to the presence of lubricating oil. The oil has a low dielectric capacity and breaks down quicker than the surrounding air or other dielectric materials. Once this voltage exceeds the dielectric capacity of the lubricating oil in the bearing, it will discharge in a pulse, causing an arc. This cycle will repeat as long as the ground brush <NUM> is lifted, causing pitting and fluting of the bearing track, ultimately leading to bearing failure. In addition, the aforementioned issues also cause damage to the bearing insulation <NUM>.

As such, the present disclosure is directed to preventing such bearing damage, such as EDM damage. More specifically, as shown in <FIG>, a flow diagram of one embodiment of the method <NUM> for preventing damage in a bearing, such as bearings <NUM>, of the generator <NUM> due to EDM is shown. In general, the method <NUM> is described herein with reference to the wind turbine <NUM>, the electrical power system <NUM>, the controller <NUM>, and the generator <NUM> illustrated in <FIG>. However, it should be appreciated that the disclosed method <NUM> may be implemented with electrical power systems having any other suitable configurations. In addition, although <FIG> depicts steps performed in a particular order for purposes of illustration and discussion, the methods discussed herein are not limited to any particular order or arrangement. One skilled in the art, using the disclosures provided herein, will appreciate that various steps of the methods disclosed herein can be omitted, rearranged, combined, and/or adapted in various ways without deviating from the scope of the present disclosure.

As shown at (<NUM>), the method <NUM> includes monitoring, via a controller (such as controller <NUM>), one or more electrical signals of the power conversion assembly <NUM>. For example, in an embodiment, the operating parameter(s) may include current and/or voltage collected from the rotor-side power converter <NUM> or the line-side converter <NUM>. More specifically, <FIG> illustrates a detail method <NUM> that, as shown at (<NUM>), includes collecting three-phase currents and/or three-phase voltages from the rotor-side power converter <NUM> (IA, IB, and IC).

Referring back to <FIG>, as shown at (<NUM>), the method <NUM> includes converting, via the controller <NUM>, the electricals signal(s) to a frequency domain. More specifically, as shown at (<NUM>) of <FIG>, the controller <NUM> is configured to convert the electricals signal(s) to the frequency domain by calculating a complex rotating vector of the one or more electrical signals, e.g., using Equation (<NUM>) below: <MAT> where <MAT> is a complex number indicating a <NUM>-degrees phase shift and IA, IB and IC are instantaneous currents in the phases A, B and C respectively. I is the instantaneous current complex number vector.

Further, as shown at (<NUM>) of <FIG>, the controller <NUM> also calculates a complex fast Fourier transform (FFT) of the complex rotating vector.

Referring to <FIG> and <FIG>, as shown at (<NUM>), the method <NUM> thus includes extracting, via the controller <NUM>, one or more spectral components in frequency bands of the frequency domain around one or more known characteristic frequencies of the bearing <NUM>. Thus, as shown at (<NUM>) of <FIG>, the controller <NUM> is also configured to calculate the bearing characteristic frequencies for the bearing dimensions and/or size.

Accordingly, as shown at (<NUM>) of <FIG>, the method <NUM> further includes determining, via the controller <NUM>, at least one characteristic of the spectral component(s) in the frequency bands. More specifically, as shown at (<NUM>) of <FIG>, the controller <NUM> may calculate one or more peak magnitudes of the spectral component(s) in the frequency bands. Moreover, as shown at (<NUM>), the controller may also calculate one or more entropies of the spectral component(s) in the frequency bands.

As shown at (<NUM>), the method <NUM> includes comparing, via the controller <NUM>, the characteristic(s) of the spectral component(s) in the frequency bands to at least one baseline value. More specifically, as shown at (<NUM>) and (<NUM>) of <FIG>, in one embodiment, the controller <NUM> is configured to compare the one or more entropies to an entropy threshold and the one or more peak magnitudes to a magnitude threshold, respectively. In certain embodiments, as an example, the entropy can be calculated using Equation (<NUM>) below: <MAT> where Ai is the ith component of the amplitude spectrum.

In alternative embodiments, the controller <NUM> may be configured to generate a high-dimensional vector including a plurality of characteristics and comparing the high-dimensional vector to a baseline cluster having similar vectors from a baseline generator. In such embodiments, deviations from the baseline cluster provides the health metric of the generator bearing.

Moreover, and referring back to <FIG>, as shown at (<NUM>), the method <NUM> includes generating, via the controller <NUM>, a fault signal or a baseline signal for the bearing based on the comparison. More particularly, as shown at (<NUM>) of <FIG>, the controller <NUM> is configured to generate the fault signal when the one or more entropies exceeds the entropy threshold. In contrast, if the one or more entropies is less than the entropy threshold, as shown at (<NUM>), the controller <NUM> is configured to compare the one or more peak magnitudes to the magnitude threshold. Moreover, as shown at (<NUM>), the controller <NUM> is configured to generate the fault signal when the one or more peak magnitudes is less than the magnitude threshold. Further, as shown at (<NUM>), the controller <NUM> is configured to generate the baseline signal when the one or more entropies is less than the entropy threshold and the one or more peak magnitudes is greater than the magnitude threshold.

In such embodiments, increases in the one or more entropies and decreases in the one or more peak magnitudes indicates worsening of EDM damage in the bearing. For example, as shown in <FIG>, a graphical representation <NUM> of the impact of EDM on the current spectrum in a generator according to the present disclosure is illustrated. In particular, as shown, the graphical representation <NUM> depicts a frequency domain of a baseline bearing spectrum <NUM> and a faulty bearing spectrum <NUM>. Thus, the graphical representation <NUM> illustrates how the faulty bearing spectrum <NUM> has increased entropy and a lower peak magnitude than the baseline bearing spectrum <NUM>.

Referring back to <FIG>, in response to the fault signal being generated, as shown at (<NUM>), the method <NUM> includes implementing, via the controller <NUM>, a control action. For example, in particular embodiments, the control action may include changing an operating set point of the wind turbine <NUM>, shutting down the wind turbine <NUM>, generating an alarm, scheduling a repair, replacing at least one of the bearing insulation <NUM> or the ground brush <NUM>, and/or any other suitable action.

Referring now to <FIG>, a flow diagram of another embodiment of a method <NUM> for preventing damage in a bearing of a generator according to the present disclosure is illustrated. As shown at (<NUM>), the method <NUM> includes collecting three-phase currents and/or three-phase voltages from the rotor-side power converter <NUM> and/or the line-side converter <NUM> (e.g., IA, IB, and IC). Moreover, as shown at (<NUM>), the method <NUM> includes calculating a complex rotating vector of the one or more electrical signals, e.g., using Equation (<NUM>) below: <MAT> where <MAT> is a complex number indicating a <NUM>-degrees phase shift and IA, IB and IC are instantaneous currents in the phases A, B and C respectively. I is the instantaneous current complex number vector. In addition to utilizing the complex rotating vector of current as described above, the present disclosure also encompasses utilizing spectral information from other quantities derived from voltages and current, such as instantaneous real power, reactive power, etc., which can be used for PMGs and/or other types of electrical machines.

Further, as shown at (<NUM>), the method <NUM> includes calculating a complex FFT of the complex rotating vector. Thus, as shown at (<NUM>), the controller <NUM> is also configured to calculate the bearing characteristic frequencies for the bearing dimensions and/or size. As shown at (<NUM>), the method <NUM> includes extracting spectral components in frequency bands around the bearing characteristic frequencies. As shown at (<NUM>), the method <NUM> then monitors the peak magnitudes of the spectral components and tracks the location of the peak magnitudes in the frequency bands around the bearing characteristic frequencies.

Accordingly, as shown at (<NUM>), the method <NUM> includes determining whether the peak location has changed. If so, as shown at (<NUM>), the controller <NUM> is configured to indicate that a friction changed has occurred in the bearing. If not, the method proceeds to (<NUM>) and determines whether the peak magnitude has changed. If so, as shown at (<NUM>), the controller <NUM> is configured to indicate that bearing damage has started. If not, as shown at (<NUM>), the controller <NUM> is configured to indicate that the bearing is representative of a baseline generator (e.g., the bearing is healthy and/or able to continue operating as-is).

Referring now to <FIG>, a graphical representation <NUM> of the impact of lubricant degradation on the current spectrum in a generator according to the present disclosure is illustrated. In particular, as shown, the graphical representation <NUM> depicts a frequency domain of spectral components in frequency bands of the bearing. More specifically, as shown, the graphical representation <NUM> illustrates arrow <NUM> that depicts the increase of the characteristic frequency with improving lubricant health.

<FIG> illustrates a flow diagram of yet another embodiment of a method <NUM> for preventing damage in a bearing of a generator according to the present disclosure. As shown, the plot <NUM> illustrates baseline <NUM> and faulty <NUM> frequency component movement for the same generator speed and compares the entropies of the healthy and faulty components. Thus, as shown at (<NUM>), the method <NUM> calculates the peak magnitude and angle of each of the healthy and faulty components. As shown at (<NUM>), the method <NUM> includes estimating the anomaly level of the peak values. As shown at (<NUM>), the method <NUM> includes determining whether the peak is high enough. If yes, as shown at (<NUM>) and (<NUM>), the method <NUM> includes considering the spectrum around the peak and determining the entropy thereof. If no, as shown at (<NUM>) and (<NUM>), the method <NUM> includes considering the full spectrum and determining the entropy thereof.

Various aspects and embodiments of the present invention are defined and summarized by the following numbered clauses:.

Claim 1:
A method (<NUM>) for preventing damage in a bearing (<NUM>) of a generator (<NUM>), the generator (<NUM>) being electrically coupled to a power conversion assembly (<NUM>) with a first converter coupled to a second converter, the method (<NUM>) comprising:
monitoring, via a controller (<NUM>), one or more electrical signals of the power conversion assembly (<NUM>);
converting, via the controller (<NUM>), the one or more electrical signals to a frequency domain;
extracting, via the controller (<NUM>), one or more spectral components in frequency bands of the frequency domain around one or more known characteristic frequencies of the bearing (<NUM>);
determining, via the controller (<NUM>), characteristics of the one or more spectral components in the frequency bands, wherein determining the characteristics comprises calculating one or more peak magnitudes of the one or more spectral components in the frequency bands, and calculating one or more entropies of the one or more spectral components in the frequency bands;
comparing, via the controller (<NUM>), the one or more entropies of the one or more spectral components in the frequency bands to an entropy threshold;
generating, via the controller (<NUM>), a fault signal or a baseline signal for the bearing (<NUM>) when the one or more entropies exceeds the entropy threshold;
if the one or more entropies is less than the entropy threshold, comparing the one or more peak magnitudes to a magnitude threshold;
generating the fault signal when the one or more peak magnitudes is less than the magnitude threshold; and
generating the baseline signal when the one or more entropies is less than the entropy threshold and the one or more peak magnitudes is greater than the magnitude threshold; and
in response to the fault signal being generated, implementing, via the controller (<NUM>), a control action.