WO2016095045A1

WO2016095045A1 - Portable electric braking system for wind turbines with induction generators

Info

Publication number: WO2016095045A1
Application number: PCT/CA2015/051337
Authority: WO
Inventors: Seyed Mahda JANABALI JAHROMI; Daryl Musselman
Original assignee: Endurance Wind Power Inc.
Priority date: 2014-12-19
Filing date: 2015-12-16
Publication date: 2016-06-23
Also published as: CA2875840A1

Abstract

Systems and processes for stopping a wind turbine having an induction generator are provided. An example process includes connecting outputs of a portable voltage sourced converter to the generator stator leads, applying a probing waveform from the portable voltage sourced converter, the probing waveform having a first amplitude and a probing frequency configured to induce a rotating magnetic field at a probing rotational velocity, measuring current and voltage on the generator leads to determine a back EMF, adjusting the probing frequency until the back EMF is substantially zero to determine an initial generator rotor rotational velocity, applying a braking waveform having a second amplitude higher than the first amplitude and having a braking frequency configured to induce a rotating magnetic field that rotates slightly slower than the generator rotor velocity, and, adjusting the braking waveform to ramp down the braking rotational velocity to a target rotational velocity.

Description

PORTABLE ELECTRIC BRAKING SYSTEM FOR WIND TURBINES WITH

INDUCTION GENERATORS

Cross-Reference to Related Application

[0001] This application claims priority from Canadian Patent Application No. 2,875,840 filed December 19, 2014, which is entitled PORTABLE ELECTRIC BRAKING SYSTEM FOR WIND TURBINES WITH INDUCTION GENERATORS. For purposes of the United States of America, this application claims the benefit under 35 U.S.C. §1 19 of Canadian Patent Application No. 2,875,840 filed December 19, 2014, which is hereby incorporated herein by reference for all purposes.

Technical Field

[0002] The present disclosure relates to wind turbines. Background

[0003] Most wind turbines have braking mechanisms that can be used to stop the rotor from spinning in the event of a malfunction, to permit maintenance or repairs, or other situations. Some wind turbines have secondary braking mechanisms to stop or slow the rotor if the primary braking mechanism fails. However, if there is no secondary braking mechanism, or if the secondary braking mechanism is not capable of stopping a malfunctioning turbine, the turbine could spin for an extended period of time before the rotor stops and the turbine may be safely accessed. For example, if a turbine's primary brakes fail, secondary braking mechanisms may be insufficient to stop the turbine or may cause excessive wear or damage to the turbine if engaged.

[0004] The inventors have identified a need for improved apparatus and methods for stopping wind turbine rotors.

Summary

[0005] The present disclosure provides systems and methods for electrically braking an induction generator-based wind turbine. Some embodiments provide portable apparatus that may be moved to the site of a wind turbine to stop the turbine, for example in the case of a malfunction in the turbine's primary brakes.

[0006] One aspect provides a process for stopping a wind turbine having an induction generator with a generator rotor connected to be driven by the wind, and a generator stator having a plurality of generator leads for connecting the generator to a power grid. The process comprises connecting outputs of a portable voltage sourced converter to the generator leads, applying a probing waveform from the portable voltage sourced converter to the generator leads, the probing waveform comprising an AC electrical signal having a first amplitude and a probing frequency configured to induce a magnetic field in the stator that rotates at a probing rotational velocity, measuring current and voltage on the generator leads to determine a back EMF from the generator in response to the probing waveform, adjusting the probing frequency until the back EMF is substantially zero to determine an initial generator rotor rotational velocity, applying a braking waveform having a second amplitude initially higher than the first amplitude and having a braking frequency configured to induce a magnetic field in the stator that rotates at a braking rotational velocity slightly lower than the initial generator rotor velocity, and, adjusting the braking waveform to ramp down the braking rotational velocity to a target rotational velocity.

[0007] Another aspect provides a portable electric braking system for a wind turbine having an induction generator with a generator rotor connected to be driven by the wind, and a generator stator having a plurality of generator leads for connecting the generator to a power grid. The system comprises a voltage sourced converter having a power unit with input connectable to a power source and an output connectable to stator leads of the wind turbine generator, a controller connected to control operation of the power unit to cause the power unit to apply a probing waveform from the portable voltage sourced converter to the generator leads, the probing waveform comprising an AC electrical signal having a first amplitude and a probing frequency configured to induce a magnetic field in the stator that rotates at a probing rotational velocity, measure current and voltage on the generator leads to determine a back EMF from the generator in response to the probing waveform, adjust the probing frequency until the back EMF is substantially zero to determine an initial generator rotor rotational velocity, apply a braking waveform having a second amplitude higher than the first amplitude and having a braking frequency configured to induce a magnetic field in the stator that rotates at a braking rotational velocity slightly lower than the initial generator rotor velocity, and, adjust the braking waveform to ramp down the braking rotational velocity to a target rotational velocity.

[0008] Another aspect provides an electric braking system for a wind turbine having an induction generator with a generator rotor connected to be driven by the wind, and a generator stator having a plurality of generator leads for connecting the generator to a power grid. The system comprises a voltage sourced converter having a power unit with input connectable to the power grid and an output connected to stator leads of the wind turbine generator, a controller connected to control operation of the power unit to cause the power unit to apply a probing waveform from the portable voltage sourced converter to the generator leads, the probing waveform comprising an AC electrical signal having a first amplitude and a probing frequency configured to induce a magnetic field in the stator that rotates at a probing rotational velocity, measure current and voltage on the generator leads to determine a back EMF from the generator in response to the probing waveform, adjust the probing frequency until the back EMF is

substantially zero to determine an initial generator rotor rotational velocity, apply a braking waveform having a second amplitude higher than the first amplitude and having a braking frequency configured to induce a magnetic field in the stator that rotates at a braking rotational velocity slightly lower than the initial generator rotor velocity, and, adjust the braking waveform to ramp down the braking rotational velocity to a target rotational velocity.

[0009] Other aspects and features of the present disclosure will become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments in conjunction with the accompanying figures.

Drawings

[0010] The following figures set forth embodiments in which like reference numerals denote like parts. Embodiments are illustrated by way of example and not by way of limitation in the accompanying figures. [0011] FIG. 1 shows an example wind power installation wherein a portable braking system is being used to stop a wind turbine.

[0012] FIG. 2 schematically illustrates circuit elements of an example portable electric braking system for stopping a wind turbine.

[0013] FIG. 3 is a flowchart showing steps of an example process for stopping a wind turbine.

[0014] FIG. 4A is a graph showing example rotational velocities over time for a generator rotor and an induced magnetic field in a generator stator of a wind turbine being braked by an electric braking system according to one embodiment.

[0015] FIG. 4B is a graph showing an example voltage over time for a generator stator of a wind turbine being braked by an electric braking system according to one embodiment.

[0016] FIG. 5A is a graph showing example rotational velocities over time for a generator rotor and an induced magnetic field in a generator stator of a wind turbine being braked by an electric braking system according to one embodiment.

[0017] FIG. 5B is a graph showing example power over rotational velocity of the wind turbine of the FIG. 5A example.

[0018] FIG. 6A schematically illustrates an example portable electric braking system according to one embodiment.

[0019] FIG. 6B schematically illustrates an example portable electric braking system according to another embodiment.

Detailed Description

[0020] The following describes a portable electric braking system for a wind turbine. The example portable electric braking systems disclosed herein may be utilized in situations in which a primary and/or secondary braking system of the wind turbine have failed, are malfunctioning, or are otherwise unable to stop the turbine.

[0021] For simplicity and clarity of illustration, reference numerals may be repeated among the figures to indicate corresponding or analogous elements.

Numerous details are set forth to provide an understanding of the examples described herein. The examples may be practiced without these details. In other instances, well- known methods, procedures, and components are not described in detail to avoid obscuring the examples described. The description is not to be considered as limited to the scope of the examples described herein.

[0022] FIG. 1 shows a wind turbine 10 coupled to an electric braking system 100. The wind turbine 10 includes a tower 12 that supports a nacelle 14, which houses an AC generator (not shown) comprising a generator rotor and a generator stator. The nacelle 14 supports a wind rotor 16 having a plurality of blades 18. Wind blowing across the blades 18 generates a rotational force or "torque" dependent on wind velocity which spins the wind rotor 16. The wind rotor 16 is coupled to a generator rotor, for example by a drive train (not shown) such that rotation of the wind rotor 16 rotates the generator rotor to convert the mechanical energy of the rotation of the wind rotor 16 to electrical energy, as is known in the art. The generator is connected to provide electrical power to an electrical grid (not shown) through a master control cabinet 20.

[0023] The generators of some wind turbines 10 are asynchronous/induction generators. In an asynchronous/induction generator, the generator stator has a plurality of coils configured such that when the stator leads are energized with AC electric power (e.g. from the grid) the stator produces an induced magnetic field that rotates at a "synchronous" speed. The synchronous speed depends on the number of poles of the generator and the frequency of the grid that the generator is connected to. For example, a 4 pole AC generator on a 50 Hz grid has a synchronous speed of 1500 rpm, whereas the same generator on a 60 Hz grid has a synchronous speed of 1800 rpm. In order to produce electrical power, the generator rotor of an asynchronous generator is driven to rotate at a rotational speed slightly greater than the synchronous speed, which causes current to flow from the generator stator. The difference between the generator rotor speed and the synchronous speed is known as the "slip," which is often expressed as a percentage of the synchronous speed. In some generators, power is typically generated with a slip of about 0.5 to 3% of the synchronous speed (e.g. about 1508- 1545 rpm for a synchronous speed of 1500 rpm). If the generator rotor speed exceeds a safe slip threshold, the turbine can exceed recommended operating conditions, which can lead to increased risk of mechanical or electrical failures. For example, some turbines have a maximum generator speed of 1 12% of the synchronous speed (e.g. 1680 rpm where the synchronous speed is 1500 rpm), and an overspeed shutdown may triggered if the generator speed exceeds the maximum, wherein the generator is disconnected from the electrical grid and brakes are applied to slow or stop the wind rotor. A generator may also be shut down for other reasons, such as for example to perform maintenance or repairs, or if an operator presses an emergency stop button. During a shutdown, a main contactor connecting the generator to the grid opens (thus de-energizing the stator and removing the stator magnetic field) and a braking mechanism is engaged (e.g. by clamping brake calipers on a braking disc, or other suitable mechanism). However, if the brakes fail to stop the turbine, for example due to worn brake pads or another malfunction, the wind rotor can continue to rotate, and may continue to increase its speed if the wind is still blowing.

[0024] Some wind turbines 10 may include a rotor 16 having an automatic mechanical blade pitching functionality that rotates the blades 18 to depower the wind turbine 10 when the rotational velocity of the wind rotor exceeds a threshold. Automatic mechanical blade pitching can advantageously depower the turbine 10 without the need for grid power or an active control system. Automatic mechanical blade pitching may be provided, for example by coupling each of the blades 18 to the wind rotor 16 (or in some implementations, coupling a tip portion of the blades to a base portion of the blades) with a coupling comprising a helical track and providing spring tension toward the center of the wind rotor 16. When the rotational velocity of the wind rotor 16 increases to the point that the centrifugal force on the blades 18 (or tips thereof) overcomes the spring tension (which may be referred to as the "blade-pitching" threshold speed), each blade 18 (or tip thereof) moves outward from the wind rotor 16 (or base of the blade) in the helical track causing the blade 18 (or tip thereof) to rotate about an axis along the length of the blade 18 such that the wind exerts less torque on the blade and the turbine 10 is depowered. Once the velocity of rotation of the wind rotor 16 is sufficiently reduced, the spring tension may pull the blades 18 (or tips thereof) back to the default position. Alternatively, the blades 18 (or tips thereof) may lock in place once they have been pitched. [0025] The electric braking system 100 is electrically coupled to the generator of the wind turbine 10, for example through an access panel or the like on the master control cabinet 20. The electric braking system 100 is preferably located at some safe distance from the base of the tower 12, as it can be dangerous to approach a spinning wind turbine. In installations where the master control cabinet 20 is located at the base of the tower 12, the system 100 can be located at a point further from the tower 12 than the master control cabinet 20 and an electrical connection to the generator leads may be established at such further point. As discussed further below, the electrical braking system 100 is typically connected between the generator stator leads and the electrical grid. In some embodiments the electrical braking system 100 may be operable without a grid connection, and could be powered by an alternate power supply such as a generator or the like, provided that the system 100 is capable of dissipating the electrical power generated by the braking, as discussed below.

[0026] The electric braking system 100 applies a braking waveform to the leads of the generator of the wind turbine 10. As described in more detail below, the braking waveform induces a rotating magnetic field that has a rotational velocity slightly lower (e.g., 0.5-1 % lower) than an initial rotational velocity of the generator rotor, to induce a braking force on the generator rotor. The rotational velocity of the braking waveform is then ramped down (e.g., by following a control setpoint or braking profile determined based on characteristics of the generator) to slow the wind rotor 16 utilizing the electric torque generated by the generator.

[0027] The electric braking system 100 shown in FIG. 1 is a self-contained portable unit that may be brought to the site of a malfunctioning wind turbine 10. The portable electric braking system 100 may be connected to the generator without an operator having to climb the tower 12 or approach the immediate area of the tower 12, which may be unsafe when the wind rotor 16 is spinning. By being provided as a portable unit, a single electric braking system 100 can be used to service multiple turbines and the additional expense of providing built-in electric braking systems in each turbine can be avoided.

[0028] As discussed in more detail below, the electric braking system 100 may be controlled remotely through, for example, connectivity over a local area network, the internet, or a cellular network. Controlling the electric braking system 100 from a remote location may be desirable when the rotor 16 is rotating at a sufficiently high velocity that approaching the wind turbine 10 would be dangerous due to, for example, the risk of mechanical failure of the rotor 16, or so that, for example, personnel may control the braking process from a remote location, without having to physically be onsite near the turbine 10.

[0029] FIG. 2 shows a schematic view of an electric braking system 100 connected to a generator 200 and to an electrical grid 300. As shown in FIG. 1 the electric braking system 100 is connected between the generator 200 and the electrical grid 300.

[0030] The electric braking system 100 includes a power module 102 for providing the braking voltage waveform to the generator, a controller 104 for controlling the power module 102, an interface 106 for the controller 104, a transformer 108 for powering the electric braking system 100 utilizing energy from the electrical grid 300. Alternatively, the electric braking system 100 may receive power from another power source (not shown) in the event the grid power is down. In the illustrated example, the electric braking system 100 also includes a network interface 1 10 for enabling the control of the electric braking system 100 remotely. The network interface 1 10 may be configured to connect the electric braking system 100 to a local area network (LAN), the internet, or a cellular network, to facilitate controlling and monitoring the electric braking system 100 remotely.

[0031] The power module 102 utilized in the electric braking system 100 may comprise, for example, a voltage sourced converter. An example of a voltage sourced converter that is suitable for use in an electric braking system 100 as described herein is the Sinamics G120 manufactured by Siemens^®. The capabilities of the voltage sourced converter may be selected based on the characteristics of the generator 200.

[0032] The electrical grid 300 shown in FIG. 2 is a three-phase electrical grid having three phase lines 302, 304, 306 and a ground line 308. The three phase lines 302, 304, 306 of the electrical grid 300 are coupled to inputs 103 of the power module 102. The phase lines 302, 304, 306 may be coupled to the power module 102 through a circuit protection element 1 12 (e.g. a circuit breaker, or three fuses with one fuse in each line) which protects the power module 102 from over current conditions. The circuit protection element 1 12 may, for example, be configured to disconnect the power module 102 from the electrical grid 300 within a certain period of time from when the current through the circuit protection element 1 12 exceeds a predetermined threshold, depending on the time-current curve of the circuit protection element 1 12. In some embodiments, the predetermined threshold may be about 150A. The illustrated example contemplates a three-phase grid, and a correspondingly configured generator, but it is to be understood that the system 100 could be adapted for use with

grids/generators with different numbers of phases/poles, different voltages, different grid frequencies, etc.

[0033] Outputs 105 of the power module 102 are electrically connected to the leads of the generator 200 (e.g., through the master control cabinet 20, possibly via a more distant connection point) such that the electric braking system 100 is in series between the generator 200 and the electrical grid 300. In some embodiments, the electric braking system 100 may take the place of the grid 300, and may provide an alternate power source and/or current sink, as discussed further below with respect to FIGS 6A and 6B.

[0034] The controller 104 is coupled to the power module 102 and controls the power module 102 by setting different operational parameters and options of the power module 102. The controller 104 may, for example, comprise a PID controller. The interface 106 is utilized to facilitate input and output of the controller 104. The interface 106 may also be utilized to set and read operational parameters and characteristics of the power module 102, the controller 104 and the generator 200. For example, the interface 106 may comprise an input device for setting parameters of the power module 102 and a display showing data associated with the electric braking system 100. In an embodiment in which the power module 102 is a Sinamics G120 unit manufactured by Siemens, the interface may utilize Siemens Total Integration Automation (TIA) software for setting the parameters through the controller 104.

[0035] Although the power module 102, the controller 104, and the interface 106 are shown in FIG. 2 as separate elements, any or all of the features of the power module 102, the controller 104, and the interface 106 may be combined as a single element.

[0036] The transformer 108 may be coupled to one or more phase lines of the electrical grid. In the FIG. 2 example, the transformer 108 is coupled to two of the phase lines 302, 304 of the electrical grid 300 and utilized for stepping down the voltage from the electrical grid 300 to a voltage that may be utilized by the power module 102, as well as other components of the electric braking system 100, in order to power the components with power from the electrical grid 300. For example, the transformer may step down the voltage of the electrical grid 300, and may include an AC/DC power adaptor to provide a suitable form of electrical power to the controller 104 and/or interface 106 (e.g., in some embodiments, the controller 104 and interface 106 are provided with 24V DC power). The transformer 108 may be adaptable such that the electric braking system 100 can be adapted to utilize the voltage and frequency of the particular electrical grid 300 at the site of the wind turbine 10. An adaptable transformer 108 enables the electric braking system 100 to be employed in various different countries, each having electrical grids operating at different voltages and frequencies.

[0037] Power the electric braking system 100 utilizing the transformer 108 as shown in FIG. 2 requires that the electrical grid 300 be live. Alternatively or additionally to the transformer 108, the electric braking system 100 may include a power source (not shown) such as for example a battery, other electrical power storage device, or portable generator, that may be utilized to power the components of the electric braking system 100 when the electrical grid is not live and no other power is available for the

transformer 108. In implementations wherein the grid is not live or the grid is not used to power the electric braking system 100, the inputs 103 would be connected to the alternative power supply rather than the grid lines. Example portable braking systems adapted to operate without a grid connection are discussed below with reference to FIGS 6A and 6B.

[0038] Referring now to FIG. 3, a flow chart of a method 400 of stopping a wind turbine 10 utilizing an electric braking system 100 is shown. Other than the connecting step at 402, the method shown in FIG. 3 may be carried out by, for example, a processor of the controller 104, the interface 106 or remotely connected through interface 1 10.

[0039] At 402, the outputs 105 of the power module 102 are connected to the leads of the generator 200. The inputs 103 are connected to the grid or an alternate power source. Connecting the power module 102 to the generator 200 may include positioning a portable electric braking system 100 in the vicinity of the generator 200 at a safe distance from the turbine. The controller 104, or other processor connected locally or remotely to control the power module 102, may be programmed with parameters of the turbine, or may have access to a memory with parameters of the turbine stored therein. The parameters of the turbine may, for example, include characteristics of the generator, the wind rotor and the drive train, as discussed further below.

[0040] At 404, a probing waveform is applied by the power module 102 to the leads of the generator 200. The probing waveform has a probing frequency that is selected to induce a magnetic field in the generator stator that rotates at a probing rotational velocity. The initial probing rotational velocity is selected to be significantly lower than the expected rotational velocity of the generator rotor. In some

embodiments, the initial probing rotational velocity is selected as 50% of the generator's synchronous speed.

[0041] The probing waveform has a relatively small amplitude compared to the waveforms utilized when electrically braking the wind rotor 16, as discussed below. For example, in some embodiments the probing waveform may have a voltage amplitude of less than 50% of the grid voltage. In some embodiments, the probing waveform may have a voltage amplitude of about 10-30% of the grid voltage. For example, in some embodiments the probing waveform may comprise an AC voltage with an initial frequency of about 25Hz and a voltage amplitude of less than 50V, for example in the range of about 10-30V.

[0042] The initial probing waveform may be applied for a time period sufficient for several (e.g. 5-10) rotations of the induced magnetic field in the stator to occur, then the back electromotive force (EMF) is determined, for example by measuring the voltage and current from the generator. The back EMF may be determined periodically or continuously. For example, in some embodiments, the probing waveform may be initially applied for approximately 0.2 seconds, following which the back EMF is determined.

[0043] At 406, a determination is made whether the measured back EMF is substantially zero. A back EMF of substantially zero indicates that the probing rotational velocity of the magnetic field induced by the probing frequency is substantially equal to the rotational velocity of the generator rotor. In an embodiment, the back EMF may be determined to be substantially zero when the measured back EMF is less than a predetermined threshold, such as for example a threshold voltage amplitude.

[0044] If the back EMF is determined not to be substantially zero at 406, the method proceeds to 408 where the probing frequency is adjusted. The probing frequency may be adjusted by stepping the probing frequency upward (or downward) by a predetermined step size. After the probing frequency is adjusted, the process returns to 404 where the probing waveform is applied at the adjusted probing frequency and a back EMF is determined.

[0045] In an example, the initial probing frequency is set to induce a rotating magnetic field at 50% of the nominal rotational velocity of the wind turbine 10 which the electric braking system 100 is connected to. Probing waveforms may adjusted by a predetermined increment such as, for example, 0.5Hz, and a "factor" parameter may be utilized to specify how long the probing waveform is applied at each incrementally adjusted frequency. In some embodiments, each frequency of the probing waveform is applied for at least 5-10 cycles in order to measure the back EMF for that frequency.

[0046] In some embodiments, the probing frequency may be adjusted to induce a rotating magnetic field at up to a maximum velocity that is higher than the maximum estimated rotor velocity in the present conditions. When the maximum velocity is reached and the measured back EMF has not been substantially zero, then the rotational rate of the generator rotor is determined to likely be lower than the range of probing frequencies utilized. In this case, the probing frequency may be reset to nearer zero and the probing waveform is applied and adjusted as described above.

[0047] When the back EMF is determined to be substantially zero at 406, the method proceeds to 410, where a generator rotor velocity is determined to be the probing velocity of the probing waveform at which the measured back EMF was measured as substantially zero.

[0048] At 412, a braking waveform having a braking rotational velocity slightly lower than the determined generator rotor velocity is applied by the power module 102 to the generator 200. For example, in some embodiments the initial braking rotational velocity may have a magnitude of about 0.5-1 % lower than the magnitude of the determined generator rotor velocity. The braking waveform exerts a negative torque (i.e. a torque that opposes the normal spinning direction of the wind rotor 16) on the generator rotor, which acts to slow down the wind rotor 16 through the drivetrain. The braking waveform has a voltage amplitude higher than the voltage amplitude of the probing waveform. The braking waveform may, for example, have a voltage amplitude similar to the grid voltage, but which is higher at braking frequencies greater than the grid frequency and lower at braking frequencies less than the grid frequency.

[0049] As discussed below, the braking waveform is adjusted to lower the frequency thereof at 414 to slow the rotor. Adjustment of the braking waveform depends on a number of factors, including how quickly rotation of the rotor 16 is to be reduced, the power limits of the power module 102 utilized in the electric braking system 100, and mechanical characteristics of the turbine (e.g. the drivetrain, the wind rotor, etc.). The power module 102 may be sized to have power limits suitable for the expected braking power generated when stopping the type of turbine that the power module 102 is connected to. For example, faster braking of the rotor 16 requires more power to be dissipated through the power module 102 (e.g. to the grid) and transferring more power than the power rating of the power module 102 could damage the power module 102. In some embodiments, the power module 102 is controlled to discontinue application of the braking waveform in the event a power dissipation threshold is exceeded. In some embodiments, the rate of adjustment of the braking waveform may be controlled to avoid extended periods of high braking power output.

[0050] At 414, the frequency of the braking waveform (and thus the rotational velocity of the braking magnetic field) is ramped down in order to slow the rotation of the rotor. In some embodiments, the frequency of the braking waveform is ramped down at a rate determined by a braking profile. Examples of how the braking waveform may be adjusted relative to the rotor velocity are discussed further below with reference to FIGS. 4A and 5A.

[0051] The braking profile may, for example, be determined based on generator parameters stored in a memory accessible to the controller. For example, the controller 104 may utilize a "turbine model" that is based on turbine parameters. The turbine model may, for example be based on customized variables determined by turbine construction and testing of the turbine, such as for example, wind rotor mass, inertia, drivetrain construction, generator configuration, etc. The turbine model may be used (e.g. by the controller) to determine the rotor speed based on the frequency of the braking waveform and the slip. When the turbine parameters are known, the slip can be determined by measurement of the current output from the generator at a given frequency of the braking waveform. Thus, the system can determine the rotor speed when applying a given braking waveform based on current measurements. In some embodiments, the frequency of the braking waveform may be ramped down according to a predetermined braking profile. The predetermined braking profile for ramping down the braking frequency (and thus rotor velocity) may be determined by the controller 104 based on turbine parameters, or alternatively entered into the controller 104 by the interface 106 of 1 10. The "shape" of the braking profile may be controlled by the controller 104. In an embodiment, the shape of the braking profile is configured to provide underdamped braking (i.e. to reduce or eliminate oscillations). In some embodiments, the power module 102 may be configured to "release" the rotor (e.g. stop applying the braking waveform) in the event of a wind gust, blade tip rotation or other event that causes a generator output power spike and corresponding power module power spike, in order to reduce the risk of damage to the power module 102.

[0052] In some situations, it is desired that the braking profile provides a smooth slowing of the rotor 16 to a safe frequency of rotation within 10-15 seconds. In some embodiments, the rotor velocity may be ramped down over a longer period of time, for example up to about one minute, in order to avoid exceeding the safe operating limits of the power module 102, and/or to reduce mechanical loads on the drivetrain and wind rotor. In general, it is not desired to have the wind rotor 16 rotating for extended periods of time at higher speeds that produce large torques or energy output as this may stress the electrical and mechanical systems of the wind turbine 10 and/or electric braking system 100.

[0053] In some embodiments, the portable electric braking system 100 may be provided with a simplified user interface, such that an on-site operator can brake a turbine with only minimal user input. For example, in some embodiments, the system 100 can be configured to require only a target speed input (e.g. 0 rpm, or some other target selected based on the situation). In some embodiments, a local or remote operator may input, such as through the interface 106 of 1 10, a target speed of rotation of the generator rotor and desired time to reach the target speed. The controller 104 determines the "shape" of the braking profile that will safely brake the generator rotor, based on the parameters and characteristics of the generator 200 and the power module 102.

[0054] In the case of a rotor 16 that includes an automatic mechanical blade pitching functionality, as described above, a two-stage ramping of the braking frequency may be desired to compensate for a power surge that may occur when the rotor blades 18 return to the normal position. Two-stage (or more than two-stage) ramping could also be used in other situations to address power surges due to the power response characteristics of the turbine. A power surge can cause the power module 102 to exceed its power limits, causing damage to the power module 102. An example of two- stage braking is discussed below with reference to FIG. 5A.

[0055] Rather than have the controller 104 automatically ramp down the braking frequency, either directly to a single target or as part of a multi-stage braking profile, as described above, the braking frequency may optionally be ramped down manually by an operator during some or part of the braking operation. The user may optionally manually ramp down the frequency of the braking waveform at the site of the electric braking system 100, or from a remote location utilizing the interface 1 10.

[0056] At 416, a determination is made whether the generator rotor rpm is substantially zero (e.g. based on the braking waveform applied, the turbine model and current measurements), which indicates that the wind rotor 16 has stopped spinning. If the wind rotor 16 is still spinning, the process may continue to cycle through 414 and 416 the wind rotor 16 has stopped. [0057] Once the wind rotor 16 has stopped, at 418 the braking waveform is maintained. In a typical emergency braking situation, there will still be a wind force causing a positive torque on the wind rotor, and in some embodiments, the ramping setpoint may be controlled to ramp down to produce a negative rotational velocity for the magnetic field induced by the braking waveform maintained at 418. This negative rotational velocity produces a negative torque which opposes the positive torque from the wind, to hold the wind rotor 16 still until on-site technicians can lock the wind rotor 16 or otherwise stop the blades 18 from spinning to secure the turbine 10. The power output is periodically or continuously monitored, and the braking waveform is adjusted at 414 as necessary to account for any changes in the wind force. In some

embodiments, the system 100 may include a power supply sufficient to maintain the braking waveform at 418 without grid power, so that the system may be decoupled from the grid once the wind rotor has stopped, as discussed below. Once the wind rotor 16 and/or blades 18 have been secured, the system 100 may be turned off and

disconnected.

[0058] FIG. 4A is a graph 500 showing example rotational velocities for a generator rotor (represented by solid trace 502) and the magnetic fields induced in a generator stator (represented by broken traces 504, 506, 508) during an example braking operation. From the start of the graph to time t₀ the generator is functioning normally, with the generator rotor velocity 502 holding steady, and the operating magnetic field 504 (dashed trace) at the stator is rotating at the synchronous speed. At time to a shutdown is initiated, at which point the stator field is turned off such that the rotor starts accelerating due to positive wind torque on the blades 18 that is not countered by back EMF induced by the stator magnetic field , and the primary brakes are applied. However, the primary brakes are malfunctioning, such that the rotor velocity increases rapidly in the presence of continued wind torque. At time ti the rotational velocity of the rotor exceeds a secondary braking threshold R_s and a secondary braking mechanism (e.g., an automatic blade pitching mechanism) is engaged, such that the rotational velocity of the rotor levels off. Alternatively of additionally, the rotational velocity of the rotor may level off due to the aerodynamic properties of the wind rotor in its nominal position. [0059] At time t₂ the stator leads are connected to a portable electric braking system according to the present disclosure, and a probing waveform is applied to generate a probing magnetic field 506 (dash-dotted trace) that initially rotates relatively slowly, the frequency of which is ramped up until the rotational velocity of the probing waveform induced magnetic field 506 substantially matches the rotational velocity of the rotor 502 at time t₃. When the rotational velocity of the probing waveform induced magnetic field 506 matches the rotational velocity of the rotor 502 at time t₃, a braking waveform is applied to generate a braking magnetic field 508 (dash-dot-dotted trade) that initially rotates at a slightly lower velocity than the rotational velocity of the rotor 502. The rotational velocity of the braking waveform induced magnetic field 508 is ramped down until the rotor stops at time t . At time t and thereafter until the turbine can be secured, the rotational velocity of the braking waveform induced magnetic field 508 is maintained at a negative value R₀ (and adjusted as necessary based on monitoring of the power output from the generator, as discussed above) to produce a negative torque sufficient to counteract a continuing positive torque from the wind.

[0060] FIG. 4B is a graph 510 showing example respective voltages used to generate the rotating magnetic fields 504, 506, 508 of FIG. 4A. The grid voltage 514 that generates the operating magnetic field 504 may, for example, be about 400V in some embodiments. The probing voltage 516 that generates the probing magnetic field 506 may, for example, be less than 50V, and about 10-30 V in some embodiments. The braking voltage 518 that generates the braking magnetic field 508 may, for example, be ramped down from an initial value which may be higher than the grid voltage to a final voltage V₀ that may depend on the amount of negative torque necessary to keep the rotor stopped in the presence of ongoing wind. In some embodiments, the voltage amplitude of the braking waveform may range from about 400V or more initially at higher frequencies (e.g. around 50 Hz) down to about 20V at lower frequencies (e.g. about 0-2 Hz).

[0061] FIG. 5A is a graph 520 similar to FIG. 4A showing example rotational velocities for a generator rotor (represented by solid trace 502) and the magnetic fields induced in a generator stator (represented by broken traces 504, 506, 508-1 , 508-2) during an example braking operation. Graph 520 is similar to graph 500 except that graph 520 illustrates an example two stage braking operation, illustrated by the two portions of the braking waveform indicated by traces 508-1 and 508-2. As discussed above, two stage braking may be useful when braking a turbine having an automatic mechanical blade pitching mechanism or other characteristics likely to cause a power surge as the rotor is electrically braked.

[0062] In a first stage of the two-stage ramping process, the braking frequency is ramped down at a first rate until time t_3.i , as indicated by trace 508-1. After time t_3.i , in a second stage the frequency of the braking waveform is further ramped down at a second rate to a target speed (e.g. to a stop), as indicated by trace 508-2. For example, in some embodiments the braking frequency may be ramped down at the first rate until the speed is just below a speed Ri at which the blades 18 will return to their normal position, which will cause a temporary increase in the power output from the generator 200.

[0063] FIG. 5B is a graph 530 illustrating an example power response of an example wind turbine, where the power output from the generator increases as the rotational velocity drops from the velocity at time t₃, then the power decreases again as the rotational velocity reaches the velocity at time t_{3 1}. The braking speed may be ramped down more quickly through this transitional region between time t₃ and time t_3.i , to avoid exceeding the power limitations of the power module 102.

[0064] The present disclosure describes a portable electric braking system for use in braking a wind turbine. The portable electric braking system may be brought to the site of a wind turbine, rather than building wind turbines with expensive, built-in electric braking systems. The disclosed portable electric braking system can slow down or completely stop a wind turbine without utilizing friction based brakes. Certain embodiments of the disclosed portable electric braking system enable fully controllable deceleration periods, enabling fast or slow braking.

[0065] The disclosed portable electric braking system may convert the kinetic energy of the turbine that is removed during braking into electric energy, which may be transferred to the electrical grid. In particular, electric energy generated during braking may be provided from the power module 102 to the grid, or to another power dump if the grid is down. As discussed above, the rate of braking may be controlled to ensure that power dissipation from the power module 102 remains within safe operational limits.

[0066] FIGS. 6A and 6B respectively show example portable braking apparatus 600A and 600B that may be configured to operate even in the absence of grid power. Each of the apparatus 600A of FIG. 6A and the apparatus 600B of FIG. 6B may be installed on a mobile platform 610 (e.g. a truck, van, trailer, etc.) to facilitate transport to a wind turbine needing braking.

[0067] The apparatus 600A of FIG 6A comprises a portable power unit such as, for example a battery/UPS module 620, that is connected to the inputs of the power module of a braking system 100 as disclosed above. In operation, the inputs of the system 100 of FIG. 6A would be connected to the grid during braking, then may be disconnected from the grid once braking is complete, and rely on the battery/UPS module 620 to maintain the braking waveform (and absorb any additional braking power produced) once the turbine is stopped. The battery/UPM module 620 may, for example, include inverters or other elements for providing the AC output used for the braking waveform. The apparatus 600A may also include a suitable switching mechanism (not shown) for breaking the connection to the grid and establishing the connection to the battery/UPS module. The apparatus 600A may safely maintain the turbine stationary so that service personnel can secure the turbine, rather than relying on continued grid power, which may be unreliable in the event of a storm or other disruption.

[0068] The apparatus 600B of FIG 6B comprises a portable AC generator 630 and a dump load 640 that are connected to the inputs of the power module of a braking system 100 as disclosed above. The AC generator 630 is configured to provide reactive power to energize the generator stator during braking operations, and the dump load 640 (e.g. a power dissipating resistor or other element for dissipating or absorbing braking power) is configured to handle the power generated during braking operations. The AC generator may, for example be configured to produce a power output of about 30% of the rating power of the turbine generator.

[0069] The examples discussed above relate to portable electric braking systems, but the teachings of the present disclosure may also be applied to built-in braking systems. Such systems may advantageously be used to provide auxiliary electric braking for wind turbines with induction generators. In some embodiments, a built-in electric braking system may include an auxiliary power supply such as a battery/UPS module similar to the portable apparatus 600A of FIG. 6A, such that the built-in electric braking system can hold the turbine stationary for a time without relying on grid power. For example, in some embodiments an electric braking system of the type shown in FIG. 2 may be incorporated into the master control cabinet 20 of the wind turbine 10, or may be incorporated into a base portion of the tower 12 supporting the wind turbine 10, with the power module 102 connected between the master control cabinet 20 and the generator 200.

[0070] In the preceding description, for purposes of explanation, numerous details are set forth in order to provide a thorough understanding of the embodiments. However, it will be apparent to one skilled in the art that these specific details are not required. In other instances, well-known electrical structures and circuits are shown in block diagram form in order not to obscure the understanding. For example, specific details are not provided as to whether the embodiments described herein are

implemented as a software routine, hardware circuit, firmware, or a combination thereof.

[0071] Embodiments of the disclosure can be represented as a computer program product stored in a machine-readable medium (also referred to as a computer- readable medium, a processor-readable medium, or a computer usable medium having a computer-readable program code embodied therein). The machine-readable medium can be any suitable tangible, non-transitory medium, including magnetic, optical, or electrical storage medium including a diskette, compact disk read only memory (CD- ROM), memory device (volatile or non-volatile), or similar storage mechanism. The machine-readable medium can contain various sets of instructions, code sequences, configuration information, or other data, which, when executed, cause a processor to perform steps in a method according to an embodiment of the disclosure. Those of ordinary skill in the art will appreciate that other instructions and operations necessary to implement the described implementations can also be stored on the machine- readable medium. The instructions stored on the machine-readable medium can be executed by a processor or other suitable processing device, and can interface with circuitry to perform the described tasks. [0072] The present disclosure may be embodied in other specific forms without departing from its spirit or essential characteristics. The described embodiments are to be considered in all respects only as illustrative and not restrictive.

Claims

1 . A process for stopping a wind turbine having an induction generator with a generator rotor connected to be driven by the wind, and a generator stator having a plurality of generator leads for connecting the generator to a power grid, the process comprising:

connecting outputs of a portable voltage sourced converter to the generator leads;

applying a probing waveform from the portable voltage sourced converter to the generator leads, the probing waveform comprising an AC electrical signal having a first amplitude and a probing frequency configured to induce a magnetic field in the stator that rotates at a probing rotational velocity;

measuring current and voltage on the generator leads to determine a back EMF from the generator in response to the probing waveform;

adjusting the probing frequency until the back EMF is substantially zero to determine an initial generator rotor rotational velocity;

applying a braking waveform having a second amplitude higher than the first amplitude and having a braking frequency configured to induce a magnetic field in the stator that rotates at a braking rotational velocity slightly lower than the initial generator rotor velocity; and,

adjusting the braking waveform to ramp down the braking rotational velocity to a target rotational velocity.

2. The process of claim 1 wherein the first amplitude is less than about 50V.

3. The process of claim 2 wherein the second amplitude is initially about 400V.

4. The process of any one of claims 1 to 3 wherein adjusting the braking waveform comprises monitoring a power output from the generator and controlling a ramping rate to limit an amount of energy output in a predetermined time period to within a

predetermined range.

5. The process of any one of claims 1 to 4 wherein the target rotational velocity is selected to generate a negative braking rotational velocity to counteract a continuing wind torque.

6. The process of any one of claims 1 to 5 wherein ramping down the braking rotational velocity comprises automatically reducing the braking rotational velocity, at a first ramping rate, to an intermediate target rotational velocity, then reducing the braking rotational velocity, at a second ramping rate, to the target rotational velocity.

7. The process of claim 6 wherein the rotor of the wind turbine comprises blades having automatic mechanical blade pitching wherein at least a portion of each blade is configured to move to present less aerodynamic torque when rotating at a velocity exceeding a blade engagement velocity, and wherein the intermediate target rotational velocity is below the blade engagement velocity.

8. The process of claim 6 wherein the first ramping rate is greater than the second ramping rate.

9. The process of any one of claims 1 to 8 comprising receiving manual input for ramping down the braking rotational velocity to the target rotational velocity.

10. The process of any one of claims 1 to 9 wherein the target rotational velocity is zero, further comprising maintaining the braking waveform at a braking frequency selected to induce the braking rotational velocity to be negative to counteract a positive wind torque, and monitoring a power output of the generator while maintaining the braking waveform and adjusting the braking frequency to maintain the generator rotor rotational velocity at substantially zero.

1 1 . The process of claim 10 comprising connecting inputs of the voltage sourced converter to the power grid until the target rotational velocity of zero is reached and then connecting the inputs of the voltage sourced converter to a portable power supply.

12. The process of any one of claims 1 to 1 1 comprising connecting inputs of the voltage sourced converter to the power grid.

13. The process of any one of claims 1 to 12 comprising connecting inputs of the voltage sourced converter to an AC generator and a dump load.

14. A portable electric braking system for a wind turbine having an induction generator with a generator rotor connected to be driven by the wind, and a generator stator having a plurality of generator leads for connecting the generator to a power grid, the system comprising:

a voltage sourced converter having a power unit with input connectable to a power source and an output connectable to stator leads of the wind turbine generator; a controller connected to control operation of the power unit to cause the power unit to:

apply a probing waveform from the portable voltage sourced converter to the generator leads, the probing waveform comprising an AC electrical signal having a first amplitude and a probing frequency configured to induce a magnetic field in the stator that rotates at a probing rotational velocity;

measure current and voltage on the generator leads to determine a back EMF from the generator in response to the probing waveform;

adjust the probing frequency until the back EMF is substantially zero to determine an initial generator rotor rotational velocity;

apply a braking waveform having a second amplitude higher than the first amplitude and having a braking frequency configured to induce a magnetic field in the stator that rotates at a braking rotational velocity slightly lower than the initial generator rotor velocity; and,

adjust the braking waveform to ramp down the braking rotational velocity to a target rotational velocity.

15. The portable electric braking system of claim 14 comprising a portable power unit connected to the input of the voltage sourced converter.

16. The portable electric braking system of claim 14 comprising an AC generator and a dump load connected to the input of the voltage sourced converter.

17. An electric braking system for a wind turbine having an induction generator with a generator rotor connected to be driven by the wind, and a generator stator having a plurality of generator leads for connecting the generator to a power grid, the system comprising:

a voltage sourced converter having a power unit with input connectable to the power grid and an output connected to stator leads of the wind turbine generator;

a controller connected to control operation of the power unit to cause the power unit to:

18. The electric braking system of claim 17 comprising an auxiliary power supply connected to the input of the voltage sourced converter.