WO2012134458A1

WO2012134458A1 - Dynamic braking and low voltage ride through

Info

Publication number: WO2012134458A1
Application number: PCT/US2011/030440
Authority: WO
Inventors: Peter Weichbold; Dejan RACA
Original assignee: Amsc Windtec Gmbh
Priority date: 2011-03-30
Filing date: 2011-03-30
Publication date: 2012-10-04

Abstract

A system for braking a generator includes a torque compensation device configured to store energy received from the generator; and a controller configured to control the torque compensation device to dissipate the stored energy such that a torque exerted by the generator is nondiscontinuously decreased from a first level to a second level lower than the first level.

Description

DYNAMIC BRAKING AND LOW VOLTAGE RIDE THROUGH

Background

[001] Wind energy has recently emerged as a fast-growing source of energy, offering a clean, renewable, and ecological-friendly alternative to fossil-based energy supplies. In addition to their traditional role in servicing rural residences in grid-isolated areas, wind turbine generators are now increasingly installed in large-scale (e.g., multi-megawatt) wind farms and integrated into power grids that can deliver electricity to consumers nationwide.

[002] The performance of a grid-connected wind energy converter can be influenced by many factors, such as voltage fluctuations on the grid. For example, a short circuit on the grid may result in a sudden voltage drop, which reduces the effective drag on the wind energy converter and may cause both the turbine and the generator to accelerate rapidly. To ensure safe operation, some wind energy converters have been designed to trip offline (i.e., disconnect from the grid and shut down) as soon as grid voltage drops below a prescribed level (e.g., 85% of nominal voltage). After fault clearance, these wind energy converters enter a restart cycle that can last several minutes before resuming power transmission to the grid.

[003] During this off-line period, the loss of power generation may impact the stability of utility grids to which wind energy converters are connected. As the number of grid- integrated wind plants/farms continues to grow, regulatory agencies in many countries have started to adopt strict interconnection standards that require large wind energy converters to remain online during disturbances and continue to operate for an extended period in a process called "low-voltage ride through" (LVRT).

Summary

[004] In a general aspect, a system for braking a generator includes a torque

compensation device configured to store energy received from the generator; and a controller configured to control the torque compensation device to dissipate the stored energy such that a torque exerted by the generator is nondiscontinuously decreased from a first level to a second level lower than the first level. [005] Embodiments may include one or more of the following.

[006] The torque compensation device comprises an energy storage device.

[007] The torque compensation device further comprises an energy dissipation device.

[008] The controller is configured to control the torque compensation device to dissipate the stored energy by connecting the energy dissipation device in parallel with the energy storage device. The energy dissipation device is configured to dissipate the stored energy when connected to the energy storage device.

[009] The controller is configured to periodically connect the energy dissipation device. The controller is configured to determine a duty cycle of the periodic connection on the basis of a heat capacity of the energy dissipation device. The controller is configured to vary the duty cycle of the periodic connection. The controller is configured to periodically connect the energy dissipation device with a switching frequency of about 1 kHz.

[010] The torque compensation device further comprises a switch connected in series with the energy dissipation device, and the controller is configured to connect the energy dissipation device by operating the switch. The switch includes an IGBT.

[011] The energy dissipation device includes a resistor. The energy storage device includes a capacitor. The system further includes a freewheeling diode connected between the generator and the torque compensation device.

[012] The controller is configured to determine a period of time of the decrease in torque. The controller is configured to determine the period of time of the decrease in torque on the basis of a heat capacity of the torque compensation device. The period of time of the decrease in torque is about 1 second.

[013] The controller is configured to determine a rate of decrease of the torque. The controller is configured to determine the rate of decrease of the torque on the basis of a heat capacity of the torque compensation device.

[014] The second level is greater than zero. The controller is configured to determine the difference in torque between the second level and zero on the basis of a heat capacity of the torque compensation device. [015] The first level is less than a steady-state torque of the generator. The controller is configured to determine the difference in torque between the first level and the steady- state torque on the basis of a heat capacity of the torque compensation device.

[016] The generator includes a wind turbine.

[017] In another general aspect, a method for braking a generator includes storing energy received from the generator in a torque compensation device; and controlling the torque compensation device to dissipate the stored energy such that a torque exerted by the generator is nondiscontinuously decreased from a first level to a second level lower than the first level.

[018] Embodiments may include one or more of the following.

[019] Controlling the torque compensation device includes controlling the torque compensation device to dissipate the stored energy by connecting the energy dissipation device in parallel with the energy storage device.

[020] Connecting the energy dissipation device includes periodically connecting the energy dissipation device. Controlling the torque compensation device includes determining a duty cycle of the periodic connection on the basis of a heat capacity of the energy dissipation device. Controlling the torque compensation device includes varying the duty cycle of the periodic connection. Periodically connecting the energy dissipation device includes periodically connecting the energy dissipation device with a switching frequency of about 1 kHz.

[021] Connecting the energy dissipation device includes operating a switch connected in series with the energy dissipation device.

[022] The method further includes determining a period of time of the decrease in torque. Determining a period of time of the decrease in torque includes determining the period of time on the basis of a heat capacity of the torque compensation device.

[023] The method further includes determining a rate of decrease of the torque.

Determining a rate of decrease of the torque includes determining the rate of decrease on the basis of a heat capacity of the torque compensation device.

[024] The second level is greater than zero. The method further includes determining the difference in torque between the second level and zero on the basis of a heat capacity of the torque compensation device. [025] The first level is less than a steady-state torque of the generator. The method further includes determining the difference in torque between the first level and the steady-state torque on the basis of a heat capacity of the torque compensation device.

[026] In another general aspect, a system includes a converter coupled between a generator and a utility power network; a crowbar circuit coupled to the generator and the converter, the crowbar circuit including a crowbar interruption device; and a controller configured to control a state of the crowbar interruption device.

[027] The crowbar interruption device includes a mechanical switch.

[028] The controller is configured to cause the crowbar interruption device to open.

[029] The controller is configured to cause the crowbar interruption device to close substantially immediately after the opening the crowbar interruption device. The controller is configured to cause the crowbar interruption device to close within about 0.5 seconds of the opening of the crowbar interruption device.

[030] The converter is configured to remagnetize the generator subsequent to the opening and closing of the crowbar interruption device.

[031] Current in the crowbar circuit is interrupted while the crowbar interruption device is open.

[032] The controller is configured to cause the crowbar interruption device to open after the crowbar circuit is activated. When the crowbar interruption device is open, the crowbar circuit is uncoupled from the generator and the converter.

[033] The crowbar circuit is coupled to the generator and the converter via the crowbar interruption device.

[034] The converter is configured to activate the crowbar circuit. The converter is configured to cause the creation of a short circuit condition in the crowbar circuit. The crowbar circuit includes a switching device, and the converter is configured to gate the switching device.

[035] The system further includes detection circuitry configured to detect an operating condition on the utility power network. The operating condition comprises a fault condition. [036] The system further includes the generator. The generator comprises a stator and a rotor, the stator connected to the utility power network via a stator interruption device, the crowbar circuit coupled to the rotor and the converter. The stator breaker is controlled by the converter. The converter is configured to cause the stator interruption device to open after activation of the crowbar circuit. The generator includes a wind turbine.

[037] In a further general aspect, a method is provided for operating a system including a generator and a converter, the converter coupled between the generator and a utility power network. The method includes activating a crowbar circuit coupled to the generator and the converter; opening a crowbar interruption device in the crowbar circuit; and closing the crowbar interruption device substantially immediately after opening the crowbar breaker.

[038] Embodiments may include one or more of the following.

[039] Closing the crowbar interruption device includes closing the crowbar interruption device within about 0.5 seconds of the opening of the crowbar interruption device.

[040] Opening the crowbar interruption device includes uncoupling the crowbar circuit from the generator and the converter.

[041] Opening the crowbar interruption device includes allowing current in the crowbar circuit to decay to zero.

[042] Activating the crowbar circuit includes gating a switching device in the crowbar circuit with the converter.

[043] Activating the crowbar circuit includes creating a short circuit condition in the crowbar circuit.

[044] The method further includes detecting an operating condition on the utility power network. Activating the crowbar circuit includes activating the crowbar circuit in response to detection of the operating condition. The operating condition includes a fault condition.

[045] The method further includes remagnetizing the generator.

[046] The generator comprises a rotor and a stator, the stator connected to the utility power network via a stator interruption device. The method further comprises opening the stator interruption device after activation of the crowbar circuit. [047] Among other advantages, the systems and methods described herein help to avoid a sudden, drastic change in torque of the generator of a wind energy converter. Without a sudden change in torque, undesirable and potentially dangerous vibrations in the drive train and oscillations of the blades of the wind energy converter can be minimized or eliminated.

[048] Other advantages include improved LVRT operation. In particular, when faults on the utility power network cause line voltage to drop, the wind energy converter can continue to operate without experiencing significant impact from transient low- voltage events.

[049] Other features and advantages of the invention are apparent from the following description and from the claims.

Brief Description of Drawings

[050] Fig. 1 is

[051] Fig. 2 is

[052] Fig. 3 is

[053] Fig. 4 is

[054] Fig. 5 is

circuit

[055] Fig. 6 is

[056] Fig. 7 is

Detailed Description

[057] Referring to Fig. 1, a wind energy converter 1 includes a tower 2 disposed on a foundation 6. A nacelle 3 on the upper end of tower 2 is rotatable around a substantially vertical axis B. A rotor head 4 provided on nacelle 3, including a hub (not shown) for fixing rotor blades 5, is rotatable around a substantially horizontal axis A. Rotor blades 5 are attached to rotor head 4 so as to be radially disposed around the rotation axis A. Thereby, wind power supplied to blades 5 from the direction of the variable rotation axis A is converted into mechanical power for rotating the rotor head 4 around the rotation axis A. [058] Referring to Figs. 1 and 2, wind energy converter 1 includes a wind power generation system 200 that converts wind power to electric power for delivery to a utility power network 204. Wind power is harvested by a double-fed or permanent magnet generator 202 located in nacelle 3 of wind energy converter 1. A rotor 206 of generator 202 is connected to rotor head 4 such that the rotation of rotor 206 is driven by wind energy supplied to rotor blades 5. A stator 208 of generator 202 is connected directly to utility power network 204. In the super-synchronous speed range of double-fed generator 202, a generator-side converter (GSC) 212 converts alternating current (AC) power generated by generator 202 into direct current (DC) power. A line-side converter (LSC) 210 then converts the DC power to AC power suitable for delivery to the utility power network 204. In the sub-synchronous speed range of double-fed generator 202, LSC 210 converts AC power from utility power network 204 to DC power, and GSC 212 covnerts the DC power to AC power which is fed to rotor 206. System components 214, described in more detail below, provide stability and robust operation of the wind power generation system 200.

Torque Compensation

[059] In the event of an unexpected contingency (e.g., connection to the utility power network is lost, one of the converters experiences a malfunction, or a failure occurs in another component of wind power generation system 200), GSC 210 is switched off to protect the components of the wind power generation system. When GSC 210 is switched off, the torque in permanent magnet generator 202 drops suddenly to zero. This sudden change in torque induces vibrations in the elements of the drive train of generator 202 and can cause swinging oscillations of rotor blades 5. Furthermore, in some cases, when GSC 210 is switched off, aerodynamic forces exerted on rotor blades 5 cause generator 202 to speed up, generating a high no load voltage which can potentially damage or destroy the converter.

[060] Referring to Figs. 3 and 4, to prevent damage to the drive train and/or the rotor blades, the torque in the generator is decreased over a period of time by a torque compensation system 400 such that the drive train vibrations are minimized or eliminated. At least a portion of the torque decrease is a non-discontinuous decrease, e.g., a continuous decrease.

[061] Referring to Figs. 2 and 4, torque compensation system 400 includes a DC link capacitor 402 connected in parallel with a dynamic braking system 404. Dynamic braking system 404 includes a dynamic brake resistor 406, an insulated-gate bipolar transistor (IGBT) 410 controlled by a controller 412, and a freewheel diode 408.

Freewheel diode 408 allows current to flow and decay to zero if there is a parasitic inductance of resistor 406 and IGBT 410 is switched off. The effective resistance of resistor 406 is controlled by pulse width modulation. That is, controller 412 controls IGBT 410 to open and close, thereby disconnecting and connecting, respectively, resistor 406. As an example, a resistor with a nominal resistance of 1 Ω, operated at 50% duty cycle, presents an effective resistance of 2 Ω (i.e., the resistor is connected for half of each cycle and disconnected for half of each cycle). GSC 212 and LSC 210 each include an IGBT 214a, 214b and a freewheel diode 216a, 216b. Freewheel diodes 216a, 216b allow current to flow and decay to zero if the associated IGBT 214a, 214b is switched off.

[062] In operation, when GSC 212 shuts down in response to a fault condition, current that arises from the no-load voltage of generator 202 flows through freewheeling diode 214b, charging capacitor 402. Current flow stops when capacitor 402 is charged to the level of the no-load voltage. Resistor 406 is then switched on (i.e., IGBT 410 is closed responsive to a command pulse from controller 412), discharging capacitor 402 and restarting current flow in the generator. That is, the energy associated with the no-load voltage of generator 202 is dissipated by resistor 406. Resistor 406 is switched into and out of the circuit by controller 412; the duty cycle of IGBT 410 affects the magnitude of current flow through GSC 212.

[063] Current flow in generator 202 induces a torque in the generator. Thus, when resistor 406 is connected, torque is induced in the generator. By controlling the duty cycle of the resistor, the current flow in the generator can be controlled, and hence the torque in the generator can also be controlled. That is, resistor 406 is used to dissipate the energy associated with the torque of the generator. Operating resistor 406 with an appropriately controlled duty cycle can thus cause a decrease in torque in the generator (e.g., the nondiscontinuous decrease shown in Fig. 3).

[064] Referring again to Fig. 3, when dynamic braking system 404 is employed with the appropriate duty cycle, the torque decreases sharply and nondiscontinously according to a curve 300. The ramp down rate of the torque (i.e., the slope of curve 300) is determined by modulation of the duty cycle of the resistor. In some instances, the torque is decreased to zero over a period of about 1 second, and may be ramped down to zero even though rotor 206 is still rotating. That is, the ramp down of the torque is independent of the rotational speed of the generator. The frequency of the drive train vibrations are often in the range of about 2-3 Hz; the torque is ramped down over a time period that is roughly comparable to one or a small number of periods of the drive train vibrations.

[065] The duty cycle of the resistor is controlled so as not to heat resistor 406 beyond its heat capacity. For instance, switching the resistor with a switching frequency of about 1 kHz causes a decrease in torque that can be sufficient to avoid damaging vibrations and oscillations without damaging the resistor. In some instances, an initial step 302 and/or a final step 304 in the torque arise because of technical constraints that set a minimum duty cycle with which the resistor can be operated. Initial step 302 can be minimized and/or eliminated if the resistance of resistor 406 is sufficiently low (i.e., such that, with a duty cycle of 1.0, the torque is at least approximately the same as the initial torque). Final step 304 is generally small enough to be neglected because the minimum allowed duty cycle of IGBT is sufficiently small.

[066] In an alternative embodiment, a resistor with high heat capacity is connected in parallel with DC link capacitor 402. If the heat capacity of the resistor is sufficiently high, the resistor can remain continuously connected. A continuously connected resistor enables continuous ramp down of the rotational speed of the rotor 206 as energy is dissipated via the resistor. As the rotation of rotor 206 slows, the voltage produced by generator 202 decreases proportionally, and the torque in turn decreases in proportion to the voltage. In this case, the rate of torque decrease is dependent upon the rotational speed of the generator and thus is slower than the rate of decrease in the dynamic braking embodiment. Referring to Fig. 3, an exemplary curve 306 shows a torque decrease over about 10 seconds as a result of a continuously connected resistor with high heat capacity.

[067] In other embodiments, other types of adjustable resistors are used as a way to control the dissipation of energy and the rate of decrease of the torque.

[068] In some instances, GSC 212 can still be used to control the current and thus the torque in generator 202 even if LSC 210 fails. In this case, the energy that is fed back to the DC link is absorbed by resistor 406. Resistor 406 is controlled, e.g., by a bang-bang controller, to maintain a constant voltage level in the DC link. The bang-bang controller causes resistor 406 to be switched in when the DC link voltage exceeds an upper limit and to be switched out when the DC link voltage falls below a lower limit. Low Voltage Ride Through with Double Fed Generators

[069] Referring to Fig. 6, a crowbar circuit 600 provides LVRT capability to wind power generation system 200. Crowbar circuit 600 includes a rectifier bridge 602, a thyristor 604 controlled by GSC 212, and a small resistor 606, such as a 0.1 Ω resistor.

[070] Referring to Figs. 6 and 7, a low voltage condition is detected on utility power network 204 (step 700). For instance, GSC continuously monitors the DC bus voltage V_DC and identifies a low voltage condition when V_DC exceeds the voltage rating of GSC 212. To prevent the components of wind power generation system 200 from damage from high current and/or voltage transients during the low voltage condition, crowbar circuit 600 is enabled (step 702), creating a short-circuit pathway 608 across the three- phase output of rotor 206. The short-circuit pathway isolates rotor 206 from GSC 212. Specifically, upon detection of the low voltage condition, GSC 212 applies a current pulse to the gate electrode of thyristor 604, which closes thyristor 604 and provides short- circuit pathway 608. Stator 208 is then also isolated from the utility power network by opening a stator breaker 610 (step 704), thus protecting elements of the wind power generation system and utility power network downstream of the stator breaker. In some embodiments, the current pulse that triggers thyristor 604 is issued simultaneously with a command to open stator breaker 610. Accounting for delays in response time, the thyristor may fire within, e.g., about 2 microseconds, while the stator breaker may open within, e.g., about 50-100 milliseconds.

[071] Once thyristor 604 has closed, short-circuit pathway 608 remains active as long as the thyristor remains forward biased. If stator breaker 610 is open, the residual magnetization current in crowbar circuit 600 decays to zero with a time constant governed by the inductance of generator 202 and the resistance of resistor 606. For instance, a typical decay time is about 1-2 seconds. Once the residual current in crowbar circuit 600 has decayed to zero, thyristor 604 is cleared and the thyristor reopens, removing the short-circuit pathway that isolates rotor 206 from GSC 212. The wind energy converter can then be restarted.

[072] Shutting down the wind energy converter for, e.g., 1-2 seconds does not meet LVRT standards that wind energy converters remain online throughout low voltage events. In order to keep the wind energy converter online, GSC 212 is reconnected to rotor 206 substantially immediately after the short-circuit pathway 608 is enabled in order to resynchronize or remagnetize generator 202. However, the existence of the short- circuit pathway precludes the reconnection of GSC 212 to rotor 206. Thus, in order to enable LVRT without a shutdown of the wind energy converter, short-circuit pathway 608 is rapidly disabled (i.e., faster than the decay time of the residual magnetization current of a shut down generator).

[073] Specifically, a set of crowbar breakers 614 are opened to disconnect crowbar circuit 600 from rotor 206 and GSC 212 (step 706). The current in the disconnected crowbar circuit rapidly clears thyristor 604, which reopens in the absence of a forward bias. Crowbar breakers 614 are then immediately reclosed (e.g., after about 0.1 seconds; step 708), allowing the crowbar circuit to be available to respond to another low voltage condition or other fault event. Because thyristor 604 has opened, short-circuit pathway 608 no longer exists, and GSC 212 can provide reactive current to generator 202 in order to remagnetize the generator and synchronize the generator to the grid (step 710). Stator breaker 610 is then closed (step 712) and the wind energy converter resumes normal operation.

[074] In some embodiments, crowbar breakers 614 are mechanical switches. In other instance, the crowbar breakers are semiconductor switches, such as insulated-gate bipolar transistors (IGBT), thyristors, or gate turn-off thyristors (GTO) capable of withstanding high short-circuit currents (e.g., current on the order of kA).

[075] The above-described procedure of detecting an overvoltage condition of the DC bus voltage, triggering crowbar circuit 600, opening stator breaker 610 and crowbar breakers 614, and reclosing the crowbar breakers is executed without regard for the duration of the low voltage condition. In some embodiments, prior to remagnetizing generator 202 and closing stator breaker 610, the condition of the utility power network is evaluated by GSC 212. For instance, in some cases, generator 202 is remagnetized fully but stator breaker 610 is closed only after the voltage on the utility power network recovers completely, which avoids the possibility of damage by voltage transients occurring during recovery. In other cases, generator 202 is partially remagnetized, e.g., in proportion to the voltage remaining on the utility power network, and stator breaker 610 is closed immediately in order to use the generator during recovery.

[076] Stator breaker 610 and crowbar breakers 614 are controlled by a controller 612, such as a programmable logic controller (PLC), which is connected to GSC 212 via a bus communication 616. PLC 612 is configured to monitor the status of the breakers and to send open and close commands to the breakers. When PLC 612 receives feedback that stator breaker 610 is opened, the PLC commands crowbar breakers 614 to open. When PLC 612 receives feedback that crowbar breakers 614 are open, the PLC commands the crowbar breakers to be closed again. When PLC receives feedback that crowbar breakers 614 are closed, GSC 212 is restarted in order to remagnetize the generator. In some embodiments, the function of controller 612 is integrated into GSC 212. In other embodiments, stator breaker 610 and crowbar breakers 614 are controlled by two separate controllers, each connected to GSC 212.

[077] In another embodiment, the current through thyristor 604 is actively controlled, e.g., by GSC 212, and the thyristor is cleared by actively driving the current in the crowbar circuit to zero.

[078] In a further alternative embodiment, the crowbar breakers 614 are placed in the DC circuit of the crowbar, e.g., in series with thyristor 604.

Low Voltage Ride Through with Full Conversion Generators

[079] Referring to Fig. 8, in a full conversion wind power generation system 800, a generator 802, e.g., a permanent magnet synchronous generator or an induction generator, is driven by a wind energy converter such as that shown in Fig. 1.

[080] In a first configuration of system 800, a generator-side converter (GSC) 804 controls the current of the generator stator, thereby controlling the torque in generator 804. Power is given as torque times speed; thus, if generator 802 is rotating, GSC 804 is capable of controlling the power output from the generator. The output power from generator 802 is fed into a DC link 806 including, e.g., a DC link capacitor 812 and a dynamic braking system 807 such as that described above.

[081] A line-side converter (LSC) 808 interposed between DC link 806 and a utility power network 810 controls the voltage across the DC link 806 via control of DC link capacitor 812. That is, for instance, if more power is fed into DC link 806 from generator 802, the DC link voltage is increased, and LSC 808 reacts by feeding more power to utility power network 810. A controller 812 communicates control signals to GSC 804 and LSC 808 to accomplish power and voltage control on the wind power generation system.

[082] In this configuration, when a disturbance (e.g., a low voltage condition) occurs on utility power network 810, LSC may be unable to sufficiently control the voltage on DC link 806. In order to avoid damage to components of the wind power generation system 800, dynamic braking system 807 may be employed as described above in order to dissipate extra power, and in some cases, the wind turbine supplying generator 802 may be shut down.

[083] In an alternative configuration, GSC 804 is used to control the DC link voltage and LSC 808 is used to control the power output from generator 802. That is, control signals from controller 812 are switched such that the functionality of GSC 804 and LSC 808 are reversed relative to that of the first configuration of the wind power generation system.

[084] In this configuration, LSC 808 receives a current command from controller 812, e.g., a command to maintain constant current output from generator 802. If a low voltage event occurs on utility power network 810, the constant current command causes the power output from generator 802 to be automatically reduced (power = current x voltage). That is, the power output from generator 802 will automatically decrease in proportion to the drop in voltage on the utility power network. The reduction in power output in turn automatically causes GSC 804 to adjust the DC link voltage in

compensation.

[085] Generator 802 acts as a stable voltage source that generates voltage of the same phase, with frequency and amplitude of the voltage varying depending on the rotational speed of the generator. Use of GSC 804 to control the DC link voltage takes advantage of the stability of the voltage sourced from generator 802. That is, although the power output from the generator is reduced as a result of a low voltage event on the utility power network, the voltage output from the generator is unaffected. Low voltage events and other disturbances that may occur on the utility power network do not affect the ability of GSC 804 to control the voltage on DC link 806. Furthermore, with more stable control of the DC link voltage, less power is dissipated in dynamic braking system 807, allowing the dynamic braking system to be implemented with a smaller resistor.

[086] In some embodiments, GSC 804 and LSC 808 cause voltage and power adjustments within, e.g., 10-50 milliseconds of the occurrence of a low voltage condition on the utility power network.

[087] It is to be understood that the foregoing description is intended to illustrate and not to limit the scope of the invention, which is defined by the scope of the appended claims. Other embodiments are within the scope of the following claims.

Claims

What is claimed is:

1. A system for braking a generator, comprising:

a torque compensation device configured to store energy received from the

generator; and

a controller configured to control the torque compensation device to dissipate the stored energy such that a torque exerted by the generator is nondiscontinuously decreased from a first level to a second level lower than the first level.

2. The system of claim 1, wherein the torque compensation device comprises an energy storage device.

3. The system of claim 2, wherein the torque compensation device further comprises an energy dissipation device.

4. The system of claim 3, wherein the controller is configured to control the torque compensation device to dissipate the stored energy by connecting the energy dissipation device in parallel with the energy storage device.

5. The system of claim 4, wherein the energy dissipation device is configured to dissipate the stored energy when connected to the energy storage device.

6. The system of claim 4, wherein the controller is configured to periodically connect the energy dissipation device.

7. The system of claim 6, wherein the controller is configured to determine a duty cycle of the periodic connection on the basis of a heat capacity of the energy dissipation device.

8. The system of claim 7, wherein the controller is configured to vary the duty cycle of the periodic connection.

9. The system of claim 6, wherein the controller is configured to periodically connect the energy dissipation device with a switching frequency of about 1 kHz.

10. The system of claim 4, wherein the torque compensation device further comprises a switch connected in series with the energy dissipation device, and the controller is configured to connect the energy dissipation device by operating the switch.

11. The system of claim 10, wherein the switch includes an IGBT.

12. The system of claim 3, wherein the energy dissipation device includes a resistor.

13. The system of claim 2, wherein the energy storage device includes a capacitor.

14. The system of claim 2, further comprising a freewheeling diode connected between the generator and the torque compensation device.

15. The system of claim 1, wherein the controller is configured to determine a period of time of the decrease in torque.

16. The system of claim 15, wherein the controller is configured to determine the period of time of the decrease in torque on the basis of a heat capacity of the torque compensation device.

17. The system of claim 15, wherein the period of time of the decrease in torque is about 1 second.

18. The system of claim 1, wherein the controller is configured to determine a rate of decrease of the torque.

19. The system of claim 18, wherein the controller is configured to determine the rate of decrease of the torque on the basis of a heat capacity of the torque compensation device.

20. The system of claim 1, wherein the second level is greater than zero.

21. The system of claim 20, wherein the controller is configured to determine the difference in torque between the second level and zero on the basis of a heat capacity of the torque compensation device.

22. The system of claim 1, wherein the first level is less than a steady- state torque of the generator.

23. The system of claim 22, wherein the controller is configured to determine the difference in torque between the first level and the steady-state torque on the basis of a heat capacity of the torque compensation device.

24. The system of claim 1 , wherein the generator includes a wind turbine.

25. A method for braking a generator, comprising:

storing energy received from the generator in a torque compensation device; and controlling the torque compensation device to dissipate the stored energy such that a torque exerted by the generator is nondiscontinuously decreased from a first level to a second level lower than the first level.

26. The method of claim 25, wherein controlling the torque compensation device includes controlling the torque compensation device to dissipate the stored energy by connecting the energy dissipation device in parallel with the energy storage device.

27. The method of claim 26, wherein connecting the energy dissipation device includes periodically connecting the energy dissipation device.

28. The method of claim 27, wherein controlling the torque compensation device includes determining a duty cycle of the periodic connection on the basis of a heat capacity of the energy dissipation device.

29. The method of claim 28, wherein controlling the torque compensation device includes varying the duty cycle of the periodic connection.

30. The method of claim 27, wherein periodically connecting the energy dissipation device includes periodically connecting the energy dissipation device with a switching frequency of about 1 kHz.

31. The method of claim 26, wherein connecting the energy dissipation device includes operating a switch connected in series with the energy dissipation device.

32. The method of claim 25, further comprising determining a period of time of the decrease in torque.

33. The method of claim 32, wherein determining a period of time of the decrease in torque includes determining the period of time on the basis of a heat capacity of the torque compensation device.

34. The method of claim 25, further comprising determining a rate of decrease of the torque.

35. The method of claim 34, wherein determining a rate of decrease of the torque includes determining the rate of decrease on the basis of a heat capacity of the torque compensation device.

36. The method of claim 25, wherein the second level is greater than zero.

37. The method of claim 36, further comprising determining the difference in torque between the second level and zero on the basis of a heat capacity of the torque compensation device.

38. The method of claim 25, wherein the first level is less than a steady-state torque of the generator.

39. The method of claim 38, further comprising determining the difference in torque between the first level and the steady-state torque on the basis of a heat capacity of the torque compensation device

40. A system comprising:

a converter coupled between a generator and a utility power network;

a crowbar circuit coupled to the generator and the converter, the crowbar circuit including a crowbar interruption device; and

a controller configured to control a state of the crowbar interruption device.

41. The system of claim 40, wherein the crowbar interruption device includes a mechanical switch.

42. The system of claim 40, wherein the controller is configured to cause the crowbar interruption device to open.

43. The system of claim 42, wherein the controller is configured to cause the crowbar interruption device to close substantially immediately after the opening the crowbar interruption device.

44. The system of claim 43, wherein the controller is configured to cause the crowbar interruption device to close within about 0.5 seconds of the opening of the crowbar interruption device.

45. The system of claim 43, wherein the converter is configured to remagnetize the generator subsequent to the opening and closing of the crowbar interruption device.

46. The system of claim 42, wherein current in the crowbar circuit is interrupted while the crowbar interruption device is open.

47. The system of claim 42, wherein the controller is configured to cause the crowbar interruption device to open after the crowbar circuit is activated.

48. The system of claim 42, wherein, when the crowbar interruption device is open, the crowbar circuit is uncoupled from the generator and the converter.

49. The system of claim 40, wherein the crowbar circuit is coupled to the generator and the converter via the crowbar interruption device.

50. The system of claim 40, wherein the converter is configured to activate the crowbar circuit.

51. The system of claim 50, wherein the converter is configured to cause the creation of a short circuit condition in the crowbar circuit.

52. The system of claim 50, wherein the crowbar circuit includes a switching device, and the converter is configured to gate the switching device.

53. The system of claim 40, further comprising detection circuitry configured to detect an operating condition on the utility power network.

54. The system of claim 53, wherein the operating condition comprises a fault condition.

55. The system of claim 40, further comprising the generator.

56. The system of claim 55, wherein the generator comprises a stator and a rotor, the stator connected to the utility power network via a stator interruption device, the crowbar circuit coupled to the rotor and the converter.

57. The system of claim 56, wherein the stator breaker is controlled by the converter.

58. The system of claim 57, wherein the converter is configured to cause the stator interruption device to open after activation of the crowbar circuit.

59. The system of claim 55, wherein the generator includes a wind turbine

60. A method for operating a system comprising a generator and a converter, the converter coupled between the generator and a utility power network, the method comprising:

activating a crowbar circuit coupled to the generator and the converter;

opening a crowbar interruption device in the crowbar circuit; and

closing the crowbar interruption device substantially immediately after opening the crowbar breaker.

61. The method of claim 60, wherein closing the crowbar interruption device includes closing the crowbar interruption device within about 0.5 seconds of the opening of the crowbar interruption device.

62. The method of claim 60, wherein opening the crowbar interruption device includes uncoupling the crowbar circuit from the generator and the converter.

63. The method of claim 60, wherein opening the crowbar interruption device includes allowing current in the crowbar circuit to decay to zero.

64. The method of claim 60, wherein activating the crowbar circuit includes gating a switching device in the crowbar circuit with the converter.

65. The method of claim 60, wherein activating the crowbar circuit includes creating a short circuit condition in the crowbar circuit.

66. The method of claim 60, the method further comprising detecting an operating condition on the utility power network.

67. The method of claim 66, wherein activating the crowbar circuit includes activating the crowbar circuit in response to detection of the operating condition.

68. The method of claim 67, wherein operating condition includes a fault condition.

69. The method of claim 60, the method further comprising remagnetizing the generator.

70. The method of claim 60, wherein the generator comprises a rotor and a stator, the stator connected to the utility power network via a stator interruption device,

wherein the method further comprises opening the stator interruption device after activation of the crowbar circuit.