WITH LOW VOLTAGE RIDE THROUGH

FIELD OF DISCLOSURE

This invention relates to generators of electrical energy, and in particular, to wind turbines.

BACKGROUND

A common type of wind-powered generator is a squirrel cage induction generator. In such generators, a current needs to be maintained in the stator windings, or else no current can be induced on the squirrel cage. This current can come from the power grid itself.

A problem arises when a voltage sag occurs in the power grid. Such voltage sags can occur as a result of a variety of disturbances, most of which are beyond anyone's control.

It is desirable in such cases for the squirrel-cage rotor to continue rotating during the voltage sag so that it can swiftly be reconnected to the grid once the voltage recovers. A generator that accomplishes this is said to "ride out" or "ride through" the low voltage event. Similarly, such a generator is said to possess "low-voltage ride through" capabilities.

One effect of a voltage sag is excessively high current in the generator. This in turn causes the rotor to spin faster. Many induction generators have protection

mechanisms that will stop or slow down the rotor when the rotor spins too fast. If, as a result of a low- voltage event, the squirrel-cage rotor spins fast enough to activate this protection system, the problem of reconnecting the induction generator to the grid will become more difficult.

Older induction generators used in wind turbines lack adequate low- voltage ride through capabilities. Because of this large installed base of induction generators, it is most economical to retrofit existing generators rather than to replace them. Known retrofit configurations rely on an uninterrupted power supplies or on voltage restorers. These are inserted in series between the wind turbine's transformer and the remainder of the electrical system.

The known retrofit methods for providing squirrel cage induction generators with low- voltage ride through tend to be costly, in part because of the many additional parts that are required. The complexity associated with their construction can lead to lack of reliability, and hence, higher maintenance costs. In addition, the voltage restorer is lossy.

SUMMARY

In one aspect, the invention features an apparatus for generating electricity from wind. Such an apparatus includes a wind-driven rotor; a transformer; a squirrel-cage induction generator, and a grid clutch. The squirrel cage induction generator includes a squirrel cage rotor driven by the wind-driven rotor for providing power to a utility grid via the transformer. The grid clutch disengages the squirrel-cage induction generator from the grid in response to a voltage sag and controls a speed of the squirrel-cage rotor during the voltage sag.

In some embodiments, the grid clutch includes an excitation condenser, a condenser switch, and a dynamic brake in parallel with the excitation condenser. The condenser switch selectively connects the excitation condenser to the stator windings. The dynamic brake includes a brake switch for selectively diverting current to a dump resistor for dissipation of energy.

Among the foregoing embodiments are those that also include a clutch controller for controlling the brake switch and the condenser switch. Some of these embodiments also include a sensor system in communication with the clutch controller. The sensor system provides information used by the clutch controller for controlling the brake switch and the condenser switch.

Also among the embodiments of the invention are those in which the grid clutch further includes a static transfer switch. Among these embodiments are those in which the grid clutch further includes a clutch controller for controlling the static transfer switch, the brake switch, and the condenser switch. Among the embodiments that include a clutch controller are those that include a sensor system connected to the clutch controller for providing the clutch controller with information for controlling the static transfer switch, the brake switch, and the condenser switch.

Yet other embodiments are those in which the grid clutch has a static transfer switch for selectively connecting the stator windings to the transformer. Some of these embodiments also include a clutch controller for controlling the static transfer switch. Among these embodiments are those that also include a sensor system for providing, to the clutch controller, information to be used in controlling the static transfer switch.

In yet another aspect, the invention features an apparatus for generating electricity from wind. Such an apparatus includes a wind-driven rotor; a transformer; a squirrel-cage induction generator having a squirrel cage rotor driven by the wind-driven collection surface for providing power to a utility grid via the transformer; means for selectively engaging and disengaging the squirrel-cage induction generator from the utility grid in response to a voltage sag; and means for controlling a speed of the squirrel-cage rotor during the voltage sag.

Another aspect of the invention is a method for causing a wind-driven squirrel cage induction generator to ride through a low- voltage event on a utility grid. Such a method includes detecting a low voltage event; in response to the low voltage event, disconnecting the squirrel cage induction generator from the utility grid; and while the squirrel cage induction generator is disconnected from the utility grid, causing the squirrel cage induction generator to maintain a voltage and current having a specified characteristic.

In some practices, causing the squirrel cage induction generator to maintain a voltage and current having a specified characteristic includes selectively causing charge to flow into stator windings of the squirrel cage induction generator.

In other practices, causing the squirrel cage induction generator to maintain a voltage and current having a specified characteristic includes selectively causing current to be induced in a squirrel cage rotor of the squirrel cage induction generator. Alternative practices of the invention also include those in which causing the squirrel cage induction generator to maintain a voltage and current having a specified characteristic includes selectively dissipating power generated by the squirrel cage induction generator. In these practices, causing the squirrel cage induction generator to maintain a voltage and current having a specified characteristic further includes selectively causing charge to flow into stator windings of the squirrel cage induction generator.

Other practices of the invention includes those in which causing the squirrel cage induction generator to maintain a voltage and current having a specified characteristic includes: monitoring electrical output of the squirrel cage induction generator; and in response to monitored electrical output, selectively connecting a condenser to the induction squirrel cage induction generator. In some of these practices, causing the squirrel cage induction generator to maintain a voltage and current having a specified characteristic further includes: in response to monitored electrical output, selectively dissipating power generated by the induction generator.

Yet other practices of the invention include the further steps of detecting restoration of line voltage on the utility grid; and in response, reconnecting the squirrel- cage induction generator to the utility grid.

These and other features of the invention will be apparent from the following detailed description and the accompanying figure.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 shows a configuration for a retrofitted squirrel-cage induction generator in a wind turbine.

DETAILED DESCRIPTION

FIG. 1 shows a wind-driven rotor 10 in mechanical communication with a gearbox 12. The gearbox 12 turns a squirrel cage rotor in a squirrel-cage induction generator for providing electric power to an electric utility grid 15 via a grid- side transformer 18. A grid clutch 20 between the generator 14 and the wind-driven rotor 10 selectively engages and disengages the squirrel-cage induction generator 14 from the transformer 18 in response to an instruction from a clutch controller 22.

The grid clutch 20 includes a static transfer switch 24 disposed to selectively interrupt current on a main bus 26 between the transformer 18 and the squirrel-cage induction generator 14. A typical static transfer switch 24 includes, for each phase, a pair of thyristors 28A, 28B in parallel but having opposite polarities, with thyristor gate currents under control of the clutch controller 22.

The grid clutch 20 also includes an excitation condenser 32 connected to the main bus 26. A condenser switch 34 under control of the clutch controller 22 selectively connects and disconnects the excitation condenser 32 from the main bus 26.

Many wind-turbine generators already have a condenser bank of power- factor control condensers for injecting reactive current, and thereby controlling power-factor. These power-factor control condensers can already be switched in and out of the circuit during normal operation. In these cases, the same condenser bank can be re-used as the excitation condenser 32 for supplying the excitation current. This is particularly advantageous since using the same condenser bank for two different functions avoids having to provide additional components.

In other wind turbine generators, the power-factor controlling condensers may not be rated for the larger currents necessary to provide excitation current. In these embodiments, a separate and additional condenser bank implements the excitation condenser 32. When this is the case, the power- factor controlling condensers should be disconnected from the squirrel-cage induction generator 14 when the excitation condenser 32 is to be connected to the squirrel-cage induction generator 14.

In parallel with the excitation condenser 32 is a dynamic brake 30 having a diode bridge 35 that rectifies the generator voltage, a brake switch 36 under control of the clutch controller 22, and a dump resistor 38 in parallel with the brake switch 36. By controlling the brake switch 36, the clutch controller 22 selectively causes current to pass through the dump resistor 38, thereby dissipating excess energy as heat. This, in turn, allows the clutch controller 22 to govern the speed of rotation of the squirrel-cage rotor at the squirrel-cage induction generator 14 when the static transfer switch 24 has disengaged the squirrel-cage induction generator 14 from the transformer 18.

The clutch controller 22 controls the static transfer switch 24, the condenser switch 34, and the brake switch 36 in response to measurements provided by three sensors: a transformer-side voltage sensor 40 for sensing transformer voltage, a generator-side voltage sensor 42 for sensing generator voltage, and a generator-side current sensor 44 for sensing generator current.

The transformer-side voltage sensor 40 is disposed between the static transfer switch 24 and the transformer 18 so that the clutch controller 22 can monitor the voltage at the grid 15 even if the static transfer switch 24 is open. The generator-side voltage sensor 42 and the generator-side current sensor 44 are disposed between the static transfer switch 24 and the squirrel-cage induction generator 14 so that these quantities can be measured even if the static transfer switch 24 has disengaged the squirrel-cage induction generator 14 from the transformer 18.

In response to detecting a voltage sag via the transformer-side voltage sensor 40, the clutch controller 22 commands the static transfer switch 24. Since a thyristor automatically stops conducting when current falls below a threshold, the static transfer switch 24 opens at the next zero crossing of the current. In addition, the clutch controller 22 closes the condenser switch 34, thus enabling reactive current for generator excitation to be supplied from charge stored on the excitation condenser 32. To maintain generator speed, the clutch controller 22 selectively opens and closes the brake switch 36, thereby selectively causing energy to be dissipated by the dump resistor 38. In a three-phase circuit, there would be one static transfer switch 24 for each phase. As a result, the static transfer switch 24 for one phase could be operated independently of the remaining two static transfer switches. The clutch controller 22 also senses, via the transformer-side voltage sensor 40, a recovery in grid voltage that would be sufficient to re-engage the squirrel-cage induction generator 14 and the grid 15. Upon sensing such a recovery, the clutch controller 22 begins the re-engagement procedure by re-synchronizing the voltage generated by the squirrel-cage induction generator 14 with the voltage prevailing at the grid 15. It does so by controlling the dynamic brake 30 so as to align the phase angles of the generator voltage and the grid voltage. Although the voltage magnitude at the squirrel-cage induction generator 14 may not be the same as that prevailing on the grid 15 at this point, re-synchronization can still be carried out by, for each phase, causing the static transfer switch 24 to re-engage that phase at its next zero crossing.

A wind-powered generator as described herein has difficulty in participating in voltage control of the grid 15 by injecting reactive current. To relieve this disadvantage, some embodiments also feature a three-phase converter that can be used to inject reactive current. To the extent such a converter is used only transiently, during a voltage sag, no cooling system would be required.

An advantage of the foregoing retro-fitted configuration arises from its low cost, which in turn results from the re-use of existing components. For example, many generators in the field already have compensation condensers that can be pressed into service as excitation condensers 32 for providing excitation current to the generator 14 after the grid clutch 20 has disengaged the generator 14 from the grid 15.

The retro-fitted configuration is also simpler than those in the prior art because it requires only a minimal number of additional components. To the extent simplicity breeds reliability, the foregoing retro-fitted configuration would also be more reliable than those in the prior art. Unlike known configurations, the foregoing configuration operates the squirrel- cage induction generator 14 in a self-excitation mode and maintains the speed of its squirrel cage rotor within circumscribed limits. In some embodiments, the clutch controller 22 is a processing system that executes clutch control instructions stored as software, firmware, or a combination of both on a tangible and non-transitory computer readable medium. Such a processing system includes various semiconductor devices that cooperate to manipulate positive and negative charge carriers contained therein.

The clutch control instructions cooperate to execute a method for controlling the switches in response to measurements from the sensors described above. As such, the method is tied to a particular machine that comprises at least the foregoing switches and sensors. The foregoing switches are clearly made of matter as that term is understood by those of skill in the art. The opening and closing of switches is clearly a transformation of matter. Therefore, the method is not only tied to a particular machine as described above, its execution also results in a transformation of matter.

Having described the invention, and a preferred embodiment thereof, what we claim as new, and secured by letters patent is: