Speed sensing circuit for a wind turbine generator

A protection circuit for a wind turbine generator that includes a PWM Brake that works in conjunction with known Brake Relays is disclosed. The Brake Relay is used to short the generator output terminals at a first threshold voltage. The PWM Brake includes one or more switching devices, coupled across the generator output. The PWM Brake is under the control of a PWM Brake Control Circuit which actuates the PWM Brake at a second threshold voltage that is relatively lower than the first threshold voltage. In accordance with an important aspect of the invention, the PWM Brake Control Circuit includes a novel speed sensing circuit for providing a signal representative of the speed of the turbine generator The novel speed sensing circuit eliminates the need to mount a speed sensor on the pole top mounted turbine generator. As such, the need for adding cabling from the pole top mounted wind turbine generator is eliminated. The novel speed sensing circuit provides a signal representative of the rotational speed of the turbine generator based upon the duty cycle of a pulse width modulated (PWM) signal that is derived from the drive signal developed by the PWM Control Circuit. This signal is used to alternatively actuate and close the Brake Relay to minimize actuation of the centrifugal switch.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a protection circuit for a wind turbine generator and more particularly to a PWM Brake Control Circuit with novel speed sensing.

2. Description of the Prior Art

Wind turbine generator systems are generally known in the art. Examples of such systems are disclosed in U.S. Pat. Nos. 4,565,929; 5,506,453; 5,907,192; 6,265,785; and 6,541,877. Such wind turbine generator systems are also described in U.S. Patent Application Publication Nos. US2002/0117861; 2005/0034937; 2005/0230979; 2005/0236839; 2006/0006658; and 2006/0012182. Such wind turbine generator systems are known to include pole mounted turbine generators. The wind turbine generator systems also include an inverter and protection circuitry.

Due to the ever-increasing demand and increasing cost for electrical power, renewable energy sources, such as wind turbine generator systems, are becoming more and more popular for generating electrical power. Such wind turbine generator systems are known to be used individually to generate supplemental or excess power for individual, residential or light industrial users to generate electrical power in the range of 1-2 kw. Such wind turbine generator systems are also known to be aggregated together, forming a wind turbine generator farm, to produce aggregate amounts of electrical power. It is also known that unconsumed electrical power generated by wind turbine generators is connected to the utility power grid.

Such wind turbine generators are known to include a wind turbine, which includes a plurality of turbine blades connected to a rotatable shaft. The rotatable shaft is rigidly connected to a direct current (DC) generator. Wind causes rotation of the wind turbine which acts as the prime mover for a DC generator. The generator, for example, a self-excited generator, generates DC electrical power.

One problem with such systems is that wind speeds are not constant. As is known in the art, the voltage output of the generator is a cubic function of the speed of the speed of the wind. As such, the effect of wind gusts on the wind turbine generator must be controlled to prevent damage to the wind turbine generator.

Some wind turbine generator systems are known to use some type of mechanical braking to protect the wind turbine generator from an over speed condition. For example, U.S. Pat. No. 5,506,453 utilizes the pitch of the wind turbine blades to protect the wind turbine from over speed. In particular, the blades of the wind turbine are mechanically coupled to a rotatable mechanical hub. The blades are configured so as to be rotatable about their longitudinal axis relative to the hub allowing the pitch of the turbine blades to be varied. The pitch of the blades is turned in such a way as to create braking of the wind turbine.

Other known systems utilize mechanical brakes, such as disclosed in U.S. Patent Application Publication No. US 2005/0034937. Yet other systems disclose the use of aerodynamic-type brakes as well as mechanical brakes, for example, as disclosed in U.S. Pat. No. 6,265,785, to protect the wind turbine from over speed.

While mechanical brakes do an adequate job of protecting the wind turbine generator from over speed, mechanical braking systems do little to optimize the operational time and thus power output of the wind turbine generator. Moreover, such mechanical braking systems are mechanically complex and are, thus, relatively expensive.

As such, electrical braking systems have been developed to control over speed of wind turbine generator systems. For example, Japanese Patent Publication JP2000-179446 discloses an electrical braking system known as a Brake Relay for a wind turbine generator. The system disclosed in the Japanese patent publication includes a Brake Relay whose contacts are connected across the output terminals of the generator. In this application, since the speed of the turbine is proportional to the voltage at the generator output, the generator voltage is used to trigger this system. When an over speed condition (i.e. over voltage) is detected, the Brake Relay is energized which, in turn, shorts out the output terminals of the generator, which loads the generator and causes it to slow down and stop.

Such Brake Relays are known to be under the control of a Brake Relay Control Circuit, which actuates the Brake Relay as a function of a voltage representative of the generator output voltage. As mentioned above, output voltage of the generator is a cubic function of the wind. Thus, wind gusts are known to cause the generator voltage to rise high enough to trip the Brake Relay, thus causing a disruption of power delivered to the AC power grid.

Referring toFIG. 1, a conventional wind turbine generator system20is illustrated, generally identified with the reference numeral20. The wind generator system20includes a generator22, such as, a self-excited DC generator, a wind turbine (not shown) and an inverter28and generator protection circuitry. The wind turbine functions as a prime-mover for the generator22. The generator22generates a DC voltage across its output terminals24,26as a cubic function of the wind. In as much as the generator22is directly coupled to the wind turbine, the rotational speed of the turbine and generator is directly proportional to the wind speed. As such, the output voltage at the generator terminals24and26is a cubic function of the wind speed.

The output terminals24,26of the generator22are coupled to an inverter, shown within the block28. The inverter28converts the DC output voltage, available at the output terminals24,26of the generator22, to an AC voltage suitable for connection to a utility AC power grid, generally identified with the reference numeral30. The AC power grid30may be a phase to phase 230/240 Volts AC, suitable for residential, commercial and industrial application. In the exemplary embodiment shown, shown, the inverter28generates a phase to phase voltage across two output phases L1and L2, for example, 230/240 Volts AC.

Depending on the configuration of the utility AC power grid30, the inverter28may also include a ground conductor for use with utility AC power grids which are 230/240 Volts AC with a center tap ground, for providing 230/240 Volts AC phase to phase and 115/120 Volts AC phase to ground. In such a system, the inverter ground conductor (not shown) would be electrically coupled to the utility center tap ground. The wind turbine generator systems20may be configured to be connected to various configurations of the utility AC power grid30.

The phase to phase output L1and L2Of the inverter28is connected to the utility AC power grid30by way of an AC grid relay32. As mentioned above, the grid relay32is used, among other things, to disconnect the wind turbine generator system from the utility AC power grid30during abnormal conditions relating to the voltage, phase or frequency of the AC power grid30. The grid relay32is under the control of an AC Relay Control Circuit34. The AC Relay Control Circuit34monitors the phase of the output of the inverter28and the phase of the utility AC power grid30. When the phase of the inverter output is synchronized with the phase of the utility AC power grid30, the AC Relay Control Circuit34causes the grid relay32to connect the two together.

In order to protect the wind turbine generator system20from damage from over speed resulting from wind gusts, as mentioned above, some wind turbine generator systems20include a Brake Relay36. The Brake Relay36is connected across the output terminals24,26of the generator22. The Brake Relay36may be an electromechanical relay that shorts the output terminals24,26of the generator22together for a nominal time period during an over speed condition. Shorting the terminals24,26of the generator22together creates a load on the generator22and slows down and eventually stops the generator22, thus acting as an electronic brake. In order to protect the generator from damage due to the variability of the wind speed, many known wind turbine generator systems20, continuously monitor the output voltage of the generator22at a DC Measurement Point. When the output voltage of the generator22exceeds a predetermined threshold voltage, for example, 320 Volts DC (“first threshold”), indicative of an over speed condition, the conventional Brake Relay Control Circuit38activates the conventional Brake Relay36, which shorts the terminals24,26of the generator22and maintains the short circuit condition, thus shutting down the generator22, for a nominal time period, such as 10 seconds or more, for example. This shut down condition causes the wind turbine generator system20to be off-line during a wind condition in which the system could be delivering maximum power to the utility AC power grid30. As such, this condition makes wind turbine energy systems20less desirable as a renewable energy source.

In order to provide a protection system which minimizes the disruptions in power while still providing sufficient protection to the turbine-generator system, it is necessary to trigger a generator protection circuit based upon the over speed of the generator However, wind turbine generators are pole mounted. Thus any speed sensors to measure the speed of the turbine generator need to be mounted on the turbine-generator and cabled from the pole top mounted generator. Considering the number of wind turbine generators in a wind turbine generator farm, such cabling would substantially add to the cost of the overall wind turbine generator system. Thus, there is a need for protection circuitry for a wind turbine generator system that is triggered on over speed of the generator which does not require a sensor mounted on the turbine generator mounted on a pole top.

SUMMARY OF THE INVENTION

The present invention relates to a protection circuit for a wind turbine generator that includes a PWM Brake that works in conjunction with known Brake Relays. The Brake Relay is used to short the generator output terminals at a first threshold voltage. The PWM Brake includes one or more switching devices, coupled across the generator output. The PWM Brake is under the control of a PWM Brake Control Circuit which actuates the PWM Brake at a second threshold voltage that is relatively lower than the first threshold voltage. In accordance with an important aspect of the invention, the PWM Brake Control Circuit includes a novel speed sensing circuit for providing a signal representative of the speed of the turbine generator The novel speed sensing circuit eliminates the need to mount a speed sensor on the pole top mounted turbine generator. As such, the need for adding cabling from the pole top mounted wind turbine generator is eliminated. The novel speed sensing circuit provides a signal representative of the rotational speed of the turbine generator based upon the duty cycle of a pulse width modulated (PWM) signal that is derived from the drive signal developed by the PWM Control Circuit. This signal is used to alternatively actuate and close the Brake Relay to minimize actuation of the centrifugal switch.

DETAILED DESCRIPTION

FIG. 1is a block diagram of a wind turbine generator system20that incorporates a prior art Brake Relay36for electronically braking the generator.22.FIG. 2is a block diagram of a wind turbine generator system20that incorporates a prior art Brake Relay36for electronically braking the generator22and additionally incorporates a PWM Brake40that is coordinated with the prior art Brake Relay36.FIGS. 3-5relate to a PWM Brake Control Circuit42minus the novel speed sensing circuit in accordance with the present invention.FIGS. 6-8relate to the novel speed sensing circuit in accordance with the present invention.

As mentioned above, some known systems utilize a brake relay, used to short out the wind turbine generator output for a nominal period, such as 10 seconds or more, any time the generator voltage exceeds a threshold level indicative of an over speed condition (i.e “first threshold voltage”). Thus, during conditions when high wind speeds exist and the opportunity to export maximum power, the brake relay in known wind turbine generator systems shuts down the generator for nominal time periods. The PWM Brake Control Circuit42with a novel speed sensing circuit in accordance with the present invention solves this problem by applying electronic braking to the generator when the output voltage of the generator voltage exceeds a second predetermined threshold voltage indicative of an over speed condition, hereinafter referred to as “the second threshold voltage”. The second threshold voltage is selected to be relatively lower than the first threshold voltage, used to trigger a conventional Brake Relay36. As such, the PWM Brake Control Circuit42minimizes the operation of the Brake Relay36, thereby maximizing the power output of the generator22over time making such wind turbine generator systems20much more practical as a renewable energy source. In order to further optimize the power output to the AC power grid, the PWM Brake Control Circuit42includes a novel speed sensing circuit which actuates the PWM Brake Relay40upon detection of a generator over speed condition. The novel speed sensing circuit is described below in connection withFIGS. 7 and 8.

Referring toFIG. 2, a protection circuit for a wind turbine generator system20is illustrated. The protection system includes a conventional Brake Relay36, a conventional Brake Relay Control Circuit38, a PWM Brake40, a PWM Brake Control Circuit42, a DC Measurement circuit58, for example, a diode44and a capacitor46, an inverter28, a grid relay32and a AC Relay Control Circuit34.

As shown, the wind turbine generator system20also includes an inverter28. Inverters are extremely well known in the art and are used to convert DC electrical power to AC electrical power. Various inverters28may be used with the present invention. Exemplary inverters which may be used are disclosed in U.S. Pat. Nos. 5,552,712; 5,907,192 and 6,256,212 and US Patent Application Publication No. US 2005/0012339 A1, all hereby incorporated by reference.

The PWM Brake40is a switching device that is electrically coupled across the generator output to short out the generator which provides electronic braking. The PWM Brake40is under the control of a PWM Brake Control Circuit42. The PWM Brake Control Circuit42incorporates the novel speed sensing circuit for actuating the Brake Relay38during an over speed condition of the generator22. In accordance with an important aspect of the invention, the novel speed sensing circuit is able to provide a signal representative of generator over speed without the need to locate a speed sensor on the pole top mounted turbine generator.

With reference toFIG. 2, the PWM Brake Control Circuit42is used to control a pulse width modulated (PWM) Brake40that is also coupled across the output terminals24,26of the generator22and is in parallel with the Brake Relay36. As discussed in more detail below, the PWM Brake40is a switching device, such as a FET. The PWM Brake Control Circuit42continuously monitors the generator output voltage at the DC Measurement Point and compares the generator output voltage with the second threshold voltage, for example, 300 Volts DC, relatively less than the first threshold voltage used to trigger the brake relay36. Thus, when the generator output voltage exceeds the second threshold voltage but is less than the first threshold voltage, the PWM Brake Control Circuit42causes the PWM Brake40to short out the generator output to slow down the generator22. The second threshold voltage is selected to slow down the generator22and prevent operation of the Brake Relay36. As such, the PWM Brake40and the PWM Brake Control Circuit42are designed to minimize if not eliminate operation of the Brake Relay36, thus optimizing the operation of the wind turbine generator system20and maximizing the power exported to the utility AC power grid30.

The DC output voltage of the generator22may be measured by a DC Measurement Circuit58or a sensor. In particular, the DC Measurement Circuit58may include a diode44and a capacitor46. With such a configuration, the DC Measurement Point (i.e. cathode of the diode44is separated from the generator22by way of the diode44. The measurement side of the diode44may be coupled to relatively large metal film hold up capacitor46, for example, 1000 microfarads, which holds the generator output voltage relatively constant during measurement once the capacitor46is fully charged defining the DC Measurement Point. When the generator output voltage at the DC Measurement Point reaches a predetermined voltage (i.e. the second threshold voltage), the PWM Brake Control Circuit42generates a drive signal to actuate the PWM Brake40. As will be discussed in more detail below, the PWM Brake40may be configured as an n-channel MOSFET, coupled across the generator output terminals24,26. In such a configuration, the drive signal from the PWM Brake Control Circuit42is applied to the gate terminal of the n-channel MOSFET. When the drive signal is pulled high, the MOSFET is turned on. This condition looks like a short to the generator22. The short across the generator22slows the turbine down with a corresponding decrease in the generator output voltage. At this point, the voltage from the generator22falls below the voltage of the DC Measurement Point (i.e. the voltage on the capacitor46). This condition back biases the series diode44, effectively isolating the generator22from the DC Measurement Point. The hold up capacitor46, coupled to the DC measurement point, is used to supply current to a fly back section of the inverter28during a flyback mode. While the capacitor46supplies current to the inverter28, the voltage at the DC Measurement Point (i.e. voltage on the capacitor46) will decrease to a point below the value of the second threshold voltage. When the voltage on the capacitor46drops below the second threshold voltage, the PWM Control circuit42generates a low signal that is applied to the gate of the MOSFET causing the MOSFET to turn off. Once the MOSFET is turned off, the turbine can now spin freely under just inverter loading and the DC input voltage from the generator will change according to the available wind speed.

The ramp-up voltage of the generator22is moderated by the load presented to the generator22through recharge of the holdup capacitor46. The recharge time of the capacitor46allows ample time for the MOSFET to turn off. The effect is to set up a PWM regulator whose duty cycle is inversely proportional to the DC voltage. The controlled voltage allows for the generator22to operate under a much wider band of wind speed than would normally be possible with the electromechanical method.

FIG. 3illustrates an exemplary analog embodiment of a PWM Brake Control Circuit, shown within the dashed box42A, is used to control the PWM Brake40, as described above. The PWM Brake Control Circuit42A is an analog circuit and includes a comparator52and a driver circuit, generally identified with the reference numeral54. The second threshold voltage or reference56is applied to an inverting input of the comparator52. The generator output (i.e. cathode of the diode44), identified inFIG. 3as the DC Measurement Point, is applied to a non-inverting input of the comparator52.

The generator output voltage may alternatively be sensed by a sensor or virtually any means for providing a signal representative of the generator output voltage. For example, the sensors may include a step down transformer.

When the output voltage of the generator22at the DC Measurement Point exceeds the second threshold voltage or Reference56, the output of the comparator52goes high, thus actuating the PWM Brake40to effectively short the output terminals24,26of the generator22. As mentioned above, the output of the comparator52will remain high until the voltage on the capacitor46(FIG. 2) drops below the second threshold voltage or Reference56plus the hysteresis set up on the comparator120and the resistors126and127to add dead band to limit the frequency of the PWM Brake40turning on and off. At that point, the output of the comparator52will go low, thus providing PWM control of the PWM Brake40.

The output of the comparator52may be applied to a driver circuit54. The driver circuit54illustrated inFIG. 3is merely exemplary and includes a pair of serially coupled resistors60and62. The output of the comparator52is applied to a node defined between the serially coupled resistors60,62. One resistor is coupled to a voltage source V1. The resistors60and62act as a voltage divider to pull up the output of the comparator52to a predetermined value. The driver circuit54also includes a pair of complementary bipolar junction transistors64and66connected in a push-pull configuration. More particularly, the transistor64is a NPN transistor while the transistor66is a PNP. The bases and emitters of the transistors64and66are coupled together. The collector of the transistor64is pulled high by way of a pull up resistor68. The collector of the transistor66is pulled low and is connected to ground. The emitters of the transistors64and66are coupled to the PWM Brake40.

In operation, when the output of the comparator52is low, the PNP transistor66is turned on, connecting the PWM Brake40n channel FET gate to ground, in which case n-channel MOSFETS used as the PWM Brake40, remain off. When the output of the comparator52goes high, the PNP transistor66turns off and the NPN transistor64turns on. This causes the PWM Brake to be pulled high, thus causing the n-channel MOSFET, used for the PWM Brake40to be turned on, effectively shorting the generator22.

An exemplary alternate digital embodiment of the PWM Brake Control Circuit is illustrated inFIG. 4and generally identified with the reference numeral42D. The PWM Brake Control Circuit42D is used to control the PWM Brake40. The PWM Brake Control Circuit42D includes a microprocessor72and a driver circuit74. A flow diagram for the microprocessor is illustrated inFIG. 5. The voltage at the DC Measurement Point (i.e. voltage at the cathode of the diode44, as illustrated inFIG. 2) is monitored by the microprocessor72.

Referring toFIG. 5, monitoring of the voltage at the DC Measurement Point may be interrupt driven, as indicated by step76. Upon an interrupt, the analog DC voltage from the DC Measurement Circuit58is converted to a digital value by an on-board analog to digital converter (not shown), as indicated in step78. The system then checks in step80if the value of the voltage at the DC Measurement Point is greater than a PWM upper limit (i.e. second threshold voltage plus a constant). If so, the PWM Brake40is actuated in step82and the n-channel MOSFET is turned on to short the generator22. The system then continues its processing in step84after servicing the interrupt.

If the system determines in step80that the voltage at the DC Measurement Point is not greater than the PWM upper limit (i.e. second threshold voltage plus a constant), the system checks in step86whether the voltage at the DC Measurement Point is less than or equal to a PWM lower limit (i.e. over speed threshold minus a constant) in step86. If not, the system returns to step84and continues its processing. If it is determined in step86that the voltage at the DC Measurement Point is less than the PWM lower limit, for example, due to a voltage on the capacitor46, the PWM Brake40is turned off in step88. The upper and lower PWM limits are used to set the duty cycle of the PWM.

The driver circuit74(FIG. 4) includes a current limiting resistor76, a pair of BJTs78,80, configured as a voltage enhancement circuit, a pair of load resistors82,84coupled to the collector terminals of the transistors78and80and a pair of complementary BJTs,86,88, connected in a push-pull configuration. The base and emitter terminals of the transistors86and88are coupled together. The base terminals of the transistors86and88are coupled to the collector of the NPN transistor80. The emitter terminals of the transistors86and88are tied to the PWM Brake40. The emitter terminals of the NPN transistors78and80are connected to ground.

In operation, whenever the microprocessor74outputs a high signal on its I/O port, the NPN transistor78is turned on, the NPN transistor80is turned off, connecting the base terminal of the PNP transistor88and the base terminal of the PNP transistor86to the high DC rail by way of the resistor84, thus turning off the PNP transistor88and turning on the NPN transistor86. As mentioned above, the PWM Brake40may be configured as an n-channel MOSFET. As such when the NPN transistor86is turned on, the MOSFET will be turned on. Thus allowing it to turn on and connect the positive voltage DC voltage rail to the DC Brake40. This causes the n-channel MOSFET, used as the PWM Brake40, to turn on. Alternatively, when the I/O port of the microprocessor72is forced low, the NPN transistor78is turned off and the PNP transistor80is turned on. During this condition, the base of the transistor86goes to Vce Saturation the transistor88is turned on and the MOSFET will be turned off.

In accordance with an important aspect of the invention, the Brake Relay36is configured to trip or short the generator22at a speed set point, relatively less than the speed at which the mechanical centrifugal switch, discussed above, actuates. By setting the speed set point lower than the speed of the centrifugal switch. The Brake Relay36is able slow the generator22down, for example, by way of the Brake Relay Control circuit38, as discussed above, prior to the rotational speed of the turbine generator reaching the speed at which the centrifugal switch operates, thus minimizing if not eliminating the operation of the mechanical over protection system and the need for manual reset of such systems.

In practical operation, however, the PWM Brake Control Circuit effectively clamps the DC voltage at the DC measurement point to the level of the second threshold voltage. The subsequent action of the PWM Control Circuit tends to make the DC voltage of this DC measurement point not change with increasing the wind speed. In times of wind gusts, the generator can continue to increase in speed while the PWM control Circuit clamps the DC voltage to the second threshold level. In such circumstances, it is possible for the generator speed to exceed the set limits of the centrifugal brake before the Brake Relay is activated since the DC voltage is clamped below the first threshold voltage of the Brake Relay. Hence, there is a need for an additional method to control the Brake Relay before the centrifugal brake threshold is reached. One method to resolve this would be to monitor the speed of the generator however, this would require adding cables to the generator.

In order to avoid the need for cabling between the remote wind turbine generator, normally pole mounted, and the PWM Brake Control Circuit42, a novel speed sensing circuit, generally identified with the reference numeral110(FIG. 6), may be used which provides a signal representative of the speed of the turbine generator. This signal is based upon based upon the duty cycle of a pulse width modulated (PWM) signal that is derived from the generator output voltage, that is available at the PWM Brake Control Circuit42, discussed above (i.e point100,FIG. 3or point112,FIG. 4). As shown inFIG. 7, the input signal to the PWM Brake40, developed by the PWM Brake Control Circuit42, is a train of pulses114,116and118having a certain magnitude and varying pulse widths or duty cycles depending on the wind speed and the second threshold voltage or reference56. As mentioned above the second threshold voltage or reference56is based upon the over speed condition which causes the PWM Brake40to actuate prior to the Brake Relay36Thus, in order to coordinate the operation of the PWM Brake40with the Brake Relay36and the centrifugal switch, mentioned above, the pulses114,116and118are fed into the speed sensing circuit110. In particular, the speed sensing circuit110is configured to provide a speed set point below the speed set point of the centrifugal switch. This speed set point is used to trip the Brake Relay36.

With such a configuration, the protection of the generator22is well coordinated. In particular, as the speed of the generator22increases, the PWM Brake40will actuate at the lowest speed set point (i.e. second threshold voltage). As mentioned above, the PWM Brake causes the generator output voltage to be clamped at the second threshold voltage. As further mentioned above, it is possible for the speed of the generator22to exceed the set point of the centrifugal switch causing the centrifugal switch to trip before the Brake Relay36is actuated since the DC voltage is clamped below the first threshold voltage. In order to prevent this condition in which the centrifugal switch trips before the Brake Relay36, the Brake Relay36is actuated at the first threshold voltage or whenever the speed of the generator22exceeds the speed set point as generated by the speed sensing circuit110. Whenever, either of those conditions are detected by the microprocessor124(FIG. 6), a signal is generated by the microprocessor124to actuate and close the Brake Relay36.

In accordance with an aspect of the invention, the duty cycle of the pulses114,116and118is used to provide a signal representative of the speed of the generator. In particular, as discussed above, the duty cycle of the input pulses to the PWM Brake40represent braking periods by the PWM Brake40. The longer braking periods represent higher wind speeds and thus higher speeds of the generator22. As such, the duty cycle of the pulses114,116and118can be used to generate a signal representative of the speed of the generator22. This signal can be used to actuate the Brake Relay36or other load across the generator windings to slow the generator down and prevent actuation of the centrifugal switch.

The speed sensing circuit110includes a comparator120, an opto-coupler or other isolation device122and a microprocessor124. The input to the PWM Brake40(i.e. point100,FIG. 3or point112,FIG. 4) is applied to a non-inverting input of the comparator120. The output of the comparator120is tied to ground by way of a grounding resistor127and shorted to the inverting terminal and pulled high by way of a pull-up resistor126. When the PWM Brake40is off, the input to the non-inverting input is low, causing the opto-coupler122to be off, for example, causing the input to the microprocessor124to be high. When the wind speed picks up, turning on the PWM Brake40, the input to the PWM Brake40toggles from low to high, causing a high input to the non-inverting terminal of the comparator120causing the output of the comparator120to toggle high, which turns on the opto-coupler122and causes it drive the input to the microprocessor124low.

The microprocessor124can now measure the duty cycle of these cycles as a signal representative of the speed of the generator22. The microprocessor124can compare the speed representative signal with a threshold signal to initiate activation of the Brake Relay36or other load across the generator22at a speed relatively less than the speed at which the centrifugal switch operates. Referring toFIG. 8, the system initially checks in step126the DC voltage at the DC Measurement Point (FIG. 1) If the DC voltage is greater than the first threshold value, for example, 320 Volts DC, the system proceeds to step132and closes the Brake Relay36. If not, the system measures the time of the duty cycle of the input pulses114,116and118at the input of the Brake Relay36in step128. In step130, the system checks whether the length of time of the duty cycle is less than a predetermined speed threshold value, for example, whether the 500 ms average value of the duty cycle is less than, for example, 49%. The predetermined speed threshold value is determined experimentally to be a value less than the speed at which the centrifugal switch, mentioned above, operates. If the measured duty cycle, which represents the speed of the generator22, is less than the predetermined speed threshold value, i.e. 49%, indicating that the generator is in an over speed condition, the system proceeds to step132and actuates and closes the Brake Relay36. If not, the system returns to step134.

FIG. 9Aillustrates the power exported by a conventional wind turbine generator system20as illustrated inFIG. 1.FIG. 9Billustrates the power exported by the wind turbine generator system20. Referring first toFIG. 9A, the curve90is an exemplary curve of the wind speed as a function time. The line92represents the lockout threshold value, for example, 10 meters per second. As shown, as the wind speed increases above the first threshold, the Brake Relay36shorts out the generator22resulting in no power being exported to the utility AC power grid30for a nominal period of 10 seconds or more. After the nominal period expires, as the wind speed drops below the threshold92, the wind turbine generator system20exports power, as indicated by the curve94, until the wind speed goes above the lockout threshold92. As shown inFIG. 9A, this occurs at about 12 minutes. The wind turbine generator system20is again shut down for a nominal period. After the second shutdown period, as the wind speed drops below the shutdown threshold, the wind turbine generator system again begins exporting power at about 21 minutes, as indicated by the curve96. Thus for the 24 minute time period illustrated inFIG. 9A, the total power exported to the utility AC power grid30is the sum of the areas under the curves94and96. For the exemplary data indicated inFIG. 9A, the total power exported is 91 watts-hours.

FIG. 9Billustrates the power exported by a wind turbine generator system20. utilizing the PWM Brake40and the PWM Brake Control Circuit with novel speed sensing in accordance with the present invention. For the same wind speed curve90illustrated inFIG. 9A. In this case, the dotted line96represents the over speed threshold, for example 10 meters per sec. The over speed threshold is selected to be lower than the shutdown threshold. As shown, any time the wind speed exceeds the over speed threshold96, the PWM Brake40electronically brakes the generator22to allow maximum power, for example, 1000 watts, to be exported by the generator from about 0.5 minutes to about 6 minutes, as indicated by the segment98of the curve100. With the conventional system, as illustrated inFIG. 9A, the wind turbine generator system was shut down during this same time period and exported no power. As the wind speed drops off during the time period from about 6 minutes to 12 minutes, the power exported drops below the maximum as a function of the wind speed. From 14 minutes to 18 minutes, the system exports maximum power, as indicated by the line segment102. During this same time period, the conventional wind turbine generator system20was shut down because the wind speeds exceeded the first threshold and thus exported no power during this period. From 18 minutes to 24 minutes, the wind turbine generator system20exported power to the utility AC power grid30as a function of the wind speed, which remained below the lockout threshold and the over speed threshold. The total power exported by the wind turbine generator is 350 Watt-hours, significantly higher than the conventional system illustrated inFIGS. 1 and 9A.

The PWM Brake40and the PWM Brake Control Circuit42in accordance with the present invention is configured to coordinate with mechanical over speed protection systems in order to minimize operation of such systems and thus reduce the need for cumbersome physical resets of such systems. In accordance with another important aspect of the invention, the PWM Brake Control Circuit42may incorporate a speed sensing circuit for providing a signal representative of the speed of the turbine generator. In order to avoid adding cabling from the remote wind turbine generator, the speed sensing circuit may be configured to determine the rotational speed of the turbine generator based upon the duty cycle of a pulse width modulated (PWM) signal, available at the output of the PWM Brake Control Circuit42, discussed above.