GB2457136A

GB2457136A - Wind turbine control system

Info

Publication number: GB2457136A
Application number: GB0802039A
Authority: GB
Inventors: Colin Lawrence Amess; Ian Robert Casselden
Original assignee: Individual
Current assignee: Individual
Priority date: 2008-02-05
Filing date: 2008-02-05
Publication date: 2009-08-12
Anticipated expiration: 2028-02-05
Also published as: GB0802039D0; GB2457136B

Abstract

A closed loop electronic control system for wind turbines to extract the maximum power available from variable wind speeds by maintaining operation of the turbine close to the Betz limit. Two control systems are claimed, both comprise two anemometers to measure the wind speed arriving 4 and leaving 5 the blades of a wind turbine and a method of transforming the impedance of a fixed resistive power load with controls 6 to alter a switched mode voltage convertor 3 that supplies power to a force cooled resistive load 8. This works to modify the braking force and thus give the optimum angular velocity of the turbine blades. The first embodiment uses computing means to calculate the optimum velocity in order to satisfy the Betz criteria. The second embodiment uses computing means to calculate the power delivered to the heating element as a function of the output voltage with respect to the incident and exhaust wind speeds. It also using a fuzzy logic system to vary the switcher voltage gain using a recursive iteration to maintain the maximum power extraction. The resistive load may be used to heat a liquid or gas 7 for domestic consumption, e.g. hot water.

Description

An adaptive small-scale wind turbine closed loop control system.

This invention relates to an electronic control system that allows small-scale wind generators to extract the maximum power available from variable wind speeds by maintaining operation of the turbine close to the Betz limit.

Description of the related art

Large wind turbines are designed to operate with the turbine rotating at a constant angular velocity and employ mechanical gear trains that are selected to drive the generator to deliver the optimum energy extracted from specific ranges of wind speeds containing thc majority energy within the wind speed/frequency profiles that are determined a priori from historical site wind speed surveys. A substantially constant angular velocity is required so that the sinusoidal output from the generator can be synchronised to the grid frequency.

Such generators may also use closed ioop systems to vary the field current of the generators according to wind speed to further regulate the turbine loading and hence the corresponding change in power delivered by the generator into the grid while keeping the generator frequency constant.

Small-scale wind generators are generally not designed to operate with the turbine blades rotating at a fixed angular velocity and are often used to either charge batteries or power fixed restive load heat storage systems. These systems are inefficient as the power delivered into the storage system provides an uncontrolled braking effect on the generator thus slowing the angular velocity of the turbine to an indeterminate velocity.

The ratio of incident and exhaust wind velocities acting on the turbine blades will vary and the turbine will operate outside the Betz criteria for optimum power extraction.

More sophisticated power delivery methods employ electronic systems that convert the power delivered by the generator into a synthesised sinusoid of constant frequency and in phase with the local power grid. National laws and directives place stringent requirements on the purity of the sinusoidal waveform and the level of harmonics that can be tolerated and this is reflected in the high capital costs of the electronics. More importantly, there are intrinsically high power losses associated with synthesis of sinusoids.

According to the present invention it is possible to extract the maximum available power from a given wind speed to a level approaching the Betz limit. The braking load presented by a generator to the turbine blades may be arranged such that the turbine blades rotate at an optimum angular velocity with respect to the given wind speed incident on the turbine blades. As the wind speed changes, a voltage converter whose input is connected to the output of the generator may be varied in order to modify the power into the load and hence modify the braking affect such that the turbine blades angular velocity is changed and permits the air behind the blades to carried away at a velocity relative to the incident wind that satisfies the Betz criteria. The output of the voltage converter is intended to produce large variations in voltage driving current into the load. This mode of operation fundamentally differs from the operation of a conventional large-scale generator where wind power feedback mechanisms are used to maintain constant angular frequency of the turbine generator and to restrict the amplitude of the generator voltage applied to the grid (load) to small changes only.

According to the invention, the output from the switching voltage converter may be connected to resistive loads within water heating or forced air heating systems electrically isolated from the grid thus providing a means of storing energy or extracting heat from a supply source of large variation in voltage amplitude and is immune from wideband harmonic content and is well suited to the use of highly efficient switched mode voltage converters.

Description of the invention

According to the present invention as set out in claim 1 and claim 2, a closed loop adaptive control system is capable of varying the impedance of a fixed electrical load as seen' by the generator coupled to a wind turbine in order to match the output power provided by the generator to the maximum wind power available to the generator to achieve optimal wind power extraction efficiency. The adaptive variation in load is set within a closed control loop with computational regard to the incident wind velocity and the angular velocity of the turbine blades and the exhaust velocity of the air escaping behind the turbine blades.

The adaptive wind power extraction system employs wind speed sensors mounted in such a position to measure the wind speed arriving and leaving the turbine blades.

Wind speed information is fed to a computational processor module that computes the angular velocity that the turbine should be rotating to achieve the optimum wind power extraction. The output from the computational processor is used to control signals to vary the output voltage of a voltage converter, the input of the voltage converter being connected to the output of the turbine generator. A fixed resistance-heating element is connected to the output of the voltage converter.

When light winds blow, the voltage converter will only supply a small voltage across the load resistance and the converter will only demand a small current from the generator and a correspondingly small braking affect will be applied to the turbine thereby only slightly slowing the rotation of the blades. If the wind were to increase in speed, the computational processor module would identify that the velocity of the air escaping behind the blades has increased too greatly compared with the incident wind speed. The computational processor then sends a demand for the voltage converter to increase the voltage across the heating element. As more current flows into the heating element, a greater braking effect is applied to the turbine generator that serves to slow the rotational velocity of the blades and as a consequence, the escape velocity of air behind the blades will reduce to satisfy the new input/output wind speed ratio to maintain maximum wind power extraction efficiency.

The computational module has interfaces and inputs that allow the measurement of the angular velocity of the turbine blades. A memory means may also hold information on specific turbine aerodynamic performance as computational input parameters used in the algorithmic computation of optimal turbine velocity.

A computational module may also provide fuzzy logic' control of the power delivered to the restive load with regard to incident and exhaust wind speed, turbine blade velocity and turbine blade aerodynamic parameters.

The adaptive wind power extraction systems described above could be used to bring any wind driven generator into a controlled loop.

In the case of DC generators with fixed permanent magnet poles, the voltage converter can take the form of a high frequency switched mode DC-to-DC converter. Such converters can operate with power transfer efficiencies in excess of 90%. and typically 95% An applied voltage under the control of the computational module may also supply the field current of DC shunt generators to pre regulate the voltage supplied to the DC to DC converter.

Alternating generators may be controlled by the use of rectifiers and the field current may also be varied as above.

The resistance-heating element may take the form of an immersion water heater, a fan cooled air heater or other restive device that is forced cooled by the heat storage medium. The primary heat storage or heated medium may be liquid or gas.

Furthermore, it should be obvious that while forced fluid cooling is employed to cope with the large variation in output power, the primary heat storage or heated medium may be used directly, for example water for bathing or warm air for space heating. The primary heat storage or heated medium may also be used as a heat exchange medium to secondarily heat large volumes of solids, for example, concrete, shale, bricks, water or earth. Storage heating systems lend themselves to allowing the heater to be driven separately from the mains supply by dedicated generators that do not have to comply with grid requirements for frequency, spectral purity or voltage amplitude. By way of illustration, it should be noted that if the output voltage of the voltage converter feeding the resistive element should be able to supply voltages that varied for example, by a factor often to one, then the output power dissipated by the heater would vary by a factor of 100. This allows the generator blades to operate at the optimal Betz limit over a corresponding 4.6 to one wind speed variation.

Description of the drawings

Figure 1 shows a schematic diagram of a closed loop control system to maintain angular blade velocity at the Betz limit for any given incident wind speed VI.Incident wind speed monitor 4 measures the incident wind speed, after the wind has passed across the turbine blade 1, some energy from the wind is translated into turbine rotational energy. The corresponding loss of kinetic energy from the exhaust wind results in a slowing of the air behind the turbine blade. Exhaust wind speed monitor 5 measures the exhaust wind speed V2'. Incident and exhaust wind speeds are fed to the computational and control module 6. The computational and control module computes the instantaneous energy available in the incident wind and computes the required exhaust wind speed required for optimum power extraction. If the measured exhaust wind speed is too low, the computational and control module decreases the demand signal to the step up voltage switcher 3. A reduced voltage across the restive heater 8, results in a decreased current flowing from the generator 3, in series with the restive load. The reduced generator current results in a linearly related reduction in generator torque opposing the rotation of the turbine blades. As the turbine blades can now turn more freely and faster, the exhaust wind speed will increase until it reaches the required optimum speed. If the exhaust wind speed should rise to a value higher than the optimum, the computing and control module will provide an increased demand signal to the step up voltage switcher that will result in increased generator current, higher torque opposing blade rotation and hence a slowing of the exhaust wind speed.

It should be noted that while the optimal exhaust wind speed may be computed a priori as a function of incident wind speed, exhaust air speed and the system transfer functions all calculated to maintain loop control over the full range of expected wind speeds as a classical real time linear closed loop system, it is also possible to provide further signal paths wherein the output current and voltage levels of the electrical generator and step up switcher are fed back to inputs into the computing and control module and the computing and control module provides a fuzzy logic' demand to the voltage switcher. In one mode of fuzzy logic operation' the power being delivered into the heater is computed from the switcher output voltage provided across the heater. The power being delivered is then compared with the predicted power absorbed by the turbine as computed from the wind speed monitors 4 and 5. The rotational velocity of the turbine may be determined by computation and control module from the magnitude of the generators output voltage as a tachometer. For any given incident wind speed, the controller may in controlled steps, increase or decrease the output of the voltage switcher and again compute the resultant generated power against the predictive power available as computed from the wind speeds measured by sensors 4 and 5. This cycle of operation may be continued with the computational module making logical deductions to increase or decrease load demands on the generator so as to optimise the maximum power extracted available from the prevailing wind. Such a system will also confer a self-calibrating mechanism to the system since the input power and output power can be equated in terms of incident winds peed, exhaust wind speed, turbine tip velocity ratio and performance parameters, output voltage switcher voltage and current supplied to the restive heater.

Figure 2a shows a generic step up boost' voltage switcher, its operation is well documented and will be obvious to one skilled in the art. The switcher comprises a bridge rectifier 10, input smoothing capacitor 11, inductor 12, shunt semiconductor switch 14, shunt switch control gate 13 and output smoothing capacitor 15.

Figure 2b shows a representational circuit that models simplified impedance transfer function of the reflected heater resistance as a function of voltage switcher gain and electrical generator output voltage. For convenience of illustration, the generator internal resistance is considered small enough to be neglected.

Equating energy in equals energy out, (Vg)'2 /Z (G*VgY2 /R1 Hence, Z = R1 /0A2 Thus the reflected load impedance of the heater element when referred to the generator is inversely proportional to the square of the gain or step up voltage ratio of the DC Voltage switcher.

Claims

CLAIMS.1/ An adaptive small-scale wind turbine closed loop control system comprising:-a means to measure the wind speed arriving and leaving the blades of a wind turbine, a computing means to calculate the optimum angular velocity of the turbine in order satisfy the Betz criteria to extract the maximum work from the wind generator over the wind generators range of operating wind speeds a means of operationally transforming the impedance of a fixed resistive power load referred to the generator wherein a computing means controls a switched mode voltage converter forming a operationally closed loop system that controls power into a force cooled resistive load that results in a braking force on the wind generator thereby modifying the turbine angular velocity with respect to wind speed and available wind power dissipated in a resistive load.

2/ An adaptive small-scale wind turbine closed loop control system comprising:-a means to measure the wind speed arriving and leaving the blades of a wind turbine, a means of operationally transforming the impedance of a fixed resistive power load referred to the generator wherein a logic means controls a switched mode voltage converter that supplies power into a force cooled resistive load thereby modifying the braking force and angular velocity of the wind turbine with respect to wind speed and available wind power dissipated in a resistive load,IDa computing means to calculate the power being delivered to the heating element as a function of measured switcher output voltage and current with respect to incident and exhaust wind speeds, a fuzzy logic' control means that systematically varies the switcher voltage gain using a recursive iteration to maintain maximum power extraction from the incident wind.

3/ An adaptive closed loop control system as in claim 2 with a further computing means to analyse incident and exhaust wind speeds correlated against measured output power achieved, a means to compute and correlate the maximum available power according to incident and exhaust wind speed, turbine blade velocity, turbine blade output power performance parameters and measured delivered power, a computing means to derive the parametric constants of the wind speed sensors and magnitude of the heating element resistance wherein the overall closed loop system can be enabled for self calibration.a memory means to store parameters derived from self calibration that may be accessed under fuzzy logic' control and be further deployed to compute optimum wind power extraction.

4/ An adaptive closed ioop control system as in claimed in any preceding claim wherein the resistive element is used to directly heat gas or liquids as part of an open heating supply for domestic consumption. Il

5/ An adaptive closed loop control system as in claimed in any preceding claim wherein the resistive element is used to directly heat gas or liquids as part of an indirect closed loop heat exchange storage system.

6/ An adaptive closed loop control system as in claimed in claim 5 wherein the indirect heat exchange system stores heat in bulk materials such as earth, shale, bricks, concrete or water.