Method in a wind power plant

Abstract

A method for preventing mechanical oscillation in a wind power plant, the method comprising the steps of determining wind speed, controlling a generator of the power plant with speed control when the wind speed is below a predefined limit to maintain the rotation speed of the generator below a predefined first limit, detecting the increase of wind speed above the predefined limit, controlling the electric drive of the wind power plant generator to torque control by using a predefined crossover torque in response to the wind speed increasing above a predefined limit, maintaining the crossover torque as the reference torque for the electric drive until the rotation speed of the generator has increased above a predefined second limit, and controlling the electric drive of the wind power plant generator with torque control while the rotation speed of the generator remains above the predefined second limit.

Description

BACKGROUND OF THE INVENTION

The present invention relates to a method in a wind power plant. In particular, this invention relates to a method for reducing or even preventing resonance-frequency mechanical oscillations of a wind power plant.

The popularity of wind power plants as sources of renewable energy has in the past few years increased partly due to the fact that the required technology has become less expensive and partly because the energy prices of non-renewable energy sources have increased. At the same time, the unit sizes of wind power plants have been increased, and today the largest working wind power plants are several megawatts in size.

With an increase in nominal electric output, the physical size and mass of the equipment in a wind power plant also increase, which means that the tower supporting the wind turbine, generator, and their peripheral equipment also needs to be strong. In addition, a high-power wind power plant requires a long-blade turbine, in which case the tower also needs to be quite high. The length of the turbine blades in turn directly restricts the rotation speed of the turbine, because the speed of the blade end cannot be allowed to grow arbitrarily already because of the noise it causes.

All the above factors bring about the quite low mechanical resonance frequency of a wind turbine structure. This resonance frequency is typically at a range that it is triggered by the rotation of the turbine. If the turbine rotates for a long time on a frequency that corresponds to the mechanical resonance frequency of the wind power plant, there is a danger that the mechanics of the wind power plant weaken or even fail, thus causing considerable costs. In addition, the noise caused by the resonance situation may disturb the surroundings of the wind power plant.

US publication 6,525,518 presents a solution with which the start of mechanical resistance of a wind power plant can be detected and the oscillations caused by the resonance reduced. Detecting the resonance is based on a spectrum analysis made on measured quantities. The oscillations are, in turn, reduced by either altering the rotation speed of the rotor or alternatively altering the dynamic load of the turbine. The solutions of the publication eliminate resonance situations, but going into resonance is, however, not actively prevented. Thus, the power plant may fall back to a resonance situation. In the solution of US publication 6 525 518, both alternative ways of reducing the oscillation caused by resonance reduce the output produced by the wind power plant.

BRIEF DESCRIPTION OF THE INVENTION

It is an object of the invention to develop a method that avoids the above-mentioned drawbacks and enables the reduction of the mechanical oscillations of a wind power plant without significantly affecting the power output of the plant. This object is achieved by a method of the invention that is characterized by what is stated in the independent claim. Preferred embodiments of the invention are set forth in the dependent claims.

The invention is based on the idea that the angular frequency range of a turbine producing mechanical resonance is passed as quickly as possible so that the oscillations caused by the resonance are not triggered. At speeds lower than the turbine rotation speed that triggers the resonance frequency, the electric drives of the wind power plant are speed-controlled, and when the wind speed increases to a value that enables the wind turbine to operate above the angular frequency triggering mechanical resonance, torque control is used in the electric drives. Shifting from one control method to another is performed in such a manner that torque control is first set to a small reference value, whereby the wind accelerates the turbine and generator quickly past the revolution range causing resonance.

The method of the invention provides the advantage that energy output improves even at low rotation speeds in comparison with using torque control. In addition, the method prevents the turbine rotation speed from remaining at the mechanics resonance range, whereby the mechanical service life of the tower becomes longer.

BRIEF DESCRIPTION OF THE FIGURES

The invention will now be described in greater detail by means of preferred embodiments and with reference to FIG. 1 that is a general description of the formation of torque as a function of wind turbine speed.

DETAILED DESCRIPTION OF THE INVENTION

The method of the invention determines the wind speed at a location that as exactly as possible corresponds to the wind speed at the location of the wind turbine. Such a location for the determination of wind speed may for instance be on the dome enclosing the generator. The determination of the wind speed is typically performed in such a manner that the momentary wind speeds are measured, and an average of the momentary speeds is calculated for a suitable time period. Calculating the average makes it possible to disregard possible momentary gusts, the utilization of which is difficult in energy production.

If, according to the invention, the wind speed is below a predefined limit w₁, the generator of the wind power plant is speed-controlled. Speed control refers to a control mode, in which the rotation speed of the generator and wind turbine is kept at a reference value. The size of this reference value may be altered depending on the prevailing wind conditions. Naturally, it is more advantageous to let the turbine rotate faster, if the prevailing wind speed permits this. In speed control, it is, however, important to make sure that the generator of the wind power plant produces sufficient torque to maintain the power production at an appropriate level. Thus, speed control can be implemented by altering the reference speed so that the wind turbine generator produces a constant torque, for instance. When utilizing the method of the invention, it is also necessary to make sure that the reference speed is maintained below a predefined limit v₁.

According to the method of the invention, wind speed is monitored continuously, and when it is detected that wind speed increases above a predefined limit w₁, control action is taken to control the electric drive of the wind power plant generator to torque control, and the torque control is at the same time given a crossover torque reference value as its reference value.

Thus, when the wind speed increases to a value that permits the operation of the wind turbine at a rotation speed that is above the frequency triggering the resonance of the wind power plant, the above-mentioned control action is taken. This action shifts control to torque control by giving as the reference torque a crossover torque reference value that is a small value and most preferably zero. The torsional moment directed to the turbine shaft by the wind then accelerates the turbine as fast as possible, because the torque of the electric drive is at the crossover torque reference value, which is zero, for instance. In other words, the revolving force directed to the wind turbine axles is used to increase the rotation speed. Because the electric torque is controlled to zero or a small value, the generator momentarily produces no power to the network.

The crossover torque is maintained as the reference torque in the generator until, due to the wind, the rotation speed of the turbine and generator has increased above a predefined second limit v₂. When this speed is reached while the wind speed is above the predefined limit w₁, the electric drive of the wind power plant generator is torque-controlled. Torque-control achieves in a manner known per se good control possibilities for the power produced with the generator and also a good power production capability. Typically, the reference torque for the electric drive is formed as a function of the generator rotation speed. It should be noted that the rotation speeds of the generator and wind turbine do not necessarily correspond, since many drives use a gear between the wind turbine and generator.

FIG. 1 is a graphical presentation of an electric torque descriptor of the invention as a function of the rotation speed of the generator. Let us first imagine a situation, in which the wind speed is below the predefined limit w₁. The rotation speed of the wind turbine and generator is then within the range of 0 to v₁ rpm, and the generator is speed-controlled as described above. As the wind speed increases, the reference speed is altered to keep the electric torque substantially constant as shown in FIG. 1. When the rotation speed has reached the limit value v₀, the torque produced by the generator is slowly increased as the wind force and the rotation speed increase. At this stage, the torque is increased to limit the rotation speed and to increase power production.

When the wind speed further increases above the predefined limit w₁, torque control mode is initiated, and crossover torque reference T₁, i.e. zero, for instance, is set as the reference torque. The generator and wind turbine then accelerate rapidly, because before shifting to torque control, the torque that slows acceleration was already increased. Now that this torque slowing acceleration is removed, the rotation speed quickly changes to a range that does not trigger the mechanical resonance of the wind power plant. The zero torque reference can be removed when the rotation speed has increased above the second predefined limit v₂. In FIG. 1, the rotation speed v_res triggering mechanical resonance is shown in a rotation speed range limited by the predefined limits v₁ and v₂. It should be noted that the range between these limits may be great or small depending on the case. In some cases, the limits v₁, v₂ can also be set to the rotation speed v_res that triggers the resonance, or to some other common rotation speed.

When the wind speed decreases so that it is necessary to shift from torque control mode to speed control mode, the shift can be done without a small crossover reference torque. The shift can for instance be done in such a manner that as the rotation speed of the wind turbine reaches the predefined second limit v₂, speed control mode is initiated directly, and the speed controller is given a reference value that is smaller than the speed triggering the resonance frequency. This also makes it possible to momentarily increase the electric torque, i.e. the produced power, whereby the generator and turbine decelerate rapidly.

The rotation speed triggering the resonance can for instance be defined during the trial run of the wind power plant by using a known method. After this definition, the required parameters can simply be set for the control system. It is also clear that the rotation speed triggering the resonance can be defined during use, in which case the equipment provides the required parameters for the control system. Likewise, it is clear that the same method can be applied to several resonance points, if the wind power plant structure has several resonance frequencies.

The method of the invention can be applied to many wind power plants of different type. The essential thing is that the wind power plant generator can be controlled to apply a reference speed and reference torque as described in the claims.

It is apparent to a person skilled in the art that as the technology advances, the basic idea of the invention can be implemented in many different ways. Thus, the invention and its embodiments are not limited to the examples described above, but may vary within the scope of the claims.

Claims

1. A method for preventing mechanical oscillation in a wind power plant, the method comprising the steps of determining wind speed, controlling a generator of the power plant with speed control when the wind speed is below a predefined limit to maintain the rotation speed of the generator below a predefined first limit, detecting the increase of wind speed above the predefined limit, controlling the electric drive of the wind power plant generator to torque control by using a predefined crossover torque in response to the wind speed increasing above a predefined limit, maintaining the crossover torque as the reference torque for the electric drive until the rotation speed of the generator has increased above a predefined second limit, and controlling the electric drive of the wind power plant generator with torque control while the rotation speed of the generator remains above the predefined second limit.

2. A method as claimed in claim 1, wherein the size of the crossover torque is zero.

3. A method as claimed in claim 1, wherein the determination of wind speed comprises the step of calculating a time average of momentary wind speeds.

4. A method as claimed in claim 1, wherein speed-controlling the wind power plant generator comprises the step of producing a reference speed for a speed control circuit in such a manner that the electric torque to be produced is essentially constant.

5. A method as claimed in claim 1, wherein speed-controlling the wind power plant generator also comprises the steps of detecting the increase of the generator speed above a predefined limit speed, and increasing the size of the electric torque produced by the generator.

6. A method as claimed in claim 1, wherein the method comprises the steps of defining in advance a generator rotation speed that triggers the mechanical resonance of the wind power plant, selecting the predefined first rotation speed limit in such a manner that it is smaller than the generator rotation speed that triggers the mechanical resonance of the wind power plant, and selecting the predefined second rotation speed limit in such a manner that it is greater than the generator rotation speed that triggers the mechanical resonance of the wind power plant.