Fan control method without measuring wind speed

Abstract

The invention provides a fan control method without wind speed measurement, which comprises the following steps of: monitoring the current rotating speed of a fan in real time, and indicating that the current wind speed is changed once the rotating speed is changed. At the moment, according to the current rotating speed and the initial rotating speed obtained through measurement, first capture power generated by the fan after the wind speed changes is calculated through an additional power control formula; and then the output power of the fan can be adjusted according to the first capture power obtained through calculation, so that the output power is equal to the current capture power of the fan, and control over the fan is completed. The method does not depend on wind speed measurement, and the first capture power generated by the fan under the action of the current wind speed is calculated only according to the current rotating speed of the fan and the initial rotating speed of the previous moment; and adjusting the output power of the fan according to the first capture power. The wind speed does not need to be measured, so that errors caused by measurement are reduced, and compared with an existing adjusting mode, the control precision is higher.

Description

Technical Field

[0001] The present invention generally relates to the technical field of fan control, and in particular to a fan control method without measuring wind speed. Background Art

[0002] As a common power generation equipment, wind turbines are used to convert wind speed into mechanical kinetic energy of fan blades, and then convert it into electrical energy using electromagnetic induction. The stable movement of wind turbines is related to the efficiency of electrical energy conversion.

[0003] During operation, the fan blades will rotate due to wind drive, thus generating captured power. The power output by the fan after conversion needs to be consistent with the captured power to ensure the stable rotation speed of the fan blades.

[0004] However, in reality, in order to effectively capture wind energy, wind turbines are generally installed in areas with high wind speeds. Due to the high wind speeds, the change in wind speed is also more obvious. The obvious change in wind speed will cause the speed of the fan blades to change synchronously, which in turn causes the captured power to change. If the output power is not adjusted, the speed of the fan blades will be difficult to control.

[0005] To solve the above problems and ensure that the wind turbine can capture more wind energy stably, it is necessary to measure the wind speed in real time and obtain the real-time captured power by looking up the table; then adjust the output power so that the output power is equal to the captured power.

[0006] Traditional methods rely heavily on wind speed measurement during operation. However, due to sensor errors, non-uniformity of wind speed spatial distribution, wake effects, and wind speed measurement delays, it is often difficult for wind farms to obtain accurate wind speed values, which affects the control accuracy of fan speed and output power, and thus affects the stable operation of wind turbines. Summary of the invention

[0007] In view of the above-mentioned defects or deficiencies in the prior art, it is desirable to provide a wind turbine control method that does not require wind speed measurement.

[0008] The present invention provides a fan control method without measuring wind speed, comprising: S1: Obtain the speed of the fan at the previous moment to obtain the initial speed; detect the speed of the fan at the current moment to obtain the current speed; S2: If the current speed is not equal to the initial speed, proceed to steps S3 to S6; otherwise proceed to step S6; S3: Obtaining an additional power control formula; the additional power control formula is used to calculate the magnitude of the captured power under the current wind speed and when the wind turbine is at the current pitch angle according to the current speed and the initial speed; the captured power is the power generated by the wind turbine under the action of external wind force; S4: Substituting the initial speed and the current speed into the additional power control formula to calculate the first captured power; S5: adjusting the output power of the wind turbine according to the first captured power; S6: Repeat steps S1 to S6 at the next moment.

[0009] According to the technical solution provided by the present invention, the additional power control formula is shown in Formula 5: Formula 5; in, Indicates the capture power, Indicates the reference coefficient, Indicates additional power, represents the proportionality factor of the additional power controller, represents the integral coefficient of the additional power controller, s represents the Laplace operator, Indicates the current speed, Indicates the initial speed.

[0010] According to the technical solution provided by the present invention, S5: adjusting the output power of the wind turbine according to the first captured power, comprising: S5-1: Obtaining a capture power threshold; S5-2: If the first captured power is greater than or equal to the captured power threshold, adjusting the output power of the wind turbine until it is equal to a set multiple of the first captured power; the set multiple is less than 1; S5-3: If the first capture power is less than the capture power threshold, reduce the value of the reference coefficient, and calculate according to the additional power control formula to obtain the second capture power; adjust the output power of the wind turbine until it is equal to the second capture power; the second capture power is less than the first capture power.

[0011] According to the technical solution provided by the present invention, obtaining the capture power threshold includes: Get the maximum captured power of the wind turbine; The capture power threshold is set according to the maximum capture power.

[0012] According to the technical solution provided by the present invention, adjusting the output power of the wind turbine until it is equal to a set multiple of the first captured power includes: During the process of adjusting the output power, if the speed of the fan reaches the rated speed, the pitch angle of the fan is adjusted so that the speed reaches the rated speed and is maintained at the rated speed during the process of adjusting the output power until the output power is equal to the first captured power.

[0013] According to the technical solution provided by the present invention, adjusting the pitch angle of a wind turbine includes: Acquire historical data of the wind turbine at rated speed under various wind speed conditions; the historical data includes: a power difference between the captured power and the output power, and a corresponding pitch angle; According to the historical data, fitting the functional relationship between the power difference and the pitch angle using a least square method to obtain a first function; Obtain the current output power of the fan to obtain the first output power; Calculating a difference between the first captured power and the first output power to obtain a first difference; Substituting the first difference into the first function to calculate a first pitch angle; Adjust the pitch angle of the wind turbine to the first pitch angle.

[0014] According to the technical solution provided by the present invention, the value of the reference coefficient is reduced, and the second capture power is calculated according to the additional power control formula, including: Obtaining the value of the reduced reference coefficient to obtain a first reference coefficient; Substituting the first reference coefficient, the initial rotation speed and the current rotation speed into the additional power control formula, the second captured power is calculated.

[0015] According to the technical solution provided by the present invention, adjusting the output power of the wind turbine until it is equal to the second captured power includes: Starting from the current rotation speed, the rotation speed is gradually reduced until the output power of the fan is equal to the second captured power.

[0016] The beneficial effects of the present invention are: Monitor the current speed of the fan in real time. Once the speed changes, it means that the current wind speed has changed. At this time, according to the measured current speed and initial speed, the additional power control formula is used to calculate the first captured power generated by the fan after the wind speed changes. Then, the output power of the fan can be adjusted according to the calculated first captured power, so that the output power is equal to the current captured power of the fan, and the control of the fan is completed. This method does not rely on the measurement of wind speed. It only needs to calculate the first captured power generated by the fan under the current wind speed according to the current speed of the fan and the initial speed of the previous moment; then adjust the output power of the fan according to the first captured power. There is no need to measure the wind speed, so the error caused by the measurement is reduced. Therefore, compared with the existing adjustment method, it has higher control accuracy. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Other features, objects and advantages of the present invention will become more apparent from the detailed description of non-limiting embodiments made with reference to the following drawings: Figure 1A flow chart of a fan control method without measuring wind speed; Figure 2 Capture the power versus speed curve for the fan. DETAILED DESCRIPTION

[0018] The present invention will be further described in detail below in conjunction with the accompanying drawings and embodiments. It is to be understood that the specific embodiments described herein are only used to explain the relevant invention, rather than to limit the invention. It should also be noted that, for ease of description, only the parts related to the invention are shown in the accompanying drawings.

[0019] It should be noted that, in the absence of conflict, the embodiments of the present invention and the features in the embodiments can be combined with each other. The present invention will be described in detail below with reference to the accompanying drawings and in combination with the embodiments.

[0020] Air flow can generate wind energy. According to Betz theory, wind turbine blades can capture part of the power generated by wind: Formula 1; Where P m is the mechanical power of the wind wheel, i.e. the captured power, ρ is the air density, S=πR 2 is the swept area of ​​the wind rotor blade, R is the blade radius, v is the wind speed, C p (λ,β) is the wind energy utilization coefficient, λ is the tip speed ratio, and β is the pitch angle. The definition of the tip speed ratio λ is as follows: Formula 2: is the wind turbine rotor speed, and the wind wheel characteristics mainly depend on C p (λ,β), which is generally expressed in the form of a fitting function. The fitting function used in this paper is as follows: Formula 3; in, α is the fitting parameter, e is the natural logarithm.

[0021] The operational relationship between output power and speed is: Formula 4; In the formula, the reference coefficient K opt =0.5C pmax πρR 5 / λ opt 3 ; C pmax is the maximum wind energy utilization coefficient; opt It is the optimal tip speed ratio of the fan.

[0022] To ensure safety and system stability, excessive rotor speed may cause the blades to over-rotate, resulting in excessive mechanical stress, or even blade breakage or other structural damage. In addition, excessively high speeds can increase the vibration of the fan, affecting its smooth operation and durability. To ensure work efficiency and avoid additional energy loss and overload risks caused by excessive speeds, most fans limit their speed to the rated speed.

[0023] Please refer to Figure 1 The present invention provides a fan control method without measuring wind speed, comprising: S1: Obtain the speed of the fan at the previous moment to obtain the initial speed; detect the speed of the fan at the current moment to obtain the current speed; In some embodiments, the interval between two adjacent moments is any time length between 1 second and 5 seconds. Thus, the output power of the wind turbine can be adjusted in real time according to the wind speed.

[0024] S2: If the current speed is not equal to the initial speed, proceed to steps S3 to S6; otherwise proceed to step S6; Specifically, if the change in wind speed causes a very small change in rotation speed, and the output power is still adjusted strictly according to the inequality, it will cause the number of adjustments to be too frequent. In the actual adjustment process, the output power may not change after adjustment due to the influence of the adjustment accuracy.

[0025] For example, within a few seconds, the wind speed increases rapidly and then returns to its original speed. In actual situations, the wind speed change that increases rapidly and then returns to its original speed in a short period of time has little effect on the wind turbine speed.

[0026] However, the traditional way of measuring wind speed will quickly adjust the output power of the fan according to the rapidly changing wind speed. The traditional control method will cause the fan speed to change rapidly, resulting in unstable operation of the fan.

[0027] In order to reduce the number of times the output power is adjusted when the speed change is small, so as to ensure the stability of the fan operation, the following methods can be used: After the initial speed and the current speed are measured, the absolute value of the difference between the initial speed and the current speed is calculated to obtain the speed difference; Then calculate the ratio of the speed difference to the initial speed; If the ratio is greater than or equal to 0.5%, they are judged to be unequal; if the ratio is less than or equal to 0.5%, they are judged to be equal.

[0028] Specifically, the speed of a general fan is about 13rpm to 20rpm; even when a small fan is actually working, the speed is about 100rpm to 300rpm. Taking 200rpm as an example, according to the above method, if the wind speed changes so that the fan speed changes by less than or equal to 0.1 revolutions per minute, it can be determined that the wind speed has not changed significantly, thereby reducing the number of times the fan output power is adjusted.

[0029] S3: Obtaining an additional power control formula; the additional power control formula is used to calculate the magnitude of the captured power under the current wind speed and when the wind turbine is at the current pitch angle according to the current speed and the initial speed; the captured power is the power generated by the wind turbine under the action of external wind force; The additional power control formula is shown in Formula 5: Formula 5; in, Indicates the capture power, Indicates the reference coefficient, Indicates additional power, represents the proportionality factor of the additional power controller, represents the integral coefficient of the additional power controller, s represents the Laplace operator, Indicates the current speed, Indicates the initial speed.

[0030] S4: Substituting the initial speed and the current speed into the additional power control formula to calculate the first captured power; S5: adjusting the output power of the wind turbine according to the first captured power, comprising: S5-1: Obtaining a capture power threshold, including: The maximum captured power of the wind turbine is obtained; the maximum captured power is: the maximum captured power that the wind turbine can obtain under full wind speed and full pitch angle.

[0031] The capture power threshold is set according to the maximum capture power.

[0032] Specifically, it is assumed that there is a capture power threshold; for load reduction operation, the capture power threshold needs to be multiplied by a set multiple to obtain the load reduction capture power threshold.

[0033] At this time, when the fan is at the load reduction capture power threshold, the maximum speed that can be achieved is exactly equal to the rated speed.

[0034] Moreover, when the fan is at the maximum capture power, after the load reduction control is performed, the fan has the maximum output power (a known fixed value for the fan of the same model); the ratio of the maximum output power to the maximum capture power is used as the value of the setting multiple; the setting multiple is less than 1 and greater than 0. Since the ratio of load reduction is generally fixed, the setting multiple is also a known fixed value.

[0035] Also, because the captured power of the wind turbine and the maximum achievable speed are in a one-to-one correspondence, the captured power threshold and the load reduction captured power threshold can be derived according to the above conditions.

[0036] Specifically, when the wind speed is constant, the captured power of the wind turbine increases first and then decreases as the rotation speed increases. When the rotation speed is constant, the higher the wind speed, the higher the captured power.

[0037] like Figure 2 As shown, o is the coordinate origin, the horizontal axis is the speed ω, 1 on the horizontal axis represents 1 times the rated speed of the fan, and the vertical axis is the captured power p, and 1 on the vertical axis represents 1 times the rated power of the fan.

[0038] L1, L2, and L3 are the curves of wind turbine captured power changing with rotation speed under low wind speed, medium wind speed, and high wind speed conditions, respectively.

[0039] Point A is the maximum capture power that can be achieved under low wind speed conditions; Point B is the maximum capture power that can be achieved under medium wind speed conditions; Point C is the maximum capture power that can be achieved under high wind speed conditions; In order to enable wind turbines to capture more wind energy and reduce failure rates: Referring to L1, when the wind speed is too low, the fan needs to be gradually reduced from the rated speed so that the fan has a certain capture power to ensure that the fan can also convert wind energy into electrical energy at low wind speeds. At this time, after load reduction control, the corresponding speed has two values; in order to facilitate the rapid increase of the speed after the subsequent wind speed increases, the fan is controlled to work at a higher speed. That is, it is adjusted from point A to point A'.

[0040] Referring to L2, when the maximum capture power corresponding to the wind speed is just equal to the capture wind speed threshold, the load reduction capture power threshold is calculated, and the output power of the wind turbine is controlled to be equal to the load reduction capture power threshold, that is, adjusted from point B to point B'.

[0041] Referring to L3, when the wind speed is too high, the speed of the wind turbine needs to be controlled not to be higher than the rated speed. Therefore, the speed is controlled to be maintained at the rated speed, and after the pitch angle is adjusted to complete the load reduction control, it is adjusted from point C to point C'.

[0042] The setting times mentioned in the present invention are the load reduction coefficient k in the existing load reduction control method. del, which is in the interval [0,1]. The smaller the load reduction factor, the more power is reserved.

[0043] Output power P of the fan after load reduction del =k del P mppt , P mppt k is the maximum power that the wind turbine can capture at the current wind speed. del The smaller the value, the more reserved power is. In addition, by changing the load reduction factor k del The value of is realized, which realizes the release and reservation of the wind turbine reserve power.

[0044] It can be seen from formula 1 that, under the concept of the present invention, since the wind speed is not measured, the output power equation of the fan is It is an uncontrollable parameter, and its value depends on the mechanical structure of the wind turbine itself and the external wind speed. Therefore, the control method of the wind turbine for load reduction operation in the present invention is essentially to control the wind energy utilization coefficient C by changing the blade tip speed ratio λ and the pitch angle β. p The size of (‌λ,β‍) is such that: Formula 6; In the formula, C pmax is the maximum wind energy utilization factor that the wind turbine can achieve; del With β del To meet the load reduction factor k del Specific values ​​of required tip speed ratio and pitch angle.

[0045] Based on the above ideas, the specific implementation steps are as follows: Case 1: High wind speed S5-2: If the first captured power is greater than or equal to the captured power threshold, adjusting the output power of the wind turbine until it is equal to a set multiple of the first captured power, including: During the process of adjusting the output power, if the speed of the fan reaches the rated speed, the pitch angle of the fan is adjusted so that the speed reaches the rated speed and is maintained at the rated speed during the process of adjusting the output power until the output power is equal to the first captured power.

[0046] In order to achieve load reduction, the pitch angle also needs to be adjusted accordingly. The higher the wind speed, the greater the added active reference value, the greater the pitch angle action, and the two change in a similar way.

[0047] By measuring the additional power P add The required load shedding pitch angle action value β is directly obtained from the value del .

[0048] The specific steps include: Acquire historical data of the wind turbine at rated speed under various wind speed conditions; the historical data includes: a power difference between the captured power and the output power, and a corresponding pitch angle; The historical data are the data recorded in the table used in the traditional table lookup method.

[0049] According to the historical data, fitting the functional relationship between the power difference and the pitch angle using a least square method to obtain a first function; Obtain the current output power of the fan to obtain the first output power; Calculating a difference between the first captured power and the first output power to obtain a first difference; Substituting the first difference into the first function to calculate a first pitch angle; Adjust the pitch angle of the wind turbine to the first pitch angle.

[0050] Specifically, the details of the fitting process include: Since the medium and high wind speeds have different benchmarks, resulting in different slopes of the curves, the upper limit of the medium wind speed is used as the boundary, and the wind speed region is fitted in sections. The additional power demarcation point threshold under a certain load reduction factor is obtained, and the functions on both sides of the demarcation point threshold are fitted respectively.

[0051] The first curve on the side of the demarcation point threshold close to the origin has a larger curvature, and a sixth-order polynomial is used to fit it; The curvature of the second curve on the side of the dividing point threshold far from the origin is smaller, and a second-order polynomial is used to fit it.

[0052] Formula 16; Formula XVII; Where a i are the coefficients of the sixth-order polynomial, b i are the coefficients of the second-order polynomial, β del1 is the load reduction pitch angle corresponding to the first curve, β del2 is the load-reducing pitch angle corresponding to the second curve, i=0, 1, 2, ...

[0053] Change the load reduction factor and repeat the above steps to complete the fitting process. Formula 16 and Formula 17 together constitute the first function.

[0054] During the actual operation of the wind turbine, the load reduction pitch angle action value can be obtained by selecting the corresponding fitting curve according to the given load reduction coefficient and the demarcation point threshold, without measuring the wind speed.

[0055] Specifically, for high wind speeds, after the wind turbine speed reaches the rated speed, the maximum value of the wind energy utilization coefficient is no longer obtained by the optimal tip speed ratio, but gradually becomes smaller than the optimal tip speed ratio. Therefore, as the wind speed increases, C pmax The power is slowly reduced but still gradually increased until the rated power is reached.

[0056] At this time, the load reduction additional power P dadd The values ​​are: Formula 14.

[0057] The corresponding load reduction pitch angle solution equation is: Formula 15.

[0058] Case 2: Low wind speed When the wind speed is low, the rotor speed is also low, so there is sufficient speed margin, and load reduction operation can be performed by reducing the reference coefficient.

[0059] S5-3: If the first captured power is less than the captured power threshold, reduce the value of the reference coefficient, and calculate according to the additional power control formula to obtain the second captured power; adjust the output power of the wind turbine until it is equal to the second captured power; the second captured power is less than the first captured power; In some embodiments, the method of reducing the value of the reference coefficient includes: When the fan operates in MPPT mode, it meets the following requirements: Formula 7; When the fan is running in load reduction mode, the following conditions are met: Formula 8; Where P del is the fan power after load reduction; K del is the corresponding suboptimal power coefficient. del Substitute into formula 3 and establish the equation as formula 9. Solve to get λ under the current load reduction factor del .

[0060] Formula 9; Combining Formula 7, Formula 8 and Formula 9, we can get the reference coefficient K opt With the best power factor K del The relationship between them is: Formula 10; From formula 10, we can know that the optimal power coefficient K del The size of λ del and k delIn the low wind speed area, first calculate the different k del The following λ del , and then substitute it into formula 10 to get K del , load reduction operation is achieved by changing the power reference curve.

[0061] The second capture power is calculated, specifically including: Obtaining the value of the reduced reference coefficient to obtain a first reference coefficient; Substituting the first reference coefficient, the initial rotation speed and the current rotation speed into the additional power control formula, the second captured power is calculated.

[0062] The process of adjusting the output power of the fan is performed by gradually reducing the rotation speed from the current rotation speed until the output power of the fan is equal to the second captured power.

[0063] It should be noted that for different k del , the corresponding upper limit wind speed v1 in the low wind speed area is different. This is because k del The smaller it is, the higher the speed required for overspeed load reduction, and the lower the wind speed to reach the rated speed. The expression of v1 is as follows: Formula XI.

[0064] In some embodiments, for medium and low wind speed conditions, the wind turbine may also be controlled in combination with the adjustment of the pitch angle. The process of calculating the pitch angle during load shedding includes: For a certain load reduction ratio k del , calculate the size of v1 through formula 9 and formula 11; V1 and the load reduction factor k del Substitute into formula 1 to calculate the load reduction additional power P required at the current wind speed dadd Size: Formula twelve.

[0065] The current wind speed, rated speed and k del Substitute into formula 3 to establish a one-variable equation for the pitch angle, and solve the equation to obtain the corresponding load-reduced pitch angle value β del .

[0066] Formula 13.

[0067] S6: Repeat steps S1 to S6 at the next moment.

[0068] Through the above control method, when the output power of the wind turbine is regulated, the problem of power oscillation can be avoided and the wind energy utilization coefficient can be improved.

[0069] The above description is only a preferred embodiment of the present invention and an explanation of the technical principles used. Those skilled in the art should understand that the scope of the invention involved in the present invention is not limited to the technical solution formed by a specific combination of the above technical features, but should also cover other technical solutions formed by any combination of the above technical features or their equivalent features without departing from the inventive concept. For example, the above features are replaced with the technical features with similar functions disclosed in the present invention (but not limited to) to form a technical solution.

Claims

1. A fan control method without measuring wind speed, characterized in that: include: S1: Obtain the speed of the fan at the last moment to obtain the initial speed; Detect the current speed of the fan and obtain the current speed; S2: If the current speed is not equal to the initial speed, proceed to steps S3 to S6; Otherwise proceed to step S6; S3: Obtaining an additional power control formula; the additional power control formula is used to calculate the magnitude of the captured power under the current wind speed and when the wind turbine is at the current pitch angle according to the current speed and the initial speed; the captured power is the power generated by the wind turbine under the action of external wind force; S4: Substituting the initial speed and the current speed into the additional power control formula to calculate the first captured power; S5: adjusting the output power of the wind turbine according to the first captured power; S6: Repeat steps S1 to S6 at the next moment.

2. A method for controlling a fan without measuring wind speed according to claim 1, characterized in that: The additional power control formula is shown in Formula 5: Formula 5; in, Indicates the capture power, Indicates the reference coefficient, Indicates additional power, represents the proportionality factor of the additional power controller, represents the integral coefficient of the additional power controller, s represents the Laplace operator, Indicates the current speed, Indicates the initial speed.

3. A method for controlling a fan without measuring wind speed according to claim 2, characterized in that: S5: adjusting the output power of the wind turbine according to the first captured power, comprising: S5-1: Obtaining a capture power threshold; S5-2: If the first captured power is greater than or equal to the captured power threshold, adjusting the output power of the wind turbine until it is equal to a set multiple of the first captured power; the set multiple is less than 1; S5-3: If the first capture power is less than the capture power threshold, reduce the value of the reference coefficient, and calculate according to the additional power control formula to obtain the second capture power; adjust the output power of the wind turbine until it is equal to the second capture power; the second capture power is less than the first capture power.

4. A method for controlling a fan without measuring wind speed according to claim 3, characterized in that: Get the capture power threshold, including: Get the maximum captured power of the wind turbine; The capture power threshold is set according to the maximum capture power.

5. A method for controlling a fan without measuring wind speed according to claim 3, characterized in that: Adjusting the output power of the fan until it is equal to a set multiple of the first captured power, comprising: During the process of adjusting the output power, if the speed of the fan reaches the rated speed, the pitch angle of the fan is adjusted so that the speed reaches the rated speed and is maintained at the rated speed during the process of adjusting the output power until the output power is equal to the first captured power.

6. A method for controlling a fan without measuring wind speed according to claim 5, characterized in that: Adjust the pitch angle of the wind turbine, including: Acquire historical data of the wind turbine at rated speed under various wind speed conditions; the historical data includes: a power difference between the captured power and the output power, and a corresponding pitch angle; According to the historical data, fitting the functional relationship between the power difference and the pitch angle using a least square method to obtain a first function; Obtain the current output power of the fan to obtain the first output power; Calculating a difference between the first captured power and the first output power to obtain a first difference; Substituting the first difference into the first function to calculate a first pitch angle; Adjust the pitch angle of the wind turbine to the first pitch angle.

7. A method for controlling a fan without measuring wind speed according to claim 3, characterized in that: Reducing the value of the reference coefficient and calculating according to the additional power control formula to obtain the second captured power includes: Obtaining the value of the reduced reference coefficient to obtain a first reference coefficient; Substituting the first reference coefficient, the initial rotation speed and the current rotation speed into the additional power control formula, the second captured power is calculated.

8. A method for controlling a fan without measuring wind speed according to claim 3, characterized in that: Adjusting the output power of the wind turbine until it is equal to the second captured power, comprising: Starting from the current rotation speed, the rotation speed is gradually reduced until the output power of the fan is equal to the second captured power.

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