CN101895249A - Maximum wind energy tracking control method for variable-speed constant-frequency wind power generation - Google Patents

Abstract

The invention relates to a maximum wind energy tracking control method for variable-speed constant-frequency wind power generation, which belongs to the field of wind power generator set control. The method does not need to previously know the optimal performance curve of a wind power generator set, uses the improved perturbation and observation process and the power signal feedback process based on different power-rotating speed curves to carry out on-line searching, and records the information of the ratio of power to cubic rotating speed at the maximum power point. When switching to the power signal feedback process from the perturbation and observation process, the power-rotating speed curve on which the power signal feedback process is based can be obtained according to the recorded information; and after finishing recording the information, an optimal power-rotating speed curve can be obtained, and at this time, only the power signal feedback process is adopted. The method can obtain the optimal power-rotating speed curve of the wind power generator set on line according to the actual operating environment of the wind power generator set, and effectively realizes the maximum wind energy tracking. The invention is suitable for variable-speed constant-frequency wind power generation systems.

Description

A maximum wind energy tracking control method for variable-speed constant-frequency wind power generation

technical field

The invention relates to a maximum wind energy tracking control method for variable-speed constant-frequency wind power generation, which belongs to the field of wind power generator control.

Background technique

Wind energy is a clean and renewable energy. With the intensification of the global greenhouse effect, the depletion of fossil energy and the maturity of wind power generation technology, countries all over the world have accelerated the pace of development and utilization of wind energy. Wind power generation technology has developed from constant speed and constant frequency to variable speed and constant frequency. One of the advantages of the latter over the former is that through the speed control of wind turbines, the maximum wind energy tracking of the wind power generation system can be realized and the utilization rate of wind energy can be improved.

"A Novel Algorithm for Fast and Efficient Maximum Power Point Tracking of "Wind Energy Conversion Systems" (a new fast and effective method for maximum power tracking of wind power generation systems) studied an improved hill-climbing method, which uses the hill-climbing method to achieve maximum wind energy tracking and records a power-speed cube ratio, the power- The speed cube ratio is a variable that determines the step size and direction of the hill climbing method. Multiple power-speed cube ratios have not been recorded or corrected, and have not been used in power signal feedback control. The power signal feedback method has not been used. The advantages. "A Hybrid Maximum PowerPoint Tracking System for Grid-Connected Variable SpeedWind- Generators” (a hybrid maximum power tracking system for grid-connected variable-speed wind turbines) This paper studies a maximum wind energy tracking control method that combines the torque signal feedback method with the disturbance observation method. The torque-speed curve controls the wind turbine. When the difference between the given torque and the actual torque of the wind turbine is within a certain range, then use the disturbance observation method to find the maximum power point based on the actual operating environment. Although this method can achieve the maximum power point based on the actual operating environment Wind energy tracking, but this method needs to know the optimal torque-speed curve, and does not record the searched maximum power point online and correct the original characteristic curve.

Zhang Xing et al. published the "Wind Power Generation Maximum Power Point Tracking Control Method" (China, publication date: May 9, 2007, publication number: CN1960159A) patent, which studies a double closed-loop control of speed and power that is set independently of each other. method, this method first performs power signal feedback control on the wind turbine according to the original maximum power curve of the wind turbine; when the wind speed changes small, the hill-climbing method is used for power optimization control to obtain the maximum power and the corresponding generator speed, and based on the original maximum power curve The revised maximum power curve is obtained by adding new data points; when the wind speed changes greatly, the power signal feedback control is implemented according to the revised latest power curve. However, this method still needs the original optimal power-speed curve of the wind turbine, and does not fully utilize the advantages of the hill-climbing method, which does not need to know the power characteristic parameters and wind speed parameters of the wind turbine.

Contents of the invention

The present invention provides a method of obtaining the optimal power-speed curve based on the actual operating environment online by using the disturbance observation method and the power signal feedback method during the actual operation of the wind turbine without knowing the optimal power-speed curve of the wind turbine in advance. A maximum wind energy tracking control method for variable-speed constant-frequency wind power generation.

The present invention adopts following technical scheme for solving its technical problem:

A maximum wind energy tracking control method for variable-speed constant-frequency wind power generation, the specific steps of the method are as follows:

a) From the beginning of the maximum wind energy tracking control to the search for the first maximum power point, record the output power P of the wind turbine and the speed ω cube ratio is k, when the disturbance makes the output power of the wind turbine increase and the value of k increases, update k value;

b) When the wind speed changes, switch from the disturbance observation method, that is, Perturb&Observe--P&O, to the power signal feedback method, that is, Power Signal Feedback——PSF. After PSF controls the stable operation of the wind turbine, if the power-speed cube ratio record table is If it is empty or the record is not completed, switch to P&O to search for the maximum power point; if the record is completed, it is considered that the best power-speed curve has been obtained online, and no longer switch to P&O;

c) When switching to PSF, use the coefficient k recorded in step a) to implement PSF according to P=k* ^ω3 before the power-speed cube ratio recording table starts recording; after the power-speed cube ratio recording table starts recording , implement PSF according to P=k _e *ω ³ , k _e is the median average filter value of k _opt1 , k _opt2 , k _opt3 ...k _optn in the power-speed cube ratio recording table.

The beneficial effects of the present invention are as follows: the P&O effectively integrates the advantages of no wind turbine power characteristic parameters and PSF control being simple, reliable and fast. In the actual operation process of wind turbines, P&O and PSF are used to obtain the optimal power-speed curve based on the actual operating environment online, and finally PSF is fully used to achieve maximum wind energy tracking control, which actually effectively improves the wind energy utilization coefficient.

Description of drawings

Fig. 1 is a schematic diagram of the P _out -ω characteristic curve and the slope at different operating points.

Fig. 2 is a schematic diagram of the control process of the maximum wind energy tracking control method.

Fig. 3 is a control flow chart of the control stage 1.

Fig. 4 is a control flow chart of the control stage 2.

Fig. 5 is a block diagram of a variable-speed constant-frequency wind power generation system based on an electrically excited double salient pole wind power generator.

Detailed ways:

Below in conjunction with accompanying drawing, technical scheme of the present invention is described in further detail:

Improvement of the disturbance observation method: 1. The method of judging the stable operation of wind turbines

In order to ensure the reliable work of P&O, it must wait for the system to run stably before implementing new disturbances. Equation (1) describes the relationship between the wind turbine output mechanical power and the system output power (excluding friction power)

${\mathrm{<span\; class="notranslate">\; P</span>}}_{\mathrm{<span\; class="notranslate">\; m</span>}}<span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; \&eta;</span>}}{\mathrm{<span\; class="notranslate">\; P</span>}}_{\mathrm{<span\; class="notranslate">\; out</span>}}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; J\&omega;</span>}\frac{\mathrm{<span\; class="notranslate">\; d\&omega;</span>}}{\mathrm{<span\; class="notranslate">\; dt</span>}}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; )</span>$

为机械储能变化率。当

的绝对值小于某一较小值时，可认为风电机组稳定运行。

又可表示为

将其离散化得

J及转速的采样时间间隔t(k)-t(k-1)为常量。因此，当：where P _m is the output power of the wind turbine, η is the efficiency from the generator to the output power measurement point, P _out is the system output power, J is the moment of inertia of the wind turbine, ω is the speed,

is the rate of change of mechanical energy storage. when

When the absolute value of is less than a certain smaller value, the wind turbine can be considered to be in stable operation.

can also be expressed as

discretize it into

The sampling time interval t(k)-t(k-1) of J and rotational speed is constant. Therefore, when:

[ω(k)+ω(k-1)][ω(k)-ω(k-1)]≤σ (2)

When it is satisfied, it is considered that the wind turbine is running stably, and the system output power can be measured to implement a new disturbance. σ is a small value set for judging stable operation.

2. Selection of disturbance step size

The absolute value of the slope at points Q ₁ , Q ₂ , Q ₃ , Q ₄ , and Q ₅ in Figure 1 gradually decreases; the greater the wind speed, the greater the range of the absolute value of the slope of the corresponding P _out -ω curve. Therefore, different disturbance step sizes can be used according to the absolute value of the slope at the operating point of the wind turbine. When the absolute value is large (such as Q ₁ , Q ₂ ), use a large step size disturbance to improve the rapidity of the control of the disturbance and observation method; when the absolute value is small (such as Q ₄ , Q ₅ ), use a small step size disturbance to improve the search speed. The accuracy of the maximum power point. According to the above analysis, the present invention improves the step size selection of the disturbance and observation method. There are n disturbance step lengths, and each disturbance step length corresponds to a power change threshold. Let n=5 for the time being to illustrate the step size selection.

假设这五个斜率值分别对应图1中Q₁、Q₂、Q₃、Q₄、Q₅处斜率的绝对值。某一时刻所用的步长大小为step_i，输出功率变化为ΔP_i，则风电机组当前工作点处斜率的绝对值约为

根据下述规则选择步长大小：Disturbance step size step ₁ >step ₂ >step ₃ >step ₄ >step ₅ >0, the corresponding power change threshold ΔP _b1 >ΔP _b2 >ΔP _b3 >ΔP _b4 >ΔP _b5 >0, and satisfy

It is assumed that these five slope values correspond to the absolute values of the slopes at Q ₁ , Q ₂ , Q ₃ , Q ₄ , and Q ₅ in FIG. 1 . The step size used at a certain moment is step _i , and the output power change is ΔP _i , then the absolute value of the slope at the current operating point of the wind turbine is about

The step size is chosen according to the following rules:

时，即当前工作点位于Q₁右侧，更加远离最大功率点，把步长设为step₁；1) when

, that is, the current operating point is located on the right side of _Q1 , farther away from the maximum power point, and the step size is set to step ₁ ;

介于上述五点的某两点之间时，如

认为当前工作点位于Q₂与Q₃点之间，把步长设为step₃；2) when

When it is between two of the above five points, if

It is considered that the current working point is between Q ₂ and Q ₃ points, and the step size is set to step ₃ ;

时，即当前工作点处的斜率已经很小，可认为已运行到最大功率点，则停止扰动，从而抑制最大功率点附近的振荡现象。3) when

When , that is, the slope at the current operating point is already very small, it can be considered that it has reached the maximum power point, and the disturbance is stopped, thereby suppressing the oscillation phenomenon near the maximum power point.

By properly setting the number of steps n, the size of the steps, and the power threshold corresponding to each step, the speed and accuracy of searching for the maximum power point by the perturbation and observation method can be improved, and at the same time, the near maximum power point can be suppressed. oscillation.

Improvement of the power signal feedback method:

The conventional PSF controls the power of the wind turbine by obtaining the system output reference power from the speed signal according to the known optimal power-speed curve. This control method is simple, reliable, fast, and less affected by wind speed changes, and is suitable for large inertia wind turbines. If these advantages are effectively integrated into the process of P&O searching for the maximum power point, the maximum wind energy tracking control effect will be improved. Therefore, the present invention improves conventional PSF:

P _out = k _e ω ³ (3)

In the formula: k _e is the power-speed cube ratio. When k _e is different, PSF is based on different power-speed curves, and the best power-speed curve is a special one.

Description of the maximum wind energy tracking control method of the present invention:

The PSF in the maximum wind energy tracking control method of the present invention is different from the conventional PSF in that it is not necessary to know the power-speed curve on which the PSF is based in advance, and the proportional coefficient k _e in formula (3) needs to be obtained online. From formula (3), k _e is the power-speed cube ratio. After the wind turbine is running stably, the output power and speed information of the wind turbine can be obtained, and then the power-speed cube ratio corresponding to the stable operating point can be obtained and recorded online . Therefore, the present invention uses the power-rotational speed cube ratio as the link between P&O and PSF. It can not only obtain the proportional coefficient k _e required for implementing PSF online, but also improve the maximum wind energy tracking control effect of P&O through PSF, and effectively integrate the two way.

The maximum wind energy tracking control method is divided into three control stages: 1. From the beginning of the maximum wind energy tracking control to the first search of the maximum power point by using P&O and PSF, the wind energy capture efficiency in this stage gradually approaches the maximum power point with the operating point of the wind turbine And gradually increase. 2. At this stage, quickly search and record the power power-speed cube ratio corresponding to the maximum power point. The wind turbine operates in the area near the maximum power point at each wind speed, and has a high wind energy capture efficiency. 3. According to the power-speed cube ratio recorded in the control stage 2, the optimal power-speed cube ratio k _opt is obtained, that is, the optimal power-speed curve is obtained, and then only PSF is used, and P&O is no longer used.

Combined with Figure 2, the description of each control stage is as follows:

Control Phase 1:

Before implementing the maximum wind energy tracking control, the wind turbine is running stably at point A in Figure 2. First, use P&O to search for the maximum power point, and record the power-speed cube ratio of the wind turbine when it is running stably, as shown in Figure 2 A and B points, and their corresponding k values are k ₁ and k ₂ respectively. If the wind speed changes (see below for the method of judging the change of wind speed), switch to PSF, and implement power signal feedback control according to P _out = kω ³ , as shown in Figure 2, when the wind speed changes from v ₃ to v ₅ , according to P _out = k ₂ ω ³ implements power signal feedback control. After the wind turbine is stabilized under PSF control, switch to P&O to search for the maximum power point. As shown in Figure 2, implement power signal feedback control according to P _out = k ₂ ω ³ and then stabilize at point C, then switch to P&O.

It can be seen from Fig. 2 that since k ₄ is closer to the power-speed cube ratio corresponding to the maximum power point than k ₂ , when the wind speed changes, the power signal feedback is implemented according to P _out = k ₄ ω ³ than according to P _out = k ₂ ω ³ The stable operating point after control is closer to the maximum power point. Therefore, when the disturbance increases the output power of the wind turbine and the value of k increases, the value of k is updated, as shown in the process of updating the value of k from k ₁ to k ₄ in Figure 2.

Figure 3 is the control flow chart of this stage. When the first maximum power point is found, the control of this stage is completed and the control stage 2 is transferred.

Control Phase 2:

The first maximum power point can be searched through the control stage 1, the power-speed cube ratio at this point can be calculated, and the power-speed cube ratio can be stored in Table 1 according to the current wind speed v provided by the anemometer (hereinafter referred to as the recording table) At the wind speed closest to the wind speed v, the average filter value of the median value of the power-speed cube ratio at the wind speed (see the appendix for its definition) is obtained, as shown in Table 1. v ₁ is the cut-in wind speed; v _n is the maximum wind speed that can implement maximum wind energy tracking control.

Table 1 Power-rotational speed cube ratio recording table

Median mean filter value definition: a set of data x ₁ , x ₂ ... x _n , where the median value mean filter value is x, if n=0, then x=0, if n=1, then x=x ₁ , If n=2, then x=min(x ₁ , x ₂ ), if n>=3, x is the average value of the remaining data after removing the maximum value and the minimum value in this group of data.

Theoretically, the optimal power-speed cube ratio k _opt is equal to the power-speed cube ratio corresponding to the maximum power point. However, due to certain errors in the P&O search for the maximum power point, the maximum power point obtained at different times under the same wind speed will be different, and the corresponding power-speed cube ratio will also be different; the corresponding power of the maximum power point obtained under different wind speeds - The speed cube ratio will also be different, as shown in Figure 2. If the maximum power point found in control stage 1 is point E, the corresponding power-speed cube ratio is directly used as the optimal power-speed cube ratio k _opt to obtain the optimal power-speed curve L ₁ , and then enter the control stage 3 , implementing power signal feedback control, the wind energy capture efficiency of wind turbines is not very high, as shown in Figure 2. Therefore, it is necessary to take appropriate methods to improve the accuracy of k _opt . First, calculate the median average filter value for the power-speed cube ratio obtained under the same wind speed, and determine a more accurate power-speed cube ratio under the wind speed, such as the power-speed cube corresponding to wind speed v ₁ in Table 1 The ratio is k _opt1 . Secondly, calculate the corresponding power-speed cube ratios for all wind speeds that can implement maximum wind energy tracking control, as shown in Table 1, find the power-speed cube ratios from wind speed v ₁ to wind speed v _n .

If at least three power-speed cube ratios are obtained at each wind speed, then more accurate k _opt1 , k _opt2 , k _opt3 ,..., k _optn can be obtained, and the median and average filter value of them can be calculated as k _opt will have a higher accuracy, and at this time it can be considered that the recording of the record table is completed.

Figure 4 describes the control process of this stage. When the record table is recorded, the control of this stage is completed and it is transferred to the control stage 3. Control mode 1: When the wind speed remains constant, the disturbance is stopped, and the wind turbine operates stably at the maximum power point. Control mode 2: When the wind speed changes, switch to PSF, and the required proportional coefficients in formula (3) are the median average filter values of k _opt1 , k _opt2 , k _opt3 ...k _optn . After PSF controls the wind turbine to be stable, it switches to P&O to search for the maximum power point under the new wind speed.

In this stage, the power-speed cube ratio corresponding to the maximum power point is used, so the wind turbine operates in the operating area near the maximum power point at each wind speed, as shown in the area between _L1 and _L2 in Figure 2, which has a relatively High wind energy capture efficiency. With the increase of the recorded power-rotational speed cube ratio, the median average filter value of k _opt1 , k _opt2 , k _opt3 , ..., k _optn is getting closer to k _opt , and the operating area of the wind turbine is gradually shrinking, gradually approaching Optimum power-speed curve.

Control Phase 3:

After recording in the recording table, calculate the average filter value of the median value of k _opt1 , k _opt2 , k _opt3 ...k _optn as k _opt , that is, find the optimal power-rotational speed curve P _{out_opt} = k _opt ω _opt ³ , P _{out_opt} Output the maximum power for the system, as shown in Figure 3. After that, the power signal feedback control is implemented according to the optimal power-speed curve, and P&O is no longer used.

As shown in Figure 5, wind energy is converted into electrical energy by wind turbines and electrically excited double salient pole wind turbines, and then converted into DC power through a three-phase uncontrolled rectifier. Electric energy is fed to the grid; for non-grid-connected wind power generation systems, DC power can be transformed for DC loads or converted into AC power for AC loads through inverters.

The disturbance variable of the disturbance observation method is the excitation current of DSEG, and the observation quantity is the change of system output power.

Switch from P&O to PSF when wind speed changes. After PSF controls the wind turbine to run stably, if the record table is empty or the record is not completed, switch to P&O to search for the maximum power point; if the record is completed, it is considered that the best power-speed curve has been obtained online, and no longer switch to P&O.

For the above-mentioned electrically excited double salient pole wind power generator system, in the process of implementing P&O, the change of wind speed can be judged by any of the following methods:

1) Two consecutive disturbances lead to a decrease in the output power. According to the original P&O theory, when the last disturbance caused the output power to decrease, the disturbance in the opposite direction should be applied this time. According to the single-peak characteristic of the P _out -ω curve, The power should be increased this time;

2) As the excitation current increases, the resistance torque of the generator increases, and the speed decreases. If the speed increases, the wind speed changes. Similarly, the excitation current decreases. If the speed decreases, the wind speed changes;

3) The excitation current is constant, and the wind turbine runs stably. If the speed changes, the wind speed change can be judged;

4) Within a rotational speed sampling time, the rotational speed change exceeds the rotational speed change threshold caused by excitation current disturbance;

5) The wind turbine has not reached a stable operating state within a predetermined long period of time.

Claims (1)

1. A maximum wind energy tracking control method for variable-speed constant-frequency wind power generation, characterized in that: a) From the beginning of the maximum wind energy tracking control to the search for the first maximum power point, record the output power P of the wind turbine and the speed ω cube ratio is k, when the disturbance makes the output power of the wind turbine increase and the value of k increases, update k value; b) When the wind speed changes, switch from the disturbance observation method to the power signal feedback method. After PSF controls the wind turbine to run stably, if the power-speed cube ratio record table is empty or has not been recorded, switch to P&O to search for the maximum power point ; If the recording is completed, it is considered that the best power-speed curve has been obtained online, and no longer switch to P&O; c) When switching to PSF, use the coefficient k recorded in step a) to implement PSF according to P=k* ^ω3 before the power-speed cube ratio recording table starts recording; after the power-speed cube ratio recording table starts recording , implement PSF according to P=k _e *ω ³ , k _e is the median average filter value of k _opt1 , k _opt2 , k _opt3 ...k _optn in the power-speed cube ratio recording table.

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