In recent years, China's wind turbine collapse accidents are not uncommon. For example, in 2008, a certain wind turbine in a wind farm in Jilin City collapsed completely after about 3 years. In 2010, a certain wind turbine of a wind farm was not fastened because the bolts on the flange were not fastened. It was blown down by the strong wind [1-3]; in 2014, a certain wind turbine in Gansu suddenly collapsed in less than one year.
Analysis of the above collapse accident, it can be seen that the looseness of the flange of the fan tower flange is the main factor leading to the collapse of the wind turbine tower, and it is also one of the common serious diseases of the wind turbine tower. The factors that cause the looseness of the flange bolt are mainly the following aspects [1, 4-6]: 1 There is a deviation in the installation of the main operating components of the fan, which causes the vibration generated during the operation of the fan to be large, resulting in the loosening of the nut of the bolt in the flange; 2 the bolt in the flange is not fastened, resulting in a nut Under the action of vibration, it is gradually loosened; 3 The material properties of the bolt are unqualified. Under the action of vibration, the diameter of the bolt is gradually thinned, resulting in the loosening of the nut. Therefore, it is helpful to prevent the collapse of the wind turbine tower as soon as possible.
The existing method mainly judges whether there are more loose bolts in the flange by manual bolt-by-bolt method [1, 7-9]. Although this method can check the bolt looseness in detail, since the number of bolts in each wind turbine tower is close to one hundred, it is time-consuming and laborious to perform individual-work inspection. Therefore, the research on the rapid identification method of flange bolt loosening in wind turbine tower has great engineering value, which is beneficial to reduce the maintenance cost of wind turbine tower, and is more conducive to the development of safety monitoring technology for wind turbine tower operation [10-11].
Fan towers are usually made up of multi-section towers with flanges at the bottom and top of each tower, and the flanges of adjacent towers are connected by a large number of high-performance bolts. If the wind turbine tower is not damaged, its structural dynamic response transfer function is fixed under linear working conditions, but when the flange bolt is loose, it can be equivalent to the stiffness reduction of the bolted joint member [6], resulting in phase The interaction force function between adjacent towers changes, which in turn may cause a change in the transfer function between adjacent towers and a change in the transfer function of the entire tower. Therefore, in theory, if the bolt looseness ratio of the wind turbine tower reaches a certain level, the vibration characteristics of the wind turbine tower will change [11]. Structural vibration modal parameters are important physical parameters of structural transfer function expression, mainly including natural frequency, vibration mode, damping ratio, phase difference, stiffness, and mass [12-13]. Therefore, the dynamic response transfer function of the wind turbine tower can be analyzed by testing and calculating the structural vibration modal parameters of the in-service fan tower, especially whether the dynamic response transfer function of the bolted joint between adjacent towers changes greatly. Further, structural damage such as loose bolts of the wind turbine tower is recognized.
In this paper, a detailed vibration test of six wind turbine towers with bolt loosening disease before and after retightening of the bolts is carried out. The spectrum of the measured data shows that the vibration energy of the wind turbine tower is mainly concentrated in the first-order natural frequency. Points, while the spectral characteristics of other frequency points are not very prominent. Therefore, the research in this paper focuses on the change of the first-order modal parameters before and after the tower maintenance, and the statistics of the first-order natural frequency value and the first-order natural frequency value before and after the maintenance are statistically detailed, and the first is The vibration signal of the measuring point along the wind direction (the bottom measuring point of the tower) is used as the vibration input signal of the wind turbine tower, and the normalized curve and phase frequency of the vibration amplitude frequency response of the other winds toward the wind direction of the first measuring point are respectively made. The response curve is then based on the normalized curve of the amplitude and frequency response of each measuring point to obtain a first-order mode curve corresponding to the first-order natural frequency value; and the phase-frequency frame corresponding to the first-order natural frequency value is picked up based on the phase-frequency response curve The value, and then the phase difference of the other measuring points with respect to the first measuring point at the first-order natural frequency point, and finally the height of the measuring point of the measuring point is the X-axis, and the corresponding natural frequency point of the first-order fan tower is drawn. Mode shape and phase difference curve.
Through the data analysis, the data of the first-order natural frequency value and the first-order natural frequency value of the six wind turbine towers are statistically analyzed. The change of the damping ratio, the vibration mode curve and the phase difference curve before and after the bolt tightening are obtained. The phase difference curve corresponding to the natural frequency value of the order is very sensitive to the looseness of the flange bolt.
1 fan tower vibration test situation
An onshore wind farm is located in Jinchang City, Gansu Province. A large number of wind turbine towers have loose flange bolts. Some bolt loosening ratios exceed 50%. Especially when a wind turbine tower is not in a hurry, the disease is not used. It collapsed in 1 year.
When the bolt is fully tightened, when the bolt (nut) is tightened by the torque method or the angle method, the torque of the assembly of the tightening bolt (nut) reaches the upper limit (the upper limit of the torque is mainly determined according to the material yield strength value of the bolt and the nut, usually taken 70% of the torque corresponding to the yield strength, the bolt (nut) no longer produces displacement (rotation angle); and the looseness of the bolt means that when tightening the nut by the torque method or the angle method, the bolt is tightened (the torque of the assembly of the nut is lower than At the upper limit, the bolt (nut) still has displacement (rotation angle) [14]. Therefore, it is possible to detect whether the bolt (nut) is loose by the torque method or the angle method and to loosen the bolt (nut) based on the initial tightening torque value. Degree. In this experiment, the number of bolts (nuts) with looseness was recorded, and the initial tightening torque value was not recorded, because the experimental plan was imperfect and the field workload was huge when the bolts (nuts) were retightened.
Vibration is one of the main factors causing loose bolts. Conversely, loose bolts can also cause changes in the vibration characteristics of the wind turbine tower. Therefore, in this experiment, six wind turbine towers with different degrees of bolt loosening in this wind farm were selected for detailed vibration test, and the influence of loose bolts on the vibration characteristics of the wind turbine tower was studied. The basic conditions of the six wind turbine towers and the bolt loosening ratio are shown in Table 1. The wind turbine towers No. 1 and No. 2 are 1.5 MW units, and the No. 3 to No. 6 wind turbine towers are 2 MW units. The bolt specifications on the tower flanges. For the M36 480 10.9 high-strength bolt, the characteristic test results meet the requirements of GB/T 3098.1-2010 "Mechanical performance bolts, screws and studs for fasteners".
12 measuring points are arranged on the wall of each fan tower from low to high. Each measuring point is arranged with a QZ2013 single-component force balance accelerometer produced by Beijing Tengyiqiaokang Technology Co., Ltd. along the main wind direction (northwest direction). ) Arrangement, the specific arrangement is shown in Table 2. The accelerometer has a frequency range of 0 to 200 Hz and has good micro-vibration and strong vibration pick-up capability. The data acquisition instrument adopts the G01HET-2 synchronous dynamic data acquisition instrument produced by Beijing Tengyiqiaokang Technology Co., Ltd., and its AD digit is 24 bits.
In order to study the sensitivity of the vibration characteristics of the wind turbine tower to the looseness of the fan tower flange, we have carried out the test conditions shown in Table 3 for each wind turbine tower. The sampling frequency is uniformly 64 Hz during the test, and the data of 20 minutes is continuously recorded in each working condition. The measured typical waveform of the vibration of the wind turbine tower is shown in Figures 1 and 2.
Analysis of vibration characteristics of 2 wind turbine tower
2.1 Analysis of natural frequency and damping ratio
The FFT average spectrum analysis (average window is 81920×64, frequency resolution is 0.00078 Hz) is obtained for the vibration data collected under various working conditions, and the natural frequency value of the wind turbine tower is obtained. At the same time, the inherent power is calculated based on the half power spectrum refinement method. The damping ratio of the frequency point, its power spectrum is shown in Figure 3. The vibration energy of the fan tower is mainly concentrated in the first-order natural frequency point, and the other natural frequencies are not prominent. Tables 4 and 5 show the statistics of the first-order natural frequency and damping ratio analysis results of each fan tower.
It can be seen from Fig. 3 that the main vibration energy of the wind turbine tower is concentrated at the first-order natural frequency point of the tower, and the other natural frequency points are not prominent. Therefore, the vibration of the wind turbine tower belongs to ultra-low frequency vibration, which also indicates that the wind turbine tower is a very flexible high-rise building.
It can be seen from Table 4 that the natural frequency values ​​of the first-order vibration of each fan tower are exactly the same before and after the flange is tightened, and the first-order natural frequency values ​​measured under shutdown and normal operation are also the same. This shows that although the looseness ratio of the bolt loosening of the flange is relatively high, for example, the looseness of the bolt on the first flange of the No. 5 wind turbine tower is as high as 64%, but the first-order natural frequency after the bolt is retightened The value is still exactly the same as before the tightening. Therefore, by analyzing the vibrational spectrum of the six wind turbine towers, the vibration energy of the tower is mainly concentrated in the first-order natural frequency value, but the change of the first-order natural frequency value is not sensitive to the looseness of the flange of the wind turbine tower flange.
It can be seen from Table 5 that the damping ratio corresponding to the first-order natural frequency value of each fan tower changes very little before and after the bolt tightening, and the damping ratio of the fan towers 1, 3, 3, 4, and 6 varies greatly. Both are less than 3%. Although the looseness ratio of the first flange bolt of the No. 5 wind turbine tower reached 63%, the damping ratio only changed by about 10%. Therefore, the damping ratio corresponding to the first-order natural frequency value of the surface tower of the six towers is not very sensitive to the looseness of the flange of the fan tower flange.
2.2 First-order vibration mode and phase difference analysis of fan tower
The first-order mode and the first-order phase difference curve of the tower are measured in each working condition. The first-order mode and the first-order phase difference curve of each fan tower are shown in Figure 4~ 9 is shown.
It can be seen from Fig. 4 to Fig. 9 that the first-order vibration mode of the fan tower measured under the different working conditions of the six fan towers has little change, and the first-order vibration mode before the bolt is retightened and the first-order after retightening The vibration modes are very similar. Therefore, by analyzing the first-order vibration modes of the six wind turbine towers before and after the bolts are retightened, the first-order vibration mode of the surface tower is not sensitive to the looseness of the flange bolts. It can be seen from Fig. 2 to Fig. 7 that the change of the first-order phase difference curve of the fan tower measured by the six wind turbine towers under various working conditions is relatively large. Before the bolt is retightened, the phase difference curve measured under the condition of 1~4 is abruptly changed at the flange of the upper bolt, that is, the distance between the two measuring points of the lower disc and the upper disc is 1 Meter elevation, but the phase difference is significantly increased or significantly reduced relative to other high-rises, and the phase difference between the measuring points near the upper and lower discs of each of the three flanges of each fan tower shows this feature, such as the No. 3 wind turbine tower Although the loose ratio of the third flange bolt is only 6%, there is a sudden change between the measuring points near the upper and lower plates. After the bolt is retightened, the phase difference curve measured under the condition of 5 to 6 is relatively stable compared with the phase difference curve before the bolt is retightened, especially on the upper and lower plates of the flange of the upper bolt. There was no significant increase or decrease between the nearby measuring points. Therefore, by analyzing the 1st-order phase difference curve of the 6 wind turbine towers before and after the bolts are tightened, it is shown that even if the flange bolt looseness ratio is low, such as 6%, the upper and lower discs of the flange will be caused. The phase difference between the two changes significantly. This fully demonstrates that the 1st order phase difference curve of the tower is very sensitive to the looseness of the flange bolts.
The 6 fan tower flanges have different degrees of looseness. By comparing the vibration characteristics of their flange bolts before and after retightening, their natural frequency, damping ratio, first-order mode, and first-order phase difference are obtained. The variation characteristics are shown in Table 6.
3 conclusions
Bolts are important components of the wind turbine tower and are used to connect the towers. However, due to the long-term vibration environment, it is easy to loosen. In severe cases, the wind turbine tower collapses. Therefore, the looseness of the bolts connecting the flanges is one of the common serious diseases of the wind turbine tower, and it is also the main object of routine maintenance of the wind turbine tower. In this paper, detailed vibration tests and analysis are carried out on the six wind turbine towers with this disease before and after maintenance. It is concluded that the following relationship exists between the vibration characteristics of the wind turbine tower and the looseness of the wind turbine tower bolts:
(1) The first-order natural frequency of the fan tower is generally lower than 0.5 Hz, and the main vibration energy is concentrated at this frequency point, but it is not sensitive to the looseness of the flange of the fan tower flange, such as the looseness ratio of a certain flange bolt has reached 64%, but the 1st order natural frequency value is exactly the same as the value measured after the bolt is retightened.
(2) The damping ratio corresponding to the first-order natural frequency of the fan tower changes before and after the flange bolt is retightened, but the change is small. For example, the first flange bolt of the No. 5 wind turbine tower has a loose ratio of 64. %, and the bolt loosening ratio on the other two flanges is also as high as 24% and 15%, respectively, but the change is only 10% compared to the damping ratio after the bolt is retightened.
(3) Compared with the first-order mode curve after the bolt is retightened, although the bolts of the six fan towers have a high proportion of looseness, the first-order mode curve does not change much.
(4) The absolute value of the phase difference between the upper and lower discs of the flange with a certain proportion of loose bolts is significantly higher than the absolute value of the phase difference after the bolts are tightened again, such as the looseness ratio of a flange bolt Although it is only 6%, the absolute value of the phase difference between the upper and lower discs is also significantly increased.
According to the above, the natural frequency, damping ratio and vibration mode of the fan tower are not sensitive to the looseness of the flange bolts, but the absolute value of the phase difference between the upper and lower discs of the fan tower flange is the flange bolt. The loosening disease is very sensitive, and the looseness of the flange bolt can be identified based on the vibration characteristics of the wind turbine tower - the change characteristics of the phase difference. Therefore, the detailed vibration test of six wind turbine towers with bolt loosening disease is carried out in this paper. The analysis results of the test data have certain reference value in the research of the rapid detection of looseness of the bolts of the wind turbine tower flange and the real-time monitoring of operational safety.
references
[1] Wei Tai, Wu Kun, Huang Junwei. Fan tower bolt anti-loose detection technology [J]. Machinery and Electronics, 2013 (8): 78-80.
WEI Tai, WU Kun, HUANG Jun wei. Prevent loosing detection technology of the wind turbinestower bolts [J]. Machinery & Electronics, 2013(8): 78-80.
[2] Li Benli, Song Xiangeng. Wind turbine structural dynamics [M]. Beijing: Beijing Aerospace University Press, 1999.
[3] Gong Yuanming, Wu Changshui. Development of high strength bolt test and test system [J]. Journal of Shanghai University of Engineering and Technology, 2011, 25(1): 27-30.
GONG Yuan ming, WUChang shui, Development of testing and measuring system for high strength bolt [J]. Journal of Shanghai University of Engineering Science, 2011, 24(1): 27-30.
[4] Yan Baiyong, Lu Qiuhai, Wang Bo, et al. Using the natural frequency anomaly analysis method to detect the bolt tightening force [J]. Vibration and Shock, 2015, 34(23): 77-82.
GOU Bai yong, LU Qiu hai, WANG Bo, etal. Bolt tightening force detection using outlier analysis of structura natural frequencies [J]. Journal of Vibration and Shock, 2015, 34(23): 77-82.
[5] Yu Jian, Xie Shousheng, Ren Litong, et al. Fractal study on vibration detection of tie rod rotor assembly [J]. Vibration and Shock, 2014, 33(14): 84-88.
YU Jian, XIE Shou sheng, REN Li tong, etal. Fractal research on the assembly vibration detection of rodfastening rotor [J]. Journal of Vibration and Shock, 2014, 33(14): 84-88.
[6] Li Yungong, Kong Xiangna, Gao Yuyong. Research on bolt loosening identification method based on probability density of two jointed vibration signals and PCA [j]. Vibration and Shock, 2015, 34(1): 63-67.
LI Yun gong, KONG Xiang na, GAO Yu yong. Method for detecting bolt looseness based on probability density of vibration signals of two connected parts and principal component analysis [J]. Journal of Vibration and Shock, 2015, 34(1): 63- 67.
[7] Li Yuan, Zeng Yu, Chen Changlin. Comparative study on vibration characteristics of fan towers of different unit types [J]. Dongfang Electric, 2012 (5): 43-46.
YI Yuan, ZENG Yu, CHEN Chang lin. Comparison with the vibration characteristics of the different wind turbine towers [J]. Dong fang Electrical Machine, 2012(5): 43-46.
[8] Rachid Y, Ismail EB, Tritsch JB, etal. Dynamic study of a wind turbine blade with horizontal axis [J]. European Journal of Mechanics A/Solids, 2001, 20(2): 216-225.
[9]Nurtagh PJ, Basu B, Broderick BM. Mode acceleration approach for rotating wind turbine blades [J]. Journal of Multi Body Dynamics, 2001, 21(8): 241-252.
[10]Murtagh PJ, Bassu B, Broderick BM. Along wind response of a wind turbine tower with blade coupling to rotationally rotationally rotationally [...] Engineering Structures, 2005, 27(8): 1209-1219.
[11] Liu Yuxiong. Dynamics and stability analysis of large wind turbine tower structure [D]. Lanzhou: Lanzhou University of Technology, 2012.
[12] Ren Wei xin, Roeck GD. Structural damage identification using modal data. I: Simulation verification [J]. Journal of Structural Engineering, ASCE, 2002, 128(1): 87-95.
[13] Ren Wei xin, Roeck GD, Structural damage identification using modal data. II: Test verification [J]. Journal of Structural Engineering, ASCE, 2002, 128(1): 96-104.
[14] Zhu Zhengde, Lin Hu. Based on the comparison and application of torque method and torque angle method in bolt assembly technology [J]. Journal of Diesel Engine Design and Manufacturing, 2005, 2(14): 39-42.
ZHU Zheng de, LIN Hu. The research of torque method and torgue/rotation method [J]. Desigh & Manufacture of Diesel Engine, 2005, 2(14): 39-42.
wire holder is made of high quality 304 stainless steel, It is easier to clean without rust, safe, healthy and durable, Prevent rust or chemicals from contaminating food and damaging health. Suitable for putting the wine glass holder.
China leading manufacturers and suppliers of Wine glass holder,Goblet Holder, and we are specialize in Wine Rack,Hanging Wine Rack, etc.
Goblet Rack,Stainless Steel Goblet Rack,Wine Glass Holder,Wine Glass Rack
Shenzhen Lanejoy Technology Co.,LTD , https://www.wirefruitbasket.com