Wind Turbine Component Damage and Microsite Selection in Wind Farm

0 Preface

The location of the wind farm is the preparatory work of the wind farm construction project and plays an important role in the success of the wind farm construction and the benefits of the wind farm. The design of micro-site selection for wind farms involves the use of wind energy resources in the site, the layout of wind turbines and current collection systems, transportation facilities, land occupation scale, and the realization of environmental protection goals, etc., as well as the construction cost and power of wind farms. The safety and reliability of production, equipment and facilities, and the ease of operation and maintenance will all have important and extensive impacts.

The micro-site selection of wind farms is directly related to the damage of the turbine components and the life of the unit. In the actual operation of wind farms, many wind turbines have caused damage to the components due to improper micro-site selection. For example, a wind farm is located in the mountains. 17 1.5MW wind turbine units, one of which will be damaged in less than 3 years, have been replaced twice, and other units installed in the same type of gearbox with the same type of wind farm have operated for 7 years. , there is little damage to the gearbox. From a number of side inspections, there is an inevitable link between the gearbox damage and the microscopic site selection of the unit.

1. Micro site selection and unit safety of wind farms

1.1 Micro site selection of wind farms

The micro-location of wind farms is the process of determining the distribution location of wind turbines in a small area selected in the macro site so that the entire wind farm can have better economic benefits.

Site selection plays a key role in the realization of the expected goals of wind energy utilization. If the site selection is unreasonable, even a high-performance wind turbine cannot generate electricity well. What is more, due to the incorrect location, it is likely to cause damage to the equipment. Therefore, how to rationally arrange wind turbines within a wind farm to obtain the maximum power generation and obtain the best economic benefits has always been the focus of micro site selection.

1.2 Micro site selection and turbulence intensity

At present, most of the micro-location software is based on the principle of maximizing power generation. The position of the unit is directly related to the magnitude of the turbulence intensity of the aircraft. Strong turbulence will cause vibration of the unit and deteriorate the stress of the unit, which will affect the failure probability of the unit and damage to components. It will affect the future maintenance and maintenance costs and the life of the unit. Therefore, the micro-selection of wind power The impact of the site on its future earnings cannot be ignored.

In order to make maximum use of the wind energy resources of specific wind farms and ensure the safe and reliable operation of wind turbines, IEC61400-1 has a safety classification for wind turbines. The turbulence intensity at hub height and extreme wind conditions are the two main parameters for wind turbine classification in the 2005 edition of IEC 61400-1. Extreme wind conditions mainly include extreme wind speed, extreme wind shear, and rapid changes in wind speed and wind direction. The maximum wind speed at a hub height of 50 years, a duration of 3 seconds, or the maximum wind speed of 10 minutes is the most important parameter for the extreme load design of wind turbines.

Select the aircraft position according to the size of the microscopically selected turbulence, or determine the type of safety class of the wind turbine being used. According to the GL specification or the IEC standard, the turbulence intensity levels of wind turbines are generally A and B. The new IEC version also has C level turbulence intensity levels.

In IEC61400, the design level of wind farm units is divided into three types of IECI, IECII and IECIII, as shown in Table 1.

Vref represents the maximum wind speed in 10 minutes of a 50-year wind farm; Class A is a high turbulence intensity, Class B is a medium turbulence intensity, C is a low turbulence intensity; I15 is the turbulence intensity characteristic value calculated when the wind speed is 15m/s. .

Turbulence intensity refers to the magnitude of random changes in wind speed within 10 minutes, which is the ratio of the standard deviation of the average wind speed within 10 minutes to the average wind speed over the same period. The turbulence intensity in the above table should be calculated according to the IEC61400-1 standard. According to IEC 61400-1 requirements, the effective turbulence intensity (the turbulence intensity generated by the wake between the units and the environmental turbulence intensity superimposed) of the wind turbine cannot exceed the designed turbulence intensity in each wind speed interval.

Turbulence intensity is an important characteristic index of wind farms, and its calculation and analysis are important contents of wind farm resource assessment. The maximum wind speed and the turbulence intensity at the height of the hub are measured at the height of the hub of the wind turbine during the 50-year period in the wind farm area. Determine the safety level of the wind farm and select the wind turbine model based on this level.

In China, wind energy resources are mainly distributed in the three northern regions and offshore areas, such as Xinjiang and Inner Mongolia, where the average wind speed is relatively large. The wind farm safety level is usually IECI or IEC II. The turbulence intensity of wind farms is usually B and C; the Shanxi and Hebei regions, Due to the large topography, wind farms usually have IEC II or IEC III safety standards. The turbulence intensity of wind farms is usually A and B. The coastal areas of Jiangsu and Zhejiang are affected by monsoons, and the wind farm safety level is usually IECI or IEC II. The wind farm turbulence intensity Usually B, C.

The adverse effect of turbulence on the performance of wind turbines is mainly to reduce the power output, increase the fatigue load of wind turbines, and ultimately weaken and destroy the wind turbines.

In order to reduce the dynamic load with strong pulsation and destructive force of the blades, wind farms should be built with or without wind farms in regions with large turbulence intensity. If the turbulence intensity of wind farms exceeds the safety design level of the turbines, the At this time, it is necessary to fully communicate with the manufacturer of the unit and fully demonstrate and evaluate the affordability of the equipment.

1.3 Alternating load and damage to the unit components

Wind turbines are vulnerable to severe fatigue loads. In a 600 kW unit, the impeller rotates 2×108 times during the 20-year life cycle. Each rotation of the impeller results in a force with a completely opposite gravitational force between the low-speed shaft and the blade, wind shear force, yaw error, and shaft tilt. Tower shadows and turbulence cause circulating blade plane loads. Therefore, the design of many wind turbine components depends on the fatigue load rather than the ultimate load.

The fluctuations or turbulence of short-term average wind speeds have a major impact on the load design because this is the source of extreme gust loads and most of the fatigue loads. The impeller rotation will continue to produce local shear gusts, which will increase the fatigue load on the blades.

Because the wind turbine operates in a very complex and changeable environment, the load on the unit is also very complex. Different load conditions have different effects on the force conditions of the various components of the unit, and the load is determined. The situation is very important and basic work for the follow-up design.

Due to different wind conditions and environmental conditions, the dynamic load of the unit varies greatly. Environmental turbulence intensity refers to the normal turbulence intensity experienced by a single wind turbine unit in a wind farm. The turbulence intensity is not affected by the wake of other units or obstacles. Determining the turbulence intensity level of the wind turbine depends not only on the turbulence intensity of the environment, but also the turbulence intensity generated by the wake of the wind turbine. The effective turbulence intensity that the unit bears in the wind farm consists of two parts: the environmental turbulence intensity and the turbulence intensity generated by the wake between the units.

The impact of different design turbulence intensity levels on the equivalent fatigue load is relatively large, basically reducing one turbulence intensity level, and the equivalent fatigue load will be reduced by 10% correspondingly. Turbulent intensity has a great influence on the equivalent fatigue load. In addition, the larger the diameter of the rotor, the more obvious the effect of reducing the turbulent intensity level on reducing the equivalent fatigue load. Therefore, the impeller diameter and the design turbulence intensity of the unit have a great influence on the bearing capacity of the unit alternating load.

In order to ensure the long-term safe and stable operation of the wind turbine, we must consider the unit's anti-fatigue load capacity when designing and manufacturing. When micro-site selection of the wind farm, fully consider the impact of wind conditions and environmental conditions on the unit to avoid the components. Damage, prolong the service life of the wind turbine.

2. Main influencing factors of micro site selection

Wind turbines operating under natural environmental conditions have complex mechanisms and causes of turbulence. The impact on equipment is also manifold. In the micro-site selection of wind farms, through the comprehensive consideration of various influencing factors, the impact and destruction of turbulence intensity on equipment will be reduced, and the most optimal location of wind farms will be achieved.

2.1 The effect of surface roughness

In the near-surface layer, the wind speed changes significantly with altitude, but the wind speed varies with height due to different ground roughness. The commonly used index formula of the lower atmosphere represents the relationship between wind speed and height and ground roughness:

In the formula, Vh: is the wind speed at the height Xh; V0: is the wind speed at the height X0; α: is the index, it is related to the roughness of the ground. The commonly used alpha values ​​in China are divided into three categories: 0.12, 0.16, 0.20. According to the formula, see Table 2.

2.2 Influence of obstacles

As the airflow passes over obstacles, disturbance zones are formed downstream of it. In the disturbed area, the wind speed not only decreases. There is also strong turbulence, which is very detrimental to the operation of the unit. Therefore, when selecting the installation position of the unit, it is necessary to avoid the disturbance area downstream of the obstacle. In theory, the length of the disturbance area is approximately 17H (H is the height of the obstacle). Therefore, when selecting the site, try to avoid it. Obstacles should generally be above 10H.

2.3 The influence of terrain

For mountain wind farms, mountain terrain and vegetation have a great influence on the turbulence caused by wind turbines. When the airflow passes through hills or mountains, it is affected by the terrain. At the lower part of the windward side of the mountain, the wind speed weakens. There are updrafts; on the top and sides of the mountain, the wind speed is strengthened; on the leeward side of the mountain, the wind speed is weakened, and there is a downdraft, gravity and inertia force will cause the leeward surface of the ridge to flow into waves.

The horizontal distance from the mountain to the wind speed is generally 5 to 10 times the height of the mountain on the windward side and 15 times the leeward side. And the higher the ridge, the more gentle the slope, the farther the impact distance on the leeward surface.

According to experience, the horizontal distance L that is affected by the leeward wind speed is roughly proportional to the product of the height of the mountain height h and the slope of the mountain's slope α half-angle, ie:

Closed valleys have smaller wind speeds than plains. In the long and straight valley, when the wind blows along the valley, its wind speed is stronger than that of the plain, ie, a narrow tube effect occurs, and the wind speed increases; but when the wind blows in the vertical valley, the wind speed is smaller than the flat ground, similar to the closed valley.

According to the actual observations, the relationship between closed valley y1 and gorge mountain y2 and flat wind speed is given by [8]:

Y1=0.712x+1.10

Y2=1.16x+0.42

Y1: Closed wind speed in valleys; y2: Wind speed in canyon passes; x: Flat wind speed.

2.4 The surrounding environment

In the course of its construction, wind farms will have an impact on the surrounding environment. Therefore, the micro site selection of wind farms also takes into account the impact on the surrounding environment. On the one hand, it is necessary to protect the ecological environment, such as the migratory routes of birds, Bird flight routes and animal habitats, etc., shall also ensure that farmland and vegetation are not occupied as much as possible; on the other hand, the impact of noise shall be considered, and the distance between the wind farm generator set and the nearest residential community shall be determined in accordance with relevant regulations. The noise size can not exceed 45db.

3. How to conduct micro site selection in wind farms

3.1 Microscopic location of flat terrain

A flat terrain can be defined as having a terrain height difference of less than 50 m within a radius of 5 km of the wind farm area and its surrounding area, while the maximum slope of the terrain is less than 3°. In fact, there is no terrain such as large hills or cliffs that can be treated as a flat terrain for the prevailing wind direction around the prevailing wind, especially the site.

When microscopically selecting sites on a flat terrain, the following two aspects are mainly considered:

First, the effect of roughness on flat terrain, in the area of ​​the site, the wind speed distribution at the same height can be seen as uniform, you can use the wind speed observation data of the neighboring meteorological station to estimate the wind energy of the site. For flat terrain, with the same impeller diameter of the wind turbine, the only way to increase the power output of the wind turbine is to increase the height of the tower.

Second, the impact of obstacles

Obstacles are relatively large objects that exist for a site, such as houses. When the air flow passes over obstacles, the flow direction and speed of the air flow are changed due to obstruction and obstruction of the air flow by obstacles. Obstacles and topographic changes affect the roughness of the ground. The average wind speed disturbance and the wind contour have a great influence on the wind structure, but this effect may be favorable (formation of the acceleration zone), or it may be unfavorable. (Cause wake, wind disturbance). Therefore, these factors must be fully taken into account when selecting a site.

As the airflow passes through obstacles, a wake disturbance zone is formed downstream of the obstacle, and then gradually weakens. In the wake area, not only the wind speed will decrease, but also a strong turbulence will be generated, which is extremely unfavorable for the operation of the wind turbine. Therefore, care must be taken when setting up the aircraft to avoid the wake area of ​​the obstacle.

3.2 Micro site selection for complex terrain

There are a large number of mountains in the coastal and inland areas of China. The flow of wind in a complex mountainous area is caused not only by large terrain and climate, but more by local topography. For simple terrain, large terrain and climate are the main causes of wind flow; for complex mountainous terrain, the acceleration of local winds and the deflection of wind direction are the difficulties in micro-site selection of wind farms.

When microsites are selected for complex terrain, there is no mature, quick and easy method for calculating the spacing of wind turbines, and it is usually necessary to finalize the plan through multiple trial calculations.

In complex terrain and climatic conditions, the factors driving wind flow are not unique. A special flow situation is that the wind speed does not increase with the elevation. After a certain height, the wind speed decreases with the elevation. The ambiguity in the criteria for microscopic siting and the adaptability of wind power equipment and sites based on or relying on foreign standards directly lead to significant differences in micro-site selection under complex terrain and harsh climatic conditions. The operational reliability of wind turbine equipment is not high, operation control becomes more difficult, and the actual output is lower than the forecasted assessment.

For complex terrains such as mountains and hills, wind turbines cannot be positioned according to the principle of simple flat terrain. Instead, after considering the wind conditions at each point according to the actual terrain, after considering various factors such as installation, terrain and geology, select the appropriate Install wind turbines at the site. In order to choose a location on a mountain with complex terrain and steep terrain, consideration should also be given to transportation, lifting, and line installation.

Because of the influence of complex terrain itself, some can only be arranged in a single row at the ridge location; some can only be placed on the location where the wind turbine can be placed; some places have valleys, hills and other terrain combinations, unit layout Affected by the combined effects of multiple factors.

3.3 Reasonable layout of the unit to avoid wake disturbance

From the point of view of making full use of wind energy resources, finding an arrangement scheme that meets the requirements of maximizing the energy utilization of wind farms as a whole is the first problem to be solved in completing microsite selection for wind farms. The key factor affecting the optimization of energy utilization in the whole field of wind farms is to recognize and solve the problem of the wake impact of the unit.

In general, when the wind passes through the rotor blades, the wind turbine will absorb some wind energy on the one hand, and at the same time, the rotating rotor will cause the turbulent kinetic energy of the wind to increase, resulting in airflow distortion and turbulence, and the wind speed will show a sudden decrease in the phenomenon. This is the so-called wind turbine wake effect. Afterwards, under the constraint of the surrounding flow field, the wind speed will gradually recover with the distance of the wind wheel. If the wind turbines in the wind farm are arranged closely, the wake effect of the winds behind the upstream units may still exist, and the wind speed may not be restored. As a result, the wind condition of the downstream units deteriorates, the input wind energy is insufficient, and the power generation output decreases.

The flow distortion and turbulence generated by the wake of the unit will generate alternating loads on the impeller of the downstream unit, which will cause damage to the blades, main shaft bearings, gear boxes and other components, shortening the life of the unit. Because the higher wake effect has a correspondingly higher turbulence intensity, a relatively small average wake effect is obtained in the entire field of the wind farm, which will help maintain the load balance of the whole plant, which in turn will increase the turbulence. Overall operation and maintenance efficiency.

However, it is difficult to absolutely avoid the wake effect in a wind farm because if the unit layout is too sparse, it will not only occupy too much land, the wind farm will be too large, and its engineering investment costs and operation and maintenance costs will also increase significantly. . Therefore, the determination of the unit spacing, or the control of the wake effect of the unit, is a technical and economical selection process that takes into account the balance of the comprehensive factors.

3.4 Microsite Selection Using Engineering Software

At present, the domestic micro-site selection usually adopts the wind power design software WASP and WindFarmer, which are more popular internationally, and the modeling process is as follows:

According to wind farm wind proofing point calibration, corrected wind measurement data, topographic map, and roughness, three input files of WindFarmer software that meet the accuracy and height requirements using hub-height wind resource raster files, including: hub height Wind resource raster file, wind resource raster file of wind measurement height and wind resource wind frequency file of wind measurement height.

The three files formed by the WASP software are input into the WindFarmer software by using an associated method, and the three-dimensional digital terrain map (1:10000 or 1:5000) is input. The mountainous terrain with complex terrain should adopt a 1:5000 topographic map and the input wind farm The power curve and thrust curve of the wind turbine under the air density, setting the layout range of the wind turbine and the number of wind turbines, setting the roughness, turbulence intensity, the minimum spacing of the wind turbine, slope, noise, etc., taking into account the various types of wind farm power generation Reduction factor, using the modified PARK wake model for optimal arrangement of wind turbines.

According to the optimized coordinates, GPS is used to inspect the fixed points on the site, fine-tuning the plane according to the site's topography and geomorphology conditions and construction and installation conditions, and using GPS to measure the new coordinates, and then input the site's fixed-point coordinates into the Windfarmer, using the viscous vortex. The swirling wake model accurately calculates the power generation and wake loss of each wind turbine unit in the wind farm.

The current micro-location technology is mainly application experience and tool software based on a linear model. All kinds of software have their applicable conditions and limitations. During use, they should fully understand the status of wind farms and software characteristics in order to optimize the layout of the aircraft.

3.5 Wind Farm Site Selection Procedures

Calculate the wind energy resources of the entire wind farm, find a good location of wind energy resources, determine the location of terrain suitable for deploying wind turbines according to the specific terrain and road conditions, and require a gentle slope (less than 10°) and convenient transportation; meet the above conditions Under the premise of a variety of programs to determine the different spacing, spacing in the main wind direction is 5 to 9 times the diameter of the unit, in the vertical main wind direction is 3 to 5 times the diameter of the unit; determine the spacing between the units in the actual layout of the wind turbine Calculate the impact of power generation, turbulence intensity, wake loss, etc.; compare the options, and select a reasonable wind turbine spacing arrangement of wind turbines.

4. Difficulties in site selection of wind farms

4.1 Lack of long-term wind data and meteorological data

The development of wind power in China is relatively late. Many wind farms and meteorological data in newly constructed wind farms are lacking or even a blank. However, the micro site selection of wind farms requires the acquisition of wind conditions data and meteorological data accumulated in the area for many years. This is correct. Site selection is particularly important. Without these data, it is difficult to have an accurate and scientific basis for site selection, which may have hidden potential risks and potential for crew safety and component damage.

For example, in Yunnan Province, due to the late start of wind power development in Yunnan Province, the lack of wind measurement data in the mountainous areas has become the first difficulty in the site selection of wind farms in Yunnan.

Turbulence intensity is an important indicator for building wind farms, and it has a direct impact on unit performance and life. There is no record of wind speed fluctuations at the Yunnan Meteorological Observatory and it is impossible to calculate the turbulence intensity. Therefore, when the wind farm is located, the influence of turbulence on the operational reliability of the wind turbine cannot be accurately predicted. Due to the lack of observational data such as glacial ice, dense fog, heavy snow, thunderstorms, and extreme temperatures in high mountains, there is no scientific basis for determining the future operating conditions of wind turbines.

It can be seen that the lack of long-term meteorological observation data has become a difficult problem for many new wind farms and wind farms in China today.

Most of the wind farms built in China are grassland or sparsely populated grasslands, and there are few relevant data and meteorological data in these regions. When wind farms are built, most of them can only be based on wind tower data of about one year. The number of wind towers built is limited, and the measured wind conditions data are limited. There is no long-term, comprehensive, and accurate data on wind conditions. Micro site selection basis.

4.2 The actual effective turbulence intensity of the aircraft position is difficult to accurately estimate

Now, the calculation of turbulence intensity for each location of a wind farm is based on the data of the wind tower. The turbulence intensity of each location is calculated using engineering software, and then the location is screened.

When estimating the turbulence intensity of the aircraft in the newly constructed wind farm, the turbulence intensity of each station cannot be accurately obtained. On the one hand, due to the fact that the wind turbines had not been built before the construction of wind farms, real data of the mutual interference between the units could not be obtained. Therefore, it was not possible to obtain actual wind data for each seat; on the other hand, Under natural conditions, the complexity, randomness, and limitations of the application of wind conditions change, the use of software to calculate the turbulence intensity of each position, there will always be theoretical and practical bias.

Therefore, it is difficult to make an accurate assessment of the actual turbulence intensity of each aircraft position through software micro site selection of wind farms. The effective turbulence intensity of some wind farms is exceeded. When the wind speed exceeds the IEC-A standard or the design standard of the unit, it will affect the safety and service life of the unit. In this case, it is necessary to take remedial measures to the unit operation. The economy and future benefits are reassessed.

5. How to handle excessive turbulent intensity on the aircraft

To avoid unit damage caused by excessive turbulence and prolong the service life of the unit. On the one hand, the wind turbines need to be screened and arranged through the engineering software in the newly-built stage of the wind farm; on the other hand, the micro-sites are improperly sited and the wind farms are operated. It is necessary to re-evaluate and optimize the operating unit's position again.

When the micro-location of wind farms is re-determined, or the actual effective turbulence intensity of wind turbines in operational wind farms exceeds their design standards, in order to avoid the fatigue damage of the components and shorten the life of the units, the environmental conditions and evaluation conditions of the specific aircraft are considered. The following measures can be taken:

(1) Replace this position with a higher turbulence intensity model.

(2) Move the unit to a position with low turbulence intensity.

(3) Adjust the arrangement around the unit, especially the upwind wind turbine unit, and increase the distance between the unit and the upwind unit so that it is as little as possible affected by the wake of other units.

(4) Since the turbulence intensity is greatly affected by the obstacles on the ground and the roughness of the ground, the tower height increases, and the effective turbulence intensity of the unit decreases. Therefore, under the conditions of project economy, base of flight, and tower tower strength, the height of the tower should be appropriately increased to adjust the effective turbulence intensity within the allowable range of the unit.

(5) Based on the on-site wind condition measurement and unit performance evaluation, based on the wind conditions, it can be selectively operated. For example, the unit will stop at a time when the turbulence intensity is very high; or the unit can be set by the unit controller program. In the operating position, the unit is only operated in the direction of small turbulence intensity, and the yaw of the unit is prohibited from yawing to the azimuth operation with large turbulence intensity.

(6) Adjust the operating mode of the wind farm unit, that is, when the downwind wind turbine is severely affected by the wake of the windward wind turbine unit, the wind turbine that is partially downwind can be shut down according to the actual situation. In this way, although a part of the power generation is sacrificed, the downwind unit can avoid the excessive turbulence intensity caused by the wake flow, which can reduce the fatigue load and prolong the service life of the downwind unit.

6 Conclusion

When constructing a new wind farm, attention is paid to the micro site selection of wind turbines to reduce the damage of the components of the wind turbine and extend the life of the turbine. For micro-site selection of wind farms, sufficient and accurate data should be used as the basis for evaluation and optimization of aircraft positions, and scientific methods should be used to achieve the most optimal location of wind farms through comprehensive consideration of various influencing factors; actual operation at wind farms In the unit, if there is a micro-site improperly located, it should be evaluated in a timely manner and measures should be taken to weigh the advantages and disadvantages so that the wind farm can achieve better economic benefits. (Author: WANG Ming-jun Liu Xiang Qi Zhu Xi Yang Liang)

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