Large wind turbine generator sets mainly consist of a wind turbine, a mechanical transmission system, power generation equipment, and a control system. The mechanical transmission system is the intermediate device that mechanically transmits the wind energy absorbed by the wind turbine to the generator. It includes drive shafts, couplings, gearboxes, clutches, and brakes. To facilitate wind energy capture and meet the performance control requirements of the unit, the unit must also be equipped with yaw drive, pitch drive, and auxiliary devices such as damping and braking. Figure 1 shows a typical large wind turbine generator set. The wind turbine on the left transmits power to the generator on the right via the main shaft and gearbox. The equipment inside the nacelle is mounted on a base and supported on the tower by yaw bearings.
The unique environment and operating conditions of large wind turbines place unusual demands on the transmission system, and numerous uncertainties exist, as shown in Figure 2.
The effects of external dynamic loads, variable wind turbine and power grid loads, strong vibrations caused by insufficient nacelle rigidity, and load spectra that can only be estimated and simulated.
The distribution of ultimate loads and other factors are all major issues that must be considered in transmission devices.
The main drive gearbox of a large wind turbine is located between the wind turbine and the generator. It is a heavy-duty gear speed-increasing transmission device that operates under irregular directional loads and instantaneous strong impact loads. The gearbox is one of the most important yet most vulnerable components in the wind turbine's drive shaft system.
Unlike on the ground, gearboxes inside the engine room cannot have a robust base. The power matching and torsional vibration factors of the entire transmission system are always concentrated on a weak point, which is often the gearbox within the generator set. Ideally, the gearbox would transmit torque and increase speed without bearing any additional loads. However, this is not only unrealistic, but also inevitably involves many additional loads due to variable wind conditions and the complex deformation of the generator set, adding considerable uncertainty to the gearbox design.
Clearly, reducing the size and weight of components within the confined engine room is crucial. Therefore, gearbox design must ensure reliability and expected lifespan while simplifying the structure and minimizing weight, while also considering ease of maintenance. Based on the parameters provided by the generator set, CAD optimization design is employed. Following the predetermined optimal transmission scheme, selecting a stable and reliable structure and materials with good mechanical properties and stability under extreme temperature variations, along with a comprehensive lubrication, cooling, and monitoring system, are essential prerequisites for gearbox design.
Therefore, the design and component selection of the transmission device must be based on the requirements of the host machine and, according to different operating conditions, be determined after analysis and comparison. The main factors to consider include:
1) Main unit operating conditions and performance parameters, and results of dynamic analysis;
2) Load distribution and structural form of the transmission system;
3) Requirements for the transmission device and its connections;
4) Safety and environmental protection requirements;
5) Lifespan requirements;
6) Economic and benefit analysis;
7) Operating and maintenance conditions.
The main gearbox in a conventional generator set is used to change speed and torque, enabling the application of a compact standard generator in the unit. Gearboxes of different power ratings employ different transmission configurations (see Figure 3).
In the 1980s, parallel shaft cylindrical gear transmissions were applied to standard wind turbines ranging from 100 to 500 kW. In the 1990s, the average power of wind turbines increased to 600 to 800 kW. To save space and obtain a larger speed ratio, cylindrical planetary gear transmissions or combinations of planetary and parallel shaft gear transmissions were adopted, achieving better results.
The single-stage planetary two-stage parallel shaft gearbox shown in Figure 4 is a widely used unit transmission structure. To achieve high power density and a large speed ratio, the planetary carrier of the planetary system distributes power to multiple planetary gears, which then converge to the sun gear and transmit the power to the parallel shaft gears. Common power ratings are below 2MW. The gearbox design is determined by the arrangement of the unit's transmission shafts and, together with the main shaft, is suitable for "two-point" or "three-point" support. A shrink-fit sleeve connects the main shaft and the gearbox input shaft (planetary carrier), with the fixed end located on the main shaft. The planetary carrier bearings (or housing) should be able to float axially. The planetary carrier uses double support to improve structural rigidity, typically employing three planetary gears, with the sun gear floating and distributing the load evenly. Helical gears are used for smooth transmission and reduced noise.
Two-stage planetary and single-stage parallel shaft gear drives also have many applications, with power reaching 3 to 3.5 MW.
For higher-power generator sets, to reduce overall size and save nacelle space, gearboxes tend to use a combination of planetary, differential, and parallel shaft gears. There are often more than three planetary gears to minimize volume and achieve greater power density.
The planetary differential and fixed-axis gear combination transmission structure shown in Figure 5 has been applied in domestically produced large-scale wind turbine units. This structure adopts a three-stage gear transmission: the first stage is a planetary differential gear transmission; the second stage is a fixed-axis gear splitting transmission; and the third stage is a parallel-axis gear transmission.
The power from the spindle is transmitted in two paths: one path, marked with a red arrow, goes directly from the second-stage internal gear ring connected to the planet carrier, through a set of circumferentially distributed fixed shaft gears to the second-stage central gear, and then back to the planet carrier through the first-stage internal gear ring connected to the central gear; the other path (indicated by a green arrow) is transmitted directly from the planet carrier, and merges with the power from the first path on the first-stage planet gears, then passes through the first-stage central gear (sun gear) to the third-stage parallel shaft gear pair.
This transmission method can achieve a total speed ratio of 200:1 (planetary/differential stage: 35:1; parallel shaft: 6:1). The power distribution ratio is 72.1% from the internal gear ring differential to the final stage driving gear; and 27.9% from the planetary gears and sun gear to the final stage driving gear. Due to the use of multiple planetary gears and a flexible planetary pin structure, its size and weight are reduced by more than 25% compared to traditional structures.
In low-speed, heavy-duty planetary gear transmissions, the planet carrier typically employs a double-support structure, as shown in Figure 6(a). The uneven load distribution on the gears is affected by manufacturing and installation errors, tooth deformation, and temperature variations. While a reasonable load-sharing mechanism (such as a floating sun gear) can partially alleviate this issue, the planetary gears still experience uneven loads, directly impacting transmission quality. Therefore, adding an elastic load-sharing mechanism can further improve load distribution; using a flexible planetary gear shaft is a simple and effective method. (Wind power materials and equipment)
As shown in Figure 6(b), the planetary carrier adopts a single support, and the elastic mandrel is interference-fitted with the planetary carrier and the elastic bushing outside the mandrel. The elastic bushing is cantilevered and fixed to the free end of the mandrel, and a rolling bearing is installed on the bushing. The planetary gears rotate on the bearing. When the load is applied to the middle or left end of the planetary gear, the mandrel and bushing deform, and the axes of the mandrel and planetary gears tilt. As a result, the load is transferred along the tooth width direction, making the load distribution along the tooth width more uniform. Figure 6(c) shows the structure of the flexible planetary gear shaft in the actual application of the above-mentioned domestic large wind turbine gearbox. Figure 7 is the relative strength distribution of the stress at the root of the planetary gear teeth measured during the prototype test. The figure shows the stress distribution under different loads.
The load distribution of the various curves is basically consistent, and the load unevenness coefficient (the ratio of the maximum load to the average load) is close to 1.1, indicating a good load-sharing effect.
The structure of the four-stage planetary differential combined transmission is shown in Figure 8, with power transmitted in two paths:
The main input stage of the gearbox consists of a single-stage planetary gear and a ring gear fixed to the gearbox housing. Unlike traditional gearbox power transmission, power is not entirely concentrated on the sun gear, but is partially transmitted through the planet carrier to the rotating internal ring gear in the second stage. In the second stage transmission, a set of gears supported on the gearbox housing, along with the meshing internal ring gear and sun gear, serves to split speed and change the direction of rotation. Torque variation occurs through the sun gear.
In the third stage of the differential planetary gear system, the power flows from the first and second stage sun gears converge. The first stage sun gear drives the planet carrier, while the second stage sun gear drives the internal ring gear. This third stage is called a three-axis planetary differential transmission, where the two power flows converge on the sun gear and are then transmitted to the fourth stage parallel shaft drive gear. The overall speed ratio can reach over 200:1, with the ratio for stages one to three being approximately 40:1 and the ratio for the parallel shaft stage being approximately 5:1.
Power distribution ratio: 61.8% from the first-stage sun gear; 38.2% from the planetary carrier and internal ring gear. The volume and weight are reduced by approximately 20% compared to traditional structures. This weight reduction is achieved through a clever power distribution transmission path. The first-stage planetary gears significantly reduce space and weight. By utilizing multi-path power transmission and the different functions of each gear stage, power is combined while balancing changing speeds and directions, thus meeting the specific requirements of the unit's drivetrain.
The final stage of the gearbox uses a fixed-axis gear pair, following the rule of non-coaxial design for wind turbine gearboxes. This is to allow for the placement of pipes or cables in the center hole to control the impeller blade pitch. Additionally, generating the necessary center offset facilitates the adjustment of different generator speeds.
As shown in the left image of Figure 9, traditional turbine units use a main shaft to transmit power between the rotor and gearbox and bear most of the abnormal load from the rotor, reducing the risk of gear damage. However, this increases the nacelle length and volume; this effect is less noticeable in smaller power units.
As power increases, the diameter and weight of the main shaft also increase. When arranging the drive shaft system for units above 3MW, the large and heavy main shaft becomes a target for weight reduction in the nacelle, and a direct-drive method is preferred in the design. As shown in the structure in Figure 9 (right), the rotor is suspended from the base via a tapered roller bearing that bears huge loads in three directions, directly transmitting power to the gear train. This brings challenges such as the development of ultra-large double-row tapered roller bearings, the high-strength, high-power-density design and manufacturing of the gear transmission device, and the setting of dynamic edge conditions for the shaft system. Effective solutions must be found for these challenges before adopting the "direct-drive" scheme.
Although direct-drive wind turbines simplify the transmission structure, the increasing size of wind turbines necessitates addressing the transportation and installation challenges caused by excessively large, low-speed generators. Coupled with high manufacturing costs, this necessitates reconsidering ways to reduce the size and weight of the mechanism and lower costs. Appropriately utilizing gear speed increase or power shunting methods are among the solutions.
There are already many application examples of so-called "semi-direct drive" or "hybrid drive" type units that use gear transmission with a smaller speed ratio between the wind turbine and the low-speed motor to reduce the size of the motor structure. The transmission form in Figure 10 adds two stages of gear transmission (one stage of planetary and one stage of fixed-axis gear transmission) between the wind turbine and the motor to increase the motor speed, enabling the unit to use a smaller permanent magnet motor and achieve a more compact structure.
Power splitting can also be used to reduce the nacelle volume. The power splitting model shown in Figure 11 has already been used abroad. In this unit, the turbine distributes power equally to four intermediate shafts via a large gear on the main shaft, and then transmits it to four motors through four sets of gears for increased speed. This allows for a smaller nacelle while achieving the capacity of a large motor. The challenge of this gear transmission structure lies in the load distribution among the four splitting shafts; if this can be reasonably resolved, it is a good solution for achieving a smaller size.
The main components of the gearbox should possess sufficient strength to withstand the dynamic and static loads of the wind turbine generator under various operating conditions. The dynamic load on the gearbox depends on the characteristics of the input end (wind rotor) and output end (generator), the mass, stiffness, and damping values of the driving and driven components (shafts and couplings), the layout of the wind turbine generator nacelle, the control and braking methods, and external operating conditions.
In reality, the gearbox is no longer considered an isolated entity, but rather a component of the entire transmission system; the operation of the transmission system...
Reliability is no longer solely determined by verifying the load-bearing capacity of individual components. The design increasingly relies on dynamic simulation results of the entire transmission system to assess operational reliability. This requires establishing a dynamic simulation model of the entire unit, simulating various operating conditions such as startup, operation, idling, shutdown, normal startup, braking, and emergency braking. For different models, corresponding dynamic power curves are derived, and specialized design software is used for analysis and calculation to determine the reliability of individual components.
Based on the design load of the components, strength calculations are performed on the main components of the gearbox.
When modeling, the following factors should be fully considered: harsh environmental conditions (extreme temperature, humidity, dust, etc.); variable wind conditions (wind direction, wind speed, storm, turbulence, etc.); frequent starts and stops, emergency stops, and sudden load impacts on the front impeller and rear motor; dynamic design and load distribution of the transmission chain; characteristics of high power density and high speed ratio speed-increasing transmissions; component design and material properties requirements; cooling and lubrication conditions; requirements for resistance to pitting and fatigue damage; noise and vibration; long service life requirements, etc.
Starting with a simplified transmission system model, actual working conditions are simulated, and the relationship between load and the stiffness of each component is analyzed. Finite element method and fracture mechanics tools are used to calculate the dynamic characteristics of the system and analyze the mode shapes and frequencies at each stage, thereby improving the transmission chain arrangement. Measures are taken to reduce gear transmission errors, reduce meshing forces, optimize tooth profile parameters, and avoid system resonance response points.
Load spectrum and ultimate load are the basis for gearbox design calculations. The load spectrum should reflect all loads that the gearbox will bear during its entire operating process throughout its design service life. This includes the normal operating load at the installation site and the highest operating load caused by extreme wind speeds or three-dimensional turbulence conditions, as well as instantaneous peak loads caused by sudden pitch adjustment, blade tip deployment, or mechanical braking.
