The function, types and components of yaw systems
The yaw system is a unique servo system of wind turbines. It has two main functions. First, it works in conjunction with the wind turbine's control system to ensure the turbine rotor is always facing the wind, maximizing wind energy utilization and improving power generation efficiency. It also provides necessary locking torque when the wind direction is relatively fixed, ensuring safe operation of the turbine. Second, because wind turbines may continuously yaw in one direction, it automatically unwinds the cables in the suspended section of the turbine to prevent excessive twisting that could lead to breakage or failure. This is because the cables reach the designed twisting value.
There are various types of yaw systems, such as damped yaw systems, fixed yaw systems with yaw brakes, soft yaw systems, damped free yaw systems, and controllable free yaw systems. Currently, the two most commonly used systems are damped yaw systems using sliding bearings and fixed yaw systems with yaw brakes.
The yaw system consists of two main parts: the yaw control mechanism and the yaw drive mechanism. The yaw control mechanism includes several components such as a wind direction sensor, a yaw controller, and a mooring release sensor, while the yaw drive mechanism includes several components such as a yaw bearing, a yaw drive device, and a yaw brake (or yaw damping device).
Damped yaw system using sliding bearings
The yaw system of a sliding bearing is a damped yaw system. This bearing is located between the tower and the nacelle, transmitting various forces from the nacelle to the tower via bearing bushes. The bearing bushes of sliding bearings are mostly made of engineering plastics. This material (such as PETP, which is better than nylon) has excellent comprehensive properties, including mechanical properties, heat resistance, wear resistance, chemical resistance, and self-lubrication. It also has a low coefficient of friction, some flame retardancy, ease of processing, and good corrosion resistance. Due to the unique mechanical properties of this material, this bearing can operate even without lubrication. The bearing bushes consist of axial thrust bushes, radial thrust bushes, and axial thrust bushes. They are used to withstand the axial forces parallel to the tower generated by the weight of the nacelle and rotor, the radial forces perpendicular to the tower transmitted from the rotor to the nacelle, and the overturning moment of the nacelle, respectively. Thus, the various forces and moments acting on the nacelle are transmitted to the tower through these three types of bearing bushes.
The structure of the sliding bearing damped yaw system is shown in Figure 1.
In the diagram: 1. Yaw motor, 2. Yaw reducer, 3. Yaw caliper, 4. Yaw pinion, 5. Tower.
Yaw reducers are generally vertical planetary reducers or planetary/worm gear reducers. The small gear at the end meshes with the large yaw gear ring and is driven by a motor to achieve yaw against the wind or to untie the mooring line.
The yaw caliper is a relatively important and complex component among the yaw components (Figure 2):
1. Nacelle chassis; 2. Caliper fixing bolts to nacelle chassis; 3. Axial thrust bearing fixing bolts; 4. Axial thrust bearing; 5. Radial thrust bearing; 6. Radial thrust bearing fixing bolts; 7. Dustproof rubber ring; 8. Connecting bolts between tower and yaw friction disc and large gear ring; 9. Disc spring inside caliper; 10. Caliper adjusting bolt; 11. Axial thrust bearing; 12. Yaw caliper; 13. Large gear ring.
There isn't a single yaw caliper; for example, there are four sets on a V52-850kW wind turbine. The axial thrust bearing acts as a sliding bearing and supports the weight of the nacelle and the downward axial force during turbine operation. The radial thrust bearing acts as a sliding bearing and supports the radial force between the nacelle and the tower during operation. The axial thrust bearing acts as a sliding bearing and withstands a certain overturning moment.
To prevent excessive vibration and resonance of the wind turbine during yaw, the yaw system must provide an appropriate damping torque. The magnitude of the damping torque is determined based on the inertial moment of the combined mass of the nacelle and rotor. The fundamental principle is to ensure smooth and stable yaw without vibration. Only under the action of the damping torque can the rotor be accurately positioned, fully utilizing wind energy for power generation.
The advantages of a sliding bearing yaw system are lower cost and easier maintenance; it uses a self-lubricating sliding bearing support method, eliminating the need for an additional lubrication system and low-speed hydraulic brake, and preventing oil leakage. The disadvantages are a relatively complex structure; a larger maintenance workload; and a larger frictional damping torque. This is because sufficient frictional damping torque is required to maintain stability against the wind and avoid vibration. This frictional torque must be overcome when yawing against the wind, meaning that under extreme yaw loads, the nacelle may slip. This type of yaw system may sometimes experience the following malfunctions:
Such problems include overheating of the yaw motor (which often occurs in winter), damage to the reducer gears, damage to the axial downward thrust bearing, detachment and breakage of the axial upward thrust bearing, breakage of the yaw caliper adjusting bolt, and detachment of the radial thrust bearing.
The use of sliding bearings in the yaw system of wind turbines is quite common. For example, VESTAS's V42-600kW and V52-850kW, GAMESA's G52-850kW, SUZLON's S.60-1000kW, S.66-1250kW, and S.88-2MW, and ZOND's Z-48 750kW are examples of this type of yaw system. In my country, Sinovel's SL1500-1.5MW (Fuhrlander technology) and Shenyang Blower Works' 2MW (Windtec technology) also use this type of yaw system.
A fixed yaw system employing rolling bearings and yaw brakes is a type of fixed yaw system. The bearing is located between the tower and the nacelle, transmitting forces from the nacelle to the tower. The installed yaw brake system acts on an annular brake disc, providing multiple hydraulic brakes (six on a 1.5MW wind turbine) to prevent unwanted yaw movements under various conditions. The brake pads are made of organic materials. This material must have a stable coefficient of friction, low wear rate, and high temperature resistance.
The structure of a fixed yaw system with a yaw brake is shown in Figure 3.
In the diagram: 1. Yaw motor, 2. Yaw reducer, 3. Yaw pinion, 4. Hydraulic brake, 5. Tower, 6. Yaw friction disc, 7. Large gear ring, 8. Nacelle chassis.
Yaw reducers are typically vertical planetary reducers, with a small gear at one end meshing with a large yaw ring gear. Driven by a motor, they achieve yaw to engage with the wind or to release the cable. The yaw brake requires a hydraulic power source and control device. In the braking state, the high hydraulic pressure keeps the engine nacelle stationary. When yawing with the wind, the brake transitions to a damped state with a back pressure of 20-30 bar, resulting in smooth movement. Under specified weather conditions, when cable release is required, the brake releases, causing the engine nacelle to reverse a full revolution and release the cable.
The advantages of rolling bearing yaw systems are simple structure, reliable wind resistance, no slippage, and easy maintenance; the frictional damping torque is small when yawing against the wind, resulting in smooth yaw. The disadvantage is higher cost.
This type of yaw system sometimes experiences sealing leaks and noise: uneven distribution of friction materials or mismatched friction pair materials can cause vibration and noise during operation; repeated yaws can cause localized wear on the friction surfaces, or misalignment of the brake calipers can lead to increased localized friction and noise. Heat-induced aging of the seals affects their lifespan; once the brake seal is damaged, oil leaks occur, affecting yaw braking force. Simultaneously, oil contamination on the friction surfaces, combined with the formation of a glaze layer on the friction pads after yaw rotation, reduces the coefficient of friction and worsens braking performance. These problems can be completely solved by selecting high-quality brakes and carefully manufacturing and installing each component of the yaw system. Currently, most yaw brakes installed on wind turbines in my country are imported, with domestically produced yaw brakes accounting for less than 20%, primarily manufactured by Jiaozuo Ruisel Disc Brake Co., Ltd. This company has been working with the wind power market for over a decade, achieving large-scale production. Its products, tested by the market for nearly ten years, meet usage requirements and can replace imports.
The use of fixed yaw systems with rolling bearings and brakes is very common in modern large-scale wind turbines. Examples include REpower's MD77-1.5MW and REpower5M-5MW; Nordex's N60-1300kW; GE's GE-1.5s-1.5MW and GE Windenergy 3.6MW; and Dewind's D6-1MW and D8-2MW. Among domestically produced wind turbines, Shenyang University of Technology...
The 1.5MW, 2MW and 3MW (more than 20 companies have already transferred ownership), Huachuang 1.5MW, Dongqi 1.5MW and 3MW, and Huarui 3MW all use this type of yaw system.
Regarding the power of the yaw drive unit: During wind turbine operation, when turning against the wind, the power of the yaw drive unit should be able to overcome the yaw drag torque M. The following aspects should be considered when calculating the yaw drag torque:
M = Mf + Mw + Mp + Mz + Mtv
In the formula: Mf -- frictional resistance torque of the slewing bearing; Mw -- wind resistance torque caused by wind pressure acting on the wind turbine and nacelle; Mp -- inertial resistance torque caused by the inertial torque in the unit when yaw starts; Mz -- damping torque caused by the damping mechanism; Mtv -- yaw resistance torque caused by the torque component generated by the tilt angle of the wind turbine main shaft.
For example, in a 1.5MW fixed rolling bearing yaw, the sum of Mf and Mz is 40%-50% of the M value. If a damped sliding bearing is used for yaw, Mf will decrease while Mz will increase, ultimately having little impact on the power of the yaw drive device.
Regarding the hydraulic system: In fixed yaw, the yaw brake and the high-speed shaft brake form a hydraulic system (hydraulic station). Even in damped yaw, the high-speed shaft brake requires a hydraulic system (hydraulic station).
Development trend
Currently and in the future, fixed yaw will remain the mainstream. Most of the aircraft models independently developed in my country use fixed yaw, especially large units of 3MW and above.
