1. Horizontal axis wind turbine
At present, the vast majority of grid-connected wind turbines use horizontal-axis wind turbines, and their shape and structure are shown in Figure 1. The rotor of the wind turbine is installed on the top of the tower where the wind is strong and the turbulence is small. At the same time, the top of the tower is also equipped with a machine cabin, which is equipped with a gear box and a generator. The rotor of the wind turbine is connected with the gearbox and the generator shaft through the shaft. The shaft of the wind turbine is in a horizontal state, hence the name “horizontal axis wind turbine”.
There are many different types of horizontal-axis wind turbines, as shown in Figure 2, which shows the most common models currently on the market. In terms of the number of blades, there are single-blade, double-blade, three-blade and multi-blade, etc. At present, three-blade is the most common in large-capacity wind turbines. The rotor of a horizontal axis wind turbine can be installed in the upwind direction (also known as the upwind type) or downwind (also known as the downwind type). The advantage of the downwind type is that it can automatically face the wind and does not require a yaw system. However, the wind must pass through the tower before reaching the wind wheel, which is greatly affected by the tower shadow effect. In addition, practical experience shows that when the wind direction changes suddenly, it is difficult for the wind wheel to adjust its direction in time. Therefore, upwind wind turbines are more common. The upwind horizontal axis wind power generation system must have a yaw mechanism to rotate the wind turbine rotor and nacelle. During normal operation, the wind turbine rotor should face the direction of the incoming wind so as to capture as much wind energy as possible. For small-capacity wind turbines, the yaw system is very simple, but for large-capacity wind turbines, the yaw system is more complicated.
2. Vertical axis wind turbine
The vertical axis wind turbine has a variety of airfoils, and the Darrie wind turbine is the most typical vertical axis wind turbine. Its structure is shown in Figure 3. It can be seen from Figure 3 that the shaft of this type of wind turbine is installed vertically, and the rotor blades rotate around the shaft, hence the name “vertical axis wind turbine”. The vertical axis wind turbine has two most prominent advantages: on the one hand, its generator and transmission system can be placed on the ground, reducing the requirements for the tower; on the other hand, it can absorb energy from wind in any direction. Therefore, the yaw-to-wind system is not required, which simplifies the system. However, vertical axis wind turbines also have very obvious shortcomings: First of all, the installation height of this type of wind turbine is limited, and it can only operate in a low wind speed environment, and the wind energy utilization rate is low; secondly, although its generator and transmission system are placed on the ground, maintenance is not easy, and the wind turbine rotor is often removed; finally, it needs to use cables to fix the tower. The cables extend far on the ground and take up a lot of ground space, as shown in Figure 4. Therefore, in large-capacity grid-connected wind power generation systems, vertical axis wind turbines are rarely used, and the maximum power of currently known vertical axis wind turbines generally does not exceed 1MW.
3. Constant speed and variable speed wind turbine
Constant speed wind turbine means that its speed is constant during normal operation. In the early days, wind turbine systems mostly used induction generators or synchronous generators, and the stator windings were connected to the grid. Therefore, the rotation speed of the generator is determined by the frequency of the power grid and cannot be reduced regularly. Although it is relatively simple to control, the utilization rate of wind energy is low.
With the development of power electronics and other technologies, doubly-fed asynchronous generators have emerged, which can maintain the constant output frequency of the stator group at different rotor speeds by controlling the frequency of the current in the rotor windings. Therefore, it allows the wind turbine speed to vary within a larger range, so it is called a variable speed constant frequency wind turbine. The recently developed low-speed direct-drive permanent magnet synchronous generator is a typical variable-speed constant-frequency wind turbine, because it outputs electrical energy from a full-capacity power electronic power rate converter, and its output frequency is determined by the inverter. Therefore, the rotation speed of the wind turbine is allowed to change in a wide range. The main advantages of variable speed wind turbines include high wind E efficiency, low mechanical stress, high power quality, and low noise.
It needs to be pointed out that whether the wind turbine is constant speed or variable speed does not depend on the wind turbine itself, but on the generator connected to it.
4. Fixed pitch and variable pitch wind turbine
The early wind turbines were mostly fixed pitch wind turbines. Its main feature is that the blades and the wheel are fixedly installed, and the blades cannot rotate around its axis. This type of wind turbine has a simple structure and low cost, but it has poor power control performance and low wind energy utilization. Therefore, fixed-pitch wind turbines are being replaced by variable-pitch wind turbines.
The blades of the variable pitch wind turbine can rotate freely relative to the wheel, thereby changing the pitch angle. The advantage of the variable pitch wind turbine is that it is easy to control the wind turbine to absorb power from the wind. Therefore, the power regulation performance is good, but it needs a set of special pitch machine (with hydraulic servo and electric servo pitch mechanism), the structure and control are more complicated, and the cost is higher.