Classification and characteristics of wind turbine generator sets
Wind turbine generator sets mainly consist of two parts:
The wind turbine section—it converts wind energy into mechanical energy;
The generator section—it converts mechanical energy into electrical energy.
Based on the different structural types of the two main parts of the wind turbine, the different characteristics of the technical solutions they employ, and their different combinations, wind turbine generator sets can be classified in many different ways.
(1) According to the direction of the fan's rotating shaft (i.e., the relative position of the shaft to the ground), it can be classified as follows:
"Horizontal axis fan" - the rotating shaft is parallel to the ground, and the impeller needs to be adjusted according to the wind direction;
"Vertical axis fan" - the rotating shaft is perpendicular to the ground, the design is relatively simple, and the impeller does not need to adjust its direction with the wind direction.
CNWPEM.COM (2) According to the force applied to the blades, they can be divided into "lift type fans" or "resistance type fans".
(3) According to the number of blades, they can be classified into "single blade", "double blade", "three blade" and "multi-blade" type fans; the number of blades is determined by many factors, including aerodynamic efficiency, complexity, cost, noise, aesthetic requirements, etc.
Large wind turbines can consist of 1, 2, or 3 blades.
Wind turbines with fewer blades typically require higher rotational speeds to extract energy from the wind, resulting in greater noise. Conversely, too many blades can cause them to interact and reduce system efficiency. Currently, three-bladed wind turbines are the mainstream design. From an aesthetic perspective, three-bladed wind turbines appear more balanced and visually appealing.
(4) According to the direction of the wind received by the wind turbine, there are two types: "upwind type" - the impeller faces the wind (i.e., it rotates in front of the tower) and "downwind type" - the impeller faces away from the wind.
Upwind wind turbines generally require some kind of directional adjustment device to keep the impeller facing the wind.
Downwind fans can automatically align with the wind direction, eliminating the need for a directional adjustment device. However, for downwind fans, some air passes through the tower before being blown onto the impeller. This causes the tower to interfere with the airflow over the blades, creating a so-called tower shadow effect, which reduces performance.
(5) According to the different mechanical connection methods of power transmission, it can be divided into "gearbox type fan" and "direct drive type fan" without gearbox.
The blades of a gearbox-type wind turbine transmit torque to the generator's drive shaft through the gearbox, its high-speed shaft, and a universal flexible coupling. The coupling has excellent damping and vibration absorption characteristics, can absorb appropriate radial, axial, and certain angular offsets, and can prevent overload of the mechanical device.
Direct-drive wind turbines take a different approach, incorporating several advanced technologies. The torque of the blades can be directly transmitted to the generator's drive shaft without going through a gearbox for speed increase, allowing the electricity generated by the wind turbine to be output to the grid. This design simplifies the structure of the unit, reduces the probability of failure, and has many advantages, and is now mostly used in large-scale units.
(6) According to the power regulation method of the blade receiving wind energy, it can be divided into:
"Fixed-pitch (stall-type) turbines" – the connection between the blades and the hub is fixed. When the wind speed changes, the blade angle cannot change accordingly. Due to their simple structure and reliable performance, fixed-pitch (stall-type) turbines have dominated wind energy development and utilization for the past 20 years.
"Variable pitch turbine" - the blades can rotate around the blade's central axis, allowing the blade's angle of attack to be adjusted within a certain range (generally 0-90 degrees). Its performance is much better than that of the fixed pitch type, but its structure is also more complex. It is now mostly used in large turbine units.
(7) According to whether the impeller speed is constant, it can be divided into:
"Constant-speed wind turbine generator set" - has a simple and reliable design, low cost, low maintenance, and can be directly connected to the grid; however, its disadvantages are: low aerodynamic efficiency, high structural load, grid fluctuations, and absorption of reactive power from the grid.
Variable-speed wind turbine generators offer high aerodynamic efficiency, low mechanical stress, minimal power fluctuation, high cost efficiency, and a lightweight support structure. Disadvantages include: power output is sensitive to voltage drop, electrical equipment is more expensive, and maintenance is more demanding. They are currently commonly used in large-capacity mainstream models.
(8) Based on the type of generator in a wind turbine generator set, they can be divided into two main categories:
"Asynchronous generator type" and "Synchronous generator type"
They can all be used in variable speed fans, provided that appropriate converters are selected.
Asynchronous generators can be further classified according to their rotor structure as follows:
(a) Squirrel-cage induction generator – The rotor is squirrel-cage. Due to its simple and reliable structure, low cost, and ease of grid connection, it is widely used in small and medium-sized generator sets;
(b) Wound-rotor doubly-fed asynchronous generator – The rotor is wound. The stator is directly connected to the power grid to transmit electrical energy, while the wound-rotor is also controlled by a frequency converter to transmit active or reactive power to the power grid.
Synchronous generators can be further classified according to the type of magnetic poles that generate the rotating magnetic field:
(a) Electrically excited synchronous generator - the rotor is a wire-wound salient pole type magnetic pole, and the magnetic field is generated by external DC current excitation.
(b) Permanent magnet synchronous generator - The rotor is made of permanent magnet poles made of ferrite material. It is usually a low-speed multi-pole type. It does not require external excitation, which simplifies the generator structure and thus has many advantages.
(9) Based on the output voltage of the fan, it can generally be divided into:
"High-voltage wind turbine" – This type of wind turbine has an output voltage of 10~20kV, or even 40kV, eliminating the need for a step-up transformer and allowing direct grid connection. Together with a direct-drive, permanent magnet pole structure, it forms a synchronous generator system, representing a promising model for wind turbines.
"Low-voltage wind turbine" - the output voltage is below 1kV, and this is the most common type of turbine on the market.
(10) According to the rated power of the fan, it can generally be divided into:
Microcomputer: below 10kW
Small units: 10kW to 100kW
Medium-sized units: 100kW to 1000kW
Large-scale turbines: 1000kW and above (MW-class wind turbines)
Wind power materials and equipment
12. Direct-drive permanent magnet synchronous wind turbine
Permanent magnet synchronous generators are widely used in small and medium-sized wind turbines due to their simple structure, lack of excitation winding, and high efficiency.
Widely used in wind turbine generators, and with the improvement of high-performance permanent magnet material manufacturing processes, large-capacity wind power generation is becoming increasingly common.
The system also tends to use permanent magnet synchronous generators. Permanent magnet wind turbines are typically used in variable speed, constant frequency wind turbines.
In the electric system, the wind turbine rotor is directly driven by the wind turbine, so the speed is very low. The elimination of the speed-increasing gearbox increases the reliability and lifespan of the unit. The use of many high-performance permanent magnets to form the magnetic poles, unlike electrically excited synchronous motors which require complex and bulky excitation windings, improves the air gap magnetic flux density and power density, reducing the motor size for the same power rating.
Permanent magnet synchronous generators are structurally divided into external rotor and internal rotor types.
In a typical external rotor permanent magnet synchronous generator structure, the inner circumference of the outer rotor has magnetic poles made of high-energy-product permanent magnet materials, while the inner stator contains three-phase windings. The external rotor design allows for more space to accommodate the permanent magnet poles, and the centrifugal force during rotor rotation makes the magnetic poles more firmly fixed.
Because the rotor is directly exposed to the outside, its cooling conditions are relatively good. The problems with external rotors are the cooling of the stator, the main heat-generating component, and the transportation of large-sized motors.
An internal rotor permanent magnet synchronous generator consists of a rotor with permanent magnet poles that rotates with the wind turbine, and an external stator core. In addition to the advantages of conventional permanent magnet motors, the internal rotor permanent magnet synchronous generator can utilize natural wind conditions outside the frame, effectively improving the cooling conditions of the stator core and windings. The airflow generated by the rotor rotation also has a cooling effect on the stator. Furthermore, if the motor's outer diameter exceeds 4 meters, it often presents transportation difficulties. Many wind farms are designed in remote areas, and the journey from the motor's factory to the installation site may involve crossing bridges and culverts. If the motor's outer diameter is too large, it often cannot pass smoothly. The internal rotor structure reduces the motor's size, often facilitating transportation.
In internal rotor permanent magnet synchronous generators, there are four common types of rotor magnetic circuits: radial, tangential, and axial. Compared to other rotor magnetic circuit structures, the radial magnetization structure has a small leakage flux coefficient because the magnetic poles directly face the air gap, and its yoke is a single piece of magnetic conductor, making it easy to manufacture. Moreover, in the radial magnetization structure, the air gap magnetic flux density is close to the operating point magnetic flux density of the permanent magnet. Although it is not as large as the air gap magnetic flux density of the tangential structure, it is not too low either. Therefore, the radial structure has obvious advantages and is the rotor magnetic circuit structure most commonly used in the design of large wind turbine generators.
