University of South Wales Master of Sciences Thesis
tải về 6.62 Mb. Chế độ xem pdf
|
| Rated output wind speed When the wind speed increases beyond the cut-in speed, the wind turbine
generates electrical power and this increases rapidly as it is cubic function to speed and this can be seen in Figure 2-9. However, depending on the type of turbines, the wind turbine reaches a point where its capability of generating the maximum output power limit is attained at some speed. This limit to the generator output is called the rated power output and the wind speed at which it is reached is called the rated output wind speed. At higher wind speeds, the design of the turbine is arranged to limit the power to this maximum level and there is no further rise in the output power. How this is done varies from design to design but typically with large turbines, it is done by adjusting the blade angles so as to keep the power at the constant level. Cut-out speed As the speed increases above the rate output wind speed, the forces on the turbine structure continue to rise and, at some point, there is a risk of damage to the rotor. As a result, a braking system is employed to bring the rotor to a standstill. This is called the cut-out speed and is usually around 25 m/s [24]. In general, if wind speed is between the rated speed and the furling (cut- out) speed of the wind turbine, the power output will be equal to the rated power of the turbine and if the wind speed is either less than the cut-in speed or greater than the furling speed of the wind turbine, then the output power will be zero [24]. 2.3.3. Variable Speed Wind Turbine Model A variable-speed wind turbine has three main regions of operation as shown in Fig. 2.10. In region 1, the wind speed is below the cut in speed (vo), which is not enough to start a turbine. Region 2 is an operational region of wind turbine where the wind speed remains between the cut in speed (vo) and cut out (vi) region. In region 3, the turbine must limit the captured wind power as the wind speed is above the cut out speed (vi), so as to ensure safe electrical and mechanical operating limits. Fig. 2.11 steady-state relationship between extracted aerodynamic power and wind speed |