① Output characteristics of wind turbine
The torque produced by a horizontal-axis wind wheel with a fixed pitch varies with wind speed and rotation speed. If the rotation speed of the blade is too low, the blade will stall and the torque output by the wind wheel will decrease. Therefore, in order to obtain the maximum power output from the airflow (when the airflow When the speed changes), the pitch angle of the blades or the speed of the blades must be changed. Nowadays, the rotors of many wind turbines are designed as variable pitch blades.
If the rotor speed of a wind turbine changes with the wind speed, the maximum power can be obtained from the air, but this is not optimal for a generator driven by the wind turbine. The solution of the optimized design is to allow the speed of the wind wheel to vary with the wind speed, while using a variable speed constant frequency generator to obtain the required constant frequency electric energy. The output power curve of the wind turbine is shown in Figure 1, where VC is the start-up wind speed, VR is the rated wind speed, at this time the wind turbine outputs the rated power, and VP is the cut-off wind speed.
When the wind speed is less than the start-up wind speed, the wind turbine cannot rotate. After the wind speed reaches the starting wind speed, the wind generator starts to rotate, drive the generator to generate electricity, output electric energy to supply the load and charge the battery. When the battery terminal voltage reaches the set maximum value, the feedback voltage signal causes the control system to enter the closed-loop voltage stabilization control, which not only keeps the battery charged, but also prevents the battery from overcharging. When the wind speed exceeds the cut-off wind speed, the wind power generator uses a mechanical speed limiting mechanism to make the wind power generator run at a certain speed or stop running at a certain speed to ensure that the wind power generator is not damaged.
Ordinary wind turbines need at least a wind speed of 3m/s to start, and a wind speed of 3.5m/s to generate electricity, which limits the application of small wind turbines in many areas of our country to a certain extent. With the full permanent magnetic levitation wind turbine, due to the use of micro-friction and small starting torque magnetic bearings, it can be started at a weak wind speed of 1.5m/s, and can generate electricity at a wind speed of 2.5m/s, increasing the energy efficiency by about 20%. Can be widely used in 80% of the country. Tested by the China National Machinery Industry Wind Power Machinery Product Quality Supervision and Inspection Center, the starting torque of the full permanent magnetic levitation wind turbine has been reduced to about 1/12 of the national standard, the starting wind speed has been reduced by 57.14%, the cut-in wind speed has been reduced by 23.81%, and the rated power Increased by 20.57%.
②The output power of the wind turbine
The rated output power of a wind generator is set in accordance with a specific rated wind speed. Since the energy is proportional to the cube of the wind speed, the power of the wind generator will vary greatly with the wind speed. Wind turbines with the same structure and rotor diameter can be equipped with different power generators. Therefore, two wind turbines with the same structure and rotor diameter may have different rated output power values, depending on whether it is designed for strong wind regions (with larger generators) or weak wind regions (with smaller generators).
The rated power of the generator refers to the power output by the generator at the rated speed. Since the wind speed is not a stable value, the generator speed will change with the wind speed, so the output power will also change with the wind speed. When the wind speed is lower than the design wind speed, the actual output power of the generator will not reach the rated value; when the wind speed is higher than the design wind speed, the actual output power will be higher than the rated value. Of course, due to the limitation of the speed limit and speed regulation device of the wind generator and the design parameters of the generator itself, the output power of the generator will not increase indefinitely, but can only change within a certain range.
When the wind speed is very low, the wind wheel of the wind turbine will remain stationary. When the cut-in wind speed is reached, the wind wheel starts to rotate and traction the generator to generate electricity. As the wind gets stronger, the output power will increase. When the wind speed reaches the rated wind speed, the wind turbine will output its rated power, and then the output power will remain roughly unchanged. When the wind speed further increases and reaches the cut-out wind speed, the wind turbine will brake and no longer output power to avoid damage to the wind turbine.
The performance of a wind turbine can be represented by a power curve, as shown in Figure 2. The power curve shows the output power of the wind turbine at different wind speeds (cut-in wind speed to cut-out wind speed).
If you need to read other knowledge about wind turbines, you can read other articles on this site — — Small wind turbines are classified according to design methods and structures, Horizontal axis wind turbine parameters and advantages, and What is a vertical axis wind turbine?