China is rich in wind energy resources and has good development prospects. According to the preliminary statistics of the latest wind energy resource survey conducted by the National Energy Administration organized by the China Meteorological Administration and other units, the total reserve of wind energy resources on China’s land at a height of 10m above the ground is about 4.35 billion kW, of which the technically developable volume is 297 million kW, the technically developable area is about 200,000 km2, and the potential technically developable volume is about 79 million kW. In addition, the offshore wind energy reserves that can be developed and utilized at a height of 10m are about 750 million kW, and the total reserves of wind energy resources that can be developed and utilized at a height of 10m across the country exceed 1 billion kW.
Wind power generation refers to the conversion of the kinetic energy of the wind into electrical energy, which is realized by wind generators, as shown in Figure 1.
Wind power includes two energy conversion processes from wind energy to mechanical energy and from mechanical energy to electrical energy.
The wind turbine blades of the wind turbine generate torque under the action of the airflow to drive the wind wheel to rotate, and the torque is input to the transmission system through Lun Yi, and the generator converts the mechanical energy from the transmission system into electrical energy using the principle of electromagnetic induction. There are two types of generators: asynchronous generators and synchronous generators.
The speed of the asynchronous generator depends on the frequency of the power grid and can only vary within a small range near the synchronous speed. When the wind speed increases and the gearbox high-speed shaft speed reaches the synchronous speed of the asynchronous generator, the unit merges into the grid and sends power to the grid. The wind speed continues to increase and the generator output power increases; when the wind speed reaches the rated value, the generator is in the rated power state ; when the wind speed increases beyond the rated value, the rotor blades of the fixed-pitch wind turbine will be in a “stall” state, the rotor speed will remain constant, the output power will be reduced, and the generator will not burn out due to overload. The wind turbine blades of the pitch-variable wind turbine can adjust the angle of attack of the airflow to the blades according to the changes in wind speed. When the wind speed exceeds the rated value, the output power can be stably maintained at the rated power.
Ordinary asynchronous generators are simple in structure and can be directly integrated into the power grid without the need for synchronous adjustment devices. However, the efficiency of the wind turbine becomes lower after the rotation speed is fixed, and it has to bear a larger load under the action of the alternating wind speed. The new variable-speed doubly-fed asynchronous generator overcomes the above shortcomings and is widely used in variable-speed constant-frequency wind turbines and megawatt-class wind turbines.
Synchronous generators rely on high-power electronic converters to achieve AC-DC-AC grid connection, and belong to variable-speed constant-frequency wind turbines. Its main feature is that the rotation speed of the generator is the same as that of the wind wheel and as the wind speed changes, the wind wheel can convert more wind energy. The gearbox-less direct-drive synchronous generator set uses the wind wheel to directly drive the synchronous multi-pole generator, which reduces the rotation loss of the gearbox and reduces the probability of failure.
The Global Wind Energy Council published the “Global Wind Power Report: Annual Market Development” on April 25, 2017 in New Delhi, India. The data shows that the newly installed capacity of global wind power in 2016 exceeded 54 GW, the cumulative capacity reached 486.8 GW, and the cumulative installed capacity increased by 12.6%, as shown in Figure 2.
In 2016, China’s cumulative grid-connected installed capacity and newly added installed capacity ranked first in the world. In 2016, the cumulative grid-connected installed capacity of wind power in China reached 149 million kW. In 2017, the newly added installed capacity of wind power was about 16 million kW, and the cumulative installed capacity was about 165 million kW, as shown in Figure 3.
With the rapid development of the wind power industry, China’s wind power generation has achieved rapid growth in recent years. In 2016, the national power generation of 6,000 kilowatts and above wind power plants was 241 billion kW·h, a year-on-year increase of 30%, an increase of approximately 14 percentage points from the previous year, achieving a substantial increase in power generation growth for two consecutive years. In 2017, under the strong support and guidance of national policies, the utilization rate of equipment was gradually improved, and wind power generation achieved steady and rapid growth.