Abstract: Wind power generation is the most technologically mature, has the greatest potential for large-scale development and commercialization among new energy sources, and its cost is now approaching that of conventional power generation. China has abundant wind energy resources. Currently, grid-connected wind turbines in China are mainly supplied by foreign manufacturers, and large wind turbines can only be imported or produced in cooperation with foreign companies. In terms of wind turbine manufacturing, the largest wind turbine produced in China has a capacity of 750 kilowatts, while international mainstream megawatt-class wind power equipment is still under development in China. However, it is expected that with the localization and successful application of megawatt-class wind power equipment, China will soon become one of the most prominent countries in the world in wind power development. China's proven wind energy reserves are approximately 3226 GW, of which approximately 253 GW are usable, mainly distributed in the grasslands and Gobi deserts of Northwest, North, and Northeast China, as well as the eastern and southeastern coastal areas and islands. Table 1. Statistics on Wind Farms and Operating Wind Turbine Units in China as of the End of 2003. As of the end of 2003, China had 40 grid-connected wind farms with a total of 1042 operating wind turbine units, with a total capacity of 567.02MW (based on the completion of turbine installation). The main distribution of the grid-connected wind farms is in Xinjiang, Inner Mongolia, Guangdong, Zhejiang, and Liaoning, as shown in Figure 1. [align=center] Figure 1: Distribution Map of Built (Under Construction) Wind Farms in China[/align] 1. Domestic and International Development Status Compared with other countries, the progress of grid-connected wind power generation in China is relatively slow. Wind power generation accounts for only 0.14% of the total installed capacity nationwide. Although 40 wind farms have been built, the average installed capacity of each wind farm is less than 15,000 kilowatts, and it has not yet reached a significant scale. Figure 2 is a comparison of wind power installed capacity in Germany, the United States, Spain, and China. [align=center]Figure 2 Wind Power Installed Capacity in Some Countries at the End of 2003 Figure 3 Domestic Production Ratio of Installed Wind Power in my country[/align] On the other hand, due to various constraints, the research and development of grid-connected wind power generation technology in my country lags far behind the world's advanced level and is far behind the requirements of wind farm construction in my country. In particular, my country's capital shortage prevents it from investing heavily in wind farm construction. Therefore, this market is currently dominated by foreign companies, with 87% of the installed capacity being imported equipment. Currently, my country's grid-connected wind turbines are mainly supplied by foreign manufacturers such as Bonus, Vestas, NEG-Micon, Nordtank, Nordex, and Gamesa. In terms of wind turbine manufacturing, megawatt-class models, which have become the international mainstream, are still under development in China. The largest domestically produced wind turbine unit has a power of only 750 kilowatts. Currently, large wind turbines can only be produced through imports or cooperation with foreign companies. The cumulative number of wind turbine units and the distribution of installed capacity in my country are shown in Figures 4 and 5. [align=center]Figure 4 Distribution of Wind Turbine Units in my country Figure 5 Distribution of Wind Turbine Unit Capacity in my country[/align] Figure 6 is a statistical chart of the distribution of newly added unit capacity in 2003. From this chart, we can clearly see that domestic production has achieved certain results, but overall, imported products still dominate. As of the end of 2003, the wind turbine units in operation in my country were mainly 600kW, totaling 497 sets, with 16 sets of units of 1MW (inclusive) and above. Units in the range of 600kW (inclusive) to 1000kW (exclusive) accounted for 65% of the total, including several power levels of 660kW, 750kW, and 850kW, as shown in Figure 7. [align=center]Figure 6: Market Share Distribution of Wind Power in 2003 Figure 7: Capacity Distribution of Wind Power Equipment[/align] Initially, wind power projects in my country cost approximately 9,000-10,000 RMB/kW. Later, with partial localization, the cost dropped to around 7,000 RMB/kW. Taking a 600 kW wind turbine as an example, the prices of several key components are as follows: ① Gearbox (increased from approximately 18 RPM to 1500 RPM). Imported gearboxes cost approximately 600,000 RMB, while domestically produced gearboxes cost approximately 180,000 RMB per unit. ② Generator (4-pole, 1500 RPM, 600 kW). Imported generators cost approximately 580,000 RMB, while domestically produced generators cost approximately 130,000-180,000 RMB. ③ Wind blades. Domestically produced wind blades cost approximately 180,000 RMB each, roughly 1/3 to 1/2 the price of imported products. For variable speed wind turbines, the price of the frequency converter must be added, approximately 1000-1500 yuan/kW. For a doubly-fed variable speed constant frequency wind power generation system, the capacity of the frequency converter is about 1/3 of the total system capacity, meaning the frequency converter accounts for less than one-tenth of the total system cost. Figure 8 shows cost statistics for wind turbine generator sets from Xinjiang Goldwind Co., Ltd., China. Figure 9 is a schematic diagram of the cost ratio of wind turbine generator sets from planning to commissioning. [align=center] Figure 8 Functional relationship between unit capacity and unit cost per kilowatt (U-shaped curve)[/align] [align=center] Figure 9 Cost of a complete wind turbine generator set[/align] 2 Key Technologies of Wind Power Equipment In a wind power generation system, the generator is the core part of energy conversion. Wind turbine generator systems are mainly divided into two categories according to the generator's operating mode: constant speed constant frequency wind turbine generator systems and variable speed constant frequency wind turbine generator systems. Constant-speed, constant-frequency wind turbine systems typically use synchronous motors or squirrel-cage induction motors as generators. By controlling the turbine's stall speed with a fixed pitch, the generator's speed is maintained at a constant value, thus ensuring a constant frequency and amplitude of the generator's output voltage. However, their operating range is relatively narrow. Variable-speed, constant-frequency wind turbine systems, on the other hand, control the turbine with variable pitch, allowing the entire system to operate at optimal efficiency over a wide speed range. This is a current trend in wind power development. Variable-speed systems are mainly divided into synchronous generator systems and asynchronous generator systems. Synchronous generator systems include permanent magnet synchronous generator systems and electrically excited synchronous generator systems; asynchronous generator systems are mainly wound-rotor asynchronous generator systems. Permanent magnet synchronous generators use permanent magnets instead of rotor excitation magnetic fields, resulting in a relatively simple and robust structure. A permanent magnet synchronous generator variable-speed, constant-frequency wind turbine system converts the variable-frequency, variable-voltage AC power output from the generator into constant-frequency, constant-voltage AC power that meets grid requirements by controlling a rectifier-inverter device. A typical structure is shown in Figure 10. Figure 11 shows a synchronous wind power generation system using electric excitation. The generator stator is connected to the power grid via a frequency converter, and the rotor uses an AC/DC rectifier to provide excitation to the generator. The generator can be driven by a gearbox or by direct drive. [align=center] Figure 10 Block diagram of permanent magnet synchronous wind power generation system Figure 11 Block diagram of electric excitation synchronous wind power generation system[/align] Currently, megawatt-level wind turbine generators generally adopt variable speed constant frequency wind power systems with wound-rotor asynchronous motors. A typical structure is a system using a doubly-fed asynchronous generator, as shown in Figure 12. This is a relatively suitable variable speed constant frequency scheme. In this structure, the stator is directly connected to the power grid, and the rotor is connected to the power converter. The power transmitted through the converter is only the slip power. The doubly-fed speed regulation feeds the slip power back to the motor shaft or the power grid, which is relatively efficient among various transmission systems. This structure is suitable for wind power generation systems with a narrow speed range, especially large and medium-capacity wind power generation systems. By the end of 2003, the proportion of variable-speed wind turbines installed in my country was still relatively low, accounting for only 6% of the total capacity. These included 12 G42 units at the Gemesa Yumen wind farm in Gansu, 4 G52 units at the Fengxian wind farm in Shanghai, and 12 V52 units each at the Zhangwu and Kangping wind farms in Liaoning Province. All installed variable-speed systems were doubly-fed variable-speed constant-frequency wind power systems. There were also some Vestas variable-slip wind power systems. Doubly-fed asynchronous variable-speed constant-frequency wind power systems can be mainly divided into three categories according to the rotor inverter topology: AC-AC inverters, matrix converters, and AC-DC-AC inverters. AC-AC inverters use thyristor natural commutation, are stable and reliable, and are suitable as power supplies for the rotor windings of doubly-fed motors. The highest output frequency of AC-AC inverters is 1/3 to 1/2 of the grid frequency, offering significant advantages in the high-power, low-frequency range. AC-AC converters have no DC link, resulting in high conversion efficiency, a simple main circuit (excluding DC circuits and filtering), and easy reactive power handling and active power feedback with the power supply. Although AC-AC converter doubly-fed systems are widely used, their application is limited by low power factor, high harmonics, low output frequency, narrow frequency range, and a large number of components. Matrix converters are a type of AC-AC direct converter, consisting of nine switches directly connected between the three-phase input and output. Matrix converters have no intermediate DC link, and the output consists of three levels with relatively low harmonic content. Their power circuit is simple and compact, and they can output sinusoidal load voltages with controllable frequency, amplitude, and phase. The input power factor of the matrix converter is controllable, and it can operate in four quadrants. While matrix converters have many advantages, the simultaneous conduction or cutoff of two switches during commutation is not allowed, making implementation difficult. The low maximum output voltage capability and high voltage withstand capability of the components are also significant drawbacks of this type of converter. In wind power generation, because the input and output of a matrix converter are not decoupled, any asymmetry on either the load or the power supply side will affect the other side. Furthermore, a filter capacitor must be connected to the input of the matrix converter. Although its capacitance is smaller than the intermediate energy storage capacitor in an AC-DC-AC converter, it is still an AC capacitor and must withstand AC current at the switching frequency, making its size not small. Currently, doubly-fed asynchronous wind power generation systems utilizing matrix converters are still in the research and development stage. AC-DC-AC inverters can be divided into voltage-type and current-type types. Due to various factors such as control methods and hardware design, voltage-type inverters are more widely used. Traditional current-type AC-DC-AC inverters use naturally commutated thyristors as power switches. Their DC-side inductors are relatively expensive, and when used in doubly-fed speed regulation, a commutation circuit is required when exceeding synchronous speed. Their performance is also relatively poor at low slip frequencies, and their application in doubly-fed asynchronous wind power generation is limited. The use of voltage-type AC-DC-AC frequency converters offers advantages such as simple structure, low harmonic content, and adjustable stator and rotor power factors. It can significantly improve the operating status and output power quality of doubly-fed generators. Furthermore, this structure completely separates the grid side and rotor side through the DC bus-side capacitor, representing a significant direction in variable-speed constant-frequency wind power generation. As of the end of 2003, all installed wind turbine units in China were 690V low-voltage units, with no medium- or high-voltage units. The frequency converters used in variable-speed units were mostly two-level voltage-type AC-DC-AC frequency converters. Rockwell's PowerFlex 7000 uses an SGCT current source converter, suitable for medium-voltage motors, but the 690V generator in wind power systems is incompatible. If applied to a doubly-fed variable-speed constant-frequency wind power system, voltage matching adjustments are necessary. To ensure the rotor-side power factor, an active front-end is required, and the grid's harmonic requirements for the generator must be met. Furthermore, doubly-fed induction generator (DFIG) wind power systems often need to operate near synchronous speeds, and the performance of PowerFlex 7000 at low and zero speeds requires improvement. 3. Future Prospects The development of China's wind power industry actually started with small wind turbine generators and has progressed from small to large. China's small wind power technology is relatively mature, and it is capable of independently developing wind turbine generators with capacities ranging from 100W to 10kW. Its cumulative installed capacity ranks first in the world. Off-grid wind power, after more than 20 years of development, has made a significant contribution to rural electrification. By the end of 2001, there were 43 units developing and producing off-grid wind turbine generators, including 16 research institutions, 17 main unit manufacturers, and 10 supporting manufacturers, with an annual production capacity exceeding 30,000 units. The main product varieties include 100W, 150W, 200W, 300W, 500W, 600W, 1kW, 2kW, 5kW, and 10kW wind turbine generators. In 2001, a total of 12,170 off-grid wind turbine units of various models (approximately 2,577 kW) were produced nationwide. The majority of products were 150W units, accounting for 41.45% of total production, followed by 100W and 300W units, accounting for 27.29% and 19.8% of total production, respectively. User demand remained primarily for units below 300W. By the end of 2001, a total of approximately 210,000 off-grid wind turbine units had been produced nationwide. Meanwhile, Chinese companies introduced off-grid wind turbine technology from foreign companies through joint ventures and cooperation. For example, Inner Mongolia Huade New Technology Co., Ltd. cooperated with the German company WENUS to produce 5kW wind turbine units, and successfully installed more than 20 units in Xinjiang, Inner Mongolia, Guangdong, and other regions. These units, along with wind/diesel/battery independent power supply systems, wind-powered water lifting systems, and television relay station wind power supply systems, effectively solved the electricity needs of local residents for daily life and production. Inner Mongolia Tianli Wind Power Machinery Factory introduced a series of seven wind turbine generator sets from the French company AEROWATT, with the 51kW unit being entirely domestically produced. Inner Mongolia Shangdu Livestock Machinery Factory partnered with the Swedish company SVIAB to produce the SVIAB065-24 wind turbine generator set. Hunan Xiangtan Electric Machinery Factory and the American company BERGEY established a joint venture to produce 600W, 1kW wind turbine generator sets. Off-grid wind power is now entering a stable period of industrial development. Compared with similar foreign units, Chinese products have advantages such as lower starting wind speed, better low-speed power generation, reliable speed limiting, and stable operation, and are also cheaper. However, there are still some gaps in appearance quality, blade material application, manufacturing process, and the production technology of larger-capacity off-grid units. To promote the commercial development of grid-connected wind power, the National Development and Reform Commission (NDRC) of the Chinese government clearly outlined the development goals for my country's wind power in September 2003: 1 million kilowatts of installed wind power capacity nationwide by 2005, 4 million kilowatts by 2010, 10 million kilowatts by 2015, and 20 million kilowatts by 2020, accounting for approximately 2% of the total installed capacity nationwide. This means that over the next five years, an average annual installed capacity of nearly 600,000 kilowatts was needed; from 2010 to 2015, nearly 1.2 million kilowatts were needed; and from 2015 to 2020, nearly 2 million kilowatts were needed. It can be predicted that China will soon become one of the world's most prominent countries in wind power development.