Currently, geared wind turbine generators mainly come in two forms: doubly-fed wind turbine generators and full-power variable frequency wind turbine generators. Among them, doubly-fed wind turbine generators account for more than 90% of the market share. The following focuses on the characteristics, market applications, and development trends of doubly-fed wind turbine generators.
I. Characteristics of Doubly Fed Wind Turbine Generators
Doubly fed wind turbine generator sets have the following characteristics:
1. Mature technology and reliable quality. Since the Industrial Revolution, gear transmission has become the most mature and mainstream transmission method, widely used in industrial and daily life fields such as aviation, aerospace, shipbuilding, automobiles, and watchmaking. Wind turbine generators operate in harsh environments, requiring high reliability. The high-power, high-ratio gearbox technology used in doubly-fed induction generators has been applied since the 1990s, and its failure rate in wind power is lower than that of electrical and generator systems. The impeller + gearbox + generator transmission chain structure is simple, with reasonable load distribution and high overall reliability.
2. High efficiency and excellent cost-effectiveness. This technology effectively allocates the parameter configurations of the mechanical transmission system and the power generation system, increasing the motor speed through a high-speed gearbox, thus significantly improving generator efficiency. Simultaneously, only 1/5 to 1/3 of the slip power (accounting for 1/5 to 1/3 of the rated power) passes through the converter, resulting in minimal energy loss. The overall unit boasts high efficiency and excellent cost-effectiveness.
3. Good maintainability. Doubly fed wind turbine generators generally adopt a transmission structure of blades + hub + gearbox + coupling + generator. In this structure, each major component is relatively independent, allowing for separate maintenance and repair. On-site maintenance is easy and the response time is timely.
4. High power quality and strong low-voltage ride-through capability. Doubly-fed induction generators (DFIGs) employ DFIG induction motors and partial power converter technology, transmitting over 70% of the generated electricity to the grid via the stator, resulting in low harmonics and high power quality. Furthermore, this technology features adjustable power factor, convenient active and reactive power control, and excellent low-voltage ride-through performance, enabling grid-friendly grid connection.
However, in the actual operation of large wind turbine generator sets, the gearbox is a component with a high failure rate. Because wind turbine generator sets are generally installed in windy locations such as mountains, wilderness, beaches, and islands, they are subjected to unpredictable wind forces with varying directions and loads, as well as the impact of strong gusts. They are constantly exposed to extreme heat and cold and extreme temperature differences. Furthermore, the inconvenient natural environment and the narrow space at the top of the tower make repair extremely difficult once a failure occurs. Therefore, the reliability and service life of the gearbox are subject to much higher requirements than those for general machinery. For example, the requirements for component materials, in addition to the mechanical properties under normal conditions, should also include resistance to cold brittleness at low temperatures; the gearbox should operate smoothly, preventing vibration and impact; and adequate lubrication should be ensured. In areas with large temperature differences between winter and summer, appropriate heating and cooling devices must be installed. Monitoring points should also be set up for remote control of operation and lubrication status.
II. Market Application Status
According to the latest 2009 Global Wind Power Industry Report released by BTM, a leading global wind energy industry research institution, 13.8 million kilowatts of new wind power capacity were installed globally in 2009. Of the installed wind turbines, 86% used geared turbines. Among the world's top ten wind power equipment manufacturers, eight companies—VESTAS, GEWIND, Sinovel, GAMESA, Dongfang Electric, SUZLON, SIEMENS, and REPOWER—adopt geared technology. In direct-drive wind power products, approximately 8.5% of the units use electrically excited direct drive, while permanent magnet direct drive accounts for less than 5%. Currently, all of the more than 800 offshore wind turbines connected to the grid worldwide use geared transmissions.
Currently in China, geared variable pitch and variable speed doubly-fed wind turbines have the largest installed capacity, with representative manufacturers including Sinovel Wind Group, Dongfang Electric, Guodian United Power, Mingyang, Shanghai Electric, and Beizhong, accounting for over 80% of the market share. Direct-drive variable pitch and variable speed wind turbines also have a certain installed capacity, with representative manufacturers including Goldwind Technology and Xiangtan Electric Wind Power.
III. Development Trends
The maturity of wind turbine technology, the stability and reliability of its quality, and timely and low-cost maintenance will be the most important criteria for market selection. Gearbox-equipped turbine technology has become the mainstream technology for large wind turbines over the past decade; permanent magnet direct-drive technology has not yet been tested in long-term market operation. Currently in my country, gearbox-equipped turbines hold a dominant position in wind power technology. Some companies believe that gearboxes are components with a high failure rate, and that adopting a gearless structure can greatly improve the reliability of wind turbines, reduce the failure rate, and extend their lifespan. Therefore, these companies are optimistic about the future development potential and market demand for direct-drive turbines, leading to a gradual increase in the number of manufacturers with direct-drive wind turbine production technology, including Enercon, Goldwind Technology, Siemens, GE, Xiangtan Electric Wind Power, Aerospace Wanyuan, and Dongfang Electric.
