Task 3　About Wind Power Technology

Ⅰ.Basic principles of wind power generation

Wind power is process that a wind turbine（fan）converts captured wind to mechanical energy，and transfers mechanical energy to generator through spindle，gear box and other transmission mechanism，and then generator converts mechanical energy into electrical energy，as is shown in Fig.1-3-1.

As a result of high damage rate of MW-class wind turbine gearbox，direct-drive wind turbine（no gear box）was invented. Commonly used power adjustment methods of wind turbine are stalling adjustment and pitch adjustment.

Ⅱ.Introduction of wind turbines

1.Classification

According to wind wheel's blades：fixed paddle wind turbine，variable pitch wind turbine.

According to wheel's speed：fixed speed wind turbine，variable speed wind turbine.

According to transmission：gearbox speed wind turbine，direct drive wind turbine.

According to generator：a synchronous wind turbine，synchronous wind turbine.

According to grid-way：grid-type wind turbine，off-grid wind turbine.

2.The development of wind power generation

In 2015，global installed wind capacity increased by 63.01GW，with a year-on-year growth of 2.41％，as is shown in Fig.1-3-2.

Until the end of 2015，the global installed wind power capacity has reached 433 million kilowatts. The Global Wind Energy Council estimated that the capacity will nearly doubled to 792 million kilowatts by the end of 2020. Wind power is moving to a more com petitive direction in terms of price，performance and reliability.

China continues to lead global wind power growing. In 2015，China's new installed capacity reached 30.5GW，which ranked top；US wind power，which was the second，continued the slow recovery trend with 8.59GW；Germany was the third with 6.01GW；Brazil and India，respectively ranked fourth and fifth with 2.75GW and 2.62MW. As is shown in Fig.1-3-3.

In 2015，with the positive drive of global wind power industry，the installed capacity in wind power machine enterprises has grown steadily. Gold wind became the world's largest wind turbine manufacturers with increment of 7.8GW global installed capacity，among which installed capacity in China accounts for 99％. Denmark's Vestas，GE of the United States，Germany's Siemens and Spain's Song Meisha ranked second to fifth，as shown in Table 1-3-1.

In 2015，China's increment of wind power cumulative installed capacity has reached 145.1GW，in which grid capacity was 129GW，accounting for 8.6％ of the total amount. In 2015，wind power generation capacity was 186.3 billion kW·h，accounting for 3.3％ of all generating capacity.

3.Development model of large wind turbines

Terrestrial wind power generation：its direction is low wind power generation technology. The main model is 2～5 MW large wind turbine unit. The key to this model is transmitting power to grid.

Offshore wind power generation：it is mainly used on relatively shallow offshore sea. Install large wind turbines above 5 MW and layout large-scale wind farms. The main constraint of this model is the planning and high construction cost，but advantages are obvious that does not occupy land and has better wind resource.

4.The development trend of large wind turbines

①Variable pitch adjustment quickly replaces stall adjustment.

②Variable speed operation quickly replaces the constant speed mode.

③Units tend to be large-scale.

④Magnetic direct-driver generator and asynchronous doubly-fed generator develop together.

Ⅲ.Structure of large horizontal axis wind turbine

In light of structure，wind turbine sets can be divided into two types：one is horizontal axis wind turbine on which the blade is mounted on horizontal axis；the other is vertical axis wind turbine that the wind turbine shaft is vertically arranged. The blade drives vertical axis to rotate，and then drives the generator to generate electricity. Accelerators，couplings，generators，brakes of vertical axis wind turbine generators are installed on the ground. And installation，commission and maintenance of the entire unit is much more convenient than the horizontal axis wind turbine. However，due to some unsolved technical problems，the development and application of vertical shaft wind turbines have been greatly limited. The following describes the structure of large horizontal axis wind generator unit.

Large horizontal axis wind turbine consists of two main parts：pneumatic mechanical parts and electrical parts. Pneumatic mechanical parts include wind wheel，low speed shaft，gear box，and high speed shaft（direct-drive variable-speed constant-frequency wind turbine unit is not equipped with gear box）. Its main function is to drive generator rotor to convert wind energy into mechanical energy.

Electrical parts include asynchronous generator，power electronic converter，transformer and power grid. Their main function is to convert mechanical energy into electrical energy wit constant frequency.

Large horizontal axis wind turbine unit is mainly composed of impeller，speed accelerator or speed limit device，yaw system，transmission mechanism，generator system，tower，control system and so on，as is shown in Fig.1-3-4～Fig.1-3-6.

1.Impeller

Impeller is also called wind wheel. The most important feature of wind power generator which is different from other machinery is that it is composed of three leaves and wheels which convert wind energy into mechanical energy，as shown in Fig.1-3-7 and 1-3-8.

Blades，which are mounted on the hub，are made of glass fiber or high strength composite. Hub connects blade root and spindle（low speed shaft）. All the force from blades are passed through the hub to drive system and then to the driven object of wind turbine. Hub is also the point controlling blade pitch by making blades move up and down. Hub is required to be able to withstand large and complex loads.

2.Speed control or speed limit device

In many cases，the wind turbine is required to maintain a constant or no more than a certain value regardless of the speed of the wind. The speed control or speed limit device is used for this purpose. When wind speed is too fast，these devices are also used to limit the power and reduce the force on blades. Speed control or speed limit device has a variety of types. There are generally three categories from their principle：one is making impeller off the main wind，another is using pneumatic resistance，and the third is changing blade pitch angle.

3.Yaw system

In order to make impeller be naturally aligned with wind direction，wind turbines usually use steering device. Large wind turbine unit usually use motor-driven wind direction tracking system. The whole yaw system consists of motor，deceleration mechanism，adjustment system and torsion cable protection device and so on. Yaw adjustment system includes wind vane and adjustment software.

For each wind direction，wind vane corresponds to a pulse output signal. It determines its yaw direction and angle by yaw system software，and then amplifies the signal to motor，and turns the wind turbine plat form through deceleration mechanism until aligning toward the wind direction.

4.Drive system

Wind turbine's drive system typically includes low speed shaft，high speed shaft，gearbox，coupling and brake. But not every wind turbine must be equipped with all these components. Some wind turbine hubs are directly connected to gearbox with no low speed drive shaft. There are also some wind turbines designed to be gearless，whose impeller is directly connected with generator.

The drive system transfers mechanical energy to generator，including a gearbox，a clutch and a brake system that can reset stopped wind turbine in an emergency.

The gearbox is used to accelerate impeller from 20～50r/min to 1000～1500r/min，the later is the speed required to drive most generators.

The gearbox can be a simple parallel shaft gearbox，where the output shaft is different，or it can be a more expensive one which allows input and output shafts to be collinear and make the structure more compact.

The drive system is designed for output power and maximum dynamic torque load.

Due to fluctuation of impeller power output，some designers attempt to control dynamic load by increasing mechanical adaptability and buffer driver，which is very important for large wind turbines because of their large dynamic loads and the buffer of induction generators is smaller than that of small wind turbine.

5.Generator system

The generator and its control system assume the task of converting mechanical energy to electrical energy.

Constant-speed constant-frequency generator system is relatively simple and two main generators are synchronous generator and squirrel cage induction generator.

Variable-speed constant-frequency generator system is a new type of wind power generation system developed after mid-1970s whose main advantage is that its impeller works in a variable speed and can maintain almost constant and best tip-speed ratio in a wide wind speed range. Thus operating efficiency is improved and the energy obtained from wind can be much higher than that of constant speed wind turbine. In addition，this kind of wind turbine have many advantages in structure and practice.

Several common types of wind turbines are shown in Fig.1-3-9，1-3-10 and 1-3-11.

6.Tower

In addition to the weight of wind turbines，the tower must withstand wind turbine and wind pressure，as well as dynamic load. Its stiffness is closely related to wind turbine vibration.

Tower of horizontal axis wind turbine can be classified into two types：tubular towerand truss tower. Tubular tower is more common and generally is large steel pipe and concrete tower，as shown in Fig.1-3-12.

To make transportation easier，medium and large towers can be divided into several segments.

Generally，cylindrical tower has less resistance to wind，especially down wind turbines，and turbulence is less than that of truss tower.

Truss tower is commonly used on small and medium wind turbines. Its advantage is low cost and convenient transportation. But this tower will make a lot of turbulence on downwind turbine blades.

Grid-connected wind turbines are heavy-duty generation equipment. The whole equipment is more than 100 meters high and its weight is hundreds of tons. So the assembly of wind turbines can not be completed in plant. If it is fully assembled in plant，transportation problems cannot be solved at present. So wind turbine is assembled partly in plant and the rest in wind farms.

Ⅳ.Training steps of wind power system

After course group contact with wind farm，learn and understand basic knowledge of wind farm and the whole process from entering into workplace to leaving.

1.Safety preparations

Implement safety regulations of wind farm，wear work clothes and cap，check the safety rope and deduction and apply for reporting procedures of needed work tools.

2.Visit and learn on the ground

Visit and learn ground substation system，control system，tower structure，foundation structure，foundation connection structure，two-story platform structure and safety assistant system structure.

3.Climb the tower and learn in the cabin

Climb to the cabin by safety devices，visit and learn hub structure，pitching mechanism，low-speed drive shaft，speed-up gearbox，high-speed drive shaft，generator set，signal and control systems and hydraulic systems，etc.

【Exercise】

（1）What are parts of large horizontal wind turbine unit? What is the role of each part?

（2）What are the shapes of middle and large wind turbine tower? Are they the overall structure?

（3）Try to draw a topology of large horizontal wind turbine system.