A Brief Discussion on Small Wind Turbines

1 Introduction

As a new type of power generation system with low price, reliable operation and no greenhouse gas emission, the installed capacity of wind power generation system is being increasingly widely used in the world with an annual growth rate of more than 30%, and has formed a global industry with an annual output value of more than 5 billion US dollars. Most of the wind power generation systems currently installed are large-scale wind turbine systems with MW level and interconnected with the power grid. The technology of this industry has been continuously improved and matured. However, small-scale wind power generation systems used for independent power supply in remote areas still need to overcome many technical difficulties before they can be widely used. With the increase in China's investment in "agriculture, rural areas and farmers" and the sustained and rapid development of the economy, the vast number of farmers, herdsmen and fishermen have urgent requirements for improving their living environment, improving their quality of life and solving the problem of electricity consumption. The use of small-scale wind power generation systems to provide electricity for local loads can not only reduce the one-time huge investment, but also avoid the greenhouse gas emissions of thermal power generation systems, improve the environment and the energy structure of rural areas, and benefit sustainable development [1][2].

2 Overall structure and working principle of small wind power generation system

Wind turbines are machines that convert wind energy into electrical energy. From the perspective of energy conversion, wind turbines consist of two major parts: one is the wind turbine, whose function is to convert wind energy into mechanical energy; the other is the generator, whose function is to convert mechanical energy into electrical energy[3].

The structure of a small wind power generation system is shown in Figure 1. It is generally composed of a wind rotor, a generator, a tail rudder and an electrical control part. Conventional small wind turbines are mostly composed of an induction generator or a permanent magnet synchronous generator plus an AC/DC converter, a battery and an inverter[4]. When the wind blows, the wind rotor rotates, converting the aerodynamic energy into mechanical energy (speed + torque). The hub of the wind rotor is fixed on the generator shaft. The rotation of the wind rotor drives the rotation of the generator shaft, driving the permanent magnet three-phase generator to generate three-phase AC power. As the wind speed changes, the current and voltage generated by the generator also change. The generated electricity is rectified by the controller, and the AC power is converted into DC power with a certain voltage, and the battery is charged. The DC power output from the battery pack is converted into 220V AC power after passing through the inverter, and is supplied to the user's household appliances.

Wind turbines are divided into grid-connected and off-grid wind turbines according to different application scenarios. Off-grid wind turbines are also called independent wind turbines. They are wind turbines used in areas without power grids and generally have low power. Independent wind turbines generally need to be combined with batteries and other control devices to form an independent wind turbine power generation system. This independent operation system can be several kW or even tens of kW to solve the power supply system of a village, or it can be a small wind turbine generator set of tens to hundreds of kW to solve the power supply of a household.

Figure 1 Independently operated small wind power generation system

3 Power conversion device for small wind turbines

Due to the randomness of wind energy, the frequency and voltage of the power generated by the generator are unstable, and the battery can only store DC power and cannot directly supply power to the AC load. Therefore, in order to provide stable, high-quality power to the load and meet the power consumption of the AC load, it is necessary to add a power conversion device between the generator and the load. This power conversion device is mainly composed of a rectifier, an inverter, a controller, a battery, etc. [5][6].

3.1 Rectifier

The main function of the rectifier is to rectify the three-phase AC output of the wind turbine, and the rectified DC is passed through the controller to charge the battery. Generally, a three-phase bridge rectifier circuit is used. Another important function of the rectifier in the wind power branch is that when the external wind speed is too low or there is basically no wind, the output power of the wind turbine is also small. Since the diode of the three-phase rectifier bridge can only be turned on from the output end of the wind turbine to the battery, the reverse power supply of the battery to the wind turbine is prevented.

In an independently operated small wind power generation system, there is an AC generator driven by a wind wheel, which needs to be equipped with a suitable rectifier to charge the battery. According to the capacity of the wind power generation system, rectifiers are divided into controllable and uncontrollable types. Controllable rectifiers are mainly used in systems with higher power, which can reduce the disadvantages of large volume and large loss caused by excessive inductance; uncontrolled rectifiers are mainly used in low-power systems.

3.2 Inverter

Inverter is a power electronic device often used in the power conversion process. Its main function is to convert the DC power stored in the battery or output by the rectifier bridge into AC power that can be used by the load. At present, most inverters for independently operated small wind power systems are voltage-type single-phase bridge inverters. The inverter used in wind power generation is required to have high efficiency, especially high efficiency under light load, because wind power generation systems often operate under light load. In addition, since the input battery voltage changes greatly with the change of charging and discharging state, this requires the inverter to be able to work normally within a large DC voltage variation range and ensure the stability of the output voltage [7].

[page] In the past, the controller and inverter of the wind turbine were separate. Now most manufacturers use the integrated controller and inverter solution. The controller rectifies the AC power generated by the generator and charges it into the battery pack. The inverter converts the DC power output by the battery pack into 220V AC power and provides it to the electrical appliances [8].

Inverters are divided into two types according to the input method:

(1) DC input type: products with the inverter input directly connected to the battery;

(2) AC input type: products with the inverter input connected to the AC output of the wind turbine generator, that is, products with integrated control and inverter.

The protection functions of the inverter are:

(1) Overcharge protection: When the wind speed is continuously high, the battery is fully charged, and the battery voltage exceeds 1.25 times the rated voltage, the controller stops charging the battery and the excess current flows to the unloader. (2) Over-discharge protection:

When the wind speed is long-term low, the battery is undercharged, and the battery voltage is lower than 0.85 times the rated voltage, the inverter stops working and no longer supplies power to the outside. When the wind speed increases again and the battery voltage returns to 1.1 times the rated voltage, the inverter automatically resumes working and supplies power to the outside.

3.3 Batteries[9][10]

In independently operated small wind power generation systems, batteries are widely used as energy storage devices. The function of batteries is to store part of the electrical energy generated by wind turbines in the batteries when the wind is strong or the load is reduced, that is, to charge the batteries. When the wind is weak, there is no wind, or the power load increases, the electrical energy stored in the batteries supplies power to the load to make up for the insufficient electrical energy generated by the wind turbine, thereby maintaining a continuous and stable power supply to the load.

There are three main types of batteries: ordinary batteries, alkaline nickel-cadmium batteries, and valve-regulated sealed lead-acid batteries. Ordinary lead-acid batteries have a very limited scope of use due to their short service life, low efficiency, complex maintenance, and acid mist pollution. They have been gradually replaced by valve-regulated sealed lead-acid batteries. The valve-regulated sealed lead-acid battery adopts a sealed structure as a whole. It does not have the gas swelling and electrolyte leakage of ordinary lead-acid batteries. It is safe and reliable to use and has a long life. During normal operation, it is not necessary to detect the electrolyte and adjust the acid and add water. It is also called a maintenance-free battery. It has been widely used in many fields such as postal and telecommunications, ship transportation, and emergency lighting. The characteristics of alkaline cadmium nickel batteries are small size, high discharge rate, simple operation and maintenance, and long life. However, due to its low monomer voltage, easy leakage, high cost and easy pollution to the environment, its use is limited. It is now mainly used in power tools and various portable electronic devices.

At present, valve-regulated sealed lead-acid batteries are used in most wind power systems or solar photovoltaic systems. The battery is a key factor affecting the life of the wind power system. The control of the charge and discharge of the valve-regulated sealed lead-acid battery directly affects the life of the battery. Unreasonable charge and discharge will directly lead to the collapse of the battery. In most wind power systems, the CPU monitors and controls the charge and discharge process of the battery, and the stage method is often used to optimize the charging process. Because the staged charging process conforms to the characteristics of valve-regulated sealed lead-acid batteries, it can well protect the batteries and extend their service life.

4 Maximum output power regulation method

In wind power generation, the unpredictable wind speed makes it difficult to utilize it. The change of wind speed causes the mechanical power output of the wind turbine to change, which causes the output power of the generator to fluctuate and the power quality to decline. The stability of the output power quality of the wind turbine generator has become an important issue in wind power generation technology. Therefore, improving wind power generation technology and improving the efficiency of wind turbine generator sets are of great significance for making the most of wind energy resources.

According to the different power supply methods of wind power generation, the power output can be qualitatively divided into two categories: regulating mechanical power, adding a regulating device to the wind turbine control loop to stabilize the mechanical power output of the wind turbine; regulating electrical power, adding feedback to the control part of the generator, and using a fast-response controller and an optimization control strategy to control the generator output power [11].

4.1 Stall control with

fixed pitch Stall control refers to the stall characteristics of the blade itself. When the wind speed is higher than the rated wind speed, the angle of attack of the airflow increases to the stall condition, causing vortices to form on the surface of the blade, reducing the aerodynamic efficiency of the blade and affecting energy capture. Stall control by the disturbance method of maximum power control of small wind power generation systems is generally used in wind turbines running at constant speed [11-13].

[page]4.2 Variable pitch adjustment

In order to improve the wind energy conversion efficiency and ensure the stable output power of the wind turbine, the wind turbine can be adapted to the change of wind speed by adjusting the pitch to achieve the optimal power output. The variable pitch wind turbine generator set does not rely entirely on the aerodynamic characteristics of the blades, but mainly relies on the change of the blade angle of attack that matches the blades to adjust the wind energy conversion efficiency. When stationary, the pitch angle is 90°. At this time, the airflow does not produce torque on the blades, and the entire blade is equivalent to a damping plate. When the starting wind speed is reached, the blades rotate in the direction of 0°, the airflow produces a certain angle of attack on the blades, and the impeller begins to rotate. When the wind speed is below the rated wind speed, the blade angle of attack is near 0°. At this time, the blade angle is affected by the control accuracy, and the range of change is very small, which can be equivalent to a fixed pitch wind turbine. When the wind speed is above the rated wind speed, the variable pitch mechanism plays a role, adjusts the blade angle of attack, and ensures that the power of the generator is within the allowable range. The starting wind speed of a variable pitch wind turbine is relatively low, which has little significance for increasing power generation. When it stops, the impact on the transmission mechanism is small. When the wind turbine is working normally, power control is mainly used [11-13].

4.3 Active stall control

This control method is a combination of the first two power control methods. At low wind speeds, variable pitch control can achieve higher aerodynamic efficiency. When the fan reaches rated power, the fan changes its pitch in the opposite direction of the variable pitch control. This adjustment will cause the blade angle of attack to change, resulting in a deeper stall, which can make the power output smoother. This control method combines the advantages of the first two control methods, similar to variable pitch control, but does not require a very sensitive adjustment speed. In strong winds, the entire unit is less impacted [13].

5 Conclusion

As a component of rural energy, the promotion and application of small wind power generation systems will play a positive role in improving the electricity structure, especially the production and living energy in remote mountainous areas, and promoting the development of ecological environment construction. Therefore, it has broad market prospects. Wind energy has randomness and uncertainty, and the wind power generation system is a complex system. Simplifying the structure of small wind power generation systems, reducing costs, improving reliability and achieving system optimization operation are of great significance to the promotion of small wind power generation systems.

References
[1] Zhang Chunyou, Tian De, Wang Haikuan. Current status and development trend of small wind turbines in China [J]. Rural and Pastoral Mechanization, 2008 (2): 38-39.
[2] Li Defu. Current status and development trend of small wind power generation industry [J]. Agricultural Engineering Technology, 2007 (1): 47-50.
[3] Deng Keyun. Use and maintenance of household small wind turbines [M]. Beijing: Agricultural Press, 2006.
[4] Liu Wankun. Wind energy and wind power generation technology [M]. Beijing: Chemical Industry Press, 2007.
[5] Ni Shouyuan. Wind power generation lectures from the first to the fifth [J]. Solar Energy, 2000 (2-4), 2001 (l-3).
[6] Tony Burton, David Sharpe, Nick Jenkins. Wind Energy Handbook [M], 2001.
[7] Zhang Xing, Chen Ling, Yang Shuying. Control of inverter for off-grid small wind power generation system [J]. Automation of Electric Power Systems. 2008(23):95-99.
[8] Xu Po, Zhang Xing, Zhang Chongwei. Small wind power grid-connected inverter system using Z-source converter [J]. Transactions of
China Electrotechnical Society. 2008(4):93-97. [9] Wang Chengxu, Zhang Yuan. Wind power generation [M]. Beijing: China Electric Power Press, 2003.
[10] Ye Hangzhi. Control technology of wind turbine generator set [M]. Beijing: Machinery Industry Press, 2002.
[11] Chen Xingying, Liu Mengjue, Shan Yuanda. Research on optimized output technology of wind power generation system [J]. Electric Power Automation Equipment. 2000(5):7-10.
[12] Pan Wenxia, ​​Chen Yunping, Shen Zuyi. Current status of wind turbine development [J]. Small and Medium Electric Machines. 2001, 28(4):38-41.
[13] Wu Xin, Zhao Bin. Regulation and control of grid-connected wind turbine generator sets [J]. Solar Energy. 2003(4):24-25.
[14] Jin Rulin, Tan Fuwa. Maximum output power of small wind turbines [J]. Small and Medium Electric Machines. 2000(2):37-39.
[15] Erickson RW. Fundamentals of power electronics [M]. 2nded. Norwell, USA: Kluwer Academic, 2001.
[16] Teodorescu R, Blaabjerg F. Flexible Control of Small Wind Turbines with Grid Failure Detection Operating in Stand Alone and Grid Connected Mode. IEEE Transactions on Power Electronics, 2004, 19(5):1323～1332.
[17] Wang Q, chang L. An Intelligent Maximum Power Extraction Algorithm for Inverter-Based Variable Speed ​​Wind Turbine System. IEEE Transaction on Power Electronics, 2004,19(5):1242～1249.