**1. Introduction**

Fossil fuels are nonrenewable and their associated prices are fluctuating sharply. Meanwhile, the increasing environmental and climatic concerns of the current times have moved the research focus from conventional electricity resources to renewable resources [1, 2]. Renewable energy resources, such as wind, solar and geothermal power, are clean alternatives to fossil fuels. Among them, wind energy is one of the most promising renewable energy resources in the world today. The main attractions of wind energy are a large resource and low environmental impact. In this condition, wind energy is developing rapidly. For example, over 51.2 GW of capacity was installed in 2014 [3].

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

In recent years, wind power industry has been flourishing all over the world, and in some countries, the focus has been gradually shifted from land-based to offshore wind farms [4]. On a global basis, the size of the annual market grew 42% year-over-year in 2014 compared to a 20% fall in 2013 [3]. By the end of 2014, the cumulative installed capacity climbed to 372 GW [5, 6], which is shown in **Figure 1**. Policy-driven accelerations play a very important role in market growth, especially in China, Germany and the United States. In these three countries, China is the world's largest wind power market with 23.2 GW of new wind power installed in 2014. The development of wind power in China is shown in **Figure 2**. **Figure 3** shows the top 10 countries of newly installed capacity from January to December in 2014 [6]. The total installed capacity is up to 51,473 MW, and the share of China is 45.1%. With more

and more wind turbines being installed, lots of potential problems still need to be solved, such as fatigue, failures, condition monitoring, operation and maintenance, which are especially true in the current circumstances where the tower height, rotor diameter and overall turbine weights have almost quadrupled in size and capacity [7]. Hence, the reliability of wind tur-

Reliability Analysis of Wind Turbines http://dx.doi.org/10.5772/intechopen.74859 171

In the late 1970s, in response to the oil price increasing around that time, a number of government programs were initiated with the objective of developing suitable wind turbines. To reduce the dependence on conventional electricity sources, the related countries carried out many programs and focused on the development of wind turbines, rating up to 4 MW and diameters up to 100 m. Some examples are included in **Table 1**. Furthermore, wind energy has witnessed rapid development in few decades, making it one of the fastest growing sources of electricity in the world today. But it is thought that wind energy is still immature these days. Due to technological advancements, policy initiatives and economic drivers, wind energy is now able to make a cost-competitive contribution to our growing energy needs. For example, over 240,000 commercial-sized wind turbines were operating in the world by 2014, producing 4% of the world's electricity [8, 9]. Wind power showed the potential for replacing natural gas in electricity generation on the cost basis. Technological innovations continue to drive new developments in the application of wind power. Until now, the cumulative installed capacity

With the growing number of the wind turbines, the industry still needs to face numerous challenges. A number of wind turbine components are prone to failure, and it is difficult and expensive to repair or replace them. For example, bearings, inverters and gearboxes raise the maintenance issues. Still, wind energy challenges still exist due to: (1) poor performance and

bines is becoming more important now than ever before.

**2. Developments and challenges**

**Figure 3.** Top 10 newly installed capacity Jan.–Dec. 2014.

has been more than 400 GW.

**Figure 1.** The global wind power capacity.

**Figure 2.** Development of wind power in China.

**Figure 3.** Top 10 newly installed capacity Jan.–Dec. 2014.

and more wind turbines being installed, lots of potential problems still need to be solved, such as fatigue, failures, condition monitoring, operation and maintenance, which are especially true in the current circumstances where the tower height, rotor diameter and overall turbine weights have almost quadrupled in size and capacity [7]. Hence, the reliability of wind turbines is becoming more important now than ever before.

## **2. Developments and challenges**

**Figure 2.** Development of wind power in China.

**Figure 1.** The global wind power capacity.

170 Stability Control and Reliable Performance of Wind Turbines

In recent years, wind power industry has been flourishing all over the world, and in some countries, the focus has been gradually shifted from land-based to offshore wind farms [4]. On a global basis, the size of the annual market grew 42% year-over-year in 2014 compared to a 20% fall in 2013 [3]. By the end of 2014, the cumulative installed capacity climbed to 372 GW [5, 6], which is shown in **Figure 1**. Policy-driven accelerations play a very important role in market growth, especially in China, Germany and the United States. In these three countries, China is the world's largest wind power market with 23.2 GW of new wind power installed in 2014. The development of wind power in China is shown in **Figure 2**. **Figure 3** shows the top 10 countries of newly installed capacity from January to December in 2014 [6]. The total installed capacity is up to 51,473 MW, and the share of China is 45.1%. With more

> In the late 1970s, in response to the oil price increasing around that time, a number of government programs were initiated with the objective of developing suitable wind turbines. To reduce the dependence on conventional electricity sources, the related countries carried out many programs and focused on the development of wind turbines, rating up to 4 MW and diameters up to 100 m. Some examples are included in **Table 1**. Furthermore, wind energy has witnessed rapid development in few decades, making it one of the fastest growing sources of electricity in the world today. But it is thought that wind energy is still immature these days. Due to technological advancements, policy initiatives and economic drivers, wind energy is now able to make a cost-competitive contribution to our growing energy needs. For example, over 240,000 commercial-sized wind turbines were operating in the world by 2014, producing 4% of the world's electricity [8, 9]. Wind power showed the potential for replacing natural gas in electricity generation on the cost basis. Technological innovations continue to drive new developments in the application of wind power. Until now, the cumulative installed capacity has been more than 400 GW.

> With the growing number of the wind turbines, the industry still needs to face numerous challenges. A number of wind turbine components are prone to failure, and it is difficult and expensive to repair or replace them. For example, bearings, inverters and gearboxes raise the maintenance issues. Still, wind energy challenges still exist due to: (1) poor performance and


**Table 1.** Some of the early prototype machines, mostly funded by governments.

reliability and (2) rising costs driven by transportation, maintenance, and so on. To achieve a longer life of wind turbines and to reduce the cost of maintenance, the development of technologies for improving the reliability of wind turbines is an important consideration for future development, especially for offshore wind turbines. Hence, measures must be taken to improve the reliability of wind turbines.

regular failure modes of gearbox are bearing failures, gear fatigue, wear, fracture, insufficient lubrication, and so on. **Figure 5** shows three common failure modes of gearbox, in which fatigue failures are the most common. The bolts connecting the front box, ring and middle box sometimes fail because of the strong and unbalanced axial forces acting on the bolts. The bolt failure is shown in **Figure 6**. The result in **Figure 5** shows that the section is smooth, so the failure is caused by fatigue fracture. However, the experiments show that a relief notch, a proper taper of thread and a thread root radius can increase its carrying capacity and reliability.

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The generator is one of the most key components with high failure rates since it connects to the high-speed shaft of the wind turbine gearbox with time-varying mechanical torques. Four failure root causes are: design issues, operations issues, maintenance practices and environmental conditions. The failure rates of wind turbine generators have a close relationship with their power rating, working environment, and so on. **Figure 7** shows failure rates of subassemblies of onshore and offshore wind turbine systems. Different failure causes may lead to different generator failure modes, including design issues, operation issues, maintenance and external environment, which is shown in **Table 3**. **Figure 8** shows three common failure

The rotor blades of wind turbine are driven by the wind energy and transform wind energy to mechanical energy. Because blades often suffer alternating stress and complex environments, they have high failure rates, with the main failure modes being fatigue, fracture, crack, wear, freezing and sensor failure. **Figure 9** shows failure modes of the blades. Due to

modes of the generator where the bearing failure is the most common.

**Figure 4.** Failure rates for wind turbine subassemblies working onshore and offshore.

**3.2. Generator failure modes**

**3.3. Rotor blade failure modes**
