**3. Wind turbine failure modes**

Nowadays, the development of wind turbines tends toward larger and heavier structures, which increases the failure frequency. In reality, the failure rates are also very different between onshore and offshore wind turbine systems for the same type. **Figure 4** shows failure rates of wind turbine systems and components [10]. The results in **Figure 4** show that some key components have higher failure rates than that of other components, and the same components working offshore have much higher failure rates than those working onshore.

For a wind turbine transmission system, key components like the generator, gearbox and blades have the highest failure rates. The gearbox failures are mainly caused by gears and bearings; the generator failures are mainly caused by bearings. **Table 2** shows the failure modes, failure causes and detection methods of wind turbine key components and subassemblies.

#### **3.1. Gearbox failure modes**

Any key components fail in the gearbox, it may result in high cost of maintenance and high production loss and may take longer time to repair, especially for offshore wind turbines. The

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

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.

#### **3.2. Generator failure modes**

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

Nowadays, the development of wind turbines tends toward larger and heavier structures, which increases the failure frequency. In reality, the failure rates are also very different between onshore and offshore wind turbine systems for the same type. **Figure 4** shows failure rates of wind turbine systems and components [10]. The results in **Figure 4** show that some key components have higher failure rates than that of other components, and the same components working offshore have much higher failure rates than those working onshore.

For a wind turbine transmission system, key components like the generator, gearbox and blades have the highest failure rates. The gearbox failures are mainly caused by gears and bearings; the generator failures are mainly caused by bearings. **Table 2** shows the failure modes, failure causes and detection methods of wind turbine key components and subassemblies.

Any key components fail in the gearbox, it may result in high cost of maintenance and high production loss and may take longer time to repair, especially for offshore wind turbines. The

improve the reliability of wind turbines.

**Location Name Rating** 

172 Stability Control and Reliable Performance of Wind Turbines

**(MW)**

Denmark Tjaereborg 2 61 3 1988 Germany Growian 3 100 2 1981 Germany Monopteros 0.64 50 1 1989

Netherlands Newecs 45 1 45 2 1985

Sweden WTS-3 3 78 2 1982

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

**Diameter (m)**

Canada Eole 3.6 64 2 1987 Vertical axis, direct drive

Italy Gamma 60 1.5 60 2 1991 Variable speed, Power control

Spain AWEC-60 1.2 60 3 1989 Variable speed

USA MOD-5B 3.2 99 2 1987 Variable speed

UK LS1 3 60 2 1987 Partial span pitch control USA WTS-4 4 78 2 1982 Similar to WTS3 (Sweden)

**Blades Date Features**

by yaw

**3. Wind turbine failure modes**

**3.1. Gearbox failure modes**

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

#### **3.3. Rotor blade failure modes**

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


the high location of rotor blades, they are difficult to repair and maintain which leads to high cost. Hence, in order to produce high-reliability blades, it is important and meaningful to

Reliability Analysis of Wind Turbines http://dx.doi.org/10.5772/intechopen.74859 175

study the relationship among failure modes, reliability and internal/external loads.

**Figure 5.** Bearings failure: (a) bearing in gearbox; (b) and (c) failure appearance of bearing.

**Figure 6.** Bolt faults of wind turbine gearbox.

**Figure 7.** Failure rates of subassemblies of onshore and offshore.

**Table 2.** Summary of failure modes of components.

**Figure 5.** Bearings failure: (a) bearing in gearbox; (b) and (c) failure appearance of bearing.

**Figure 6.** Bolt faults of wind turbine gearbox.

**Objects Function Failure mode Cause Detection method**

Fracture Fatigue loads

Fatigue loads higher than anticipated, extreme loads, environment influences, imbalance

underestimated; operation of WTG at off-design conditions; material properties below

Cracked roller; galled surface; lack of lubrication

Fatigue loads underestimated; exceeding design load; improper material; loss of

lubricating oil

corrosion

Cut or wear in lip Installation damage; wear

Excessing design loads; excessive preload; stress

Case leakage Damage to case or seals Low oil; level switch

Overload; no excitation; environmental effects; misalignment; fatigue; mechanical failure; loss of drivetrain control

Pump failure; leakage; diverting valve failure; ambient temperature above or below design conditions; excessive friction losses; diverting

valve failure

Environment effect Tachometer

specs

Excessive vibration sensed by rotor bearing accelerometer in hub; high stresses recorded by operating instrumentation

vibration sensor

Yaw error signal

Vibration sensor

Rotor bearing accelerometer; periodic inspection for loose or missing bolts

Low oil switch

Protective relays; overspeed

detection; testing

Oil flow switch; oil temperature sensor; air

temperature

Low-speed sensor; bearing

crack, stuck, motor failure, pitch bearing failure

Increased bearing

Low or higher brake torque

Internal gear tooth

Structure failure; bolt failure

Overheat; fault; jammed bearing; bearing seizure; overspeed;

Loss of oil; overheating; oil under temperature

friction

failure

Blades Capture wind Fracture, edge

174 Stability Control and Reliable Performance of Wind Turbines

Main shaft Transmit large torque

Yaw system Enable the nacelle to

Gearbox Transmit torque

Hub assembly Transmit torque

Oil seals Retain oil in main

Filters To extract and

Generator Generate electric power

Lubrication Lubricate gearbox

from blades

bearing housing; exclude foreign matter

hold all particulate contaminants from hydraulic fluid

and rotor bearing

**Table 2.** Summary of failure modes of components.

High-speed shaft

rotate on the tower

with speed increase

Stop and hold the shaft during shutdown and operation

> the high location of rotor blades, they are difficult to repair and maintain which leads to high cost. Hence, in order to produce high-reliability blades, it is important and meaningful to study the relationship among failure modes, reliability and internal/external loads.

**Figure 7.** Failure rates of subassemblies of onshore and offshore.


wind turbines. **Figure 10** shows the failure rates of different subassemblies and its downtime after failure. The results in **Figure 10** show that the lower the subassembly's reliability, the

Reliability Analysis of Wind Turbines http://dx.doi.org/10.5772/intechopen.74859 177

The reliability of wind turbine system is becoming more and more important with the continued growth and expansion of markets for wind turbine technology. In addition, wind turbines with reduced repair and maintenance (R&M) requirements and higher reliability are needed emergently. However, wind turbines produced by different companies have different reliability. There is no unified evaluation criterion. The current reliability analysis methods mainly focus on gear transmission systems of wind turbines and ignore the influences of other systems. The effects of the reliability model are limited if the system is simplified and seen as a series or parallel connection. Due to high costs of repair and maintenance, it is essential to study the health management systems of wind turbines and develop maintenance strategies in order to improve reliability and reduce unexpected repair and maintenance. The

high-reliability systems can be achieved from three aspects, as shown in **Figure 11**.

There are two kinds of reliability analysis methods: statistical method based on database and

The failure rates of wind turbines are time-varying during its lifetime, but the failure rates of repairable systems follow a bathtub curve. With a service life of around 20 years, wind turbine failure rates are assumed to follow the famous bathtub curve, as shown in **Figure 12**. Weibull

longer is the downtime of the corresponding subassembly.

**4.1. Reliability analysis methods**

*4.1.1. Statistical method based on database*

stress-strength interference theory based on loads.

**Figure 10.** Failure rates and downtime for different subassemblies (DFIG).

**Table 3.** Failure modes of the generator.

**Figure 8.** Generator failure: (a) bearing, (b) magnetic wedge loss and (c) contamination.

**Figure 9.** Failure modes of the blades. (a) Trailing edge crack; (b) leading edge failure and (c) blade fracture.
