**3. Wind Turbines**

tion. The interest in wind energy was renewed in the mid-1970s following the oil crises and increased concerns over resource conservation. Initially, wind energy started to gain popu‐ larity in electricity generation to charge batteries [17] in remote power systems, residential scale power systems, isolated or island power systems, and utility networks. These wind turbines themselves are generally small (rated less than 100kW) but could be made up to a large wind farm (rated 5MW or so). It was until the early 1990s when wind projects really took off the ground, primarily driven by the governmental and industrial initiatives. It was also in 1990s there seemed a shift of focus from onshore to offshore development in major

Offshore wind turbines were first proposed in Germany in 1930s and first installed in Swe‐ den in 1991 and in Denmark in 1992. By July 2010, there were 2.4 GW of offshore wind tur‐ bines installed in Europe. Compared to onshore wind energy, offshore wind energy has some appealing attributes such as higher wind speeds, availability of larger sites for devel‐ opment, lower wind sheer and lower intrinsic turbulence intensity. But the drawbacks are associated with harsh working conditions, high installation and maintenance costs. For off‐ shore operation, major components should be marinized with additional anti-corrosion measures and de-humidification capacity [24]. In order to avoid unscheduled maintenance, they should also be equipped with fault-ride-through capacity to improve their reliability.

Over the last three decades, wind turbines have significantly evolved as the global wind market grows continuously and rapidly. By the end of 2009, the world capacity reached a total of 160 GW [7]. In the global electricity market, wind energy penetration is projected to rise from 1% in 2008 to 8% in 2035 [45]. This is achieved simply by developing larger wind turbines and employing more in the wind farm. In terms of the size, large wind turbines of

wind development countries, especially in Europe.

178 Advances in Wind Power

**Figure 1.** Ever-growing size of horisontal-axis wind turbines [36].

Clearly, wind energy is high on the governmental and institutional agenda. However, there are some stumbling blocks in the way of its widespread.

Wind turbines come with different topologies, architectures and design features. The sche‐ matic of a wind turbine generation system is shown in Fig. 3. Some options wind turbine topologies are as follows [35],


This chapter focuses only on horizontal-axis wind turbines (HAWTs), which are the prevail‐ ing type of wind turbine topology, as is confirmed in Fig. 4.

**Figure 3.** Schematic of a wind turbine generation system [50].

Wind turbines include critical mechanical components such as turbine blades and rotors, drive train and generators. They cost more than 30% of total capital expenditure for offshore wind project [24]. In general, wind turbines are intended for relatively inaccessible sites placing some constraints on the designs in a number of ways. For offshore environments, the site may be realistically accessed for maintenance once per year. As a result, fault toler‐ ance of the wind turbine is of importance for wind farm development.

**Figure 4.** Commonly agreed wind turbine type and its divergence [24].

**•** Rotor axis orientation: horizontal or vertical;

**•** Rotor speed: fixed or variable;

**•** Rigidity: still or flexible;

180 Advances in Wind Power

**•** Yaw control: active or free.

**•** Rotor position: upwind or downwind of tower;

**•** Hub: rigid, teetering, gimbaled or hinged blades;

**•** Number of blades: one, two, three or even more;

**Figure 3.** Schematic of a wind turbine generation system [50].

**•** Power control: stall, pitch, yaw or aerodynamic surfaces;

ing type of wind turbine topology, as is confirmed in Fig. 4.

This chapter focuses only on horizontal-axis wind turbines (HAWTs), which are the prevail‐

Wind turbines include critical mechanical components such as turbine blades and rotors, drive train and generators. They cost more than 30% of total capital expenditure for offshore wind project [24]. In general, wind turbines are intended for relatively inaccessible sites placing some constraints on the designs in a number of ways. For offshore environments, the site may be realistically accessed for maintenance once per year. As a result, fault toler‐

ance of the wind turbine is of importance for wind farm development.

One of key components in the wind turbine is its drive train, which links aerodynamic rotor and electrical output terminals. Optimization of wind turbine generators can not be realized without considering mechanical, structural, hydraulic and magnetic performance of the drive train. An overview of the drive train technologies is illustrated in Fig. 5 for compari‐ son. Generally, they can be broken down into four types according to their structures [24]:


Drive train topologies may raise the issues such as the integration of the rotor and gearbox/ bearings, the isolation of gear and generator shafts from mechanical bending loads, the in‐ tegrity and load paths. Although it may be easier to service separate wind turbine compo‐ nents such as gearboxes, bearings and generators, the industry is increasingly in favor of system design of the integrated drive train components.
