**3. Wind turbine technology**

supplied [2, 5]. With the recent advancement of variable speed wind turbines, power electronics, drives and control system technologies, wind energy is now competitive with traditional coal and natural gas power. However, one of the shortcomings of wind energy is that it is stochastic in nature, thus making its availability sporadic, consequently, needs back up by

The solar photovoltaic systems have the merit of being static and require less repair and maintenance. However, it is five times more expensive than wind power, although recently, there is huge research and development to produce low cost photovoltaic solar panels for widespread applications. Solar power conversion performance efficiency is basically 16%, and its

Hydrogen gas is the primary fuel for fuel cell energy or a fossil fuel type like gasoline or methanol, with a reformer. Fuel cells are static and have high conversion efficiency of about 60% compared to wind and solar power. However, fuel cells are heavy, expensive and possess poor transient response in their current state. Although, fuel cells show tremendous promise for the future, especially for electric cars, however, a tremendous amount of research and

Wind energy is the indirect form of solar energy which is always being replenished by the sun. The differential heating of the earth's surface by the sun causes wind. It has been estimated that about 10 million MW of energy are continuously available in the earth's wind [6–8]. Wind energy could act as an environmental friendly alternative and national energy security especially during times of limited global reserves of fossil fuels, which threatens the

The technology of wind turbine has a technical identity and demand that is unique in terms of the design methods. Recently, remarkable advances and improved reliability in wind power design have been achieved owing to developments in modern technology. The structural dynamics advances and aerodynamics along with micrometeorology since 1980, have led to about 5% increase annually in wind turbines energy yield [9–13]. Present science and engineering research methods are producing better, stronger, lighter and more efficient blades for wind turbines. Wind turbines annual energy output has increased enormously and the weights, emitted noise of the wind turbine during operation have been halved over the last few years. A considerable amount of power can be obtained from wind energy technology by establishing more wind monitoring stations, effective selection of wind farm site with proper wind generator, enhanced maintenance procedures and practices of wind turbines, increase the wind generator availability, the use of large capacity wind generator, low wind regime turbine, higher heights of tower, wider rotor blade swept area, improved structural design and aerodynamics, proficient and enhanced technique for computer-based machining, improved power factor and better policies of the government.

other conventional power sources.

4 Stability Control and Reliable Performance of Wind Turbines

availability is also sporadic like the wind power.

development is needed to achieve this aim.

long-term sustainability of global economy.

**2. Overview of wind energy technology**

There are three main types of wind turbines used nowadays [15]: the Fixed Speed Wind Turbines with Squirrel-Cage Induction Generator (FSWT SCIG), the Variable Speed Wind Turbines with Doubly Fed Induction Generator (VSWT DFIG) and the Variable Speed Wind Turbines with Permanent Magnet Synchronous Generator (VSWT PMSG). Wind energy technology has experienced important improvement in several last decades [16] due to the technological improvement of wind turbines from fixed speed to variable speed. A brief distinction of the three types of wind turbine driven generators is given below.

The SCIG are used in general as fixed speed wind turbine generators due to their superior characteristics such as brushless and rugged construction, low cost, maintenance free and operational simplicity. However, this type of wind turbine technology requires large reactive power to recover the air gap flux when a short circuit fault occurs or grid disturbances in the power system. SCIG wind turbine technology has limited ability to provide voltage and frequency control, except it is supported with some expensive external power electronic control strategy, hence not commonly used in modern wind turbines.

The variable speed turbines are becoming the norm for new wind farm installations, because of high energy capture efficiency couple with reduced drive train stresses [17]. The PMSG VSWT, also known as the direct-drive synchronous generator with permanent magnet excitation and the DFIG VSWT with doubly fed back-to-back power converter type technologies, have become the two generator alternatives. The former has the disadvantage of cost mainly due to a power converter rated for the full power. Although in the latter, a gear box is needed, the DFIG requires a converter of only 20–30% of the generator rating for an operating speed range of 0.7–1.3 per unit (p.u) resulting in a lower cost.

Although the DFIG is not as rugged and robust as the squirrel-cage wind turbine type, however, the brushes have little wear and sparking when compared to DC machines and is the only acceptable option for alternative energy conversion in the megawatts power range. With the help of modern power electronic devices, it is possible to recover the slip power dissipated in resistances [2]. The DFIG wind turbine use a back-to-back power inverter system connected between the rotor side and the grid side of the machine, while the stator is directly connected to the grid. The DFIG can effectively operate at a wide range of speed based on the available wind speed and other specific operation requirements. Consequently, better capture of wind energy [2, 18, 19], with dynamic slip and pitch control could lead to effective rebuilding of the wind turbine terminal voltage, at the same time maintaining the power system stability after clearance of an external short circuit fault [20]. Besides, DFIG wind turbine has shown better behavior regarding system stability during short circuit fault in comparison to SCIG, because of its ability to decouple the active and reactive power output control. The DFIG superior dynamic performance is achieved from the frequency or power converters which typically operates with sampling and switching frequencies of above 2 kHz [21]. The Insulated Gate Bipolar Transistors (IGBTs) of the DFIG converter system are normally off, during lower voltages down to 0% and the system remains in standby mode [22–26]. During grid disturbances, if the voltages are high above a set cutoff or threshold value, the DFIG wind turbine system is very quickly synchronized and is back in operation again.

The VSWT PMSG is connected through a back-to-back converter to the grid. This provides maximum flexibility, enabling full real and reactive power control and fault ride through capability during voltage dips, as compared to the VSWT DFIG technology. However, the use of this wind turbine technology is limited when compared to the DFIG technology due to high cost. The schematic diagrams of the three wind turbine technologies are shown in **Figures 1**–**3**, while a comparison between the fixed and variable speed wind turbine technology is given in **Table 1**.

**Figure 2.** Doubly fed induction generator variable speed wind turbine.

**Figure 3.** Permanent magnet synchronous generator variable speed wind turbine.

**Fixed speed wind generator Variable speed wind generator**

1. This type of wind turbine has variable speed operation, making it possible to achieve a high efficiency of energy conversion compared to constant speed operation especially in

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2. The technology of this turbine has ability to decouple control of active and reactive powers rapidly and independently by secondary excitation control. Thus, the grid connected turbine system tends to be more stable during network disturbances.

low wind speed areas.

1. This type of wind turbine has fixed speed operation, thus the power captured is in limited

2. The technology of this turbine has limited ability to provide voltage and frequency control.

range.

**Figure 1.** Fixed speed induction generator wind turbine.

Introductory Chapter: Stability Control and Reliable Performance of Wind Turbines http://dx.doi.org/10.5772/intechopen.80237 7

**Figure 2.** Doubly fed induction generator variable speed wind turbine.

dissipated in resistances [2]. The DFIG wind turbine use a back-to-back power inverter system connected between the rotor side and the grid side of the machine, while the stator is directly connected to the grid. The DFIG can effectively operate at a wide range of speed based on the available wind speed and other specific operation requirements. Consequently, better capture of wind energy [2, 18, 19], with dynamic slip and pitch control could lead to effective rebuilding of the wind turbine terminal voltage, at the same time maintaining the power system stability after clearance of an external short circuit fault [20]. Besides, DFIG wind turbine has shown better behavior regarding system stability during short circuit fault in comparison to SCIG, because of its ability to decouple the active and reactive power output control. The DFIG superior dynamic performance is achieved from the frequency or power converters which typically operates with sampling and switching frequencies of above 2 kHz [21]. The Insulated Gate Bipolar Transistors (IGBTs) of the DFIG converter system are normally off, during lower voltages down to 0% and the system remains in standby mode [22–26]. During grid disturbances, if the voltages are high above a set cutoff or threshold value, the DFIG wind turbine system is very quickly synchronized and is back

The VSWT PMSG is connected through a back-to-back converter to the grid. This provides maximum flexibility, enabling full real and reactive power control and fault ride through capability during voltage dips, as compared to the VSWT DFIG technology. However, the use of this wind turbine technology is limited when compared to the DFIG technology due to high cost. The schematic diagrams of the three wind turbine technologies are shown in **Figures 1**–**3**, while a comparison between the fixed and variable speed wind turbine technol-

in operation again.

6 Stability Control and Reliable Performance of Wind Turbines

ogy is given in **Table 1**.

**Figure 1.** Fixed speed induction generator wind turbine.

**Figure 3.** Permanent magnet synchronous generator variable speed wind turbine.



be imperative to perform new studies to evaluate the behavior of the wind farms during and after severe faults, in order to improve the design of the wind farms in an efficient and economy way. Hence, the most demanding requisite for wind farm is the Fault Ride Through

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**Figure 5.** The rule of voltage support during grid fault as set by E.ON NETZ GmbH.

**Figure 6.** Grid frequency requirement of wind farms as set by E.ON NETZ GmbH.

**Table 1.** Comparative study of fixed and variable speed wind generators.
