**Chapter 2**

### **Wind Energy**

The aim of this chapter is to introduce the reader to the wind energy. In this way, as the primary source of wind energy, how the wind is created and its characteristics are evaluated.

Due to its nature, the wind is an un-programmable energy source. However, it is possible to estimate the wind speed and direction for a specific location using wind patterns. Therefore, in the present chapter, how to describe the wind behavior for a specific location, the kinetic energy contained in the wind and its probability to occur is described.

To convert the wind energy into a useful energy has to be harvested. The uptake of wind energy in all the wind machines is achieved through the action of wind on the blades, is in these blades where the kinetic energy contained in the wind is converted into mechanic energy. Thus, the different ways to harvest this energy are evaluated, such as: different kind of blades, generators, turbines… wind in

Once, the wind and the fundamentals of the wind machines are familiar, the advantages / disadvantages between offshore and onshore energy are discussed.

#### **2.1 The wind**

The unequal heat of the Earth surface by the sun is the main reason in the generation of the wind. So, wind energy is a converted form of solar energy.

The sun's radiation heats different parts of the earth at different rates; this causes the unequal heat of the atmosphere. Hot air rises, reducing the atmospheric pressure at the earth's surface, and cooler air is drawn in to replace it, causing wind. But not all air mass displacement can be denominate as wind, only horizontal air movements. When air mass has vertical displacement is called as "convection air current"

The wind in a specific location is determinate by global and local factors. Global winds are caused by global factors and upon this large scale wind systems are always superimposed local winds.

#### Global or geostrophic winds

The geostrophic wind is found at altitudes above 1000 m from ground level and it's not very much influenced by the surface of the earth.

The regions around equator, at 0° latitude are heated more by the sun than the regions in the poles. So, the wind rises from the equator and moves north and south in the higher layers of the atmosphere. At the Poles, due to the cooling of the air, the air mass sinks down, and returns to the equator. 

Figure 2.2 Illustration of the sea breezes direction.

Figure 2.3 Illustration of the mountain / valley breezes direction.

night the wind direction is reversed, and turns into a down-slope wind.

turbulences are high influenced by the roughness of the area.

open terrain, and will have even less influence on the wind.

*V V*

*wind wind*

described with the following equation (1) [11]:

and wind blows in the opposite direction [10].

**2.1.1The roughness of the wind** 

will be slowed down.

Land masses are heated by the sun more quickly than the sea in the daytime. The hot air rises, flows out to the sea, and creates a low pressure at ground level which attracts the cool air from the sea. This is called a sea breeze. At nightfall land and sea temperatures are equal

Wind Energy 11

A similar phenomenon occurs in mountain / valleys. During the day, the sun heats up the slopes and the neighboring air. This causes it to rise, causing a warm, up-slope wind. At

About 1 Km above the ground level the wind is hardly influenced by the surface of the earth at all. But in the lower layers of the atmosphere, wind speeds are affected by the friction against the surface of the earth. Therefore, close to the surface the wind speed and wind

In general, the more pronounced the roughness of the earth's surface, the more the wind

Trees and high buildings slow the wind down considerably, while completely open terrain will only slow the wind down a little. Water surfaces are even smoother than completely

The fact that the wind profile is twisted towards a lower speed as we move closer to ground level is usually called wind shear. The wind speed variation depending on the height can be

> *h h*

 ´ ´ *w*

(1)

 

If the globe did not rotate, the air would simply arrive at the North Pole and the South Pole, sink down, and return to the equator. Thus, the rotation with the unequal heating of the surface determines the prevailing wind directions on earth. The general wind pattern of the main regions on earth is depicted in Figure 2.1

Figure 2.1 Representation of the global wind on the earth.

Besides the earth rotation, the relative position of the earth with the sun also varies during the year (year seasons). Due to these seasonal variations of the sun's radiation the intensity and direction of the global winds have variations too.

#### Local Winds

The wind intensity and direction is influenced by global and local effects. Nevertheless, when global scale winds are light, local winds may dominate the wind patterns. The main local wind structures are sea breezes and mountain / valley breezes. The breeze is a light and periodic wind which appears in locations with periodic thermal gradient variations.

If the globe did not rotate, the air would simply arrive at the North Pole and the South Pole, sink down, and return to the equator. Thus, the rotation with the unequal heating of the surface determines the prevailing wind directions on earth. The general wind pattern of the

Besides the earth rotation, the relative position of the earth with the sun also varies during the year (year seasons). Due to these seasonal variations of the sun's radiation the intensity

The wind intensity and direction is influenced by global and local effects. Nevertheless, when global scale winds are light, local winds may dominate the wind patterns. The main local wind structures are sea breezes and mountain / valley breezes. The breeze is a light and periodic wind which appears in locations with periodic thermal gradient variations.

main regions on earth is depicted in Figure 2.1

Figure 2.1 Representation of the global wind on the earth.

and direction of the global winds have variations too.

Local Winds

Figure 2.3 Illustration of the mountain / valley breezes direction.

Land masses are heated by the sun more quickly than the sea in the daytime. The hot air rises, flows out to the sea, and creates a low pressure at ground level which attracts the cool air from the sea. This is called a sea breeze. At nightfall land and sea temperatures are equal and wind blows in the opposite direction [10].

A similar phenomenon occurs in mountain / valleys. During the day, the sun heats up the slopes and the neighboring air. This causes it to rise, causing a warm, up-slope wind. At night the wind direction is reversed, and turns into a down-slope wind. into

#### **2.1.1The roughness of the wind wind**

About 1 Km above the ground level the wind is hardly influenced by the surface of the earth at all. But in the lower layers of the atmosphere, wind speeds are affected by the friction against the surface of the earth. Therefore, close to the surface the wind speed and wind turbulences are high influenced by the roughness of the area.

In general, the more pronounced the roughness of the earth's surface, the more the wind will be slowed down.

Trees and high buildings slow the wind down considerably, while completely open terrain will only slow the wind down a little. Water surfaces are even smoother than completely open terrain, and will have even less influence on the wind.

The fact that the wind profile is twisted towards a lower speed as we move closer to ground level is usually called wind shear. The wind speed variation depending on the height can be described with the following equation (1) [11]:

$$\frac{\dot{V}\_{\text{wind}}}{V\_{\text{wind}}} = \left(\frac{h}{h}\right)^{\alpha\_{\text{w}}} \tag{1}$$

**2.1.2The general pattern of wind: Speed variations and average wind** 

investors to estimate their incomes from electricity generation.

density (3) depends on two adjustable parameters.

*v*

10 m/s.

*0.02*

*0.04*

*0.06*

*0.08*

*probability density*

*0.1*

*0.12*

*0.14*

*0.16*

Wind is an un-programmable energy source, but this does not mean unpredictable. It is possible to estimate the wind speed and direction for a specific location. In fact, wind predictions and wind patterns help turbine designers to optimize their designs and

Wind Energy 13

The wind variation for a typical location is usually described using the so-called "Weibull" distribution. Due to the fact that this distribution has been experimentally verified as a pretty accurate estimation for wind speed [14], [15] The weibull's expression for probability

*wind e*

Where: *ф(v)*= Weibull's expression for probability density depending on the wind, *vwind* =

The curves for weibulls distribution for different average wind speeds are shown in Figure 2.5. This particular figure has a mean wind speed of 5 to 10 meters per second, and the

*weibull's distribution* 

Figure 2.5 Curves of weibull's distribution for different average wind speeds 5, 6, 7, 8, 9 and

*<sup>0</sup> <sup>5</sup> <sup>10</sup> <sup>15</sup> <sup>20</sup> <sup>25</sup> <sup>30</sup> <sup>0</sup>*

*wind speed*

1

*c*

the velocity of the wind measured in m/s, *c* = scale factor and *k* = shape parameter.

*v*

*c k*

shape of the curve is determined by a so called shape parameter of 2.

*k v*

*k wind c*

( ) (3)

*10 m/s 9 m/s 8 m/s 7 m/s 6 m/s 5 m/s*

 

Figure 2.4 Illustration of the wind speed variation due to the obstacles in the earth surface.

Where: *V'wind*= the velocity of the wind (m/s) at height *h*' above ground level. *Vwind* = reference wind speed, i.e. a wind speed is already known at height *h*. *h*' = height above ground level for the desired velocity, αw = roughness length in the current wind direction. *h* = reference height (the height where is known the exact wind speed, usually =10m).

As well as the wind speed the energy content in the wind changes with the height. Consequently, the wind power variations are described in equation (2) [12]:

$$\frac{P\_{\text{wind}}^{\cdot}}{P\_{\text{wind}}} = \left(\frac{h}{h}\right)^{3\alpha\_w} \tag{2}$$

Where: *P'wind* = wind power at height *h*' above ground level. *Pwind* = reference wind power, i.e. a wind power is already known at height *h*. *h*' = height above ground level for the desired velocity, αw = roughness length in the current wind direction. *h* = reference height (the height where is known the exact wind speed, usually =10m).

At the following table, the different values of αw (The roughness coefficient) for different kind of surfaces, according to European Wind Atlas [13] are shown.


Table 2.1 Different α values for different kind of surfaces.

Figure 2.4 Illustration of the wind speed variation due to the obstacles in the earth surface. Where: *V'wind*= the velocity of the wind (m/s) at height *h*' above ground level. *Vwind* = reference wind speed, i.e. a wind speed is already known at height *h*. *h*' = height above ground level for the desired velocity, αw = roughness length in the current wind direction. *h* = reference height (the height where is known the exact wind speed, usually =10m).

As well as the wind speed the energy content in the wind changes with the height.

*h h*

Where: *P'wind* = wind power at height *h*' above ground level. *Pwind* = reference wind power, i.e. a wind power is already known at height *h*. *h*' = height above ground level for the desired velocity, αw = roughness length in the current wind direction. *h* = reference height

At the following table, the different values of αw (The roughness coefficient) for different

0,0024-0,5 *Completely open terrain with a smooth surface, e.g. concrete runways in airports,* 

0,03-1 *Open agricultural area without fences and hedgerows and very scattered* 

0,4-3 *Villages, small towns, agricultural land with many or tall sheltering hedgerows,* 

 '´

> 

*w*

(2)

3

 

Consequently, the wind power variations are described in equation (2) [12]:

*wind*

*P P*

(the height where is known the exact wind speed, usually =10m).

kind of surfaces, according to European Wind Atlas [13] are shown.

*buildings. Only softly rounded hills* 

*forests and very rough and uneven terrain*  1,6-4 *Very large cities with tall buildings and skyscrapers* 

0 0,0002 *Water surface* 

*mowed grass, etc.* 

Table 2.1 Different α values for different kind of surfaces.

*wind*

#### **2.1.2The general pattern of wind: Speed variations and average wind**

Wind is an un-programmable energy source, but this does not mean unpredictable. It is possible to estimate the wind speed and direction for a specific location. In fact, wind predictions and wind patterns help turbine designers to optimize their designs and investors to estimate their incomes from electricity generation.

The wind variation for a typical location is usually described using the so-called "Weibull" distribution. Due to the fact that this distribution has been experimentally verified as a pretty accurate estimation for wind speed [14], [15] The weibull's expression for probability density (3) depends on two adjustable parameters.

$$\phi(\nu) = \frac{k}{c} \cdot \left(\frac{\nu\_{\text{wind}}}{c}\right)^{k-1} \cdot e^{-\left(\frac{\nu\_{\text{wind}}}{c}\right)^{k}} \tag{3}$$

Where: *ф(v)*= Weibull's expression for probability density depending on the wind, *vwind* = the velocity of the wind measured in m/s, *c* = scale factor and *k* = shape parameter.

The curves for weibulls distribution for different average wind speeds are shown in Figure 2.5. This particular figure has a mean wind speed of 5 to 10 meters per second, and the shape of the curve is determined by a so called shape parameter of 2.

Figure 2.5 Curves of weibull's distribution for different average wind speeds 5, 6, 7, 8, 9 and 10 m/s.

The rotor area

equation (5)

in meters.

swept area.

m/s.

Equation of the winds kinetic energy

The rotor area determines how much energy a wind turbine is able to harvest from the wind. Due to the fact that the amount of the air mass flow upon which the rotor can actuate is determined by this area, this amount increases with the square of the rotor diameter,

Wind Energy 15

Where: *Ar* = the rotor swept area in square meters and *r* = the radius of the rotor measured

The input air mass flow of a wind turbine with a specific rotor swept area determined by *Ar* is given by equation (6). This input air mass flow depends on the wind speed and the rotor

Where: *M* = Air mass flow, *ρ* = the density of dry air ( 1.225 measured in kg/m3 at average atmospheric pressure at sea level at 15° C) and *Vwind* = the velocity of the wind measured in

> 2 1

Where: *Pwind* = the power of the wind measured in Watts, *ρ* = the density of dry air ( 1.225 measured in kg/m 3 at average atmospheric pressure at sea level at 15° C), *Vwind* = the velocity of the wind measured in m/s and *r* = the radius of the rotor measured in meters.

The wind speed determines the amount of energy that a wind turbine can convert to electricity. The potential energy per second in the wind varies in proportion to the cube of

The more kinetic energy a wind turbine pulls out of the wind, the more the wind will be slowed down. In one hand if the wind turbines extract all the energy from the wind, the air could not leave the turbine and the turbine would not extract any energy at all. On the other hand, if wind could pass though the turbine without being hindered at all. The turbine

Therefore is possible to assume that there must be some way of breaking the wind between

*<sup>r</sup> wind M* 

*wind <sup>r</sup> wind wind P Mv A v* <sup>2</sup> <sup>3</sup>

<sup>1</sup>

Therefore, the winds kinetic energy is given by equation (7).

the wind speed, and in proportion to the density of the air.

**2.2.2Usable input power, Betz law** 

would not extract any energy from the wind.

2

these two extremes, to extract useful mechanical energy from the wind.

(5)

*A v* (6)

(7)

<sup>2</sup> *A r <sup>r</sup>* 

The graph shows a probability density distribution. Therefore, the area under the curve is always exactly 1, since the probability that the wind will be blowing at some wind speed including zero must be 100 per cent.

The statistical distribution of wind speed varies from one location to another depending on local conditions like the surfaces roughness. Thus to fit the Weibull distribution to a specific location is necessary to set two parameters: the shape and the wind speeds mean value. a

If the shape parameter is 2, as in Figure 2.5, the distribution is known as a Rayleigh distribution. Wind turbine manufacturers often give standard performance figures for their machines using the Rayleigh distribution.

The distribution of wind speeds is skewed, is not symmetrical. Sometimes the wind presents very high wind speeds, but they are very rare. On the contrary, the probability of the wind to presents slow wind speeds is pretty high.

To calculate the mean wind speed, the wind speed value and its probability is used. Thus, the mean or average wind speed is the average of all the wind speeds measured in this location. The average wind speed is given by equation (4) [16]:

$$
\overline{\nu}\_{wind} = \int\_0^\infty \nu\_{wind} \cdot \phi(\nu\_{wind}) \cdot d\nu\_{wind} \tag{4}
$$

Where: *ф( vwind )* = Weibull's expression for probability density depending on the wind, *vwind*  =the velocity of the wind measured in m/s.

#### **2.2 The power of the wind**

The uptake of wind energy in all the wind machines is achieved through the action of wind on the blades, is in these blades where the kinetic energy contained in the wind is converted into mechanic energy. Therefore, in the present section the analysis of the power contained in the wind is oriented to those devices.

#### **2.2.1The kinetic energy of the wind**

The input power of a wind turbine is through its blades, converting wind power into a torque. Consequently, the input power depends on the rotor swept area, the air density and the wind speed.

#### Air density

The kinetic energy of a moving body is proportional to its mass. So, the kinetic energy of the wind depends on the air density, the air mass per unit of volume. At normal atmospheric pressure (and at 15° C) air weigh is 1.225 kg per cubic meter, but the density decreases slightly with increasing humidity.

Also, the air is denser when it is cold than when it is warm. At high altitudes, (in mountains) the air pressure is lower, and the air is less dense.

The graph shows a probability density distribution. Therefore, the area under the curve is always exactly 1, since the probability that the wind will be blowing at some wind speed

The statistical distribution of wind speed varies from one location to another depending on local conditions like the surfaces roughness. Thus to fit the Weibull distribution to a specific location is necessary to set two parameters: the shape and the wind speeds mean value.

If the shape parameter is 2, as in Figure 2.5, the distribution is known as a Rayleigh distribution. Wind turbine manufacturers often give standard performance figures for their

The distribution of wind speeds is skewed, is not symmetrical. Sometimes the wind presents very high wind speeds, but they are very rare. On the contrary, the probability of the wind

To calculate the mean wind speed, the wind speed value and its probability is used. Thus, the mean or average wind speed is the average of all the wind speeds measured in this

*wind wind wind wind <sup>v</sup> <sup>v</sup> <sup>v</sup> dv*

Where: *ф( vwind )* = Weibull's expression for probability density depending on the wind, *vwind* 

The uptake of wind energy in all the wind machines is achieved through the action of wind on the blades, is in these blades where the kinetic energy contained in the wind is converted into mechanic energy. Therefore, in the present section the analysis of the power contained

The input power of a wind turbine is through its blades, converting wind power into a torque. Consequently, the input power depends on the rotor swept area, the air density and

The kinetic energy of a moving body is proportional to its mass. So, the kinetic energy of the wind depends on the air density, the air mass per unit of volume. At normal atmospheric pressure (and at 15° C) air weigh is 1.225 kg per cubic meter, but the density decreases

Also, the air is denser when it is cold than when it is warm. At high altitudes, (in mountains)

(4)

including zero must be 100 per cent.

machines using the Rayleigh distribution.

to presents slow wind speeds is pretty high.

=the velocity of the wind measured in m/s.

in the wind is oriented to those devices. **2.2.1The kinetic energy of the wind** 

slightly with increasing humidity.

the air pressure is lower, and the air is less dense.

**2.2 The power of the wind** 

the wind speed.

Air density

location. The average wind speed is given by equation (4) [16]:

 0

#### The rotor area

The rotor area determines how much energy a wind turbine is able to harvest from the wind. Due to the fact that the amount of the air mass flow upon which the rotor can actuate is determined by this area, this amount increases with the square of the rotor diameter, equation (5) amount

$$A\_r = \pi \cdot r^2 \tag{5}$$

Where: *Ar* = the rotor swept area in square meters and *r* = the radius of the rotor measured in meters.

#### Equation of the winds kinetic energy

The input air mass flow of a wind turbine with a specific rotor swept area determined by *Ar* is given by equation (6). This input air mass flow depends on the wind speed and the rotor swept area.

$$M = \rho A\_r \mathbf{v}\_{\text{wind}} \tag{6}$$

Where: *M* = Air mass flow, *ρ* = the density of dry air ( 1.225 measured in kg/m3 at average atmospheric pressure at sea level at 15° C) and *Vwind* = the velocity of the wind measured in m/s.

Therefore, the winds kinetic energy is given by equation (7).

$$P\_{\rm wind} = \frac{1}{2} M \nu^2 \iota\_{\rm wind} = \frac{1}{2} \rho A\_r \nu^3 \iota\_{\rm wind} \tag{7}$$

Where: *Pwind* = the power of the wind measured in Watts, *ρ* = the density of dry air ( 1.225 measured in kg/m 3 at average atmospheric pressure at sea level at 15° C), *Vwind* = the velocity of the wind measured in m/s and *r* = the radius of the rotor measured in meters.

The wind speed determines the amount of energy that a wind turbine can convert to electricity. The potential energy per second in the wind varies in proportion to the cube of the wind speed, and in proportion to the density of the air.

#### **2.2.2Usable input power, Betz law**

The more kinetic energy a wind turbine pulls out of the wind, the more the wind will be slowed down. In one hand if the wind turbines extract all the energy from the wind, the air could not leave the turbine and the turbine would not extract any energy at all. On the other hand, if wind could pass though the turbine without being hindered at all. The turbine would not extract any energy from the wind.

Therefore is possible to assume that there must be some way of breaking the wind between these two extremes, to extract useful mechanical energy from the wind.

**2.3 Fundamentals of wind machines** 

of air over the surface of the blades.

*Incident free wind*

blades.

Wind machines convert the kinetic energy contained in the wind into mechanic energy through the action of wind on the blades. The aerodynamic principle in this transformation

Wind Energy 17

According to this principle, the air is forced to flow over the top and bottom of a blade (see Figure 2.7) generating a pressure difference between both sides. The pressure difference causes a resultant force upon the blade. This force can be decomposed in two components:

**b)** *Drag force***,** which is parallel to the direction of the wind. This force helps the circulation

*Drag force*

(kinetic to mechanic energy) is similar to the principle that makes airplanes fly.

Figure 2.7 Representation of lift force and drag force generated on the blades.

position of the blades with the axis and the wind.

Figure 2.8), to make maximum lift / drag force ratio [12].

The force which will generate a torque is lift force or drag force depending on the relative

drag

In wind turbines with horizontal axis, the lift component of the force is the only one that gives the torque. Therefore, as the lift force gives torque, the profile of the blade has to be designed setting the attack angle (α), the relative position of the blade with the wind (see

This simple analysis is only valid when the blades of a wind turbine are at rest. If the rotation of the rotor is allowed, the resultant force on the blades will be the result from the combination of direct action of the real wind and the action of the wind created by the

The incident wind on the blades is called apparent wind (Figure 2.8), is the result from the

composition of the vector of the true wind vector and the wind created by the blade.

**a)** *Lift force*, which is perpendicular to the direction of the wind.

*Lift force*

Betz law

Betz law says that it's only possible convert less than 16/27 (or 59%) of the kinetic energy in the wind to mechanical energy using a wind turbine. This law can be applied to any kind of wind generators with disc turbines. Besides this limit, also must be considered the aerodynamic and mechanic efficiency from the turbines.

#### **2.2.3Useful electric energy from wind**

As said before, from the winds kinetic energy it's only possible convert less than 16/27 (Betz's law). However, the process to harvest the wind also has other losses, even the best blades have above 10% of aerodynamic losses [17].

So, the electric power that can be extracted from the kinetic energy of the wind with a turbine is given by the well-known equation (8).

$$P\_r(\nu) = \frac{1}{2} \cdot \rho \cdot \pi \cdot r^2 \cdot V\_{\text{vod}}^3 \cdot Cp \tag{8}$$

Where: *Pt (v)* = the input power of the generator, *ρ* = the density of dry air ( 1.225 measured in kg/m 3 for average atmospheric pressure at sea level with 15° C), *r* = the radius of the rotor measured in meters, *Vwind* = the velocity of the wind measured in m/s and *Cp* = the power coefficient.

As any machine in movement, the generator has mechanic losses whether they are: at the bearings, brushes, gear...Equally any electric machine has electric losses. Hence, only a part of the winds kinetic energy can be converted to electric power Figure 2.6.

Figure 2.6 Representation of the power losses at different steps of electric wind energy generation.

#### **2.2.3.1The power coefficient**

The power coefficient tells how efficient is a turbine capturing the energy contained in the wind. To measure this efficiency, the energy captured by the rotor is divided by the input wind energy. In other words, the power coefficient is the relation between the kinetic energy in the rotor swept area and the input power of the generator.

Betz law says that it's only possible convert less than 16/27 (or 59%) of the kinetic energy in the wind to mechanical energy using a wind turbine. This law can be applied to any kind of wind generators with disc turbines. Besides this limit, also must be considered the

As said before, from the winds kinetic energy it's only possible convert less than 16/27 (Betz's law). However, the process to harvest the wind also has other losses, even the best

So, the electric power that can be extracted from the kinetic energy of the wind with a

Where: *Pt (v)* = the input power of the generator, *ρ* = the density of dry air ( 1.225 measured in kg/m 3 for average atmospheric pressure at sea level with 15° C), *r* = the radius of the rotor measured in meters, *Vwind* = the velocity of the wind measured in m/s and *Cp* = the power

As any machine in movement, the generator has mechanic losses whether they are: at the bearings, brushes, gear...Equally any electric machine has electric losses. Hence, only a part

Figure 2.6 Representation of the power losses at different steps of electric wind energy

The power coefficient tells how efficient is a turbine capturing the energy contained in the wind. To measure this efficiency, the energy captured by the rotor is divided by the input wind energy. In other words, the power coefficient is the relation between the kinetic energy

(8)

*vP CpVr <sup>t</sup> wind* <sup>32</sup>

2 1

of the winds kinetic energy can be converted to electric power Figure 2.6.

in the rotor swept area and the input power of the generator.

aerodynamic and mechanic efficiency from the turbines.

blades have above 10% of aerodynamic losses [17].

turbine is given by the well-known equation (8).

**2.2.3Useful electric energy from wind** 

Betz law

coefficient.

generation.

**2.2.3.1The power coefficient** 

#### **2.3 Fundamentals of wind machines**

Wind machines convert the kinetic energy contained in the wind into mechanic energy through the action of wind on the blades. The aerodynamic principle in this transformation (kinetic to mechanic energy) is similar to the principle that makes airplanes fly.

According to this principle, the air is forced to flow over the top and bottom of a blade (see Figure 2.7) generating a pressure difference between both sides. The pressure difference causes a resultant force upon the blade. This force can be decomposed in two components:

**a)** *Lift force*, which is perpendicular to the direction of the wind.

**b)** *Drag force***,** which is parallel to the direction of the wind. This force helps the circulation of air over the surface of the blades.

Figure 2.7 Representation of lift force and drag force generated on the blades.

The force which will generate a torque is lift force or drag force depending on the relative position of the blades with the axis and the wind.

In wind turbines with horizontal axis, the lift component of the force is the only one that gives the torque. Therefore, as the lift force gives torque, the profile of the blade has to be designed setting the attack angle (α), the relative position of the blade with the wind (see Figure 2.8), to make maximum lift / drag force ratio [12].

This simple analysis is only valid when the blades of a wind turbine are at rest. If the rotation of the rotor is allowed, the resultant force on the blades will be the result from the combination of direct action of the real wind and the action of the wind created by the blades.

The incident wind on the blades is called apparent wind (Figure 2.8), is the result from the composition of the vector of the true wind vector and the wind created by the blade.

**2.4.1Horizontal or vertical axes classification** 

it is self-regulated at high wind speeds

means tearing the whole machine down.

a horizontal axis (i.e. a horizontal main shaft), Figure 2.9.

Figure 2.9 Commercial wind turbine with horizontal axis.

The machine is not self-starting.

axis turbines are [15]:

for the machine.

The basic disadvantages are:

axes machines.

Vertical axis wind turbines are the machines where drag force causes the torque in the perpendicular direction of the rotation axis. The basic theoretical advantages of a vertical

Wind Energy 19

Do not need a yaw mechanism to turn the rotor against the wind.

The possibility to place the generator, gearbox etc. on the ground avoiding a tower

Vertical axes machine does not needs regulation with wind speed variations since

The overall efficiency of the vertical axis machines is usually worst than horizontal

 To replace the main bearing for the rotor, it requires removing the rotor on both horizontal and vertical axis machines. But, in the case of vertical axes machine, this

worst the

Today, all grid-connected commercial wind turbines are built with a propeller-type rotor on

Figure 2.8 Wind created by the blade and the apparent wind.

Each section of the blade has a different speed and the wind speed is higher in terms of the height, thus, the apparent wind in each section is different. To obtain the same resultant force along its length, the profile of the blade has to have different dimensions. Therefore, to achieve this homogeneous resultant force, the rotor blade is twisted. The wing does not change its shape, but changes the angle of the wing in relation to the general direction of the airflow (also known as the angle of attack).

To start a wind turbine, wind speed must exceed the so-called cut in speed (minimum value needed to overcome friction and start producing energy) usually between 3-5 m/s. With higher speeds the turbine starts generating power depending on the known equation (8) of section 2.2.3.

This will be so until it reaches the nominal power. At this point the turbine activates its regulation mechanisms to maintain the same output power. At very high wind speeds the turbine stops in order to avoid any damage. This stop wind speed is called the cut out wind speed.

#### **2.4 Wind turbine classification**

According to the most of the authors [12], [15] and [17] wind turbines can be classified by three parameters: the direction of the rotor axis, the number of rotor blades and the rotor position.

Each section of the blade has a different speed and the wind speed is higher in terms of the height, thus, the apparent wind in each section is different. To obtain the same resultant force along its length, the profile of the blade has to have different dimensions. Therefore, to achieve this homogeneous resultant force, the rotor blade is twisted. The wing does not change its shape, but changes the angle of the wing in relation to the general direction of the

To start a wind turbine, wind speed must exceed the so-called cut in speed (minimum value needed to overcome friction and start producing energy) usually between 3-5 m/s. With higher speeds the turbine starts generating power depending on the known equation (8) of

of and

This will be so until it reaches the nominal power. At this point the turbine activates its regulation mechanisms to maintain the same output power. At very high wind speeds the turbine stops in order to avoid any damage. This stop wind speed is called the cut out wind

According to the most of the authors [12], [15] and [17] wind turbines can be classified by three parameters: the direction of the rotor axis, the number of rotor blades and the rotor

Figure 2.8 Wind created by the blade and the apparent wind.

airflow (also known as the angle of attack).

**2.4 Wind turbine classification** 

section 2.2.3.

speed.

position.

#### **2.4.1Horizontal or vertical axes classification**

Vertical axis wind turbines are the machines where drag force causes the torque in the perpendicular direction of the rotation axis. The basic theoretical advantages of a vertical axis turbines are [15]:


The basic disadvantages are:


Today, all grid-connected commercial wind turbines are built with a propeller-type rotor on a horizontal axis (i.e. a horizontal main shaft), Figure 2.9.

Figure 2.9 Commercial wind turbine with horizontal axis.

The main drawback on downwind machines is that they are influenced by the wind shade behind the tower. When blades cross the wind shade behind the tower, they lose torque and get it back again, causing periodic effort variations in the rotor [17]. Therefore, by far the

Wind Energy 21

A general outline of the components of a wind turbine is given by the following figure:

**Rotor blades:** Device to harvest the energy for the wind. At this part the kinetic energy of

**Gearbox:** To convert the slowly rotating, high torque power from the wind turbine rotor to a

**Associated power electronics:** The part of the wind turbine where electric power is adapted

**Transformer:** The turbines have their own transformer to step-up the voltage level of the

An electric generator converts mechanical energy into electrical energy. Synchronous generators are used in most traditional generators (hydro, thermal, nuclear ...). But if these kinds of generators are directly connected to the main grid, they must have fixed rotational speed in synchronism to the frequency of the grid. Thus, torque fluctuations in the rotor (like the fluctuations caused by the wind speed variations) are propagated through the

Furthermore, with fixed speed of the rotor, the turbine cannot vary the rotational speed in order to achieve the optimum speed and extract the maximum torque from the wind. So,

**Anemometer and wind vane**: Devices for measuring wind speed and direction.

**Electrical generator**: Device to transform mechanical energy into electrical energy.

vast majority of wind turbines have upwind design.

Figure 2.11 Illustration of wind turbine components.

the wind is transformed into a mechanical torque.

to the frequency and the voltage amplitude of the grid.

high speed, low torque power rotation.

wind turbine to the medium voltage line.

machine to the output electric power.

with fixed speed the aerodynamic losses are bigger.

**2.5.2 Electric generator** 

**2.5 Wind turbines** 

**2.5.1 Wind turbine components** 

#### **2.4.2Classification by the number of blades**

A wind turbine does not give more power with more blades. If the machines are well designed, the harvested power is more or less the same with different number of blades [17].

Wind turbines do not harvest power from the aerodynamic resistance; they do from the blades shape. So, the difference between two wind turbines with a different number of blades is the torque generated by each blade and consequently, the rotational speed of the rotor. Besides, wind turbines with multiple blades starts working at low wind speeds, due to their high start-up torque.

A rotor with an odd number of blades (and at least three blades) can be considered as a disk when calculating the dynamic properties of the machine.

In the other hand, a rotor with an even number of blades will give stability problems for a machine with a stiff structure. At the very moment when the uppermost blade bends backwards, because it gets the maximum power from the wind, the lowermost blade gets the minimum energy from the wind, which generates mechanic stress to the structure. Thus most of the modern wind turbines have three blades [12].

#### **2.4.3Upwind or downwind classification**

In this classification the machines can be upwind or downwind depending on the position of the rotor, Figure 2.10. Upwind machines have the rotor facing the wind, on the contrary downwind machines have the rotor placed on the lee side of the tower.

Figure 2.10 Illustration of upwind and downwind turbines.

Downwind machines have the theoretical advantage that they may be built without a yaw mechanism. If the rotor and the nacelle have a suitable design that makes the nacelle follows the wind passively. Another advantage is that the rotor may be made using more flexible materials. Thus, the blades will bend at high wind speeds, taking part of the load off the tower. Therefore, downwind machines may be built somewhat lighter than upwind machines.

A wind turbine does not give more power with more blades. If the machines are well designed, the harvested power is more or less the same with different number of blades [17]. Wind turbines do not harvest power from the aerodynamic resistance; they do from the blades shape. So, the difference between two wind turbines with a different number of blades is the torque generated by each blade and consequently, the rotational speed of the rotor. Besides, wind turbines with multiple blades starts working at low wind speeds, due

A rotor with an odd number of blades (and at least three blades) can be considered as a disk

In the other hand, a rotor with an even number of blades will give stability problems for a machine with a stiff structure. At the very moment when the uppermost blade bends backwards, because it gets the maximum power from the wind, the lowermost blade gets the minimum energy from the wind, which generates mechanic stress to the structure. Thus

In this classification the machines can be upwind or downwind depending on the position of the rotor, Figure 2.10. Upwind machines have the rotor facing the wind, on the contrary

Downwind machines have the theoretical advantage that they may be built without a yaw mechanism. If the rotor and the nacelle have a suitable design that makes the nacelle follows the wind passively. Another advantage is that the rotor may be made using more flexible materials. Thus, the blades will bend at high wind speeds, taking part of the load off the tower. Therefore, downwind machines may be built somewhat lighter than upwind

**2.4.2Classification by the number of blades** 

when calculating the dynamic properties of the machine.

most of the modern wind turbines have three blades [12].

Figure 2.10 Illustration of upwind and downwind turbines.

machines.

downwind machines have the rotor placed on the lee side of the tower.

**2.4.3Upwind or downwind classification** 

to their high start-up torque.

The main drawback on downwind machines is that they are influenced by the wind shade behind the tower. When blades cross the wind shade behind the tower, they lose torque and get it back again, causing periodic effort variations in the rotor [17]. Therefore, by far the vast majority of wind turbines have upwind design.

#### **2.5 Wind turbines**

#### **2.5.1 Wind turbine components**

A general outline of the components of a wind turbine is given by the following figure:

Figure 2.11 Illustration of wind turbine components.

**Rotor blades:** Device to harvest the energy for the wind. At this part the kinetic energy of the wind is transformed into a mechanical torque.

**Anemometer and wind vane**: Devices for measuring wind speed and direction.

**Gearbox:** To convert the slowly rotating, high torque power from the wind turbine rotor to a high speed, low torque power rotation.

**Electrical generator**: Device to transform mechanical energy into electrical energy.

**Associated power electronics:** The part of the wind turbine where electric power is adapted to the frequency and the voltage amplitude of the grid.

**Transformer:** The turbines have their own transformer to step-up the voltage level of the wind turbine to the medium voltage line.

#### **2.5.2 Electric generator**

An electric generator converts mechanical energy into electrical energy. Synchronous generators are used in most traditional generators (hydro, thermal, nuclear ...). But if these kinds of generators are directly connected to the main grid, they must have fixed rotational speed in synchronism to the frequency of the grid. Thus, torque fluctuations in the rotor (like the fluctuations caused by the wind speed variations) are propagated through the machine to the output electric power.

Furthermore, with fixed speed of the rotor, the turbine cannot vary the rotational speed in order to achieve the optimum speed and extract the maximum torque from the wind. So, with fixed speed the aerodynamic losses are bigger.

The concept exists in both single and double speed versions. The double speed operation gives an improved performance and lower noise production at low wind speeds [18].

Wind Energy 23

Limited variable Speed wind turbines used by Vestas in the 80s and 90s are equipped with a 'wound rotor' induction generator (WRIG). Power electronics are applied to control the rotor electrical resistance, which allows both the rotor and the generator to vary their speed

This system combines advantages of previous systems with advances in power electronics. The DFIG is a wound rotor induction generator whose rotor is connected through frequency converter. In the other hand, stator is directly connected to the grid. As a result of the use of the frequency converter, the grid frequency is decoupled from the mechanical speed of the machine allowing a variable speed operation. Thus maximum absorption of wind power is

Approximately 30% - 40% of the output power goes through the inverter to the grid, the other part goes directly through the stator. The speed variations window is approximately 40% up and down from synchronous speed. The application of power electronics also provides control of active and reactive power, i.e. the DFIG wind turbine has the capability

European market share: 30% (2005)

**2.5.3.2Limited variable speed** 

up and down to ± 10% [18].

European market share: 10% (2005)

possible.

Manufacturers: Vestas (V27, V34, V47). [18].

**2.5.3.3Improved variable Speed with DFIG** 

to control independently active and reactive power.

Manufacturers: Suzlon, Nordex, Siemens Bonus, Ecotecnia. [18].

Figure 2.13 The main scheme of limited variable speed wind turbine.

Due to these drawbacks, synchronous generators are only used in wind turbines with indirect grid connection. The synchronous generator is controlled electronically (using an inverter), as a result the frequency of the alternating current in the stator of the generator may be varied. In this way, it is possible to run the turbine at variable rotational speed. Consequently, the turbine will generate alternating current at exactly the variable frequency applied to the stator.

On the other hand, asynchronous generators can be used directly or indirectly connected to the grid. Due to the fact that this kind of generators allows speed variations (little) when is connected directly to the grid. Hence, until the present day, most wind turbines in the world connected directly to the grid use a so-called three phase asynchronous generator (also called induction generator) to generate electric power.

#### **2.5.3 Wind turbine systems**

#### **2.5.3.1 Fixed Speed (one or two speeds)**

Introduced and widely used in the 80s, the concept is based on a 'squirrel cage' asynchronous generator (SCIG), the rotor is driven by the turbine and its stator is directly connected to the grid. Its rotation speed can only vary slightly (between 1% and 2%), which is almost "fixed speed" in comparison with other wind turbine concepts. So, as its name says, this type of generators cannot vary the speed of the turbine to the optimum speed and extract the maximum torque from the wind.

Aerodynamic control is mostly performed using passive stall, and as a result only a few active control options can be implemented in this kind of wind turbines.

SCIGs directly connected to the grid do not have the capability of independent control of active and reactive power, therefore, the reactive power control is performed usually by mechanically switched capacitors.

Their great advantage is their simple and robust construction, which leads to lower capital cost. In contrast to other generator topologies, FSIGs (Fixed Speed Induction Generators) offer no inherent means of torque oscillation damping which places greater burden and cost on their gearbox.

Figure 2.12 The main scheme of fixed speed wind turbine.

Due to these drawbacks, synchronous generators are only used in wind turbines with indirect grid connection. The synchronous generator is controlled electronically (using an inverter), as a result the frequency of the alternating current in the stator of the generator may be varied. In this way, it is possible to run the turbine at variable rotational speed. Consequently, the turbine will generate alternating current at exactly the variable frequency

On the other hand, asynchronous generators can be used directly or indirectly connected to the grid. Due to the fact that this kind of generators allows speed variations (little) when is connected directly to the grid. Hence, until the present day, most wind turbines in the world connected directly to the grid use a so-called three phase asynchronous generator (also

Introduced and widely used in the 80s, the concept is based on a 'squirrel cage' asynchronous generator (SCIG), the rotor is driven by the turbine and its stator is directly connected to the grid. Its rotation speed can only vary slightly (between 1% and 2%), which is almost "fixed speed" in comparison with other wind turbine concepts. So, as its name says, this type of generators cannot vary the speed of the turbine to the optimum speed and

Aerodynamic control is mostly performed using passive stall, and as a result only a few

SCIGs directly connected to the grid do not have the capability of independent control of active and reactive power, therefore, the reactive power control is performed usually by

Their great advantage is their simple and robust construction, which leads to lower capital cost. In contrast to other generator topologies, FSIGs (Fixed Speed Induction Generators) offer no inherent means of torque oscillation damping which places greater burden and cost

active control options can be implemented in this kind of wind turbines.

Figure 2.12 The main scheme of fixed speed wind turbine.

applied to the stator.

**2.5.3 Wind turbine systems** 

**2.5.3.1 Fixed Speed (one or two speeds)** 

extract the maximum torque from the wind.

mechanically switched capacitors.

on their gearbox.

called induction generator) to generate electric power.

The concept exists in both single and double speed versions. The double speed operation gives an improved performance and lower noise production at low wind speeds [18].

European market share: 30% (2005)

Manufacturers: Suzlon, Nordex, Siemens Bonus, Ecotecnia. [18].

#### **2.5.3.2Limited variable speed**

Limited variable Speed wind turbines used by Vestas in the 80s and 90s are equipped with a 'wound rotor' induction generator (WRIG). Power electronics are applied to control the rotor electrical resistance, which allows both the rotor and the generator to vary their speed up and down to ± 10% [18].

Figure 2.13 The main scheme of limited variable speed wind turbine.

European market share: 10% (2005)

Manufacturers: Vestas (V27, V34, V47). [18].

#### **2.5.3.3Improved variable Speed with DFIG**

This system combines advantages of previous systems with advances in power electronics. The DFIG is a wound rotor induction generator whose rotor is connected through frequency converter. In the other hand, stator is directly connected to the grid. As a result of the use of the frequency converter, the grid frequency is decoupled from the mechanical speed of the machine allowing a variable speed operation. Thus maximum absorption of wind power is possible.

Approximately 30% - 40% of the output power goes through the inverter to the grid, the other part goes directly through the stator. The speed variations window is approximately 40% up and down from synchronous speed. The application of power electronics also provides control of active and reactive power, i.e. the DFIG wind turbine has the capability to control independently active and reactive power.

Manufacturers: Enercon, MEG (Multibrid M5000), GE (2.x series), Zephyros, Winwind,

Jeumont.

Wind Energy 25

The generator uses a two stage gearbox to connect the low-speed shaft to the high-speed shaft, with all the problems associated to the gearbox, like the maintenance or the torque

Figure 2.15 The main scheme of variable speed geared with full scale frequency converter

This kind of solutions avoids the gearbox and brushes, so, the implementation of the direct

Figure 2.16 The main scheme of variable speed direct drive with full scale frequency

On a pitch controlled wind turbine, an electronic controller checks the output power of the turbine several times per second. If the output power is bigger than the rated power, it sends an order to the blade pitch mechanism to pitches (turns) the rotor blades out of the wind. In the other hand, if the output power is lower than the rated power the blades are

turned back into the wind, in order to harvest the maximum energy.

drive in a wind turbine improves the mechanic reliability and produces less noise.

Siemens (2.3 MW), Made, Leitner, Mtorres, Jeumont. [18].

**2.5.3.4.1 Full scale frequency converter with gearbox** 

**2.5.3.4.2 Full scale frequency converter with direct drive** 

losses.

wind turbine.

converter wind turbine.

Pitch controlled

**2.5.4Active power control** 

Figure 2.14 The main scheme of improved variable speed with DFIG wind turbine.

European market share: 45% (2005)

Manufacturers: General Electric (series 1.5 y 3.6), Repower, Vestas, Nordex, Gamesa, Alstom, Ecotecnia, Ingetur, Suzlon. [18].

#### **2.5.3.4Variable Speed with full scale frequency converter**

The stator of the generator is connected to the grid through a full-power electronic converter. Various types of generators are being used: SCIG, WRIG (Wound rotor induction generator), PMSG (permanent magnet synchronous generator) or WRSG (wound rotor synchronous generator). The rotor has excitation windings or permanent magnets. Being completely decoupled from the grid, it can provide even a more wide range of operating speeds than DFIGs. This kind of wind turbines has two variants: direct drive and with gearbox.

The basic theoretical characteristics of a variable speed with full scale frequency converters are [19]:

> The DC link decouples completely the generator from the Grid. As the grid frequency is completely decoupled, the generator can work at any rotational speed. Besides changes in grid voltage does not affect the dynamics of the generator.

> The converters have equal rated power as the generator does, not 30% - 40% like DFIG wind turbines.

The converters have full control over the generator.

 This kind of wind turbine provides complete control over active and reactive power exchanged with the grid. Moreover, it is possible to control the voltage and reactive power in the grid without affecting the dynamics in the generator. As long as there is not a grid fault.

European market share: 15% (2005)

Figure 2.14 The main scheme of improved variable speed with DFIG wind turbine.

Manufacturers: General Electric (series 1.5 y 3.6), Repower, Vestas, Nordex, Gamesa,

The stator of the generator is connected to the grid through a full-power electronic converter. Various types of generators are being used: SCIG, WRIG (Wound rotor induction generator), PMSG (permanent magnet synchronous generator) or WRSG (wound rotor synchronous generator). The rotor has excitation windings or permanent magnets. Being completely decoupled from the grid, it can provide even a more wide range of operating speeds than DFIGs. This kind of wind turbines has two variants: direct drive and with

The basic theoretical characteristics of a variable speed with full scale frequency converters

The converters have full control over the generator.

 The DC link decouples completely the generator from the Grid. As the grid frequency is completely decoupled, the generator can work at any rotational speed. Besides changes in grid voltage does not affect the dynamics of the

The converters have equal rated power as the generator does, not 30% - 40%

 This kind of wind turbine provides complete control over active and reactive power exchanged with the grid. Moreover, it is possible to control the voltage and reactive power in the grid without affecting the dynamics in the generator.

European market share: 45% (2005)

generator.

European market share: 15% (2005)

like DFIG wind turbines.

As long as there is not a grid fault.

gearbox.

are [19]:

Alstom, Ecotecnia, Ingetur, Suzlon. [18].

**2.5.3.4Variable Speed with full scale frequency converter** 

Manufacturers: Enercon, MEG (Multibrid M5000), GE (2.x series), Zephyros, Winwind, Siemens (2.3 MW), Made, Leitner, Mtorres, Jeumont. [18].

#### **2.5.3.4.1 Full scale frequency converter with gearbox**

The generator uses a two stage gearbox to connect the low-speed shaft to the high-speed shaft, with all the problems associated to the gearbox, like the maintenance or the torque losses.

Figure 2.15 The main scheme of variable speed geared with full scale frequency converter wind turbine.

#### **2.5.3.4.2 Full scale frequency converter with direct drive**

This kind of solutions avoids the gearbox and brushes, so, the implementation of the direct drive in a wind turbine improves the mechanic reliability and produces less noise.

Figure 2.16 The main scheme of variable speed direct drive with full scale frequency converter wind turbine.

#### **2.5.4Active power control**

Pitch controlled

On a pitch controlled wind turbine, an electronic controller checks the output power of the turbine several times per second. If the output power is bigger than the rated power, it sends an order to the blade pitch mechanism to pitches (turns) the rotor blades out of the wind. In the other hand, if the output power is lower than the rated power the blades are turned back into the wind, in order to harvest the maximum energy. the

Besides, the sea has huge spaces to place wind turbines, thus it is possible to install much larger wind farms than in land. The Arklow Bank wind farm has plans to expand its rated power to 520 MW and in Germany and France are proposals to create wind farms with over

Wind Energy 27

**Less roughness:** At sea the roughness is lower than in land. As seen in section 2.1.1, the power coefficient (alpha) is much smaller and wind power potential at the same height (equation (2)) is bigger. Moreover, the wind at sea is less turbulent than on land, as a result, wind turbines located at sea may therefore be expected to have a longer lifetime than land

In the same way, at sea there are not obstacles to disturb the wind. Consequently, it is possible to build wind turbines with smaller towers, only the sum between the maximum

**Easier to transport big structures:** To transport very large turbine components from the place of manufacturing by road to installation sites on land are logistical difficulties. However, the

**Less environmental impact:** Offshore wind farms are too far from the populated areas and they do not have visual impact. Thus they have less noise restrictions than in land, making possible higher speeds for the blade. As a result, it is possible a weight reduction of the blades and mechanical structures, achieving a significant reduction in manufacturing cost. On the other hand, offshore wind farms present the following disadvantages in comparison

**Operation and maintenance more complicated than in land:** It is not easy access to a facility installed many kilometers into the sea. Therefore it's more complicated the ensemble

**Corrosive environment:** At sea the salinity and humidity increases the corrosion rate of

**Bigger investment cost:** The cost of the foundations and the transmission system of these facilities is more expensive than onshore wind farms. So the cost per MW installed offshore

**The energy transmission system to shore:** The electrical facilities to connect the areas with big offshore wind energy potential with the energy consumption areas are not prepared to

**The depth of the seabed:** The cost and construction difficulties for an offshore wind farm

pieces for offshore wind farms are easily transported by special vessels called Jack-ups.

**Operation and maintenance more complicated than in land.** 

1,000 MW.

based turbines.

with onshore wind farms:

 **Corrosive environment. Bigger invest cost.** 

**The depth of the seabed.** 

and maintenance of the facility.

transport huge amount of energy.

increases with the water depth.

**The energy transmission system to shore.** 

is about 2.5 times bigger than the cost of installed MW in land.

Disadvantages:

materials.

height of the expected wave and the rotor radius.

(alpha) other the

#### Stall controlled

The stall controlled wind turbines are regulated by the aerodynamic loss in the blades. The geometry of the rotor blade profile is aerodynamically designed, to create turbulences on the side of the rotor blade which is not facing the wind, at the moment which the wind speed becomes too high. In this way, it is possible to waste the excess energy in the wind.

#### Control using ailerons (flaps)

Some older wind turbines use ailerons (flaps) to control the power of the rotor, just like aircraft use flaps to alter the geometry of the wings. But mechanical stress caused by the use of these flaps can damage the structure. Therefore this kind of control only is used in low power generators.

#### **2.6 Offshore wind energy vs onshore wind energy**

Offshore wind energy in comparison with onshore wind energy has the following advantages / disadvantages [20], [21]:

Advantages:


**Bigger resource:** Winds are typically stronger at sea than on land. In the European wind atlas is clearly shown how the wind resource is more abundant in the sea Figure 2.17.

Figure 2.17 The average wind speed in Europe, in land (a) and offshore (b) [22].

The stall controlled wind turbines are regulated by the aerodynamic loss in the blades. The geometry of the rotor blade profile is aerodynamically designed, to create turbulences on the side of the rotor blade which is not facing the wind, at the moment which the wind speed

Some older wind turbines use ailerons (flaps) to control the power of the rotor, just like aircraft use flaps to alter the geometry of the wings. But mechanical stress caused by the use of these flaps can damage the structure. Therefore this kind of control only is used in low

Offshore wind energy in comparison with onshore wind energy has the following

**Bigger resource:** Winds are typically stronger at sea than on land. In the European wind atlas is clearly shown how the wind resource is more abundant in the sea Figure 2.17.

European

*(a) (b)* 

Figure 2.17 The average wind speed in Europe, in land (a) and offshore (b) [22].

becomes too high. In this way, it is possible to waste the excess energy in the wind.

Stall controlled

power generators.

Advantages:

Control using ailerons (flaps)

**2.6 Offshore wind energy vs onshore wind energy** 

 **Easier to transport big structures. Less environmental impact.** 

advantages / disadvantages [20], [21]:

 **Bigger resource. Less roughness.**

Besides, the sea has huge spaces to place wind turbines, thus it is possible to install much larger wind farms than in land. The Arklow Bank wind farm has plans to expand its rated power to 520 MW and in Germany and France are proposals to create wind farms with over 1,000 MW.

**Less roughness:** At sea the roughness is lower than in land. As seen in section 2.1.1, the power coefficient (alpha) is much smaller and wind power potential at the same height (equation (2)) is bigger. Moreover, the wind at sea is less turbulent than on land, as a result, wind turbines located at sea may therefore be expected to have a longer lifetime than land based turbines.

In the same way, at sea there are not obstacles to disturb the wind. Consequently, it is possible to build wind turbines with smaller towers, only the sum between the maximum height of the expected wave and the rotor radius.

**Easier to transport big structures:** To transport very large turbine components from the place of manufacturing by road to installation sites on land are logistical difficulties. However, the pieces for offshore wind farms are easily transported by special vessels called Jack-ups.

**Less environmental impact:** Offshore wind farms are too far from the populated areas and they do not have visual impact. Thus they have less noise restrictions than in land, making possible higher speeds for the blade. As a result, it is possible a weight reduction of the blades and mechanical structures, achieving a significant reduction in manufacturing cost. than blade. a transmission expensive is about 2.5 times bigger than the cost of installed MW in land.

On the other hand, offshore wind farms present the following disadvantages in comparison with onshore wind farms:

Disadvantages:


**Operation and maintenance more complicated than in land:** It is not easy access to a facility installed many kilometers into the sea. Therefore it's more complicated the ensemble and maintenance of the facility.

**Corrosive environment:** At sea the salinity and humidity increases the corrosion rate of materials.

**Bigger investment cost:** The cost of the foundations and the transmission system of these facilities is more expensive than onshore wind farms. So the cost per MW installed offshore is about 2.5 times bigger than the cost of installed MW in land.

**The energy transmission system to shore:** The electrical facilities to connect the areas with big offshore wind energy potential with the energy consumption areas are not prepared to transport huge amount of energy.

**The depth of the seabed:** The cost and construction difficulties for an offshore wind farm increases with the water depth.

**Chapter 3** 

**Offshore Wind Farms** 

In this chapter an overview of the current technology of the offshore wind farms is performed. This survey is focused into the two main parts of the offshore wind farms electric connection infrastructure: the energy collector system (the inter-turbine medium voltage grid) and the energy transmission system, which are separately evaluated in the

Firstly, the AC and DC transmission options to carry the energy from the offshore wind farm to the main grid are described and then, a discussion about the advantages /disadvantages of those AC and DC transmission options is performed. The discussion about the best transmission option is based on the rated power of the wind farms and their

As for the energy transmission system, for the energy collector system of the wind farm, the different configuration options are described. However, for the energy collector grid only

In this way, the spatial disposition of the wind turbines inside the inter-turbine grid, the cable length between two wind turbines or the redundant connections of the inter-turbine

The fast growth of the onshore wind power in Europe, a small and populated area, has led to a situation where the best places to build a wind farm onshore are already in use. However, in the sea, there is not a space constraint and it is possible to continue installing

The first country to install an offshore wind farm was Denmark in 1991. In the same decade, Netherlands also installed some wind farms very close to shore. So, offshore wind farms are

At the early 90s they were very little 6 MW of average rated power, built in very low water

However, after this first steps, offshore wind farms are being installed in deeper and deeper waters. Thus, at the end of the 2000s the average water depth of new wind farms multiplied

present chapter.

distance to shore.

grid are analyzed.

wind power capacity.

by three, see Figure 3.1.

a recently developed technology.

depths and with small wind turbines.

AC configurations are taken into account.

**3.1 Historic overview of offshore wind farms** 

#### **2.7 Chapter conclusions**

Offshore wind presents great advantages to develop wind energy, due to the fact that it has a high potential that today still remains largely untapped. However, the opportunities for advancing offshore wind technologies are accompanied by significant challenges, such as: the exposure of the components to more extreme open ocean conditions, the long distance electrical transmission systems on high-voltage submarine cables or turbine maintenance at sea.

Despite of those technological challenges, also have significant advantages. Turbine blades can be much larger without land-based transportation / construction constraints and the blades also are allowed to rotate faster offshore (no noise constraints), so at sea can be installed wind turbines with higher rated powers. Furthermore, the wind at sea is less turbulent than on land.

Thus, the bigger capital costs (twice as high as land-based) can be partially compensated by the higher energy of the wind at sea. In this way, in recent years the average rated power of installed new offshore wind farms has been multiplied by 15. In conclusion, offshore wind is a real opportunity to develop wind energy in the upcoming years.

## **Chapter 3**

### **Offshore Wind Farms**

In this chapter an overview of the current technology of the offshore wind farms is performed. This survey is focused into the two main parts of the offshore wind farms electric connection infrastructure: the energy collector system (the inter-turbine medium voltage grid) and the energy transmission system, which are separately evaluated in the present chapter.

Firstly, the AC and DC transmission options to carry the energy from the offshore wind farm to the main grid are described and then, a discussion about the advantages /disadvantages of those AC and DC transmission options is performed. The discussion about the best transmission option is based on the rated power of the wind farms and their distance to shore.

As for the energy transmission system, for the energy collector system of the wind farm, the different configuration options are described. However, for the energy collector grid only AC configurations are taken into account.

In this way, the spatial disposition of the wind turbines inside the inter-turbine grid, the cable length between two wind turbines or the redundant connections of the inter-turbine grid are analyzed.

#### **3.1 Historic overview of offshore wind farms**

28 Energy Transmission and Grid Integration of AC Offshore Wind Farms

Offshore wind presents great advantages to develop wind energy, due to the fact that it has a high potential that today still remains largely untapped. However, the opportunities for advancing offshore wind technologies are accompanied by significant challenges, such as: the exposure of the components to more extreme open ocean conditions, the long distance electrical transmission systems on high-voltage submarine cables or turbine maintenance at

Despite of those technological challenges, also have significant advantages. Turbine blades can be much larger without land-based transportation / construction constraints and the blades also are allowed to rotate faster offshore (no noise constraints), so at sea can be installed wind turbines with higher rated powers. Furthermore, the wind at sea is less

Thus, the bigger capital costs (twice as high as land-based) can be partially compensated by the higher energy of the wind at sea. In this way, in recent years the average rated power of installed new offshore wind farms has been multiplied by 15. In conclusion, offshore wind is

a real opportunity to develop wind energy in the upcoming years.

**2.7 Chapter conclusions** 

turbulent than on land.

sea.

The fast growth of the onshore wind power in Europe, a small and populated area, has led to a situation where the best places to build a wind farm onshore are already in use. However, in the sea, there is not a space constraint and it is possible to continue installing wind power capacity.

The first country to install an offshore wind farm was Denmark in 1991. In the same decade, Netherlands also installed some wind farms very close to shore. So, offshore wind farms are a recently developed technology.

At the early 90s they were very little 6 MW of average rated power, built in very low water depths and with small wind turbines.

However, after this first steps, offshore wind farms are being installed in deeper and deeper waters. Thus, at the end of the 2000s the average water depth of new wind farms multiplied by three, see Figure 3.1.

Figure 3.3 Evolution of the offshore wind farms average capacity in MW [23].

*Name Country Year* 

Table 3.1 Biggest constructed offshore wind farms in EU.

and

during 2010, see Table 3.1

*0*

*Average capacity (MW)*

*100*

away into the sea (> 45 Km).

As a result, all the biggest offshore wind farms currently in operation are been opened the last few years. Furthermore, four of the five biggest offshore wind farms have been opened

*Offshore wind farms average capacity*

Offshore Wind Farms 31

*Thanet* UK 2010 100 7.75 300 *Horns Rev 2* Denmark 2009 91 30 209.3 *Nysted II/ Rødsand II* Denmark 2010 90 23 207 *Robin Rigg* UK 2010 60 9.5 180 *Gunfleet Sands* UK 2010 48 7 172.8 *Nysted / Rødsand 1* Denmark 2003 72 8 165.6 *Belwind phase 1* Belgium 2010 55 48,5 165 *Horns Rev 1* Denmark 2002 80 14 160 *Prinses Amalia* Netherlands 2008 60 23 120 *Lillgrund* Sweden 2007 48 10 110.4 *Egmondaan Zee* Netherlands 2007 36 10 108 *Inner Dowsing* UK 2008 27 5 97.2 *Lynn* UK 2008 27 5.2 97.2

Despite those examples, today the most of the offshore wind farms have a relatively small capacity (<60 MW) and are located relatively close to shore (less than 20 Km), but as is been listed before, also pretty huge wind farms (100-300MW) have been built al locations far

*Nº of turbines* 

*Length to shore (Km)* *Rated power (MW)* 

*Average offshore wind farm water depth*

Figure 3.1 Evolution of the average offshore wind farms water depth [23].

But, not only is increasing the average water depth of the wind farms. As the developers are gaining experience / technology in this field and there are more constructed examples. The wind farms are being constructed with bigger rated powers and at locations with longer distances to shore. Thus, from 90s to the next decade the average rated power of installed new offshore wind farms has been multiplied by 15, see Figure 3.3. In parallel with the growth of the average wind farms capacity, the average distance to shore of the wind farms increases as well, see Figure 3.2.

*Average offshore wind farms distance to shore*

Figure 3.2 Evolution of the average offshore wind farms distance to shore in km [23].

*Average offshore wind farm water depth*

Figure 3.1 Evolution of the average offshore wind farms water depth [23].

increases as well, see Figure 3.2.

*0*

*5*

*10*

*15*

*Average distance to shore (km)*

*20*

*25*

*30*

*Water depth (m)*

But, not only is increasing the average water depth of the wind farms. As the developers are gaining experience / technology in this field and there are more constructed examples. The wind farms are being constructed with bigger rated powers and at locations with longer distances to shore. Thus, from 90s to the next decade the average rated power of installed new offshore wind farms has been multiplied by 15, see Figure 3.3. In parallel with the growth of the average wind farms capacity, the average distance to shore of the wind farms

*Average offshore wind farms distance to shore*

Figure 3.2 Evolution of the average offshore wind farms distance to shore in km [23].

#### *Offshore wind farms average capacity*

Figure 3.3 Evolution of the offshore wind farms average capacity in MW [23].

As a result, all the biggest offshore wind farms currently in operation are been opened the last few years. Furthermore, four of the five biggest offshore wind farms have been opened during 2010, see Table 3.1


**3.2 Offshore wind farms energy transmission system** 

submarine cables for the energy transmission.

depending on its shunt capacitance (see section 4.3).

The frequency of the applied voltage.

to transform the evacuated energy are not necessary.

wind turbines and to evacuate the generated power.

The applied voltage.

field.

**3.2.1 AC Configurations** 

AC) transmission systems.

power and charging currents depending on three factors:

Thus, the main variable which can be changed is the frequency.

The length (magnitude of the shunt capacitive component).

The key difference between onshore wind farms and offshore wind farms is the different environment of their locations. As a result, offshore wind farms must be provided with

Offshore Wind Farms 33

On the contrary of overhead power cables, subsea power cables have a high capacitive shunt component due to their structure [25]. When a voltage is applied onto a shunt capacitance, capacitive charging currents are generated. These charging currents increase the overall current of the cable reducing the power transfer capability of the cable (which is thermally limited). Therefore, the power transfer capability for a specific cable decreases

Like the capacitors, the shunt capacitive component of the cables generates more reactive

The length is determined by the location of the wind farm and it cannot have big changes. The transmission voltage is directly related to the current and the wind farms rated power.

Consequently, there are two different types of transmission system configurations: AC and DC. In DC (zero frequency) there are not charging reactive currents, but the energy

Therefore, the main drawback of the DC configurations is that they need to transform the energy from AC to DC and vice versa. So, until now, any offshore wind farm with HVDC transmission system has been build. However, a lot of studies are been conducted in this

AC cable systems are a well understood, mature technology. For this reason, all the built wind farms up to now have an AC transmission system to connect the wind turbines to the distribution grid. The distribution grid and the generators are AC, thus, DC/AC converters

With regards to different types of AC configurations, the different options are divided into two families: HVAC (High Voltage AC) transmission systems and MVAC (Medium Voltage

HVAC transmission system has a local medium voltage wind farm grid (20-30kV) connected to a transformer and a high voltage transmission system. Thus the transmission system requires an offshore platform for the step-up transformer. On the contrary, in MVAC configurations the local medium voltage wind farm grid is used both for connecting all

distribution, the energy consumption and the energy generation is AC voltage.

The new offshore wind farms and offshore wind farm projects are bigger and bigger and at longer distances. As can be seen in Figure 3.4 based on the characteristics of built offshore wind farms which summarize the previous Figure 3.2 and Figure 3.3.

### *Constructed offshore wind farms*

Figure 3.4 Constructed offshore wind farms, rated power (size of the bubbles) depending on the distance to shore and opening year. Offshore wind farms opened in 90s (Red bubbles) and offshore wind farms opened in 2000s (blue bubbles).

Furthermore, according to [24] this trend will continue or will increase in upcoming years, as shown in Table 3.2.


Table 3.2 Evolution of the offshore wind energy and future trend.

The new offshore wind farms and offshore wind farm projects are bigger and bigger and at longer distances. As can be seen in Figure 3.4 based on the characteristics of built offshore

*Constructed offshore wind farms*

Figure 3.4 Constructed offshore wind farms, rated power (size of the bubbles) depending on the distance to shore and opening year. Offshore wind farms opened in 90s (Red bubbles)

*0 10 20 30 40 50 60*

*Distance to shore (km)*

Furthermore, according to [24] this trend will continue or will increase in upcoming years,

*Countries with offshore wind* 3 7 20+ *Average wind farm size* 6 MW 90 MW >500 MW *Average yearly installed capacity* 3 MW 230 MW 6000 MW *Average turbine size* < 0.5 MW 3 MW 5-6 MW *Average rotor diameter* 37 m 98 m 125-130 m *Average water depth* 5 m 15 m >30 m

energy

*90s 2000s 2010-2030* 

and offshore wind farms opened in 2000s (blue bubbles).

Table 3.2 Evolution of the offshore wind energy and future trend.

as shown in Table 3.2.

*wind* 

*Opening year*

wind farms which summarize the previous Figure 3.2 and Figure 3.3.

#### **3.2 Offshore wind farms energy transmission system**

The key difference between onshore wind farms and offshore wind farms is the different environment of their locations. As a result, offshore wind farms must be provided with submarine cables for the energy transmission.

On the contrary of overhead power cables, subsea power cables have a high capacitive shunt component due to their structure [25]. When a voltage is applied onto a shunt capacitance, capacitive charging currents are generated. These charging currents increase the overall current of the cable reducing the power transfer capability of the cable (which is thermally limited). Therefore, the power transfer capability for a specific cable decreases depending on its shunt capacitance (see section 4.3).

Like the capacitors, the shunt capacitive component of the cables generates more reactive power and charging currents depending on three factors:


The length is determined by the location of the wind farm and it cannot have big changes. The transmission voltage is directly related to the current and the wind farms rated power. Thus, the main variable which can be changed is the frequency.

Consequently, there are two different types of transmission system configurations: AC and DC. In DC (zero frequency) there are not charging reactive currents, but the energy distribution, the energy consumption and the energy generation is AC voltage.

Therefore, the main drawback of the DC configurations is that they need to transform the energy from AC to DC and vice versa. So, until now, any offshore wind farm with HVDC transmission system has been build. However, a lot of studies are been conducted in this field.

#### **3.2.1 AC Configurations**

AC cable systems are a well understood, mature technology. For this reason, all the built wind farms up to now have an AC transmission system to connect the wind turbines to the distribution grid. The distribution grid and the generators are AC, thus, DC/AC converters to transform the evacuated energy are not necessary.

With regards to different types of AC configurations, the different options are divided into two families: HVAC (High Voltage AC) transmission systems and MVAC (Medium Voltage AC) transmission systems.

HVAC transmission system has a local medium voltage wind farm grid (20-30kV) connected to a transformer and a high voltage transmission system. Thus the transmission system requires an offshore platform for the step-up transformer. On the contrary, in MVAC configurations the local medium voltage wind farm grid is used both for connecting all wind turbines and to evacuate the generated power.

In the collecting point the voltage is increased to the required level in the transmission system (Barrow 132kV, Nysted 132kV, Horns Rev 150kV). The energy is then transmitted from the wind farm to the grid interface (substation) over the transmission system. The substation adapts the voltage, frequency and the reactive power of the transmission system to the voltage level, frequency and reactive power required by the main grid (in the PCC) in

Offshore Wind Farms 35

This configuration only has one electric three-phase connection to shore, consequently, if

In this configuration the wind farm is divided into smaller clusters and each cluster is connected by its own three-phase cable to shore. This connection is made in medium voltage, at the same voltage level of the wind farms local inter-turbine grid 24-36kV. Therefore, due to the fact that the voltage level of the transmission system and the interturbine grid is the same, this electrical configuration avoids the offshore platform (where is

MVAC electrical configurations are used by small wind farms located near to the shore. For example: Middelgrunden (30kV-40MW) at 3Km to shore, Scroby Sands (33kV-60MW)

The lay-out of the grid connection scheme for an MVAC transmission system is shown in

MVAC configuration avoids offshore substantiation and its associated cost, but the energy transmission is through more than one three-phase connection and with lower voltage level than in HVAC configurations. As a result, the energy transmission needs to be made with

As an example, in equations (9)-(14) are compared MVAC configurations conduction losses with HVAC configurations conduction losses for the same cable resistivity and the same

this does not work, the whole offshore wind farm is disconnected.

located 2.3Km seaward o North Hoyle (33kV -60MW) at 7-8Km offshore.

Figure 3.6 Typical layout of multiple MVAC transmission system.

more current and it is also possible to increase the conduction losses.

order to integrate the energy.

placed the step-up transformer) and its cost.

**3.2.1.2Multiple MVAC** 

Figure 3.6

As discussed before, AC cables have limited their power transfer capability by the length. But this fact does not mean that wind farms power transfer capability must be limited. If one three-phase connection cannot evacuate the rated power to the required length, is possible to use multiple three-phase connections. For example, Kriegers flak offshore wind farms have planned a transmission system with multiple HVAC connections [26].

On the other hand, for medium voltage configurations (according to [27]), the maximum practical conductor size for operation at 33 kV appears to be 300 mm2, giving a cluster rating in the range of 25 to 30 MW. For wind farms with bigger rated powers more threephase connections are used. Wind farm is divided into clusters and each cluster is fed by its own, 3 core, cable from shore. At the same voltage level of the wind farms local inter-turbine grid 24-36kV [28].

Therefore, an AC configuration presents multiple design options depending on the transmission voltage level and the number of three-phase connections. In this work, 3 different AC-systems are investigated: HVAC, MVAC and multiple HVAC.

#### **3.2.1.1High voltage AC transmission HVAC**

The first configuration to be discussed is the HVAC. This lay-out is commonly used by large offshore wind farms such as: Barrow - 90MW, Nysted - 158MW or Horns Rev - 160MW.

To transmit the energy produced in the wind turbines to the point where the electric grid is strong enough to absorb it, the HVAC transmission systems follows roughly the same lines for the grid connection scheme. A typical layout for such a scheme is depicted in Figure 3.5.

As shown, the wind turbines are connected to a medium voltage local inter-turbine grid, where the energy of the wind farm is collected to be transmitted to shore, beyond is the transmission system.

The transmission system is made up by: an offshore substation (step-up transformer), submarine cables, the interface between the wind farm and the point of common coupling (substation) and the main grid.

Figure 3.5 Typical layout of HVAC transmission system.

The inter-turbine network is extended from each wind turbine to the collecting point, and due to the fact that it may have a length of kilometers [ 10 ], is typically medium voltage 20- 36kV ( Horns Rev 24kV, Barrow 33kV or Nysted 34kV).

As discussed before, AC cables have limited their power transfer capability by the length. But this fact does not mean that wind farms power transfer capability must be limited. If one three-phase connection cannot evacuate the rated power to the required length, is possible to use multiple three-phase connections. For example, Kriegers flak offshore wind farms

On the other hand, for medium voltage configurations (according to [27]), the maximum practical conductor size for operation at 33 kV appears to be 300 mm2, giving a cluster rating in the range of 25 to 30 MW. For wind farms with bigger rated powers more threephase connections are used. Wind farm is divided into clusters and each cluster is fed by its own, 3 core, cable from shore. At the same voltage level of the wind farms local inter-turbine

Therefore, an AC configuration presents multiple design options depending on the transmission voltage level and the number of three-phase connections. In this work, 3

The first configuration to be discussed is the HVAC. This lay-out is commonly used by large offshore wind farms such as: Barrow - 90MW, Nysted - 158MW or Horns Rev - 160MW.

To transmit the energy produced in the wind turbines to the point where the electric grid is strong enough to absorb it, the HVAC transmission systems follows roughly the same lines for the grid connection scheme. A typical layout for such a scheme is depicted in Figure 3.5. As shown, the wind turbines are connected to a medium voltage local inter-turbine grid, where the energy of the wind farm is collected to be transmitted to shore, beyond is the

The transmission system is made up by: an offshore substation (step-up transformer), submarine cables, the interface between the wind farm and the point of common coupling

The inter-turbine network is extended from each wind turbine to the collecting point, and due to the fact that it may have a length of kilometers [ 10 ], is typically medium voltage 20-

have planned a transmission system with multiple HVAC connections [26].

different AC-systems are investigated: HVAC, MVAC and multiple HVAC.

**3.2.1.1High voltage AC transmission HVAC** 

Figure 3.5 Typical layout of HVAC transmission system.

36kV ( Horns Rev 24kV, Barrow 33kV or Nysted 34kV).

grid 24-36kV [28].

transmission system.

(substation) and the main grid.

In the collecting point the voltage is increased to the required level in the transmission system (Barrow 132kV, Nysted 132kV, Horns Rev 150kV). The energy is then transmitted from the wind farm to the grid interface (substation) over the transmission system. The substation adapts the voltage, frequency and the reactive power of the transmission system to the voltage level, frequency and reactive power required by the main grid (in the PCC) in order to integrate the energy.

This configuration only has one electric three-phase connection to shore, consequently, if this does not work, the whole offshore wind farm is disconnected.

#### **3.2.1.2Multiple MVAC**

In this configuration the wind farm is divided into smaller clusters and each cluster is connected by its own three-phase cable to shore. This connection is made in medium voltage, at the same voltage level of the wind farms local inter-turbine grid 24-36kV. Therefore, due to the fact that the voltage level of the transmission system and the interturbine grid is the same, this electrical configuration avoids the offshore platform (where is placed the step-up transformer) and its cost.

MVAC electrical configurations are used by small wind farms located near to the shore. For example: Middelgrunden (30kV-40MW) at 3Km to shore, Scroby Sands (33kV-60MW) located 2.3Km seaward o North Hoyle (33kV -60MW) at 7-8Km offshore.

The lay-out of the grid connection scheme for an MVAC transmission system is shown in Figure 3.6

Figure 3.6 Typical layout of multiple MVAC transmission system.

MVAC configuration avoids offshore substantiation and its associated cost, but the energy transmission is through more than one three-phase connection and with lower voltage level than in HVAC configurations. As a result, the energy transmission needs to be made with more current and it is also possible to increase the conduction losses. associated

As an example, in equations (9)-(14) are compared MVAC configurations conduction losses with HVAC configurations conduction losses for the same cable resistivity and the same

2 2

reliability of the wind farm is improved.

configurations is depicted in Figure 3.7.

**3.2.1.3Multiple HVAC** 

under construction.

In this type of configurations with more than one three-phase connections to shore, if a fault occurs in one of those connections, the faulted clusters can remain connected sharing another connection with other cluster, while the faulty cable is repaired. In this way the

Offshore Wind Farms 37

These redundant connection/s are important when around the location of the wind farm is a lot of marine traffic which can damage submarine cables or if the wind farm location has an extreme climate in winter which makes impossible to repair a cable on this year station [29].

This electrical configuration is a combination of the previous two configurations. The wind farm is divided into smaller clusters but these clusters are not connected directly to shore. In this case, each cluster has a collector point and step-up transformer. At this point the voltage is increased to the required level in the transmission system. Typical layout for these

Offshore wind farms with multiple HVAC transmission systems have large rated power, for example Kriegers Flak (640MW) or Robin Rigg (East/west – 180MW). These wind farms are

In this electrical configuration, the wind farm is divided into clusters and the rated power of these clusters is a design option. In the same way, the transmitted power through a threephase connection which depends on the number of clusters can be selected by design also. Moreover, the transmission voltage can be different to the local inter-turbine grids voltage,

Figure 3.7 Typical layout of multiple HVAC transmission system.

making possible multiple and different combinations.

*active reactive III* (17)

cable

transmitted energy (without considering the charging currents due to the capacitive component). The compared cases are 150kV HVAC configuration and a 30kV MVAC configuration with two connections to shore.

$$P = \text{Constant} \; ; \; \; P\_{HVAC} = P \; ; \; \mathcal{D} \cdot P\_{MVAC} = P \; ; \; \; R\_{cable} = \text{Constant} \tag{9}$$

$$\left|V\right|\_{HVAC} = 1\,\text{50} \\ kV\left|\mathcal{V}\_{\text{MVAC}}\right. = \text{30} \\ kV \tag{10}$$

$$\boldsymbol{V}\_{\rm{HVAC}} = \mathfrak{S} \cdot \boldsymbol{V}\_{\rm{MVAC}} \tag{11}$$

$$P = V \cdot I \tag{12}$$

$$\left| I\_{M\text{VAC}} = \left(\frac{\mathfrak{S}}{2}\right) \cdot I\_{M\text{VAC}} \right. \tag{13}$$

$$P\_{\rm loss} = I\_{\rm active}^2 \cdot R \tag{14}$$

$$P\_{loss\\_HVAC} = I\_{HVAC}^2 \cdot R \tag{15}$$

$$P\_{loss\\_MVAC} = 2 \cdot \left( I\_{MVAC}^2 \cdot R \right) = 2 \cdot \left( \left( \frac{\mathfrak{S}}{2} \right)^2 \cdot I\_{HVAC}^2 \cdot R \right) \tag{16}$$

Where: *P* =Rated power of the offshore wind farm, *PHVAC* = Rated power of the HVAC threephase connection, *PMVAC* = Rated power of each MVAC three-phase connection, *VHVAC* = HVAC connections voltage level, *VMVAC* = MVAC connections voltage level, *IHVAC* = The necessary active current to transmit the rated power of the three-phase HVAC connection at the rated transmission voltage, *IMVAC* = The necessary active current to transmit the rated power of the three-phase MVAC connection at the rated transmission voltage, *Ploss* = Conduction active power losses, *Ploss\_HVAC* = Conduction active power losses for HVAC connection and *Ploss\_MVAC* = Conduction active power losses for MVAC connection.

The calculated conduction losses are referring only to the active current, thus to calculate total conduction losses, the conduction losses produced by reactive power (charge/discharge currents) must be taken into account.

MVAC configurations have less voltage (MV) than HVAC configurations (HV), as a result, the reactive power/charging currents generated in submarine cables are lower.

Depending on the transmitted reactive power, the conduction loses increases. Therefore, in cases with big capacitive shunt component of the cable (which depends on the length and cable characteristic) high transmission voltage levels increases significantly reactive currents. Reached such a point where high voltage three-phase connections have more conduction losses than medium voltage three-phase connections

transmitted energy (without considering the charging currents due to the capacitive component). The compared cases are 150kV HVAC configuration and a 30kV MVAC

;Constant 2; ; Constant *P HVAC MVAC RPPPP cable* (9)

*MVAC MVAC <sup>I</sup> <sup>I</sup>* 2 5

*loss HVAC HVAC RIP* <sup>2</sup>

*<sup>P</sup> RI RI loss MVAC MVAC HVAC*

2 \_ 2

(charge/discharge currents) must be taken into account.

conduction losses than medium voltage three-phase connections

 

Where: *P* =Rated power of the offshore wind farm, *PHVAC* = Rated power of the HVAC threephase connection, *PMVAC* = Rated power of each MVAC three-phase connection, *VHVAC* = HVAC connections voltage level, *VMVAC* = MVAC connections voltage level, *IHVAC* = The necessary active current to transmit the rated power of the three-phase HVAC connection at the rated transmission voltage, *IMVAC* = The necessary active current to transmit the rated power of the three-phase MVAC connection at the rated transmission voltage, *Ploss* = Conduction active power losses, *Ploss\_HVAC* = Conduction active power losses for HVAC

connection and *Ploss\_MVAC* = Conduction active power losses for MVAC connection.

the reactive power/charging currents generated in submarine cables are lower.

The calculated conduction losses are referring only to the active current, thus to calculate total conduction losses, the conduction losses produced by reactive power

MVAC configurations have less voltage (MV) than HVAC configurations (HV), as a result,

Depending on the transmitted reactive power, the conduction loses increases. Therefore, in cases with big capacitive shunt component of the cable (which depends on the length and cable characteristic) high transmission voltage levels increases significantly reactive currents. Reached such a point where high voltage three-phase connections have more

*VHVAC VkV MVAC* 30;150 *kV* (10)

*HVAC VV MVAC* 5 (11)

*IVP* (12)

*loss active RIP* <sup>2</sup> (14)

\_ (15)

 

2

2

<sup>5</sup> <sup>2</sup> <sup>2</sup> (16)

 

(13)

configuration with two connections to shore.

$$\left| I \right| = \sqrt{I\_{\text{active}}^2 + I\_{\text{reactive}}^2} \tag{17}$$

In this type of configurations with more than one three-phase connections to shore, if a fault occurs in one of those connections, the faulted clusters can remain connected sharing another connection with other cluster, while the faulty cable is repaired. In this way the reliability of the wind farm is improved.

These redundant connection/s are important when around the location of the wind farm is a lot of marine traffic which can damage submarine cables or if the wind farm location has an extreme climate in winter which makes impossible to repair a cable on this year station [29].

#### **3.2.1.3Multiple HVAC**

This electrical configuration is a combination of the previous two configurations. The wind farm is divided into smaller clusters but these clusters are not connected directly to shore. In this case, each cluster has a collector point and step-up transformer. At this point the voltage is increased to the required level in the transmission system. Typical layout for these configurations is depicted in Figure 3.7.

Offshore wind farms with multiple HVAC transmission systems have large rated power, for example Kriegers Flak (640MW) or Robin Rigg (East/west – 180MW). These wind farms are under construction.

Figure 3.7 Typical layout of multiple HVAC transmission system.

In this electrical configuration, the wind farm is divided into clusters and the rated power of these clusters is a design option. In the same way, the transmitted power through a threephase connection which depends on the number of clusters can be selected by design also. Moreover, the transmission voltage can be different to the local inter-turbine grids voltage, making possible multiple and different combinations.

In addition, a LCC converter needs a minimum reactive power to work (a minimum current), consequently, this type of converters need voltage on both sides (offshore and

Offshore Wind Farms 39

Voltage Source Converter (VSC) solution is comparatively new compared to the LCC solution. As the main advantage, the semiconductors switching is decoupled to the grid voltage. Thus, the VSC solutions are able to supply and absorb reactive power to the system independently and may help to support power system stability. As a result, VSCs are

In VSC configurations the substation requires fewer components to filter, due to this task can be performed by the converter itself. On the negative side of these converters is their

To date, any of the built offshore wind farms have a HVDC transmission system, but a lot of studies are been conducted in this field. Furthermore, a small-scale demonstration of HVDC technology is working successfully in Tjaereborg Enge (Denmark). This wind farm has four

An AC configuration presents a sort of advantages and disadvantages in comparison with DC configurations. For example, with regard for the energy transmission and integration

AC configurations do not need to convert all the transmitted energy, due to the

The components for AC configurations are more standard, consequently the cost of

AC configurations have well proven and reliable technology. All the built offshore

o Long AC cables generate large amounts of reactive power, due to their high

o The associated charging currents to the reactive power reduce the transfer capacity of the cable. This reduction is in proportion to the capacitive shunt component,

o All faults in the main power grid affect the collecting AC offshore grid directly and vice versa. So depending on the grid requirements, system may require fast voltage

DC configurations have several features that make them attractive. For example, in these kind of configurations, there are not charging currents. Consequently, the power transfer capability for long cables is not reduced and the reactive power compensation is not needed.

Besides, a DC configuration needs AC/DC converters at both ends of the line. Thus, the transmission system has the capability to control both the voltage and the power injected to

wind turbines of different types with total rated capacity of 6.5MW.

generators and the main grids have AC voltage.

and reactive power control during fault operation.

the offshore platform is lower.

wind farms until now are AC.

capacitive shunt component.

mainly for the cable length

As a result, they are more suitable for long distances.

the main grid.

into the main grid has the following advantages and disadvantages [30]:

In the other hand, the disadvantages for an AC transmission configuration are:

onshore) to start working.

suitable for systems with low short circuit power.

high cost (higher than LCC converters).

**3.2.3AC vs. DC Configurations** 

In this type of configurations also are more than one three-phase connections to shore. As a result, it is possible to make redundant connections and if a fault occurs in one of those connections, the wind farm does not have to disconnect.

With regards to the drawbacks, this electrical configuration needs several offshore platforms (substations) to increase the voltage to the required level by the transmission system. Another option is place several transformers in the same platform, but the size of the platform increases. Thus, in both cases the cost of the platform increases.

#### **3.2.2DC Configurations**

DC configurations do not generate charging currents or reactive power due to the capacitive shunt component of the submarine cable. This is a huge advantage in comparison with AC configurations, but they also have a big drawback: The distribution grid, the energy consumption and the generators are AC voltages. Therefore these configurations must be provided with AC/DC converters to adequate the voltage to the energy transmission.

Figure 3.8 Generic layout of bipolar HVDC transmission system.

The AC/DC converters technology and the transmission voltage characteristics are the main features of this kind of configurations:

The transmission can be through mono-polar voltage (Using a single cable) with return to earth or bipolar (Using two cables), Figure 3.8. Due to the extra cable the transmission system can evacuate more power to shore and improve the reliability of the system providing redundancy.

With regards to the AC/DC converters technology, these can be LCC (Line commutated converters) based on thyristors or VSC (Voltage source converter) based on switching devices with the capability to control their turn on and off.

Line Commutated Converter (LCC) devices have been installed in many power transmission systems around the world, so it is a mature technology.

In a line commutated converter, it is possible to control the turn on instant of the thyristor, but the turn off cannot be controlled. Therefore, systems based on this technology for the converters are more susceptible to potential AC grid faults than VSC converters.

In this type of configurations also are more than one three-phase connections to shore. As a result, it is possible to make redundant connections and if a fault occurs in one of those

With regards to the drawbacks, this electrical configuration needs several offshore platforms (substations) to increase the voltage to the required level by the transmission system. Another option is place several transformers in the same platform, but the size of the

DC configurations do not generate charging currents or reactive power due to the capacitive shunt component of the submarine cable. This is a huge advantage in comparison with AC configurations, but they also have a big drawback: The distribution grid, the energy consumption and the generators are AC voltages. Therefore these configurations must be provided with AC/DC converters to adequate the voltage to the energy transmission.

The AC/DC converters technology and the transmission voltage characteristics are the main

The transmission can be through mono-polar voltage (Using a single cable) with return to earth or bipolar (Using two cables), Figure 3.8. Due to the extra cable the transmission system can evacuate more power to shore and improve the reliability of the system

With regards to the AC/DC converters technology, these can be LCC (Line commutated converters) based on thyristors or VSC (Voltage source converter) based on switching

Line Commutated Converter (LCC) devices have been installed in many power

In a line commutated converter, it is possible to control the turn on instant of the thyristor, but the turn off cannot be controlled. Therefore, systems based on this technology for the

converters are more susceptible to potential AC grid faults than VSC converters.

connections, the wind farm does not have to disconnect.

**3.2.2DC Configurations** 

platform increases. Thus, in both cases the cost of the platform increases.

Figure 3.8 Generic layout of bipolar HVDC transmission system.

devices with the capability to control their turn on and off.

transmission systems around the world, so it is a mature technology.

features of this kind of configurations:

providing redundancy.

In addition, a LCC converter needs a minimum reactive power to work (a minimum current), consequently, this type of converters need voltage on both sides (offshore and onshore) to start working.

Voltage Source Converter (VSC) solution is comparatively new compared to the LCC solution. As the main advantage, the semiconductors switching is decoupled to the grid voltage. Thus, the VSC solutions are able to supply and absorb reactive power to the system independently and may help to support power system stability. As a result, VSCs are suitable for systems with low short circuit power.

In VSC configurations the substation requires fewer components to filter, due to this task can be performed by the converter itself. On the negative side of these converters is their high cost (higher than LCC converters).

#### **3.2.3AC vs. DC Configurations**

To date, any of the built offshore wind farms have a HVDC transmission system, but a lot of studies are been conducted in this field. Furthermore, a small-scale demonstration of HVDC technology is working successfully in Tjaereborg Enge (Denmark). This wind farm has four wind turbines of different types with total rated capacity of 6.5MW.

An AC configuration presents a sort of advantages and disadvantages in comparison with DC configurations. For example, with regard for the energy transmission and integration into the main grid has the following advantages and disadvantages [30]:


In the other hand, the disadvantages for an AC transmission configuration are:


DC configurations have several features that make them attractive. For example, in these kind of configurations, there are not charging currents. Consequently, the power transfer capability for long cables is not reduced and the reactive power compensation is not needed. As a result, they are more suitable for long distances.

Besides, a DC configuration needs AC/DC converters at both ends of the line. Thus, the transmission system has the capability to control both the voltage and the power injected to the main grid.

**3.2.4The optimum layout depending on rated power and distance** 

powers until 200MW and distances to shore shorter than 100 Km.

the optimum option for big power and long distances.

This report considered these limits flexible.

optimum configuration (striped area).

*MVAC*

another one in the middle of the submarine cable.

*P*

*300 MW*

*200 MW*

Several studies about which transmission configuration is the optimum depending on the distance to shore and rated power are been analyzed in this chapter [33], [34] and [35].

Offshore Wind Farms 41

In [36] the analysis is focused on HVAC and HVDC configurations with pretty high rated powers (until 900MW) and long distances to shore (up to 300 Km). As a conclusion, this survey determine AC technology like the optimum for offshore wind farms with rated

However, the survey does not determine the optimum technology in the range between 200MW-400MW with distances to shore between 100Km-250Km. As says in the report AC technology is losing attractiveness increasing the distance to shore. So DC configurations are

A similar study is presented in [27], which determines the limits of each technology for the transmission system of offshore wind farms as follows: Until 100MW and 100Km to shore the optimum configuration is MVAC, between 100MW-300MW and 100Km-250Km the

Finally in [37], the report places the limits of each technology as follows: The distance limit for using AC transmission configuration is between 80 and 120 km, if the reactive power generated in submarine cables is compensate at both ends. But if the transmission system has a reactive power compensator in the middle of the submarine cable besides the reactive power compensators at the both ends, AC technology may be the optimum until to 180 km.

The summarize of the previous studies is shown in Figure 3.9, in this picture are depicted the areas where each type of transmission is the optimum configuration (white) and the areas where depending on the characteristics of each wind farm can be any of them the

*HVDC*

*20 40 60 80 100 120 140 160 180*

Figure 3.9 Optimum configuration depending on distance to shore and rated power: (1) With reactive compensation at both ends, (2) With reactive compensation at both ends and

*HVAC*

*2*

*HVAC*

*1*

*D (Km)*

optimum is HVAC and for bigger rated powers and longer distances HVDC.

These converters provide also electrical decoupling between the collecting AC offshore grid and the main power grid for faults in the main power grid.

On the other hand, there are several disadvantages associated to this technology, mainly the cost:



Table 3.3 Comparison of AC and DC transmission systems.

These converters provide also electrical decoupling between the collecting AC offshore grid

On the other hand, there are several disadvantages associated to this technology, mainly the cost: These configurations have higher substation cost, both offshore and onshore.

It is possible to increase the injected harmonic level to the main grid, especially

platform. Needs an offshore platform. Needs an offshore platform.

platform. AC

**DC** 

**LCC VSC** 

Switching losses at AC/DC converters.

Not need reactive power compensation.

Electrical decoupling between the collecting AC offshore grid and the main power grid.

There are not charging currents

Power transmission capability in both directions.

Well proven technology but in other applications.

High cost.

The advantages and disadvantages of each transmission configuration can be seen

Switching losses at AC/DC converters.

Not need reactive power compensation.

Electrical decoupling between the collecting AC offshore grid and the main power grid.

There are not charging currents

To change the direction of the power flow need to change the polarity.

Well proven technology but in other applications.

applications.

no-

Needs a minimum reactive power to work. Also in nowind conditions

and the main power grid for faults in the main power grid.

with LCC technology [31], [32].

summarized in Table 3.3

**AC** 

Is possible to avoid offshore

Is possible to avoid switching losses (to avoid converters).

Need reactive power compensation

All faults in the main power grid affect the collecting AC offshore grid directly and vice versa

charging currents reduces the power transfer capacity

Power transmission capability in both directions.

> Mature and reliable technology in offshore applications.

Less cost due to the standard components

Table 3.3 Comparison of AC and DC transmission systems.

and

 Higher overall losses (switching losses in power converters) Limited offshore experience with VSC-transmission systems

#### **3.2.4The optimum layout depending on rated power and distance**

Several studies about which transmission configuration is the optimum depending on the distance to shore and rated power are been analyzed in this chapter [33], [34] and [35].

In [36] the analysis is focused on HVAC and HVDC configurations with pretty high rated powers (until 900MW) and long distances to shore (up to 300 Km). As a conclusion, this survey determine AC technology like the optimum for offshore wind farms with rated powers until 200MW and distances to shore shorter than 100 Km.

However, the survey does not determine the optimum technology in the range between 200MW-400MW with distances to shore between 100Km-250Km. As says in the report AC technology is losing attractiveness increasing the distance to shore. So DC configurations are the optimum option for big power and long distances.

A similar study is presented in [27], which determines the limits of each technology for the transmission system of offshore wind farms as follows: Until 100MW and 100Km to shore the optimum configuration is MVAC, between 100MW-300MW and 100Km-250Km the optimum is HVAC and for bigger rated powers and longer distances HVDC.

Finally in [37], the report places the limits of each technology as follows: The distance limit for using AC transmission configuration is between 80 and 120 km, if the reactive power generated in submarine cables is compensate at both ends. But if the transmission system has a reactive power compensator in the middle of the submarine cable besides the reactive power compensators at the both ends, AC technology may be the optimum until to 180 km. This report considered these limits flexible.

The summarize of the previous studies is shown in Figure 3.9, in this picture are depicted the areas where each type of transmission is the optimum configuration (white) and the areas where depending on the characteristics of each wind farm can be any of them the optimum configuration (striped area).

Figure 3.9 Optimum configuration depending on distance to shore and rated power: (1) With reactive compensation at both ends, (2) With reactive compensation at both ends and another one in the middle of the submarine cable.

The maximum number of wind turbines on each string is determined by the rated power of the generators and the rated power of the submarine cable. The lay-out of this kind of

Offshore Wind Farms 43

collector systems is shown in Figure 3.10.

Figure 3.10 Layout of the radial design for local inter-turbine grid.

additional cable connects the last wind turbine with the collector.

must be able to evacuate all the energy generated in the string.

A single-sided ring design is similar to radial design, but with an extra connection. The

In comparison with radial inter-turbine grid design, the additional cable incorporates a redundant path to improve the reliability of the system. Therefore, this additional cable

The main drawback of this lay-out is the required cable length, longer than in the radial

To justify the use of redundancy in the collecting system, it is considered the energy that will be saved with this redundant cable during its useful life (usually 20 years) and this benefit is confronted with the cost of redundant cable. In this way, In [41] is reported that the most internal power networks of existing offshore wind farms have very little

**3.3.2Single-sided ring design** 

design with its associated cost increase.

redundancy or none at all.

#### **3.3 Offshore wind farms electrical collector system**

The local inter-turbine grid can be AC or DC, although this characteristic does not determinate the transmission systems technology. The transmission system can be made with the same technology or not. To date all the inter-turbine grids are AC and a lot of studies of HVDC transmission systems have AC inter-turbine grids. Thus, in the present book, only AC collector grids are considered.

The design of the wind farms collector system begins with the selection of the inter-turbine cable and inter-turbine grids (collection grid) voltage level. The use of voltage levels above 36kV for the inter-turbine grid becomes uneconomic. Due to the impossibility to accommodate switchgear and transformers in each turbine tower, so 33-36kV is widely used for collection schemes [38].

The number of turbines connected to the same cable and the rated power of these wind turbines with the voltage level determines the inter-turbines cable section. This aspect is not trivial, as cables have to be landed on seabed, and a larger section means a larger bending radius (higher stiffness), with consequent difficult maneuvering of cable posing ships and larger mechanical protection components [39].

With regards to the spatial disposition of the wind turbines inside the inter-turbine grid, most offshore wind farms to date, have had simple geometric boundaries and have adopted a straightforward rectangular or rhomboid grid [38].

The cable length between two wind turbines is determined mainly for the aerodynamic efficiency of each turbine. Due to the length between two wind turbines must be enough to avoid turbulences generated at the turbines around it. According to [40] the length between two wind turbines is in the range of 500-1000 m.

In onshore wind farms, the electrical system configuration is usually decided by the turbine and substation positions, and the site track routes. Offshore, on the contrary, to design the inter-turbine grid is more freedom, and at first sight is not clear how to choose from the wide range of possible options [41].

Nevertheless, for wind farm collector systems employed in existing offshore wind farms are various standard arrangements. In this way, four basic designs are identified [40], [34]:


#### **3.3.1Radial design**

The most straightforward arrangement of the collector system in a wind farm is a radial design. The wind turbines are connected to a single cable feeder within a string. This design is simple to control and also inexpensive because the total cable length is the smallest to connect all the wind turbines with the collecting point [41]

The radial design is not provided by redundant connections. Consequently if a fault occurs in a cable or at the hub, the entire radial string collapses and all of the wind turbines in the string are disconnected.

The local inter-turbine grid can be AC or DC, although this characteristic does not determinate the transmission systems technology. The transmission system can be made with the same technology or not. To date all the inter-turbine grids are AC and a lot of studies of HVDC transmission systems have AC inter-turbine grids. Thus, in the present

The design of the wind farms collector system begins with the selection of the inter-turbine cable and inter-turbine grids (collection grid) voltage level. The use of voltage levels above 36kV for the inter-turbine grid becomes uneconomic. Due to the impossibility to accommodate switchgear and transformers in each turbine tower, so 33-36kV is widely used

use

The number of turbines connected to the same cable and the rated power of these wind turbines with the voltage level determines the inter-turbines cable section. This aspect is not trivial, as cables have to be landed on seabed, and a larger section means a larger bending radius (higher stiffness), with consequent difficult maneuvering of cable posing ships and

With regards to the spatial disposition of the wind turbines inside the inter-turbine grid, most offshore wind farms to date, have had simple geometric boundaries and have adopted

The cable length between two wind turbines is determined mainly for the aerodynamic efficiency of each turbine. Due to the length between two wind turbines must be enough to avoid turbulences generated at the turbines around it. According to [40] the length between

In onshore wind farms, the electrical system configuration is usually decided by the turbine and substation positions, and the site track routes. Offshore, on the contrary, to design the inter-turbine grid is more freedom, and at first sight is not clear how to choose from the

Nevertheless, for wind farm collector systems employed in existing offshore wind farms are various standard arrangements. In this way, four basic designs are identified [40], [34]:

The most straightforward arrangement of the collector system in a wind farm is a radial design. The wind turbines are connected to a single cable feeder within a string. This design is simple to control and also inexpensive because the total cable length is the smallest to

The radial design is not provided by redundant connections. Consequently if a fault occurs in a cable or at the hub, the entire radial string collapses and all of the wind turbines in the

**3.3 Offshore wind farms electrical collector system** 

book, only AC collector grids are considered.

larger mechanical protection components [39].

a straightforward rectangular or rhomboid grid [38].

two wind turbines is in the range of 500-1000 m.

wide range of possible options [41].

 Single side radial design. Double-sided radial design.

connect all the wind turbines with the collecting point [41]

Radial design.

Star design.

**3.3.1Radial design** 

string are disconnected.

for collection schemes [38].

The maximum number of wind turbines on each string is determined by the rated power of the generators and the rated power of the submarine cable. The lay-out of this kind of collector systems is shown in Figure 3.10.

Figure 3.10 Layout of the radial design for local inter-turbine grid.

#### **3.3.2Single-sided ring design**

A single-sided ring design is similar to radial design, but with an extra connection. The additional cable connects the last wind turbine with the collector.

In comparison with radial inter-turbine grid design, the additional cable incorporates a redundant path to improve the reliability of the system. Therefore, this additional cable must be able to evacuate all the energy generated in the string.

The main drawback of this lay-out is the required cable length, longer than in the radial design with its associated cost increase.

To justify the use of redundancy in the collecting system, it is considered the energy that will be saved with this redundant cable during its useful life (usually 20 years) and this benefit is confronted with the cost of redundant cable. In this way, In [41] is reported that the most internal power networks of existing offshore wind farms have very little redundancy or none at all.

Connecting the last wind turbines of each string saves cable length, but in the other side, if a fault occurs, the whole output power of two strings is deviated through the same cable.

Offshore Wind Farms 45

The star design has a large number of connections, because each turbine is connected directly to the collector point in the center. So this design provides a high level security for the wind farm as a whole. If one cable have a fault, only affects to one turbine (the turbine

The additional expense in longer subsea cables is compensated at least in part with less cable sections required by this design. Due to the fact that trough the inter-turbine cable is only transmitted the energy generated by one wind turbine. So the biggest cost implication of this arrangement is the more complex switchgear requirement at the wind turbine in the

biggest

Thus, the inter-turbine cable has to be sized for that purpose.

Figure 3.13 Layout of the star design for local inter-turbine grid.

**3.3.4Star design** 

centre of the star.

connected trough this cable).

Figure 3.11 Layout of the single sided ring design for local inter-turbine grid.

#### **3.3.3Double-sided ring design**

A double-sided design is similar to single-sided ring design but in this case the extra connection is between the last wind turbines of two strings, as is illustrated in Figure 3.12

Figure 3.12 Layout of the double sided ring design for local inter-turbine grid.

Figure 3.11 Layout of the single sided ring design for local inter-turbine grid.

*Submarine cable*

Figure 3.12 Layout of the double sided ring design for local inter-turbine grid.

A double-sided design is similar to single-sided ring design but in this case the extra connection is between the last wind turbines of two strings, as is illustrated in Figure 3.12

> *Offshore substation*

**3.3.3Double-sided ring design** 

*GRID*

Connecting the last wind turbines of each string saves cable length, but in the other side, if a fault occurs, the whole output power of two strings is deviated through the same cable. Thus, the inter-turbine cable has to be sized for that purpose.

#### **3.3.4Star design**

The star design has a large number of connections, because each turbine is connected directly to the collector point in the center. So this design provides a high level security for the wind farm as a whole. If one cable have a fault, only affects to one turbine (the turbine connected trough this cable). connected

The additional expense in longer subsea cables is compensated at least in part with less cable sections required by this design. Due to the fact that trough the inter-turbine cable is only transmitted the energy generated by one wind turbine. So the biggest cost implication of this arrangement is the more complex switchgear requirement at the wind turbine in the centre of the star.

Figure 3.13 Layout of the star design for local inter-turbine grid.

**Chapter 4** 

**Power AC Transmission Lines** 

The submarine power AC cables have an important role in offshore wind energy. Furthermore, the submarine cables are the main difference between the offshore wind farms

Therefore, a proper submarine cable model is crucial to perform accurate evaluations of the offshore wind farms collector and transmission systems. So, in the present chapter the different options to model a submarine cable are evaluated and their accuracy is discussed. Then based on an accurate and validated submarine cable model, an analysis about the reactive power management in submarine power transmission lines is carried out. Thus, taken into account active power losses, the reactive power generated in the transmission system and the voltage drop for three different reactive power management options, a

The purpose of a power cable is to carry electricity safely from the power source to different loads. In order to accomplish this goal, the cable is made up with some components or parts.

Conductor: The conductor is referred to the part or parts of the cable which carry the electric power. Electric cables can be made up by one conductor (mono-phase cables), three (three-

transmission system and onshore wind farms transmission system.

reactive power compensation option is proposed.

Figure 4.1 Generic representation of a electric power cable.

Figure 4.1 shows a description of the cable components, which are:

**4.1 Basic components of electric power cables** 

phase cables), four, etc.

#### **3.4 Chapter conclusions**

Due to the fact that the energy distribution grid, the energy consumption and the energy generation are AC voltages, all the built offshore wind farms to date have AC transmission system. So, AC alternative is well proven and feasible.

AC and DC configurations have their advantages and disadvantages. All the considered studies in this chapter are agreed that for long-distances to shore DC option would be the optimum if not the only viable. But at present DC transmission options are proposals to adapt the technology to the offshore environment.

The analyzed studies are not agreed about the limits on distance and the rated power for the optimal transmission option (AC or DC). These studies neither are agreed about within each family which transmission configuration (HVAC, MVAC or Multiple HVAC) is the optimum.

In general, the studies are agreed in rough lines. Highlighting that for short distances to shore and low rated power the optimum option is AC and for long distances to shore and big rated power DC. Consequently, depending on the rated power of the wind farm and its features any transmission option (AC or DC) can be the optimum.

Therefore, it is necessary a more detailed analysis, which takes into account features of specific cases in order to define the optimum transmission option and configuration for each offshore wind farm.
