# Wind Turbine Earth

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CHAPTER THREE

3.1 Introduction

As the installations of the wind turbines have been spread around the world, the lightning strikes to a wind turbine have become a major problem. Due to their open air structure and very tall. However, the wind turbines become the fastest growing electrical energy source with an average growth rate of 28% since 1990. By the end of the year 2006 the wind turbine generating capacity installed in the world was more than 74GW [1]. Within the same period 1990 -2006 the typical size of the wind turbine has increased in capacity from 300kW to more than 2MW and from 50 m to 120 m in total height. As a consequence of these developments, there is an increase in fault current associated with windfarms and an increase in the probability of a lightning strike a wind turbine. Therefore, the protection of human beings and livestock becomes more important due to the trend towards large scale high capacity wind farms. A brief review of the available literature relating to the performance of earth electrodes including the wind turbine earthing system under both power frequency faults and transient conditions have been carried out in chapter 2. The earthing system behaviour under power frequencies current injection is well understood [ ]. The previous work on characterisation of earth electrodes under high magnitude transient current was concentrated on a simple earth electrode []. However, few researchers [ ] have studied the performance of wind turbine earthing system subjected to high frequency faults. However, windfarm earthing systems have different features and requirements, compared to earthing system of conventional electrical installations such as a substation. Due to the physical layout of turbines, the earthing systems comprise concentrated electrode systems at the individual turbines which are interconnected and often extend over several kilometres. Often, due to their location on high rocky terrain, the underlying soil resistivity is high and the turbine may be subjected to frequent lightning strikes. As a result high transient potentials may be developed on the earthing system and it is important the earthing system to be designed to dissipate high magnitude transient currents safely to ground. In this chapter an investigation is carried out using simulation techniques to calculate the wind turbine earth impedance for both power frequency and transient conditions. The earthing system models are developed to include representation of the above - ground structure of wind turbine.

3.2 Wind turbine earthing system modelling

The literature clearly identified the difficulties of the measurement and the test procedures of the earthing system of the wind turbines and the extended earth electrode of the windfarm. Different earthing system arrangements representing wind turbine earthing system were adopted by different investigators [] which include octagonal, rectangular and concentric rings surrounded by a rectangular as can be seen in Figure 1. All models presented in the literature are considered the under ground part of the wind turbine earthing system. The above ground structure (steel tower) was not considered in all previous simulation studies. Due to the height of the wind turbine steel tower may contain inductive element which may be has an effect when modelling the wind turbine earthing system.

However, an improved model was developed in order to investigate the effect of the steel tower on the wind turbine earthing system. A comprehensive simulations were carried out on both models which include the above structure and not included, a comparison between these models are conducted for frequencies ranged from 0 to 3MHz.

Wind turbine foundation

In general, the wind turbine foundation design is based on the weight and configuration of the proposed turbine. However, wind turbine foundation depends on the size of the wind turbine and the type of the ground to be mounted in. In the previous studies the size of the turbines investigated between 400kW to 1.5 MW and their foundation radius ranged between 4m to 6.5 m. In this study full details of 1.5MW wind turbine with tower and foundation was modelled based on an actual working wind turbine. Full details of the foundation are shown in Figure 2 [9].

a) Octagonal b) Rectangular c) Rectangular

d) Rectangular e) Concentric rings

Figure 1 different earthing configuration models for wind turbine.

a) Wind turbine layout and dimensions

b) Wind turbine Concrete foundation

Figure 2 foundation of 1.5MW wind turbine

3.3 Computer model

In order to calculate the wind turbine impedance and impedance phase angle, a wind turbine model is constructed, based on an approximation of the physical wind turbine dimension. The model consists of reinforced concrete has an outer dimension 15mX15mX2.65m as shown in Figure 2. The tower was 60m in height and bonded to the reinforcement steel and to the outer ring. The models investigated are shown in figure 3. As can be seen a model without tower, and a model with tower. A ring may be placed around each model. The investigation is carried out in order to investigate the importance of the tower in accurate modelling particularly in the high frequency current injection. As the previous simulation studies did not consider the steel tower in their transient studies. Also the foundation contains reinforcement embedded steel. However, in this model the embedded steel net and the steel tower are modelled as a pipe constructed from copper conductors as can be seen in Figure3.

Injection point

Injection point

b) Wind turbine base without tower with ring electrode around the base

a) Wind turbine base without tower

Injection point

c) Wind turbine base with tower and ring electrode around the base

Figure 3 Wind turbines models

3.4 Simulations

In order to calculate the impedance, impedance phase angle and scalar potential around the base of the wind turbine an injection of 1 A with wide range of frequency to the top of the tower. Also, the same injection to the models without the tower. The soil resistivity considered for this study is 400Wm the soil resistivity was chosen slightly high due to wind farm sites have high soil resistivity.

3.5 Results

The impedance magnitude shown in Figure 4 indicates that each curve has lower frequency range over which the impedance magnitude is almost constant for all models with and without above ground structure. Therefore, the wind turbine earthing system for low frequencies can be considered resistive without significant error for all models. A higher frequencies range where an inductive and capacitive effect become evident. The impedance magnitude increases suddenly above particular frequency higher impedance magnitude for models with above ground structure. The results clearly indicate that above ground structure has a significant inductive effect in which the values remarkably higher compared to the models that not considered the above ground structure. Moreover, the benefit of placing ring electrode around the tower base in reducing the resistance for low frequency to the value recommended by the standards [2-3]. In the other hand the models were used for representing the tower the results show same values obtained for the tow types of the models the pipe model and the conductor model. For these models the impedance increases to higher values up to 1.5 MHz then start to decrease due to the capacitive effect at this frequency then become inductive and increased.

With tower

Without tower

Figure 4 impedance vs. frequency of one turbine injected with 1A from the top

Angle vs. frequency

-90

-70

-50

-30

-10

10

30

50

70

90

1

10

100

1000

10000

100000

1000000

10000000

Frequency Hz

Phase angle degree

With tower

Without tower

Figure 6 wind turbine impedance phase angle vs. frequency

Figure 6 shows the impedance phase angle vs. frequency, the impedance phase angle for models shown in figure 4 a) and b) show increase with the frequency due to the inductive effects. However, for the models shown in figure 4 c) and d) were the tower is considered the phase angle increases with frequency up to 500 kHz then at 1MHz starts to decrease to negative values to reach -83 at 1.5 MHz due to the capacitive effects at this frequency. Then the angle start to oscillate between positive and negative values this may be attributed to the interaction between the inductive and capacitive effects. This may explain the increase and decrease of the impedance magnitude.

3.6 Effect of soil resistivity

Figure 7 shows a comparison of impedance curves obtained at different values of soil resistivities for models representing the above ground structure. The curves show the significant effect of soil on the impedance value at low frequency. As the frequency increased to the difference in the impedance become low until it seems to be equal values for higher frequency. For very high soil resistivity the impedance curves exhibit a different trend constant value up to frequency of 1.2 MHz then start to decrease. This is may be attributed to the high dc resistance of the earthing system and higher capacitance of this resistivity value. It reaches values lower than the impedance of the earthing system for low soil resistivity particularly at high frequency. However, the effect of the ring electrode in reducing the impedance value is clearly shown in low frequency depending on the soil resistivity value. For high frequency the effect of the ring is not pronounced.

The impedance angle shown in Figure 8 the curves clearly show that the impedance angle could be divided into three regions. Firstly low frequency region where the angle magnitude starts with very low values, this can be attributed to the resistive behaviour of the wind turbine earthing system at low frequency for all soil resistivity considered. Secondly the phase angle starts to increase at different frequencies depends on the soil resistivity values the low resistivity starts to increase before the higher soil resistivity. For very high soil resistivity (10kWm) the earthing system continuo behaving resistively. The third region for (100Wm to 1000Wm) starts to decrease at 600 kHz to reach negative values at frequency 1.2MHz. Then the angle starts to oscillate between positive and negative values. This could be explained by the interaction between inductive and capacitive effects. At higher frequency higher soil resistivity will behave in capacitive manner and the angle become negative.

100Wm soil resistivity

400Wm soil resistivity

1000Wm soil resistivity

10000Wm soil resistivity

Figure 7 Comparison of computed impedance value for different values of soil resistivity.

With tower

Without tower

1000Wm soil resistivity

10000Wm soil resistivity

400Wm soil resistivity

Figure 8 Comparison of computed impedance phase angle different values of soil resistivity.

3.7 Surface potential

The surface potentials distributed at the surface of the soil were calculated along profile around the wind turbine foundation. Figures 9 to 11 show selected results of the potential distribution at the surface of the soil at different frequencies for all considered models the potential is higher at the injection point in all cases. As can be seen in Figure 8 a comparison of dc potential around the wind turbine base the is the same for all models except the models equipped with ring electrode have lower potential magnitude. Therefore for low frequencies the models are producing the same surface potential and the same value of impedance. The results clearly show the benefit of placing ring electrode around the wind turbine base in reducing the potential and impedance in considered frequencies. However, the potential increases significantly for higher frequencies for the models with above ground structure compared with models without above ground structure. Moreover, the ring slightly reduced the surface potential at higher frequencies as can be seen in Figure 10. This increase can be attributed to the inductive effect of the tower and the foundation. This explains why the models with tower have higher impedance and potential.

0

2

4

6

8

10

12

0

10

20

30

40

50

Distance of origin (m)

Potential (V)

No Tower no ring

No Tower with ring

Tower without ring

Tower with ring

Figure 8 Comparison of potential surface of different models (DC)

0

2

4

6

8

10

12

14

0

10

20

30

40

50

Distance of origin of profile (m)

Potential (V)

Tower and ring

Tower no ring

No tower no ring

Figure 9 Comparison of potential surface of different models (100 kHz)

0

20

40

60

80

100

120

0

10

20

30

40

50

Distance from origin of profile

Surface Potential V

No T no ring

No Tower with ring

Tower no ring

Tower with ring

Figure 10 Comparison of potential surface of different models (1MHz)

3.8 Effect of downleads in improving the performance of the earthing system

A 10m and 100m conductors buried at depth of 0.5m injected at the end with a 1A current with wide range of frequencies and soil resistivities of 100Wm and 1kWm. However, in order to investigate the effect of the downleads conductors on the earthing system a conductor above ground and have a several downleads connecting it to the buried conductor. To analyse the effect of the leads a number of downleads are connected as shown in the figure 11.

The results are summarised in figure 12 a and 12 b for low soil resistivity 100Wm and high soil media 1kWm. it is clear form figure 12 that as increasing the downleads results in a reduction of earth impedance magnitude, depending on soil resistivity, frequency and the number of downleads. For low soil resistivity the percentage of reduction is low for frequency up to 1MHz and after that the reduction in impedance magnitude increased gradually. For high soil resistivity the reduction can be seen in low frequency higher that in the high frequency range. The reduction can be explained by the reduction in the inductance by the short lengths connected the above ground conductor and buried conductor. In conclusion the downleads enhancing the earthing system in reduction of the impedance magnitude, hence this reduction will result in potential rise mitigation.

0.5m

0.2m

Soil

10 m conductor with 10m conductor above ground

Injection lead

Figure 11; 10m conductor with 10m conductor above ground with 10 down leads

100Wm soil

1000Wm soil

a) 10m conductor with different number of down leads

100Wm soil

1000Wm soil

b) a) 100m conductor with different number of down leads

Figure 12: effect of above ground conductor in improving an earthing system performance