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Wind farm grounding system design for transient currents
Renewable Energy 36 (2011) 2004e2010
Contents lists available at ScienceDirect
Renewable Energy journal homepage: www.elsevier.com/locate/renene
Wind farm grounding system design for transient currents Olatz Ukar a, *, Inmaculada Zamora b,1 a b
Deusto Institute of Technology, DeustoTech Energy, University of Deusto, Avda. Universidades 24, 48007 Bilbao, Spain Dept. of Electrical Engineering of the Univ. of the Basque Country (UPV/EHU), ETSI of Bilbao, Alda. Urquijo sn, 48013 Bilbao, Spain
a r t i c l e i n f o
a b s t r a c t
Article history: Received 9 September 2010 Accepted 30 December 2010 Available online 1 February 2011
Wind turbines are often very high structures that are usually installed in high keraunic level areas. The keraunic level is the number of storm days per year. Therefore, wind farms are very vulnerable to lightning discharge. The damage due to a lightning strike can be reduced if the high current is quickly conducted to the ground. To date, wind turbine grounding system designs have been based on prior experience, without accurately studying transient grounding system behavior. In this work, typical wind farm grounding system geometries are analyzed in the context of lightning strikes. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: Renewable energies Lightning Grounding systems Wind farm
1. Introduction Since the early 1990s, the number of wind farm installations has increased, because wind energy is a nonpolluting source of electrical power. Today, more than 74,767 MW of wind power are installed in Europe. Although wind turbines of different sizes and types exist, all of them are in open spaces linked with strong winds, but where they are also exposed to frequent atmospheric discharges. The negative effect of lightning on wind turbines is becoming more significant as their height increases [1,2]. Table 1 details the high power wind turbine models of three major companies. When a lightning strikes a wind turbine, high frequency and high magnitude current passes through the wind turbine before being dissipated through the grounding system to the soil. The electromagnetic fields generated by this type of discharge leads to high currents and voltages that can be dangerous for people in the area and/or can damage the electrical equipment, making the wind turbine unusable. Moreover, lightning cannot be avoided, because its behavior, as well as the place of impact, cannot be predicted. Instead, statistical probabilities and average current magnitudes are used to design safe grounding systems for the turbines. In atmospheric discharge contexts, the grounding system is the most important part of the wind turbine protection system. The better the grounding system dissipates lightning, the smaller the damage will be to the equipment.
* Corresponding author. Tel.: þ34 944139000. E-mail addresses: [email protected] (O. Ukar), [email protected] (I. Zamora). 1 Tel.: þ34 946014063. 0960-1481/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.renene.2010.12.026
There have been limited studies of the transient behavior of wind farm grounding systems. However, there are a few works that indicate the necessity for further study [3e5]. In summary, wind farm grounding systems have been designed based on knowledge of other electrical systems, such as substations or transformer centers, without taking into account the particular characteristics of wind farms. They do not consider grounding system impedance or its configuration, which can modify grounding system behavior. In this paper, an analysis of wind farm grounding systems for dissipating lightning current is presented. The analysis modifies parameters of the grounding system and evaluates the resulting systems in terms of human and equipment safety. 2. Transient grounding system behavior 2.1. Transient current phenomena As described in Ref. [6], most studies of grounding system behavior treat soil as a conductive medium; therefore, they do not consider displacement currents. This assumption is only reasonable for slow currents, as in the case of short circuits. For fast currents, like atmospheric discharge currents, the capacitive current may be on the same order of magnitude as the resistive current, especially in high resistivity soils. Experimental data show that the relationship between the conductive and capacitive current along the soil varies significantly in terms of the typical frequencies of lightning currents. Because there is no numerical expression to describe the relationship between soil parameters and frequency, it is assumed, in a conservative approach, the same values for the parameters as those obtained for low frequency (LF) signals.
O. Ukar, I. Zamora / Renewable Energy 36 (2011) 2004e2010
waves are usually characterized by the time to crest (T1) and the time to 50% of the maximum value (T2) (Fig. 1).
Nomenclature T1 T2 Es Cs
rs r
hs ts
2005
Time to reach the maximum value in s Time to reach half of the maximum value in s Step voltage in V Empirical value Resistivity of surface material in Um Resistivity of the earth beneath the surface material in Um Surface material thickness in m Duration of the shock current in s
When current or voltage waves are applied to a long conductor buried in soil, the corresponding electromagnetic wave propagates along the conductor. This system works like a transmission line in a lossy medium. As the wave propagates, the wave amplitude is attenuated due to the energy loss. Moreover, each frequency component of the wave has a different propagation speed, and consequently each has a different attenuation rate. This attenuation increases with frequency and soil conductivity, in the same manner in which the energy is reduced. In summary, the amplitude of the current and voltage waves that propagate along the electrode are attenuated and distorted (the wave front slope decreases) in the direction of propagation. Due to attenuation, the current dispersed along the electrode exhibits a non-uniform density. The current density (A/m) decreases along the electrode, so the effective length of the electrode is limited. For electrode lengths longer than the effective length the system impedance does not decrease. The bigger the conductivity and frequency are, the shorter the effective length is. This happens because conductivity and frequency parameters are responsible for increases in soil losses and this impacts the attenuation of current and voltage waves that propagate along the electrodes. On the other hand, for a wide range of current magnitudes, soil exhibits linear behavior. This means that the relationship between the applied voltage and the resulting current is constant. Nevertheless, when the magnitude of the applied current is large and if the electrodes are concentrated, the current density in soil near the electrode surface can reach very large values. Therefore, the corresponding electric field in this region can exceed the critical limit. Over this value, ionization takes place and there are electrical discharges in the soil. The critical electric field (ECR) varies from 0.2 to 1.7 MV/m, depending on the resistivity and humidity of the soil. This phenomenon is similar to the Corona Effect, although it is much more irregular due to soil features. For lightning-type currents, it is difficult to understand the transient behavior because, usually, all relevant phenomena happen simultaneously. Each phenomenon influences grounding behavior; however, one effect may dominate, based on excitation current characteristics. This makes it very difficult to correctly identify the cause of a certain behavior. Thus, in order to study transient grounding behavior, it is necessary to develop models that take into account all the factors previously mentioned. The first step involves defining the lightning discharge waveform based on its maximum amplitude and duration. The commonly used
2.2. The grounding system Fault or atmospheric discharge current circulation along the grounding system can lead to potential differences between certain points, for example, between the grounding installation and the surrounding soil or between two grounding system points. Therefore, this grounding system, even if there is a potential difference, has to guarantee:
Human safety Electric and electronic equipment protection Service quality improvement A reference potential
The step voltage and touch voltage are used to determine if a grounding system is safe for people and/or equipment. These facilities will not be dangerous for people if the step voltages are guaranteed to be less than the maximum allowable step voltage, which is defined according to existing standards. Once again, there is no specific criterion for lightning currents. The most often internationally used guide for substation grounding system design is proposed in Ref. [7], which is also used for wind farms grounding system design. However, this guide does not apply for atmospheric discharge current protection. The formulation proposed in Ref. [7] for the maximum allowable step voltage calculation is shown in Eqs. (1) and (2). Expressions (1) and (2) express provide the maximum allowable step voltages for a 50 and 70 kg person:
0:116 ES50 ¼ ð1000 þ 6Cs rs Þ pffiffiffiffi ts
(1)
0:157 ES70 ¼ ð1000 þ 6Cs rs Þ pffiffiffiffi ts
(2)
where Cs is:
Cs ¼ 1
0:09 1 rr s
(3)
2hs þ 0:09
Supposing that the discharge duration is about 0.1 s, the soil resistivity is 1000 Um, and the surface material is the same as the soil, Cs would be 1. Thus, the maximum step voltage for a 50 and 1.4 1.2
Imax
1.0 0.8 0.6
Imax/2 0.4 0.2
Table 1 Wind turbines height.
0 Manufacturer
Model
Power
Height
Vestas Gamesa Enercon
V-112 G-128 E-126
3 MW 4.5 MW 7.5 MW
84e94e119 m 120 m 135 m
T1
Time T2
Fig. 1. 1.2/50 ms shock wave shape.
2006
O. Ukar, I. Zamora / Renewable Energy 36 (2011) 2004e2010
a 70 kg body would be 2.57 and 3.48 kV. These values, compared with the analysis presented in this paper, are very small. Therefore, it is possible to say that the standards establish calculations and criteria for industrial frequency electrical faults, but not for atmospheric discharges. Thus, it is very difficult to specify what the maximum allowable step voltage should be. 2.3. Wind turbines and wind farm grounding systems Wind farms are normally located in high wind incidence places. In general, these locations have low-conductivity soils and are high keraunic level regions, because they are usually elevated areas. On the other hand, wind farms are usually located in places where human access is not forbidden. Atmospheric discharges can produce overvoltage that, besides injuring people, can cause serious failures in the installation. Thus, wind farm grounding systems must be able to dissipate lightning current as fast as possible. According to Ref. [8], a wind turbine grounding system usually has a circular electrode around the foundation, connecting the electrode to the tower base through the foundation. The foundation is also connected directly or through the wind turbine tower to the circular electrode. Thanks to this practice, it is possible to treat the resistivity of concrete as similar to that of the surrounding soil. Nevertheless, the concrete is usually ignored to analyze the worstcase scenario. The grounding mesh geometry varies by manufacturer. The grounding systems used by the main manufacturers can be summarized as circular or square geometries, although some other manufacturers use an octagonal geometry. These grounding system geometries are usually combinations of simple geometries. When the resistivity is not small enough, the manufacturers recommend adding rods. On the other hand, a wind farm consists of several wind turbines, where the wind farm grounding system is the interconnection of each wind turbine grounding system. Usually, a wind farm has a variable number of wind turbines that is around 30 units. They are separated from each other by about three times the rotor diameter, to achieve greater wind incidence and fewer visual impact points. In addition, wind turbines grounding systems are arranged in groups of five units. 3. Mathematical models for transient grounding system analysis Designing a good grounding system, for permanent or temporary installment, is a complex task. There are multiple factors that affect the grounding behavior, including such as the soil parameter variation with the frequency, the field distribution, and the soil propagation effects and ionization phenomenon. The first analysis of the problem was formulated, among others, by Rüdemberg [9] and Sunde [10], some years ago. However, the solution complexity and the mathematical methods involved such as the use of numerical methods for grounding system analysis. It was then that the electrostatic field equations developed by Maxwell were considered and the method of images was applied. Thus, at the beginning of the 1990s, several computational models for transient groundings behavior calculations appeared [11e13]. These models were developed some time ago and, today, allow calculation of all these effects. These models can be classified as [14]: Empirical Approaches, Circuits Theory Approaches, Transmission Lines Approaches, and Electromagnetic Field Approaches. Because the electromagnetic model is the most precise, it was used for the analysis developed and presented in this paper. The first step for solving the electromagnetic model consists of dividing each
Fig. 2. Current distribution in segments.
conductor into short segments and calculating the current distribution along these segments (Fig. 2). After the conductor has been segmented, the contribution of the tangential electric field in the conductor surface is calculated. This electric field is due to the currents that circulate along the segments. Thus, a system of linear equations is obtained where the unknown parameters are the currents in each segment. Once the current distribution along the electrode segments is calculated, it is possible to find the electromagnetic field due the current distribution at any point and the Ground Potential Rise (GPR). In order to analyze how the electromagnetic variables vary, it is necessary to define some points in the soil surface. This process is repeated for all current signal harmonic frequencies to obtain the representation in the frequency domain. Finally, the inverse Fourier transform is used to obtain the signal in the time domain. 4. Transient analysis wind farm grounding systems In order to design a good grounding system, it is necessary to have a transient impedance value that ensures that neither humans nor the electrical and electronic devices experience high voltages and electrical currents. Therefore, the maximum step voltage and the transient impedance have been chosen as representative parameters in the analysis. The transient impedance is obtained from the point of the current injection. In addition, this analysis has been divided in two parts. First, a single wind turbine grounding system is analyzed to establish the optimal parameters in the grounding system design. Second, a wind farm system is analyzed, which considers the interconnection and influence of neighboring wind turbines. There is no consistency in the grounding systems between different manufacturers. Therefore, it has been necessary to establish some reference parameters for comparative analysis. Although each manufacturer uses a different grounding system, all of them have an average diameter of about 11 m.
Soil 2 m rod
2 m rod 1m
11 m
11 m
11 m
11 m
a
b Circular geometry
c
d Square geometry
Fig. 3. Wind turbine grounding system geometries.
O. Ukar, I. Zamora / Renewable Energy 36 (2011) 2004e2010
∅11 m
∅11 m
∅11 m
2007
∅11 m
a
b
11 m
11 m
11 m
11 m
c
d Fig. 4. Interconnection configurations.
Thus, four configurations have been selected to analyze each wind turbine grounding system: a circle (Fig. 3.a), a circle with two rods diametrically opposed (Fig. 3.b), a square (Fig. 3.c), and a square with two rods diametrically opposed (Fig. 3.d). Moreover, to quantify the effect of interconnecting wind turbine grounding systems, the configurations shown in Fig. 4 have also been analyzed. The soil resistivity has a strong effect on grounding impedance behavior. The impedance also depends on the dimensions of the grounding electrodes and their physical distribution. The magnitude of the injected current and the soil resistivity are uncontrollable parameters from a design point of view. Thus, in this analysis, the magnitudes that geometrically characterize the grounding system have been modified. The reference parameters used in the analysis are shown in Table 2. In the following sections, the results of the analysis are presented. 4.1. Influence of grounding system burying depth In order to determine the influence of the depth of the grounding system, five burying depths are analyzed. The two most common depths 0.5 m, the minimum value as determined by standards, and 0.8 m, the depth recommended by some manufacturers, and also 1, 1.2, and 1.5 m depths are analyzed, although they are not typical depths, they are recommended by some manufacturers. As shown in Figs. 5 and 6, the deeper the grounding system is buried, the smaller the impedance and step voltages are. If the grounding system is buried 0.8 m instead of 0.5 m, the step voltage is reduced by about 30%. This step voltage reduction is not so remarkable when the grounding system is buried at greater depths. In these cases, the reduction is about 10%. However, the step voltages for 1, 1.2 and 1.5 depths are very high (they are around 275 and 350 kV). On the other hand, increasing the burying depth increases the grounding installation price. Nevertheless, for burying depths of 0.5 m or 0.8 m, the cost difference is not very significant, while the step voltage difference is significant for all analyzed geometries. Therefore, increasing the depth to 0.8 m is an inexpensive way to increase safety.
Fig. 5. Step voltages at 0.5, 0.8, 1.0, 1.2 and 1.5 m depth.
4.2. Influence of grounding system perimeter length From Figs. 7 and 8 observe that the bigger the diameter or side, the smaller the transient impedance and step voltage. This holds as long as the effective length of the electrode is not exceeded. Thus, when the diameter/side increases up to 11 m the step voltage diminishes around 30% (compared to that of 8 m). The maximum step voltage is 800e600 kV. When the diameter/side is 13 m, the maximum step voltage is approximately 10e15% lower than for the 11 m diameter/side. For a 13 m diameter/side, the maximum step voltage is around 700 kV. When the diameter increases up to 11 m for a circle with rods, the step voltage diminishes by about 25%, and as a result, the step voltage is about 800 kV. When the diameter is 13 m, the step voltage is 16% smaller than for an 11 m circle, and the maximum step voltage is about 630 kV. Therefore, as far as possible, a greater dimension is recommended.
Table 2 Reference parameters. Diameter Depth Electrode section Material conductivity Soil type
er Lightning wave Lightning current
11 m 0.5 m 50 mm2 56 MS/m (Cu) 1000 Um 10 1.2/50 ms 100 kA Fig. 6. Transient impedance at 0.5, 0.8, 1.0, 1.2 and 1.5 m depth.
2008
O. Ukar, I. Zamora / Renewable Energy 36 (2011) 2004e2010
Fig. 7. Transient impedance for 8, 9, 10, 11, 12 and 13 m diameter/side. Fig. 9. Transient impedance for an isolated and two interconnected (at 50 m) wind turbine grounding system.
4.3. Influence of geometry 4.4. Influence of grounding system interconnection Adding rods to a geometry at the same depth reduces the maximum step voltage by about 8%. Although the difference is less significant at greater depths, the maximum step voltage is about 20% smaller for the square grounding mesh for all cases. As the depth increases, the influence of adding rods on the step voltage decreases. Therefore, it is more effective to install square electrodes instead of circular electrodes, because the square geometries without rods provide comparable or smaller step voltage values than the circle with rods. Therefore, to diminish the step voltage, it is more advantageous, from the constructive point of view, to increase the size of the grounding mesh rather than to add rods.
Fig. 8. Step voltages for 8, 9, 10, 11, 12 and 13 m diameter/side.
In Figs. 9 and 10, the transient impedance and step voltage are shown for an isolated wind turbine grounding system and for two interconnected wind turbines grounding systems. The distance between interconnected wind turbines is about 50 m. When the grounding systems of two wind turbines are interconnected, the step voltage diminishes between 20% and 30%. This variation depends on the geometry used. The smaller the step voltage of an insolated wind turbine grounding system, the smaller the reduction. Therefore, the interconnection of grounding systems is highly recommended when poor conditions are expected. Figs.11 and 12, show the transient impedance and step voltage for the interconnection of the configurations shown in Fig. 4. The
Fig. 10. Step voltage for an isolated and two interconnected (at 50 m) wind turbine grounding system.
O. Ukar, I. Zamora / Renewable Energy 36 (2011) 2004e2010
2009
Fig. 13. 11 m side square transient impedance.
Fig. 11. Transient impedance for two interconnected wind turbine grounding systems at 50, 75 and 150 m.
distances of the geometries analyzed are 50, 75, and 100 m. From Figs. 11 and 12, it can be concluded that the distance between the wind turbines grounding meshes that are interconnected has significant influence on the transient impedance and the step voltage.
In addition, the instruments are designed for 50e60 Hz currents. Therefore, the obtained measurements would have questionable validity. Wind farm constructors and operating companies do not provide construction information. Therefore, it would be complicated to get permission to take measures in a wind farm with suitable devices. Thus, to validate the analysis presented in this paper, the methodology proposed by Montaña [15] was chosen. Montaña analyzed the variation of the electrical parameters of soil as a function of frequency. He calculated the potentials in the electrodes, but not at the surface of soil. So, the transient impedance is used for the validation. As shown in Fig. 13, the value of the transient impedance at LF is very similar as is the behavior across frequencies. 5. Conclusions
4.5. Validation In order to correctly validate the method used in this analysis, it would be necessary to have real measurements to compare with the obtained values. At the present time, this option is impractical. To artificially reproduce an atmospheric discharge would require taking equipment from the laboratory to a wind farm. Such experiments would be expensive and would only be a replication of a random physical phenomenon.
Wind farms are relatively modern facilities with very particular characteristics. It is necessary to correctly install the grounding mesh to ensure the safety of a wind farm. It is also necessary to consider the grounding system behavior when an atmospheric discharge occurs. Lightning strikes are natural phenomena that are random and hardly controllable. In fact, it is practically impossible to prevent the discharges. Once a discharge is produced, a large current pulse is generated and must be dissipated by the grounding system. Thus, the prediction of the grounding system behavior is a very important but complex problem. There are four numerical approaches for calculating the transient behavior of grounding systems, and the electromagnetic approach is the most computationally intensive, but also the most precise. Today’s computers are powerful enough to handle the computation required to analyze the behavior of wind farms grounding systems. In this paper, a theoretical study of wind farm grounding systems is presented. This study consists of modifying the geometries and parameters of the wind farm grounding systems and studying their effect on the maximum step voltage and transient grounding impedance. According to the IEEE s80 guide, the maximum step voltages are very high. Therefore, it is necessary to pay careful attention to wind farm grounding system designs. In addition, it would be very useful to have real measurements to design safer wind turbines grounding systems. References
Fig. 12. Step voltages for two interconnected wind turbine grounding systems at 50, 75 and 150 m.
[1] Fox Neil I. A tall tower study of Missouri winds. Renewable Energy; July 2010. [2] O’Gairbhith, Colm. Assessing the viability of high altitude wind Resources in Ireland. HAWP 2009 conference; November 2009. [3] Cotton I, Jenkins N, Haigh S, Hancock M, Hatziargyriou N, Lorentzou M, Jaquemotte P. Lightning protection of wind turbines. EWEC:97; 1997.
2010
O. Ukar, I. Zamora / Renewable Energy 36 (2011) 2004e2010
[4] Hatziargyriou N, Lorentzou M, Cotton I, Jenkins N. Transferred overvoltages by wind farm grounding systems. ICHQP conference; 1998. [5] Lorentzou M, Hatziargyriou N, Effective dimensioning of extended grounding systems for lightning protection. 25th ICLP; 2000. [6] Visacro S. A didactic general approach of grounding behavior for lightning currents. 27th ICLP: Avignon; Sep.2004, pp. 508e13. [7] Guide for safety in AC substation grounding. IEEE Std; 2000. 80e2000. [8] Lorentzou M, Hatziargyriou N, Papadias B. Analysis of wind turbine grounding systems. In: Proc. 10th Mediterranean Electrotechnical Conference (MELECON 2000), vol. 3; 2000. p. 936e9. [9] Rüdemberg R. Grounding principles and practice, part I: fundamental considerations on ground circuits. Electrical Eng Jan. 1945;64(1):1e13. [10] Sunde ED. Earth conduction effects in transmission systems. Copyright Ó by. Bell Telephone Laboratories, Incorporated; 1949. pp. 146e48.
[11] Grcev L, Dawalibi F. An electromagnetic model for transients in grounding system. IEEE Trans Power Deliv 1990;5:1773e81. [12] Grcev L. Computer analysis on transient voltages in large grounding systems. IEEE Trans Power Deliv April 1996;11:815e23. [13] Visacro SF, Portela C. Investigation of grounding systems behavior on the incidence of atmospheric discharges at electrical systems. In: Proc. of 20th ICLP; Sept. 1990. Interlaken, Switzerland. [14] Saraj cev P, Vujevi c S. A review of methods for grounding grid analysis. In: Proceedings of the 17th international conference on software, telecommunications and computer networks; 2009. p. 42e9. Hvar, Croatia. [15] Johny Hernan Montaña Chaparro. Puestas a Tierra. Variación de los Parámetros Eléctricos del Terreno con la Frecuencia y Software para el Cálculo de Potenciales Transitorios (Tesis Doctoral), Universidad Nacional de Colombia: Bogotá; April 2006.
Contents lists available at ScienceDirect
Renewable Energy journal homepage: www.elsevier.com/locate/renene
Wind farm grounding system design for transient currents Olatz Ukar a, *, Inmaculada Zamora b,1 a b
Deusto Institute of Technology, DeustoTech Energy, University of Deusto, Avda. Universidades 24, 48007 Bilbao, Spain Dept. of Electrical Engineering of the Univ. of the Basque Country (UPV/EHU), ETSI of Bilbao, Alda. Urquijo sn, 48013 Bilbao, Spain
a r t i c l e i n f o
a b s t r a c t
Article history: Received 9 September 2010 Accepted 30 December 2010 Available online 1 February 2011
Wind turbines are often very high structures that are usually installed in high keraunic level areas. The keraunic level is the number of storm days per year. Therefore, wind farms are very vulnerable to lightning discharge. The damage due to a lightning strike can be reduced if the high current is quickly conducted to the ground. To date, wind turbine grounding system designs have been based on prior experience, without accurately studying transient grounding system behavior. In this work, typical wind farm grounding system geometries are analyzed in the context of lightning strikes. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: Renewable energies Lightning Grounding systems Wind farm
1. Introduction Since the early 1990s, the number of wind farm installations has increased, because wind energy is a nonpolluting source of electrical power. Today, more than 74,767 MW of wind power are installed in Europe. Although wind turbines of different sizes and types exist, all of them are in open spaces linked with strong winds, but where they are also exposed to frequent atmospheric discharges. The negative effect of lightning on wind turbines is becoming more significant as their height increases [1,2]. Table 1 details the high power wind turbine models of three major companies. When a lightning strikes a wind turbine, high frequency and high magnitude current passes through the wind turbine before being dissipated through the grounding system to the soil. The electromagnetic fields generated by this type of discharge leads to high currents and voltages that can be dangerous for people in the area and/or can damage the electrical equipment, making the wind turbine unusable. Moreover, lightning cannot be avoided, because its behavior, as well as the place of impact, cannot be predicted. Instead, statistical probabilities and average current magnitudes are used to design safe grounding systems for the turbines. In atmospheric discharge contexts, the grounding system is the most important part of the wind turbine protection system. The better the grounding system dissipates lightning, the smaller the damage will be to the equipment.
* Corresponding author. Tel.: þ34 944139000. E-mail addresses: [email protected] (O. Ukar), [email protected] (I. Zamora). 1 Tel.: þ34 946014063. 0960-1481/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.renene.2010.12.026
There have been limited studies of the transient behavior of wind farm grounding systems. However, there are a few works that indicate the necessity for further study [3e5]. In summary, wind farm grounding systems have been designed based on knowledge of other electrical systems, such as substations or transformer centers, without taking into account the particular characteristics of wind farms. They do not consider grounding system impedance or its configuration, which can modify grounding system behavior. In this paper, an analysis of wind farm grounding systems for dissipating lightning current is presented. The analysis modifies parameters of the grounding system and evaluates the resulting systems in terms of human and equipment safety. 2. Transient grounding system behavior 2.1. Transient current phenomena As described in Ref. [6], most studies of grounding system behavior treat soil as a conductive medium; therefore, they do not consider displacement currents. This assumption is only reasonable for slow currents, as in the case of short circuits. For fast currents, like atmospheric discharge currents, the capacitive current may be on the same order of magnitude as the resistive current, especially in high resistivity soils. Experimental data show that the relationship between the conductive and capacitive current along the soil varies significantly in terms of the typical frequencies of lightning currents. Because there is no numerical expression to describe the relationship between soil parameters and frequency, it is assumed, in a conservative approach, the same values for the parameters as those obtained for low frequency (LF) signals.
O. Ukar, I. Zamora / Renewable Energy 36 (2011) 2004e2010
waves are usually characterized by the time to crest (T1) and the time to 50% of the maximum value (T2) (Fig. 1).
Nomenclature T1 T2 Es Cs
rs r
hs ts
2005
Time to reach the maximum value in s Time to reach half of the maximum value in s Step voltage in V Empirical value Resistivity of surface material in Um Resistivity of the earth beneath the surface material in Um Surface material thickness in m Duration of the shock current in s
When current or voltage waves are applied to a long conductor buried in soil, the corresponding electromagnetic wave propagates along the conductor. This system works like a transmission line in a lossy medium. As the wave propagates, the wave amplitude is attenuated due to the energy loss. Moreover, each frequency component of the wave has a different propagation speed, and consequently each has a different attenuation rate. This attenuation increases with frequency and soil conductivity, in the same manner in which the energy is reduced. In summary, the amplitude of the current and voltage waves that propagate along the electrode are attenuated and distorted (the wave front slope decreases) in the direction of propagation. Due to attenuation, the current dispersed along the electrode exhibits a non-uniform density. The current density (A/m) decreases along the electrode, so the effective length of the electrode is limited. For electrode lengths longer than the effective length the system impedance does not decrease. The bigger the conductivity and frequency are, the shorter the effective length is. This happens because conductivity and frequency parameters are responsible for increases in soil losses and this impacts the attenuation of current and voltage waves that propagate along the electrodes. On the other hand, for a wide range of current magnitudes, soil exhibits linear behavior. This means that the relationship between the applied voltage and the resulting current is constant. Nevertheless, when the magnitude of the applied current is large and if the electrodes are concentrated, the current density in soil near the electrode surface can reach very large values. Therefore, the corresponding electric field in this region can exceed the critical limit. Over this value, ionization takes place and there are electrical discharges in the soil. The critical electric field (ECR) varies from 0.2 to 1.7 MV/m, depending on the resistivity and humidity of the soil. This phenomenon is similar to the Corona Effect, although it is much more irregular due to soil features. For lightning-type currents, it is difficult to understand the transient behavior because, usually, all relevant phenomena happen simultaneously. Each phenomenon influences grounding behavior; however, one effect may dominate, based on excitation current characteristics. This makes it very difficult to correctly identify the cause of a certain behavior. Thus, in order to study transient grounding behavior, it is necessary to develop models that take into account all the factors previously mentioned. The first step involves defining the lightning discharge waveform based on its maximum amplitude and duration. The commonly used
2.2. The grounding system Fault or atmospheric discharge current circulation along the grounding system can lead to potential differences between certain points, for example, between the grounding installation and the surrounding soil or between two grounding system points. Therefore, this grounding system, even if there is a potential difference, has to guarantee:
Human safety Electric and electronic equipment protection Service quality improvement A reference potential
The step voltage and touch voltage are used to determine if a grounding system is safe for people and/or equipment. These facilities will not be dangerous for people if the step voltages are guaranteed to be less than the maximum allowable step voltage, which is defined according to existing standards. Once again, there is no specific criterion for lightning currents. The most often internationally used guide for substation grounding system design is proposed in Ref. [7], which is also used for wind farms grounding system design. However, this guide does not apply for atmospheric discharge current protection. The formulation proposed in Ref. [7] for the maximum allowable step voltage calculation is shown in Eqs. (1) and (2). Expressions (1) and (2) express provide the maximum allowable step voltages for a 50 and 70 kg person:
0:116 ES50 ¼ ð1000 þ 6Cs rs Þ pffiffiffiffi ts
(1)
0:157 ES70 ¼ ð1000 þ 6Cs rs Þ pffiffiffiffi ts
(2)
where Cs is:
Cs ¼ 1
0:09 1 rr s
(3)
2hs þ 0:09
Supposing that the discharge duration is about 0.1 s, the soil resistivity is 1000 Um, and the surface material is the same as the soil, Cs would be 1. Thus, the maximum step voltage for a 50 and 1.4 1.2
Imax
1.0 0.8 0.6
Imax/2 0.4 0.2
Table 1 Wind turbines height.
0 Manufacturer
Model
Power
Height
Vestas Gamesa Enercon
V-112 G-128 E-126
3 MW 4.5 MW 7.5 MW
84e94e119 m 120 m 135 m
T1
Time T2
Fig. 1. 1.2/50 ms shock wave shape.
2006
O. Ukar, I. Zamora / Renewable Energy 36 (2011) 2004e2010
a 70 kg body would be 2.57 and 3.48 kV. These values, compared with the analysis presented in this paper, are very small. Therefore, it is possible to say that the standards establish calculations and criteria for industrial frequency electrical faults, but not for atmospheric discharges. Thus, it is very difficult to specify what the maximum allowable step voltage should be. 2.3. Wind turbines and wind farm grounding systems Wind farms are normally located in high wind incidence places. In general, these locations have low-conductivity soils and are high keraunic level regions, because they are usually elevated areas. On the other hand, wind farms are usually located in places where human access is not forbidden. Atmospheric discharges can produce overvoltage that, besides injuring people, can cause serious failures in the installation. Thus, wind farm grounding systems must be able to dissipate lightning current as fast as possible. According to Ref. [8], a wind turbine grounding system usually has a circular electrode around the foundation, connecting the electrode to the tower base through the foundation. The foundation is also connected directly or through the wind turbine tower to the circular electrode. Thanks to this practice, it is possible to treat the resistivity of concrete as similar to that of the surrounding soil. Nevertheless, the concrete is usually ignored to analyze the worstcase scenario. The grounding mesh geometry varies by manufacturer. The grounding systems used by the main manufacturers can be summarized as circular or square geometries, although some other manufacturers use an octagonal geometry. These grounding system geometries are usually combinations of simple geometries. When the resistivity is not small enough, the manufacturers recommend adding rods. On the other hand, a wind farm consists of several wind turbines, where the wind farm grounding system is the interconnection of each wind turbine grounding system. Usually, a wind farm has a variable number of wind turbines that is around 30 units. They are separated from each other by about three times the rotor diameter, to achieve greater wind incidence and fewer visual impact points. In addition, wind turbines grounding systems are arranged in groups of five units. 3. Mathematical models for transient grounding system analysis Designing a good grounding system, for permanent or temporary installment, is a complex task. There are multiple factors that affect the grounding behavior, including such as the soil parameter variation with the frequency, the field distribution, and the soil propagation effects and ionization phenomenon. The first analysis of the problem was formulated, among others, by Rüdemberg [9] and Sunde [10], some years ago. However, the solution complexity and the mathematical methods involved such as the use of numerical methods for grounding system analysis. It was then that the electrostatic field equations developed by Maxwell were considered and the method of images was applied. Thus, at the beginning of the 1990s, several computational models for transient groundings behavior calculations appeared [11e13]. These models were developed some time ago and, today, allow calculation of all these effects. These models can be classified as [14]: Empirical Approaches, Circuits Theory Approaches, Transmission Lines Approaches, and Electromagnetic Field Approaches. Because the electromagnetic model is the most precise, it was used for the analysis developed and presented in this paper. The first step for solving the electromagnetic model consists of dividing each
Fig. 2. Current distribution in segments.
conductor into short segments and calculating the current distribution along these segments (Fig. 2). After the conductor has been segmented, the contribution of the tangential electric field in the conductor surface is calculated. This electric field is due to the currents that circulate along the segments. Thus, a system of linear equations is obtained where the unknown parameters are the currents in each segment. Once the current distribution along the electrode segments is calculated, it is possible to find the electromagnetic field due the current distribution at any point and the Ground Potential Rise (GPR). In order to analyze how the electromagnetic variables vary, it is necessary to define some points in the soil surface. This process is repeated for all current signal harmonic frequencies to obtain the representation in the frequency domain. Finally, the inverse Fourier transform is used to obtain the signal in the time domain. 4. Transient analysis wind farm grounding systems In order to design a good grounding system, it is necessary to have a transient impedance value that ensures that neither humans nor the electrical and electronic devices experience high voltages and electrical currents. Therefore, the maximum step voltage and the transient impedance have been chosen as representative parameters in the analysis. The transient impedance is obtained from the point of the current injection. In addition, this analysis has been divided in two parts. First, a single wind turbine grounding system is analyzed to establish the optimal parameters in the grounding system design. Second, a wind farm system is analyzed, which considers the interconnection and influence of neighboring wind turbines. There is no consistency in the grounding systems between different manufacturers. Therefore, it has been necessary to establish some reference parameters for comparative analysis. Although each manufacturer uses a different grounding system, all of them have an average diameter of about 11 m.
Soil 2 m rod
2 m rod 1m
11 m
11 m
11 m
11 m
a
b Circular geometry
c
d Square geometry
Fig. 3. Wind turbine grounding system geometries.
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∅11 m
∅11 m
∅11 m
2007
∅11 m
a
b
11 m
11 m
11 m
11 m
c
d Fig. 4. Interconnection configurations.
Thus, four configurations have been selected to analyze each wind turbine grounding system: a circle (Fig. 3.a), a circle with two rods diametrically opposed (Fig. 3.b), a square (Fig. 3.c), and a square with two rods diametrically opposed (Fig. 3.d). Moreover, to quantify the effect of interconnecting wind turbine grounding systems, the configurations shown in Fig. 4 have also been analyzed. The soil resistivity has a strong effect on grounding impedance behavior. The impedance also depends on the dimensions of the grounding electrodes and their physical distribution. The magnitude of the injected current and the soil resistivity are uncontrollable parameters from a design point of view. Thus, in this analysis, the magnitudes that geometrically characterize the grounding system have been modified. The reference parameters used in the analysis are shown in Table 2. In the following sections, the results of the analysis are presented. 4.1. Influence of grounding system burying depth In order to determine the influence of the depth of the grounding system, five burying depths are analyzed. The two most common depths 0.5 m, the minimum value as determined by standards, and 0.8 m, the depth recommended by some manufacturers, and also 1, 1.2, and 1.5 m depths are analyzed, although they are not typical depths, they are recommended by some manufacturers. As shown in Figs. 5 and 6, the deeper the grounding system is buried, the smaller the impedance and step voltages are. If the grounding system is buried 0.8 m instead of 0.5 m, the step voltage is reduced by about 30%. This step voltage reduction is not so remarkable when the grounding system is buried at greater depths. In these cases, the reduction is about 10%. However, the step voltages for 1, 1.2 and 1.5 depths are very high (they are around 275 and 350 kV). On the other hand, increasing the burying depth increases the grounding installation price. Nevertheless, for burying depths of 0.5 m or 0.8 m, the cost difference is not very significant, while the step voltage difference is significant for all analyzed geometries. Therefore, increasing the depth to 0.8 m is an inexpensive way to increase safety.
Fig. 5. Step voltages at 0.5, 0.8, 1.0, 1.2 and 1.5 m depth.
4.2. Influence of grounding system perimeter length From Figs. 7 and 8 observe that the bigger the diameter or side, the smaller the transient impedance and step voltage. This holds as long as the effective length of the electrode is not exceeded. Thus, when the diameter/side increases up to 11 m the step voltage diminishes around 30% (compared to that of 8 m). The maximum step voltage is 800e600 kV. When the diameter/side is 13 m, the maximum step voltage is approximately 10e15% lower than for the 11 m diameter/side. For a 13 m diameter/side, the maximum step voltage is around 700 kV. When the diameter increases up to 11 m for a circle with rods, the step voltage diminishes by about 25%, and as a result, the step voltage is about 800 kV. When the diameter is 13 m, the step voltage is 16% smaller than for an 11 m circle, and the maximum step voltage is about 630 kV. Therefore, as far as possible, a greater dimension is recommended.
Table 2 Reference parameters. Diameter Depth Electrode section Material conductivity Soil type
er Lightning wave Lightning current
11 m 0.5 m 50 mm2 56 MS/m (Cu) 1000 Um 10 1.2/50 ms 100 kA Fig. 6. Transient impedance at 0.5, 0.8, 1.0, 1.2 and 1.5 m depth.
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Fig. 7. Transient impedance for 8, 9, 10, 11, 12 and 13 m diameter/side. Fig. 9. Transient impedance for an isolated and two interconnected (at 50 m) wind turbine grounding system.
4.3. Influence of geometry 4.4. Influence of grounding system interconnection Adding rods to a geometry at the same depth reduces the maximum step voltage by about 8%. Although the difference is less significant at greater depths, the maximum step voltage is about 20% smaller for the square grounding mesh for all cases. As the depth increases, the influence of adding rods on the step voltage decreases. Therefore, it is more effective to install square electrodes instead of circular electrodes, because the square geometries without rods provide comparable or smaller step voltage values than the circle with rods. Therefore, to diminish the step voltage, it is more advantageous, from the constructive point of view, to increase the size of the grounding mesh rather than to add rods.
Fig. 8. Step voltages for 8, 9, 10, 11, 12 and 13 m diameter/side.
In Figs. 9 and 10, the transient impedance and step voltage are shown for an isolated wind turbine grounding system and for two interconnected wind turbines grounding systems. The distance between interconnected wind turbines is about 50 m. When the grounding systems of two wind turbines are interconnected, the step voltage diminishes between 20% and 30%. This variation depends on the geometry used. The smaller the step voltage of an insolated wind turbine grounding system, the smaller the reduction. Therefore, the interconnection of grounding systems is highly recommended when poor conditions are expected. Figs.11 and 12, show the transient impedance and step voltage for the interconnection of the configurations shown in Fig. 4. The
Fig. 10. Step voltage for an isolated and two interconnected (at 50 m) wind turbine grounding system.
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Fig. 13. 11 m side square transient impedance.
Fig. 11. Transient impedance for two interconnected wind turbine grounding systems at 50, 75 and 150 m.
distances of the geometries analyzed are 50, 75, and 100 m. From Figs. 11 and 12, it can be concluded that the distance between the wind turbines grounding meshes that are interconnected has significant influence on the transient impedance and the step voltage.
In addition, the instruments are designed for 50e60 Hz currents. Therefore, the obtained measurements would have questionable validity. Wind farm constructors and operating companies do not provide construction information. Therefore, it would be complicated to get permission to take measures in a wind farm with suitable devices. Thus, to validate the analysis presented in this paper, the methodology proposed by Montaña [15] was chosen. Montaña analyzed the variation of the electrical parameters of soil as a function of frequency. He calculated the potentials in the electrodes, but not at the surface of soil. So, the transient impedance is used for the validation. As shown in Fig. 13, the value of the transient impedance at LF is very similar as is the behavior across frequencies. 5. Conclusions
4.5. Validation In order to correctly validate the method used in this analysis, it would be necessary to have real measurements to compare with the obtained values. At the present time, this option is impractical. To artificially reproduce an atmospheric discharge would require taking equipment from the laboratory to a wind farm. Such experiments would be expensive and would only be a replication of a random physical phenomenon.
Wind farms are relatively modern facilities with very particular characteristics. It is necessary to correctly install the grounding mesh to ensure the safety of a wind farm. It is also necessary to consider the grounding system behavior when an atmospheric discharge occurs. Lightning strikes are natural phenomena that are random and hardly controllable. In fact, it is practically impossible to prevent the discharges. Once a discharge is produced, a large current pulse is generated and must be dissipated by the grounding system. Thus, the prediction of the grounding system behavior is a very important but complex problem. There are four numerical approaches for calculating the transient behavior of grounding systems, and the electromagnetic approach is the most computationally intensive, but also the most precise. Today’s computers are powerful enough to handle the computation required to analyze the behavior of wind farms grounding systems. In this paper, a theoretical study of wind farm grounding systems is presented. This study consists of modifying the geometries and parameters of the wind farm grounding systems and studying their effect on the maximum step voltage and transient grounding impedance. According to the IEEE s80 guide, the maximum step voltages are very high. Therefore, it is necessary to pay careful attention to wind farm grounding system designs. In addition, it would be very useful to have real measurements to design safer wind turbines grounding systems. References
Fig. 12. Step voltages for two interconnected wind turbine grounding systems at 50, 75 and 150 m.
[1] Fox Neil I. A tall tower study of Missouri winds. Renewable Energy; July 2010. [2] O’Gairbhith, Colm. Assessing the viability of high altitude wind Resources in Ireland. HAWP 2009 conference; November 2009. [3] Cotton I, Jenkins N, Haigh S, Hancock M, Hatziargyriou N, Lorentzou M, Jaquemotte P. Lightning protection of wind turbines. EWEC:97; 1997.
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[4] Hatziargyriou N, Lorentzou M, Cotton I, Jenkins N. Transferred overvoltages by wind farm grounding systems. ICHQP conference; 1998. [5] Lorentzou M, Hatziargyriou N, Effective dimensioning of extended grounding systems for lightning protection. 25th ICLP; 2000. [6] Visacro S. A didactic general approach of grounding behavior for lightning currents. 27th ICLP: Avignon; Sep.2004, pp. 508e13. [7] Guide for safety in AC substation grounding. IEEE Std; 2000. 80e2000. [8] Lorentzou M, Hatziargyriou N, Papadias B. Analysis of wind turbine grounding systems. In: Proc. 10th Mediterranean Electrotechnical Conference (MELECON 2000), vol. 3; 2000. p. 936e9. [9] Rüdemberg R. Grounding principles and practice, part I: fundamental considerations on ground circuits. Electrical Eng Jan. 1945;64(1):1e13. [10] Sunde ED. Earth conduction effects in transmission systems. Copyright Ó by. Bell Telephone Laboratories, Incorporated; 1949. pp. 146e48.
[11] Grcev L, Dawalibi F. An electromagnetic model for transients in grounding system. IEEE Trans Power Deliv 1990;5:1773e81. [12] Grcev L. Computer analysis on transient voltages in large grounding systems. IEEE Trans Power Deliv April 1996;11:815e23. [13] Visacro SF, Portela C. Investigation of grounding systems behavior on the incidence of atmospheric discharges at electrical systems. In: Proc. of 20th ICLP; Sept. 1990. Interlaken, Switzerland. [14] Saraj cev P, Vujevi c S. A review of methods for grounding grid analysis. In: Proceedings of the 17th international conference on software, telecommunications and computer networks; 2009. p. 42e9. Hvar, Croatia. [15] Johny Hernan Montaña Chaparro. Puestas a Tierra. Variación de los Parámetros Eléctricos del Terreno con la Frecuencia y Software para el Cálculo de Potenciales Transitorios (Tesis Doctoral), Universidad Nacional de Colombia: Bogotá; April 2006.