Wind Farm Grounding Systems Design Regarding the Maximum Permissible Touch & Step Voltage A. Esmaeilian, A.A. Shayegani Akmal
M. Salay Naderi, Senior Member, IEEE
School of Electrical & Computer Engineering University of Tehran Tehran, Iran
[email protected]
School of Electrical Engineering & Telecommunications University of New South Wales Sydney, Australia
Abstract—Nowadays Wind turbines are used over a wide area as they are environmentally favorably means to product energy without emissions and moreover the fuel is wind which is free for use. Wind turbines are often installed in mountainous area where the soil resistivity and isokeraunic level is high. When the soil resistance is high, the potential rise caused by lightning strike to a wind turbine is more remarkable at the wave tail than at the wave front. On the other hand, the potential rise at the wave tail depends on the steady-state grounding resistance. In this condition the steady-state grounding resistance becomes more important than the transient grounding resistance. In this paper a comprehensive steady-state study on grounding system of wind turbines is presented. Different grounding system configuration will be analyzed. At first a single wind turbine ground system will be considered and the configuration which decrease the step voltage, touch voltage and equivalent grounding resistance of wind turbine grounding system more than other will be selected, then simulations expanded to wind farm which their grounding systems connected to each other. Different grounding methods are analyzed using CYMEGRD which is well known grounding system analysis software. Keywords-component; Ground Potential Rise (GPR), Touch Voltage, Step Voltage, Wind turbine.
I.
INTRODUCTION
Electricity windfarms are every day increasing in number. Windfarms are usually situated in rocky and mountainous areas where the wind potential is high [1]. So wind turbines are exposed to lightning strokes for two main reasons: One is the fact that they have a distinctive shape and they are very tall, open-air structures, and the other is the high isokeraunic level over windfarm locations. When lightning strikes a wind turbine, current flowing through its main parts: blades, rotor brushes and tower. To prevent from dangerous overvoltages that can damage parts of the wind turbine and human hazards, proper grounding system should be designed for each wind turbine in an electricity windfarm. Grounding system is one of the most important points inside the transmission systems and electric power distribution design. In power system, for operation and safety demand, some part of the power system and electric equipment should be connected to the grounding device in the earth ground. The grounding system is used to dissipate current into the ground. The grounding systems resistance must be low enough to
dissipate fault currents. If the grounding resistant is bigger, when there is big current through the resistance, the ground electric potential will ascend. Previous studies [1-4] used models based on various geometric arrangements of wind turbine earthing system. However, all of them neglected to exactly model the electrodes used as foundation concrete reinforcement and model the grounding system electrodes as a number of simple horizontal electrodes arranged in square or ring shapes, while the effect of reinforcement bars is too much to be neglected. In this paper, firstly, the different configuration for a single wind turbine grounding system is analyzed by adding the effect of electrodes in foundation concrete to establish the optimal configuration in the grounding system design. Secondly, a windfarm grounding system is analyzed, which considers the interconnection and influence of neighboring wind turbines grounding system. The paper is prepared in four sections, regarding clear explanation of the concepts. Section II gives basic information on grounding system design. Section III has been dedicated to the grounding system analysis and evaluation. Finally, section IV remarks the conclusion of the study. II.
GENERAL INFORMATION ON GROUNDING SYSTEM
In grounding system study there are some definitions which according to IEEE Std 80-2000 are defined as below [5]: Ground Potential Rise (GPR) is the maximum voltage that a station grounding grid may attain relative to a distant grounding point assumed to be at the potential of remote earth. This voltage, GPR, is equal to the maximum grid current times the grid resistance. Touch Voltage is the potential difference between the ground potential rise (GPR) and the surface potential at the point where a person is standing, while at the same time having his hands in contact with a grounded structure. Step Voltage is the difference in surface potential experienced by a person bridging a distance of 1 m with his feet without contacting any other grounded object. According to [5], the step and touch voltage can be determined from the two equations 1 and 2. These two equations are calculated using the resistance from a 70 Kg person.
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157
V
0.236C ρ √t
157
V
0.942C ρ √t
(1) (2)
where ρ is the surface material resistivity in Ω.m and C is the surface layer derating factor which is determined from below equation: 0.09 1 C
1
2h
ρ ρ 0.09
(3)
where h is the height surface layer in m. In general, grounding system analysis classified into two methods known as integral approach and differential approach. The former is based on integral equation to determine the ground potential [6, 7] and the later applies the finite element method (FEM) for calculating ground potential [8, 9]. In this paper analysis utilized by the software program CYMGRD which is based on finite element method. One of the most critical factors in grounding system design is the soil resistivity which has a strong effect on grounding impedance behavior. However the impedance depends on the dimensions of the grounding electrodes and their physical distribution. From designing view, the magnitude of the injected current and the soil resistivity are uncontrollable parameters. But it is necessary to measure and model the soil resistivity correctly. In below subsections the conventional method to measuring the soil resistivity and a two layer soil resistivity modeling which is utilized by CYMGRD are described. A. Resistivity Measurements Fig. 1 shows the Wenner method scheme which is the most conventional way to determine the soil resistivity. Wenner method consists of four electrodes; two are for current injection and two for potential measurement. Four measurements with the spacing a = 1, 2, 4 and 8 m are carried out; b is the depth of the electrodes in m. Each measurement applied by injecting a specified current between the current probes, then the voltage between voltage probes is measured [10].
Fig. 1. Wenner method resistivity measurement
The soil resistivity formula associated with Wenner method is shown in equation (4) where R is the resistance measured between probes: 4 1
2 √
(4) 4
√
In practice, four rods are usually placed in a straight line at intervals a, driven to a depth not exceeding 0.1 a. Then we can neglect b in equation (4) [11], so this equation changed to below simple equation: 2
(5)
Equation (5) gives an approximate for average resistivity of the soil to the depth of . B. Two-Layer Soil Apparent Resistivity The earth's crust is a non-homogeneous, anisotropic, nonlinear and layered medium. Its electrical behavior is a function of these different characteristics as well as moisture content, chemical composition, temperature, and pressure. Resistance of two-layer soil structure or multi-layer can be calculated by mirror method or Finite elements method. A resistivity determination using the Wenner-method (Fig. 1) result in an apparent resistivity which is a function of the electrode separation can be shown to be: 1
4 1
2
4
2
(6)
where: h: First layer height. ρ : First layer resistivity, in Ω.m ρ : Deep layer resistivity, in Ω.m A two-layer soil model can be represented by an upper layer soil of a finite depth above a lower layer of infinite depth. The abrupt change in resistivity at the boundaries of each soil layer (Fig. 1) can be described by means of a reflection factor. The reflection factor is defined by Equation (7) [12]: (7)
III.
WIND TURBINE GROUNDING SYSTEM DESIGN
Wind turbines consist of electrical equipment mounted in steel structures. So to avoid from hazardous, all parts of wind turbine must be connected to grounding system. A conventional grounding system for a wind turbine is consists of a steel ring shape electrode around the foundation and bonding it through the foundation to the turbine tower. Different manufactures utilize various grounding mesh geometry that can be categorized as circular, square or octagonal geometry. Adding rods are recommended by manufacture in the case of highly resistive soils. The foundation reinforcement steel bar is connected to the grounding mesh electrode. Since the surrounding concrete between the steel bars and ground is roughly 15 cm, it can be considered to have a resistivity equal to that of the surrounding soil, so the foundation steel bar resistance will be parallel to grounding system mesh and consequently decrease the equivalent grounding system resistance. Another factor that should be considered in grounding system design is the effect of step up transformer grounding system which can be connected to wind turbine grounding system to reduce the equivalent resistance.
On the other hand, a windfarm consists of several wind turbines, where the windfarm grounding system is the interconnection of each wind turbine grounding system. They are separated from each other by about three times the rotor diameter. So the designing procedure should be divided in two parts. At 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 are two main features that must be satisfied in grounding system design. One of them is lightning strike safety limits and the other one is short circuit safety limits. Grounding resistance of a wind turbine connected to the local transformer grounding system is required to be below or equal to 10 Ohms. This is the only requirement being suitable for lightning protection. A resistance of 10 ohm or less (before it is connected to any other system) is stated in international standards recommendations. In case of short circuit the step and touch voltage safety limits are the only requirements that must be satisfied [1]. When a wind turbine generator system is constructed in a region with high resistivity soil, the steady-state grounding resistance becomes more important than the transient grounding resistance. A potential rise caused by a lightning strike to a wind turbine generator system is more remarkable at the wave tail than at the wave front. The potential rise at the wave tail depends on the steady-state grounding resistance [13]. So in this paper we design and analyze the grounding system regarding to steady-state condition. A. Single Wind Turbine Grounding System Analysis To analysis different grounding system configuration, using wenner method and the typical soil resistivity data, CYME software prepares a two layer soil model. Table I shows the input soil data to software. From equations (6) and (7) the software calculates an upper layer with 202.84 Ω.m resistivity and depth of 2.16 m while the lower layer with 22.25 Ω.m has an infinite depth.
Table II: Grounding electrode parameters
Mesh electrode diameter Rod electrode diameter Foundation dimension (W×L×H) Mesh loop in concrete Square shape Grounding mesh Circular shape length Octagonal shape
16 mm 16 mm 7×7×2 (m×m×m) 20×20 (cm×cm) 4×10=40 m 2π×6=37.7 m 41 m
Fig. 2. Three-dimensional scheme of square shape grounding system
Fig. 3. Two-dimensional scheme of circular shape grounding system
At first we consider three different configurations for designing the grounding system of a single wind turbine. Fig. 2 show the three dimensional scheme of the grounding system for square shape electrode. Fig. 3 and Fig. 4 also show the two dimensional schemes for circular and octagonal shape electrodes. In each three cases there are four rods with length of 3 meters which is connected to grounding mesh. The foundation reinforcement bar is also connected directly to grounding electrode and will be effective in acting as a ground electrode since the surrounding concrete can be considered to have a resistivity equal to that of the surrounding soil. The grounding electrodes data is illustrated in Table II. Table I: Soil data
Probe distance (m) Resistivity (Ω.m) Surface layer thickness and resistivity
1 200
2 150
4 80
20cm / 3000Ω.m
8 30
Fig. 4. Two-dimensional scheme of octagonal shape grounding system
The WT grounding arrangement is usually connected to the local transformer grounding. This makes use of the existing path for the connection between the WT tower and the transformer, lowering the total grounding resistance. The obtained 3D results related to these above three configurations are shown in Fig. 5 to Fig. 7. Fig. 5 shows the touch voltage profile for square shape grounding system. In this case, however, the touch voltage is relatively low in the middle of wind tower foundation; it reaches to nearly 1800 volt
in the circle of 5meter around the tower. Fig. 6 and Fig. 7 also show the touch voltage profile for Circular and octagonal shape grounding system, respectively. As it is obvious from these figures the square shape design has the lowest touch voltage profile. On the other hand the circular shape design has the worst one.
Fig. 8. Touch and Step voltage profile related to square shape configuration
0
310.59
621.18
931.77
1242.36
Fig. 5. Touch and Step voltage profile related to square shape configuration
Fig. 9. Touch and Step voltage profile related to circular shape configuration
0
310.59
621.18
931.77
1242.36
Fig. 6. Three-dimensional touch voltage profile related to circular shape grounding system
Fig. 10. Touch and Step voltage profile related to octagonal shape configuration
0
310.59
621.18
931.77
1242.36
Fig. 7. Three-dimensional touch voltage profile related to octagonal shape grounding system
To have better comparison between these design cases, more outputs are acquired. Fig. 8 to Fig. 10 shows the touch and step voltage regarding to maximum permissible touch and step voltage for each configuration. The maximum permissible step voltage is 3534.17 V and the maximum permissible touch voltage is 1050.07 V. By investigating in these figures, one can conclude that using the square shape electrodes is the best design because of its lower touch potential compare to two
other configuration. However the square shape grounding system is the best choice, according to Figs. (5 and 8), the touch voltage exceeds the maximum permissible touch voltage line. Therefore, it is necessary to find another way to reduce the touch voltage under the maximum permissible voltage. As noted before a step-up transformer is installed near each wind turbine. By connecting the step-up transformer grounding system to wind turbine, the touch voltage according to Fig. 11 will be decreased, however the touch voltage still exceeds the maximum value at borders. The equivalent resistance is 7.236 Ω which is lower than the limit defined by international standards as lightning protection requirement (10 Ω).
Fig. 13. Touch and Step voltage related to windfarm
Fig.11.Touch and Step voltage profile, considering the step-up transformer
B. Windfarm Grounding System Analysis According to Fig. 12, the safe area around the wind turbine tower for touch voltage is more than 4 m. But there is still some hazardous if any fence not exists to show the safe area around the wind turbine. The final solution for decreasing the touch potential is to connect the grounding system of each wind turbine to others. In this simulation we consider a windfarm consist of 16 turbine (Fig. 13). Each turbine grounding system is designed as the above mentioned configuration and connected to each other to obtain an integrated grounding system. From Fig. 14, it is obvious that the touch and step voltage are highly decreased when the grounding system of wind turbines connected to each other. As can be seen, the touch and step voltage are lower than permissible ones.
Fig. 12. Windfarm grounding system configuration
0
310.59
621.18
931.77
1242.36
Fig. 14. Three dimensional Touch voltage related to windfarm
Nowadays wind turbines grounding systems are connected to each other through the armor of the main power cable running between the turbines. Hence, this way is a suitable way to reduce the step and touch voltages. More simulation results show that the above final windfarm grounding system design is safe if the soil resistivity increase to 500 Ω.m for a single layer soil modeling. IV.
CONCLUSION
In this paper the grounding system of wind turbines was analyzed by CYMGRD software and showed that for a single wind turbine, the step and touch voltages may be above the permissible step and touch voltages, if the soil has a high resistivity. To solving this problem, several routine grounding system configurations were analyzed to determine the best configuration. For the purpose of better designing, an exact model of foundation concrete reinforcement bars also added and connected to grounding electrodes. Finally the grounding system of a windfarm is analyzed. This windfarm consists of 16 wind turbine which their grounding systems is connected to each other. Simulation shows that connecting of the grounding systems has a significant effect on decreasing the touch and step voltage profiles.
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