Influence of Design Parameters on Grounding Grid Performance: Alkhoms Power Substation 220/400 kV Case Study

– Many design parameters are known to affect the characteristics of a substation grounding system, in terms of its quality and efficiency. The most relevant of these parameters are soil resistivity, number of meshes within a grounding grid, and the use of grounding rods in the grid. Grounding rods may contribute to quality and efficiency of substations in terms of their dimensions, distribution within the grid, and material. In this study, the effects of the number meshes and grounding rods on the quality and efficiency of Al khoms substation have been investigated.


Introduction
The goal of excellent design of grounding systems in power stations is to protect people from dangerous voltages.Foe safety purpose, connections all metallic frames inside the area of substation with a common ground system.This will limit the potential differences, which can occur during the ground fault event and provide a low resistance path for the fault current.
Grounding is commonly used as an electrical conductor for system returns.Although resistivity of the earth is high compared to a metal conductor, the overall resistance can be quite low due to the large cross-sectional area of the electrical path.Many devices have been made to all metallic power station with earth including grounding grids, ground rods, or both [1].The resistance to earth of these devices varies proportionally with earth resistivity, which in turn depends on the earth composition, moisture content, temperature, and compactness.
A substation ground grid is generally designed to limit step and touch voltages, results a safe environment for people who walk or touch inside and at around the substation during a ground fault.Field measurements of soil resistivity are required to determine a suitable soil model for design purpose.The interpretation of these measurements consists of finding a set of soil parameters defining itself in such a way as attain a good fitness between the measured and the calculated resistivity values [2].Wenner method has been widely used for field measurements of soil resistivity.In the present paper, as the first time on our analog, a Matlab algorithm has been developed on the finite Wenner method for interpretation of soil resistivity measurements.The second part centered on the study of ground grid safety at Al Khoms 220/400 kV substation.The study focused on the influence of design parameters on the grounding system performance, such as number of meshes and grounding rods.

Measurement and Modeling for Soil Resistivity
Soil is an important parameter for the grounding system performance.To obtain an ideal model for soil resistivity of power substations, actual resistivity tests are imperative.These tests should be made at a number of places inside the station.In addition, soil resistivity tests is importantly to be determined at different depth to investigate any variations in the resistivity [3].As a rule, the number of such reading taken should be greater where the variations are large, especially if some readings are so high as to suggest a possible safety problem.Efficiency of grounding system depends on the soil and its characteristics.
Grounding grid performance can be measured in terms of grounding resistance as primary parameter.In addition, it is preferable to include step and touch voltages as secondary parameters for grounding quality.Many techniques have been proposed for soil resistivity measurements [4].The Wenner's four-pin method is the most widely used technique, which has been used in this study.

Wenner's Four Point Method
The four-point method is the most widely used method in practice of measurement the average resistivity of soil.This method can be classified into the following methods:

Equally Spaced (Wenner Arrangement)
In this method four probes (pins) are driven into the earth as a row, at equal distance (a), and driven to a depth (b).Mutual resistance (R) is the ratio of the voltage between the two inner electrodes and the current between the two outer electrodes (i.e.R = potential electrodes/current electrodes) [4].The soil resistivity can be obtained as given in Eq.1. Where: : The apparent resistivity.

R:
The voltmeter reading divided on the ammeter reading.For b = 0, the Eq. 1 can be simplified to read Eq. 2:

Unequally Spaced (Schlumberger Arrangement)
One shortcoming of the Wenner method is rapid decrease in magnitude of potential between the inner electrodes when their spacing is increased to relatively large values.The formula to be used in this case can be determined if the depth of burial of the electrodes (b) is small compared to their separation (d) and (c) as seen in Figure 1.Therefore, the resistivity can be calculated as given in Eq. 3. [5]: (1) =

Selection of Soil Model
The most accurate representation of a grounding system should be based on the actual variations of soil resistivity that present at the substation.The obtained apparent resistivity in the field tests must be modeled as a uniform soil model or non-uniform soil model based on resistivity varies with respect to depth [6].These two models have been studied in details in this study.

Uniform Soil Model
In some cases, the variation in apparent soil resistivity with respect to depth is not too great.
For such studies, a soil can be considered a uniform with average resistivity value selected for design purpose.In general, a soil can be considered uniform if the difference between two extreme values of apparent resistivity is less than 30 % [7].

Non-Uniform Soil (Two Layers Soil)
In non-uniform soil, the two-layer model has an upper layer of definite depth and resistivity, whereas the lower layer has an infinite depth and definite resistivity.The two-layer model is the more exact theoretical approach to situations where resistivity varies markedly with depth.It is reasonably valid for actual soil conditions and the range of resistivity variations found on the site.In this model, the soil divided into two layers of different resistivity ⍴1 and ⍴2 and there is a reflection absorbed current between the two layers, expressed by the reflection factor (K), which describes the abrupt changes in resistivity at the boundaries of each soil layer [8].This reflection factor can be obtained using Eq. 4.

Interpretation of Soil Measurements
The interpretation of the results obtained in the substation field is perhaps the most difficult parts, because the earth resistivity variation is complex [9].Such a soil can be interpreted as uniform soil when a soil composition is found where the resistivity varies little with respect to depth [7].A two layer soil model is generally an adequate representation of nonhomogeneous soil for ground system design.Evaluation of two layer soil model from the measured data can be done either by computer programming or graphical methods.

Expressions
To evaluate the two layer soil parameters, various parameters should be known.Among of then, the sequence of operations in the iterative search values of ⍴1 and ⍴2 and h.
1-Selection of the initial values ⍴1 and ⍴2 and h.

2-
The iteration count k is set to be 0.
3-For the present values of soil parameters, the matrixes elements in Eq.5 can be computed.
The new values of soil parameters vectors, X T = [⍴1 and ⍴2 and h] were obtained as given in Eq. 6.
s Where: Mn : is measured apparent resistivity for n electrode spacing.
Cn: it is calculated apparent resistivity at n electrode spacing computed by finite expression.
: it is a scalar, which is usually chosen for ⍴1 and ⍴2 and h values to be changed by less than 50% in the second iteration for each iteration.
A developed first program is used to determine the equivalent two-layer earth model from the measured apparent resistivity data obtained by the equally-spaced four point (Wenner) method.It is based on (finite Wenner resistivity expression) method in case of ⍴1>⍴2.The program flow chart is shown in Figure (2).

Introduction
High voltage installations require an earthing system to protect human life against excessive touch voltage and to keep transferred potential to minimum.The increase of fault currents to earth affects the importance of earthing systems and the need for low resistance of earthing grid.Planning, calculations and measurements of earthing systems can be performed according based on IEEE standard 80-1986 or IEEE 80-2000 [10].

Description of Grounding System under Study
The 400 kV GIS Alkhoms substation is located next to the existing 220 kV substation.The area covered by the total area of extension and existing substation is about 200 m x 200 m.

The extension consists basically of:
1. 400 kV switchgear building with 400 kV GIS.

OHL gantry
The connection to 400 kV network at first stage will be carried out via one single circuit overhead line.The area of existing 220 kV substation and extension will be located on interconnected area surrounded by a boundary fence.The general grounding system of Alkhoms substation consists of copper wire connected together in form of a grid buried at a depth of 0.5m [10].

Formula for Calculating Ground Resistance
One of the important steps in determining the size and layout of a grounding system for an ac substation is the estimation ground resistance of ground grid.

Two-Layer
For grounding system in two-layer soil model with ⍴1 and ⍴2 , the reflection factor K and two-layer soil resistivity can be simplified as one-layer apparent resistivity as follows [11]: Where:

‫األسمرية‬ ‫الجامعة‬ ‫مجلة‬
The ground resistance can be calculated using the Sverakʼs Equation as given in Eq. 15 [11].
Also it can be estimated by Empirical Equation as defined in Eq. 16. Where: Rg: Two-Layer soil ground resistance of grid conductor in ohm.
Lp: The peripheral length of grid in m.
If the grid is sunken in the first layer, the grid resistance can be obtained from Eq. 17.

Grid Current
Grid current is the maximum portion of symmetrical ground fault current that flows between the grounding grid and surrounding earth.The earth fault entering the earth grid returns on different ways: 1-Parts of earth fault current return to the transformer neutral within the substation area.The remaining part flows between the earthing system of the substation and surrounding earth [10].

Transformer Neutral Current
The total transformer neutral current for the two 400/220 kV auto transformers comes: Where: n: Number of transformers in parallel.U0 : Zero sequence voltage.Z0 : Zero sequence impedance of 400 MVA transformer.

Current Division Factor (Sf)
The ratio between the grid current and the earth fault is described by the current division factor.Under consideration of all above mentioned parts of all earth fault current the current division factor can be calculated from Eq.20.[10].
Where:  The grid current can be calculated from Eq.21.[12].

Ground Potential Rise (GPR)
The ground grid potential rise is the maximum potential rise of a grounding installation, with respect to remote earth, created by fault current that flows through the grid.The ground grid potential rise can be obtained from Eq.22.

Step and Touch Voltages
The Step voltage and the touch voltage are two important parameters for the grounding system of any structure, and especially for electrical substations.They are necessary to assess grounding efficiency [13].

Step Voltage
The step voltage defined as the potential difference between a person's outstretched feet, normally one meter apart, without the person touching any earth structure [14].To ensure that the earth grid design is safe the calculated step voltage for the earth grid has to be compared with the tolerable step voltage.

Tolerable Step Voltage
The tolerable step voltage can be calculated from the following equation based on IEEE standard 80-2000 considering a body resistance of 1000 , body weights of 50 kg and 70 kg.

Soil Resistivity Measurements Data for Alkhoms Substation
Several measurements were obtained for the soil resistivity at different locations, labeled site #1 to site #3.Table 1 shows soil resistivity using Wenners method.

Study of the Ground Grid Safety
The real grounding grid size of Alkhoms substation is 200 x 200 m 2 .The grid was buried at a depth of 0.5 m below ground level.Table (3) tabulated the required data for run the second MatLab program.

Zch
Tower footing chain impedance of transmission lines connected to the substation.For both types of soil models, the ground resistance and the ground potential rise at Akhoms substation are computed using the data in Table (3) and the obtained results are summarized in Table (4).The obtained results indicated that the ground resistance of onelayer soil model was 0.2615  based on Sverakʼs equation, while, the ground resistance for the two-layer soil model were 0.1474 , 0.1072  and 0.1253  for grid buried in upper layer soil, Sverakʼs equation, and empirical equation, respectively.It is clear that the results that obtained using the three methods for two layer have very small variation.However, the value that estimated using one layer is a double of the value obtained using two layer method.
Therefore, the obtained results using two layer soil model describes the real soil resistivity in the substation.5 represents minimum touch voltages (Et) for two persons having 50 kg and 70 kg using one layer model and two layer models (Model I: grid resistance computed by using Sverakʼs equation and Model II: grid resistance computed by using empirical equation).As seen in Table (5), the Et values of Akhoms substation using the one -layer soil model were 1.0875 kV and 1.4719 kV for 50 and 70 kg persons, respectively.In addition, the GPR value of 2.3235 (see Table 4) for one layer model is greater than the tolerable voltages for 50 and 70 kg person of 1.0875 kV and 1.4719 kV, respectively (see Table 5).Furthermore, as seen in The study of minimum values of step voltages (Es) and the maximum/mesh step voltages (EM) were a part of the present work.Table (6) illustrates the obtained results for Es and EM using one-layer soil model and two-layer soil model.As seen in Tables 5 and 6, it is obvious that the tolerable touch voltages (Et,tol) for 50 kg and 70 kg persons were greater than the mesh voltage (EM) that obtained by the one-layer soil model.In addition, the minimum step voltage (Es) values were smaller than the tolerable step voltage (Es,tol) values for 50 and 70 kg persons.In summary, from the previous investigations, it can be reported that the grounding design of Alkhoms substation was found to be in safe according to the theories of one-layer and two-layer soil models.

The Influence of Meshes Number on the Grounding System Performance
The number of meshes within the grounding grid has a large effect on the grounding system performance.By increasing the number of meshes in the grid increase the number of the grounding conductors which causes a simultaneous reduction in ground potential rise and the grounding resistance.As a result, the touch and step voltages also decrease.However, the rate at which grounding resistance decrease less significant for large number of meshes, i.e. for small parallel conductor spacing [4].To assess the role of this parameter, seven cases have been studied, namely: one mesh grid, 2x2 meshes grid, 4x4 meshes grid, 8x8 meshes grid, 16x16    5) and ( 6), the values of grid resistance, maximum touch, and step voltage decrease with an increase in the number of meshes.In addition, the increasing of the number of meshes within the grid result a decrease in the step voltage until the number of meshes reaches a minimum of 1000.

Grounding Rods
The presence of grounding rods in the grounding grid has significant effect on the value of grounding resistance.A rod electrode discharges approximately two times greater than the current into the ground per unit length a horizontal buried conductor of the same size [9].This fact is valid for individual and multi electrodes; therefore, in this study multi rods was used to investigate the relationship between rod length and grounding parameters.Rods were characterized by their material, diameter, length, and its arrangement within the grid.

Length of Grounding Rods
The rod length is more important parameter to characterize the grounding parameters; therefore, it was studied in details.The previous studies indicated that the increasing of the rod length leads to more current that discharged through it.In addition, the increase in the contact surface area results a decrease in both of the soil resistivity and the grounding parameters values [16].In this study, four values of rod length have been studied, namely: 3

a:
Rod (pin) spacing in m. b: Rods depth in m.In practice, four rods (pins) are usually placed in a straight line at different spacing (a), driven to a depth (b), b < 0.1a.

113
According to the IEEE std.80-2000 edition, the Schwarz's equation gives a simple formula for calculating the ground resistance in the uniform soil at a substation[11].Where:⍴: Uniform soil resistivity in ohm.m Lc: Total length grid conductor in m.Lr: Length of rod in m.Ag: Total area enclosed by ground grid m 2 .nr: Quantity of ground rods placed in area.a : Radius of ground rod in m. hg: Depth of ground grid conductor in m.
Measured soil resistivity data in ohm.m. n: Number of measurements.L: Length of area occupied by the ground grid in m.W: Width of area occupied by the ground grid in m.

⍴a:
Apparent soil resistivity in ohm.m. ⍴1: Upper-layer soil resistivity in ohm.m. ⍴2: Lower-layer soil resistivity in ohm.m. hr: Depth of reflection boundary in m. hg: Depth of ground grid conductor in m.

18 ) 2 -
Due to inductive coupling parts of earth fault current return via earth wires.3-Parts of the earth fault current flows via earthing impedances which are in parallel to the considered earthing current.

Zp:
Impedance to earth of the parallel earthing system in ohm.Rg: Grid resistance in ohm.rE: Earth wire reduction factor.
32x32 meshes grid, and finally a grid of 64x64 meshes grid.These seven case have been applied individually in the MatLab second program to investigate the effect of meshes number on the grounding system performance.Figures4, 5, and 6 represent the plot of the meshes number versus grounding resistance, step voltage, and touch voltages, respectively.

Figure ( 4 )
Figure(4) The number of meshes within the grid versus ground resistance.

Figure ( 5 )Figure ( 6 )
Figure (5) Mesh versus the number of meshes within the grid m, 9 m, 12 m and 15 m.The obtained results are shown in Figures )7), (8), and (9).The results indicated that the increase in the length of grounding rods in the grid leads to a decrease in the grounding resistance of the grid and also a reduction in both the step and touch voltages.

Table (
(2)this part of research work, a MatLab program was developed to determine the equivalent two-layer earth model from the measured apparent resistivity data that obtained by the Equally-Spaced Four Point (finite Wenner resistivity expression).This method is used for ⍴ 1 >⍴ 2 .Table(2)illustrates the summary of obtained results of soil resistivity parameters using the developed program.

Table 5 ,
the Et values using Models (I) and (II) were 1.4612 kV and 1.4612 kV, respectively for two layer model.Whereas, GPR values were 1.4646 and 1.5657 for Sverakʼs and empirical equations (see Table4).