Substation grounding grid design using Alternative Transients Program-ATP and ASPIX

GROUNDING GRID DESIGN

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Substation grounding grid design using Alternative Transients Program-ATP and ASPIX This example shows the method for designing a grounding grid following the standard IEEE 80 safety criteria. It shows the procedure for designing the grounding grid of a substation with voltage levels of 115 kV and 34.5 kV.

1 Software This example of the design was developed with the aid of the following software:  Alternative Transients Program (ATP): Licensing to use ATP is free of all charges for all who are not engaged in EMTP commerce. For information on the licensing and user groups, visit the website: http://www.emtp.org/  Aspix: You may obtain a trial version for 30 days. This version allows you to run simulations and observe the results; however, you cannot save or generate reports. You can request a 30-day free license without functionality limitations. The program can be downloaded from the following website: http://www.spartalightning.com/products/aspix  Aspix Resistivity Analyzer: It is an Excel spreadsheet that allows the processing of the resistivity measurements to obtain a two-layer model. This spreadsheet sheet is available for free and can be downloaded from the website: http://www.spartalightning.com/download

2 Design method The method for designing the grounding grids includes the following steps: 1. 2. 3. 4. 5. 6. 7.

Measuring and analyzing the soil resistivity. Plotting the grounding grid layout. Calculating the grounding grid resistance with the initial design. Calculating the earth current distribution. Calculating the touch and step voltages. Verifying the touch and step voltages. Redesign of the grounding grid when the calculated touch and step voltages are higher than the tolerated values; the procedure is repeated from point 2.

3 Input information The following information is required for designing the grounding grid:     

Single line diagram of the power system Short-circuit levels Parameters of the power transformers Substation plan view Measurements of the soil resistivity

http://www.spartalightning.com/


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4 Description of the case This is an example of the design of the grounding grid of a substation with voltage levels of 115 kV and 34.5 kV, with three lines of 115 kV, a 25-MVA power transformer, and three 34.5-kV distribution circuits. The single line diagram is shown in Figure 1, and the substation for which the grounding grid is designed is the load 1 substation. Short Circuit Equivalent Isc1 = 7 kA Isc3 = 9 kA

Source Substation 230 kV

150 MVA Ynyn0D11 Z1 = 14% Z0= 14%

115 kV

Double circuit line 36 km

Load 1 substation

Single circuit line 58 km

115 kV

34,5 kV

115 kV 25 MVA Dyn5 Z1 = 12% Z0= 12%

34,5 kV

Load 2 substation 25 MVA Dyn5 Z1 = 12% Z0= 12%

Figure 1. Single line diagram

The substation is composed of three line bays of 115 kV, one 25 MVA transformer, one 115 kV transformer bay, and four 34.5 kV bays (three for the distribution circuits and one for the power transformer), as shown in Figure 2.



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Figure 2. Substation plan view

The substation has a layer of crushed rock of 20 cm thickness and with a resistivity of 2500 m. The maximum time of fault clearance is 500 ms. The cables of the mesh have a depth of burial of 50 cm.

5 Resistivity measurements The measurements are obtained using the Wenner method for a separation of the electrodes of up to 8 m and using the Schlumberger-Palmer method for a separation of the voltage electrodes of up to 32 m. The soil resistivity measurement data are summarized in Table 1.



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Table 1. Resistivity measurements Wenner Method

Schlumberger-Palmer Method

a (m)

a (m)

c(m)

a (m)

c(m)

16

4

32

4

2

4

8

193.1

168.5

139.6

101.7

210.7

167.7

185.3

185

211.4

66.9

222.3

129.5

117.9

164.4

123.4

245.2

175.6

84

147.2

93.2

The Aspix Resistivity Analyzer is used for processing the resistivity measurements and to obtain the two-layer model. This processor is an Excel spreadsheet that generates a curve of resistivity with a probability of non-exceedance of 70%. From this curve, Aspix Resistivity Analyzer tries to find the values of the upper-layer resistivity, bottom-layer resistivity, and depth of the top layer that best suit to the obtained measurements. This tool displays a graph where the user can observe the resistivity measurements and the values calculated by the analyzer, allowing comparison of the model with the measurements. Table 2 summarizes the processing of the resistivity measurements in Table 1. Table 2. Earth resistivity measurement processing Resistivity Analyzer Version 1.0 http://www.spartalightning.com/

Resistivity Analyzer Equally Spaced - Wenner Method

Unequally Spaced - Schlumberger-Palmer Method

a (m)

a (m) c(m) a (m) c(m) a (m) c(m) a (m) c(m)

2

4

8

16

4

32

P1 (Ohm_m)

193.1 168.5 139.6

101.7

P2 (Ohm_m)

167.7 185.3

211.4

66.9

P3 (Ohm_m)

222.3 129.5 117.9

164.4

123.4

P4 (Ohm_m)

245.2 175.6

147.2

93.2

185 84

Resistivity_70%

250

4

200

P5 (Ohm_m) P6 (Ohm_m) P7 (Ohm_m) P8 (Ohm_m) P9 (Ohm_m) P10 (Ohm_m)

150

100

50

Average

207.1 164.7 131.6

Std Dev

33.82 24.47

45.334

62.518

Resistivity_70%

223.6 177.4 150.4

177.134

145.573

Resistivity 2LModel 223.3 176.7 146.8

165.02

162.309

156.175

42.3

123.55 0 0

5

Estimated 2 layer soil parameters Upper Limit Lower Limit  1 (Ohm_m) 256.14 10000.00 1.00  2 (Ohm_m) 136.35 10000.00 1.00 k H(m)

Resistivity 2LModel

210.7

Resistivity (ohm-m)

Profile

-0.31 1.82

0.99 20.00

-0.99 0.10

10 15 20 25 30 Voltage Electrodes Separation Distance a (m)

I V Analyze

a

a a Wenner Method

The resistivity analyzer provides the following parameters for the two-layer model:  Resistivity of the upper layer (1) = 256.14 m  Resistivity of the bottom layer (2) = 136.35 m  Depth of the top layer (H) = 1.82 m


I V c c a Schulumberger-Palmer Method

35


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6 Initial design 6.1 Grounding grid layout The design of the grounding grid must be such that it ensures the safety of persons against failures, and requires the minimum amount of materials and work. In the design method, initially a mesh is drawn on the substation layout covering all the substation equipment. This first layout of the grounding grid must take into account the following:  All the substation equipment require at least one ground pigtail for their structures.  When the substation has a metallic fence, a ground conductor can be buried outside the fence in order to control the touch voltages. Normally, it is a cable installed 1 m outside the fence, buried at 50-cm depth. This cable must be interconnected with the internal grounding grid every 20-50 m, depending on the soil resistivity.  A cable around the transformers, outside the transformer foundation.  A cable outside the gantry foundations.  The minimum number of cables required to facilitate grounding of all the equipment and their structures are drawn. In the case of the above example, the fence is a brick wall; therefore, it is not necessary to install a buried cable outside the fence. Figure 3 shows the plan view of the substation and the grounding grid drawn according to the above mentioned criteria.



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Figure 3. Grounding grid layout

Below is the analysis of the grounding grid to know whether this first design meets the safety requirements or whether it is necessary to improve it.

6.2 Grounding resistance Once the soil resistivity data and the geometry of the grounding grid are known, the grounding resistance value can be calculated. Initially, the value of the maximum current that circulates through the grounding grid is not known. This current is calculated later, and it is not required to know its value for the calculation of the resistance of the grounding grid. As the program requires this current value, we can select a value of 1000 A. Table 3 lists the parameters that are used for the simulation.



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Table 3. Simulation parameters

Aspix parameter

Value

Upper layer resistivity (Ωm)

256.14

Lower layer resistivity (Ωm)

136.35

Upper layer thickness (m)

1.82

Crushed rock resistivity (Ωm)

2500

Thickness crushed rock surfacing (m)

0.2

Fault duration (s)

0.5

Maximum grid current (A)

1000

These parameters are entered in the Aspix program using the “Simulation” option in the “Simulation Settings” menu. Figure 4 shows the parameters entered in the program.

Figure 4. Aspix simulation settings

The next step is to enter the physical data of the grounding grid in the program. For this, the wires and rods that make up the mesh of the grounding are added. In this example, the mesh of the grounding is composed only of horizontal conductors, which are added by right-clicking on “Horizontal Conductors” and then clicking on “New Conductor”. Figure 5 shows the data that must be entered by each conductor.



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Figure 5. Horizontal conductor data

Once the conductor details are entered, the user can observe the plan view of the grounding grid. The plan view is displayed with the “Grid Plan View” option in the “View” menu, and it appears as shown in Figure 6.

Figure 6. Grounding grid—initial design

The simulation is run using the “Run” option in the “Simulation” menu. The results are displayed using the “Results” option in the “Simulations Results” menu. The program displays a window with the results table as shown in Figure 7.



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Figure 7. Simulation results

Thus far, only the grounding grid resistance value (2.267 ) has been calculated. The touch and step voltages have not yet been calculated given that the value of the current passing through the grounding grid is required.

6.3 Earth fault current distribution The calculation of the touch and step voltages requires the knowledge of the maximum grid current value. A first approximation is to assume the value of this current as the maximum value of the ground fault current in the substation, by taking into account all the voltage levels. This approach can work in some cases; however, it can be very conservative and expensive. For a less conservative design, the value of the maximum grid current can be more accurately calculated. The ground fault current flows through both the grounding grid and the ground wires of the transmission lines and distribution circuits. There are different methodologies to determine the earth fault current distribution; in this example, the detailed simulations of the ground faults are performed using the ATP program. In the case under analysis, it is important to take into account the fault current to the ground that is derived through the ground wires of the 115-kV lines. Therefore, the tower footing resistances closest to the substation should be modeled in detail. Table 4 summarizes the data of the phase conductors and ground wires and the tower configuration.



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Table 4. 115 kV-lines parameters

By assumed an average span of 300 m and a tower footing resistance of 30 , the 20 sections of the line closest to the substation (6 km) are modeled in detail, and the remaining part is modeled as a single section. Figure 8 shows the network modeled using the ATP program. Source substation

Load 1 substation

Load 2 substation

Tower footing resistance

Grounding grid resistance

Figure 8. ATP simulated network

A single phase and two phases to the ground faults are simulated, at the levels of 115 kV and 34.5 kV. The case in which more current passes through the grounding grid corresponds to a two-phase ground fault on a 34.5-kV circuit at a point very close to the substation, by assuming that the 34.5-kV circuits do not have a ground wire. Figure 9 shows the distribution of the ground fault current.



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115 kV Ground wire

34.5 kV

2720 A 875 A Grounding in the fault point

Towers footing resistances

Substation grounding grid 2720 A

1121 A 815 A Ground wire


Figure 9. Most critical earth current distribution

The currents cannot be added arithmetically because there is an angle of deviation between them mainly caused by the inductive component of the ground wires; this lag can be observed in Figure 10. 4000 [A] 3000

Fault ground current and transformer neutral current

2000 1000

Substation grounding grid current

0

Lines ground wires current

-1000 -2000 -3000 -4000 0.00

0.02

0.04

(f ile Ejemplo_Aspix.pl4; x-v ar t) c:MALL -NEUT c:FALL -

c:

0.06 -MALL

c:CG1

0.08 -MALL

c:CG2

[s]

0.10

-MALL

Figure 10. Earth currents

The maximum current value through the grounding grid is 1121 A, and it is the value that is used to calculate the touch and step voltages.



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The effect of the direct current component of the short-circuit current is negligible given that the X/R relationship of the system in the substation analyzed is less than 10 and the fault duration is 500 ms.

6.4 Touch and step voltages For calculating the touch and step voltages, the value of the current through the grounding grid must be set to 1121 A. This value is changed by the “Simulation Settings” option in the “Settings” menu, and the parameter “Maximum Grid Current (A)” should be changed (see Figure 4). The regions or areas in which the touch and step voltages are calculated must be added. These regions are selected by taking into account the following criteria:  The step voltages must be controlled both inside and outside the substation; however, it is not necessary to calculate them in very large areas because the highest step voltages appear on the perimeter of the grounding grid. Therefore, when we control the step voltages on the perimeter, we control them elsewhere. Therefore, a good approach is to calculate the step voltages in a region that covers the total area of the grounding grid.  The touch voltages must be controlled at all the sites where people can touch the grounded structures (steel structures, transformers, electric panels, metal poles, etc.). Usually, these grounded structures are located within the area covered by the grounding grid. Therefore, a valid criterion is calculating the touch voltages in a region that covers the entire area of the grounding grid. The areas in which the touch and step voltages are calculated are added by right-clicking on “Chart Areas” and then clicking on the “New Chart Area.” The program displays a window that is shown in Figure 11.

Figure 11. Touch and step voltages chart area data

The areas that can be defined using the Aspix program are rectangular in shape and are defined by the coordinates of the starting point (x, y), number of points in the “X” and “Y” directions, and distance between the points or resolution. For the case that is being analyzed, the five areas that are shown in Figure 12 can be defined.



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Area 5

Area 4 Area 3 Area 2

Area 1

Figure 12. Areas for simulation

Once the chart areas for simulation are defined, the simulation is run with the “Run” option from the “Simulation” menu. Further, the results are observed using the “Simulations Results” option of the “Results” menu, and the results table is shown in Figure 13. The results table displays a summary of the simulation; we can observe the values of the grounding resistance and the calculated maximum touch and step voltages values in the defined chart areas. In addition, we can observe the tolerable touch and step voltages for people of 50 kg and 70 kg of weight in accordance with the standard IEEE 80. In this example, the safety criterion is not to exceed the tolerable touch and step voltages for persons of 50 kg of weight. Aspix also generates the touch and step voltages charts for all the configured areas. These charts can be twodimensional or three-dimensional and can be displayed using the options “Touch Voltage 3D Chart,” “Step Voltage 3D Chart,” “Touch Voltage Chart 2D,” and “Step Voltage Chart 2D” in the “Results” menu. These charts are shown in Figure 14, Figure 15, Figure 16, and Figure 17, respectively.



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Figure 13. Simulation results

Figure 14. Touch voltage—3D view



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Figure 15. Touch voltage—2D view



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Figure 16. Step voltage—3D view

Figure 17. Step voltage—2D view

As can be observed in the results table shown in Figure 13, the tolerable touch voltage for a 50-kg person is 677.8 V, and this value is exceeded by the calculated touch voltage of 914.2 V. The step tolerable voltage is



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2219.1 V and is above the calculated step voltage of 406.5 V. In summary, with this initial design, the tolerable touch voltage is exceeded, i.e., the design does not meet the safety criteria and should therefore be modified.

7 Modified design 7.1 Grounding grid layout The modifications to the initial design must be oriented to the reduction in the touch voltages, for which there are different alternatives such as follows:  The ground potential rise (GPR) is decreased, for which the alternatives are as follows: reduce the grounding grid resistance or decrease maximum current through the grounding grid.  The space between the parallel conductors is reduced by adding conductors on the inside of the grid of the initial design. The first option is to attempt to decrease the touch voltage by reducing the space between the parallel conductors. This requires identifying the points where the tolerable touch voltage is exceeded. The regions where the tolerable touch voltage is exceeded appear circled as shown in Figure 18, and the modification of the design consists of adding cables in these regions. Figure 19 shows the modified design.



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Figure 18. Most critical touch voltages

Figure 19. Grounding grid—modified design

7.2 Grounding resistance The program is run and a grounding grid resistance of 2.09  is obtained, as shown in Figure 20.



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Figure 20. Results—modified design

7.3 Distribution of currents to the earth The value of the grounding grid resistance of the substation 1 (see Figure 8) is modified in the ATP program, and the simulation is run. Figure 21 shows the distribution of the ground fault current.



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115 kV Ground wire

34.5 kV

2732 A 856 A


Substation grounding grid 1188 A

Grounding in the fault point

2732 A

793 A Ground wire


Figure 21. Distribution of ground fault current—modified design

The maximum current value through the grounding grid is 1188 A, and it is the value that is used to calculate the touch and step voltages.

7.4 Touch and step voltages For calculating the touch and step voltages, the current value of 1121 A passing through the grounding grid must be changed to 1188 A, which is obtained for the modified design. This value is changed by the “Simulation Settings” option in the “Settings” menu, and the parameter “Maximum Grid Current (A)” should be changed (see Figure 4). The areas for the calculation of the touch and step voltages are the same as those used for the simulation with the initial design. The program is run with the “Run” option from the “Simulation” menu, and with the option “Simulation Results” in the “Results” menu, we can observe the results table as shown in Figure 22.



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Figure 22. Results—modified design

Figure 23 and Figure 24 and Figure 25 and Figure 26 show the charts of the calculated touch and step voltages in two dimensions and three dimensions. As can be observed in Figure 22, the tolerable touch voltage for a 50-kg person is 677.8 V. This value is greater than the calculated touch voltage in the grid, which is 657.1 V. The tolerable step voltage is 2219.1 V, and it is greater than the calculated step voltage of 396.1 V. This modified design controls touch and step voltages, rendering them lower than the tolerable values, i.e., it meets the safety criteria and can be considered as the final design.



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Figure 23. Touch voltage—modified design—3D view

Figure 24. Touch voltage—modified design—2D view



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Figure 25. Step voltage—modified design—3D view

Figure 26. Step voltage—modified design—2D view


Substation grounding grid design using Alternative Transients Program-ATP and ASPIX

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