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

GROUNDING GRID DESIGN PAGE 1
http://www.spartalightning.com/
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. Measuring and analyzing the soil resistivity.
2. Plotting the grounding grid layout.
3. Calculating the grounding grid resistance with the initial design.
4. Calculating the earth current distribution.
5. Calculating the touch and step voltages.
6. Verifying the touch and step voltages.
7. 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

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
115 kV
150 MVA
Ynyn0D11
Z1 = 14% Z0= 14%
230 kV
Double circuit line
36 km
34,5 kV
25 MVA Dyn5
Z1 = 12%
Z0= 12%
Single circuit line
58 km
34,5 kV
25 MVA Dyn5
Z1 = 12%
Z0= 12%
Source
Substation
Load 1
substation
Load 2
substation
115 kV 115 kV
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.

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.

Table 1. Resistivity measurements
Wenner Method Schlumberger-Palmer Method
a (m) a (m) c(m) a (m) c(m)
2 4 8 16 4 32 4
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
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
a (m) c(m) a (m) c(m) a (m) c(m) a (m) c(m)
2 4 8 16 4 32 4
P1 (Ohm_m) 193.1 168.5 139.6
P2 (Ohm_m) 167.7 185.3 185
P3 (Ohm_m) 222.3 129.5 117.9
P4 (Ohm_m) 245.2 175.6 84
P5 (Ohm_m)
P6 (Ohm_m)
P7 (Ohm_m)
P8 (Ohm_m)
P9 (Ohm_m)
P10 (Ohm_m)
Average 207.1 164.7 131.6
Std Dev 33.82 24.47 42.3
Resistivity_70% 223.6 177.4 150.4
Resistivity 2LModel 223.3 176.7 146.8
Estimated 2 layer soil parameters
1 (Ohm_m)
2 (Ohm_m)
k
H(m) Wenner Method Schulumberger-Palmer Method
256.14
136.35
-0.31
1.82
101.7
164.4
10000.00 1.00
10000.00 1.00
0.99 -0.99
20.00
210.7
211.4 66.9
Resistivity Analyzer
a (m)
Profile
Unequally Spaced - Schlumberger-Palmer
Method
Equally Spaced - Wenner Method
123.4
147.2 93.2
45.334 62.518
177.134 145.573
0.10
Upper Limit Lower Limit
156.175 123.55
162.309165.02
ResistivityAnalyzer
Version1.0
0
50
100
150
200
250
0 5 10 15 20 25 30 35
Resistivity(ohm-m)
Voltage Electrodes Separation Distance a (m)
Resistivity_70% Resistivity 2LModel
a a a
I
V
c a
V
c
I
Analyze

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.

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.

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.

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.

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.

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.
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.
Load 1 substation
Load 2 substation
Source substation
Grounding grid
resistance
Tower footing
resistance

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.
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.
Substation
grounding grid
Grounding in
the fault pointTowers footing resistances
Towers footing resistances
115 kV
34.5 kV
2720 A
1121 A
815 A
875 A
2720 A
Ground wire
Ground wire
(f ile Ejemplo_Aspix.pl4; x-v ar t) c:MALL -NEUT c: -MALL c:CG1 -MALL c:CG2 -MALL
c:FALL -
0.00 0.02 0.04 0.06 0.08 0.10[s]
-4000
-3000
-2000
-1000
0
1000
2000
3000
4000
[A]
Fault ground current and
transformer neutral
current
Substation grounding
grid current
Lines ground wires
current

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.

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 two-
dimensional 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.
Area 1
Area 2
Area 3
Area 4
Area 5

Figure 13. Simulation results
Figure 14. Touch voltage—3D view

Figure 15. Touch voltage—2D view

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

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.

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.

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.

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.
Substation
grounding grid
Grounding in
the fault pointTowers footing resistances
Towers footing resistances
115 kV
34.5 kV
2732 A
1188 A
793 A
856 A
2732 A
Ground wire
Ground wire

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.

Figure 23. Touch voltage—modified design—3D view
Figure 24. Touch voltage—modified design—2D view

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