ARCHIVES OF ELECTRICAL ENGINEERING VOL. 70(3), pp.

601 –613 (2021)

DOI 10.24425/aee.2021.137576

Reduction of the step voltages of MV/LV substation

grounding system based on shaping electric field

ROMAN SIKORAo , PRZEMYSŁAW MARKIEWICZo

Institute of Electrical Power Engineering

Lodz University of Technology
Stefanowski str. 18/22, 90-924 Lodz, Poland
e-mail: {roman.sikora/przemyslaw.markiewicz}@p.lodz.pl

(Received: 14.10.2020, revised: 21.02.2021)

Abstract: The article presents the analysis results of the effectiveness limitation of the step
voltage by forming an electric field on the ground surface. For shaping the electric field,
a method consisting of screens placed around the point of the earth current flow was used.
The analysis was performed using an example of an MV/LV substation grounding system.
This research was conducted applying a mathematical model of the grounding system and
screens by means of the finite element method. The influence of metal, insulating screens
and surface material on the step/touch voltage values for the considered grounding system
was estimated. Most of the methods described can be applied in practice. In the opinion of
the authors, the method of using screens made of insulating and conductive materials has
not been sufficiently described in the literature. Moreover, in the available literature there
is no in-depth analysis of the described electric field shaping methods.
Key words: grounding system, step voltage, touch voltage, IEEE 80-2013 standard

1. Introduction

One of the elements of the power system responsible for its proper operating and safety of
use are grounding systems. The basic part of such installations is the earth electrode located
below the ground level. For the simplest structural solutions, it is implemented in the form of
a metal strip buried horizontally, vertical rods or both. The metal strip is placed underground
parallel to the ground surface. When there is a need for a grounding system with low resistance
RE for power substations, it is made in the form of a grid made of steel tape or rods with
dimensions depending on the planned or existing power substation. The basic grounding system

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602 Roman Sikora, Przemysław Markiewicz Arch. Elect. Eng.

parameter, which is the resistance of the grounding system electrode, depends on such factors as
the geometrical dimensions of the buried grid and the soil resistivity ρ. Local factors such as the
chemical composition of soil, its temperature or humidity result in large ranges in the resistivity
values.
The second parameter characterizing the grounding system is the value of the step voltage
Vstep . This value is defined as the difference of potentials on the ground surface between points
spaced from each other by the size of the contractual step (1 m). From the point of view of
designing the grounding installation, the step voltage cannot exceed the set limit value in order to
eliminate the possibility of the electric shock of a person standing on the ground surface.
The next characteristic parameter describing the degree of electric shock risk is the touch
voltage Vtouch , which is measured as a potential difference between the conductive part of the
grounding system accessible to touch and a point on the ground surface at 1m from it. Figure 1
depicts the step voltage Vstep and the touch voltage Vtouch .

Fig. 1. Definition of step voltage Vstep and touch voltage Vtouch

The method of measuring the step/touch voltage is described in [1]. Another method of
estimating the step/touch voltage is calculating the distribution of the electrical potential on the
ground and its resistance. The value of the step/touch voltage cannot exceed the recommendations
contained in the standards [2, 3]. The essence of the problem is to find a solution to the flow field
equation in a conductive environment around an electrode buried in the ground. For stationary
fields, this is the solution of the Laplace Equation (1) for the electric potential of VE .

∇2VE = 0. (1)

The accurate solution of Equation (1) can be obtained using analytical methods for simple
theoretical cases and assuming the uniformity of soil parameters. In the case of complex grounding
system geometry, other methods of solving Equation (1) are used. Analytical methods used to
Vol. 70 (2021) Reduction of the step voltages of MV/LV substation grounding system 603

design a grounding system are an issue raised in the subject standards [2–4]. Due to the difficulty in
determining the boundary conditions, with the complex shapes of the search areas of the electric
field, analytical methods are used for theoretical cases such as the hemisphere or roller. The
solutions obtained for such geometry make it possible to verify the results obtained by numerical
methods of calculation and estimation errors. In numerical methods, the shape of the considered
area is virtually arbitrary, and the only limit is computational effort.
When initiating a grounding system project, one should first take the following input data
into account: the soil resistivity ρ (Ωm), earth current IE (A), earth current flow time t C (s),
geometric dimensions of the area to be occupied by the grounding system, the depth of burying
the grounding system hGS (m), the material used for its construction, the type and thickness of
the surface material.
The most reliable method of evaluating the resistivity of soil is to perform measurements on
the grounding system location. In the absence of such possibilities, the tables contained in the
standards can be used [2–4]. The value of the current IE and the duration of its flow t C depend on
the structure of the power network and the adopted protection system settings. The burial depth
of the earth electrode hGS is usually between 0.5 and 1 m. The permissible values of voltage
are derived from the assumed time of the earth current flow and they are given in a graphical or
analytical form in [2–4].
Paper [5] promotes the awareness of hazards connected with the potential on the surface
of the ground. It has been proven that the earth potential values may be above the permissible
value if the voltage is up to 220 V AC. Paper [6] presents the method of measuring the existing
grounding systems and provides the results and statistical data for 6 selected substations that
have been recently designed. To calculate the ground resistance quickly and conveniently with
the required accuracy, paper [7] proposes an algorithm that uses the method of fitting the least
squares curve and hyperbolic matching functions to match the resistivity and soil resistance of
the layered soil model. Article [8] presents research on the impact of various soil types and shock
wave characteristics in the aspect of human safety. The test focuses on the step/touch voltage
and transferred potentials generated by lightning striking the grounding system and the potential
electrical shock hazard for humans. The problem of finding the potential in the amorphous soil
is discussed in [9], where an analytical method has been proposed. This method is used to study
the electrical properties of a ground electrode connected to a metal mesh which is embedded in
concrete. Article [10] contains the development of a new methodology for measuring earthing
resistance and the step voltage in the supply network of urban power substations. The article
includes the mathematical model for estimating and predicting the work of the earth electrode,
based on the polynomial regression from the database method (PRED).
A grounding system modelling is a multi-stage and relatively complicated process. In the
literature [11–16], a number of mathematical models and computational methods are described.
In the first modelling stage, the input parameters such as the geometry of the earth electrode, the
physical and chemical properties of the soil and the number of earth current flow points should
be selected. In the next stage, a mathematical model is formulated, the form of which depends
on the chosen solution (analytic or numerical). The mathematical model is then implemented in
a given programming environment if a numerical method is used for the solution. In the fourth
stage calculations are made. The calculation time is closely related to the numerical method used.
In the last stage, the obtained results of calculations should be validated. The choice of the input
604 Roman Sikora, Przemysław Markiewicz Arch. Elect. Eng.

parameters of the model should be made very carefully because the adoption of imprecise or
incorrect values results in obtaining erroneous results. The most difficult aspect related to the
selection of input parameters of the grounding system model is the determination of the right soil
parameters. This issue is discussed, among others, in [17]. The typical values of the basic soil
parameter, which is its specific resistivity, are included in many subject standards and manuals
e.g. [2–4, 18]. The problem of choosing the right value of soil resistivity is closely related to the
problem of whether the soil can be treated as an area with homogeneous properties or whether
it is a heterogeneous area. This is described in the literature in [12–14]. In the article [12], the
authors discuss in detail the equivalence of the use of a significantly simplified homogeneous
or two-layer soil model with a multi-layered model. In [12], an analysis and comparison of
the grounding resistance and the touch voltage was made for various equivalent soil models. A
solution to the issue of ensuring that a one or two-layer model is representative of a multi-layered
structure is presented. An attempt was made to answer the question whether one can develop
a good soil model that allows to properly determine various parameters defining the grounding
system as resistance, touch voltage and current distribution simultaneously. The authors of the
article [12] state that it is difficult to find a single-layer model that could successfully replace
a complex multi-layered model. The selected one-layer model compared to the multi-layered
one gives similar results for the resistance of the earth electrode, but the obtained values of
the touch voltage are divergent. Paper [19] presents a numerical technique on the basis of the
Boundary Element Method (BEM) for the analysis of horizontal and vertical grounding systems.
The proposed method has been demonstrated by its application to the actual grounding network,
taking into account different types of soil models. Realizing the needs of engineers for a flexible
and reliable tool for estimating and predicting the behaviour of a grounding system, paper [20]
has developed a model that accurately describes and predicts the variability of soil resistance. The
main purpose of [21] is to develop an optimization model for designing grounding systems for
electrical substations. The design of the grounding grid in the substation is formulated as a linear
integer programming problem. The developed optimization model takes into account structural
features, in addition to technical and safety requirements related to the construction, installation
and operation of these power networks. Typical grounding systems configurations are used in
MV/LV distribution substations often without assessing their performance in terms of safety in
the event of a critical electric shock due to step/touch voltages which arise in the event of a ground
fault. Thanks to this, the safety of an existing or new MV/LV substation can be easily assessed
using a safety curve. In [22] the simple safety assessment method of typical MV/LV substation
groundings is introduced on the basis of simple calculations.
In practical applications, different models should be used to calculate resistance and touch
voltage. A grounding grid can be replaced with a substitute diagram with fixed constants, which
is presented in [23]. The model consists of resistances representing individual grid segments
and current sources. To determine the flow of current in the circuit constructed in this way, the
node potential method is used. Paper [24] presents an advanced method for calculating grounding
systems based on the calculation of a substitute diagram of the buried electrodes forming one unit.
In the case of designing the grounding system for lightning protection, analysis of its operation
in the transient states can be used [15].
This paper proposes the use of screens made of conductive and non-conductive materials to
properly shape the electric field on the ground level. Thanks to this, lower values of step voltages
Vol. 70 (2021) Reduction of the step voltages of MV/LV substation grounding system 605

can be obtained, which significantly improves the effectiveness of shock protection. Analytical
methods known from the literature [2, 3] used in the design of earth electrodes make it possible
to include, as a technical measure only, the surface material. If screens are used to shape the
electric field, it is virtually impossible to obtain a solution using analytical methods. To analyse
this problem, a model of a grounding grid with assumed parameters was made. To determine the
distribution of the electric field around the working earth electrode, the numerical method called
Finite Element Method (FEM) was used. This method is used to calculate the distribution of
electromagnetic field in the analysis of various engineering problems [25, 26]. The calculations
of the electric field distribution were made in the steady state. Both step and touch voltages were
analysed in the paper.
Most of the methods described can be applied in practice. In the opinion of the authors, the
method of using screens made of insulating and conductive materials has not been sufficiently
described in the literature. Moreover, in the available literature there is no comparison of the
effectiveness of the described electric field shaping methods.

2. Limiting the value of the step/touch voltage by forming an electric field

on the ground surface using conductive and insulating screens
Ground grids are the integral elements of power substations. Their proper design, realization
and maintenance reduce the risk of electric shock to the station’s staff and even to an ordinary
person. Such a risk exists with MV/LV stations as they are most numerous and often accessible
to ordinary people. Therefore, the analysis of the effectiveness of using screens to limit the values
of step/touch voltages was performed for the MV/LV substation grounding system.
The considered grounding system is made in the form of a grounding grid made around the
substation. Due to the design constraints, the current flow point is located on the outer edge of the
grid. This positioning of the current flow point is least favourable due to the occurrence of a large
gradient of the electric field potential. The value of the earth current results from the assumption
of a ground fault on the MV side. In this case, a part of this current closes via the grounding
system of the substation.
The change in the way the neutral point works in the power system from isolated to grounded
causes a significant increase in the value of the earth current (from several dozen to even three
hundred amperes). The existing MV/LV grounding grid was not designed for such a significant
increase in the value. There is therefore a supposition that the permissible values of the step
and touch voltages defined in [4] will be exceeded. Obtaining the required values of touch and
step voltages requires a modification of the grounding grid. The occurrence of hazardous shock
voltages can be effectively limited by reducing the resistance to earth RE . This can be achieved,
for example, by increasing the size of the grounding grid or its modification. In practice, the
modernization consists in making additional vertical electrodes connected to the existing grid,
and then performing a series of necessary measurements, which is not technically possible in every
case and usually involves a lot of investment. Another method of reducing the step voltages is to
shape the electric field on the ground surface. Proper shaping of the electric field makes it possible
to obtain the smallest step voltage values [2–4]. As mentioned above, surface material is applied
on the ground surface. The application of screens in the most hazardous areas is an alternative
606 Roman Sikora, Przemysław Markiewicz Arch. Elect. Eng.

to using the surface material. The applied surface material can be made of materials with higher
or lower specific resistivity than the native soil, which causes a change in the permissible step
voltage [2, 3]. The technical method in the form of screens does not change the permissible value
of shock voltages.
For the analysis, the authors assumed the dimensions of MV/LV grounding grids, soil re-
sistivity and the grounding current value. To analyse the effect of screens on the shaping of the
electric field, a simple grid with dimensions 6.3 × 6.3 m, buried at the depth of hGS = 0.8 m
was used during the work of the earth electrode. The grid was made of steel with mesh number
n = 4. The model of the grounding system is shown in Figure 2. The flow point is located on the
edge of the grid. For the calculations, it has been assumed that the ground has specific resistivity
ρ = 100 Ωm. For the analysis a system with isolated neutral was chosen. The value of the earth
current is computed as:

IE = r · IC , (2)
√
IC = 3 · ωCC · VR . (3)

Fig. 2. Ground grid layout (soil 60 × 60 × 30 m)

According to [4] for single-core XLPE cables (10 kV and 20 kV) with copper screen
r = 0.5−0.6. For calculations r = 0.6, ωCC = 7.70 mS and VR = 15 kV were assumed. Thus,
the earth current equals 40 A.
For easy presentation of the results obtained, an auxiliary coordinate system was used (the
coordinate axes are marked as a, b and c). The coordinate system is located on the ground surface,
and its beginning is 3 m from the point of the current flow. The results presented further in
Figures 5 to 7 are shown for axis a, which was introduced in the place of the expected maximum
value of the electric potential on the ground level VE (step voltage Vstep or touch voltage Vtouch ).
For the flow field, the earth current flows into the point of the current flow with values resulting
from the previously assumed assumptions. The soil model has the dimensions of 60 × 60 × 30 m.
The boundary condition is the acceptance of the reference earth potential on the outer edges of
the soil model (VE = 0).
Vol. 70 (2021) Reduction of the step voltages of MV/LV substation grounding system 607

The calculations of the electric field distribution have been made for the following grounding
grids variants:
1) the grounding grid in homogeneous soil,
2) the grounding grid with surface material 8.3 × 8.3 m, hs = 3 cm, ρs = 1 Ωm,
3) the grounding grid with surface material 8.3 × 8.3 m, hs = 3 cm, ρs = 2000 Ωm,
4) the grounding grid with isolation screen 8.3 × 8.3 m, hsc = 0.4 m, ρs = 10e + 12 Ωm,
5) the grounding grid with isolation screen 3 × 3 m, hsc = 0.4 m, ρs = 10e + 12 Ωm,
6) the grounding grid with metal screen 3 × 3 m, hsc = 0.4 m.
In the proposed modernization variants through the application of a surface material, the
value of the coefficient Cs [2] is 60.4 for ρS = 1 Ωm and is equal to 0.43 for ρS = 2000 Ωm. For
such assumptions, the grounding grid model made in the three-dimensional space in the ANSYS
environment was made using the FEM method. The assumed aim of the simulation calculations
is to select such a modernization variant of the grounding grid as to obtain the minimum value of
the step voltage without changing its geometry and/or construction.
The cross sections of the grounding grid models for variants 1, 2 and 3 are shown in Figure 3.
The cross-section of the grounding grid model for screen variants 4, 5 and 6 is shown in Figure 4.

(a) (b)

Fig. 3. The cross section of the grounding grid model for variant 1(a); the cross section of the grounding
grid model for variants 2 and 3(b)

Fig. 4. The cross section of the grounding grid model for variants 4, 5 and 6

The surface material on the ground level is made of a material layer with a given resistivity
of proper thickness hs = 3 cm, in the shape of a square with a side wider than the outer edge of
the earth electrode by 1 meter. The surface material has been symmetrically arranged over the
buried earth electrode. The screens are in the shape of a square buried at half the depth of the
earth electrode symmetrically around the current flow point.
608 Roman Sikora, Przemysław Markiewicz Arch. Elect. Eng.

3. Analysis of the obtained simulation results

For the above assumptions, calculations of the electric potential distribution VE were made.
The calculation results of the electric potential distribution VE along the axis a on the ground level
for homogeneous soil (variant 1) are shown in Figure 5(a). The highest value of the VE potential
occurs at the point of the earth current flow and it is 240.94 V. The distribution of potential on
the ground over the buried earth electrode is fairly even. A large increase in the potential value
occurs on the ground along the edges of the earth electrode. After the application of the surface
material with resistivity ρs = 1 Ωm (variant 2), the distribution of the potential has changed, as
illustrated in Figure 5(b).

(a) (b)

Fig. 5. Distribution of the electric potential VE along the axis a on the ground level for homogeneous
soil (a); distribution of the electric potential VE along the axis a on the ground level for surface material
with ρs = 1 Ωm (b)

The maximum value of the potential at the point of the current flow decreases slightly.
Noticeable changes occur in the area of 2 m from the edge of the earth electrode. Potential
values do not decrease as fast as for variant 1. The use of the surface material with resistivity
ρs = 2000 Ωm does not change the distribution of the potential on the ground. It is practically
identical to that for homogeneous soil (Figure 6(a)). The application of the surface material, with
both low (variant 2) and large (variant 3) resistivity, did not bring the expected results, because
the potential of the ground surface around the current outlet point was not reduced. Therefore,
calculations were made for the modernization variants in which shields of insulating and metal
(steel) materials were used to shape the electric field on the ground level. The calculation results
of the electric potential on the ground surface for the insulating screen of dimensions 8.3 × 8.3 m
(variant 4) are shown in Figure 5(b) and for the insulation screen with dimensions 3 × 3 m
(variant 5) in Figure 7(a).
Unfortunately, the use of insulation screens also did not bring the expected effects in the form
of the electrical potential at the runoff point and its even distribution. In addition, for variant 4, the
highest value of the electric potential was 254.04 V (Table 1). Based on the calculations made for
Vol. 70 (2021) Reduction of the step voltages of MV/LV substation grounding system 609

(a) (b)

Fig. 6. Distribution of the electric potential VE along the axis a on the ground level for surface material with
ρs = 2000 Ωm (a); distribution of the electric potential VE along the axis a on the ground level for isolation
screen 8.3 × 8.3 m (b)

(a) (b)

Fig. 7. Distribution of the electric potential VE along the axis a on the ground level for isolation screen
3 × 3 m (a); distribution of the electric potential VE along the axis a on the ground level for steel screen
3 × 3 m (b)

variant 6 (Figure 7(b)), in which the steel screen is used, the distribution of the electric potential
on the ground surface is more uniform in the vicinity of the current outlet point. The maximum
value of the potential is lower than in the previously analysed cases. For the area on the ground
level, the value of the electric potential drops markedly with increasing distance from the current
outlet point. This phenomenon occurs for all variants of the grounding system modification.
The main aim of the proposed methods for the modification of the grounding system is
to strive for a possibly homogeneous distribution of the electric potential on the soil surface.
610 Roman Sikora, Przemysław Markiewicz Arch. Elect. Eng.

Table 1. Maximum values of the electric potential VE , step voltage Vstep and touch voltage Vtouch for the
analysed variants of modernization

The maximum
The maximum The percentage
Variant of value of the step The percentage
value of the value of the
moderniza- voltage V step (V) value of the step
electrical electric potential
tion (touch voltage voltage V PU
step (%)
potential V E (V) V PU
E
(%)
V touch )
1 240.94 112.32 100 100
2 230.86 162.64 195.82 155.77
3 244.78 117.49 101.59 104.60
4 254.04 129.66 105.44 115.44
5 244.58 115.18 101.51 102.55
8 210.80 160.45 187.49 153.82

Consequently, this will provide the desired step voltage Vstep limitation. The best effects on the
shape of the electric field (the distribution of the potential) are obtained for the surface material
with resistivity ρs = 1 Ωm (variant 2) and a steel screen of dimensions 3.3 × 3.3 m (variant 6). In
the case of using the surface material with a resistivity of ρs = 1 Ωm (variant 2), however, it was
not possible to limit the occurrence of a large potential gradient in the immediate vicinity of the
current flow point. The step voltage values Vstep have been determined on the basis of the obtained
electric potential distribution. Step voltages were determined in accordance with the definition
presented in [2] as the difference in the electrical potential between the points separated by 1 m
from each other. The value of the step voltage clearly shows the effectiveness of the electric shock
protection. If this value is lower, the protection against electric shock is more effective. The most
unfavourable case occurs when a person is standing at a distance of 1 m from the point of current
flow and touches the metal conductive elements of the grounding system.
The percentage reduction/increase values of the electrical potential, the step and touch voltage
were calculated using (5) and (6) and the results are given in Table 1. The values of the step voltage
were calculated from its definition.
The maximum step voltage Vstep max for the installation under analysis is also the touch voltage

Vtouch (Equation 4).

Vtouch = Vstep
max
, (4)
VEv
VEPU = , (5)
VE1
v
Vstep
PU
Vstep = 1
, (6)
Vstep
where:
VEv is the maximum value of the electrical potential for a given variant, in (V),
Vol. 70 (2021) Reduction of the step voltages of MV/LV substation grounding system 611

VE1 is the maximum value of the electrical potential for homogeneous soil (variant 1), in (V),
v
Vstep is the maximum value of the step voltage for a given variant, in (V),
1
Vstep is the maximum value of the step voltage for homogeneous soil (variant 1), in (V),
Based on the results presented in Table 1, it can be concluded that the step voltage increased
for the following modernization variants of the grounding system: 3, 4 and 5. Due to the assurance
of the proper effectiveness of the protection against electric shock, these variants should not be
used. For variant 2, the maximum step voltage value drops to 62.64 V, i.e. its value was reduced
by 55.77%. In the case of variant 6, the maximum step voltage value decreases to 60.45 V.
In comparison with variant 1, its value was reduced by 53.82%. During the design process,
the obtained maximum values of the step/touch voltage are compared with the limit values. To
determine them, the values of the shock current flow time t s are required. A properly designed
grounding system should ensure that the permissible values are not exceeded, thus eliminating
the possibility of electric shock. Due to the limited paper length, the dependence on limit values
of the shock voltages determined on the basis of other standards was not considered.

4. Conclusions

The paper presents the analysis results of applying conductive and insulating screens to
reduce the step/touch voltage values. The maximum value of the step and touch voltage reduction
is realized by the appropriate shaping of the electric field on the ground surface. Reducing the
step and touch voltages improves the effectiveness of the shock protection. As the structure of
a distribution network changes frequently, it may cause an increase/decrease in the earth current
value. An increase in the earth current value heightens the probability of exceeding the permissible
step/touch voltage values, in which case it is necessary to take action to reduce this value. As this
paper proves, one of the most effective and economically viable methods of reducing step/touch
voltages is the use of screens. The concept of screen application is not widely described in the
literature and its effectiveness has not been proved. The aim and the main achievement of the
research described in the paper is to accurately estimate the possibility of reducing the step/touch
voltage by means of using screens.
This paper presents a comparison of the electric field shaping method using surface material
with the method involving insulating and metallic screens. The investigations were carried out for
a typical MV/LV pole substation grounding system. The value of earth fault current (single-phase
short-circuit) on MV side was assumed in considerations. The single-phase short-circuit current
on the LV side will have a much higher value than that assumed for the analysis by the authors.
Due to the assumed linearity of the grounding system, the percentage reduction in shock voltages
(step and touch) compared to variant 1 (baseline) will be the same. Based on the analysis, the best
method to reduce the step/touch voltage is to use a metal screen as it limits the maximum values
of the electric potential and the step/touch voltage to a large extent (by up to 50%). In addition,
it causes a more even distribution of the electrical potential in the area surrounding the ground
current flow point, which results in improved effectiveness of the shock protection. The only
drawback of this solution is the need for adequate anti-corrosion protection of the metal screen.
To protect the screen against corrosion it is possible to use galvanic protection (zinc plating) or
anti-corrosive conductive paint.
612 Roman Sikora, Przemysław Markiewicz Arch. Elect. Eng.

The analysis of kiosk substation grounding systems is a very complex issue. The modelling of
the foundation grounding system given its complexity in terms of structure and the heterogeneity
of the concrete is a complex computational problem. The resistance of the foundation grounding
system will influence the resulting resistance of the substation grounding. In this case, the values
of touch and step voltages will be less. The paper analyzes the variant, in which there is a higher
risk of negative effects of electric shock.

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