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CDEGS versus WinIGS: Soil Modelling Applications
S. D. Buba, W. F. Wan Ahmad, M. Z. A. Ab Kadir, C. Gomes, J.
Jasni1
M. Osman2
1. Department of Electrical and Electronic Engineering
Faculty of Engineering
Universiti Putra Malaysia
43400 UPM Serdang, Selangor, Malaysia.
2. Department of Electrical Power Engineering
College of Engineering
Universiti Tenaga Nasional
43000 Kajang, Selangor, Malaysia.
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Abstract
It is a well-known fact that the resistance of an earth
electrode system which could be as
complex as an earth grid or lightning protection earthing system
or single electrode system
largely depends on the site specific soil resistivity.
Therefore, to establish an effective earth
electrode system with sufficiently low resistance, an accurate
measurement to obtain soil
resistivity data must be done at the site and interpreted
correctly. This paper compares the
performances of two softwares typically used to develop such
soil model, i.e. CDEGS and
WinIGS. Soil resistivity data was collected at two sites where
distribution substations are to be
installed which was used as input to both CDEGS and WinIGS to
determine the soil models.
Results from the two softwares indicated that the soils at the
two sites had the same number of
layers but with slightly differing soil resistivity values.
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Introduction
In every electrical installation, adequate earthing is
considered to have utmost importance
particularly, the earthing of high-voltage substations in order
to protect people and equipment in
the event of an electrical fault. Well designed earthing systems
ensure correct operation and
performance of the power system and safety of personnel. It is
desirable that the substation
earthing provides a near zero resistance to remote earth. The
prevailing practice of most utilities
is to install earth grid comprising of horizontal earth
electrodes (buried bare copper conductors)
supplemented by a number of vertical earth rods connected to the
grid, and by a number of
equipment grounding mats and cable interconnections. The
earthing grid then provides a
common earth for electrical equipment and all metallic
structures at the station [1].
The main objective of earthing electrical systems is to provide
a suitably low resistance
connection to the earth electrode system such as a substation or
other earthed facility where low
resistance is required to limit the earth potential rise (EPR)
of a substation from the potential of
the surrounding environment. This EPR must be limited to an
extent that there is no danger to
person or livestock standing on the ground but touching, for
example, the substation fence. In
order to ensure that the EPR, touch and step voltages are within
safety limits, an accurate soil
model is needed to ensure that the resistance of the earthing
grid through the soil is sufficiently
low. The soil model is obtained after performing soil
resistivity measurement at the proposed
substation location [2], and typically using softwares, such as
CDEGS and WinIGS.
Soil resistivity is technically referred to as the resistance of
the soil to the passage of
electric current. Soils with low resistivity are generally
assumed to consist of abundance of
highly mobile ions that are capable of conducting electric
current and thus offer low resistance to
the flow of current. On the other hand, soils with high
resistivity typically lack an abundance of
mobile ions and are less able to conduct electric current [3-4].
Considering semiconductor
resistivity models and descriptions, resistivity depends on both
the number and mobility of free
charge carriers. In a typical soil model, the number of mobile
ions is primarily dependent on the
number and type of water-soluble compounds available in the
soil, and the amount of moisture
present in the soil. The mobility of the ions is therefore
governed by a combination of soil
moisture, soil grain size, temperature, and soil compaction, as
well as the surface
electrochemistry of the soil grains [3].
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Soil resistivity data has extensive applications in civil
engineering, electrical engineering,
geology and archaeology to mention but a few. In electrical
engineering, soil resistivity data are
useful for locating the best depth for installing low resistance
earth electrode system and are very
necessary when new electrical facilities such as generating
stations, substations, transmission
line towers, telephone exchange and mobile communication base
transceiver station are being
constructed. In addition, soil resistivity data is used to
indicate the expected degree of corrosion
in underground pipelines for water, oil and gas, which
facilitates the installation of cathodic
protection systems [4]. The purpose of soil resistivity
measurement is to obtain a set of data
which may be interpreted to yield an equivalent soil model to
facilitate the design of an earthing
system. Note that, when defining the electrical properties of a
portion of the soil, a distinction
between the geoelectric and geologic model is necessary. In the
geoelectric model the boundaries
between layers are determined by changes in resistivity which
primarily depends upon water and
chemical content, and the soil texture, while the geologic model
on the other hand is based upon
such criteria as fossils and texture, may contain several
geoelectric sections [4].
There are several techniques by which soil resistivity or
subsurface soil information could
be obtained, some of the common techniques includes,
self-potential (SP), four-electrode probe
method, vertical electrical sounding (VES), electrical profiling
(EP) and non-contact
electromagnetic profiling principles such as the ground
penetrating radar (GPR). VES and EP
techniques measure electrical resistivity or conductivity of
soil to any depth when a constant
electrical field is artificially created on the surface. VES and
EP techniques as well as other
laboratory techniques of measuring electrical resistivity in
soil samples are based on four-
electrode method, but vary considerably in electrode array
lengths and arrangements, which
make the methods very suitable for different applications. The
VES, EP, and SP techniques
evaluate parameters of the stationary electrical fields in
soils. All the techniques based on
stationary electrical fields require inserting electrodes into
the soil surface, therefore,
measurements using these principles could be made only in the
fields, rural areas, or in the
laboratory in soil samples. EM, NEP, and GPR on the other hand
introduce electromagnetic
waves of different frequencies into the soil. The EM, NEP, and
GPR evaluate properties of the
non-stationary electromagnetic fields in soils, they are mobile
as they do not require a physical
contact with the soil surface and can measure electrical
resistivity or conductivity in soils
covered with firm pavement [5-7]. Airborne resistivity mapping
was also reported in [8].
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Soil resistivity measurement is normally conducted using these
three methods namely,
Wenner, Schlumberger and Driven rod methods. Many factors
influence the selection of a
particular method, but normally, maximum probe depths, lengths
of cables required, efficiency
of the measuring technique, cost which is usually determined by
duration and the size of the
survey crew and ease of interpretation of the data are the main
factors that are considered when
selecting a particular test method [5]. In the Wenner method,
all four electrodes are moved for
each test with the spacing between each adjacent pair remaining
the same. Wenner method is the
most efficient in terms of the ratio of received voltage per
unit of transmitted current. In the
Schlumberger method, the potential electrodes remain stationary
while the current electrodes are
moved for a series of measurements. The driven rod method,
(three pin or Fall-of-Potential
method) is normally suitable for use in circumstances such as
transmission line tower earthing,
or areas of difficult terrain, because of the shallow
penetration that can be achieved in practical
situations, the local measurement area, and the inaccuracies
encountered in two layer soil
conditions. In all the three methods, the depth of penetration
of the electrodes has been
recommended to be less than 5% of the separation distance to
ensure that the approximation of
point sources, required by the simplified formula remains valid
[9-10]. The apparent soil
resistivity values could be calculated for any probe spacing
using Equations (1) and (2) for
Wenner and Schlumberger methods and Equation (3) for driven rod
method.
maRa 2 (1)
Where, a is the apparent soil resistivity in (-m), a is the
probe spacing in (m), R is the
resistance reading in () displayed by the measuring instrument
and is constant equal to 3.142.
ml
RLa
2
2 (2)
Where, a is the apparent resistivity in (-m), l is the distance
from centre line to inner probes
(m), L is the distance from centre line to inner probes (m), R
is the resistance reading displayed
by the measurement () and is a constant equal to 3.142.
m
d
l
lRa 8
ln
2 (3)
Where, a is the apparent resistivity in (-m), l is the length of
driven rod in contact with the
soil (m), d is the driven rod diameter (m), R is the measured
value of resistance displayed by the
instrument () and is a constant equal to 3.142.
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Soil resistivity measurements are often performed using suitable
measuring instruments
such as Megger Earth Tester DET3TC, Fluke 1631 Geo Earth Tester,
and Kyoritsu Earth Testers
etc. In principle, soil resistivity measurement is performed
using any of these instruments by
injecting electric current into the soil through two outer
current probes and the resulting voltage
between the two inner potential probes is measured and
displayed. The probes (stakes) are
normally arranged at equal distances and along the same straight
line. When the adjacent spacing
between the current and potential probes is small, the measured
soil resistivity is indicative of
local surface soil characteristics. On the other hand, when the
probe spacing is large, the
measured soil resistivity is indicative of the soil
characteristics at the extent of the depth. In
principle, soil resistivity measurements are made using spacing
(between adjacent current and
potential probes) that are, at least, on the same order as the
maximum size of the earthing
system(s) under study.
The results of soil resistivity measurement in its raw form does
not carry much
information without interpretation, thus, it is imperative to
interpret the results to obtain
meaningful information. IEEE Std. 80-2000 [11] has recommended
some equations for
averaging all the values which represent the measured apparent
resistivity data obtained at
different probe spacing and the total number of measurements
prior to interpretation. The
methods used for interpreting the results of soil resistivity
measurements are basically grouped
into empirical, analytical and computer based techniques.
Empirical methods are typically
developed through a combination of interpolation and field
measurements. The earliest method
of interpretation of soil resistivity field data is a graphical
method used to approximate a two-
layer soil model based on the interpretation of a series of
curves commonly called the Sunde
curves which allow for a rough approximation of the soil model
parameters without the use of a
computer or sophisticated equations. However, Sundes curve was
found to be in accurate as it
relied on the visual interpolation of the curves to determine
the soil model parameters [12].
Several other methods for interpretation of soil resistivity
data has been reported in
literature. A practical method for the interpretation of driven
rod test results that relies upon
simple hand calculations based on the semi-empirical expressions
for the resistance of a rod in
two layer soils was reported in [13]. The interpretation of
resistivity sounding measurements in
N-layer soil using electrostatic images method was proposed in
[14]. Also in [15-18] a new
method was proposed by deriving the theoretical equations for
calculation of apparent resistivity
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standard curves of horizontally multi-layered models, stating
that, for known soil parameters, the
apparent resistivity distribution could be computed efficiently
using the proposed method.
A statistical method and a computer program for interpreting
soil measurement data obtained
from four pin or three pin measurements was presented in [19].
Also, a simple analytical
formula was derived for the Sunde curves in [20] by generating
an infinite series of multiple
images in two-layer soil and replaced by their asymptotes which
was used to determine a two-
layer soil model through numerical optimization. However, due to
inaccuracies associated with
empirical and analytical methods, the use of computer programs
such as CDEGS and WinIGS
for soil resistivity data interpretation and modelling have
gained popularity in recent times and
are used in this study.
Brief Description of Softwares
The program WinIGS is an analysis/design tool for grounding
system design, multiphase power
system analysis, induced/transferred voltages, etc. With regard
to grounding system design, it
enables design of typical power system substation grounding,
overhead line tower/pole
grounding and any other grounding systems. The program WinIGS
supports the IEEE Standard
80 safety criteria as well as the IEC criteria for grounding
system safety. A number of other
specialized studies can be performed with the program WinIGS
[21].
The CDEGS software package (Current Distribution,
Electromagnetic Fields, Grounding
and Soil Structure Analysis) is a versatile set of integrated
engineering tool designed to analyse
problems involving earthing, electromagnetic fields,
electromagnetic interference including
AC/DC interference mitigation studies and various aspects of
cathodic protection. CDEGS
software computes conductor currents and electromagnetic fields
generated by an arbitrary
network of energized conductors above or below ground for normal
fault, lightning and transient
conditions. In this paper, the MultiGround package comprising of
RESAP, MALT and FCDIST
modules which are specialized for low frequency earthing
analysis and design was used.
Methodology
Two sites for installation of distribution substations were
identified in Universiti Putra Malaysia
(UPM) Serdang, Selangor, Malaysia. Site 1 occupies a land of
size of 12m x 12m and located
near the Surau at College 12 in UPM, while Site 2 occupies a
land size of 15m x 15m and
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located adjacent to the Canteen at Faculty of Engineering, UPM.
Soil resistivity measurement
was conducted at the two sites using a Megger Earth Tester
according to Wenner method. The
measurement traverse followed the four sides of a rectangle and
a diagonal for each probe
spacing. The field data from the measurement traverse was
initially averaged using Equation (4)
yielding Tables 1a and 2a which was used as input to the RESAP
module of CDEGS software.
The same data was also used as input to WinIGS software. The
soil models produced by the two
softwares were then compared for similarity and difference.
iNi 121
....
(4)
Results and Discussion
Table 1a lists the soil resistivity field data collected at Site
1 which served as input to both
CDEGS and WinIGS softwares. Table 1b depicts the soil model
developed by RESAP module of
CDEGS. It indicates that the soil structure at Site 1 consists
of two layers. Layer 1 being the top
soil layer has a resistivity of 2231.9-m and a thickness of
1.11m, while the second layer, i.e.
bottom layer has a resistivity of 752.4-m and infinite
thickness. For earthing purposes, the
earth grid would normally be installed within the bottom layer
to take advantage of lower
resistivity and also to allow for installation of vertical earth
rods.
Table 1a, Average of measured soil resistivity field data Site
1
Probe Spacing
(m)
Average Apparent
Resistance () Average Apparent
Resistivity (-m)
1.0
2.0
3.0
4.0
5.0
6.0
308.6
110.2
52.8
33.6
32.0
20.0
1,938
1,384
995
844
1,005
754
Table 1b, Soil structure/model developed by RESAP Site 1
Layer Number Resistivity (-m) Thickness (m)
1
2
2231.913
752.4407
1.113013
infinite
Figure 1 shows the soil model developed by WinIGS which
indicates that the soil structure at
Site 1 consists of two layers. The upper layer soil has a
resistivity of 2279.1-m and a thickness
of 3.5ft, while the second layer, i.e. bottom layer has a
resistivity of 770.3-m and infinite
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thickness. Comparing the performance of CDEGS and WinIGS, it
could be observed that the
resistivity of the top soil layer is lower in CDEGS and higher
in WinIGS indicating a difference
of 47.2-m which may not be neglected. Considering the second
soil layer, converse is the case,
the resistivity of the bottom layer is higher in WinIGS than
CDEGS with a difference of
approximately 18-m which may be neglected. Figure 2 illustrates
the model fit for Site 1.
Case Name DETERMINATION-OF-SOIL-MODEL-FOR-SITE-1
2279.1
Soil Resistivity Model
Upper Soil Resistivity Ohm Meters
770.3
Upper Layer Thickness Feet3.5
Lower Soil Resistivity Ohm Meters
29.5Results are valid to depth of Feet
Grounding System / Geometric Model
Description
CloseWenner Method Soil Parameters
635.6
127.2
1.0
90.0At Confidence Level %
ToleranceExp. Value
Error:Error:Error:Conf: Conf: Conf:
Program WinIGS - Form SOIL_RA
Close
2279.1
Measured
Soil Resistivity Model
Upper Soil Resistivity Ohm Meters
3.5Upper Layer Thickness Feet
770.3Lower Soil Resistivity Ohm Meters
Plot Cursors
File:
Description: Grounding System / Geometric Model
Computed
Wenner Method Model Fit Report
Separation Distance Linear
Log
X Scale
Program WinIGS - Form SOIL_RB
Figure 1 Soil parameters for Site 1using WinIGS
Figure 2 Soil model developed by WinIGS
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Table 2a depicts the soil resistivity field data collected at
Site 2 also used as input to CDEGS and
WinIGS softwares. The soil model developed by CDEGS is listed in
Table 2b which reveals that
the soil structure also consist of two layers. The top layer
i.e. the first layer has a resistivity of
60.4-m with a thickness of 0.55m, while the second layer, i.e.
the bottom layer has a resistivity
of approximately 42-m. Although, Site 2 has obviously low soil
resistivity, it is recommended
that earth grid should be installed within the second layer to
benefit from the lower value of
resistivity.
Table 2a, Average measured soil resistivity field data Site
2
Probe Spacing
(m)
Average Apparent
Resistance () Average Apparent
Resistivity (-m)
1.0
2.0
3.0
4.0
5.0
6.0
7.0
8.5
3.2
2.3
1.8
1.6
1.0
1.0
53.4
40.2
43.3
45.2
50.2
38.0
44.0
Table 2b, Soil structure/model developed by RESAP Site 2
Layer Number Resistivity (-m) Thickness (m)
1
2
60.43904
41.95911
0.5563730
infinite
Figure 3 illustrates the soil parameters produced by WinIGS
indicating that the soil structure at
Site 2 comprise of two layers. The upper layer has a resistivity
of 79.7-m and a thickness of
1.2ft, while the lower layer has a resistivity 42.8-m and
infinite thickness. Comparing CDEGS
and WinIGS with regard to Site 2, it could be observed that
upper layer resistivity in Table 2b is
lower than the upper layer resistivity in Figure 3 with a
difference of 19.3-m, however, the
resistivity of the lower soil layer is almost the same where
approximately 42-m was recorded
from CDEGS and 42.8-m from WinIGS. Figure 4 shows the model fit
for Site 2 which
indicates the accuracy of the measured resistivity data.
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Apart from the output functions, there are other features of the
two softwares that could be
compared. CDEGS software has enjoyed a popular usage by
electricity utility companies and
academic institutions in Malaysia but the WinIGS software is
rarely mentioned despite its
potentials. In terms of cost, CDEGS is very expensive but
consists of many packages where
customers have a choice based on their budgets, unfortunately,
the cost of WinIGS could not be
obtained for comparison. Considering the ease of usage, CDEGS
consists of modules within
packages and each module could be accessed directly from the
desktop but, WinIGS is an
Case Name DETERMINATION-OF-SOIL-MODEL-FOR-SITE-2
79.7
Soil Resistivity Model
Upper Soil Resistivity Ohm Meters
42.8
Upper Layer Thickness Feet1.2
Lower Soil Resistivity Ohm Meters
34.4Results are valid to depth of Feet
Grounding System / Geometric Model
Description
CloseWenner Method Soil Parameters
54.1
4.8
90.0At Confidence Level %
ToleranceExp. Value
Error:Error:Error:Conf: Conf: Conf:
Program WinIGS - Form SOIL_RA
Close
79.7
Measured
Soil Resistivity Model
Upper Soil Resistivity Ohm Meters
1.2Upper Layer Thickness Feet
42.8Lower Soil Resistivity Ohm Meters
Plot Cursors
File:
Description: Grounding System / Geometric Model
Computed
Wenner Method Model Fit Report
Separation Distance Linear
Log
X Scale
Program WinIGS - Form SOIL_RB
Figure 3 Soil parameters for Site 2 using WinIGS
Figure 4 Soil model fit using WinIGS
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integrated software in which access to soil resistivity platform
could only be gained by invoking
the substation grounding tool. WinIGS software have excellent
features with regard to output
features for soil resistivity as it adequately takes care of bad
data, it also shows the degree of
confidence for soil models, tolerance level and the depth at
which the results are valid as
indicated in Figures (1) to (4), these features are not provided
by CDEGS results as far as I
know, however the percentage discrepancy between measured and
calculated values of soil
resistivity is indicated. In summary, the soil models developed
by CDEGS and WinIGS are
closely related and may be considered to be almost the same
within the limits of instrument and
simulation errors.
Conclusion
The performance of CDEGS and WinIGS softwares for soil modeling
has been presented
considering the result produced by each software. Other issues
such as cost, user friendliness,
popularity and unique features were also compared. It was found
that there was no much
difference in the values of soil resistivity for bottom (lower
soil layers) for both Sites 1 and 2
from the results produced by CDEGS and WinIGS, however, there
was variation of soil
resistivity for the top soil (upper soil layer) in both cases
but not extreme in value. Therefore, it
could be concluded that any of the two softwares is recommended
for soil modeling applications.
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