seasonal influences on safety of substation grounding systems seminar report
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Seasonal Influence on Safety of Substation Grounding
1. INTRODUCTION
With the development of modern power system to the direction of
extra-high voltage, large capacity, far distance transmission and application of
advanced technologies the demand on the safety, stability and economic
operation of power system became higher. A good grounding system is the
fundamental insurance to keep the safe operation of the power system. The
good grounding system should ensure the following:
To provide safety to personnel during normal and fault conditions by
limiting step and touch potential.
To assure correct operation of electrical devices.
To prevent damage to electrical apparatus.
To dissipate lightning strokes.
To stabilize voltage during transient conditions and therefore to
minimize the probability of flashover during the transients
As it is stated in the ANSI/IEEE Standard 80-1986 “IEEE Guide for
Safety in AC substation grounding,” a safe grounding design has two
objectives:
To provide means to carry electric currents into the earth under normal
and fault condition without exceeding any operational and equipment
limit or adversely affecting continuity of service.
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Seasonal Influence on Safety of Substation Grounding
To assure that a person in the vicinity of grounded facilities is not
exposed to the danger of critical electrical shock.
A practical approach to safe grounding considers the interaction of
two grounding systems: The intentional ground, consisting of ground
electrodes buried at some depth below the earth surface, and the accidental
ground, temporarily established by a person exposed to a potential gradient at
a grounded facility.
An ideal ground should provide a near zero resistance to remote
earth. In practice, the ground potential rise at the facility site increases
proportionally to the fault current; the higher the current, the lower the value
of total system resistance which must be obtained. For most large substations
the ground resistance should be less than 1 Ohm. For smaller distribution
substations the usually acceptable range is 1-5 Ohms, depending on the local
conditions.
When a grounding system is designed, the fundamental method is to
ensure the safety of human beings and power apparatus is to control the step
and touch voltages in their respective safe region. step and touch voltage can
be defined as follows.
1.1 STEP VOLTAGE
It is defined as the voltage between the feet of the person standing in
near an energized object. It is equal to the difference in voltage given by the
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voltage distribution curve between two points at different distance from the
electrode.
1.2 TOUCH VOLTAGE
It is defined as the voltage between the energized object and the feet
of the person in contact with the object. It is equal to the difference in voltage
between the object and a point some distance away from it.
In different season, the resistivity of the surface soil layer would be
changed. This would affect the safety of grounding systems. The value of step
and touch voltage will move towards safe region or to the hazard side is the
main concerned question
The grounding system of power plants and substations is usually
formed by several vertical ground rods connected to each other and to all
equipment frames, neutrals and structures that are to be grounded. Such a
system that combines a horizontal grid and a number of vertical ground rods
penetrating lower soil layers has several advantages in comparison to a grid
alone. Sufficiently long ground rods stabilize the performance of such a
combined system making it less dependent on seasonal and weather variations
of soil resistivity. Rods are more efficient in dissipating fault currents because
the upper soil layer usually has a higher resistivity than the lower layers. The
current in the ground rods is discharged mainly in lower portion of the rods.
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Seasonal Influence on Safety of Substation Grounding
Therefore, the touch and step voltages are reduced significantly compared to
that of the grid alone.
In areas where the soil resistivity is rather high or the facility space
is at premium, it may be not possible to obtain the required low impedance of
the grounding system by spreading the ground rods and grid over a large area.
The possible solutions of that rather complicated problem may be summarized
as follows:
To change the soil resistivity in the limited area of interest by
implementation of the chemically charged ground rods with or without
an additional backfill.
To establish remote ground grid connected to the main ground system.
To use the deep-driven ground rods reaching underground water table or
lower soil layers with low resistivity.
To use main remote ground mats.
To analyze the technical and economical aspects of each one of
alternatives mentioned above, first one must examine the components of the
grounding electrode resistance. There are three general components affecting
grounding electrode resistance:
The resistance of the electrode
The resistance of the electrode-to-soil interface area
The soil resistivity.
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Out of which the resistance of the electrode itself is negligible,
although it varies with the length, diameter and deployment of the electrode.
The resistance of the electrode-to-soil interface area is nearly negligible at
temperatures above freezing. However, when the temperature of soil drops
below freezing point, a veneer of ice may form on the ground electrode,
adding resistance to the electrodeearth interface. Another that affects
electrode soil interface resistance is soil compactness around the ground
electrode. A loose backfill or non-compact soil around the electrode will
reduce the contact area and increase resistance. The soil resistivity is the
single most important factor affecting the resistance of the ground system.
That is why the most economically sound solution is lowering the soil
resistivity to the level required to obtain the specified resistance impedance of
the ground system. In order to work out a practical approach of the soil
treatment, the soil characteristics related to electrical conductivity at different
seasons are to be studied.
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2. INFLUENCE OF RAINING AND FREEZING SEASON
ON SOIL RESISTIVITY
The resistivity of soil in nature is decided by the following factors:
Soil type
Soil resistivity varies widely depending on soil type, from as low as
1 Ohm- meter for moist loamy topsoil to almost 10,000 Ohm-meters for
surface limestone.
Moisture content
It is one of the controlling factors in earth resistance because electrical
conduction in soil is essentially electrolytic. The resistivity of most soils
rises abruptly when moisture content is less than 15 to 20 percent by
weight, but is affected very little above 20 percent. It must be
recognized, however, that the moisture alone is not the predominant
factor influencing the soil resistivity. If the water is relatively pure, it
will be of high resistivity and may not provide the soil with adequate
conductivity.
The soluble salts, acids or alkali
The presence of these components influences considerably the soil
resistivity.The most commonly used salting materials are sodium
chloride (common salt), copper sulfate and magnesium sulfate (Epsom
salt). Different types of salts have varying depletion rates; consequently,
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different types may be combined to produce the optimum depletion and
conditioning characteristics. Sodium chloride and magnesium sulfate are
the most commonly used salting materials. Magnesium sulfate is
considered to be the least corrosive. Salting materials will inhibit the
formation of ice and will lower the resistivity of the soil. It may take
some time for the salting effects to be noticed, although the earth
connection will continue to improve over time until the salt content
reaches about six per cent by weight. Higher resistivity soils take longer
to condition. It takes topsoil about two months, clay four months and
sand gravel five months for the salt minerals concentration to reach
about six per cent. Such concentration of salts poses a negligible
corrosion threat.
The temperature
Effect on soil resistivity is almost negligible for temperatures above the
freezing points. When temperature drops below water freezing point the
resistivity increases rapidly.
Compactness and granularity
It affects soil resistivity in that denser soils generally have lower
resistivity. These factors do not vary over time. Once the resistivity has
been assessed these factors can usually be ignored. From all the factors
mentioned above, two factors—moisture and salt content—are the most
influential ones on soil resistivity for a given type of soil. Therefore the
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chemical treatment of soil surrounding ground rods is preferable and in
some cases the only economically sound solution in obtaining low
impedance of the ground system
The resistivity of affected soil layer and surface material used in the
analysis of this paper is shown in Table I. When the influence of season factor
is not considered, the soil is homogeneous, the resistivity of normal soil is
200Ώm.This is called as the normal condition, the ground resistance, step
voltage and touch voltage in the normal condition is used to measure the
influence of the raining and freezing season.
In raining season, there is a wet surface soil layer with decreased
resistivity; the resistivity of the affected surface soil layer is changed in the
range of 10 to 200 Ώ m to consider the influence of raining season on the
surface soil layer. In freezing season, there is a freezing surface soil layer with
increased resistivity, the resistivity of the affected surface soil layer is
changed in the range of 200 to 5000 Ώm to consider the influence of freezing
season on the surface soil layer. The resistivity of the granite layer in raining
season is assumed as 5000 Ώ m and its resistivity in normal condition and in
freezing season is assumed as 15000 Ώ m.
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3. ANALYSIS MODEL OF GROUNDING SYSTEM
For the analysis of the seasonal influence on the grounding system
we are considering two models. Model 1 and 2 which are as given in the
figure below:
Two different cases are analyzed in this paper as shown in Figs. I
and 2. There is not surface material with high resistivity in model I as shown
in Fig. 1. The surface material with high resistivity was considered in model
II as shown in Fig. 2. A granite layer with thickness of 0.1 m is used for the
surface material. A 100 x I00 m horizontal grounding system is assumed in
our analysis.
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The main function of the surface material with high resistivity is to
increase the limits of step and touch voltages. According to IEEE Standards
the limits of step and touch voltages in V for a body weight of 50 kg can be
calculated by using the formula given below
Estep = (1000+6 Cs Ps) 0.116/t
Etouch = (1000+1.5 Cs Ps) 0.116/t
Ps =resistivity of surface layer
t =time duration of shock current in seconds
Cs =surface layer derating factor
Cs =1- 0.09 (1-P/P)/2hs+0.09
hs =thickness of the surface material in meters
P =resistivity of earth beneath the surface material
The limit of step and touch voltage in different seasons when surface
material is used and not used is given below
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Considered limits of step and touch voltages decrease in raining
season and increase in freezing season. In normal condition, the grounding
grid is buried in homogeneous soil with resistivity of 200 Ώ m, the maximum
step and touch voltages are 547 and 1481 V. if the surface material is not
used, and the limits of step and touch voltages are 316 and 187 V, the
maximum step and touch voltages exceed limits. If a granite layer is used, the
limits of step and touch voltages are 9131 and 2890 V, the grounding system
is safe. So the surface granite layer should be used, then the grounding system
in normal condition is safe.
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4. INFLUENCES OF RAINING SEASON ON SAFETY
OF GROUNDING SYSTEMS
INFLUENCES ON GROUNDING RESISTANCE
The influence of the thickness it of the low-resistivity soil layer
formed in raining season on the grounding resistance is shown in Fig below
The vertical dot line illustrated in Fig. shows the burial depth of the
grounding system If the thickness of this low-resistivity soil layer is smaller
than the burial depth of the grounding system (left side of the vertical dot line
as shown in Fig.), the grounding resistance of the grounding system decreases
with the burial When the low-resistivity soil layer exits, if a current is injected
into the grounding system, a part of this current would disperse into the low-
resistivity surface soil layer, the thicker the low-resistivity soil layer is, the n
the dispersed current from the low-resistivity soil layer is, and the more the
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grounding resistance decreases. When the thickness of the low-resistivity soil
layer is very thin, its influence on the grounding resistance can be neglected.
But when the thickness of the low-resistivity soil layer changes from smaller
than the burial depth of the grounding g to higher than the burial depth (right
side of the vertical dot as shown in Fig.), the grounding resistance would
decrease sharply.
INFLUENCE ON TOUCH VOLTAGE
The influence of the thickness of the low resistivity soil layer formed
on the touch voltage is shown in figure below.
The maximum touch voltage on the ground surface in normal
condition is 1481 V. When the thickness of the low-resistivity soi1 layer is
smaller than the burial depth (left side of the vertical dot line as shown in
Fig.) the touch voltage on the ground surface wou1d increase comparing with
the normal condition, the smaller the thickness of the affected soil layer is, the
higher the influence is. When the thickness of the low-resistivity soil layer
changes from smaller than the burial depth of the grounding systems to higher
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than the burial depth. The touch voltage would decrease rapidly, and be
smaller than the respective value in normal condition.
INFLUENCE ON STEP VOLTAGE
The influence of the thickness of the low resistivity soil layer formed
in raining season on the step voltage is shown in the figure below:
The step voltage decreases with increase of the thickness of the low
resistivity soil layer. The higher the resistivity smaller is the influence on step
voltage. If the resistivity of the affected soil layer is small, and its thickness h
is smaller than 0.5H ( H is the burial depth of the grounding system. 0.8 m is
used in this paper), the step voltage decreases very quickly. In raining season,
the step voltage is smaller than the respective normal value; it is safe for
human beings
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5 . INFLUENCES OF FREEZING SEASON ON SAFETY
OF GROUNDING SYSTEMS
INFLUENCE ON GROUNDING RESISTANCE
In freezing season, a high-resistivity surface soil layer is formed,
which is called as freezing soil layer in this paper. The influence of the
thickness of the freezing soil layer on the grounding resistance of the
grounding system is shown in figure below:
The grounding resistance increases with the thickness of the freezing
soil layer. When its thickness is less than the burial depth of the horizontal
grounding system (the left side of the vertical dot line in figure), if a current is
injected-into the grounding system, the current would mainly flow into the
bottom normal soil with resistivity of 200 Ώ m, and there is a very small part
of current that would flow through the upper freezing soil layer with high
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resistivity formed in freezing season, so the grounding resistance is almost
kept unchanged.
When the thickness of the high resistivity soil layer exceeds the
burial depth of the grounding grid (the right side of the vertical dot line in
figure), if a current is injected into the grounding system, the current has to
flow through the freezing soil layer with high resistivity to the bottom normal
soil layer with resistivity of 200 Ώ m, so the grounding resistance increases
with the thickness of the freezing soil layer. We observed that the grounding
resistance increases with the resistivity of the high resistivity soil layer. When
the thickness of the freezing soil layer is in the range of 1 to 1.6 m, and the
resistivity Pa of the freezing soil layer is 5000 m, in this case, the grounding
resistance will increase to 1.7 to 3.0 times of that of grounding system in
normal condition, so the influence of freezing season on the grounding
resistance is very high and we have to pay attention to it.
INFLUENCE OF FREEZING SEASON ON THE
TOUCH VOLTAGE
Influence of the thickness h of the freezing soil layer on the touch
voltage on the grounding surface is shown below
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When thickness h is less than the burial depth of the grounding
resistance (the left side of the vertical dot line in figure), the influence of the
freezing surface soil layer on the touch voltages very small. And touch
voltage also increases slightly. When the thickness of the freezing soil layer
exceeds the burial depth of the grounding grid, the touch voltage increases
quickly with the thickness of the freezing soil layer.
We can observe from figure, the higher the resistivity of the freezing
soil layer is, the more largely the touch voltage increases. When its thickness
exceeds 1 m and its resistivity is 5000 Ώ m. then the touch voltage would
increase to 12 times of the respective value in normal condition, the influence
is very serious, this condition must he considered in the design of grounding
system.
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INFLUENCE OF FREEZING SEASON ON THE STEP
VOLTAGE
The influence of the thickness h of the freezing soil layer on the step
voltage is illustrated in figure below:
The step voltage increases with the thickness, when there is a
freezing surface layer, the step voltage would be higher than the normal value.
When the thickness of the freezing soil layer is smaller than the burial depth
of the grounding system (left side of the vertical dot line in figure), the
influence of the freezing soil layer on the step voltage is very small. When the
thickness of the freezing soil layer exceeds the burial depth of the grounding
grid (right side of the vertical dot line in figure), the step voltage increases
rapidly.
So, in the highly freezing area, the design of the grounding grids
should strictly analyze whether the step and touch voltages are smaller than
the safe values, and reduce the safety of the grounding grids. The influence of
the freezing soil layer on step and touch voltages is very weak when its
thickness is smaller than the burial depth of the grounding grid, and the safety
of the grounding grid is not affected. When its thickness is larger than the
burial depth of the grounding grid, the safe characteristics of the grounding
grid would he reduced strongly.
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6. MEASURES TO BE TAKEN
In raining season the low resistivity soil layer formed leads the
ground resistance and step voltage smaller than the respective value in normal
condition .it is good for human beings but the raining season perhaps leads the
touch voltage higher than its limit values. When the surface soil layer forms
high resistivity layer in freezing season, the grounding resistance of
grounding grid increases with the thickness of the high resistivity layer. In
order to nullify the effect the effect we can use the following methods:
6.1 USE OF LONG VERTICAL GROUNDING ELECTRODES
The use of long electrodes will effectively increace the burial depth
and hence decreases the effect of seasonal influence
6.2 THE CHEMICAL TREATMENT OF THE SOIL
It may be implemented by any one of the following three ways:
TO USE CONDUCTIVE BACKFILL MATERIALS
Several materials exist on the market that are used to replace poorly
conducting soil near the ground electrodes. The impact of putting these
materials around the electrodes is significant, since that is where the majority
of connection to the earth takes place. Four such materials used for conductive
backfills around ground electrodes are described below.
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Concrete: has a resistivity range of 30 to 90 Ohm-meters. Since it is
hydroscopic by nature it will tend to absorb moisture when available and keep
it up to 30 days, thus maintaining a resistivity lower than the surrounding soil.
However, during a long dry season concrete will dry out with a subsequent
rise in resistivity. Also, if a substantial amount of fault or lightning current is
injected into a concrete encased electrode, the moisture in the concrete may
become steam, dramatically increasing in volume and placing a substantial
stress on the concrete.
Carbon-based backfills materials: have generally a resistivity lower than
clay-based mixtures. Some of these materials can be mixed with concrete to
make concrete more conductive. However, these materials tend to be the most
expensive and do not retain moisture nearly as well as clay-based materials.
The amount of the backfill material required is determined in most cases by
the Interfacing Volume and Critical Cylinder principles. A ground electrode
establishes a connection to earth by affecting only a certain volume of earth,
called the Interfacing
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TO USE THE CHEMICALLY CHARGED GROUND RODS
(CCGR)
Used instead of the conventional ground rods. A CCGR is a copper
tube of 2-2.5 inches in diameter with several small holes perforated along the
length of the tube. The tube is filled with metallic salt evenly distributed along
the entire length of the tube. The moisture absorbed from the air and soil form
a solution of the contained metallic salt within the CCGR which seeps out
through the holes into the surrounding soil, thus lowering the soil’s resistivity
and increasing the efficiency of the electrode.
TO IMPLEMENT A COMBINATION OF THE CCGR WITH
BACKFILL.
Grounding system based on the implementation of the CCGRs with
backfill appears to be the most economical solution in the given conditions of
poor conducting soil and low values of the required ground resistance, even
without taking into consideration the cost of real estate. Where the appropriate
space is not available or too expensive, the CCGRs the only solution in
establishing required ground system.
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7. CONCLUSION
The use of long vertical electrodes and chemically charged ground
electrodes (CCGR) with or with out backfills establishes relatively low
ground resistance and ground impedances which are not subjected to seasonal
and weather variations much.
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8. REFERENCES
Grounding of AC electrical devices, electrical power industry
ministry.19997,98
IEEE Guide for safety in AC substations grounding ANSI/IEEE 80-
2000
R.S GUSTAFSONS,R.PURSELY “Seasonal grounding resistance
variation on grounding system” IEEE transaction on power delivery.
Roy B.CARPENTER ,DRABKIN “BETTER GROUNDING ”
Lightning eliminator and consultants inc,USA
WWW.OSHA.GOV
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Seasonal Influence on Safety of Substation Grounding
ABSTRACT
The low or high resistivity soil layer formed in raining or freezing
season affects the safety of grounding system, and leads the changes of
grounding resistance of the grounding system, step and touch voltages on the
ground surface. This paper systematically discusses the seasonal influence on
the safety of grounding system and the different methods to be adopted
inorder to nullify the effect of seasonal changes.
Inorder to nullify the effect of the seasonal changes we can use long
vertical grounding electrodes or by using chemically charged ground
electrodes (CCGR) with or without backfill which can effectively attenuate
the seasonal influence and improve the safety of the grounding system.
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Seasonal Influence on Safety of Substation Grounding
ACKNOWLEDGEMENT
I express my sincere gratitude to Dr.Nambissan, Prof. & Head,
Department of Electrical and Electronics Engineering, MES College of
Engineering, Kuttippuram, for his cooperation and encouragement.
I would also like to thank my seminar guide Mrs. Sobha M (Asst.
Professor, Department of EEE), Asst. Prof. Gylson Thomas. (Staff in-charge,
Department of EEE) for their invaluable advice and wholehearted cooperation
without which this seminar would not have seen the light of day.
Gracious gratitude to all the faculty of the department of EEE &
friends for their valuable advice and encouragement.
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Seasonal Influence on Safety of Substation Grounding
CONTENTS
1. INTRODUCTION 1
1.1 STEP VOLTAGE 2
1.2 TOUCH VOLTAGE 3
2. INFLUENCE OF RAINING AND FREEZING 6
SEASON ON SOIL RESISTIVITY
3. ANALYSIS MODEL OF GROUNDING SYSTEM 9
4. INFLUENCES OF RAINING SEASON ON 12
SAFETY OF GROUNDING SYSTEMS
5 . INFLUENCES OF FREEZING SEASON ON 15
SAFETY OF GROUNDING SYSTEMS
6. MEASURES TO BE TAKEN 19
6.1 USE OF LONG VERTICAL GROUNDING ELECTRODES 19
6.2 THE CHEMICAL TREATMENT OF THE SOIL 19
7. CONCLUSION 22
8. REFERENCES 23
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