Seasonal Influences on Safety of Substation Grounding Systems Seminar Report

Seasonal Influence on Safety of Substation Grounding

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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 ♦

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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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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 lowresistivity 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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Seasonal Influence on Safety of Substation Grounding 

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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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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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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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CONTENTS 1.

2.

INTRODUCTION

1

1.1

2

STEP VOLTAGE

1.2 TOUCH VOLTAGE

3

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

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