000000000001013900

Electric Power Research Institute 3420 Hillview Avenue, Palo Alto, California 94304-1338PO Box 10412, Palo Alto, California 94303-0813 USA

[email protected]

Guide for Transmission Line GroundingA Roadmap for Design, Testing, and Remediation: Part I - Theory Book

EPRI Project Manager A. Phillips

ELECTRIC POWER RESEARCH INSTITUTE 3420 Hillview Avenue, Palo Alto, California 94304-1338 PO Box 10412, Palo Alto, California 94303-0813 USA

800.313.3774 650.855.2121 [email protected] www.epri.com

Guide for Transmission Line Grounding A Roadmap for Design, Testing, and Remediation: Part ITheory Book 1013900

Final Report, December 2007

DISCLAIMER OF WARRANTIES AND LIMITATION OF LIABILITIES THIS DOCUMENT WAS PREPARED BY THE ORGANIZATION(S) NAMED BELOW AS AN ACCOUNT OF WORK SPONSORED OR COSPONSORED BY THE ELECTRIC POWER RESEARCH INSTITUTE, INC. (EPRI). NEITHER EPRI, ANY MEMBER OF EPRI, ANY COSPONSOR, THE ORGANIZATION(S) BELOW, NOR ANY PERSON ACTING ON BEHALF OF ANY OF THEM:

(A) MAKES ANY WARRANTY OR REPRESENTATION WHATSOEVER, EXPRESS OR IMPLIED, (I) WITH RESPECT TO THE USE OF ANY INFORMATION, APPARATUS, METHOD, PROCESS, OR SIMILAR ITEM DISCLOSED IN THIS DOCUMENT, INCLUDING MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE, OR (II) THAT SUCH USE DOES NOT INFRINGE ON OR INTERFERE WITH PRIVATELY OWNED RIGHTS, INCLUDING ANY PARTY'S INTELLECTUAL PROPERTY, OR (III) THAT THIS DOCUMENT IS SUITABLE TO ANY PARTICULAR USER'S CIRCUMSTANCE; OR

(B) ASSUMES RESPONSIBILITY FOR ANY DAMAGES OR OTHER LIABILITY WHATSOEVER (INCLUDING ANY CONSEQUENTIAL DAMAGES, EVEN IF EPRI OR ANY EPRI REPRESENTATIVE HAS BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGES) RESULTING FROM YOUR SELECTION OR USE OF THIS DOCUMENT OR ANY INFORMATION, APPARATUS, METHOD, PROCESS, OR SIMILAR ITEM DISCLOSED IN THIS DOCUMENT.

ORGANIZATION(S) THAT PREPARED THIS DOCUMENT KEMA Nederland B.V.

Kinetrics

J. Anderson

NOTE For further information about EPRI, call the EPRI Customer Assistance Center at 800.313.3774 or e-mail [email protected].

Electric Power Research Institute, EPRI, and TOGETHERSHAPING THE FUTURE OF ELECTRICITY are registered service marks of the Electric Power Research Institute, Inc.

Copyright 2007 Electric Power Research Institute, Inc. All rights reserved.

iii

CITATIONS

This report was prepared by

KEMA Nederland B.V. P.O. Box 9035 Arnhem, 6800 ET The Netherlands

Principal Author C. Engelbrecht

Kinetrics 800 Kipling Avenue KL206 Toronto, Ontario M8Z 6C4 Canada

Principal Investigator W. Chisholm

J. Anderson 525 Old Windsor Road Dalton, MA 01226

Principal Investigator J. Anderson

This report describes research sponsored by the Electric Power Research Institute (EPRI).

The report is a corporate document that should be cited in the literature in the following manner:

Guide for Transmission Line Grounding: A Roadmap for Design, Testing, and Remediation: Part ITheory Book. EPRI, Palo Alto, CA: 2007. 1013900.

v

PRODUCT DESCRIPTION

Electrical utilities have a duty to provide effective grounding for managing steady-state and fault currents, whether near a large generating station or at a remote distribution pole ground. For transmission lines, this imperative is usually met with investment in overhead ground wires and grounding electrodes. Effective grounding at each tower improves reliabilityby providing low path impedance to lightning strokesand contributes to safety. However, the fundamental physical parameters in ground electrode engineering vary with climate and location, so tower-by-tower testing and validation are needed. Existing standards for successful testing are better suited to substations or concentrated electrodes than to transmission towers, which can have several large, effective foundation grounding electrodes in parallel. This leads to a wide discrepancy in treatment and testing options from one utility to another.

Results and Findings Tower-to-tower differences in soil resistivity are so large that each tower needs a different design and execution method. This report facilitates good grounding engineering practice, showing the users how to make effective choices, considering performance and life cycle costs. In particular, the techniques in this report can help utilities decide whether to go deep or go wide and flat and can also improve estimates of how deep or what ring size electrodes are required in order to achieve design targets. Pre-engineering and pre-staging of materials have been shown to improve the overall effectiveness of this approach, leading to the possibility of reduced overall project cost despite the use of expensive resistivity surveys before or after tower spotting.

Challenges and Objectives Most utility design guides and industry standards offer a bewildering set of equations, one for every electrode shape and none suitable for a four-legged transmission tower with extra rods or radial wire. This report treats complex electrode shapes and two-layer soil effects using methods that are simple, accurate, easy to teach, and easy to use, even for a high school graduate with a math credit and a scientific calculator. Stakeholders include the following:

Transmission line planners who need simple methods to evaluate the relative merits of resistivity profiles in route and site selection

Transmission line designers and structural engineers with limited appreciation of how minor design choices can improve the performance and longevity of electrical grounding

Protection and control designers who rely on effective grounding data to improve distance relaying and fault location

vi

Construction and inspection staff who must bridge the gaps between a 20- specification and a rock-anchored tower, a spool of wire, and a pile of ground rods

Asset managers who can use the test methods and equations to calculate the remaining life of existing grounding

Risk managers who need to understand why the risk of electrocution near transmission towers has proven to be so low compared with other public and worker exposures

Applications, Value, and Use One near-term development described in this report is a method to measure the transient impedance of grounding electrodes without isolating the overhead ground wires. Low-cost, fast, and portable digital oscilloscopes with built-in memory have already made this approach practical, leading to a factor-of-three improvement in test time. Development of the equipment, refinement of the interpretation, and additional experience can reduce test time even more.

This report also highlights electromagnetic methods that can provide tower-by-tower measurements of two-layer soil resistivity. The analysis and design methods in this report take full advantage of the new data in the forward direction, computing resistance from two-layer resistivity and electrode size. The methods also support an evaluation of footing condition in a reverse direction, using simultaneously measured values of impedance and local resistivity to establish performance benchmarks. This opens the possibility of using electromagnetic surveys to assess ground electrode conditions.

EPRI Perspective EPRI has been in the business of consolidating improvements in the analysis and design of grounding systems for substations for more than 20 years. This project takes advantage of continuous improvements in modeling and experimental data and maintains a focus on making these technologies easier to use by providing simple applets and worked examples. In several areas, especially remote sensing of resistivity and advanced measurements of local tower impedance, EPRI will play an increasing role in improving the raw data needed for effective grounding analysis and design, possibly taking this expert advice right to the base of every tower at which there is work is to be done.

Approach This report consolidates approaches to testing and modeling of grounding electrodes, identifies appropriate simplifications, and adapts the methods specifically for transmission line grounding. The report is supported by the EPRI Transmission Line Grounding Guide Software (EGGS), Version 1.01 (1011654). There are nine modules in the EGGS that are intended either to implement complex algorithms presented in this EPRI report or to explain complex concepts.

Keywords Grounding Two-layer resistivity Surge impedance Ionization Transmission lines

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ACKNOWLEDGMENTS

EPRI would like to thank the following contributors for their reviews and suggestions to improve the quality and usability of this report:

Eric Engdahl American Electric Power Ben Howat National Grid UK Rita Jo Livezey Tennessee Valley Authority (TVA) Gene Nelson TVA Allen Van Leuven Bonneville Power Administration Members of the EPRI Lightning and Grounding Task Force

In addition, the following individuals and organizations provided illustrations for the report:

J. L. Bermudez Arboleda W. Chisholm and W. Janischewskyj Fugro Airborne Surveys Geophex International Telecommunication Union (ITU) Mineralogical Research Company Hydro-One NB Power K. Nixon, University of the Witwatersrand, South Africa J. P. Reilly Tennessee Department of Environment and Conservation Tennessee Valley Authority (TVA)

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CONTENTS

1 INTRODUCTION ....................................................................................................................1-1 1.1 General ........................................................................................................................1-1 1.2 Relationship to Line Design .........................................................................................1-2 1.3 History and Past Reports .............................................................................................1-3 1.4 Purpose and Structure of This Report .........................................................................1-4

2 ROLES OF TRANSMISSION LINE GROUNDING ................................................................2-1 2.1 Lightning ......................................................................................................................2-2 2.2 Correct Operation of the Transmission System...........................................................2-3 2.3 Safety...........................................................................................................................2-3

2.3.1 Normal Operation................................................................................................2-4 2.3.2 Fault Conditions ..................................................................................................2-4

2.4 Electromagnetic Interference.......................................................................................2-5

3 DEFINITIONS .........................................................................................................................3-1

4 ELECTRICAL CHARACTERISTICS OF SOIL ......................................................................4-1 4.1 Introduction ..................................................................................................................4-1 4.2 Electrical Characteristics of Homogeneous Soil ..........................................................4-2

4.2.1 Basic Parameters That Influence Soil Resistivity................................................4-2 4.2.1.1 Type of Soil .....................................................................................................4-4 4.2.1.2 The Composition and Type of Salts Dissolved in the Ground Water..............4-4 4.2.1.3 Moisture Content ............................................................................................4-5 4.2.1.4 Temperature ...................................................................................................4-6 4.2.1.5 Variation with Electric Field.............................................................................4-7 4.2.1.6 Variation with Frequency ................................................................................4-7

4.2.2 Seasonal Variations ............................................................................................4-9 4.3 Electrical Characteristics of Nonhomogeneous Soil ..................................................4-10

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4.3.1 Soil Resistivity as a Function of Location..........................................................4-10 4.3.2 Horizontal and Vertical Layering .......................................................................4-12

4.3.2.1 Horizontal Layering.......................................................................................4-12 4.3.2.2 Vertical Layering ...........................................................................................4-13

4.4 Measurement of Soil Resistivity.................................................................................4-14 4.4.1 Wenner Four-Electrode Method........................................................................4-16

4.4.1.1 Required Equipment .....................................................................................4-17 4.4.1.2 Measurement Procedure ..............................................................................4-18 4.4.1.3 Analysis and Interpretation of the Results ....................................................4-23 4.4.1.4 Uniform Soil ..................................................................................................4-24 4.4.1.5 Layered Soil ..................................................................................................4-24

4.4.2 Other Multiple-Electrode Methods.....................................................................4-25 4.4.3 Driven Ground Rod Methods (Two- and Three-Electrode Methods) ................4-26

4.4.3.1 Required Equipment .....................................................................................4-27 4.4.3.2 Measurement Procedure ..............................................................................4-27 4.4.3.3 Analysis and Interpretation of the Results ....................................................4-28 4.4.3.4 Alternative Method........................................................................................4-29

4.4.4 Passive Electromagnetic Methods ....................................................................4-31 4.4.4.1 Radio Wave Attenuation ...............................................................................4-31 4.4.4.2 Lightning Location System Observations......................................................4-33

4.4.5 Active Induction Methods..................................................................................4-34 4.4.5.1 Theoretical Background of Inversion Problem..............................................4-34 4.4.5.2 Electromagnetic Induction.............................................................................4-35 4.4.5.3 Ground-Based Two-Coil Multifrequency Electromagnetic Surveys ..............4-35 4.4.5.4 Aerial Multifrequency to 100 kHz or Transient ..............................................4-35 4.4.5.5 Active Transient Current Injection at Tower Base Using the EPRI Zed-Meter ..................................................................................................................4-36

4.4.6 Choosing an Appropriate Method for Soil Resistivity Measurements ...............4-39 4.4.6.1 Resistivity Profile from Aerial Surveys ..........................................................4-41 4.4.6.2 Resistivity Information from Tower Footing Resistance Measurements .......4-41 4.4.6.3 Detailed Soil Resistivity Measurements........................................................4-41 4.4.6.4 Variations in Resistivity After Construction ...................................................4-42

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5 CHARACTERISTICS OF A GROUND ELECTRODE............................................................5-1 5.1 Introduction ..................................................................................................................5-1 5.2 Low-Frequency Ground Electrode Impedance ............................................................5-2

5.2.1 Derivation of the Ground Electrode Resistance of a Hemispheric Electrode......5-4 5.2.2 Analytical Expressions for the Calculation of Ground Electrode Resistance ......5-6

5.2.2.1 Dwight and Sunde Equations..........................................................................5-6 5.2.2.2 Equations for Calculating the Resistance of Typical Transmission Line Tower Electrodes.........................................................................................................5-8

5.2.3 The Geometric and Contact Resistance Method ..............................................5-11 5.2.3.1 The Derivation of Geometric Resistance of Solid Spheroid Electrodes........5-11 5.2.3.2 Derivation of the Contact Resistance Term ..................................................5-15 5.2.3.3 Geometric and Contact Resistance Equations for Basic Electrode Types .........................................................................................................................5-16

5.2.4 Calculation of Electrode Resistance in Two-Layer Soil.....................................5-17 5.2.5 Calculation of Resistance of Multiple Electrode Systems .................................5-18 5.2.6 Choosing an Equation to Calculate the Ground Electrode Resistance.............5-19

5.2.6.1 Single-Rod Electrode....................................................................................5-19 5.2.6.2 Hemispheric Electrode:.................................................................................5-20 5.2.6.3 Round Plate Electrode..................................................................................5-21 5.2.6.4 An Ellipsoid of Revolution Electrode.............................................................5-21 5.2.6.5 Summary ......................................................................................................5-23

5.2.7 Numerical Methods for Calculating Ground Electrode Resistance ...................5-24 5.3 Surface Potential Gradients .......................................................................................5-24

5.3.1 Calculation of Potential Gradients Around Grounding Electrodes ....................5-25 5.3.1.1 Theoretical Background................................................................................5-25 5.3.1.2 Numerical Methods to Evaluate the Surface Potential Gradients .................5-27

5.3.2 Step and Touch Potential Around Transmission Line Towers ..........................5-28 5.3.2.1 Basic Principles.............................................................................................5-28 5.3.2.2 Evaluation of Step and Touch Potentials ......................................................5-29 5.3.2.3 Mitigation of Step and Touch Potentials .......................................................5-31

5.4 The Behavior of Grounding Electrodes When Discharging Lightning Current...........5-31 5.4.1 The Surge Impedance of a Ground Electrode System .....................................5-34

5.4.1.1 Surge Impedance of the Buried Ground Wires .............................................5-34 5.4.1.2 Surge Impedance of the Ground Plane ........................................................5-37

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5.4.2 Soil Ionization Effects at High-Voltage Gradients .............................................5-41 5.4.2.1 Liew-Darveniza Dynamic Model for Rod Electrodes.....................................5-44 5.4.2.2 Korsuncev Dimensionless Parameter Model ................................................5-47

5.4.3 Step and Touch Potentials Under Lightning Conditions....................................5-50 5.5 Electrical Properties of Concrete Foundations...........................................................5-51 5.6 Procedures for Testing Tower Grounding Electrodes................................................5-54

5.6.1 Introduction .......................................................................................................5-54 5.6.1.1 Motivation for Testing Grounding Electrodes................................................5-54 5.6.1.2 The Basic Principle of Measuring the Electrode Resistance ........................5-55 5.6.1.3 Effect of the Connected Ground Wires .........................................................5-56 5.6.1.4 Common Methods for Electrode Resistance Measurement .........................5-57

5.6.2 Fall-of-Potential Method....................................................................................5-58 5.6.2.1 The Test Setup .............................................................................................5-59 5.6.2.2 Premeasurement Checks .............................................................................5-61 5.6.2.3 Performing the Measurement .......................................................................5-62 5.6.2.4 Analysis of the Results..................................................................................5-62

5.6.3 Oblique-Probe Method......................................................................................5-63 5.6.3.1 The Test Setup .............................................................................................5-63 5.6.3.2 Performing the Measurement .......................................................................5-65 5.6.3.3 Analysis of the Results..................................................................................5-66 5.6.3.4 Accuracy of the Results ................................................................................5-67

5.6.4 Use of Stray Tower Current and Voltage for Footing Resistance .....................5-69 5.6.4.1 The Test Setup .............................................................................................5-70 5.6.4.2 Performing the Measurement .......................................................................5-71 5.6.4.3 Analysis of the Results..................................................................................5-71 5.6.4.4 Use of Stray Tower Current and Voltage for Resistivity................................5-71

5.6.5 Directional Impedance Measurements..............................................................5-72 5.6.6 Simulated Fault Method ....................................................................................5-74

5.6.6.1 The Test Setup .............................................................................................5-76 5.6.6.2 Performing the Measurements......................................................................5-76

5.6.7 High-Frequency (26-kHz) Impedance ...............................................................5-77 5.6.8 Active Transient Current Injection at Tower Base (Zed-Meter).........................5-78

5.6.8.1 The Test Setup .............................................................................................5-79 5.6.8.2 Performing the Measurements......................................................................5-79

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5.6.8.3 Analysis of the Results..................................................................................5-83 5.6.8.4 Accuracy of the Results ................................................................................5-84

5.6.9 Direct Method for Measuring Structure Resistance ..........................................5-86 5.6.10 Ground Electrode Integrity Assessment ........................................................5-87

5.6.10.1 Continuity Measurements ...........................................................................5-87 5.6.10.2 Use of Footing Resistance and Resistivity to Assess Intact Rod Length........................................................................................................................5-88

5.6.11 Step and Touch Potential Measurements......................................................5-90 5.6.12 Assessment of the Interference to Other Infrastructure .................................5-91 5.6.13 Precautions Under Power Lines When Doing Measurements.......................5-92

5.6.13.1 Electrostatic, Induction, and Stray Ground Current PickUp ........................5-92 5.6.13.2 Signal-to-Noise Ratio in Selection of Equipment ........................................5-92 5.6.13.3 Additional Considerations Near Substations...............................................5-93

5.6.14 Choosing an Appropriate Method for Soil Resistivity Measurements ............5-94

6 USEFUL GUIDELINE DOCUMENTS AND RESOURCES ....................................................6-1

7 REFERENCES .......................................................................................................................7-1 7.1 Cited References .........................................................................................................7-1 7.2 Other References ........................................................................................................7-4

7.2.1 EPRI Reports ......................................................................................................7-4 7.2.2 International Standards .......................................................................................7-5 7.2.3 Books ..................................................................................................................7-5 7.2.4 Technical Papers ................................................................................................7-6 7.2.5 U.S. Military Publications ....................................................................................7-8

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LIST OF FIGURES

Figure 1-1 Components of the Grounding System of a Transmission Line ...............................1-2 Figure 3-1 Definition of Ground Resistivity ................................................................................3-2 Figure 4-1 Definition of Resistivity and Resistance....................................................................4-2 Figure 4-2 The Effect of Grain Packing on the Volume of Voids in the Soil...............................4-4 Figure 4-3 Resistivity of Materials as a Function of Moisture Content .......................................4-5 Figure 4-4 Soil Resistivity as a Function of Temperature ..........................................................4-6 Figure 4-5 Typical Sand Fulgurite from East Texas...................................................................4-7 Figure 4-6 Ratio of Material Resistivity at 100 kHz and 100 Hz Versus Moisture Content ........4-8 Figure 4-7 Resistivity Distribution Between Electrodes at Tournemire, France.......................4-10 Figure 4-8 Generalized Geologic Map of Tennessee ..............................................................4-11 Figure 4-9 Areas in Tennessee Where Mean Resistivity Is Less than 150 m.......................4-11 Figure 4-10 Areas in Tennessee Where Mean Resistivity Exceeds 1000 m ........................4-12 Figure 4-11 Complex Soil Model with Various Types of Soil Layering.....................................4-12 Figure 4-12 Vertical and Horizontal Distribution of Resistivity Values from an Aerial

Electromagnetic Survey Near a 345-kV Power Line ........................................................4-14 Figure 4-13 Wenner Probe Technique for Measurement of Apparent Resistivity, a...............4-16 Figure 4-14 Wenner Probe Arrangement Effect of Probe Spacing on the Depth of

Current Penetration..........................................................................................................4-17 Figure 4-15 Wenner Probe Positioning Strategy to Reduce the Amount of Work ...................4-21 Figure 4-16 Nonuniform Surface Probe Spacing for Multilayer Resistivity Survey ..................4-25 Figure 4-17 General Setup of the Driven Ground Rod Method to Determine Soil

Resistivity 4.4.3.1 Required Equipment ...........................................................................4-26 Figure 4-18 Vertical Rod Penetration Giving R () = Upper Layer Resistivity 1 (m)............4-29 Figure 4-19 Electrode Setup for the Three-Terminal Setup for Measuring the Ground

Resistivity .........................................................................................................................4-30 Figure 4-20 Map of Extra-Low Frequency (

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Figure 4-24 Surge Impedance () of Insulated Wire on Ground Surface Versus Soil Resistivity for Various Frequencies..................................................................................4-37

Figure 5-1 The Potential Profile of a Hemispherical Electrode in Uniform Soil, Showing the Parameters for Calculating Ground Resistance and Step and Touch Potentials.........5-4

Figure 5-2 Round Plate Electrode..............................................................................................5-9 Figure 5-3 Rod Electrode...........................................................................................................5-9 Figure 5-4 An Ellipsoid of Revolution.......................................................................................5-10 Figure 5-5 Three Types of Solid Spheroid Electrodes .............................................................5-12 Figure 5-6 General Two-Layer Soil Model with Horizontal Layering........................................5-17 Figure 5-7 Comparison of Equation 5-11 with Analytical Expressions for Rod,

Hemisphere, and Plate Electrodes...................................................................................5-22 Figure 5-8 Ratio of Expressions for Geometric Resistance to Equation 5-9............................5-23 Figure 5-9 The Potential Profile of a Hemispherical Electrode in Uniform Soil, Showing

the Parameters for Calculating the Step and Touch Potentials in Uniform Soil ...............5-26 Figure 5-10 Surface Potential Distribution for Rod and Mesh Electrodes................................5-28 Figure 5-11 Standard Values for AC Ventricular Fibrillation Current .......................................5-30 Figure 5-12 A Lightning Strike to a Transmission Line ............................................................5-32 Figure 5-13 Lightning Flashover Rate of Single-Circuit Lines Versus Footing Resistance......5-33 Figure 5-14 Example of the Time Variation of the Surge Impedance, the Leakage

Resistance, and the Resultant Effective Impedance of a Buried Counterpoise ...............5-35 Figure 5-15 Bewley Equivalent Circuit of a Counterpoise........................................................5-36 Figure 5-16 Experimentally Measured Currents on Peissenberg Tower at the Top and

Bottom of the Tower.........................................................................................................5-38 Figure 5-17 Tower-to-Base Reflection Coefficient g() as a Function of Frequency for

Three Experimental Records in Figure 5-16 ....................................................................5-39 Figure 5-18 Modified Bewley Equivalent Circuit of a Counterpoise (Figure 5-15) with the

Addition of an Inductor to Represent the Surge Impedance of the Ground Plane ...........5-42 Figure 5-19 Resistance of a 48- Driven Rod for Various Impulse Currents for 2.5/15 s

Impulse Current (Typical of a Subsequent Stroke) ..........................................................5-43 Figure 5-20 Liew-Darveniza Ground Rod Surrounded by Concentric Shells of Earth .............5-44 Figure 5-21 Variation of the Soil Resistivity of Each Current Shell as a Function of the

Current Density ................................................................................................................5-45 Figure 5-22 Observed Relationships Between Dimensionless Parameters for Ionized

Resistance of Grounding Electrodes from Popolansk and Korsuncev...........................5-48 Figure 5-23 Ventricular Fibrillation Current Versus Duration of a 60-Hz Stimulus for a

Wide Range of Exposure Durations.................................................................................5-51 Figure 5-24 Typical Transmission Line Concrete Foundations................................................5-52 Figure 5-25 Effect of Humidity on Concrete Weight Loss and Shrinkage................................5-53 Figure 5-26 Effect of Water Saturation on Concrete Resistivity...............................................5-54

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Figure 5-27 Principle of the Resistance Measurement of a Transmission Line Tower Ground .............................................................................................................................5-55

Figure 5-28 Current Sharing Between Transmission Line Towers If the Overhead Ground Wires Are Disconnected......................................................................................5-56

Figure 5-29 Current Sharing Between Transmission Line Towers If the Overhead Ground Wires Are Connected ..........................................................................................5-57

Figure 5-30 Fall-of-Potential Method for Measuring Structure Resistance ..............................5-58 Figure 5-31 Top View of the Preferred Probe Layout for the Fall-of-Potential Method for

Measuring Structure Resistance ......................................................................................5-60 Figure 5-32 Measurement Error as a Function of the Voltage Probe Position in Two-

Layer Soil .........................................................................................................................5-61 Figure 5-33 General Probe Layout for the Oblique-Probe Method ..........................................5-63 Figure 5-34 Top View of the Ideal Potential Probe Layout for the Oblique-Probe Method ......5-64 Figure 5-35 Top View of a Practical Potential Probe Layout for the Oblique-Probe

Method .............................................................................................................................5-65 Figure 5-36 Typical Data Analysis for Oblique-Probe Measurement of Resistance and

Resistivity with Probes at 90 ...........................................................................................5-67 Figure 5-37 Measured Resistance for Fall-of-Potential and 90 Oblique-Probe Methods .......5-68 Figure 5-38 Typical Data Analysis for Three Angles (22.5, 45, and 90) in Oblique-

Probe Method...................................................................................................................5-68 Figure 5-39 Stray Tower Current Method for Testing of Ground Rods....................................5-69 Figure 5-40 Setup of Tower Footing Resistance Measurement with Split-Core Current

Transformers Around the Tower Legs ............................................................................5-73 Figure 5-41 Excavation of Tower Leg to Allow Correct Installation of Big Norma Current

Transformer......................................................................................................................5-74 Figure 5-42 Setup for the Simulated Fault Method ..................................................................5-75 Figure 5-43 The Setup for the Active Transient Current Injection Method (Zed-Meter)...........5-78 Figure 5-44 Time Sequence of the Current Wave Injected into the Transmission Tower

Base .................................................................................................................................5-81 Figure 5-45 Waveforms of the Current Injected into the Tower (I1) and Current Lead (I2)

and the Voltage Measured at the Tower Base.................................................................5-82 Figure 5-46 Calculated Impedance from the Voltage and Current Waveforms Shown in

Figure 5-45.......................................................................................................................5-82 Figure 5-47 Typical Equivalent Circuit Seen by the Zed-Meter During the Optimal Time

of Measurement ...............................................................................................................5-83 Figure 5-48 Comparison of Low-Frequency Resistance with Zed-Meter Impedance for

Compact Electrodes.........................................................................................................5-85 Figure 5-49 Comparison of Low-Frequency Resistance with Zed-Meter Impedance for

Distributed Electrodes ......................................................................................................5-86 Figure 5-50 Setup for the Direct Method for Measuring the Structure Resistance ..................5-87 Figure 5-51 Setup for Continuity Measurement on a Looped, Continuous Counterpoise........5-88

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Figure 5-52 Resistance Test Method for Towers with Continuous Counterpoise....................5-90 Figure 5-53 Typical Variation in Tower-to-Tower Resistance for TVA 500-kV Line.................5-94

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LIST OF TABLES

Table 3-1 Symbols Used in This Report ....................................................................................3-3 Table 4-1 Low-Frequency Resistivity, , for Soil, Earth, and Concrete Material........................4-3 Table 4-2 Comparison of the Methods for Determining Soil Resistivity ...................................4-39 Table 5-1 Low-Frequency Ground Resistance of Electrodes ....................................................5-7 Table 5-2 Footing Resistance Expressions ..............................................................................5-8 Table 5-3 Equations for Calculating the Resistance of a Rod Electrode .................................5-10 Table 5-4 Equations Describing the Resistance of a Rod Electrode with Length L and

Radius a ...........................................................................................................................5-20 Table 5-5 Equations Describing the Resistance of a Hemispheric Electrode with Radius

a .......................................................................................................................................5-20 Table 5-6 Equations Describing the Resistance of a Round Plate Electrode with Radius

d .......................................................................................................................................5-21 Table 5-7 Comparison of Methods for Determining Soil Resistivity.........................................5-96

1-1

1 INTRODUCTION

1.1 General

All electrical installations need a grounding system for safe and reliable operation. A grounding system is defined as the total set of steps taken to provide a low-impedance connection between the transmission line structures and the general mass of earth and to limit the buildup of potential gradients around it. Typically, a transmission line grounding system comprises the following components (see Figure 1-1): A set of buried metallic conductors, called the ground electrode. An overhead ground wire, which can also be called a static wire or shield wire. Overhead

ground wires are not necessarily installed on all transmission lines or line sections. Connections between the components of the grounding system and the electrical installation

are made with ground conductors.

Introduction

1-2

Figure 1-1 Components of the Grounding System of a Transmission Line

1.2 Relationship to Line Design

The provision of an effective grounding system on overhead lines can be challenging and costly. The design is usually determined by the lightning performance requirements of the line. Other aspects that can influence the grounding system design are the requirement to manage the steady-state fault current along the line and the necessity to avoid the buildup of high potential gradients around the tower base during line fault conditions.

Individual tower grounding must be considered both with respect to its impact on the performance of the line and on the specific conditions near each tower. This can result in different grounding designs from tower to tower because of the variation in grounding parameters and conditions along the line route. Utilities manage this by generally following a pragmatic approach based on a mixture of experience and empirical methods to design and remediate transmission line grounding.

Introduction

1-3

1.3 History and Past Reports

EPRI has strived to support its members for more than 20 years in sponsoring research and development of methods and tools that can help utilities in their pursuit of a well-performing and cost-effective grounding system. A long-standing aim is to provide members with practical and easy-to-use methodologies and tools that are based on the latest developments, theories, and experimental data. This report serves as a consolidation of this knowledge base with the aim of providing readers with guidelines and tools necessary for good grounding engineering practice. This report and its companion volume Part II Practical guidelines, replaces the EPRI report Guide for Transmission Line Grounding: A Roadmap for Design, Testing, and Remediation (1002021).

An important resource for the development of this report was the EPRI report Transmission Line Grounding (EL-2699) [1]. In 768 pages, the two-volume report presents substantial theoretical background and more than 340 design curves based on the EPRI Grounding Analysis of Transmission Lines (GATL) software package. The approach was found to be accurate when compared with measured results from staged-fault tests at three utilities on 765-kV and 500-kV lines.

Since the publication of that report in 1982, advances have been made in both analysis and computation technology, including the following:

Successful adaptation of moment methods described in Harringtons Field Computation by Moment Methods and variational calculus described in Chow and Yovanovics The Shape Factor of the Capacitance of a Conductor to reduce the need for tiny segmentation of electrodes [2, 3]. This leads to simple, accurate expressions for ground resistance that can be inverted more easily to determine how large the electrode must be for the local soil conditions.

Development of vastly improved computer-based instrumentation that averages multiple measurements to give an increased accuracy and noise rejection in grounding measurements.

Use of computer-based electromagnetic measurements to characterize the variation of soil resistivity over large areas, along with validation programs by federal organizations such as the United States Geological Service and the Geological Service of Canada.

Widespread availability of Microsoft Excel and other easy-to-use spreadsheets that contain all the relevant and difficult mathematical functions used in grounding and allow users to generate their own design curves and values efficiently.

An Internet-savvy generation of people who are comfortable using a web browser to navigate through a logical series of web pages, entering data and evaluating visual output as they go.

Introduction

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1.4 Purpose and Structure of This Report

A rich and bewildering set of technical resources is already available to the transmission line designer who must specify a ground electrode. This report focuses mainly on electrode treatments that reduce the ac and lightning impulse impedance of transmission towers to remote earth. Other guides and standards listed in Section 6, References, provide details on managing large fault currents in substations and coordinating power system fault current with nearby railroads, pipelines, or telephone services.

This report is the first of two parts. Part ITheory Book, provides the theory and basic principles necessary for a solid understanding of good grounding engineering practice. Part IIPractical Guidelines, provides easy-to-use guidelines, tools, and worked examples for several tower configurations.

These reports are complemented by a set of software applications, EPRI Transmission Line Grounding Guide Software (EGGS), Version 1.01 (1011654). The applets provide a set of simple and easy-to-use tools that are intended as tutorials to illustrate and amplify the text of the reports to help readers gain a better understanding of the underlying principles.

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2 ROLES OF TRANSMISSION LINE GROUNDING

The grounding system is an essential part of both high- and low-voltage electric power networks and serves at least four crucial electrical roles:

To protect against lightning and improve the lightning outage performance of the line by performing the following functions: Provide a low-impedance path to earth using mechanically and electrically robust

grounding connections Limit potential differences across electrical insulation on towers that are struck by

lightning Reduce the number of flashovers that occur

To ensure correct operation of the transmission system control and protection equipment by performing the following functions: Provide increased fault current levels to allow a rapid and unambiguous identification of

fault conditions for efficient relay and fuse coordination Provide low zero-sequence impedance for the return of the unbalanced fraction of

three-phase ac to eliminate hazards associated with ungrounded systems To ensure electrical safety for exposed humans by performing the following functions:

Allow the quick identification of faults, which leads to reduced fault duration Limit touch or step potentials to levels that restrict body currents to safe values

To lower the electromagnetic interference of the line by having a grounding configuration that is designed to suppress induced voltage and current on nearby conducting objects

All these functions are provided by an integrated grounding system made up of conductors, hardware, foundations, and the local soil or rock. Each element of the system has its specific purpose, but all the elements must function together in an electrically interconnected system that must be designed and analyzed as a whole.

Roles of Transmission Line Grounding

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2.1 Lightning

Transmission line grounding forms an integral part of the lightning protection system that typically includes the following components:

Overhead ground wires (also called static wires or shield wires) Grounding conductors or down conductors Grounding electrodes Surge arresters

Other connectors or fittings required for a complete system

On overhead lines, lightning can cause line outages in two ways: 1) as a result of induction when it strikes in the vicinity of the line, and 2) by direct contact when it terminates either on a grounded structure or shield wire or onto phase conductors.

Induction is not considered important for transmission lines because the level of induced voltage, which is generally lower than 300 kV on unshielded lines, is lower than the line insulation level and is unlikely to cause a flashover. On lines with overhead ground wires, lightning-induced overvoltages are even lower; therefore, induction is not considered further in this report.

Direct strokes, or flashes, to the line can cause flashover in two ways.

1. By terminating on the phase conductor, which is called a shielding failure. Flashovers as a result of shielding failure are prevented by the correct placement of the overhead ground wires or shieldwires to intercept the lightning stroke and direct it to ground.

2. By terminating on the tower or shielding arrangement, which causes a so-called back-flashover as a result of the voltage buildup over the grounding system. The most common remedy for back-flashover is to lower the tower footing impedance.

The overhead ground wires intercept the lightning strokes and prevent them from terminating on the phase conductors or other equipment that needs to be protected. The rest of the grounding system provides a low-impedance path for the lightning current to discharge into the general mass of the earth. It must do so without developing high voltages on the tower that could lead to flashover of the line insulation.

From a transmission line grounding perspective, back-flashover is the most important lightning condition that must be considered. Other aspects regarding the improvement of the lightning performance of lines are treated in the EPRI report Handbook for Improving Overhead Transmission Line Lightning Performance (1002019) [4].


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2.2 Correct Operation of the Transmission System

An important aspect of electric power system reliability is how well its protection can identify and isolate fault locations. Relay systems are grouped into primary and backup protection, with instantaneous or timed response as follows:

Instantaneous primary-protection relays detect system faults that could destroy system integrity or equipment. These relays are set to respond quickly to basic parameters such as overcurrent, changes in impedance, or differential currents. The clearing times vary with the fault current magnitude, the type of breakers, and the fault location in the monitored zone. Primary protection relays usually operate within approximately 200 ms for 69-kV, 115-kV, and 138-kV systems and within approximately 100 ms for higher system voltages.

Timed primary-protection relays respond after a fixed or inverse time delay to system conditions (such as overvoltage, undervoltage, loss of excitation, negative-sequence current, voltage parameters, or frequency parameters) that will lead to deterioration or failure of equipment.

Backup-protection relays trip breakers when the primary protection relays fail to clear a fault. These situations re associated with breaker failure and transformer and bus backup protections. The operating times are intentionally longer than those of the instantaneous primary protection. Typical values range from 150 ms to 200 ms, independent of the system voltage, but values as large as 800 ms are also used.

The transmission line grounding plays the following roles in helping the protection to locate and isolate faults in the electrical system effectively:

The grounding system (overhead ground wires and grounding electrodes) provides a well-defined and stable zero-sequence impedance against which protection settings can be made.

It provides a low impedance to ground that will present a greater contrast between normal and fault conditions, improving speed and accuracy of identification for the instantaneous primary protection.

It provides a return path for fault current to the source. It helps to prevent single-phase faults from escalating into multi-phase faults.

2.3 Safety

Permanent and temporary grounds can be used to ensure that transmission line structures and the immediate surroundings pose a low risk, in terms of step and touch potentials, to humans and livestock in the vicinity of the line both during normal operation of the line and during fault conditions. Procedures, designs, and requirements are governed by regulations and company policy. These aspects are outside the scope of this report.


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2.3.1 Normal Operation

During normal operation, transmission line structures carry a voltage that is the result of the following factors:

Electrostatic coupling between the energized phase conductors and the overhead ground wires, and between the phase conductors and the tower.

Magnetically induced current caused by the flux linkage between the load current in the phase conductors and the loop formed by the overhead shield wire, tower, and ground. The magnitude of this current is a function of the phase configuration, the magnitudes of the phase current, the level current imbalance between the three phases, and the length of the span. Differences between the current induced in adjacent spans are conducted to ground through the earth electrode and its impedance, causing the tower base to rise in voltage. These differential currents are most pronounced at sections with unequal span lengths or where there is a change in the conductor configuration, such as at phase transposition points.

Unbalanced circuits that can result a steady-state neutral potential with its associated zero-sequence current, which varies with line loads. This neutral potential is transferred to the line towers in the vicinity of the substation through the overhead shield wires if they are connected to the substation ground mat (as is the usual case).

Leakage currents as the result of insulator contamination that must also be shunted to ground through the earth electrode, which will cause a momentary rise in potential.

For normal operation in which an effective grounding system is present, the sum of these ground potentials is usually limited to less than 10 V.

2.3.2 Fault Conditions

Fault conditions, especially phase-to-ground faults, can result in large current magnitudes in the grounding system. Most of the line-to-ground fault current is transported back to the substations, sourcing the fault, through the overhead ground wires. However, a significant portion of the current is still shunted to ground through the ground electrode, which can result in large potential gradients in the soil surrounding the towers. The potential gradients are expressed in terms of step and touch potentials, which can be evaluated in terms of the safety limits imposed by regulations and company policy. The requirement to limit step and touch potentials around transmission line towers could govern the ground electrode design and layout in some cases.


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2.4 Electromagnetic Interference

Transmission lines transport a large amount of energy over long distances. The resulting high voltage, high current levels, and the electromagnetic field around the lines can cause interference on nearby conducting objects such as other infrastructure (pipelines, fences, railways, and other lines) that parallel the transmission lines. Of concern are the levels of induced voltages and currents that might cause the following:

Exposure of people to unexpected discharges when they make contact with the supposedly non-energized structures

Damage to the infrastructure itself

Damage to electronic equipment that is in contact with the infrastructure

Induced voltage and current into parallel running lines, pipelines, and railroads

The transmission lines themselves are immune to electromagnetic interference from other lines in the vicinity because the induced voltages and currents are much lower than the normal operating voltage or current. However, coupled voltages and currents are a concern when work is performed on deenergized lines. In these cases, temporary grounds must be applied to ensure the safety of personnel. The application of temporary grounds falls outside the scope of this report. It is covered in the EPRI report Survey of Utility Practices for Establishing Equipotential Zones During De-Energized Work (1001752) [5].

The mitigation of electromagnetic interference is a specialized study area. It requires a combination of shielding, configuration changes, and grounding. This report does not cover electromagnetic interference as a whole, but rather mentions it when necessary as a factor that should be taken into account when deciding how to treat the grounding around transmission line structures. Guidance on the electromagnetic interference to pipelines can be found in the Cigr Guide on the influence of high voltage ac power systems on metallic pipelines (Technical brochure 95) [58] and for railroad systems in the EPRI Power System and Railroad Electromagnetic Compatibility Handbook (1005572) [55].

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3 DEFINITIONS

Counterpoise. Horizontal buried wires installed for grounding of transmission lines. They can be continuous from tower to tower or radial outward from the tower.

Earth surface potential Vx. The voltage between a point x on the earths surface and remote

earth.

Electrical resistivity . A physical property with units of ohm-meters (m) of all materials that relate the electric field E (V/m) to the current density J (A/m2) using E = J.

Electrode voltage (electrode potential) VE. The voltage occurring between the grounding system and reference earth at a given value of the impressed earth current.

Equipotential zone. A concept that was introduced to protect people and animals from hazardous or annoying potentials because of inadvertent energization or induction. Ensuring that all equipment, conductors, anchors, and structures within a defined area are electrically connected creates an equipotential zone.

Grounding conductor. A conductor that connects a part of an electrical installation, exposed conductive parts, or extraneous conductive parts to a ground electrode or that interconnects grounding electrodes. The grounding conductor is laid above the soil, or if it is buried in the soil, it is insulated from it, meaning it does not function as a ground electrode.

Grounding electrode. A metal conductor or a system of interconnected metal conductors or other metal parts acting in the same manner, embedded in the earth and electrically connected to it or embedded in concrete that is in contact with the earth over a large area.

Grounding (grounding system). The total of all means and measures by which part of an electrical circuit, accessible conductive parts of electrical equipment (exposed conductive parts), or conductive parts in the vicinity of an electrical installation (extraneous conductive parts) are connected to earth.

Grounding impedance. The overall impedance between the grounding system and reference earth.

Ground potential rise (GPR). The potential with respect to far earth to which a ground electrode will rise when discharging current into the soil.

Definitions

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Ground resistance. The resistive part of the grounding impedance between the grounding system and reference earth. In most cases, the ground resistance is a good approximation of the grounding impedance.

Ground resistivity (specific earth resistance) . The resistance measured between two opposite faces of a 3.3-ft (1-m) cube of earth (see Figure 3-1), expressed in m.

Figure 3-1 Definition of Ground Resistivity

Reference earth. That part of the ground, particularly on the earth surface, located outside the sphere of influence of the considered ground electrode. One test (real or conceptual) for sphere of influence is to impress a current through the electrode and establish the locations where there is no perceptible voltage between two random points resulting from this grounding current flow. The potential of reference earth is always assumed to be zero.

Step potential. The potential difference between two points on the ground that are 3.3 ft (1 m) apart.

Touch potential. The potential difference between a persons outstretched hand, which is touching an earthed structure, and the persons foot. A persons maximum reach is assumed to be 3.3 ft (1 m). The touch potential can be equal to the full ground potential rise if the object is grounded at a point remote from the place where the person is in contact with it.

Definitions

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Table 3-1 defines the symbols used in this report.

Table 3-1 Symbols Used in This Report

Symbol Definition

pi, 3.14159

A complex number expressing the skin depth, which is defined as the depth at which the eddy current density has decreased to 1/e, or approximately 36% of that at the ground surface

The soil conductivity in siemens per meter (S/m), equal to 1/, where is the soil resistivity in m

The soil resistivity

The frequency in radians/s

A time constant defined as the time at which a signal has fallen to 1/e (36%) of its initial value.

4 10-7 H/m

The permittivity of free space, 8.854x10-12 farads per meter

r

The relative earth permittivity (dielectric constant)

A Surface area

a Radius

ax

Horizontal dimension from the center of an electrode along a line

ay Horizontal dimension from the center of an electrode across a line

az Vertical dimension from the center of an electrode to the surface of the ground

c The velocity of light, 3 x 108 m/s

C1, C2 The current terminals

D Distance

dT The depth of the upper soil layer

E0 The critical dielectric ionizing gradient of the soil, typically 300400 kV/m

f The test frequency (Hz)

g 222 zyx aaa ++ , geometric radius

h The height above ground level

Definitions

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Table 3-1 (continued) Symbols Used in This Report

Symbol Definition

I The current through an object

J The current density

kcmil 1000 circular mils, where a circular mil is the area of a circle with a diameter of 0.004 in. (0.0025 cm) L The length of an object

ln Natural logarithm (to base e), or loge

P1, P2 The potential terminals

r The radius of an overhead conductor

R The resistance of an object

s The characteristic distance from the center of an electrode to its outermost point

t Time

T Temperature

V Voltage

X Reactance

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4 ELECTRICAL CHARACTERISTICS OF SOIL

4.1 Introduction

The ability of soil to conduct current is of fundamental importance to the grounding of electrical systems. Conduction of current through soil can take place by electronic or electrolytic current flow. Electronic conduction is characterized by the movement of free electrons in the material itself as a result of the electric field impressed on it. Electrolytic conduction is characterized by the movement of ions through a solution. In soil, electronic conduction typically takes place through conductive metal ores, metallic objects such as pipelines, or carbon deposits. Electrolytic conduction takes place through water that is carrying dissolved minerals and salts.

Of these two, electrolytic conduction is predominant because soil always contains some amount of moisture thatin combination with salts present in the soilcan serve as an electrolyte for current flow. Electronic conduction becomes important deep in the earth where the deep rocks are subjected to a high overburden pressure. Electronic conduction can also be important in surface deposits that contain conducting minerals such as magnetite, graphite, or pyrite.

The electrical conduction in soil is expressed in terms of the soil resistivity, , in ohm-meters (m). Resistivity is defined as the relationship between the resistance (R) of an object, its length (L), and its cross-sectional area (A). Resistance is defined as the relationship between the voltage (V) across the object and the current (I) through the object. See Equation 4-1 and Figure 4-1.

IA

LV

LAR

=

=

Equation 4-1 Definition of Resistivity

Electrical Characteristics of Soil

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Figure 4-1 Definition of Resistivity and Resistance

When calculating the grounding resistance of electrode systems, the soil resistivity features as a linear term, usually in the following form:

size electrodefactor shape electrodeyresistivit soil the toalproportiondirectly is resistance electrode Ground

The following sections describe soil resistivity and its measurement.

4.2 Electrical Characteristics of Homogeneous Soil

4.2.1 Basic Parameters That Influence Soil Resistivity

Because electrolytic conduction is the predominant process by which current flows through soil, the resistivity of soil is highly dependent on the presence and factors that influence the electrolyte in the soil. Some of the primary factors that influence the electrolyte in the soil are the following:

The type of soil The grain size and grain size distribution of the material The closeness of the packing of the grains and the pressure on the soil The size and shape of voids and interconnecting passages in the soil The extent to which the passages are linked by water paths


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The chemical composition of the salts dissolved in the ground water The concentration of the salts dissolved in the ground water The moisture content of the soil

The temperature of the soil

Secondary factors that influence the conduction through soil, and by implication the soil resistivity, are the level of the electric field in the soil (which is a function of the current density in the soil) and the frequency of the applied voltage.

Variations in these parameters result in differences in soil resistivity of nearly four orders of magnitude. This is shown in Table 4-1, which lists typical ranges of resistivity for different soils and concrete at room temperature. Although some of these parameters remain constant over time, others such as moisture content and temperature can vary over time as a result of seasonal changes. This can introduce large seasonal variations in the soil resistivity, depending on the type of climate.

Table 4-1 Low-Frequency Resistivity, , for Soil, Earth, and Concrete Material

Resistivity in m Material

Range of Values Average Value Type of Soil or Earth Boggy ground 250 30 Adobe clay 2200 40 Silt and sand-clay ground, humus 20260 100 Sand and sandy ground 503000 200 (moist) Peat 1200 200 Gravel 503000 1,000 (moist) Stony and rocky ground 1008000 2,000 Type of Concrete Electrically conductive concrete (see note) 510 8 Concrete with 1:3 cementsand mix 50300 150 Concrete with 1:5 cementgravel mix 1008000 400

Note: Measured after a 30-day cure on rectangular slabs.

In the past, it was common practice to assume that the soil resistivity is equal to 100 m when no better information was available. However, actual soil resistivity can differ from this assumption, and it is often significantly higher. As a result, ground electrode designs based on the 100-m assumption are often undersized.


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4.2.1.1 Type of Soil

In terms of soil resistivity, the most important characteristic of the soil is the grain size distribution because it influences how the soil carries moisture. For example, clay retains moisture and dissolved chemicals in a tight matrix. Compared with sandy ground, clay has low, relatively constant resistivity that changes only by a factor of 10 over a 020% range of moisture content. As explained by Tagg in Earth Resistances, coarse gravel of uniform size has a very high resistivity [6].

Another important soil characteristic is the amount of free space between sand grains. This determines at what moisture level the soil becomes saturated, which is the point at which all the voids in the soil are filled with water. The size of the voids in the soil depends on the effectiveness of the packing in the soil, which in turn depends on the shape and size distribution of the grains in the soil. This is illustrated in Figure 4-2, which shows the different packing arrangements of spherical grains. In tightly packed soil (Figure 4-2a), the volume of the voids amount to approximately 26% of the total volume, and in the least compact soil (Figure 4-2b), the volume of the voids amount to 48% of the total. In typical soils (Figure 4-2c), the grains are not exactly spherical or of uniform size, which results in generally lower volumes of voids. As an indication, the volume of free space in the soil can vary from 25% for porous conglomerates, to 815% for ordinary clays and sand, to 0.22 % for rocks.

Figure 4-2 The Effect of Grain Packing on the Volume of Voids in the Soil. Please note that the circles in the diagram represent spheres.

4.2.1.2 The Composition and Type of Salts Dissolved in the Ground Water

Because the conduction of electric current through soil is largely electrolytic, the amount and types of salts dissolved in the soils water are an important factor in determining soil resistivity. Especially at low salt concentrations, the amount of salt dissolved has a large influence on the resistivity of the solution [6]. Furthermore, the type of salt present in the solution has a marked influence on the conductivity of an electrolyte. For example, common sea salt (sodium chloride)


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results in a more conductive solution than sodium sulphate and copper sulphate do. Sulphuric acid, on the other hand, results in a higher conductivity solution than sodium chloride does at the same concentration [6].

4.2.1.3 Moisture Content

The amount of electrolyte in the soil, which is related to the moisture content, is one of the most important factors that influence the soil resistivity. Generally, soil resistivity decreases with an increase in the moisture content.

Typical moisture levels can range from 5% for desert regions up to approximately 80% for temperate, swampy regions. The moisture content rarely exceeds 40% in the typical conditions in which transmission lines are located [6]. The influence of moisture on resistivity tends to decrease at moisture levels greater than 20%, which corresponds to the moisture saturation level of the soil. Figure 4-3 shows that resistivity increases rapidly at less than 20% moisture content.

Figure 4-3 Resistivity of Materials as a Function of Moisture Content


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4.2.1.4 Temperature

Temperature variations influence the resistivity of the soil. For practical transmission lines, the effect is most prominent when the temperature is less than 32F (0C). When the water contained in the soil freezes, ion mobility is greatly reduced, leading to a dramatic increase in soil resistivity, as Hoekstra and McNeill described in Electromagnetic Probing of Permafrost (see Figure 4-4a) and Keller and Frischknecht explained in Electrical Methods in Geophysical Prospecting (see Figure 4-4b) [7, 8].

Figure 4-4 Soil Resistivity as a Function of Temperature Sources: Hoekstra and McNeill, Keller and Frischknecht

In most areas, only the topmost layer of the soil freezes when the ambient temperature drops to less than 32F (0C). Deeper down, where the soil is insulated from the ambient, the soil temperature assumes the average yearly temperature of the region. The increase in resistivity typically does not coincide precisely with 32F (0C) because the salts dissolved in the ground water lower its freezing point [1].


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4.2.1.5 Variation with Electric Field

Ionization, or electrical breakdown, of the soil occurs when the electrical field exceeds approximately 400 kV/m. This typically happens around concentrated electrodes when they conduct high currents (for example, during a lightning strike). This important aspect of soil behavior is described further in Section 5.4, The Behavior of Grounding Electrodes When Discharging Lightning Current.

The breakdown of soil under high-current conditions can lead to the formation of fulgurites, also known as fossilized lightning, which are hollow glass tubes that are fused as a result of the passage of lightning current through sand. Figure 4-5 shows the hollow tube formed in sandy soil when the lightning current heats and fuses the silicon dioxide (SiO2) into glass at a temperature in excess of 1600C. After triggered lightning experiments in Florida, Martin Uman excavated a pair of fulgurites that were 16 and 17 ft (about 5 m) long and donated another specimen to EPRI in appreciation of EPRIs support.

Figure 4-5 Typical Sand Fulgurite from East Texas Source: Mineralogical Research Company, www.minresco.com/fulgurites/fulgurites.htm

High electric fields lead to a high current density, which generates heat as a result of the resistive losses (I2R). This can lead to drying of the soil in areas with a high current density, when relatively high currents are conducted for a prolonged period. As a result, the local resistivity of the soil increases. The current densities in typical grounding electrodes are low enough that this effect is not a major concern.

4.2.1.6 Variation with Frequency

Measurements have shown that the resistivity of most types of soil does not remain constant with frequency. Rather, the resistivity decreases as the frequency increases. Because lightning is a transient phenomenon, having a fundamental frequency of 80 to 120 kHz, this dependence can result in a different ground electrode impulse response than one would predict using soil resistivity measurements performed at a single low frequency.


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Nevertheless, most engineers currently use low-frequency resistivity or ground electrode resistance measurements (approximately 100 Hz) to predict the response of transmission line grounding electrodes to lightning, although frequencies of around 100 kHz are of more interest. This issue is addressed by instruments, such as the EPRI Zed-meter, that measure the response of grounding electrodes using higher frequencies or impulses.

The data from Visacro and Portellas Soil Permittivity and Conductivity Behavior on Frequency Range of Transient Phenomena in Electric Power Systems (illustrated in Figure 4-6) suggest that the change in impedance with frequency varies, depending on soil type[9]. This is because, physically:

Sandy soil is made up of uniform, intermediate-sized particles, classed from very coarse (< 0.08 in. [


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4.2.2 Seasonal Variations

No two sets of ground measurements will ever agree precisely, especially if they are taken days, weeks, or months apart. In Germany, Rudolph and Winter (in EMV nach VDE 0100) reported a 30% sinusoidal variation around average values of grounding resistance [10]. They found that the maximum resistivity is reached in February and the minimum occurs during August. The values measured in May and November correspond to the yearly average value.

These seasonal variations are a result of changes in humidity and changes in temperature. Seasonal variations in rainfall and humidity affect the moisture content of the soil. The soil resistivity can vary by a factor of 10 with high resistivity values in the dry season and lower values during the wet season. Studies documented in Scott and Mays Earth Resistivities of Canadian Soils show that soil resistivity varies by a factor of 10 or more in areas where the soil freezes [11]. This can result in significantly higher values during the cold months of the year. Freezing, like drying, increases resistivity by one to two orders of magnitude, but the effect is generally restricted to the top 3.9 in. to 3.3 ft (0.1 to 1 m) of soil.

Seasonal variations are more pronounced in the resistivity of the upper layer compared with that of the bottom layer. In deeper layers, the moisture content stabilizes and the temperature approaches the yearly average values. Measurements in France have shown that these changes can reach as deep as 65.6 ft (20 m) below the surface [12]. Figure 4-7 (redrawn from [14]) shows the results from measurements on two arrays of 21 electrodes each, spaced at 7.9-in. (0.2-m) intervals, which were installed in boreholes approximately 4 ft (1.2 m) apart. The derived resistivity values presented in Figure 4-7 show a feature at a depth of 59 ft (18 m) with a resistivity of 89113 m in January 1999, 5469 m in April 1999, and 2633 m in September 2000.


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Figure 4-7 Resistivity Distribution Between Electrodes at Tournemire, France

4.3 Electrical Characteristics of Nonhomogeneous Soil

In the preceding sections, the basic parameters that influence soil resistivity have been presented for homogeneous soil. In practice, however, soil can rarely be considered homogeneous because geological features cause soil types to vary from location to location, and the presence of bedrock or ground water can result in significant changes in resistivity as a function of depth. For transmission lines with their concentrated grounding electrodes at each tower base, it is important to be aware of the impact that the vertical and horizontal variations in soil resistivity can have on the performance of a line.

4.3.1 Soil Resistivity as a Function of Location

Most maps of local geology, such as the example for Tennessee (see Figure 4-8), show areas where geology is relatively constant over distances of 20 mi (40 km) and other areas where geology changes over a distance of 15 mi (28 km) [13]. Moving between different geological zones, such as from Precambrian to Paleozoic areas, the resistivity can drop by four orders of


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magnitude. Overhead lines that traverse two or more geologic areas can require different grounding treatments in each area. Generally, geologic maps give an indication of the underlying layer resistivity.

In the Tennessee Valley Authority (TVA) network, the ground resistivity is managed as a corporate resource through a TTHOR database of local values [14]. Two examples extracted from this database show areas where the mean resistivity is less than 150 m (see Figure 4-9) and areas where the values exceed 1000 m (see Figure 4-10) [14]. The areas with low resistivity are associated with sand, silt, clay, and gravel. The areas of high resistivity include regions identified in Figure 4-8 as Mississippian limestone and Cambrian and Precambrian rock.

Figure 4-8 Generalized Geologic Map of Tennessee Source: Tennessee Department of Environment and Conservation

Figure 4-9 Areas in Tennessee Where Mean Resistivity Is Less than 150 m Source: Tennessee Valley Authority


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Figure 4-10 Areas in Tennessee Where Mean Resistivity Exceeds 1000 m Source: Tennessee Valley Authority

4.3.2 Horizontal and Vertical Layering

From the perspective of transmission line grounding, variations in soil resistivity take the form of horizontal and vertical layering (see Figure 4-11). These formations are closely related to the local soil type and geological structures, as described in Section 4.3.1, Soil Resistivity as a Function of Location.

Figure 4-11 Complex Soil Model with Various Types of Soil Layering. (A) Horizontal layering (B) Vertical layering.

4.3.2.1 Horizontal Layering

Horizontal layering in the soil (indicated by A in Figure 4-11) can be present as a result of differences in levels of moisture and temperature and as a result of the geological soil structure. Surface evaporation increases resistivity near the surface, even in areas where the soil is relatively uniform. A distinct drop in resistivity can occur at the depth of the water table. In climates with cold winters, a drop in resistivity occurs during the cold months in the frozen top


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layer, and the deeper soil layer converges to the local mean annual average temperature (if it is greater than 32F (0C). The local geology is composed of horizontal strata of different soil types with different resistivity values, such as bedrock that has a high resistivity covered by an overbunden of material that has a lower soil resistivity.

Horizontal layering plays an important part in the behavior of grounding electrodes. In areas where a low-resistivity overburden covers high-resistivity rock, the top layer might be the only material that provides a low-resistance path for current. Deep grounding electrodes might not be effective in such areas; therefore, consideration should be given to wide, shallow electrodes such as ring electrodes or a counterpoise. In areas where a top layer of high-resistivity soil covers a low-resistivity layer, the grounding electrode should consist of several vertical rods to make contact with the low resistivity layer.

For transmission line grounding, it is important to take horizontal layering into account, but it is usually sufficient to use a simplified soil model consisting of one or more layers in grounding calculations, even if more layers might be present.

4.3.2.2 Vertical Layering

Changes in resistivity can also be present as vertically orientated soil layers (indicated by B in Figure 4-11). The most obvious of these layers is where there is a rapid change of the depth of the overburden over a rock layer or at the borders of geological zones.

Generally speaking, these vertical layers do not affect the design of individual tower footing electrodes unless the electrode is located on a vertical border. More important, however, are the changes in soil conditions that can occur from tower to tower. As a result, modeling all towers on a line or line segment with the same model is an oversimplification that can lead to inaccurate results on individual towers.

In actual conditions, the soil structure is a combination of vertical and horizontal or slanted layers. Figure 4-12 shows a resistivity profile along the route of a 345-kV line based on the results from an aerial electromagnetic survey. The technique is described in Section 4.4.5, Active Induction Methods.


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Figure 4-12 Vertical and Horizontal Distribution of Resistivity Values from an Aerial Electromagnetic Survey Near a 345-kV Power Line Source: Fugro Airborne Surveys, NB Power

Each color in the figure represents a different soil resistivity; warmer colors have lower resistivity. Figure 4-12 illustrates the following conditions, all of which occur within 3 mi (5 km) or approximately 15 transmission spans:

In the areas marked O, there is a conductive overburden of 300 m (typically 316 ft [15 m] thick) on top of poorly conducting 3000 m rock. These are areas in which wide, flat electrodes provide an advantage.

In the area marked D, the overburden is approximately 230 ft (70 m) thick and has a resistivity of 500 m.

In the area marked G, the relatively thin overburden covers low-resistivity soil (approximately 100 m). In this area, it would be practical to apply driven rods to reach the lower resistivity soil.

The blue areas marked H are areas of extremely high resistivity (7000 m). These areas present difficult grounding conditions because there is no low-resistivity earth in the vicinity that can be used to establish a good contact with the earth.

4.4 Measurement of Soil Resistivity

As the fundamental parameter, it is important to obtain some information about the soil resistivity when designing grounding electrodes and transmission lines. More specifically, the reasons for obtaining information about soil resistivity are the following:

Soil resistivity has a direct influence on tower potential rise from lightning flashes. Uncertainties in estimates of resistivity dominate our ability to compute transmission line back-flashover rates. For typical lines, it can be shown that a 10% change in resistivity will lead to a 10% change in the lightning tripout rate because there is a direct relationship between soil resistivity and the value of footing resistance.


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Soil resistivity has a marked influence on the earth return impedance (or zero-sequence impedance) of a transmission line at 60 Hz, as reported by Carson in Wave Propagation in Overhead Wires with Ground Return and by Deri in The Complex Ground Return PlaneA Simplified Model for Homogenous and Multi-Layer Earth Return [15, 16]. Therefore, it directly affects the efficiency of power transfer and the magnitude of imbalance current in the line, and it must be accounted for in overvoltage simulations and protection settings.

The value of soil resistivity, as well as the nature of its layering near the surface, has important safety implications. The resistance under the foot, which depends on upper-layer soil resistivity (1), acts as an important electrical barrier around transmission lines. The touch potentials near electrical systems and the zone of electrical influence to nearby communication, pipeline, and rail systems are strongly affected by the ratio of upper- to lower-layer resistivity.

When considering the measurement of soil resistivity, several points must be considered. The electrical resistivity of the different materials contained in soil can vary by over 20 orders of magnitude. In addition, the soil resistivity depends on temperature, moisture content, and frequency for many materials, including most soils and rock types. No single technique or instrument can measure soil resistivity over such a wide range. Therefore, it is necessary to select techniques and instruments that enable a practical measurement of soil resistivity and an assessment of the related experimental errors.

Soil resistivity is typically measured at low voltage gradients, low current densities, and relatively low frequencies. These are generally considered as secondary effects and are normally not taken into account. However, the effects become important when grounding electrodes discharge lightning currents. They are described in detail in Section 5.4, The Behavior of Grounding Electrodes When Discharging Lightning Current.

Over the years, several methods have been developed for the measurement of soil resistivity, including the following:

Resistivity measurements directly on soil core samples at the same time they are being evaluated for civil engineering work. Often, a time-domain reflectometer instrument is used to carry out this measurement.

Direct galvanic measurement of the soil resistivity with an electrode array such as the Wenner, Schlumberger, or Lee array.

Passive electromagnetic methods. Active induction methods.


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Of these methods, the most well known within the electrical utility industry is the Wenner four-electrode method. For this reason, it is described from a theoretical point of view in the following subsection. The other methods are treated in less detail, and the indirect methods are introduced as candidates for future development with a view of making large-scale soil resistivity surveys possible. Several methods are described in EPRI report EL-2699 [1]. This section focuses on the theory behind these methods; the practical aspects are covered more thoroughly in Part II of this report.

4.4.1 Wenner Four-Electrode Method

Four-electrode methods are used to determine the soil resistivity based on the measurement of the potential profile that is established around a pair of electrodes when they are used to circulate current through the soil. The most well known four-electrode method is the Wenner array, which consists of four equally spaced surface probes at spacing s (see Figure 4-13). Current is circulated between the two outer, or current, electrodes (C1 and C2), and the potential difference is measured between the two inner, or voltage, probes (P1 and P2). The ratio between the current and the voltage can then be used to calculate the apparent soil resistivity at a depth that is related to the probe spacing.

Figure 4-13 Wenner Probe Technique for Measurement of Apparent Resistivity, a

This four-terminal g

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