METHODOLOGY AND TECHNOLOGY FOR POWER SYSTEM GROUNDING

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3 METHODOLOGY AND TECHNOLOGY FOR POWER SYSTEM GROUNDING

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5 METHODOLOGY AND TECHNOLOGY FOR POWER SYSTEM GROUNDING Jinliang He Rong Zeng Bo Zhang Department of Electrical Engineering, Tsinghua University, China

6 This edition first published 2013 # 2013 John Wiley & Sons Singapore Pte. Ltd. Registered office John Wiley & Sons Singapore Pte. Ltd., 1 Fusionopolis Walk, #07-01 Solaris South Tower, Singapore For details of our global editorial offices, for customer services and for information about how to apply for permission to reuse th e copyright material in this book please see our website at All Rights Reserved. No part of this publication may be reproduced, stored in a retrieval system or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording, scanning, or otherwise, except as expressly permitted by law, without either the prior written permission of the Publisher, or authorization through payment of the appropriate photocopy fee to the Copyright Clearance Center. Requests for permission should be addressed to the Publisher, John Wiley & Sons Singapore Pte. Ltd., 1 Fusionopolis Walk, #07-01 Solaris South Tower, Singapore , tel: , fax: , enquiry@wiley.com. Wiley also publishes its books in a variety of electronic formats. Some content that appears in print may not be available in electronic books. Designations used by companies to distinguish their products are often claimed as trademarks. All brand names and product names used in this book are trade names, service marks, trademarks or registered trademarks of their respective owners. The Publisher is not associated with any product or vendor mentioned in this book. This publication is designed to provide accurate and authoritative information in regard to the subject matter covered. It is sold on the understanding that the Publisher is not engaged in rendering professional services. If professional advice or other expert assistance is required, the services of a competent professional should be sought. Library of Congress Cataloging-in-Publication Data He, Jinliang. Methodology and technology for power system grounding / Jinliang He, Rong Zeng, Bo Zhang. pages cm Includes bibliographical references and index. ISBN (cloth) 1. Electric currents Grounding. 2. Electric power systems Protection. I. Zeng, Rong, II. Zhang, Bo, III. Title. TK3227.H dc ISBN Set in 9/11 pt Times by Thomson Digital, Noida, India

7 Contents Preface Acknowledgements xiii xv 1 Fundamental Concepts of Grounding Conduction Mechanism of Soil Soil Structure Conduction Mechanism of Soil Functions of Grounding Devices Concept of Grounding Classification of Grounding Purpose of Grounding Definition and Characteristics of Grounding Resistance Definition of Grounding Resistance Relationship between Grounding Resistance and Capacitance Shielding Effect among Grounding Conductors Grounding Resistance of Grounding Devices Grounding Resistance of General Grounding Devices Grounding Resistance of Grounding Device in Non-Homogeneous Soil Body Safety and Permitted Potential Difference Allowable Body Current Limit Allowable Body Voltage Allowable Potential Difference Influence of Resistivity of Surface Soil Layer on Body Safety Standards Related to Power System Grounding 25 References 26 2 Current Field in the Earth Electrical Property of Soil Soil Resistivity Influence of Different Factors on Soil Resistivity Permittivity of Soil Frequency Characteristics of Soil Parameters Basic Properties of a Constant Current Field in the Earth Current Density in the Earth Continuity of Earth Current Field Potential of Stable Current Field Current Field at the Interface of Layered Soil 37

8 vi Contents 2.3 Current Field Created by a Point Source in Uniform Soil Laplace s Equation Current Field Created by a Point Source in Soil Earth Current Field Produced by Two Opposite Point Current Sources on the Ground Surface Earth Current Field in Non-Uniform Soil Potential Produced by a Point Source on the Ground Surface in Non-Uniform Soil Horizontally Layered Soil Horizontal Double-Layer Soil Horizontal Triple-Layer Soil Vertically Layered Soil Potential Produced by a Point Source in Multi-Layered Soil Analysis of Potential Produced by a Point Current Source Numerical Integral Method to Calculate Green s Function of a Point Current Source Computer Program Derivation Method of Green s Function Method of Obtaining Analytic Expression Expression of Green s Function Derived from Software Program Calculation of Current Field in Multi-Layered Soil Fast Calculation Method of Green s Function in Multi-Layered Soil Development of a Two-Stage Fitting Method Application of the Fast Calculation of Green s Function in Multi-Layered Soils Current and Potential Distributions Produced by a DC Ground Electrode Current and Potential Distributions of DC in Uniform Soil Current and Potential Distributions of DC Current in Non-Uniform Soil 72 References 78 3 Measurement and Modeling of Soil Resistivity Introduction to Soil Resistivity Measurement Measurement Methods of Soil Resistivity Sampling Analysis Method of Soil Resistivity Electrical Sounding Methods Test Probe Configuration for Four-Probe Method Field Test Technique of Soil Resistivity Electromagnetic Sounding Method Simple Analysis Method for Soil Resistivity Test Data Electrical Sounding Curve Method for Two-Layered Horizontal Soil Model Analysis of a Three-Layered Horizontal Geological Structure Resistivity of Vertically Layered Soil Structure Estimation of Soil Model Parameter using the Three-Probe Method Numerical Analysis for a Multi-Layered Soil Model Typical Curves of Multi-Layered Soil Apparent Resistivity Expression of Apparent Soil Resistivity Inverting Soil Parameters Numerical Analysis Method for Two-Layered Soil Model Multi-Layered Soil Model by Solving Fredholm s Equation Solving the Forward Integral Equation Inversing Parameters of Soil Models Application in Estimation of Soil Parameters 116

9 Contents vii 3.6 Estimation of Multi-Layered Soil Model by Using the Complex Image Method Estimation of Multi-Layered Soil Structure Fast Calculation of the Soil Apparent Resistivity Partial Derivatives of Calculated Apparent Resistivity The Partial Derivative Expressions of f(l) Determination of the Initial Soil Parameters Engineering Applications 123 References Numerical Analysis Method of Grounding Calculation Method for Parameters of Substation Grounding Systems Calculation of Grounding Parameters with Empirical Formulas Numerical Analysis Method for Grounding System Parameters Equal Potential Analysis of Grounding Grid Approach of Green s Function for Calculating Grounding Parameters Superposition Method Under the Assumption of Nodal Leakage Current Multi-Step Method Under the Assumption of Nodal Leakage Current Integration Method Under the Assumption of Branch Leakage Current Unequal Potential Analysis of a Large-Scale Grounding System Analysis Model of a Grounding System with Unequal Potential Problems in the Analytical Method for Solving a Mutual Resistance Coefficient Numerical Integration Method for Mutual Resistance Coefficient Calculation Multi-Step Method for Uniform Soil Analyzing Grounding Grid with Grounded Cables Principles of Setting up Equations Calculating Self-Admittances of Conductors and Cables MoM Approach for Grounding Grid Analysis in Frequency Domain Basis Functions of MoM Setting up the Equations Green s Functions and Generalized Sommerfeld Integral Finite Element Method for a Complex Soil Structure Time Domain Method for Electromagnetic Transient Simulation of a Grounding System Generalized MMC Method under EMQS Assumption Numerical Approach Based on Time Domain Integral Equation in a Lossy Medium Finite Difference Time Domain Method 181 References Ground Fault Current of a Substation Power Station and Substation Ground Faults Types of Power Station and Substation Ground Faults Principle to Determine Maximum Ground Fault Current Location of the Maximum Ground Fault Current Maximum Fault Current through a Grounding Grid to the Earth Maximum Grounding Grid Fault Current Zero-Sequence Fault Current Determining the Fault Current Division Factor Determining the Decrement Factor 196

10 viii Contents Determining the Correction Coefficient for Future Planning Impact of Substation Grounding Resistance Impact of Fault Resistance Impact of Overhead Ground Wires and Neutral Lines Impact of Buried Conduits and Cables Steps to Determine a Proper Design Value of the Maximum Grounding Grid Current Simplified Calculation of a Fault Current Division Factor Fault Current Division Factor Within a Local Substation Fault Current Division Factor Outside a Local Substation Numerical Calculation of the Fault Current Division Factor Numerical Calculation Method of the Fault Current Division Factor Matrix Method to Calculate the Fault Current Division Factor Phase Coordinate Transformer Model for Calculating the Fault Current Division Factor Typical Values of the Fault Current Division Factor Influence of Substation Grounding Resistance Influence of Transmission Towers Influence of Fault Location Influence of Incoming Cables Influence of Transmission Line Number Influence of Transmission Line Length Influence of Transformer Influence of Seasonal Freezing on the Fault Current Division Factor Influence of Seasonally Frozen Soil on the Fault Current Division Factor Influence of Transmission Line Numbers Affected by Frozen Soil 221 References Grounding System for Substations Purpose of Substation Grounding Function of Substation Grounding Design Objective of a Substation Grounding System Requirement on the Grounding System of a Substation Specificity of Power Plant Grounding Requirements for Grounding System Design Design and Construction Procedures for a Grounding System Safety of Grounding Systems for Substations and Power Plants Design Criteria of Grounding Systems Calculation of the Grounding Resistance of a Grounding System Analysis of Grounding in Inhomogeneous Soil Simplified Formula for Calculating Step, Touch and Mesh Voltages Formulas in IEEE Standard for Calculating Mesh and Step Voltages Formulas to Calculate Touch and Step Voltages in Chinese Standards Transfer Potential Methods for Improving the Safety of a Grounding System Methods for Decreasing the Grounding Resistance of a Substation Basic Methods for Decreasing Grounding Resistance Using Long Vertical Ground Rods to Decrease Grounding Resistance Explosion Grounding Technique 248

11 Contents ix Deep Ground Well Slanting Grounding Electrode Equipotential Optimal Arrangement of a Grounding Grid Principle of the Unequal-Spacing Arrangement Regularity of the Unequal-Spacing Arrangement Unequal-Spacing Arrangement with Exponential Distribution Influence of Vertical Grounding Electrodes on OCR Numerical Design of a Grounding System Grounding System Design of a 220-kV Substation Grounding System Design of a 1000-kV UHV Substation 270 References Grounding of Transmission and Distribution Lines Requirement for a Tower Grounding Device Requirement of Transmission Tower Grounding Resistance Seasonal Factor for the Grounding Resistance of a Tower Grounding Device Structures of Tower Grounding Devices Basic Structures of Tower Grounding Devices Using Natural Footings as Tower Grounding Devices Properties of a Concrete-Encased Grounding Function of a Concrete-Encased Grounding Device Hygroscopic Properties of Concrete Permissible Current through a Concrete-Encased Grounding Device Computational Methods for Tower Grounding Resistance Equivalent Cylindrical Conductor Method Grounding Resistance of a Vertical Ground Rod Covered with Concrete Grounding Resistance of a Fabricated Concrete-Encased Footing Grounding Resistance of a Tower Grounding Device with Different Structures Utilization Coefficient Step and Touch Voltages Near a Transmission Tower Step Voltage and Touch Voltage Shock Accident Possibilities Caused by Step and Touch Voltages Short-Circuit Fault on Transmission Tower Fault Current of Transmission Line Distribution of Ground Potential around Transmission Towers Methods to Improve Potential Distribution Grounding Device of Distribution Lines Vertically Driven Rods Grounding of Wood Poles Requirement for Grounding the Distribution Line 301 References Impulse Characteristics of Grounding Devices Fundamentals of Soil Impulse Breakdown Electric Field Strength of Soil Breakdown Phenomenon of Electrical Breakdown in Soil Impulse Breakdown Delay Characteristics of Soil Mechanism of Electrical Breakdown in Soil Residual Resistivity of Ionized Soil 323

12 x Contents 8.2 Numerical Analysis of the Impulse Characteristics of Grounding Devices Equivalent Circuit Model MoM Coupled with Circuit Theory An Interpolation Model to Accelerate the Frequency Domain Response Calculation Impulse Characteristics of Tower Groundings Field Test of Grounding Devices Impacted by a Large Impulse Current Lightning Current Decay Along a Grounding Electrode Definition of Impulse Grounding Resistance Influence of Different Factors on the Impulse Grounding Resistance of Grounding Devices Influence of Different Factors on Impulse Coefficient Regressive Formulas to Calculate Impulse Coefficients Impulse Coefficient and Utilization Efficient Suggested in the Literature Low Resistivity Material Effects to Decrease Impulse Grounding Resistance Impulse Effective Length of Grounding Electrodes Phenomenon of Impulse Effective Length Regressive Formulas to Calculate the Effective Length of Counterpoise Wires Influence of LRM on the Impulse Effective Length of Counterpoise Wires Impulse Characteristics of a Grounding Grid Influence of the Structure of the Grounding Grid Influence of Soil Parameters Influence of Impulse Current Waveform on the Transient Performance of Grounding Grids Impulse Effective Regions of Grounding Grids Lightning Electromagnetic Field Generated by a Grounding Electrode Computation Methodologies Disposal of a Lightning Current Influence of Soil Ionization 383 References DC Ground Electrode Technical Requirements of a DC Ground Electrode Technical Characteristics of a DC Ground Electrode Basic Principles of DC Ground Electrode Design Structure Types of DC Ground Electrodes Land Electrode Shore Ground Electrode Sea Electrode Main Design Aspects of a DC Ground Electrode Main Design Items Determination of DC Ground Electrode Size Determination of Coke Section Diameter of Feeding Rod Burial Depth of Electrode Selection of Ground Electrode Material Numerical Analysis Methods for a Ground Electrode Numerical Analysis of a Ground Electrode by MoM and BEM Simplified Numerical Analysis Method 417

13 Contents xi 9.5 Heat Generation Analysis of a DC Ground Electrode Numerical Analysis of the Heat Dissipation of a Ground Electrode Maximum Temperature Rise Limit Common Ground Electrode of a Multiple Converter System Demands on a Common Ground Electrode Parameters of the Common Ground Electrode Common Ground Electrode Design Influence of DC Grounding on AC System Influence of DC Electrode s Current Field on AC System Numerical Analysis of DC Current Entering a Neutral Grounded Transformer Allowable DC Current of a Transformer Methods to Decrease Winding DC Current of a Neutral Grounding Transformer Injecting Reverse DC Current Method Inserting Capacitor Method Inserting Resistor Method Corrosion of Underground Metal Pipes Caused by a DC Ground Electrode Mechanism of Electrochemical Corrosion of Underground Metal Pipes Leakage Current through a Metal Pipe Caused by Ground Electrodes Protection Measures 456 References Materials for Grounding Choice of Material and Size for Conductors Requirement on Material and Size of Grounding Conductors Materials for a Grounding Conductor Determination of Conductor Size Grounding Conductor Size Determined by Ground Fault Protection Soil Corrosion of Grounding Conductor Features of Soil Corrosion Natural Corrosion Electrical Corrosion in Soil Corrosion of Concrete-Encased Electrodes Low-Resistivity Material Principle of Reducing Grounding Resistance by LRM Ingredients of LRM Basic Requirements for LRM Evaluation of LRM Performance of LRM Power Frequency Performance of LRM Lightning Impulse Performance of LRM Construction Method of LRM Influence of LRM Bulk Shape on Reducing the Grounding Resistance Effect Amount of LRM and Construction Method Construction of a Complex Ground Device 497 References Measurement of Grounding Methods for Grounding Resistance Measurement Simple Methods for Measuring the Grounding Resistance of Small Grounding Devices 500

14 xii Contents Principle of the Fall of Potential Method Method of Far Placed Current Probe for Fall of Potential Method Compensation Location of a Potential Probe for the Fall of Potential Method Compensation Method for the Fall of Potential Method Instruments for Measuring Grounding Resistance Ammeter Voltmeter Method Ammeter Wattmeter Method Ratio Meter Method Bridge Method Potentiometer Method Single Equilibrium Transformer ZC-8 Grounding Resistance Tester Digital Measurement System of Grounding Resistance Factors Influencing the Results from the Fall of Potential Method Electromagnetic Interferences During Measurements Impact and Elimination of Power Frequency Interference Components of the Measured Voltage Signal for the Grounding Resistance Test Mutual Inductance Between Potential and Current Lead Wires Short Measuring Leads Method Accurate Location of Test Probe Positioning by GPS Influence of a Metal Structure Buried Nearby Method to Eliminate Measuring Interference Grounding Resistance Test in Vertically Layered Soil Grounding System Built in a Middle Low Resistivity Region Grounding System Built in a Middle High Resistivity Region Discussion of Analysis Results Influence of Overhead Ground Wires on Substation Grounding Resistance Measurement General Analysis Model General Discussion Analysis of a 500 kv Substation Measurement of Potential Distribution Equipotential Line Measurement of Equipotential Lines Measurement of Step Voltage and Touch Voltage Corrosion Diagnosis of Grounding Grids Corrosion Diagnosis Model of a Grounding Grid Implementation of the Diagnosis System Field Test Results 547 References 550 Index 553

15 Preface The development of modern power systems for the direction of extra-high voltage, large capacity, far distance transmission and the application of advanced technologies, is placing higher demands on the safety, stability and economic operation of power systems. A sound grounding system for substations is a very important and fundamental countermeasure to guarantee the safe and reliable operation of power systems and to ensure the safety of human being in the situation of a grounding fault in the power system. It is also a key method to decrease electromagnetic interferences in substations. Considerable operation results show that, if the grounding system has not been designed suitably, then control cables will be destroyed and a high voltage will be led into the control room of the substation. This could make control devices misfunction or reject operating instructions, which could then cause huge economical loss and social effects. Further, the ground device directly decides the lightning protection characteristics of transmission lines. With the rapid expansion of the capacity of power systems, the short-circuit fault current rises enormously. Under such situations, the grounding resistance should be low enough to guarantee the safety of the power system. However, the locations of those substations constructed in urban areas are not always in good sites with low soil resistivity. They are often on hills or in other regions with high soil resistivity, which means we cannot always simply regard the soil as homogeneous. Since the 1980s, with the development of computer technology and progress of the numerical analysis technology of electromagnetic fields, the method of moments, boundary element method, complex image method, finite element method and other direct numerical analysis methods have been widely applied in the calculation of grounding system parameters. Now the design of grounding systems has been moved from simple calculations based on the methods provided in standards to full numerical analysis. Currently, grounding technology has become an interdiscipline related to electrical engineering, electric safety, electromagnetic theory, numerical analysis method, techniques of measurement and geological prospecting. Up to the present, the grounding technology of power systems has achieved much, in both methodology and technology: Grounding system analysis has moved from a simple estimation based on homogeneous soils and empirical formulas to a numerical analysis based on complicated soil models. How to decrease the grounding resistance has become a shoo-in by adding vertical ground rods, based on realizing the multi-layer structure of soil, rather than simply expanding the area occupied by the ground grid. We had gotten to the heart of the lightning impulse characteristics of tower ground devices based on deeper experimental results of soil ionization performances. This book contains 11 chapters. First, all fundamental and theoretical knowledge is introduced and highlighted, including fundamental concepts of grounding, current field in the Earth, modeling of soil

16 xiv Preface resistivity, numerical analysis method of grounding, ground fault current of a substation and impulse characteristics of grounding devices. Second, design guidelines for substations, transmission towers and converter stations are presented, including grounding systems for substations, grounding of a transmission line tower, DC ground electrodes and materials for grounding. Third, measurement methods and techniques for grounding are introduced, including the measurement and modeling of soil resistivity, grounding resistance, potential distribution and corrosion diagnosis of grounding grids for power substations. This book covers all main aspects of the grounding technologies for power systems, including substations, converter stations and transmission towers. It introduces fundamental and advanced theories and technologies related to power system groundings and the research achievements of the past 20 years. This reflects the recent research work of the authors and their students and colleagues at Tsinghua University, especially the Ph.D. dissertations of Dr. Zeng Rong, Dr. Sun Weimin, Dr. Gao Yanqing, Dr. Gong Xuehai, Dr. Kang Peng, Dr. Zhang Baoping and Dr. Wang Shunchao and the M.Sc. theses of Ms. Li Siyun, Mr. Zhang Bo, Mr. Pan Xiyuan, Mr. Ding Qiangfeng, Mr. Yuan Jingping and Mr. Du Xin. The authors have tried to cover all aspects of power system grounding, but it is hard to avoid those that may have been left out. Numerous references have been cited in our book, each listed in the appropriate chapter, but it is hard to avoid accidental omission, in which case we beg your pardon. We are so sorry, but some formulas could not be traced back to their original references.

17 Acknowledgements Numerous references have been cited in our book, each listed in the appropriate chapter, but it is hard to avoid accidental omission, in which case we beg your pardon. We are so sorry, but some formulas could not be traced back to their original references. During the drafting of this book, Prof. Chen Xianlu of Chongqing Univeristy, who was the director of my M.Sc. thesis has led me into the door of grounding, provided many valuable comments and allowed me to refer to his lecture notes and his book manuscript of Grounding. Mr. Du Shuchun, the famous grounding and lightning protection expert in China, who works in China EPRI, read the manuscript and gave many modification suggestions. Many colleagues have provided us with materials and suggestions. I would like to extend my sincere thanks to them. Special thanks also go to my students for their assistance in preparing the draft of this book, and to my colleagues for their generous help in many ways so as to allow me to allocate time for working on the book. Great gratitude is given to Mr. Wu Jinpeng for preparing the part manuscript of Chapter 5, to Dr. Wang Shunchao for preparing the part manuscript of Chapter 4 and to Miss Wang Xi for her assistance in the formatting and editing of the book. A particular acknowledgment is given to Profs. Zeng Rong and Zhang Bo, the coauthors of this book. They are the perfect choice for the task. Prof. Zeng has done excellent work in grounding measurement, and Prof. Zhang has made many contributions in the numerical analysis of grounding systems. Gratitude is extended to Ms. Shelley Chow, Project Editor at John Wiley & Sons, for her editorial and technical review of this book. Her professionalism and experience have greatly enhanced the quality and value of this book. Last, but not least, my most special gratitude goes to my supporting and understanding family, to my mother, Yang Ruiru, who taught me to enjoy this wonderful life, to my wife, Prof. Tu Youping, who has done and is still doing a great job of supporting the family. Most of all, I am indebted to my son, Ziyu, I have not given much time to enjoy his growing-up process. Jinliang He

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19 1 Fundamental Concepts of Grounding 1.1 Conduction Mechanism of Soil Soil Structure Soil is a complex system, consisting of solid, liquid and gas components. The solid phase of normal soil usually includes minerals and organic matter; the liquid phase means the water solution and the gas phase is the air between the solid particles. The solid phase makes up of the basic structure of the soil, the liquid and gas phases fill the voids within the structure, as shown in Figure 1.1. Different from normal soil, a new kind of solid material, ice, is present in frozen soil. Soil conductivity is strongly determined by water content and water state. According to the distance from solid particles and the electrostatic force received from solid particles, the water in soil can be classified into the following types [1]: Strongly Associated Water. Near the surface of soil particles, the water molecules cram together closely and cannot move freely due to the great intensity of the electrostatic field. This type of water is called strongly associated water. Weakly Associated Water. Being farther from the soil particles, the intensity of the electrostatic field has comparatively decreased, so the water molecules are more active and weakly oriented. This type of water is still mainly affected by the electrostatic field and is called weakly associated water. Capillary Water. As the distance between soil particles and water molecules increases, the water molecules become mainly affected by gravity. Although the electrostatic field still plays a role, it does not have a primary function. This type is called capillary water. Gravity Water. As the distance between soil particles and water molecules continues to increase, the effect of the electrostatic field becomes negligible to the water molecules, and the water molecules are only controlled by gravity. This type is called gravity water or ordinary liquid water Conduction Mechanism of Soil Research has shown that soil conductivity falls with dropping temperature. This can be explained by the theory of electrochemistry [2,3], because the electrical conduction in soil is predominantly electrolytic conduction in the solutions of water-bearing rocks and soils. Accordingly, the resistivity of soil or rock normally depends on the degree of porosity or fracturing of the material, the type of electrolyte Methodology and Technology for Power System Grounding, First Edition. Jinliang He, Rong Zeng and Bo Zhang. Ó 2013 John Wiley & Sons Singapore Pte. Ltd. Published 2013 by John Wiley & Sons Singapore Pte. Ltd.

20 2 Methodology and Technology for Power System Grounding Figure 1.1 Photo showing the microstructure of soil. and the temperature. Metallic conduction, electronic semiconduction and solid electrolytic conduction can occur but only when specific native metals and minerals are present [28]. Similar to the solid medium, frozen soil is obviously distinguished from normal soil. Because of the charges and ions attracted onto the surface of soil particles, soil can be considered as a polyvalent electrolyte. Soil conductance is the contribution of both charged soil particles (known as colloidal particle conductance, mainly decided by the amount of charge on the surface of soil particles) and ions in solution (known as ion conductance, mainly decided by the diffusion velocity of ions). When ions diffuse into the soil solution, the diffusion velocity is affected by the resistance of the water molecules. As the temperature drops, the water becomes more viscous and its diffusion becomes slower because the resistance of water molecules increases. In contrast, the ions are affected by the soil electrostatic resistance. As the temperature lowers, the average kinetic energy of ions decreases and the capacity to overcome the soil electrostatic resistance also decreases and the diffusion velocity slows up. So, ion conductance decreases and soil resistivity increases as the temperature drops. When the soil temperature decreases to 0 8C or even lower, most of the water in the soil is frozen gradually and the ice (with high resistivity) fills the voids between the soil particles in the form of grains or laminas, so the conductive cross-section of soil reduces. The thickness of the water film coating the soil particles is reduced and the activity of the water molecules becomes weak. So, the resistivity of frozen soil is significantly higher than that of normal soil. When the soil is chilled to a much lower temperature, most of the soil water is frozen and the ion conductance created by ion movements gradually disappears. Finally, there would be only colloidal particle conductance created by the charges on the surface of soil particles, which is not related to temperature, so a saturation phenomenon appears. 1.2 Functions of Grounding Devices Concept of Grounding Grounding is provided to connect some parts of electrical equipment and installations or the neutral point of a power system to the earth. This provides dispersing paths for fault currents and lightning currents in order to stabilize the potential and to act as a zero potential reference point to ensure the safe operation of the power system and electrical equipment and the safety of power system operators and other persons. Grounding is achieved by grounding devices (or ground devices) buried in soil. The grounding devices of a power system can be divided into a relatively simple one for transmission line towers, such as a horizontal grounding electrode (or ground electrode), vertical ground rod, or ring grounding electrode, and the other is the grounding grid (or ground grid) for a substation or power plant.

21 Fundamental Concepts of Grounding 3 The grounding device is a single metal conductor or a group of metal conductors buried in soil, including horizontally or vertically buried metal conductors, metal components, metal pipes, reinforced concrete foundations of structures, metal equipment, or a metal grid in soil. The grounding system refers to the whole system, including the grounding device of a substation or power plant, and all metal tanks for the power apparatus and electrical equipment, towers, overhead ground wires, neutral points of transformers and the metal sheaths of cables connected with the grounding device. The basic parameter to indicate the electrical property of a grounding device is grounding resistance (or ground resistance), which is defined as the ratio of the voltage on the grounding device with respect to the zero potential point at infinity and the current injected into earth through the grounding device. If the current is a power-frequency alternating current (AC), the grounding resistance is called a power-frequency grounding resistance. If the current is an impulse current, such as a lightning current, then it is called an impulse grounding impedance, which is a timevariant transient resistance. The impulse grounding resistance of a grounding device is usually defined as the ratio of the peak value V m of the voltage developed at the feeding point to the peak value I m of the injected impulse current into the grounding device Classification of Grounding The grounding devices of AC electrical equipment for a power system can be divided into three categories according to their functions: working grounding, lightning protection grounding and protective grounding. Further, the instrumentation and control equipment of the substation should also be grounded Working Grounding Based on whether the neutral point is grounded, an AC power system can be classified into a neutralpoint effective grounding system or a neutral-point ineffective grounding system (including neutral-point ungrounded system, neutral-point resistance grounding system and neutral-point reactance grounding system). In order to reduce the operating voltage on the insulation of the power apparatus, the neutralpoints of power systems of 110 kv and above are solidly grounded. This grounding mode is called a working grounding. For the neutral-point effectively grounded operation mode, under normal situations, the voltage on the insulation of the power apparatus (such as a power transformer) is the phase voltage. If the neutral-point is insulated, when the single-phase grounding fault occurs, the voltage on the pffiffi insulation of the power apparatus is the line voltage before the breaker cuts off the fault, which is 3 times as high as the phase voltage. The neutral-point effectively grounded operation mode can effectively reduce the voltage on the insulation of the power apparatus and the insulation level of the power apparatus is reduced, so the purpose of reducing the insulation size and lowering the cost of the equipment is achieved. For the neutral-point solidly grounded system, the current through the grounding device is the unbalanced current of the system under normal situations and, when a short circuit fault occurs, a short-circuit current of tens of kilo-amperes (ka) will flow through the grounding device, and usually the short-circuit current will last about 0.5 s. Usually, the neutral point of the double-pole DC transmission system is grounded, which can operate under single pole mode by using the earth as the return path. Operating in single pole mode, a current of several ka will flow through the grounding electrode over a long period of time and we should pay particular attention to the electrochemical corrosion of the grounding electrode. For a power distribution system, a step-down transformer is used to connect the high-voltage system with the low-voltage system and, according to whether the neutral point of the transformer is grounded, the low-voltage distribution system can be classified into a grounded system (either solidly or through impedance) or an ungrounded system. Figure 1.2 shows a low-voltage distribution system with neutral point grounded. If someone touches the low-voltage conductor, a loop will be formed in which the current through the body is related to the contact resistance between the body and earth. If the contact resistance is small, a dangerous current will flow through the body and harm it.

22 4 Methodology and Technology for Power System Grounding Figure 1.2 Solid grounding of a low-voltage distribution system. For water lighting and other power supply lines, it is necessary to add an insulated transformer with a secondary side neutral point not grounded. This kind of system is called a neutral point ungrounded system. As shown in Figure 1.3, when a person contacts the secondary circuit of the neutral point ungrounded system, only a very small current flows through the loop circuit, which is formed by the distributed capacitance, and it passes through the body, so it is much safer. One shortcoming of an ungrounded system is that there is no way to inhibit this abnormal voltage and it will cause a hazard on the secondary side, when the system voltage is increased for some special reason, such as the mixed contact of the high and low voltage circuits, a lightning impulse, a switching voltage and so on. Another shortcoming is that an ageing insulation will possibly break down, thus leading to a grounding accident Protective Grounding When the insulation of electrical equipment fails, its enclosure becomes live and a person will suffer an electric shock if he or she contacts its enclosure. In order to guarantee personal safety, the enclosures of all electrical equipment should be grounded. This kind of grounding is called protective grounding. When the enclosure of electrical equipment is live due to insulation damage, the fault current flowing through the protective grounding device should trigger a relay protection device to cut off the faulty equipment, and we can also reduce the grounding resistance to make sure that the voltage on the enclosure is lower than the value of the body safety voltage, so that electric shock accidents caused by the live enclosure can be avoided. Figure 1.3 Ungrounded low-voltage distribution system.

23 Fundamental Concepts of Grounding Lightning Protection Grounding In order to prevent the hazard of lightning to power systems and human beings, lightning rods, shielding wires, surge arresters and other lightning protection equipment are usually adopted. Such lightning protection equipment should all be connected to suitable grounding devices to lead the lightning current into the earth. This kind of grounding is called lightning protection grounding. The lightning current through the lightning protection grounding device is huge and can reach hundreds of kilo-amperes, but it has a very short duration, tens of microseconds in general Signal Reference Grounding A large number of instrumentation and control devices based on solid electronic devices are widely used in modern power systems, but these devices need a signal reference point when in operation. Signal reference grounding plays a very important role in making sure that the electronic devices and the computer control system work regularly. But in the modern power system, it is very difficult to provide a pure signal reference ground without interference. So, how to improve the anti-interference ability of the signal ground is one of the important issues that should be considered during signal ground design. From the functional point of view, the signal reference grounding is a kind of special working grounding Purpose of Grounding Reducing Insulation Level of Electrical Equipment. As mentioned earlier, the working grounding formed by grounding the power system neutral point can decrease the operating voltage on the power apparatus and thereby reduces the insulation level of the power apparatus. Ensuring Safe Operation of Power System. The grounding resistance of transmission line towers must be lower than a certain value to reduce the potential difference between the transmission tower top and the phase conductor. A value of less than 50% of the impulse flashover voltage of the insulator can guarantee the safe operation of transmission lines. If the grounding resistance is too large, it could possibly cause a tower top potential which is high enough to trigger an insulators string flashover and a power outage might happen. In addition, as mentioned before, lightning protection systems in substations, such as lightning rods, shielding wires and surge arresters, must be grounded to the grounding devices to discharge the lightning energy to the earth. Ensuring Personal Safety. As mentioned above, the protective grounding is intended to make the enclosures of all electrical equipment grounded. When damage or the aging of equipment insulations make the enclosures live, it can ensure the safety of any person who contacts the shell of the equipment. However, the grounding devices of substations can make sure that the personal touch voltage and step voltage meet the desired safety requirements by reducing the grounding resistance and taking voltage equalization measures. The touch voltage is the potential difference between one hand and one foot when a person contacts the equipment shell or metal components under power system failure, while the step voltage is the potential difference between two feet. Eliminating Electrostatic Accidents. Static electricity may cause an explosion and fire, and oil storage tanks and natural gas pipelines are particularly susceptible to an explosion caused by electrostatic discharge. Further, static electricity may interfere with the normal work of solid electronic devices. Through grounding, the static charges generated and collected by friction and other factors can be released to the earth as soon as possible to prevent accidents and damage caused by static interference. Detecting Ground Faults. In order to ensure personal and property safety, leakage breakers and other fault leakage protection devices are used in low-voltage circuits. If a ground fault happens at one point in the circuit, there must be a very large ground fault current to bring the protection device into action. In order to meet this condition, the neutral point on the secondary side of the step-down transformer should be grounded. In contrast, for a neutral point grounded circuit, if the enclosure of the

24 6 Methodology and Technology for Power System Grounding Figure 1.4 Grounding of the enclosure of electrical equipment to ensure the protection device is triggered. electrical equipment is not grounded, when the electrical equipment enclosure is charged due to insulation damage or other reasons, the current generated in the circuit by the distributed capacitors cannot trigger the protection device, so the equipment enclosure should be grounded, as shown in Figure 1.4. The current I is: U I ¼ R 0 þ R E ð1:1þ where U is the phase voltage of the circuit, R 0 is the grounding resistance of the neutral point (for a 380/220 V low-voltage AC circuit, the value of the grounding resistance is generally selected as 4 V) and R E is the grounding resistance of the electrical equipment. Equipotential Bonding. Equipotential bonding is a kind of connection mode to ensure that externally exposed conductive bodies of a device have the same potential. The electrical equipment inside a building can achieve equipotential bonding through grounding the equipment enclosure with the main ground bus, as shown in Figure 1.5. The purpose of equipotential bonding is to prevent dangerous potential differences between different devices or to avoid forming a loop, because the loop formed by grounding connection is vulnerable to external electromagnetic fields, and the loop current will interfere with the normal operation of equipment. Reducing Electromagnetic Interference. External electromagnetic interference may cause electronic devices to malfunction, or may interfere with a signal transmitted by cable. This can be reduced or eliminated by grounding the shielding shell of the electrical equipment and the cable shielding sheath. Further, in order to prevent the high-frequency energy generated by electronic devices from interfering with other devices, the electronic devices should also be grounded. Grounding to prevent electromagnetic interference has different types, such as grounding of shielding rooms or shielding layers, grounding of cable-shielding sheaths, grounding of transformer electrostatic shields, grounding of the protection devices for precision instrumentations and so on. The power line filters at the entrances of electric or electronic devices should also be grounded. In short, grounding against electromagnetic interference provides a channel for energy to be released into the earth. Function Grounding. Some equipment needs to be grounded functionally, for example cathodic protection makes use of electrochemistry to prevent metal corrosion. In order to make the corrosion current flow into the earth, the cathodic protection system should be grounded. Additionally, a reference point with a stable potential must be adopted to ensure the regular operation of computers and other electrical equipment, which can be achieved by grounding.

25 Fundamental Concepts of Grounding 7 Figure 1.5 Equipotential bonding of electrical equipment. Work Grounding. When operating personnel work on transmission lines under power outages, the energy stored in the transmission line and other equipment should be discharged by grounding, to prevent any hazards to the operating personnel from the induced current through the transmission line. Further, any fatal harm to operating personnel caused by anothers false operation can be prevented. 1.3 Definition and Characteristics of Grounding Resistance Definition of Grounding Resistance Grounding resistance is the ratio between the potential of the grounding device and the current flowing into the earth through the grounding device, which is related to the soil characteristics and the size and shape of the grounding device. The soil resistance encountered when a current flows into the soil is called the current-dispersing resistance. The grounding resistance consists of the ground lead resistance, the contact resistance between the ground lead and the grounding device, the resistance of grounding conductors themselves, the contact resistance between the grounding conductors and soil and the current-dispersing resistance of the soil. Because the current-dispersing resistance is much greater than the other four kinds of resistance, the grounding resistance of a grounding device approximates to the current-dispersing resistance. Usually, the grounding resistance of a grounding device calculated by numerical methods or empirical formulas is the current-dispersing resistance of the soil, but the actually measured value is generally greater than the calculated result. This is because the actual contact between grounding conductors and the soil is not a complete surface-like contact, but a point-like contact. This leads to a contact resistance between the grounding conductors and the soil, especially in rocky areas, where the contact resistance is sometimes quite high. This contact resistance has an uncertain value, which is related to the soil compression degree during construction, the soil particle status, the soil moisture and so on, but the contact resistance cannot be reflected in the calculation formula. For example, as shown in Figure 1.6, the radius of a hemispherical grounding device is r 0, the current flowing into the earth through the grounding device is I, assuming the terra firma is an homogenous soil

26 8 Methodology and Technology for Power System Grounding Figure 1.6 Hemispherical grounding device in homogeneous soil and the respective potential distribution. with resistivity of r. The potential of the point with a distance r to the center of the hemispherical grounding device can be calculated by the potential formula of a point current source, which is: v ¼ Ir 2pr ð1:2þ The potential v 0 of the grounding device can be calculated by Equation 1.2 when r ¼ r 0 : v 0 ¼ Ir 2pr 0 The potential distribution of the hemispherical grounding device is shown in Figure 1.6. According to the definition of grounding resistance, the grounding resistance of a hemispherical grounding device is: R ¼ v 0 I ¼ r 2pr 0 ð1:3þ ð1:4þ Relationship between Grounding Resistance and Capacitance According to the similarity between the electrostatic field and the constant current field, it is very easy to obtain the relationship between the grounding resistance and the capacitance of a grounding device: R ¼ re C ð1:5þ where e is the dielectric coefficient of the soil (with units of F/m) and C is the capacitance of the grounding device with respect to infinity (with units of F). When the resistivity r and the dielectric coefficient e of soil are constants, the capacitance is in inverse proportion to its size. From Equation 1.5, we can ascertain that the grounding resistance of the grounding device is inversely proportional to its capacitance. Thus, the larger the size of the grounding device, the greater is its capacitance and the lower is its grounding resistance. For an actual grounding

27 Fundamental Concepts of Grounding 9 Figure 1.7 Grounding grid with short vertical ground rods. project, the grounding resistance of a grounding grid is basically determined once the area of the grounding grid is defined. A grounding grid consisting of many horizontal conductors can be approximated to an isolated plane, whose capacitance is mainly determined by its area. As shown in Figure 1.7, if short vertical ground rods are connected to this plane, they have little influence on the capacitance and so the grounding resistance decreases just slightly. According to analysis, the grounding resistance is highly reduced only when the length of the vertical ground rods can match the equivalent radius of the grounding grid Shielding Effect among Grounding Conductors A grounding device usually consists of a group of grounding conductors and, when the current diffuses into soil through one conductor, it is affected by the other conductors. Adding more horizontal conductors to a grounding grid, or adding short vertical rods to a grounding grid, can only reduce the grounding resistance by a little and this is because the internal conductors of the grounding grid are shielded by the peripheral conductors. Strictly speaking, it is only when the distance between two conductors is infinite that the electric field generated by one grounding conductor is not affected by the other one. Considering this shielding effect, the grounding resistance of the grounding device is not equal to the parallel value of the grounding resistances of all the grounding conductors. Different grounding conductors of a grounding device diffuse currents with the same polarity into soil, so we can use two adjacent point sources (as shown in Figure 1.8) to analyze the interaction between them. Both point sources inject the same current I into the earth at the same time, but the Figure 1.8 Shielding effect between two neighboring point sources with the same polarity.