Methodology and Technology for Power System Grounding E-Book

Jinliang He

• Herausgeber: John Wiley & Sons
• Kategorie: Wissenschaft und neue Technologien
• Serie: Wiley - IEEE
• Sprache: Englisch

Seitenzahl: 1067

Veröffentlichungsjahr: 2012

Beschreibung

Grounding is the fundamental measures to ensure the safe operation of power systems, including power apparatus and control/monitoring systems, and guarantee the personal safety. Grounding technology is an interdiscipline involving electrical engineering, high voltage technology, electric safety, electromagnetics, numerical analysis, and geological exploration Methodology and Technology for Power System Grounding: * Covers all topics related to power system grounding * Presents fundaments and theories of grounding systems * Well balances technology and methodology related to grounding system design * Helps to understand the grounding analysis softwares * Highlights the advanced research works in the field of grounding systems * Comprehensively introduces numerical analysis methods * Discovers impulse ionization phenomenon of soil around the grounding conductors * Touches on lightning impulse characteristics of grounding devices for towers and buildings As a comprehensive treatment of the topic, Methodology and Technology for Power System Grounding is ideal for engineers and researchers in power system, lightning protection, and grounding. The book will also better equip postgraduates, senior undergraduate students in electrical engineering.

Leseprobe

Contents

Cover

Title Page

Copyright

Preface

Acknowledgements

Chapter 1: Fundamental Concepts of Grounding

1.1 Conduction Mechanism of Soil

1.2 Functions of Grounding Devices

1.3 Definition and Characteristics of Grounding Resistance

1.4 Grounding Resistance of Grounding Devices

1.5 Body Safety and Permitted Potential Difference

1.6 Standards Related to Power System Grounding

References

Chapter 2: Current Field in the Earth

2.1 Electrical Property of Soil

2.2 Basic Properties of a Constant Current Field in the Earth

2.3 Current Field Created by a Point Source in Uniform Soil

2.4 Potential Produced by a Point Source on the Ground Surface in Non-Uniform Soil

2.5 Potential Produced by a Point Source in Multi-Layered Soil

2.6 Computer Program Derivation Method of Green's Function

2.7 Fast Calculation Method of Green's Function in Multi-Layered Soil

2.8 Current and Potential Distributions Produced by a DC Ground Electrode

References

Chapter 3: Measurement and Modeling of Soil Resistivity

3.1 Introduction to Soil Resistivity Measurement

3.2 Measurement Methods of Soil Resistivity

3.3 Simple Analysis Method for Soil Resistivity Test Data

3.4 Numerical Analysis for a Multi-Layered Soil Model

3.5 Multi-Layered Soil Model by Solving Fredholm's Equation

3.6 Estimation of Multi-Layered Soil Model by Using the Complex Image Method

3.7 Engineering Applications

References

Chapter 4: Numerical Analysis Method of Grounding

4.1 Calculation Method for Parameters of Substation Grounding Systems

4.2 Equal Potential Analysis of Grounding Grid

4.3 Unequal Potential Analysis of a Large-Scale Grounding System

4.4 Analyzing Grounding Grid with Grounded Cables

4.5 MoM Approach for Grounding Grid Analysis in Frequency Domain

4.6 Finite Element Method for a Complex Soil Structure

4.7 Time Domain Method for Electromagnetic Transient Simulation of a Grounding System

References

Chapter 5: Ground Fault Current of a Substation

5.1 Power Station and Substation Ground Faults

5.2 Maximum Fault Current through a Grounding Grid to the Earth

5.3 Simplified Calculation of a Fault Current Division Factor

5.4 Numerical Calculation of the Fault Current Division Factor

5.5 Typical Values of the Fault Current Division Factor

5.6 Influence of Seasonal Freezing on the Fault Current Division Factor

References

Chapter 6: Grounding System for Substations

6.1 Purpose of Substation Grounding

6.2 Safety of Grounding Systems for Substations and Power Plants

6.3 Methods for Decreasing the Grounding Resistance of a Substation

6.4 Equipotential Optimal Arrangement of a Grounding Grid

6.5 Numerical Design of a Grounding System

References

Chapter 7: Grounding of Transmission and Distribution Lines

7.1 Requirement for a Tower Grounding Device

7.2 Structures of Tower Grounding Devices

7.3 Properties of a Concrete-Encased Grounding

7.4 Computational Methods for Tower Grounding Resistance

7.5 Step and Touch Voltages Near a Transmission Tower

7.6 Short-Circuit Fault on Transmission Tower

7.7 Grounding Device of Distribution Lines

References

Chapter 8: Impulse Characteristics of Grounding Devices

8.1 Fundamentals of Soil Impulse Breakdown

8.2 Numerical Analysis of the Impulse Characteristics of Grounding Devices

8.3 Impulse Characteristics of Tower Groundings

8.4 Impulse Effective Length of Grounding Electrodes

8.5 Impulse Characteristics of a Grounding Grid

8.6 Lightning Electromagnetic Field Generated by a Grounding Electrode

References

Chapter 9: DC Ground Electrode

9.1 Technical Requirements of a DC Ground Electrode

9.2 Structure Types of DC Ground Electrodes

9.3 Main Design Aspects of a DC Ground Electrode

9.4 Numerical Analysis Methods for a Ground Electrode

9.5 Heat Generation Analysis of a DC Ground Electrode

9.6 Common Ground Electrode of a Multiple Converter System

9.7 Influence of DC Grounding on AC System

9.8 Methods to Decrease Winding DC Current of a Neutral Grounding Transformer

9.9 Corrosion of Underground Metal Pipes Caused by a DC Ground Electrode

References

Chapter 10: Materials for Grounding

10.1 Choice of Material and Size for Conductors

10.2 Soil Corrosion of Grounding Conductor

10.3 Corrosion of Concrete-Encased Electrodes

10.4 Low-Resistivity Material

10.5 Performance of LRM

10.6 Construction Method of LRM

References

Chapter 11: Measurement of Grounding

11.1 Methods for Grounding Resistance Measurement

11.2 Instruments for Measuring Grounding Resistance

11.3 Factors Influencing the Results from the Fall of Potential Method

11.4 Grounding Resistance Test in Vertically Layered Soil

11.5 Influence of Overhead Ground Wires on Substation Grounding Resistance Measurement

11.6 Measurement of Potential Distribution

11.7 Corrosion Diagnosis of Grounding Grids

References

Index

This edition first published 2013

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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 978-1-118-25495-0 (cloth)

1. Electric currents–Grounding. 2. Electric power systems–Protection. I. Zeng, Rong, 1971- II. Zhang, Bo, 1976- III. Title.

TK3227.H425 2012

621.319′2–dc23

2012024667

ISBN 9781118254950

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 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.

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

1

Fundamental Concepts of Grounding

1.1 Conduction Mechanism of Soil

1.1.1 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.

Figure 1.1 Photo showing the microstructure of 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.

1.1.2 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 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 °C 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

1.2.1 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.

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 time-variant transient resistance. The impulse grounding resistance of a grounding device is usually defined as the ratio of the peak value of the voltage developed at the feeding point to the peak value of the injected impulse current into the grounding device.