Methodology and Technology for Power System Grounding

Preface xiii Acknowledgements xv 1 Fundamental Concepts of Grounding 1 1.1 Conduction Mechanism of Soil 1 1.1.1 Soil Structure 1 1.1.2 Conduction Mechanism of Soil 1 1.2 Functions of Grounding Devices 2 1.2.1 Concept of Grounding 2 1.2.2 Classification of Grounding 3 1.2.3 Purpose of Grounding 5 1.3 Definition and Characteristics of Grounding Resistance 7 1.3.1 Definition of Grounding Resistance 7 1.3.2 Relationship between Grounding Resistance and Capacitance 8 1.3.3 Shielding Effect among Grounding Conductors 9 1.4 Grounding Resistance of Grounding Devices 11 1.4.1 Grounding Resistance of General Grounding Devices 11 1.4.2 Grounding Resistance of Grounding Device in Non-Homogeneous Soil 14 1.5 Body Safety and Permitted Potential Difference 19 1.5.1 Allowable Body Current Limit 19 1.5.2 Allowable Body Voltage 20 1.5.3 Allowable Potential Difference 21 1.5.4 Influence of Resistivity of Surface Soil Layer on Body Safety 23 1.6 Standards Related to Power System Grounding 25 References 26 2 Current Field in the Earth 27 2.1 Electrical Property of Soil 27 2.1.1 Soil Resistivity 27 2.1.2 Influence of Different Factors on Soil Resistivity 29 2.1.3 Permittivity of Soil 30 2.1.4 Frequency Characteristics of Soil Parameters 31 2.2 Basic Properties of a Constant Current Field in the Earth 36 2.2.1 Current Density in the Earth 36 2.2.2 Continuity of Earth Current Field 36 2.2.3 Potential of Stable Current Field 37 2.2.4 Current Field at the Interface of Layered Soil 37 2.3 Current Field Created by a Point Source in Uniform Soil 38 2.3.1 Laplace s Equation 38 2.3.2 Current Field Created by a Point Source in Soil 39 2.3.3 Earth Current Field Produced by Two Opposite Point Current Sources on the Ground Surface 41 2.3.4 Earth Current Field in Non-Uniform Soil 41 2.4 Potential Produced by a Point Source on the Ground Surface in Non-Uniform Soil 43 2.4.1 Horizontally Layered Soil 44 2.4.2 Horizontal Double-Layer Soil 45 2.4.3 Horizontal Triple-Layer Soil 46 2.4.4 Vertically Layered Soil 46 2.5 Potential Produced by a Point Source in Multi-Layered Soil 48 2.5.1 Analysis of Potential Produced by a Point Current Source 48 2.5.2 Numerical Integral Method to Calculate Green s Function of a Point Current Source 52 2.6 Computer Program Derivation Method of Green s Function 55 2.6.1 Method of Obtaining Analytic Expression 55 2.6.2 Expression of Green s Function Derived from Software Program 59 2.6.3 Calculation of Current Field in Multi-Layered Soil 62 2.7 Fast Calculation Method of Green s Function in Multi-Layered Soil 62 2.7.1 Development of a Two-Stage Fitting Method 63 2.7.2 Application of the Fast Calculation of Green s Function in Multi-Layered Soils 66 2.8 Current and Potential Distributions Produced by a DC Ground Electrode 69 2.8.1 Current and Potential Distributions of DC in Uniform Soil 69 2.8.2 Current and Potential Distributions of DC Current in Non-Uniform Soil 72 References 78 3 Measurement and Modeling of Soil Resistivity 81 3.1 Introduction to Soil Resistivity Measurement 81 3.2 Measurement Methods of Soil Resistivity 83 3.2.1 Sampling Analysis Method of Soil Resistivity 83 3.2.2 Electrical Sounding Methods 83 3.2.3 Test Probe Configuration for Four-Probe Method 88 3.2.4 Field Test Technique of Soil Resistivity 90 3.2.5 Electromagnetic Sounding Method 93 3.3 Simple Analysis Method for Soil Resistivity Test Data 94 3.3.1 Electrical Sounding Curve Method for Two-Layered Horizontal Soil Model 94 3.3.2 Analysis of a Three-Layered Horizontal Geological Structure 99 3.3.3 Resistivity of Vertically Layered Soil Structure 101 3.3.4 Estimation of Soil Model Parameter using the Three-Probe Method 102 3.4 Numerical Analysis for a Multi-Layered Soil Model 102 3.4.1 Typical Curves of Multi-Layered Soil Apparent Resistivity 102 3.4.2 Expression of Apparent Soil Resistivity 105 3.4.3 Inverting Soil Parameters 107 3.4.4 Numerical Analysis Method for Two-Layered Soil Model 108 3.5 Multi-Layered Soil Model by Solving Fredholm s Equation 109 3.5.1 Solving the Forward Integral Equation 109 3.5.2 Inversing Parameters of Soil Models 113 3.5.3 Application in Estimation of Soil Parameters 116 3.6 Estimation of Multi-Layered Soil Model by Using the Complex Image Method 118 3.6.1 Estimation of Multi-Layered Soil Structure 119 3.6.2 Fast Calculation of the Soil Apparent Resistivity 119 3.6.3 Partial Derivatives of Calculated Apparent Resistivity 121 3.6.4 The Partial Derivative Expressions of f(l) 123 3.6.5 Determination of the Initial Soil Parameters 123 3.7 Engineering Applications 123 References 128 4 Numerical Analysis Method of Grounding 131 4.1 Calculation Method for Parameters of Substation Grounding Systems 131 4.1.1 Calculation of Grounding Parameters with Empirical Formulas 131 4.1.2 Numerical Analysis Method for Grounding System Parameters 132 4.2 Equal Potential Analysis of Grounding Grid 135 4.2.1 Approach of Green s Function for Calculating Grounding Parameters 135 4.2.2 Superposition Method Under the Assumption of Nodal Leakage Current 139 4.2.3 Multi-Step Method Under the Assumption of Nodal Leakage Current 141 4.2.4 Integration Method Under the Assumption of Branch Leakage Current 143 4.3 Unequal Potential Analysis of a Large-Scale Grounding System 146 4.3.1 Analysis Model of a Grounding System with Unequal Potential 147 4.3.2 Problems in the Analytical Method for Solving a Mutual Resistance Coefficient 148 4.3.3 Numerical Integration Method for Mutual Resistance Coefficient Calculation 148 4.3.4 Multi-Step Method for Uniform Soil 149 4.4 Analyzing Grounding Grid with Grounded Cables 151 4.4.1 Principles of Setting up Equations 151 4.4.2 Calculating Self-Admittances of Conductors and Cables 153 4.5 MoM Approach for Grounding Grid Analysis in Frequency Domain 153 4.5.1 Basis Functions of MoM 153 4.5.2 Setting up the Equations 154 4.5.3 Green s Functions and Generalized Sommerfeld Integral 155 4.6 Finite Element Method for a Complex Soil Structure 159 4.7 Time Domain Method for Electromagnetic Transient Simulation of a Grounding System 161 4.7.1 Generalized MMC Method under EMQS Assumption 161 4.7.2 Numerical Approach Based on Time Domain Integral Equation in a Lossy Medium 171 4.7.3 Finite Difference Time Domain Method 181 References 186 5 Ground Fault Current of a Substation 191 5.1 Power Station and Substation Ground Faults 191 5.1.1 Types of Power Station and Substation Ground Faults 191 5.1.2 Principle to Determine Maximum Ground Fault Current 191 5.1.3 Location of the Maximum Ground Fault Current 193 5.2 Maximum Fault Current through a Grounding Grid to the Earth 194 5.2.1 Maximum Grounding Grid Fault Current 194 5.2.2 Zero-Sequence Fault Current 194 5.2.3 Determining the Fault Current Division Factor 195 5.2.4 Determining the Decrement Factor 196 5.2.5 Determining the Correction Coefficient for Future Planning 198 5.2.6 Impact of Substation Grounding Resistance 199 5.2.7 Impact of Fault Resistance 200 5.2.8 Impact of Overhead Ground Wires and Neutral Lines 200 5.2.9 Impact of Buried Conduits and Cables 200 5.2.10 Steps to Determine a Proper Design Value of the Maximum Grounding Grid Current 200 5.3 Simplified Calculation of a Fault Current Division Factor 201 5.3.1 Fault Current Division Factor Within a Local Substation 201 5.3.2 Fault Current Division Factor Outside a Local Substation 202 5.4 Numerical Calculation of the Fault Current Division Factor 203 5.4.1 Numerical Calculation Method of the Fault Current Division Factor 203 5.4.2 Matrix Method to Calculate the Fault Current Division Factor 207 5.4.3 Phase Coordinate Transformer Model for Calculating the Fault Current Division Factor 207 5.5 Typical Values of the Fault Current Division Factor 213 5.5.1 Influence of Substation Grounding Resistance 213 5.5.2 Influence of Transmission Towers 214 5.5.3 Influence of Fault Location 216 5.5.4 Influence of Incoming Cables 216 5.5.5 Influence of Transmission Line Number 216 5.5.6 Influence of Transmission Line Length 217 5.5.7 Influence of Transformer 217 5.6 Influence of Seasonal Freezing on the Fault Current Division Factor 219 5.6.1 Influence of Seasonally Frozen Soil on the Fault Current Division Factor 219 5.6.2 Influence of Transmission Line Numbers Affected by Frozen Soil 221 References 221 6 Grounding System for Substations 223 6.1 Purpose of Substation Grounding 223 6.1.1 Function of Substation Grounding 223 6.1.2 Design Objective of a Substation Grounding System 224 6.1.3 Requirement on the Grounding System of a Substation 225 6.1.4 Specificity of Power Plant Grounding 225 6.1.5 Requirements for Grounding System Design 226 6.1.6 Design and Construction Procedures for a Grounding System 226 6.2 Safety of Grounding Systems for Substations and Power Plants 227 6.2.1 Design Criteria of Grounding Systems 227 6.2.2 Calculation of the Grounding Resistance of a Grounding System 228 6.2.3 Analysis of Grounding in Inhomogeneous Soil 230 6.2.4 Simplified Formula for Calculating Step, Touch and Mesh Voltages 234 6.2.5 Formulas in IEEE Standard 80-2000 for Calculating Mesh and Step Voltages 235 6.2.6 Formulas to Calculate Touch and Step Voltages in Chinese Standards 237 6.2.7 Transfer Potential 238 6.2.8 Methods for Improving the Safety of a Grounding System 238 6.3 Methods for Decreasing the Grounding Resistance of a Substation 240 6.3.1 Basic Methods for Decreasing Grounding Resistance 240 6.3.2 Using Long Vertical Ground Rods to Decrease Grounding Resistance 241 6.3.3 Explosion Grounding Technique 248 6.3.4 Deep Ground Well 250 6.3.5 Slanting Grounding Electrode 253 6.4 Equipotential Optimal Arrangement of a Grounding Grid 254 6.4.1 Principle of the Unequal-Spacing Arrangement 254 6.4.2 Regularity of the Unequal-Spacing Arrangement 256 6.4.3 Unequal-Spacing Arrangement with Exponential Distribution 263 6.4.4 Influence of Vertical Grounding Electrodes on OCR 267 6.5 Numerical Design of a Grounding System 268 6.5.1 Grounding System Design of a 220-kV Substation 268 6.5.2 Grounding System Design of a 1000-kV UHV Substation 270 References 272 7 Grounding of Transmission and Distribution Lines 275 7.1 Requirement for a Tower Grounding Device 275 7.1.1 Requirement of Transmission Tower Grounding Resistance 276 7.1.2 Seasonal Factor for the Grounding Resistance of a Tower Grounding Device 276 7.2 Structures of Tower Grounding Devices 277 7.2.1 Basic Structures of Tower Grounding Devices 277 7.2.2 Using Natural Footings as Tower Grounding Devices 280 7.3 Properties of a Concrete-Encased Grounding 280 7.3.1 Function of a Concrete-Encased Grounding Device 280 7.3.2 Hygroscopic Properties of Concrete 281 7.3.3 Permissible Current through a Concrete-Encased Grounding Device 283 7.4 Computational Methods for Tower Grounding Resistance 284 7.4.1 Equivalent Cylindrical Conductor Method 284 7.4.2 Grounding Resistance of a Vertical Ground Rod Covered with Concrete 285 7.4.3 Grounding Resistance of a Fabricated Concrete-Encased Footing 286 7.4.4 Grounding Resistance of a Tower Grounding Device with Different Structures 287 7.4.5 Utilization Coefficient 289 7.5 Step and Touch Voltages Near a Transmission Tower 290 7.5.1 Step Voltage and Touch Voltage 290 7.5.2 Shock Accident Possibilities Caused by Step and Touch Voltages 292 7.6 Short-Circuit Fault on Transmission Tower 294 7.6.1 Fault Current of Transmission Line 294 7.6.2 Distribution of Ground Potential around Transmission Towers 295 7.6.3 Methods to Improve Potential Distribution 296 7.7 Grounding Device of Distribution Lines 299 7.7.1 Vertically Driven Rods 300 7.7.2 Grounding of Wood Poles 301 7.7.3 Requirement for Grounding the Distribution Line 301 References 301 8 Impulse Characteristics of Grounding Devices 303 8.1 Fundamentals of Soil Impulse Breakdown 303 8.1.1 Electric Field Strength of Soil Breakdown 303 8.1.2 Phenomenon of Electrical Breakdown in Soil 312 8.1.3 Impulse Breakdown Delay Characteristics of Soil 319 8.1.4 Mechanism of Electrical Breakdown in Soil 321 8.1.5 Residual Resistivity of Ionized Soil 323 8.2 Numerical Analysis of the Impulse Characteristics of Grounding Devices 325 8.2.1 Equivalent Circuit Model 325 8.2.2 MoM Coupled with Circuit Theory 331 8.2.3 An Interpolation Model to Accelerate the Frequency Domain Response Calculation 335 8.3 Impulse Characteristics of Tower Groundings 346 8.3.1 Field Test of Grounding Devices Impacted by a Large Impulse Current 346 8.3.2 Lightning Current Decay Along a Grounding Electrode 348 8.3.3 Definition of Impulse Grounding Resistance 350 8.3.4 Influence of Different Factors on the Impulse Grounding Resistance of Grounding Devices 352 8.3.5 Influence of Different Factors on Impulse Coefficient 355 8.3.6 Regressive Formulas to Calculate Impulse Coefficients 357 8.3.7 Impulse Coefficient and Utilization Efficient Suggested in the Literature 359 8.3.8 Low Resistivity Material Effects to Decrease Impulse Grounding Resistance 360 8.4 Impulse Effective Length of Grounding Electrodes 362 8.4.1 Phenomenon of Impulse Effective Length 362 8.4.2 Regressive Formulas to Calculate the Effective Length of Counterpoise Wires 364 8.4.3 Influence of LRM on the Impulse Effective Length of Counterpoise Wires 368 8.5 Impulse Characteristics of a Grounding Grid 370 8.5.1 Influence of the Structure of the Grounding Grid 370 8.5.2 Influence of Soil Parameters 374 8.5.3 Influence of Impulse Current Waveform on the Transient Performance of Grounding Grids 375 8.5.4 Impulse Effective Regions of Grounding Grids 378 8.6 Lightning Electromagnetic Field Generated by a Grounding Electrode 381 8.6.1 Computation Methodologies 381 8.6.2 Disposal of a Lightning Current 383 8.6.3 Influence of Soil Ionization 383 References 385 9 DC Ground Electrode 391 9.1 Technical Requirements of a DC Ground Electrode 391 9.1.1 Technical Characteristics of a DC Ground Electrode 391 9.1.2 Basic Principles of DC Ground Electrode Design 392 9.2 Structure Types of DC Ground Electrodes 394 9.2.1 Land Electrode 394 9.2.2 Shore Ground Electrode 400 9.2.3 Sea Electrode 401 9.3 Main Design Aspects of a DC Ground Electrode 401 9.3.1 Main Design Items 401 9.3.2 Determination of DC Ground Electrode Size 403 9.3.3 Determination of Coke Section 405 9.3.4 Diameter of Feeding Rod 406 9.3.5 Burial Depth of Electrode 407 9.3.6 Selection of Ground Electrode Material 407 9.4 Numerical Analysis Methods for a Ground Electrode 413 9.4.1 Numerical Analysis of a Ground Electrode by MoM and BEM 414 9.4.2 Simplified Numerical Analysis Method 417 9.5 Heat Generation Analysis of a DC Ground Electrode 418 9.5.1 Numerical Analysis of the Heat Dissipation of a Ground Electrode 419 9.5.2 Maximum Temperature Rise Limit 422 9.6 Common Ground Electrode of a Multiple Converter System 423 9.6.1 Demands on a Common Ground Electrode 424 9.6.2 Parameters of the Common Ground Electrode 427 9.6.3 Common Ground Electrode Design 429 9.7 Influence of DC Grounding on AC System 433 9.7.1 Influence of DC Electrode s Current Field on AC System 433 9.7.2 Numerical Analysis of DC Current Entering a Neutral Grounded Transformer 436 9.7.3 Allowable DC Current of a Transformer 443 9.8 Methods to Decrease Winding DC Current of a Neutral Grounding Transformer 445 9.8.1 Injecting Reverse DC Current Method 445 9.8.2 Inserting Capacitor Method 446 9.8.3 Inserting Resistor Method 447 9.9 Corrosion of Underground Metal Pipes Caused by a DC Ground Electrode 455 9.9.1 Mechanism of Electrochemical Corrosion of Underground Metal Pipes 455 9.9.2 Leakage Current through a Metal Pipe Caused by Ground Electrodes 455 9.9.3 Protection Measures 456 References 458 10 Materials for Grounding 461 10.1 Choice of Material and Size for Conductors 461 10.1.1 Requirement on Material and Size of Grounding Conductors 461 10.1.2 Materials for a Grounding Conductor 463 10.1.3 Determination of Conductor Size 464 10.1.4 Grounding Conductor Size Determined by Ground Fault Protection 470 10.2 Soil Corrosion of Grounding Conductor 470 10.2.1 Features of Soil Corrosion 471 10.2.2 Natural Corrosion 471 10.2.3 Electrical Corrosion in Soil 474 10.3 Corrosion of Concrete-Encased Electrodes 476 10.4 Low-Resistivity Material 478 10.4.1 Principle of Reducing Grounding Resistance by LRM 478 10.4.2 Ingredients of LRM 482 10.4.3 Basic Requirements for LRM 485 10.4.4 Evaluation of LRM 487 10.5 Performance of LRM 488 10.5.1 Power Frequency Performance of LRM 488 10.5.2 Lightning Impulse Performance of LRM 493 10.6 Construction Method of LRM 495 10.6.1 Influence of LRM Bulk Shape on Reducing the Grounding Resistance Effect 495 10.6.2 Amount of LRM and Construction Method 495 10.6.3 Construction of a Complex Ground Device 497 References 497 11 Measurement of Grounding 499 11.1 Methods for Grounding Resistance Measurement 499 11.1.1 Simple Methods for Measuring the Grounding Resistance of Small Grounding Devices 500 11.1.2 Principle of the Fall of Potential Method 501 11.1.3 Method of Far Placed Current Probe for Fall of Potential Method 502 11.1.4 Compensation Location of a Potential Probe for the Fall of Potential Method 504 11.1.5 Compensation Method for the Fall of Potential Method 506 11.2 Instruments for Measuring Grounding Resistance 510 11.2.1 Ammeter Voltmeter Method 510 11.2.2 Ammeter Wattmeter Method 510 11.2.3 Ratio Meter Method 511 11.2.4 Bridge Method 513 11.2.5 Potentiometer Method 514 11.2.6 Single Equilibrium Transformer 514 11.2.7 ZC-8 Grounding Resistance Tester 515 11.2.8 Digital Measurement System of Grounding Resistance 516 11.3 Factors Influencing the Results from the Fall of Potential Method 519 11.3.1 Electromagnetic Interferences During Measurements 519 11.3.2 Impact and Elimination of Power Frequency Interference 520 11.3.3 Components of the Measured Voltage Signal for the Grounding Resistance Test 521 11.3.4 Mutual Inductance Between Potential and Current Lead Wires 521 11.3.5 Short Measuring Leads Method 527 11.3.6 Accurate Location of Test Probe Positioning by GPS 529 11.3.7 Influence of a Metal Structure Buried Nearby 529 11.3.8 Method to Eliminate Measuring Interference 531 11.4 Grounding Resistance Test in Vertically Layered Soil 532 11.4.1 Grounding System Built in a Middle Low Resistivity Region 532 11.4.2 Grounding System Built in a Middle High Resistivity Region 534 11.4.3 Discussion of Analysis Results 535 11.5 Influence of Overhead Ground Wires on Substation Grounding Resistance Measurement 535 11.5.1 General Analysis Model 536 11.5.2 General Discussion 536 11.5.3 Analysis of a 500 kV Substation 538 11.6 Measurement of Potential Distribution 539 11.6.1 Equipotential Line 539 11.6.2 Measurement of Equipotential Lines 540 11.6.3 Measurement of Step Voltage and Touch Voltage 541 11.7 Corrosion Diagnosis of Grounding Grids 542 11.7.1 Corrosion Diagnosis Model of a Grounding Grid 543 11.7.2 Implementation of the Diagnosis System 546 11.7.3 Field Test Results 547 References 550 Index 553