ELECTRIC POWER TRANSMISSION AND DISTRIBUTION S. Sivanagaraju Associate Professor Department of Electrical Engineering JNTU College of Engineering Anantapur, Andhra Pradesh S. Satyanarayana Professor and Head Department of Electrical Engineering Tenali Engineering College Tenali, Andhra Pradesh Delhi • Chennai • Chandigarh Contents Preface Acknowledgement 1 Transmission and Distribution: An Introduction 1.1 Overview 1.2 Various Levels of Power Transmission 1.3 Conventional Sources of Electrical Energy 1.3.1 Hydro Power Stations 1.3.2 Thermal Power Stations 1.3.3 Nuclear Power Stations 1.3.4 Diesel Power Stations 1.4 Load Forecasting 1.4.1 Purpose of Load Forecasting 1.4.2 Classification of Load Forecasting 1.4.3 Forecasting Procedure 1.4.4 Load Characteristics 1.5 Load Modelling 1.5.1 Characteristics of Load Models 1.6 Star-Connected Loads 1.6.1 Constant Power Model 1.6.2 Constant Current Model 1.6.3 Constant Impedance Model 1.7 Deregulation 1.7.1 Need for Restructuring 1.7.2 Motivation for Restructuring the Power Industry 1.8 Distribution Automation 2 Transmission-Line Parameters 2.1 Introduction 2.2 Conductor Materials 2.3 Types of Conductors 2.4 Bundled Conductors 2.5 Resistance 2.6 Current Distortion Effect 2.6.1 Skin Effect 2.6.2 Proximity Effect 2.6.3 Spirality Effect 2.7 Inductance 2.7.1 Inductance of a Conductor Due to Internal Flux 2.7.2 Inductance of a Conductor Due to External Flux 2.8 Inductance of a Single-Phase Two-Wire System 2.9 Flux Linkages With One Sub-Conductor of a Composite Conductor 2.10 Inductance Of a Single-Phase System (With Composite Conductors) 2.11 Inductance of Three-Phase Lines 2.11.1 Equivalent (Symmetrical) Spacing 2.11.2 Unsymmetrical Spacing (Untransposed) 2.11.3 Transposition of Overhead Lines 2.11.4 Unsymmetrical Spacing (Transposed) 2.12 Inductance of Three-Phase Double Circuit Line 2.12.1 Inductance of Three-Phase Double-Circuit Line with Symmetrical Spacing (Hexagonal) 2.12.2 Inductance of a Three-Phase Transposed Double-Circuit Line with Unsymmetrical Spacing 2.13 Capacitance 2.14 Potential Difference Between Two Points Due to a Charge 2.15 Capacitance of a Single-Phase Line (Two-Wire Line) 2.16 Potential Difference Between Two Conductors of a Group of Charged Conductors 2.17 Capacitance of Three-Phase Lines 2.17.1 Equilateral Spacing 2.17.2 Capacitance of an Unsymmetrical Three-Phase System (Transposed) 2.18 Capacitance of a Three-Phase Double-Circuit Line 2.18.1 Hexagonal Spacing 2.18.2 Flat Vertical Spacing (Unsymmetrical Spacing) 2.19 Effect of Earth on Transmission Line Capacitance 2.19.1 Capacitance of a Single Conductor 2.19.2 Capacitance of a Single-Phase Transmission Line 2.19.3 Capacitance of Three-Phase Line 3 Performance of Short and Medium Transmission Lines 3.1 Introduction 3.2 Representation of Lines 3.3 Classification of Transmission Lines 3.4 Short Transmission Line 3.4.1 Effect of Power Factor On Regulation and Efficiency 3.5 Generalised Network Constants 3.6 A, B, C, D Constants for Short Transmission Lines 3.7 Medium Transmission Line 3.7.1 Load End Capacitance Method 3.7.2 Nominal-T Method 3.7.3 Nominal-π Method 4 Performance of Long Transmission Lines 4.1 Introduction 4.2 Rigorous Solution 4.3 Interpretation of the Long Line Equations 4.3.1 Propagation Constant 4.3.2 Wave Length and Velocity of Propagation 4.4 Evaluation of Transmission Line Constants 4.5 Regulation 4.6 Equivalent Circuit Representation of Long Lines 4.6.1 Representation of a Long Line by Equivalent- π Model 4.6.2 Representation of a Long Line by Equivalent-T Model 4.7 Tuned Transmission Lines 4.8 Characteristic Impedance 4.9 Surge Impedance Loading (Sil) 4.10 Ferranti Effect 4.11 Constant Voltage Transmission 4.12 Charging Current in Lines 4.12.1 Power Loss Due to Charging Current (or Open-Circuited Line) 4.13 Line Loadability 4.14 Power Flow Through a Transmission Line 4.15 Circle Diagram 4.15.1 Receiving-End Phasor Diagram 4.15.2 Receiving-End Power Circle Diagram 4.15.3 Analytical Method for Receiving-End Power Circle Diagram 4.15.4 Sending-End Power Circle Diagram 4.15.5 Analytical Method for Sending-End Power Circle Diagram 5 Transmission Line Transients 5.1 Introduction 5.2 Types of System Transients 5.3 Travelling Waves on a Transmission Line 5.4 The Wave Equation 5.5 Evaluation of Surge Impedance 5.6 Importance of Surge Impedance 5.7 Travelling Wave 5.8 Evaluation of Velocity of Wave Propagation 5.9 Reflection and Refraction Coefficient (Line Terminated Through a Resistance) 5.9.1 Line Open-Circuited at the Receiving End 5.9.2 Line Short-Circuited at the Receiving End 5.10 Line Connected to A Cable 5.11 Reflection and Refraction at A T-Junction 5.12 Reactance Termination 5.12.1 Line Terminated Through Capacitance 5.12.2 Line Terminated Through Inductance 5.13 Bewley’s Lattice Diagram 5.14 Attenuation of Travelling Waves 6 Corona 6.1 Introduction 6.2 Theory of Corona Formation (Corona Discharge) 6.3 Electric Stress 6.4 Critical Disruptive Voltage 6.5 Visual Critical Voltage 6.6 Power Loss Due to Corona 6.7 Factors Affecting Corona Loss 6.7.1 Electrical Factors 6.7.2 Atmospheric Factors 6.7.3 Factors Related to the Conductors 6.8 Methods For Reducing Corona Loss 6.9 Advantages and Disadvantages of Corona 6.10 Effect of Corona on Line Design 6.11 Radio Interference 6.12 Audio Noise 6.13 Interference with Communication Lines 6.13.1 Electromagnetic Effect 6.13.2 Electrostatic Effect 6.14 Corona Phenomena in HVDC Lines 7 Mechanical Design of Transmission Line 7.1 Introduction 7.2 Factors Affecting Mechanical Design 7.3 Line Supports 7.3.1 Wooden Poles 7.3.2 Tubular Steel Poles 7.3.3 RCC Poles 7.3.4 Latticed Steel Towers 7.4 Sag 7.4.1 Calculation of Sag at Equal Supports 7.4.2 Effect of Ice Covering and Wind Pressure 7.4.3 Safety Factor 7.4.4 Calculation of Sag at Different Level Supports 7.5 Stringing Chart 7.6 Effects and Prevention of Vibrations (Vibrations and Dampers) 7.7 Sag Template 7.8 Conductor Spacing and Ground Clearance 8 Overhead Line Insulators 8.1 Introduction 8.2 Insulator Materials 8.3 Types of Insulators 8.3.1 Pin Type Insulators 8.3.2 Suspension Type Insulator 8.3.3 Strain Insulator 8.3.4 Shackle Type Insulator 8.4 Potential Distribution Over a String of Suspension Insulators 8.4.1 Mathematical Expression for Voltage Distribution 8.5 String Efficiency 8.6 Methods of Improving String Efficiency 8.6.1 Selection of m 8.6.2 Grading of Units 8.6.3 Guard Ring or Static Shielding 8.7 Arcing Horn 8.8 Testing of Insulators 8.8.1 Flashover Tests 8.8.2 Performance Test 8.8.3 Routine Tests 8.9 Causes of Failure of Insulators 9 Underground Cables 9.1 Introduction 9.2 General Construction of a Cable 9.3 Types of Cables 9.3.1 Low Tension Cables 9.3.2 High Tension Cables 9.3.3 Super Tension Cables 9.3.4 Extra High Tension Cables 9.4 Advantages and Disadvantages of Underground Cables Over Overhead Lines 9.5 Properties of Insulating Materials for Cables 9.5.1 Insulating Materials 9.6 Insulation Resistance of Cables 9.7 Capacitance of a Single-Core Cable 9.8 Dielectric Stress in a Cable 9.9 Economical Core Diameter 9.10 Grading of Cables 9.10.1 Capacitance Grading 9.10.2 Intersheath Grading 9.10.3 Practical Aspects of Cable Grading 9.11 Power Factor in Cables (Dielectric Power Factor) 9.12 Capacitance of a Three-Core Cable 9.12.1 Measurement of C and C 9.13 Heating of Cables 9.13.1 Generation of Heat Within the Cables 9.14 Thermal Characteristics 9.14.1 Current Capacity 9.15 Testing of Cables 9.15.1 Acceptance Tests at Works c s 9.15.2 Sample Tests at Working 9.15.3 Performance Tests 9.15.4 Tests On Oil-Filled and Gas-Filled Cables 9.15.5 Tests When Installed 9.15.6 Tests On Pressurized Cables 9.16 Laying of Cables 9.16.1 Direct System 9.16.2 Draw-In System 9.16.3 Solid Systems 9.17 Cable Faults 9.18 Determination of Maximum Current Carrying Capacity of Cables 10 Power Factor Improvement 10.1 Introduction 10.2 Power Factor 10.2.1 Causes of Low Power Factor 10.2.2 Effects or Disadvantages of Low Power Factor 10.3 Advantages of Power Factor Improvement 10.4 Methods of Improving Power Factor 10.4.1 Static Capacitor 10.4.2 Synchronous Condenser 10.4.3 Phase Advancers 10.5 Most Economical Power Factor When the Kilowatt Demand is Constant 10.6 Most Economical Power Factor When the kVA Maximum Demand is Constant 11 Voltage Control 11.1 Introduction 11.2 Necessity of Voltage Control 11.3 Generation and Absorption of Reactive Power 11.4 Location of Voltage Control Equipment 11.5 Methods of Voltage Control 11.5.1 Excitation Control 11.5.2 Shunt Capacitors and Reactors 11.5.3 Series Capacitors 11.5.4 Tap Changing Transformers 11.5.5 Booster Transformers 11.5.6 Synchronous Condensers 11.6 Rating of Synchronous Phase Modifier 12 Electric Power Supply Systems 12.1 Introduction 12.2 Comparison of Conductor Efficiencies for Various Systems 12.2.1 Overhead Lines 12.2.2 Cable Systems 12.3 Choice of System Frequency 12.4 Choice of System Voltage 12.5 Advantages of High-Voltage Transmission 12.6 Effect of Supply Voltage 12.7 Economic Size of Conductor (Kelvin’s Law) 12.7.1 Modification of Kelvin’s Law 12.7.2 Practical Limitations to the Application of Kelvin’s Law 13 Substations 13.1 Introduction 13.2 Factors Governing the Selection of Site 13.3 Classification of Substation 13.3.1 According to Service 13.3.2 According to Design 13.4 Merits and Demerits of Indoor and Outdoor Substations 13.5 Substation Equipment 13.6 Types of Bus Bar Arrangements 13.6.1 Single Bus Bar 13.6.2 Single-Bus Bar System with Sectionalization 13.6.3 Double Bus Bar with Single Breaker 13.6.4 Double Bus Bar with Two Circuit Breakers 13.6.5 Breakers and a Half with Two Main Buses 13.6.6 Main and Transfer Bus Bar 13.6.7 Double Bus Bar with Bypass Isolator 13.6.8 Ring Bus 13.7 Pole and Plinth-Mounted Transformer Substations 13.8 Optimal Substation Location 13.8.1 Perpendicular Bisector Rule 13.9 Basic Terms of Earthing 13.10 Grounding or Neutral Earthing 13.11 Earthing of Substations 13.12 Methods of Neutral Grounding 13.12.1 Solid Grounding or Effective Grounding 13.12.2 Resistance Grounding 13.12.3 Reactance Grounding 13.12.4 Peterson-Coil Grounding 13.12.5 Grounding Transformer 13.13 Grounding Grid 14 Distribution Systems 14.1 Introduction 14.2 Primary and Secondary Distribution 14.2.1 Primary Distribution 14.2.2 Secondary Distribution 14.3 Design Considerations in a Distribution System 14.4 Distribution System Losses 14.4.1 Factors Effecting Distribution-System Losses 14.4.2 Methods for the Reduction of Line Losses 14.5 Classification of Distribution System 14.5.1 Type of Current 14.5.2 Type of Construction 14.5.3 Type of Service 14.5.4 Number of Wires 14.5.5 Scheme of Connection 14.6 Radial Distribution System 14.7 Ring or Loop Distribution System 14.8 Interconnected Distribution System 14.9 Dc Distribution 14.9.1 Distributor Fed at One End with Concentrated Loads 14.9.2 Distributor Fed at Both Ends with Concentrated Loads 14.9.3 Uniformly Loaded Distributor Fed at One End 14.9.4 Uniformly Distributed Load Fed at Both Ends at the Same Voltage 14.9.5 Uniformly Distributed Load Fed at Both Ends at Different Voltages 14.10 Ring Distribution 14.10.1 Advantages of Using Interconnector 14.11 Stepped Distributor 14.12 Ac Distribution 14.12.1 Power Factor Referred to the Receiving-End 14.12.2 Power Factor Referred to Respective Load Voltages 14.13 Ac Three-Phase Distribution 15 EHV and HVDC Transmission Lines 15.1 Introduction 15.2 Need of EHV Transmission Lines 15.3 Advantages and Disadvantages of EHV Lines 15.4 Methods of Increasing Transmission Capability of EHV Lines 15.5 HVDC Transmission System 15.6 Comparison Between AC and DC Transmission Systems 15.6.1 Economic Advantages 15.6.2 Technical Advantages 15.7 Advantages and Disadvantages of HVDC Systems 15.8 HVDC Transmission System 15.8.1 Monopolar Link 15.8.2 Bipolar Link 15.8.3 Homopolar Link 15.9 Rectification 15.10 Three-Phase Bridge Converter 15.11 Inversion 15.12 Components of HVDC Transmission System 15.13 Harmonic Filters 15.14 Application of HVDC Transmission System 16 Flexible AC Transmission Systems 16.1 Introduction 16.2 Facts 16.3 Facts Controllers 16.3.1 Shunt-Connected Controllers 16.3.2 Series-Connected Controllers 16.3.3 Combined Shunt and Series-Connected Controllers 16.4 Control of Power Systems 16.4.1 FACTS Devices 16.4.2 Benefits of Control of Power Systems 16.4.3 FACTS Technology: Opportunities 16.5 Basic Relationship For Power-Flow Control 16.5.1 Shunt Compensator 16.5.2 Thyristor-Controlled Reactor (TCR) 16.5.3 Thyristor-Switched Capacitor (TSC) 16.5.4 Series Compensator 16.5.5 Unified Power-Flow Controller (UPFC) 16.6 “Facts” for Minimizing Grid Investments 16.7 Voltage Stability 16.7.1 Voltage Stability – What is it? 16.7.2 Derivation of Voltage Stability Index Appendix 1: Datasheets Appendix 2: Answers to Problems Appendix 3: Answers to Odd Questions Appendix 4: Solutions Using MATLAB Programs Glossary Bibliography TO MY PARENTS Preface Electric Power Transmission and Distribution has been designed for undergraduate courses in electrical and electronics engineering in Indian universities. Tailored to provide an elementary knowledge of power systems, a foundation in electric circuits and engineering is a pre- requisite for starting this course. The organization of the topics and the pace at which they unfold have been planned to enable students to proceed from the basic concepts to the difficult ones with ease. The text can ideally be taught at the sixth or seventh semester, and can also be used as a reference book by BE, BTech and AMIE students. The contents of this book have been developed with emphasis on clarity, with equal stress on the basic concepts as well as advanced ideas, in detail over sixteen chapters. Chapter 1 introduces the conventional sources of electrical energy, and explains about the load forecasting and its various aspects, the various levels of power transmission, and the need as well as the motivation for restructuring the power industry. Chapter 2 explains the four transmission line parameters, namely, resistance and inductance in a series combination and a shunt combination of capacitance and conductance. Materials used for the manufacture of conductors and the various types of conductors are explained in detail. It is explained in detail about the current distortion effect and the effect of earth on transmission line capacitance to give a lucid understanding of the constituent elements of the transmission system.