Underground Distribution System Design Guide

(1)

Underground Distribution

System Design Guide

(2)

(3)

Prepared by

Edward S. Thomas, PE Utility Electrical Consultants, PC 620 N.West St., Suite 103 Raleigh, NC 27603-5938

and

Bill Dorsett Booth & Associates, Inc. 1011 Schaub Drive Raleigh, NC 27606

for

Cooperative Research Network National Rural Electric Cooperative Association 4301 Wilson Boulevard Arlington, Virginia 22203-1860

Underground Distribution

System Design Guide

(4)

© Underground Distribution System Design Guide

Reproduction in whole or in part is strictly prohibited without prior written approval of the National Rural Electric Cooperative Association, except that reasonable portions may be reproduced or quoted as part of a review or other story about this publication.

Legal Notice

This work contains findings that are general in nature. Readers are reminded to perform due diligence in applying these findings to their specific needs, as it is not possible for NRECA to have sufficient understanding of any specific situation to ensure applicability of the findings in all cases.

Neither the authors nor NRECA assume liability for how readers may use, interpret, or apply the information, analysis, templates, and guidance herein or with respect to the use of, or damages resulting from the use of, any information, apparatus, method, or process contained herein. In addition, the authors and NRECA make no warranty or representation that the use of these contents does not infringe on privately held rights.

This work product constitutes the intellectual property of NRECA and its suppliers, as the case may be, and contains Confidential Information. As such, this work product must be handled in accordance with the CRN Policy Statement on Confidential Information.

Contact:

Edward S. Thomas, PE

Utility Electrical Consultants, PC 620 N.West St., Suite 103 Raleigh, NC 27603-5938 Phone: 919.821.1410 Fax: 919.821.2417

Bill Dorsett

Booth & Associates, Inc. 1011 Schaub Drive Raleigh, NC 27606 Phone: 919.851.8770 Fax: 919.859.5918

(5)

Section 1 Design of an Underground Distribution System 1

System Components 2

Types of UD Systems 6

Reliability of UD Systems 14

Design Considerations for System Operation and Maintenance 17

Future Upgrades and Replacements 19

Economic Comparison of System Configurations 20

UD Loss Economics 32

Steps for Layout of a UD System 38

Summary and Recommendations 50

Section 2 Cable Selection 51

Typical Cable Configuration 51

Conductor Size Designations 53

Conductor Materials and Configuration 53

Cable Insulation Materials 57

Insulation Fabrication 60

Conductor Shields and Insulation Shields 64

Cable Specification and Purchasing 74

Cable Acceptance 77

Section 3 Underground System Sectionalizing 79

General Sectionalizing Philosophy 79

Overcurrent Protection of Cable System 88

Effect of Inrush Current on Sectionalizing Devices 96 Selection of Underground Sectionalizing Equipment 100

Faulted-Circuit Indicators 105

Section 4 Equipment Loading 121

Primary Cable Ampacity 121

Pad-Mounted Transformer Sizing 144

Section 5 Grounding and Surge Protection 165

Cable Grounding System Function 166

Factors Affecting Cable Grounding System Performance 177 Counterpoise Application for Insulated Jacketed Cable 188 System Ground Resistance Measurement and Calculation 192

Underground System Surge Protection 207

(6)

Section 6 Ferroresonance 239

Allowable Overvoltages During Ferroresonance 240

Distribution Transformer Connections 241

Qualitative Description of Ferroresonance 242

Ferroresonance When Switching at the Primary Terminals of Overhead

and Underground Transformer Banks 252

Ferroresonance with Cable-Fed Three-Phase Transformers with Delta

or Ungrounded-Wye Connected Primary Windings 254 Ferroresonance with Cable-Fed Three-Phase Transformers with

Grounded-Wye Primary Winding and Five-Legged Core 260 Ferroresonance with Cable-Fed Three-Phase Transformers with

Grounded-Wye Primary Windings and Triplex Construction 266 Ferroresonance in Underground Feeders Having More Than

One Transformer 270

Summary of Techniques for Preventing Ferroresonance in

Underground Systems 273

References 279

Section 7 Cathodic Protection Requirements 281

Special Note 281

Introduction 281

What to Protect 282

Where to Protect 282

Types of Cathodic Protection Systems 285

Amount of Cathodic Protection 286

Cathodic Protection Design with Galvanic Anodes 287 Cathodic Protection Installation and Follow-Up 294

Calculation of Resistence to Ground 296

Section 8 Direct-Buried System Design 299

Trench Construction Considerations 299

Trench Design Components 300

Trench Layout/Routing Considerations 303

Depth of Burial 304

Joint-Occupancy Trenches 307

Section 9 Conduit System Design 311

Conduit System Design 311

Cable Pulling 332

(7)

Section 10 Joints, Elbows, and Terminations 343 Joints, Elbows, and Terminations for 200-Ampere Primary Circuits 344 Joints, Elbows, and Terminations for 600-Ampere Primary Circuits 353 Joints, and Terminations for Secondary Circuits 355

Section 11 Cable Testing 359

Reasons for and Benefits of Cable Testing by the User 359

Primary Cable Tests by the User 359

Secondary Cable Tests by the User 369

Tests by the Cable Manufacturer 370

Appendix A Calculations for Reliability Studies 373

Reliability Index 373

Acceptability Criteria 374

Calculation of Reliability 374

Importance of Sectionalizing 375

Appendix B Transformer and Secondary Voltage Drop 377

Voltage Flicker 385

Appendix C Sample Specification UGC2 for 600-Volt

Secondary Underground Power Cable 389

Scope 389 General Specifications 390 Referenced Specifications 390 Conductor 391 Insulation 391 Tests 392 Miscellaneous 393 Markings 393

Multiconductor Cable Assemblies 393

Appendix D Checklist for Information Requirements 395

(8)

Appendix E Sample Specification for 15-, 25-, and 35-kV Primary Underground

Medium Voltage Concentric Neutral Cable (Specification UGC1) 397

Purpose 397

General Specifications 397

Referenced Specifications 398

Conductor 399

Conductor Shield (Stress Control Layer) 400

Insulation 400

Insulation Shielding 400

Concentric Neutral Conductor 401

Overall Outer Jacket 401

Dimensional Tolerances 402

Tests 402

Miscellaneous 403

Appendix F Allowable Short Circuit Currents for Solid Dielectric Insulated Cables 405

Appendix G Ampacity Tables 415

Appendix H Industry Specifications 425

Appendix I Component Manufacturers 427

Appendix J Cable-Pulling Examples 431

(9)

1.1 UD System Components 2

1.2 Schematics for Different Types of Switchgear 3

1.3 Flat Pad for Equipment Mounting 5

1.4 Ground Sleeve 5

1.5 Box Pad for Equipment Mounting 5

1.6 Underground Substation Circuit Exit 6

1.7 Radial Main Feeder 7

1.8 Radial Main Feeder with Faulted Cable Section 8

1.9 Open-Loop Feeder 9

1.10 Open-Loop Feeder with Faulted Cable Section 9

1.11 Radial Feeder 10

1.12 Open-Loop Feeder in Shopping Center 11

1.13 Multiple-Loop System 11

1.14 Area Lighting System 12

1.15 Loop-Feed Design of UD System Under Normal Conditions 16 1.16 Loop-Feed Design of UD System with Damaged Cable Section 16

1.17 Open-Loop System, 37-Lot Subdivision 21

1.18 Open-Loop System, Single Residential Consumer 22

1.19 Single-Phase Sub-Feeder 24

1.20 Three-Phase Sub-Feeder 25

1.21 Front Property Placement 28

1.22 Back Property Placement 28

1.23 Methods for Providing Secondary Service 31

1.24 Road Crossing to Feed Secondary Pedestal 40

1.25 Service and Transformer Layout for 75-Lot Subdivision 40

1.26 Primary Cable Layout for 75-Lot Subdivision 42

1.27 Minimum Required Working Space 43

1.28 Sample Easement 47

1.29 Staking Sheet for Service to a Commercial Consumer 49

2.1 Jacketed Concentric Neutral Cable 52

2.2 Bare Concentric Neutral Cable 52

2.3 Medium-Voltage Power Cable with Tape Shield and L.C. Shield 52

2.4 Concentric Lay Strand Options 56

2.5 Standard Strand Arrangements for Multilayer Conductors 56 2.6 Comparative Hot Creep vs. Temperatures for Cable Insulation Materials 60

2.7 General Layout of a Cable Extrusion Line 62

2.8 Typical Extrusion Methods 63

2.9 Capacitive and Dielectric Loss Current Flow in Insulation Shield 66

2.10 Cable Identification Markings 73

(10)

3.1 Symmetrical Current 82

3.2 Asymmetrical Short-Circuit Current 82

3.3 Sample Distribution Circuit with Typical Locations of Sectionalizing

Devices Show 86

3.4 Cross Section of Cable Showing Components Subject to

Through-Fault Damage 88

3.5 Example of 70-Ampere, Type “L” Recloser Curves for Cable Protection 90 3.6 Current Limiting Fuses for Padmounted Switching Cabinets 104 3.7 Inrush Current Resulting from Operation of Three-Phase Recloser 107 3.8 Inrush Current Resulting from Operation of Single-Phase Recloser 107

3.9 Trip Response for Peak-Current-Sensitive Units 108

3.10 Trip Response for 450A and 800A FCIs 109

3.11 Trip-Set Characteristics for Adaptive-Trip FCI 110

3.12 FCI Placement on Overhead Feeder with Underground Segment 111

3.13 FCI Placement on Three-Phase Underground Feeder 111

3.14 FCI Placement for Single-Phase Open Loop 112

3.15 FCI Placement for Underground Subdivision with Three-Phase Source 112

3.16 Current-Reset FCI 113

3.17 Low-Voltage-Reset FCI 114

3.18 High-Voltage-Reset FCI 114

3.19 Time-Reset FCI 115

3.20 Correct Placement of FCI Sensor 116

3.21 Incorrect Placement of FCI Sensor 116

3.22 Reset FCI 117

4.1 Ratio of Shield Loss to Conductor DC Loss at 90°C as a Function of Shield Resistance, 1/C 35-kV Aluminum Power Cables in

Triplexed Formation 124

4.2 Relationship Between Load Factor and Loss Factor Per Unit 125 4.3 Thermal Resistivity vs. Moisture Content for Various Soil Types 127

4.4 Thermal Resistivity of Soil at Various Locations 127

4.5 Effect of Depth on Soil Temperatures as Influenced by Seasonal

Temperature Variations 128

4.6 Trefoil or Triangular Cable Configuration 130

4.7 Flat Conductor Configuration, Maintained Spacing 130

4.8 Direct-Buried Duct Bank Installation Using Rigid Nonmetallic Conduit 132

4.9 Single-Phase U-Guard Installation with Vented Base 136

4.10 Three-Phase Cable Installation Configurations 138, 423

4.11 Typical Dead-Front, Single-Phase, Pad-Mounted Transformer 145

4.12 Actual Load Cycle and Equivalent Load Cycle 147

4.13 Thermal Equivalent Load Cycle 147

4.14 Case Temperature Measurement Location—Pad-Mounted Distribution

Transformer 159

4.15 Relationship Among NEMA Starting Code Letters, Starts per Hour, and

Transformer kVA per Motor HP for Transformer Thermal Considerations 160 4.16 Maximum Motor Starts per Hour for Transformer Mechanical Considerations 162

(11)

5.1 Typical Distribution Transformer Core Form Design and Neutral

Grounding Circuit 169

5.2 Variation of Surge Impedance with Surge Current for Various Values

of 60-Cycle Resistance 171

5.3 Surge Characteristics of Various Ground Rods 171

5.4 Arrester Lead Length for Two Riser Pole Installations 173 5.5 Three-Phase Installation Showing Optimum Riser Pole Arrester

Lead Connections 173

5.6 Typical Primary and Secondary Underground Installation 174 5.7 Schematic Diagram Showing Surge Current Paths After Lightning

Arrester Discharge 175

5.8 Maximum Jacket Voltage (Neutral to Ground) Produced by Lightning

Current Surge in Ground Rod 175

5.9 BCN Cable Riser Pole Installation Surge Arrester Discharge Paths 178

5.10 Ground Rod Being Driven by Hydraulic Tool 180

5.11 Resistance of Vertical Ground Rods as a Function of Length

and Diameter 181

5.12 Resistance of Multiple Ground Rods 182

5.13 Installation of Three Rods for a Riser Pole Ground 183

5.14 Installation of Four Rods for a Riser Pole Ground 183

5.15 Grounding Assembly for Pad-Mounted Single-Phase Transformers 185 5.16 Grounding Grid for Pad-Mounted Equipment Installation 185 5.17 Installation of JCN Connection in Above-Grade Pedestal 186 5.18 Grounding Assembly for JCN Underground Primary Cable 187 5.19 Intermediate Grounding Assembly, Underground Primary Cable 187 5.20 Counterpoise 60-Hz Resistance Variation with Length and Different

Soil Resistivities 188

5.21. Effect of Length on Transient Surge Impedance of Counterpoise 189

5.22 Counterpoise Application to Reduce Jacket Voltage 190

5.23 Earth Resistance 193

5.24 Correct Ground Resistance Test Setup 193

5.25 Incorrect Ground Resistance Test Setup 193

5.26 Clamp-On Ground Resistance Tester 195

5.27 Circuit Diagram for Multigrounded System 195

5.28 Ground Resistance Test Setup for Clamp-On Tester 195

5.29 Setup for Soil Resistivity Test 196

5.30 Effects of Moisture on Soil Resistivity 198

5.31 Effects of Salt Content on Resistivity in Soil Containing

30 Percent Moisture 198

5.32 Coefficient K1for Ground Resistance Calculations 201

5.33 Grouping of Four Ground Rods with 16-Foot Spacing 203

5.34 Grouping of Four Ground Rods with 5-Foot Spacing 203

5.35 Types of Arresters and Their Construction 208

5.36 Comparison of Nonlinear Characteristics of SiC and MOV Valve Elements 209

5.37 Effect of Fast Rise Times on IR Discharge 210

5.38 Series- and Shunt-Gapped MOV Distribution Arresters 210

(12)

5.39 Dead-Front Arrester Elbow Configuration 211

5.40 Dead-Front Surge Arresters 212

5.41 Temporary 60-Hz Overvoltage Capability Curves—Typical MOV

Distribution Arrester 215

5.42 Typical Test Current Waveshape—Sinusoidal Wavefront 217

5.43 Lightning Rise Time to Peak 218

5.44 Arrester Lead Length Equal to Three Feet 219

5.45 Arrester Lead Length Equal to 1.5 Feet 220

5.46 Zero Arrester Lead Length 221

5.47 Representation of Distributed Parameter Distribution Line 222 5.48 Change in Surge Impedance at a Junction Point—Effect on Traveling

Voltage Wave 223

5.49 Traveling Wave Behavior at Junction Points Terminated with Various

Surge Impedances 224

5.50 Traveling Waves at a Cable Open-End Point Terminated by an

MOV Arrester 225

5.51 Arrester Locations 227

5.52 Cable-End Arresters at Open Point 230

5.53 Arrester Upstream from Open Point (Third Arrester) 231

5.54 Two Elbow Arresters and a Feed-Through 231

5.55 Elbow Arrester and Parking Stand Arrester 232

5.56 Bushing Arrester and Parking Stand Arrester 232

5.57 Elbow Arrester on Feed-Through Insert on Transformer Upstream

from Open Point 232

5.58 Bushing Arrester on Transformer Upstream from Open Point 232 5.59 Lateral Tap Cable-End Arrester (Radial Feed Circuit) 232

5.60 Tap-Point Arrester 232

5.61 Typical Underground Subdivision Loop Feed with Open Point 232

6.1 Transformer Connections for Four-Wire Wye and Four-Wire

Delta Services 242

6.2 Series RLC Circuit with Sinusoidal Excitation 243

6.3 Cable-Fed Three-Phase Transformer Susceptible to Ferroresonance 245 6.4 Conductor Spacings for an Overhead Line on an Eight-Foot Crossarm 247 6.5 Equivalent Capacitance Network for an Overhead Multigrounded

Neutral Line 247

6.6 Cross Section of a Multiwire Concentric Neutral Cable 248 6.7 Floating-Wye/Delta Transformer Bank with Fused Cutouts at

Primary Terminals 253

6.8 Three-Phase Cable-Fed Transformer with a Delta-Connected

Primary Winding 255

6.9 Voltage and Current Waveforms During Ferroresonance with

a 150-kVA Delta Grounded-Wye Bank 255

6.10 Five-Legged Wound-Type Core with Grounded-Wye Primary Windings 260 6.11 Three-Phase Cable-Fed Transformer with a Grounded-Wye Primary

Winding on a Five-Legged Core 262

(13)

6.12 Open-Phase Voltage Waveforms with Five-Legged Core

Grounded-Wye Transformers 262

6.13 Overhead System Supplying a Cable-Fed Grounded-Wye

Transformer on a Five-Legged Core 267

6.14 Triplex-Type Wound Core with Grounded-Wye Primary Windings 269 6.15 Cable-Fed Triplex-Core Transformer with Grounded-Wye

Primary Windings 269

6.16 Circuit with “S” Cable Sections and “N” Five-Legged Core

Grounded-Wye Primary Transformers 270

6.17 Circuit Configuration for Switching Example 6.2 271

6.18 Single-Line Diagram of a Portion of a UD System 274

7.1 Dissimilar Metal Effects Between Buried Metals Connected to the

Neutral of an Electric Distribution Line 282

7.2 Electric System Map Shaded to Show Corrosive Soil Locations 283 7.3 Measurement of Potential to a Copper-Copper Sulfate Half Cell 283

7.4 Dissimilar Metal Effects Between Copper and Steel 284

7.5 Dissimilar Soil Effects on Buried Copper Wires 284

7.6 Measurement of Earth Resistivity with a Four-Terminal Ground Tester 284 7.7 Potentials of a Copper-Steel Couple Before and After Connecting

a Zinc Anode 285

7.8 Equivalent Circuit for a Galvanic Anode Connected to the Electric Neutral 287

7.9 Anode Positioning 295

7.10 Anode Connector 295

7.11 Test Station Connector 295

8.1 Typical Trench Warning Tape 301

8.2 Cable Route Marker 302

8.3 Burial Depth Requirements 305

8.4 Joint Trench Use 308

9.1 Typical Duct Configurations 316

9.2 Typical Duct Line and Manhole Arrangement 319

9.3 Typical Arrangements for System in Figure 9.2 319

9.4 Preferred Location of Duct Lines in Roadways 326

9.5 Typical Manhole Configurations 326

9.6 Rectangular Manhole Construction Details 327

9.7 Rectangular Manhole Installation Details 328

9.8 Octagonal Manhole Construction Details 329

9.9 Octagonal Manhole Installation Details 330

9.10 Cable/Conduit Friction and Pulling Tension 333

9.11 Cable Configurations in Conduit 334

9.12 Sidewall Bearing Pressure 336

(14)

10.1 Voltage Stress Concentration 344 10.2 Voltage Stress Distribution in a Typical Premolded Joint Housing 344 10.3 Premolded Permanent Straight Joint for Primary Cables 345

10.4 Jacket Replacement Assembly (Method C) 346

10.5 Premolded Permanent Wye Joint for Primary Cables 347

10.6 Dead-Break Elbow for Primary Cables 348

10.7 Load-Break Elbow for Primary Cables 348

10.8 Typical 200-Ampere Elbow Accessories 349

10.9 Heat-Shrink Jacket Seal at Elbow 349

10.10 Premolded Indoor Termination (Slip-On Stress Cone) for Primary Cables 351 10.11 Premolded Integral Indoor/Outdoor Termination for Primary Cables 351 10.12 Premolded Modular Indoor/Outdoor Termination with Separate Skirts

for Primary Cables 351

10.13 Porcelain Indoor/Outdoor Terminal for Primary Cables 352 10.14 Cold-Shrink Indoor/Outdoor Termination for Primary Cables 352

10.15 Stick-Operable, Dead-Break Elbows 353

10.16 Dead-Break 600-Ampere Elbow Connector and Accessories for

Primary Cables 354

10.17 Housing Assembly Joint for Secondary Cables 355

10.18 Cold-Shrink Joint for Secondary Cables 355

10.19 Heat-Shrink Joint for Secondary Cables 355

10.20 Sealed Stud Termination for Secondary Cables 356

10.21 Bus and Rubber Cover Termination for Secondary Cables 356 10.22 Housing and Sleeve Assembly Termination for Secondary Cables 356

11.1 Test Setup for the Hot Silicone Oil Test 364

11.2 Typical Test Setup for the Stripping Test of the Insulation Shield 365 11.3 Typical High-Voltage Proof Tester Showing a Sectionalized Discharge

Stick for Grounding the Cable 368

A.1 Components Affecting Outage Rate to the Consumer 374

A.2 Sectionalized UD Area 376

B.1 Distance for Various Conductor Arrangements 381

B.2 Permissible Voltage Flicker Limits 386

(15)

F.1 Aluminum Conductor/Thermoplastic Insulation (PE/HMWPE)— Allowable Short Circuit Currents Based on 75°C Initial Conductor

Temperature and 150°C Final Temperature 406

F.2 Copper Conductor/Thermoplastic Insulation (PE/HMWPE)— Allowable Short Circuit Currents Based on 75°C Initial Conductor

Temperature and 150°C Final Temperature 407

F.3 Aluminum Conductor/Thermoset Insulation (TR-XLPE/EPR)— Allowable Short Circuit Currents Based on 90°C Initial Conductor

Temperature and 250°C Final Conductor Temperature 408 F.4 Copper Conductor/Thermoset Insulation (TR-XLPE/EPR)—

Allowable Short Circuit Currents for 90°C Rated Insulation Based on 90°C Initial Conductor Temperature and 250°C Final

Conductor Temperature 409

F.5 Aluminum Conductor/Thermoplastic Insulation (PE/HMWPE)— Allowable Short Circuit Currents for Conductor to Not Exceed Insulation Emergency Operating Temperature Rating Based on 75°C Initial Conductor Temperature and 90°C Final

F.6 Copper Conductor/Thermoplastic Insulation (PE/HMWPE)— Allowable Short Circuit Currents for Conductor to Not Exceed Insulation Emergency Operating Temperature Rating Based on 75°C Initial Conductor Temperature and 90°C Final

F.7 Aluminum Conductor/Thermoset Insulation (TR-XLPE/EPR)— Allowable Short Circuit Currents for Conductor to Not Exceed Insulation Emergency Operating Temperature Rating Based on 90°C Initial Conductor Temperature and 130°C Final

F.8 Copper Conductor/Thermoset Insulation (TR-XLPE/EPR)— Allowable Short Circuit Currents for Conductor to Not Exceed Insulation Emergency Operating Temperature Rating Based on 90°C Initial Conductor Temperature and 130°C Final

(16)

1.1 Lamp and Ballast Characteristics—240 Volts 14

1.2 Front Versus Rear Property Line Placement 17

1.3 Additional Materials for an Open-Loop System 20

1.4 Sample Spare Cable Cost, Single Residential Consumer 22

1.5 Sample Radial System Cost, Commercial Consumer 23

1.6 Additional Cost per Kilowatt, Open-Loop and Spare Cable Systems 23

1.7 Single-Phase Sub-Feeder Cost 24

1.8 Three-Phase Sub-Feeder Cost 25

1.9 25-kV Versus 15-kV Cable and Components 26

1.10 Added Cost of Dual-Voltage Transformers 26

1.11 Voltage Conversion Cost at Year 10 26

1.12 Voltage Conversion Cost at Year 20 27

1.13 Option 1—Direct-Buried Cable 30

1.14 Option 2—PVC Rigid Conduit 30

1.15 Option 3—Cable in HDPE Flexible Conduit 31

1.16 Present Worth of Cable Installation Options 31

1.17 Separate Service Cables 32

1.18 Secondary Pedestal 32

1.19 Sample Cable Loss Analysis 35

1.20 Sample Secondary Cable Data 36

1.21 Savings from Deferred Transformer Energization 37

1.22 Savings from Deferred Transformer Installation 38

2.1 Dimensional Characteristics of Common Conductors

(Standard Concentric-Lay) 53

2.2 Conductor Physical and Electrical Characteristics 54

2.3 Configurations of 4/0 AWG Aluminum Conductor 57

2.4 RUS Insulation Thickness 59

2.5 Insulation Shield Strippability Ratings 66

2.6 Concentric Neutral Configurations for Common Aluminum Cables 67

2.7 Comparison of Jacketing Material Test Data 71

2.8 Static Coefficient of Friction for Jacketing Materials in PVC Conduit 72

3.1 Multiplying Factors to Determine Asymmetrical Fault Currents

Where Symmetrical Fault Currents Are Known 83

3.2 Effective Cross-Sectional Area of Shield 91

3.3 Values of T1, Approximate Shield Operating Temperature, °C, at

Various Conductor Temperatures 92

3.4 Values of T2, Maximum Allowable Shield Transient Temperature, °C 92 3.5 Values of M for the Limiting Condition Where T2= 200°C 92 3.6 Values of M for the Limiting Condition Where T2= 350°C 92 3.7 Approximate Levels of I2t (Amperes2x Seconds) That May Result in

Destructive Transformer Failure for Internal Faults 95 3.8 Approximate Levels of Fault Current Symmetrical (Amperes) That May

Result in Destructive Transformer Failure for Internal Faults 95

(17)

4.1 Ampacities for Single-Phase Primary Underground Distribution Cable—

XLPE, TR-XLPE, and EPR Insulated 123

4.2 Typical Ambient Soil Temperatures at a Depth of 3.5 Feet 128 4.3 Ampacity for 15-kV Copper Conductor, Direct Buried, Single Circuit,

75% and 100% Load Factor 130

4.4 Ampacity Table for 15-kV Aluminum Conductor, Direct Buried, Single

Circuit, 75% and 100% Load Factor 131

4.5 Pros and Cons of Installing Cable Circuits in Conduit 133 4.6. Ampacity Values—15-kV Cable, Trefoil Configuration,

Copper Conductor 135

4.7 Ampacity Values—15-kV Cable, Trefoil Configuration,

Aluminum Conductor 135

4.8 Abstract of ICEA Standards for Maximum Emergency-Load and

Short-Circuit-Load Temperatures for Various Insulations 137 4.9 Correction Factors to Convert from 25°C Ambient Soil Temperature

to 20°C and 30°C 139

4.10 Correction Factors for Various Ambient Air Temperatures 139 4.11 Typical Ampacities for Various Sizes and Types of 600-Volt Secondary

UD Cable—Stranded Aluminum Conductors 143

4.12 Average Temperatures for July and August Averaged for the Previous

10 Years 146

4.13 Daily Peak Loads Per Unit of Nameplate Rating for Self-Cooled

Oil-Immersed Transformers to Give Minimum 20-Year Life Expectancy 148 4.14 Application of Single-Phase Distribution Transformers to Serve

Residential Consumers—Sample Loading Guide 150

4.15 Typical Watts-Per-Square-Foot Factors for Commercial Buildings 153

4.16 Typical Electrical Load Power Factor Values 153

4.17 Typical Electrical Load Demand Diversity Factor Values 154 4.18 Estimated Electrical Demand (Summer) and Energy Consumption

(Sample Family Restaurant) 155

4.19 Estimated Peak Duration 156

4.20 Transformer Loading Capability Table 156

4.21 Typical Three-Phase Pad-Mounted Transformer Capacities—

Short-Term Overload Capabilities (in kVA) 156

4.22 Surface Temperatures Measured at Various Locations on the

Cases of Pad-Mounted Transformers. 159

4.23 Surface Contact Time to Produce Burning 160

4.24 NEMA Starting Code Letters 161

5.1 Surge Withstand Strengths of Polyethylene Insulating Jackets for

15-kV, 25-kV, and 35-kV Class JCN Cable 176

5.2 2007 NESC Ground Rod Requirements for JCN Cable Installations 184 5.3 Spacing of Test Probes for Testing Resistance of a Single Ground Rod 194 5.4 Spacing of Test Probes for Testing Resistance of an Electrode System 194 5.5 Soil Resistivities for Different Soil Types and Geological Formations 197

5.6 Effect of Temperature on Soil Resistivity 198

(18)

5.7 Ground Resistance in Varying Soil Resistivities 204 5.8 Comparison of Protective Characteristics of Heavy-Duty Distribution

Class Silicon Carbide, MOV, and Riser Pole MOV Arresters 209 5.9 Typical Electrical Ratings and Characteristics of Dead-Front

Surge Arresters 213

5.10 Comparison of Standard Requirements for Surge Arrester Classifications 214

5.11 Metal Oxide Surge Arrester Ratings in (kV) rms 215

5.12 Protective Margin, 24.9-kV Underground Distribution System: 125-kV BIL Insulation, 18-kV Arresters at Riser Pole Only,

10-kA Lightning Discharge, Surge Voltage Doubled by Reflection 219 5.13 Protective Margin, 12.47-kV Underground Distribution System:

95-kV BIL Insulation, 9-kV Arresters at Riser Pole Only,

10-kA Lightning Discharge, Surge Voltage Doubled by Reflection 220

5.14 Recommended Arrester Locations 229

5.15 MOV Riser Pole Arrester: Arrester Rating, 10 kV; Equipment BIL,

95 kV; Aged BIL, 76 kV 234

5.16 MOV Riser Pole Arrester and Dead-Front Cable-End Arrester (No. 4):

Arrester Rating, 10 kV; Equipment BIL, 95 kV; Aged BIL, 76 kV 234 5.17 MOV Riser Pole Arrester: Arrester Rating, 21 kV; Equipment BIL,

125 kV; Aged BIL, 100 kV 235

5.18 MOV Riser Pole Arrester and Dead-Front Cable-End Arrester (No. 4):

Arrester Rating, 21 kV; Equipment BIL, 125 kV; Aged BIL, 100 kV 235 5.19 MOV Riser Pole Arrester Plus Dead-Front Cable-End Arrester (No. 4)

and Dead-Front Third Arrester (No. 3): Arrester Rating, 21 kV;

Equipment BIL, 125 kV; Aged BIL, 100 kV 236

5.20 Ground Resistance Testers 237

6.1 Values for Equivalent Capacitances of an Overhead Line with

4/0 ACSR Phase Conductors and a 1/0 ACSR Neutral Conductor 248 6.2 Representative Capacitance and Three-Phase Charging for XLPE

Insulated Cables with 175 Mils Insulation 249

6.3 Representative Capacitance and Three-Phase Charging or XLPE

6.4 Representative Capacitance and Three-Phase Charging for XLPE

6.5 Representative Capacitance and Three-Phase Charging for XLPE

6.6 Phase-to-Ground Capacitance of Three-Phase Grounded-Wye

Capacitor Banks 251

6.7 Maximum Allowed Cable Lengths in 12.47-kV Systems to Limit

Open-Phase Voltages to 1.25 PU 265

6.10 Transformer and Cable Data for the System of Figure 6.17 272

(19)

7.1 Typical DC Potentials in Soil 283

7.2 Suggested DC Potentials for Cathodic Protection 286

7.3 Calculated Resistance and Conductance to Ground of Individual

Ground Rods as Related to Soil Resistivity 288

7.4 Potentials to a Copper-Copper Sulfate Half Cell 289

7.5 Sacrificial Anode Resistance, Output Current, and Estimated Life 290 7.6 Conductance to Ground of BCNs with Effective Diameters as Indicated 291

8.1 Minimum Cover Requirements 304

8.2 Requirements for Random-Lay Joint Trench 309

9.1 Classifications of Plastic Conduit 314

9.2 PVC Duct Dimensions—Minimum Wall Thickness 314

9.3 Comparison of Characteristics for Four-Inch Size PVC Duct 314

9.4 PVC Duct—Impact Strength (Foot-Pounds) 315

9.5 PVC Duct Collapse Pressure (PSI) 318

9.6 Conduit Fill 320

9.7 Conductor Shield Thickness 320

9.8 Insulation Shield Thickness 320

9.9 Concentric Neutral Thickness—Aluminum Cables 320

9.10 Concentric Neutral Thickness—Copper Cables 321

9.11 Secondary Cable Insulation Thickness 321

9.12 220-Mil Primary Cable: Minimum Size of Conduit Necessary to Accommodate Primary Underground Power Cable: 15-kV Cable—

220-Mil Insulation Wall, Concentric Neutral Construction 322 9.13 260-Mil Primary Cable: Minimum Size of Conduit Necessary to

Accommodate Primary Underground Power Cable: 25-kV Cable—

260-Mil Insulation Wall, Concentric Neutral Construction 323 9.14 345-Mil Primary Cable: Minimum Size of Conduit Necessary to

Accommodate Primary Underground Power Cable: 34.5-kV Cable—

345-Mil Insulation Wall 324

9.15 Conduit Fill—Secondary Cable: Minimum Size of Conduit Necessary

to Accommodate 600-Volt Secondary Underground Power Cable 325 9.16 Recommended Dynamic Friction Coefficients for Straight Pulls and

Bends Using Soap/Water or Polymer Lubricants 333

9.17 Inside Bend Radius for 90° Schedule 40 Conduits 335

9.18 Recommended Maximum Sidewall Bearing Pressures 337

9.19 Cable Configuration for Various Jam Ratios 338

9.20 Recommended Maximum Pulling Tension Stress for Pulling Eyes

on Copper and Aluminum Conductors 339

9.21 Recommended Maximum Pulling Tension Limits for Basket-Type

Pulling Grips 339

(20)

10.1 Electrical Rating of Elbows 350 10.2 Relative Corrosion Resistance of Metal Combinations for

Outdoor Terminations 353

11.1 Dimensions for Primary Cables to ICEA Specification S-94-649-2000

with Concentric Neutral (Concentric Stranding) 361 11.2 Dimensions for Primary Cables to ICEA Specification S-94-649-2000

with Concentric Neutral (Compressed Stranding) 362

11.3 Cable Diameter Tolerances 363

11.4 Adders for Extruded Insulation Shield (Mils) to Obtain Nominal

Diameter Over Insulation Shield of Cable 363

11.5 DC Proof-Test Voltages (Conductor to Ground) for Primary Cables 367

11.6 Insulation Thickness of Secondary Cables 369

11.7 Manufacturers’ Voltage Withstand Tests on Completed Cable 371 11.8 Manufacturers’ Voltage Tests on Cables Rated 0 to 600 Volts 371

A.1 Acceptable Outage Hours Per Year Per Consumer 374

B.1 Allowable Voltage Drop on a 120-Volt Base 377

B.2 Resistance of Class B Concentric-Strand Aluminum Cable with Thermosetting and Thermoplastic Insulation for Secondary Distribution Voltages (to 1 kV) at Various Temperatures and

Typical Conditions of Installation 380

B.3 Corrections for Multiconductor Cables 382

B.4 Comparison of Conductor Diameter and Approximate Cable Outside Diameter of Typical Single, Class B Concentric-Strand

Aluminum Cables 382

B.5 60 Hz Reactance of Conductors in the Same Conduit 384

C.1 Nominal Composite Insulation Layer Thickness (Ruggedized) 392

C.2 Nominal Insulation Thickness (Non-Ruggedized) 392

E.1 Extruded Conductor Shield Thickness 400

E.2 Nominal, Minimum, and Maximum Insulation Thickness 400

E.3 Insulation Shield Thickness for Cables with Wire Neutral 401

E.4 Extruded-to-Fill Jacket Thickness 402

(21)

G.1 Configuration No. 1—15-kV Copper 415

G.2 Configuration No. 1—15-kV Aluminum 415

G.9 Configuration No. 2, 3-Inch Type DB Conduit—15-kV Aluminum 417 G.10 Configuration No. 2, 3.5-Inch Type DB Conduit—25-kV Aluminum 417

G.27 Configuration No. 6, 6-Inch Type EB Conduit—15-kV Aluminum 422 G.28 Configuration No. 6, 6-Inch Type EB Conduit—25-kV Aluminum 422

I.1 Cable Installation Equipment Manufacturers (Trenchers, Backhoes, Cable Plow, Guided Boring Tools, Piercing Tools, Hydraulic Pipe Pusher, Track-Mounted Cable Plows, Trench Compactors,

Auger-Type Boring Tools) 427

I.2 Cable Installation Equipment Manufacturers (Primary Circuit Joints,

Elbows, and Terminations; Secondary Circuit Joints and Terminations) 428 I.3 Manufacturers of Joint, Elbow, and Termination Accessories and Kits 429 I.4 Partial Listing of Cable Testing Equipment Suppliers 429

(22)

1.1 Cable Loss Calculations 35

1.2 Calculating Losses on Secondary Cables 36

1.3 Typical Costs Associated with Transformer Losses 37

3.1 Device Rated in Maximum Asymmetrical Current Capacity 83

3.2 Device Rated for Maximum Circuit X/R Ratio 84

3.3 Determine Minimum Shield Size for Known Through-Fault Current 93

4.1 Comparing the Ampacity of Trefoil and Flat-Spaced Configurations 131

4.2 Single-Phase UD Cable Ampacities 140

4.3 Emergency Overload Rating Cable in Protective Riser 141

4.4 Three-Phase Substation Exit Ampacity 141

4.5 Average Daily Temperature Selection for a Summer-Peaking Utility 146 4.6 Selection of Maximum Permissible Transformer Per-Unit Loading 149 4.7 Pad-Mounted Transformer Sizing for New UD Residential Consumers 151

4.8 Sizing Commercial Transformers 157

4.9 Dedicated Transformer Load 160

5.1 No Counterpoise Added (Switches S1, S2, and S3 Open) 191 5.2 Attaching a 100-Foot Counterpoise to the Riser Pole Ground Rod and

the Other End to a Remote, Smaller Resistance (Switch S2 Closed;

S1 and S3 Open) 191

5.3 Continuous or Full-Length Counterpoise (Switches S1 and S3 Closed;

S2 Open) 191

5.4 A Single 8-Foot × 3/4-Inch Ground Rod Driven in Soil with a

Resistivity of 250 Ohm-M 201

5.5 Two 8-Foot × 3/4-Inch Ground Rods Placed 5 Feet Apart 202

5.6 Two Rods Spaced 16 Feet Apart 202

5.7 Group of Four Rods 203

5.8 Increase in Rod Length 204

5.9 Change in Soil Resistivity 204

5.10 The Effect of a Two-Layer Soil with a Top-Layer Resistivity of

250 Ohm-M and a Bottom-Layer Soil Resistivity of 50 Ohm-M 205 5.11 Counterpoise of #2 AWG Conductor Buried 30 Inches Deep for a

Distance of 100 Feet 206

5.12 More Conductive Soil 206

5.13 Counterpoise Burial Depth 206

5.14 Protective Margin Calculation for Riser Pole Application— Industry Standard 4 kA/µs Average Rise Time for Lightning

Strokes Assumed 217

5.15 MOV Riser Pole Arrester: Arrester Rating, 10 kV 234

5.16 MOV Riser Pole Arrester and Dead-Front Cable-End Arrester

(No. 4): Arrester Rating, 10 kV 234

5.17 MOV Riser Pole Arrester: Arrester Rating, 21 kV 235

(23)

5.18 MOV Riser Pole Arrester and Dead-Front Cable-End Arrester

(No. 4): Arrester Rating, 21 kV 235

5.19 MOV Riser Pole Arrester Plus Dead-Front Cable-End Arrester (No. 4)

and Dead-Front Third Arrester (No. 3): Arrester Rating, 21 kV 236

6.1 Maximum Lengths of Cable Circuit Possible 264

6.2 Energizing Multiple-Transformer System with Single-Pole 272

7.1 Measuring Earth Resistivity 284

7.2 Calculating the Neutral Conductance to Ground Per 1,000 Feet of Cable 288

7.3 Determining Required Shift in Potential 289

7.4 Calculating Required Anode Output Current 289

7.5 Selecting Anode Types, Sizes, and Numbers 291

7.6 Estimating Neutral Conductance to Ground of BCN Cable 292

7.7 Determining Required Shift in Neutral Potential 292

7.8 Determining Output Current and Anodes Required 293

11.1 Diameter Calculation 363

B.1 Transformer Voltage Drop Calculation 379

B.2 Secondary Cable Resistance and Reactance 383

B.3 Complete Secondary Voltage Drop Calculation 385

B.4 Voltage Flicker Calculation 387

G.1 Ampacity Reduction for Direct-Buried Versus Conduit Encasement

for Flat-Spaced Installation 417

G.2 Increase in Ampacity for Duct Bank Installation When Type EB

Conduit is Used Versus Schedule 40 422

J.1 Cable Pulling Example 1: Maximum Straight-Pull Distance for Three

25-kV Cables Installed in Five-Inch PVC Conduit 431 J.2 Cable Pulling Example 2: Feasibility of Pulling Three 25-kV Cables

into a Six-Inch PVC Conduit 432

(24)

(25)

Design of an Underground

Distribution System

1

In This Section:

Since their introduction, underground distribution (UD) systems have proved generally popular with electric consumers. Although some of this popu-larity is due to aesthetics—eliminating pole lines and overhead conductors and “ugly” tree trim-ming—greater reliability is the greater attraction. Consumers facing outages due to wildlife, falling tree limbs, and ice storms think underground sys-tems more desirable. Unfortunately, many of the present UD systems are less reliable and have more operational problems than do comparable overhead distribution systems. To reverse this trend, cooperatives must undertake several comprehensive steps:

1. Specify high-quality materials and components, 2. Stipulate every safety provision to ensure

reliability of the system,

3. Design efficient systems that will have the lowest reasonable cost for both installation and operation, and

4. Plan carefully to minimize problems during construction and provide for future opera-tion and replacement of these systems.

This section gives the engineer guidelines for designing a high-quality UD system. Before starting a design, the engineer must have com-prehensive knowledge of the components of a UD system. Next, the engineer must under-stand how these components can be config-ured to form different types of UD systems and the special design concerns of each. During the design process, the engineer must consider the following:

• UD system safety,

• UD system reliability,

• UD system operation and maintenance,

• Future upgrades or replacement,

• The economics of different system configurations, and

• The economics of UD losses.

The final design task is layout of the UD system. On completing this task, the engineer will have a final plan and staking sheets to give to construction crews.

System Components Types of UD Systems Reliability of UD Systems Design Considerations

Future Upgrades and Replacements

Economic Comparison of System Configurations

UD Loss Economics

Steps for Layout of a UD System Summary and Recommendations

(26)

In the past, some UD systems were total underground systems with all components located below ground. Placing trans-formers, sectionalizing devices, and switches below ground re-quires buried vaults. Because water often accumulates in these vaults, the equipment has to be suitable for operation under water. Moisture also

ac-celerates the corrosion of this equipment and leads to premature equipment failure.

This type of system is very difficult to operate and maintain. Maintenance and operation of the equipment usually require a person to enter the underground enclosure. If the enclosure is full of water, the water must be pumped out before

anyone enters. This require-ment increases the time needed to access the equip-ment and, thus, also increases the duration of any outage.

Because of these problems, a total underground system is impractical and unreliable. A more reliable system consists of underground cables and pad-mounted equipment (transformers, sectionalizing devices, and switches). The pad-mounted equipment is placed on the surface instead of below ground. As a result, the equipment is easier to operate and subject to fewer corrosion problems. This type of UD system, with its major system com-ponents, is shown in Figure 1.1.

System

Components

A typical UD system

consists of buried

cables and

pad-mounted

equipment.

Underground Cable, Primary Voltage Underground Cable, Secondary Voltage Dead-Front Surge Arrester Flat Pad Box Pad Ground Line

Ground Electrode Ground Electrode Service Ground

Cable Splice Cable Terminations

Underground Cable Riser

Pad-Mounted Switchgear/ Junction Cabinet Pad-Mounted Transformer Surge Arrester Cable Termination

(27)

UNDERGROUND CABLE

The most extensive component of a UD system is the underground cable. The primary-voltage (15-, 25-, or 35-kV class) cable carries power from a source to the primary bushing of a trans-former. The secondary-voltage (600-Volt class) cable carries power from the secondary bushings of the transformer to the consumer.Section 2, Cable Selection, describes cable construction and gives guidelines for specifying high-quality cable.

PAD-MOUNTED EQUIPMENT

The main types of pad-mounted equipment are transformers, protective devices, and switching devices. Pad-mounted transformers function the same as those overhead. Pad-mounted switchgear usually functions as a combination of switches and sectionalizing devices. For example, a single enclosure can provide switching on the main feed and fusing on two taps off the main feed. Figure 1.2 shows the schematics for several types of switchgear.Section 3, Underground System Sectionalizing, reviews the different types of pad-mounted switchgear. Because of the many

configurations possible, this component pro-vides the engineer with many options in the design of a UD system.

CABLE TERMINATIONS AND JOINTS

Cable terminations and joints are other impor-tant components of a UD system. The joints pro-vide a way to connect two underground cables. The terminations provide a way to connect underground cables to transformer bushings, switches, fuses, and other devices.Section 10 describes the different types of terminations and how to use them on a UD system.

SURGE ARRESTERS AND GROUNDING ELECTRODES

Surge arresters are used to protect underground systems from overvoltages induced by lightning and other transients. To operate effectively, ar-resters must be properly connected to the cable grounding system. The grounding system must have ground electrodes that are in optimum contact with the soil. Examples of ground electrodes are:

FIGURE 1.2: Schematics for Different Types of Switchgear. Adapted from S&C Electric Company, 2005.

kV Ampere, RMS Short-Circuit

Fuse Mini-Rupter Load Max Cont. Dropping

14.4 17.0 95 200 600 600 350 25 27 125 200 600 400 540

Nom. Max BIL

MVA 3-Phase Sym. at Rated Voltage PME-4 COMPARTMENT-2 COMPARTMENT-1 COMPARTMENT-2 COMPARTMENT-1 COMPARTMENT-3 COMPARTMENT-2 COMPARTMENT-4 COMPARTMENT-1 COMPARTMENT-3 COMPARTMENT-2 COMPARTMENT-4 COMPARTMENT-1 COMPARTMENT-3 COMPARTMENT-2 COMPARTMENT-4 COMPARTMENT-1 COMPARTMENT-3 COMPARTMENT-2 COMPARTMENT-4 COMPARTMENT-1 COMPARTMENT-3 COMPARTMENT-2 COMPARTMENT-4 COMPARTMENT-1

PME-9 PME-10 PME-11 PME-12

(28)

• Driven ground rods,

• Buried counterpoise wires,

• Semiconducting jacketed cables, and

• Metallic water or sewer systems.

Figure 1.1 shows driven ground rods as the ground electrodes. Detailed information on cable grounding systems and surge protection is contained inSection 5.

EQUIPMENT MOUNTINGS

Equipment mountings provide a flat, rigid sur-face for supporting pad-mounted equipment. It is very important to mount the

bottom edge of pad-mounted equipment flush to the flat sur-face of the supporting pad. Doing so prevents persons from poking a wire or other object into the interior com-partment of pad-mounted equipment and meets the

requirements of American National Standards Institute/Institute of Electrical and Electronics Engineers (ANSI/IEEE) C57.12.28 (Standard for Pad-Mounted Equipment-Enclosure Integrity) and ANSI/IEEE37.74 (Standard Requirements for Subsurface, Vault and Pad-Mounted Load Interrupter Switchgear and Fused Load-Inter-rupter Switchgear for Alternating Current Sys-tems Up to 38 kV). The former code has become a standard for specifying tamper-resis-tant pad-mounted equipment enclosures. This tamper-resistant design helps prevent vandalism to utility equipment and protect the public from contact with energized parts.

The equipment must also attach securely to the mounting surface to prevent it from being moved or tipped over by people, animals, lawn mowers, or vehicles. Secure attachment is partic-ularly important when polyethylene pads are used. The pad’s slick surface makes it easy for an unsecured piece of equipment to slide.

Another important factor in a stable installa-tion is proper soil compacinstalla-tion beneath the pad. Without proper compaction, the soil will settle and erode, leaving the pad with little support. When this happens, pads can tilt or warp (if made of polyethylene) and expose the interior

compartments of transformers, fuse cabinets, or switchgear. If the settling is severe, the pad may not support all the equipment weight. If some of the equipment weight is transferred to the attached cables, this settling can damage transformer bushings, connectors, and switch terminals.

Types of Equipment Mountings

The most basic type of equipment mounting is a flat, or monolithic, pad. The flat pad provides a uniform surface for mounting equipment and has openings for cable access into the equipment

en-closure as shown in Figure 1.3. Because this pad is placed directly on the ground, there is limited space for cable train-ing and cable terminations. However, this type of pad is usually adequate for single-phase pad-mounted transform-ers and small single-phase sectionalizing devices.

Some types of cable installations require more space than is available with a flat pad. For ex-ample, large-diameter cables are stiffer and have a larger minimum bending radius than do small-diameter cables. Thus, the large-small-diameter cables require more space for cable training. Another consideration is cold weather. Low temperatures make cables stiffer and more difficult to install or operate. Providing additional cable space helps minimize these problems. Therefore, co-operatives in areas with extended periods of cold weather may prefer using a ground sleeve (“basement”) or a box pad instead of a flat pad. A ground sleeve or box pad also provides the extra space needed for large-diameter cables.

Typical installation of a ground sleeve is shown in Figure 1.4. The ground sleeve is in-stalled below the ground surface, with the equipment mounting surface elevated two to three inches above final grade. This type of mounting provides additional space for cables below grade, but is suitable for equipment with only one entry compartment such as three-phase pad-mounted transformers and junction cabinets. Ground sleeves are generally limited in their ability to support heavier pieces of equipment.

The soil beneath

the pad must be

well compacted.

(29)

The third type of mounting is a box pad (see Figure 1.5). The box pad is placed in the ground rather than on the sur-face, with typically three to six inches exposed above grade. A perimeter lip supports the pad-mounted equipment. The remaining space is open and

FIGURE 1.3: Flat Pad for Equipment Mounting.

FIGURE 1.4: Ground Sleeve. Source: Nordic

Fiberglass Inc., Warren, Minn., 2002.

FIGURE 1.5: Box Pad for Equipment Mounting.

provides plenty of room to work with the ca-bles. This type of pad is ideal for supporting pad-mounted switchgear that has multiple cable entry compartments.

Pad Materials

Manufacturers offer a varied selection of pad materials, including the following:

• Steel-reinforced concrete,

• Fiberglass-reinforced concrete,

• Fiberglass, and

• Polyethylene.

Because these materials have very different properties, the engineer must carefully select the material type suitable for the intended ap-plication. The material and pad design must have the strength required to support the equipment weight. This is of particular con-cern with box pads, because all the equipment weight is supported by the outside pad walls,

and is especially important, for example, when box pads are used for transformers 500-kilovolt amperes (kVA) and larger. Care must be ex-ercised in making sure the box pad manufacturer clearly states the strength rating of the box pad walls.

Pad material must

be suitable for the

intended application.

(30)

Also of concern are polyethylene pads with wooden braces. A puncture through the poly-ethylene allows water to enter the pad and rot the wooden braces. When the wooden braces rot, part of the pad strength is lost, and war-page results.

A second property to review is the performance of the material outdoors where it is exposed to frost and ultraviolet radiation. The pad materials must not break down or crack from ultraviolet exposure or frigid conditions. Cracks or material breakdown lead to a loss of mechanical strength.

A final property to review is pad buoyancy. Some of the polyethylene pads tend to float and can overturn pad-mounted equipment. There-fore, these pads would not be suitable for use in areas that are subject to flooding.

In summary, pads must be of a design that will have long-term durability under adverse conditions, meet system operating needs, and maintain equipment security. All these factors must be balanced when selecting a pad design for a particular UD system.

Types of UD

Systems

SUBSTATION CIRCUIT EXITS

Underground cable is often used for substation circuit exits from distribution substations. Under-ground circuit exits help reduce congestion on poles just outside a substation, making the area around a substation more attractive and work-able. As an added benefit, underground substa-tion circuit exits are protected from ice loading, wildlife contacts, and vehicle damage, and, thus, may be more reliable than overhead exits.

In most cases, each underground substation circuit exit will terminate on a riser pole and feed overhead circuit conductors. Therefore, this type of UD system consists of underground pri-mary-voltage cable, cable terminations, surge ar-resters, and grounding electrodes. The conduit, cable terminations, surge arresters, grounding electrodes, and disconnect switches are commonly referred to as a riser assembly. See Figure 1.6.

When designing underground substation circuit exits, the engineer must be particularly concerned with reliability. If the underground cable fails, the circuit outage interrupts power to many consumers. Placing the cable in a conduit system or concrete-encased duct bank helps protect it from mechan-ical damage.Section 9contains information on duct bank installations. Another way to improve reliability is to install a spare

cable or provide backup capa-bility from another source. Al-though spare cables or backup options do not change the risk of cable failure, they do reduce the power restoration time if only one cable is damaged.

A special concern for un-derground circuit exits is cable ampacity. These cables carry large loads and may operate close to their ampacity rating. Therefore, the engineer must

Design concerns for

substation circuit exits

are reliability, system

growth, and ampacity.

FIGURE 1.6: Underground Substation Circuit Exit. Disconnect Switches Surge Arrester Cable Termination Neutral Riser Vent Undergroung Circuit Exit Cable Ground Electrode

(31)

carefully determine the cable operating condi-tions, system growth, and the resulting ampacity.

MAIN FEEDERS

Underground cable can serve as a distribution main feeder. A main feeder is that portion of a distribution circuit between the substation and the first in-line overcurrent protective device. The protective device in the substation clears a fault on a main feeder. Therefore, a main feeder fault causes an outage to the entire circuit. Be-cause most faults on an underground main feeder are cable failures and are permanent, power to the circuit may remain off until the cable is repaired. The utility engineer must con-sider this characteristic when designing a main feeder, particularly when deciding between a ra-dial or open-loop feeder.

The engineer must also determine the maxi-mum load to be carried by the main feeder in order to select a cable with adequate ampacity

and choose the 200-ampere or 600-ampere class of cable terminations.Section 4provides de-tailed information on cable ampacity, and Sec-tion 10provides information on the types of cable terminations.

Radial Main Feeder

The radial main feeder has one source and de-livers power to a load area along a single path. This feeder can also serve several load areas by using a junction box or sectionalizing switch with fused taps. This type of arrangement is shown in Figure 1.7 and may have the following components:

• Underground primary-voltage cable,

• Cable terminations,

• Pad-mounted junction box or sectional-izing switch,

• Surge arresters, and

• Grounding electrodes.

FIGURE 1.7: Radial Main Feeder.

Junction Box or Sectionalizing Switch Junction Box or Sectionalizing Switch Substation Junction Box or Switching Cabinet

Primary Voltage Cable

To Load Area To Load Area To Load Area

(32)

The junction box or sectionalizing switch provides sectionalizing of the load areas and limited sectionalizing of the main feeder. For ex-ample, consider a fault in the second line sec-tion as shown in Figure 1.8. This fault trips the protective device at the substation and interrupts power to all consumers on the faulted circuit. The cooperative can restore power to the first load area by placing the faulted cable(s) in a parking stand, or by opening the load-side switch on the first sectionalizing switch to isolate the faulted cable. Figure 1.8 shows this option. Because the radial feeder has no alternative source or path, the cooperative cannot restore power to the other consumers until crews repair the cable fault.

It is possible to improve the reliability of a radial system by installing the cable in a con-crete-encased duct bank or in a conduit system, or by installing a spare cable or conduit in the trench. A concrete-encased duct bank provides

substantial mechanical protection from dig-ins and should be considered in areas congested with other underground utilities. A conduit sys-tem provides limited mechanical protection. However, it does decrease outage time by allow-ing the cooperative to replace a section of faulted cable without disturbing the earth sur-face. This saves substantial time, particularly when the main feeder is located beneath a road-way. The spare cable or conduit provides no mechanical protection but does decrease restoration time if only one cable is faulted. Be-cause the costs of these installation methods vary significantly, each cooperative must weigh the advantages of these more expensive installa-tions against their costs.

Under any circumstance, the simple radial does have limited operational flexibility and should not be used to serve a large number of consumers. Information on comparative system reliability may be found inAppendix A.

FIGURE 1.8: Radial Main Feeder with Faulted Cable Section.

Junction Box or Sectionalizing Switch Power Off Substation Open Load-Side Switch Fault Power Off Power On Open Power On

(33)

Open-Loop Feeder

In dense load areas, an underground main feeder may tie together two substations. A main feeder may also tie two circuits from the same substation. This type of arrangement would operate as an open-loop system. The components of this system are the same as those of a radial system. However, the open-loop feeder has two sources, unlike the radial feeder that has only one source. Each source provides power along a single path to the desig-nated open point in a junction box or a section-alizing switch. In a junction box, the open point results from placing one set of cables in a parking

stand. In a sectionalizing switch, leaving one of the switches open creates an open point.

The open-loop feeder (see Figure 1.9) provides much higher system availability than does the radial system. With an open-loop system, utility crews can isolate a faulted cable section and re-store power to all consumers. A cable fault in the second line section interrupts power to all consumers on that circuit. After isolating the faulted cable section, as shown in Figure 1.10, crews can feed the first section from Substation No. 1 and remaining line sections from Substation No. 2. Because crews can restore power to all

FIGURE 1.9: Open-Loop Feeder.

Substation No. 2

N.O.

Sectionalizing Switch N.O. = Normally Open Point

Looped-Primary Circuit Substation No. 1 Three-Phase, Pad-Mounted Transformer

FIGURE 1.10: Open-Loop Feeder with Faulted Cable Section.

Substation No. 2

N.O. N.O.

Fault

Sectionalizing Switch N.O. = Normally Open Point

Looped-Primary Circuit Substation No. 1 Three-Phase, Pad-Mounted Transformer

(34)

load areas before repairing the cable fault, the outage time is much shorter than with a radial feeder. As a result, it is not critical to install the cable in a concrete-encased duct bank or conduit. However, as already noted, in areas congested with underground utilities, the concrete-encased duct bank will help protect cables from dig-ins. Again, it is important to judge the benefits of in-stalling duct bank or conduit against the addi-tional cost. An open-loop feeder also requires that the designer consider the ampacity of the feeder cables while serving all possible loop segments, which may dictate the use of a larger cable size than otherwise needed.

Open-loop feeders provide much more oper-ating flexibility than do simple radial feeders. System reliability considerations generally dictate open-loop feeders as the preferred design.

SUB-FEEDERS

The more common underground feeder is the sub-feeder, also called a load area feeder. This type of feeder has at least one stage of sectionalizing between it and the protective device at the sub-station. As a result, a fault on a sub-feeder does not interrupt power to the

en-tire circuit and, thus, affects fewer consumers than does a similar fault on a main feeder.

The two types of feeders also have different functions. The basic function of a main feeder is to deliver power to load area feeders. The main function of a sub-feeder is to

deliver power to consumers. Therefore, sections of cable on a sub-feeder often terminate in pad-mounted transformers. The sub-feeder can have several configurations ranging from a simple ra-dial feeder to a complex multiloop feeder.

Radial Feeder

The simplest type of load area feeder is a radial feeder. The radial feeder is usually the most practical way to serve a single consumer. How-ever, a single consumer with critical loads, such as a hospital or police station, often requires a more reliable system. Methods for improving re-liability include the following:

• Changing to an open-loop configuration,

• Adding a spare cable or conduit to the trench, and

• Placing the cable in a conduit or duct bank. The radial feeder can be extended to serve multiple consumers as shown in Figure 1.11. However, a cable fault interrupts power to all consumers beyond the fault location. For exam-ple, a fault between transformers T1 and T2

re-sults in a power outage to transformers T2 through T5. The power remains off until the cable is repaired. As the number of consumers increases, it becomes more practical to consider an open-loop system. The subsectionEconomic Comparison of System Con-figurations, which comes later

A cable fault on a

sub-feeder affects

fewer consumers than

does a similar fault

on a main feeder.

FIGURE 1.11: Radial Feeder.

T1 T2 T3 T4 T5

Fault

Power On Power Off

Single-Phase, Pad-Mounted Transformers

(35)

in this section, provides information on the economics of radial versus open-loop systems.

Open-Loop Feeder

As mentioned earlier, the open-loop feeder has two sources and, therefore, provides better system availability. Large subdivisions or com-mercial shopping areas are ideal applications of loop systems. Figure 1.12 shows an open-loop feeder in a shopping center. Utility crews can isolate any section of faulted cable and restore power to all transformers. This feature makes the open-loop feeder a preferred design

for UD systems serving multiple or critical consumers. An open-loop feeder also requires that the designer consider the ampacity of the primary cables and devices while serving all possible loop segments, which may dictate the use of a larger cable size than otherwise needed.

Multiple-Loop Feeder

In heavy load areas, multiple-loop feeders are necessary to improve sectionalizing and to allow the coordination of overcurrent protective devices. A typical multiple-loop system is shown in Fig-ure 1.13. This type of system usually has a

sub-FIGURE 1.12: Open-Loop Feeder in Shopping Center.

Three-Phase Feeder

Normally Open Point

Three-Phase, Pad-Mounted Transformers

Riser Pole Riser Pole

FIGURE 1.13: Multiple-Loop System.

Riser Pole Riser Pole

Sectionalizing Switch Sectionalizing Switch N.O. N.O. N.O. N.O. Legend

N.O. Normally Open Point Single-Phase, Pad-Mounted Transformer

N.O. Three-Phase, Pad-Mounted Transformer

(36)

feeder that serves as an open-loop system be-tween two sources. The sectionalizing switches on the sub-feeder have fused taps that serve other open-loop feeders. This arrangement provides excellent system availability. It also speeds up fault location because the large load area has been sectionalized into small load groups. A multiple-loop feeder also requires that the designer consider the ampacity of the feeder cables and devices while serving all possible loop segments, which may dictate the use of a larger cable size than otherwise needed.

TRANSFORMER AND SECONDARY SYSTEMS

Pad-mounted transformers and underground secondary-voltage cable constitute the final seg-ment of a UD system. To properly design this part of the system, the engineer must first select the appropriate equipment rating and cable ampacity.Section 4provides information for making these selections.

Second, the engineer must consider reliability. Most secondary cable faults are the result of me-chanical damage to the cable. Utilities can mini-mize mechanical damage by following the prop-er installation techniques described inSection 9 and by specifying cable with an abrasion-resistant

Ground Electrode Cable Riser Pole Lighting Package Underground Secondary-Voltage Cable

FIGURE 1.14: Area Lighting System.

or self-healing insulating jacket (seeSection 2). Cable dig-ins by other utilities or consumers also damage cable. To minimize dig-ins by con-sumers, cable should be installed two to three feet off the property line. Doing so helps pre-vent cable damage if the consumer installs a fence on the property line. Another method for minimizing dig-in damage is to use conduit. The conduit offers some mechanical protection, par-ticularly from hand digging. As noted, the coop-erative may particularly want to use conduit in areas congested with other utilities.

A third design concern with secondary systems is voltage drop and voltage flicker. The engineer must design a system that provides the consumer with acceptable voltage levels throughout the day and during motor starting.Appendix Blists the acceptable voltage levels and gives methods for calculating voltage drop and flicker.

STREET AND AREA LIGHTING

Public safety and consumer convenience require street and area lighting in the area served by a large percentage of underground projects. Most cooperatives furnish this service, so the engineer must make accommodations in underground sys-tems to include it. The engineer needs to devel-op a plan at the start of the project for eventual (if not actual) street and area lighting. Conduits and pedestals can then be installed at strategic locations that will minimize future trenching in lawns or around consumer facilities.

This type of UD system is shown in Figure 1.14. It uses a combination of overhead components (poles and a lighting package) and underground components (underground secondary-voltage cable, surge arresters, and grounding electrodes).

Street and area lights are generally self-con-tained units with an integral photoelectric cell for control. These standard light packages usually operate from 120-Volts single phase or 120/240-Volts single phase. The cooperative may want to consider using the same lighting package that it uses in overhead areas. Doing so will avoid unnecessary duplication of stock and minimize confusion during installation and maintenance.

If the lighting package requires a 120-Volt, two-wire power supply, service may be pro-vided through a two-wire duplex underground