2012 TECHNICAL REPORT

Electric Power Research Institute 3420 Hillview Avenue, Palo Alto, California 94304-1338 PO Box 10412, Palo Alto, California 94303-0813 USA

800.313.3774 650.855.2121 askepri@epri.com www.epri.com

Aging Power Cable Maintenance Guideline Medium-Voltage Cables, 5 to 35 kV

EPRI Project Manager R. Chambers

ELECTRIC POWER RESEARCH INSTITUTE 3420 Hillview Avenue, Palo Alto, California 94304-1338 PO Box 10412, Palo Alto, California 94303-0813 USA

800.313.3774 650.855.2121 askepri@epri.com www.epri.com

Aging Power Cable Maintenance Guideline Medium-Voltage Cables, 5 to 35 kV 1024044

Final Report, November 2012

DISCLAIMER OF WARRANTIES AND LIMITATION OF LIABILITIES THIS DOCUMENT WAS PREPARED BY THE ORGANIZATION NAMED BELOW AS AN ACCOUNT OF WORK SPONSORED OR COSPONSORED BY THE ELECTRIC POWER RESEARCH INSTITUTE, INC. (EPRI). NEITHER EPRI, ANY MEMBER OF EPRI, ANY COSPONSOR, THE ORGANIZATION(S) BELOW, NOR ANY PERSON ACTING ON BEHALF OF ANY OF THEM:

(A) MAKES ANY WARRANTY OR REPRESENTATION WHATSOEVER, EXPRESS OR IMPLIED, (I) WITH RESPECT TO THE USE OF ANY INFORMATION, APPARATUS, METHOD, PROCESS, OR SIMILAR ITEM DISCLOSED IN THIS DOCUMENT, INCLUDING MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE, OR (II) THAT SUCH USE DOES NOT INFRINGE ON OR INTERFERE WITH PRIVATELY OWNED RIGHTS, INCLUDING ANY PARTY'S INTELLECTUAL PROPERTY, OR (III) THAT THIS DOCUMENT IS SUITABLE TO ANY PARTICULAR USER'S CIRCUMSTANCE; OR

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REFERENCE HEREIN TO ANY SPECIFIC COMMERCIAL PRODUCT, PROCESS, OR SERVICE BY ITS TRADE NAME, TRADEMARK, MANUFACTURER, OR OTHERWISE, DOES NOT NECESSARILY CONSTITUTE OR IMPLY ITS ENDORSEMENT, RECOMMENDATION, OR FAVORING BY EPRI.

THE FOLLOWING ORGANIZATION PREPARED THIS REPORT:

Electric Power Research Institute (EPRI)

NOTE For further information about EPRI, call the EPRI Customer Assistance Center at 800.313.3774 or e-mail askepri@epri.com.

Electric Power Research Institute, EPRI, and TOGETHERSHAPING THE FUTURE OF ELECTRICITY are registered service marks of the Electric Power Research Institute, Inc.

Copyright 2012 Electric Power Research Institute, Inc. All rights reserved.

This publication is a corporate document that should be cited in the literature in the following manner:

Aging Power Cable Maintenance Guideline: Medium-Voltage Cables, 5 to 35 kV. EPRI, Palo Alto, CA: 2012. 1024044.

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ACKNOWLEDGMENTS

The following organization prepared this report:

Electric Power Research Institute (EPRI) 1300 West W. T. Harris Blvd. Charlotte, NC 28262

Principal Investigators A. Mantey G. Toman

This report describes research that was prepared for and managed by G. Toman and A. Mantey of EPRI and that was documented in the report, Medium-Voltage Cable Aging Management Guide, Revision 1 (1021070). The report was edited by R. Chambers to remove material not applicable to fossil-fueled plants.

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PRODUCT DESCRIPTION

Medium-voltage cables (5- to 35-kV rated cables) have provided reasonable service in power plants. However, there is a concern that cables that have experienced long periods of wet service might degrade and fail in service. Because most plants have had few problems with medium-voltage cables, little on-staff experience with medium-voltage cables exists at most sites. This report provides information that will be of practical use when questions concerning medium-voltage cable longevity in adverse environments and service conditions arise at fossil-fired plants.

Objectives This report is meant for persons on plant staffs who are responsible for cable system maintenance, operation, and design. This report describes the currently available information concerning the types of cables in service in plants and the best way to address aging and monitoring concerns.

Approach This report is unique in that it focuses on the cable types used in the power industry and the conditions that challenge them. The report supports the needs of plant staffs, allowing them to understand the specifics of their cable systems without being confused by the much broader range of cable information associated with power distribution systems that do not apply to the plants.

The report was developed by consultants with extensive experience in cable design, manufacturing, installation, troubleshooting, and replacement. They were guided by industry task group meetings that helped maintain focus on the issues facing plant cable personnel. This revision provides additional information and formalizes the report.

Results The report provides information on cable system design, cable construction, insulation systems and their aging characteristics, condition assessment, cable installation, and preparation for repair and replacement. The report provides practical information to help choose the correct assessment tests to apply and to help understand critical issues about the selection and installation of cables and terminations that affect cable longevity.

Applications, Value, and Use This report provides a wide range of information that can affect the aging of cables and the management of that aging. Information is provided on grounding system designs that, if not understood, could lead to the misapplication of cable types or increase the likelihood of a potentially devastating phase-to-phase fault that could cause failures of transformers and buses

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in addition to the cable. Guidance is provided on the selection of an appropriate test method, depending on the cable design, and how the cable is expected to degrade with time. Effects of aging of the insulation and metallic shield system must be considered to allow appropriate selection of the test method.

Keywords Aging Cable insulation assessment Cable systems Cross-linked polyethylene (XLPE) Ethylene-propylene rubber (EPR) Medium-voltage cable

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CONTENTS

1 INTRODUCTION .................................................................................................................... 1-1 1.1 History and Background of Polymer-Insulated Cables ................................................ 1-2

1.2 In-Plant Cables ............................................................................................................ 1-2

1.3 Underground Cables .................................................................................................... 1-3

1.4 Abbreviations and Acronyms ....................................................................................... 1-4

1.5 Terminology ................................................................................................................. 1-5

2 UNDERSTANDING THE DESIGN OF POWER PLANT CABLE SYSTEMS ........................ 2-1 2.1 Shielded and Nonshielded Cables ............................................................................... 2-1

2.2 Grounding Systems, Protection, and Alarms ............................................................... 2-2

2.2.1 Grounding Systems ............................................................................................. 2-2

2.2.2 Phase-to-Phase Faults ........................................................................................ 2-4

2.2.3 Number of Grounds on a Cable Insulation Shield ............................................... 2-4

2.3 Multiple Cables per Phase and Balanced Magnetic Fields .......................................... 2-6

2.3.1 Configuration ....................................................................................................... 2-7

2.3.2 Eddy Currents ..................................................................................................... 2-8

2.3.3 Circulating Currents ............................................................................................ 2-8

2.4 Protective Relay and Annunciation Alarm Systems ..................................................... 2-9

3 UNDERSTANDING THE PHYSICAL CONDITION OF THE SYSTEM .................................. 3-1 3.1 Conditions in Manholes and Ducts .............................................................................. 3-1

3.1.1 Insulation Deterioration ....................................................................................... 3-1

3.1.2 Pumping and Dryness ......................................................................................... 3-3

3.2 Correcting Adverse Conditions .................................................................................... 3-4

3.2.1 Adverse Dry Conditions ...................................................................................... 3-4

3.2.2 Physical Stress .................................................................................................... 3-4

3.2.3 Vertical Support ................................................................................................... 3-5

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3.2.4 Adverse Environments ........................................................................................ 3-5

3.2.4.1 Temperature-Related Aging .......................................................................... 3-5

3.2.4.2 Radiation-Related Aging ............................................................................... 3-5

3.2.4.3 High Conductor Temperature from Ohmic Heating ....................................... 3-6

3.2.4.4 High-Resistance Connections ....................................................................... 3-6

3.2.5 Surface Corona and Partial Discharge ................................................................ 3-6

4 CABLE DESIGNS .................................................................................................................. 4-1 4.1 Cable Design Summary ............................................................................................... 4-1

4.2 Medium-Voltage Cable Constructions ......................................................................... 4-1

4.2.1 Voltage Rating..................................................................................................... 4-4

4.2.2 Conductors .......................................................................................................... 4-5

4.2.3 Conductor Shield ................................................................................................. 4-6

4.2.4 Insulation ............................................................................................................. 4-6

4.2.5 Insulation Shield .................................................................................................. 4-8

4.2.5.1 Semiconducting or High-Permittivity Shield Layer ........................................ 4-8

4.2.5.2 Metallic Shield Layer ..................................................................................... 4-9

4.2.6 Nonshielded Cables ............................................................................................ 4-9

4.2.7 Jacket ................................................................................................................ 4-10

5 SPLICING AND TERMINATING ............................................................................................ 5-1 5.1 Cable Splicing and Terminating Theory ....................................................................... 5-1

5.2 Gradients ..................................................................................................................... 5-1

5.2.1 Electric Fields ...................................................................................................... 5-1

5.2.2 Stress Cones....................................................................................................... 5-3

5.2.3 Voltage Gradient Design ..................................................................................... 5-4

5.3 Splices for Shielded Cables ......................................................................................... 5-5

5.3.1 Cable Preparation for Splices and Terminations ................................................. 5-5

5.3.2 Hand-Taped Splices ............................................................................................ 5-8

5.3.3 Premolded Splices ............................................................................................ 5-10

5.3.4 Cold-Shrink Splices ........................................................................................... 5-12

5.3.5 Heat-Shrink Splices ........................................................................................... 5-13

5.4 Terminations .............................................................................................................. 5-16

5.5 Lugs and Connectors ................................................................................................. 5-18

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5.6 Installation Considerations ......................................................................................... 5-18

5.6.1 Connecting the Conductors ............................................................................... 5-18

5.6.2 Insulation for Splices ......................................................................................... 5-19

5.6.3 Semiconducting Insulation Shield Materials for Splices and Terminations ....... 5-19

5.6.4 Metallic Insulation Shield for Splices and Terminations .................................... 5-19

5.6.5 Jackets for Splices ............................................................................................ 5-20

5.7 Selection of Splices and Terminations ....................................................................... 5-20

6 FUNDAMENTALS OF CABLE INSULATION SYSTEMS ...................................................... 6-1 6.1 Primary Insulations ...................................................................................................... 6-1

6.2 Elastomer Basics ......................................................................................................... 6-1

6.2.1 Cross Linking ...................................................................................................... 6-2

6.2.2 Fillers Used in Rubber Insulations ...................................................................... 6-2

6.2.3 Crystallinity .......................................................................................................... 6-3

6.2.4 Cable Conductor and Insulation Shields ............................................................. 6-3

6.3 Butyl Rubber ................................................................................................................ 6-4

6.3.1 Material Description ............................................................................................ 6-4

6.3.2 Butyl Wire and Cable Insulation .......................................................................... 6-4

6.3.3 Fillers and Other Additives .................................................................................. 6-5

6.4 Ethylene-Propylene Rubber ......................................................................................... 6-5

6.4.1 Material Description ............................................................................................ 6-5

6.4.2 Ethylene-Propylene Rubber Cross Linking ......................................................... 6-6

6.4.3 Fillers for Ethylene-Propylene Rubber Insulation ................................................ 6-7

6.4.4 Compounding (Mixing) of Ethylene-Propylene Rubbers ..................................... 6-7

6.4.5 Shielded Cable Constructions for Medium-Voltage Ethylene-Propylene Rubber Cables for Plant Applications ................................................................. 6-8

6.5 Historical Review of Medium-Voltage Cable Constructions ......................................... 6-9

6.5.1 General ............................................................................................................... 6-9

6.5.2 Ethylene-Propylene Rubber Types ................................................................... 6-10

6.5.2.1 Black Ethylene-Propylene Rubber .............................................................. 6-11

6.5.2.2 Pink Ethylene-Propylene Rubber ................................................................ 6-12

6.5.2.3 Brown Ethylene-Propylene Rubber ............................................................. 6-14

6.6 Cross-Linked Polyethylene ........................................................................................ 6-14

6.6.1 Material Description .......................................................................................... 6-14

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7 AGING AND DEGRADATION OF BUTYL, ETHYLENE-PROPYLENE RUBBER, AND CROSS-LINKED POLYETHYLENE CABLES DUE TO ADVERSE ENVIRONMENTS ........... 7-1

7.1 Aging and Degradation of Butyl Rubber ...................................................................... 7-1

7.1.1 General ............................................................................................................... 7-1

7.1.2 Water-Related Degradation of Butyl Rubber ....................................................... 7-1

7.1.3 Thermal Degradation of Butyl Rubber ................................................................. 7-2

7.1.4 Radiation Degradation of Butyl Rubber ............................................................... 7-2

7.1.5 Conclusions......................................................................................................... 7-3

7.2 Aging and Degradation of Ethylene-Propylene Rubber ............................................... 7-3

7.2.1 General ............................................................................................................... 7-3

7.2.2 Water-Related Degradation of Ethylene-Propylene Rubber ............................... 7-4

7.2.3 Thermal Degradation of Ethylene-Propylene Rubber ......................................... 7-6

7.2.4 Radiation Degradation of Ethylene-Propylene Rubber ....................................... 7-9

7.2.5 Conclusions......................................................................................................... 7-9

7.3 Aging and Degradation of Cross-Linked Polyethylene ................................................ 7-9

7.3.1 General ............................................................................................................... 7-9

7.3.2 Water-Related Degradation of Cross-Linked Polyethylene ............................... 7-10

7.3.3 Thermal Degradation of Cross-Linked Polyethylene ......................................... 7-11

7.3.4 Radiation Degradation of Cross-Linked Polyethylene ....................................... 7-11

7.3.5 Conclusions....................................................................................................... 7-11

7.4 Other Degradation Causes ........................................................................................ 7-11

7.4.1 General ............................................................................................................. 7-11

7.4.2 Corona Discharge ............................................................................................. 7-11

7.4.3 Partial Discharge ............................................................................................... 7-12

8 TESTING: MANUFACTURING, INSTALLATION, AND MAINTENANCE OR IN-SERVICE TESTS ....................................................................................................................... 8-1

8.1 Introduction .................................................................................................................. 8-1

8.2 Purpose of Tests .......................................................................................................... 8-1

8.3 Manufacturing Tests .................................................................................................... 8-2

8.3.1 Standards and Test Methods .............................................................................. 8-2

8.3.1.1 Tests of Special Interest ................................................................................ 8-5

8.3.1.2 Final Electrical Tests for Shielded Cables ..................................................... 8-7

8.3.1.3 Final Electrical Tests for Nonshielded Cables, 20015000 Volts Without Metallic Sheath or Armor ................................................................ 8-10

8.3.1.4 Qualification Tests for 5 kV35 kV Shielded Cables ................................... 8-10

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8.4 Acceptance Tests ...................................................................................................... 8-12

8.5 Installation Tests ........................................................................................................ 8-12

8.6 Maintenance and In-Service Testing ......................................................................... 8-13

8.6.1 Introduction ....................................................................................................... 8-13

8.6.1.1 Historical Perspective on In-Service Testing ............................................... 8-14

8.6.1.2 Withstand Versus Diagnostic Testing .......................................................... 8-15

8.6.1.3 Global Versus Local Assessment................................................................ 8-15

8.6.1.4 60 Hz Versus Other Frequencies ................................................................ 8-15

8.6.2 Off-Line Diagnostic and Withstand Testing ....................................................... 8-16

8.6.2.1 Dissipation Factor (Tan ) Testing .............................................................. 8-16 8.6.2.2 Dielectric Spectroscopy ............................................................................... 8-19

8.6.2.3 Off-Line Partial Discharge Measurement .................................................... 8-21

8.6.2.4 AC Withstand Testing.................................................................................. 8-25

8.6.2.5 Tests Under Development ........................................................................... 8-27

8.6.3 On-Line Diagnostics Assessment ..................................................................... 8-27

8.7 Applicability of Tests .................................................................................................. 8-31

9 CABLE AGING MANAGEMENT PROCESS ......................................................................... 9-1 9.1 Strategies and Philosophies ........................................................................................ 9-1

9.1.1 Test Strategies for Shielded Cable Circuits ........................................................ 9-1

9.1.1.1 Run to Failure ................................................................................................ 9-2

9.1.1.2 Diagnostic Testing ......................................................................................... 9-2

9.1.1.3 Withstand Testing.......................................................................................... 9-3

9.1.2 Nonshielded Cable Circuits ................................................................................. 9-4

9.2 Prioritization of Cables for Assessment and Testing ................................................... 9-5

9.2.1 Risk Ranking Methodology ................................................................................. 9-6

9.2.1.1 Maintenance Rule and Criticality Screening .................................................. 9-6

9.2.1.2 Insulation Type .............................................................................................. 9-7

9.2.1.3 Significance Factor for Jacket Types or Water-Impervious Designs ............. 9-7

9.2.1.4 Significance Factor for Operating Experience ............................................... 9-7

9.2.1.5 Significance Factor for Diagnostic Test Results ............................................ 9-7

9.2.1.6 Significance Factor for Voltage and Insulating Level .................................... 9-7

9.2.1.7 Significance Factor for Operating Conditions ................................................ 9-7

9.2.1.8 Weighting Factor for Adverse Environment ................................................... 9-8

9.2.1.9 Weighting Factor for Current and Amperage Level ....................................... 9-8

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10 RESPONSE TO CABLE FAILURES .................................................................................. 10-1 10.1 Electrical Tests of Circuit ....................................................................................... 10-1

10.1.1 Logic for Testing to Be Performed ................................................................. 10-1

10.1.2 Logistical Issues for Electrical Testing ........................................................... 10-2

10.2 Fault Location ........................................................................................................ 10-2

10.3 Forensics and Failure Assessments ..................................................................... 10-3

10.4 Repair Options ...................................................................................................... 10-6

10.4.1 Replacement of Failed Section ...................................................................... 10-6

10.4.2 Total Replacement ......................................................................................... 10-7

10.4.3 Acceptance Testing ....................................................................................... 10-7

11 REFERENCES ................................................................................................................... 11-1

A MEDIUM-VOLTAGE CABLE FAILURES AND FIELD EXPERIENCE ................................. A-1 A.1 Introduction ................................................................................................................. A-1

A.2 Dry Condition Failures ................................................................................................ A-1

A.2.1 Failure of a Cable at the Point of a Shield Crimp ............................................... A-1

A.2.2 Thermal Deterioration of a Butyl Rubber Insulated Cable .................................. A-3

A.2.3 Failure of a Brown Ethylene-Propylene Rubber Insulated Cable Due to Failure of a Zinc Shield Tape ............................................................................. A-5

A.3 Water-Related Failures ............................................................................................... A-8

A.3.1 Failure of a 38-Year-Old Butyl Rubber Cable Due to Water-Induced Degradation........................................................................................................ A-8

A.3.2 Failure of a Wet Okonite Black Ethylene-Propylene Rubber Cable ................. A-16

A.3.3 Failure of a Cross-Linked Polyethylene Insulated Cable from Localized Water Treeing .................................................................................................. A-17

A.4 Failures of Terminations and Splices ........................................................................ A-22

A.4.1 Failure from Use of an Oversized Termination Sleeve .................................... A-22

A.5 Summary of Lessons Learned .................................................................................. A-24

A.5.1 Dry Cable Compression-Related Event ........................................................... A-24

A.5.2 Overheated Butyl Rubber Failure ..................................................................... A-24

A.5.3 Failure of Continuity of a Zinc Tape Shield ...................................................... A-24

A.5.4 Water-Related Failure of a Butyl Rubber Cable ............................................... A-25

A.5.5 Failure of an Ethylene-Propylene Rubber Insulated Cable from Long-Term Wetting ............................................................................................................. A-26

A.5.6 Failure of Cross-Linked Polyethylene Insulation from Water Treeing .............. A-26

A.5.7 Failure from Use of an Oversized Molded Termination .................................... A-26

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B RESULTS OF NUCLEAR ENERGY INSTITUTE SURVEY AND NUCLEAR REGULATORY COMMISSION GENERIC LETTER 2007-01 .................................................. B-1

B.1 Nuclear Energy Institute Underground Medium-Voltage Cable Survey ...................... B-1

B.1.1 Survey Purpose.................................................................................................. B-1

B.1.2 Survey Scope ..................................................................................................... B-1

B.2 Survey Results Evaluation .......................................................................................... B-2

B.2.1 Contributors........................................................................................................ B-2

B.2.2 Underground Circuit Quantities .......................................................................... B-2

B.2.3 Installed Cable Types ......................................................................................... B-2

B.2.4 Shielding ............................................................................................................ B-5

B.2.4.1 Underground Wet-Duty Failure Assessment ............................................... B-5

B.3 Results of Utility Responses to Generic Letter 2007-01 ........................................... B-13

B.3.1 Summary of Results ......................................................................................... B-14

B.3.2 Assessment Methods ....................................................................................... B-16

B.4 Comparison of the Nuclear Energy Institute Survey and Generic Letter 2007-01 Results ....................................................................................................... B-17

C TAN DATA FOR ETHYLENE-PROPYLENE RUBBER AND BUTYL RUBBER INSULATED CABLES .............................................................................................................. C-1

C.1 60-Hz Tan Data from 1972 Black Okoguard Insulation ........................................... C-1 C.2 Very-Low-Frequency Tan Results for Okonite Black Ethylene-Propylene

Rubber Insulation ....................................................................................................... C-4

C.3 Tan for Anaconda Pink Ethylene-Propylene Rubber UniShield ............................... C-6 C.4 Anaconda Black UniShield Ethylene-Propylene Rubber .......................................... C-11

C.5 Tan Results for Butyl Rubber Insulation ................................................................ C-12 C.6 Conclusions Related to Rubber Insulated Cables and Tan Results ...................... C-16 C.7 Effects of Mixed Shielded and Nonshielded Segments on Tan ............................. C-17

D ADDITIONAL POLYMER MATERIALS INFORMATION ..................................................... D-1 D.1 Fundamentals of Elastomers ...................................................................................... D-1

D.2 Cross Linking .............................................................................................................. D-2

D.3 Butyl Rubber Compositions ........................................................................................ D-3

D.4 Ethylene-Propylene Rubber and Ethylene-Propylene-Diene Monomer ...................... D-4

D.5 Dicumyl Peroxide Cross-Linking Agent Byproducts .................................................... D-4

D.6 Ethylene-Propylene Rubber Formulations .................................................................. D-5

D.7 Influence of Clay on Properties of Ethylene-Propylene Rubber .................................. D-7

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E OFF-LINE TESTS THAT ARE UNDER DEVELOPMENT .................................................... E-1 E.1 Introduction ................................................................................................................. E-1

E.2 Isothermal Return Current .......................................................................................... E-1

E.3 Return Voltage ............................................................................................................ E-2

E.4 Oscillating Wave ......................................................................................................... E-3

F INSULATION RESISTANCE TEST MEASUREMENTS: THEIR VALUE AND LIMITATIONS ............................................................................................................................ F-1

F.1 Introduction .................................................................................................................. F-1

F.2 Determining Minimum Insulation Resistance Value and Improving Interpretation of Results .................................................................................................................... F-1

F.3 Insulation Resistance of Good Insulation..................................................................... F-3

F.4 Conclusions ................................................................................................................. F-4

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LIST OF FIGURES

Figure 2-1 Shielded Cable Components .................................................................................... 2-2 Figure 2-2 Single-Point Grounding of a Shield, Showing Voltage of Shield to Ground as

a Function of Distance ....................................................................................................... 2-5 Figure 2-3 Shield Losses and Voltages for Single-Conductor Cables ....................................... 2-5 Figure 2-4 Acceptable Configurations: Two Cables per Phase ................................................. 2-7 Figure 2-5 Acceptable Configurations: Three Cables per Phase ............................................... 2-7 Figure 2-6 Acceptable Configurations: Four Cables per Phase ................................................. 2-8 Figure 3-1 Electrostatic Flux Lines and Charged Particles ........................................................ 3-3 Figure 3-2 Example of Manhole Drainage System .................................................................... 3-4 Figure 3-3 White Powder Indicates a Location of Corona Discharge Between a Cable

and a Ground Cable in Close Proximity ............................................................................. 3-7 Figure 4-1 Shielded, Single-Conductor, Medium-Voltage Cable Design ................................... 4-2 Figure 4-2 Medium-Voltage Shielded Cable, Water-Impervious Design.................................... 4-3 Figure 4-3 Medium-Voltage Nonshielded Cable Design ............................................................ 4-3 Figure 4-4 UniShield Construction, Shield Wires Embedded in the Semiconducting

Outer Jacket ....................................................................................................................... 4-4 Figure 4-5 Conductor Stranding Configuration, Showing Compressed and Compacted

Conductor Configurations .................................................................................................. 4-5 Figure 5-1 Equipotential and Flux Lines in a Cable ................................................................... 5-2 Figure 5-2 Electrical Stress Fields, Shield Removed ................................................................. 5-2 Figure 5-3 Termination of an Insulation Shield with a Stress Cone ........................................... 5-3 Figure 5-4 Stress Relief with High Dielectric Constant or High Resistivity Materials ................. 5-4 Figure 5-5 Projection from Semiconducting Layer and Cut into Insulation ................................ 5-5 Figure 5-6 Two Types of Semiconducting Layer Scoring or Stripping Tools ............................. 5-6 Figure 5-7 Measure Carefully to Achieve the Proper Length of Cable ...................................... 5-7 Figure 5-8 Slip Applicable Sleeves over One or Both Cable Ends Before Installing

Connector ........................................................................................................................... 5-7 Figure 5-9 Connectors of Several Lengths and Diameters for the Same Conductor Size ......... 5-7 Figure 5-10 Shear-Bolt Connectors for Copper Conductor ........................................................ 5-7 Figure 5-11 Clean the Insulation, Using an Approved Solvent .................................................. 5-8 Figure 5-12 Semiconducting Tape Is Applied over the Connector to Form a Smooth

Interface ............................................................................................................................. 5-8 Figure 5-13 Insulating Tape Is Applied Until the Proper Thickness Is Achieved over

the Connector ..................................................................................................................... 5-9

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Figure 5-14 Another Layer of Semiconducting Tape Is Applied over the Insulating Tape ......... 5-9 Figure 5-15 A Metallic Braid Is Installed, a Ground Strap Is Attached, and the Jacket

Tape Is Installed over the Entire Splice Area ................................................................... 5-10 Figure 5-16 Cutaway of a Premolded Splice ........................................................................... 5-11 Figure 5-17 Cold-Shrink Splice, Showing the Direction of the Shrinking Process ................... 5-12 Figure 5-18 Carefully Position the Housing Before Removing the White Core Support .......... 5-12 Figure 5-19 Continue Removing the Core While Holding Its Position on the Cable ................ 5-13 Figure 5-20 High-Permittivity Mastic Material Is Placed over the Connector and

Conductor, Without Concern for Smoothness .................................................................. 5-14 Figure 5-21 The Semiconducting Tube Is Slid into Place and Heated Until Properly

Shrunk Down to the Cable ............................................................................................... 5-14 Figure 5-22 An Insulation Tube Is Slid into Place and Shrunk Down ...................................... 5-14 Figure 5-23 A Tube That Is Both Insulating on the Inside and Semiconducting on the

Outside Is Positioned and Shrunk into Place ................................................................... 5-14 Figure 5-24 Tinned Copper Braid is Wrapped Around the Splice to Replace the Metallic

Portion of the Insulation Shield ........................................................................................ 5-14 Figure 5-25 A Ground Strap Spring Is Placed Under One Side of the Cables Taped

Metallic Insulation Shields ................................................................................................ 5-15 Figure 5-26 The Ground Strap Is Placed Across the Splice and Connected to the Factory

Metallic Shield on the Opposite Side of the Splice ........................................................... 5-15 Figure 5-27 An Overall Rejacketing Tube Is Placed Around the Entire Area .......................... 5-15 Figure 5-28 Cold-Shrink Outdoor Termination with Rain Sheds or Skirts ................................ 5-16 Figure 5-29 Dead Break, Premolded Termination ................................................................... 5-17 Figure 5-30 Stub-Type Motor Connection ................................................................................ 5-17 Figure 5-31 In-Line Type Connection ...................................................................................... 5-17 Figure 5-32 Improperly Crimped Connector ............................................................................ 5-19 Figure 6-1 Butyl Rubber Molecule ............................................................................................. 6-4 Figure 6-2 Copolymer of Ethylene and Propylene ..................................................................... 6-6 Figure 6-3 Banbury Mixer Used for Preparing Ethylene-Propylene Rubber Compounds .......... 6-8 Figure 6-4 Various Representations of Crystallinity in Polyethylene ....................................... 6-14 Figure 7-1 Ethylene-Propylene Rubber Cable Failures as Function of Color of the

Ethylene-Propylene Rubber Insulations ............................................................................. 7-5 Figure 7-2 Generational Differences in Life Expectancy for Ethylene-Propylene Rubber

and Other Insulations ......................................................................................................... 7-6 Figure 7-3 AC Breakdown Strength of Medium-Voltage, Field-Aged Ethylene-Propylene

Rubber Insulated Cables at V0 ........................................................................................... 7-8 Figure 7-4 AC Voltage Breakdown Strength of Combined Ethylene-Propylene Rubber

Cables Aged in the Laboratory and in Service at Rated Voltage ....................................... 7-8 Figure 7-5 Unfilled and Mineral-Filled Cross-Linked Polyethylene Cable Failures

Compared to All Failures .................................................................................................. 7-10 Figure 8-1 Wafer Examination for Voids, Inclusions, and Conductor Shield Protrusions .......... 8-6 Figure 8-2 Hot Oil Test ............................................................................................................... 8-6

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Figure 8-3 Vertical Tray Flame Test .......................................................................................... 8-7 Figure 8-4 An Acceptable Partial Discharge Plot ....................................................................... 8-8 Figure 8-5 An Unacceptable Partial Discharge Plot ................................................................... 8-8 Figure 8-6 Partial Discharge Requirement History from AEIC 5 ................................................ 8-9 Figure 8-7 Dry Specimen Design Qualification Tests .............................................................. 8-10 Figure 8-8 Wet Specimen Design Qualification Tests ............................................................. 8-11 Figure 8-9 Derivation of Dissipation Factor (Tan ) Measurement in Insulation ...................... 8-16 Figure 8-10 Typical Variable-Frequency, Very-Low-Frequency Portable Test Equipment

for Performing 0.1-Hz Dissipation Factor (Tan ) Testing ................................................ 8-17 Figure 8-11 Voltage Dependence of Dissipation Factor for New and Aged Cross-Linked

Polyethylene Cable .......................................................................................................... 8-18 Figure 8-12 0.1-Hz Dissipation Factor of Cross-Linked Polyethylene-Insulated Cables .......... 8-18 Figure 8-13 Dielectric Spectroscopy Measurements for Shelf-Aged 15-kV Cross-Linked

Polyethylene Cable .......................................................................................................... 8-20 Figure 8-14 Calibration Equipment for Off-Line Partial Discharge Testing .............................. 8-21 Figure 8-15 Test Setup for Performing 60-Hz Partial Discharge Measurements ..................... 8-22 Figure 8-16 Test Profile for Short Duration Off-Line 60-Hz Partial Discharge Testing ............. 8-23 Figure 8-17 60-Hz Partial Discharge One-Step Diagnostic Data Capture Approach ............... 8-23 Figure 8-18 Nominal Very-Low-Frequency (0.1-Hz) Sinusoidal Waveform ............................. 8-25 Figure 8-19 Trapezoidal (Bipolar Rectangular) Very-Low-Frequency (0.1-Hz) Waveform ...... 8-26 Figure 8-20 Sensors for On-Line Signal Detection and Data Acquisition System ................... 8-29 Figure 8-21 On-Line Condition Assessment Data Acquisition System .................................... 8-29 Figure 8-22 Influence of Defect Type on Signal Patterns Detected During On-Line

Evaluation ........................................................................................................................ 8-30

Figure A-1 External Cable Condition at the Location of the Fault ............................................. A-2 Figure A-2 Condition of the Shield After the Jacket and Burn Hole Were Removed from

the Fault ............................................................................................................................ A-2 Figure A-3 Damage to the Conductor from the Fault ................................................................ A-3 Figure A-4 Inner Surface of the Insulation with Severe Indentation from the

Semiconducting Tape ....................................................................................................... A-4 Figure A-5 Zinc Shield .............................................................................................................. A-6 Figure A-6 Tracking Path and Failure Location on Interior of Jacket ........................................ A-6 Figure A-7 Tracking Path and Failure Site on Insulation .......................................................... A-7 Figure A-8 Damage to Conductor from Fault ............................................................................ A-7 Figure A-9 Transformer Connection of One of the Cables ....................................................... A-8 Figure A-10 Circuit Breaker Connection of One of the Cables ................................................. A-9 Figure A-11 Cable Configuration in the Manhole Adjacent to the Transformer ........................ A-9 Figure A-12 Location of Failed, Faulted, and At-Risk Cables Within Duct Leading

to Building ....................................................................................................................... A-10 Figure A-13 Duct Sealing and Water Leaking from Failed Conductor B-5 ............................. A-11

xviii

Figure A-14 Cables A-5, B-5, and C-5 .................................................................................... A-12 Figure A-15 Close-Up of B-5 Cable Failure ............................................................................ A-12 Figure A-16 Comparison of Conductor Corrosion ................................................................... A-13 Figure A-17 Marked Low-Resistance Channel in the Insulation with Breakdown Hole

in Center .......................................................................................................................... A-13 Figure A-18 Micrograph of Insulation Wall at Low-Resistance Channel, Showing

Swelling and Fissures ..................................................................................................... A-14 Figure A-19 Temporary, Aboveground Cable System ............................................................ A-15 Figure A-20 Permanent Aerial Structure Routing on Second Unit .......................................... A-15 Figure A-21 Damage to the Copper Shield at the Damage Site ............................................. A-18 Figure A-22 Cross Section of Insulation at the Fault Tube, Showing Carbonization of the

Wall of the Fault Tube ..................................................................................................... A-19 Figure A-23 Large Water Tree Viewed Through Hot Oil Bath ................................................ A-20 Figure A-24 Cross Section of the Water Tree Shown in Figure A-23 ..................................... A-20 Figure A-25 Embedded Particle at Base of Water Tree Shown in Figures A-23 and A-24 ..... A-21 Figure A-26 As-Found Condition of Terminations ................................................................... A-22 Figure A-27 Burnthrough of Stress Relief Adaptor ................................................................. A-23 Figure A-28 Burned-Through Insulation Found After Removal of the Stress Relief

Adaptor ............................................................................................................................ A-23 Figure B-1 Manufacturers of 5-kV Cable .................................................................................. B-3 Figure B-2 Age Distribution of Units with No Failures ............................................................... B-6 Figure B-3 Age Distribution of Wet Cable Failures for All Insulation Types .............................. B-7 Figure B-4 Age Distribution of Wet Cross-Linked Polyethylene Cable Failures ....................... B-8 Figure B-5 Age Distribution of Wet Ethylene-Propylene Rubber Cable Failures ...................... B-9 Figure B-6 Age Distribution of Butyl Rubber Cable Failures ................................................... B-10 Figure B-7 Age of Wet Cable at Failure Versus Year of Failure ............................................. B-11 Figure B-8 Age at Time of Failure for Wet Ethylene-Propylene Rubber Cables ..................... B-12 Figure B-9 Age at Time of Failure for Wet Cross-Linked Polyethylene Cables ...................... B-13 Figure B-10 Inaccessible Cable Failures by Plant, from Responses to Generic Letter

2007-01 ........................................................................................................................... B-14 Figure B-11 Number of Failures of Wet Cable by Year .......................................................... B-15 Figure B-12 Wet Medium-Voltage Cable Failures Versus Age at Time of Failure .................. B-15 Figure B-13 Age of Cable at Time of Failure By Insulation and Design .................................. B-16 Figure C-1 AC Breakdown Strength Versus 60-Hz Tan Results............................................ C-2 Figure C-2 60-Hz Tan Results by Test Voltage ..................................................................... C-4 Figure C-3 Very-Low-Frequency Tan Result for a 5-kV Okonite Black Ethylene-

Propylene Rubber Cable ................................................................................................... C-5 Figure C-4 Very-Low-Frequency Tan Results for 15-kV Okonite Black Ethylene-

Propylene Rubber Cable ................................................................................................... C-6 Figure C-5 60-Hz Tan for Anaconda UniShield Pink Ethylene-Propylene Rubber ................ C-7

xix

Figure C-6 Laboratory-Measured Very-Low-Frequency Tan for Anaconda Pink Ethylene-Propylene Rubber UniShield .............................................................................. C-7

Figure C-7 Tan Results for Anaconda Pink UniShield Cables (Ordered by 8-kV result) ....... C-8 Figure C-8 Tan Results for Anaconda Pink UniShield Cables (Ordered by Difference in

16 kV and 8kV Results) ..................................................................................................... C-9 Figure C-9 Tan Results for Anaconda Pink UniShield Cables (Showing Moderate to

High Results) ................................................................................................................... C-10 Figure C-10 Tan Measurements for Anaconda UniShield Cables Taken One Year

Apart ................................................................................................................................ C-11 Figure C-11 Tan Measurements for Anaconda UniShield with Black Insulation .................. C-12 Figure C-12 Tan Versus Breakdown Strength for Okonex Butyl Rubber ............................. C-13 Figure C-13 Comparison of 60-Hz Tan Versus Breakdown Strength of Okonite Butyl

Rubber to Black Ethylene-Propylene Rubber ................................................................. C-14 Figure C-14 AC Breakdown Voltage (in V/mil) for Okonite Butyl and Black Ethylene-

Propylene Rubber Insulations ......................................................................................... C-15 Figure C-15 Tan Results for Mixed Shielded and Nonshielded Black Okonite Ethylene-

Propylene Rubber Circuits .............................................................................................. C-17 Figure C-16 Tan Results for Mixed Shielded and Nonshielded Black Okonite Ethylene-

Propylene Rubber Circuits .............................................................................................. C-18 Figure C-17 Average Tan Value Versus Percent of Nonshielded Cable Section ................ C-19 Figure D-1 A Cross-Linked Elastomer ...................................................................................... D-2 Figure D-2 Ethylene-Propylene-Diene Monomer Terpolymer ................................................... D-4 Figure E-1 Isothermal Return Current Plots for Cross-Linked Polyethylene Cable Having

Undergone Different Degrees of Aging ............................................................................. E-1

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LIST OF TABLES

Table 6-1 Typical Components in Medium-Voltage Ethylene-Propylene Rubber Compounds ...................................................................................................................... 6-13

Table 6-2 Percentage of Nuclear Plants with Cross-Linked Polyethylene Insulated Cables by Cable Voltage Rating ...................................................................................... 6-16

Table 8-1 History of Specifications and Testing of Shielded and Nonshielded Medium-Voltage Cables ................................................................................................................... 8-3

Table 8-2 Common Production Tests ........................................................................................ 8-4 Table 8-3 Dielectric Constant and Dissipation Factor Acceptance Criteria................................ 8-5 Table 8-4 IEEE Standard 400 Criteria for Assessment for Cross-Linked Polyethylene

Insulated Cables .............................................................................................................. 8-17 Table 8-5 Matrix of Applicability of Tests During the Life of a Medium-Voltage Cable ............ 8-31

Table B-1 Originally Installed 5-kV Insulation Types ................................................................ B-3 Table B-2 Distribution of 8-kV Cable Insulation Materials ......................................................... B-4 Table B-3 Distribution of 15-kV Cable Insulation Materials ....................................................... B-4 Table B-4 Distribution of 25-kV to 35-kV Cable Insulation Materials ........................................ B-4 Table B-5 Number of Failures per Plant Reporting Failures ..................................................... B-7 Table B-6 Summary of Assessment and Testing Responses from Generic Letter

2007-01 ........................................................................................................................... B-17 Table D-1 Butyl Rubber Insulation Components ....................................................................... D-3 Table D-2 Butyl Rubber Wire Insulation Components .............................................................. D-3 Table D-3 Typical Black Wire and Cable Insulation Ethylene-Propylene Rubber

Compound Components from the 1970s .......................................................................... D-5 Table D-4 Properties of Black Ethylene-Propylene Rubber Compound Shown in

Table D-3 .......................................................................................................................... D-5 Table D-5 Medium-Voltage Black Ethylene-Propylene Rubber Insulation Using Ethylene-

Propylene-Diene Monomer ............................................................................................... D-6 Table D-6 Compounds of EPR Using Calcined and Coated Clay ............................................ D-7 Table D-7 Influence of Clay Nature on Ethylene-Propylene Rubber Cable Properties:

Calcined Versus Coated Clay ........................................................................................... D-7

1-1

1 INTRODUCTION

This report is based on the research performed for the Nuclear Sector and its membership. GenMAC acknowledges their work as comprising the content of this product and its applicability to the Generation Sector. It is our intent to partner where ever possible on this important research to avoid duplication of work or seek to overload limited resources providing this critical information.

The medium-voltage cable system (cable rated from 5 kV to 35 kV, for the purpose of this report) is composed of cable, terminations, splices, trays, conduits, ducts, and, in some cases, trenches. Vertical support systems, manholes, fire stops, and water drainage systems also affect the cable. All of these play a role in the longevity of the cable system.

This report provides a wide range of information in support of medium-voltage cable aging management. Information is provided on topics such as electrical system grounding practices and operational issues because they affect failure mechanisms, especially in the final stages of failure. For example, some safety-related distribution systems are ungrounded or have high-resistance grounds so that, during accident conditions, they could sustain a phase-to-ground fault and continue to operate for some period of time. However, under normal operating conditions, a cable with a suspected or identified ground fault should be de-energized as soon as possible to prevent the fault from converting to a phase-to-phase fault, which is much more damaging. A phase-to-ground fault in an ungrounded or high-resistance ground system is likely to generate heat or increase voltage stress across the adjacent phase insulation so that deterioration of the insulation of a second phase in a relatively short period of time is possible. After the fault converts to a phase-to-phase fault, extremely high currents result, causing severe damage to the cable and connected equipment. Understanding the need for de-energizing cables having a phase-to-ground fault, even though the system is designed to withstand the condition for a short period, is important to limiting damage and stress to the remainder of the system.

Topics included in this report related to cable aging management include the following:

The grounding system design, which determines the way in which ground currents and transient voltages affect cables under fault conditions and affects operating practices at the time of faults

Types of cables and cable designs, including nonshielded and shielded, and the effects of different cable designs on aging and testability

The physical layout of the cable and the various service conditions and stresses imparted to the cable beyond operating voltage and current

Cable design, aging mechanisms, and failure mechanisms Splice and termination issues that can lead to early failure

Introduction

1-2

Cable insulation materials and changes and improvements that have been made to them since the 1970s

Cable condition assessment tests This report is an update of the Electric Power Research Institute (EPRI) report Medium Voltage Cable Aging Management Guide (1016689), which was published in 2008 [1]. This update reflects the consensus guidance on aging management of medium-voltage power cables developed in 20092010, as well as other developments since the initial report was published.

Due to industry attention to submergence of cable, much of the report concerns aging of medium-voltage cables under wet energized conditions. However, cables and terminations can fail under dry conditions if defects exist in the insulation, if damage or installation errors occur, or if the cable is affected by an adverse local environment. Appendix A provides insights into some of the conditions that have resulted in failures under both wet and dry conditions.

1.1 History and Background of Polymer-Insulated Cables In the early 1960s, high-molecular-weight polyethylene, cross-linked polyethylene (XLPE), and various synthetic rubber insulations (black ethylene-propylene rubber [EPR] and black butyl systems [2, 3]) began to be deployed in medium-voltage distribution networks. Blodgett and Fisher described the development of one such system and provided a brief review of the status of the full range of alternative polymers [2].

With the exception of high-molecular-weight polyethylene, these materials were used in generating stations in the 1960s. A key feature of these materials was their expected enhanced moisture stability compared with legacy insulations. As a consequence, most distribution utilities adopted high-molecular-weight polyethylene or XLPE due to their low dielectric losses and costs, and then routed such circuits with little concern for moisture. Although these cables have provided good service under dry conditions, none of these materials lived up to their promised moisture stability, and the early designs were superseded by more robust systems.

Some utilities followed the distribution industrys lead and installed XLPE insulated cables. However, most chose to use rubber insulated cables due to their greater flexibility and tighter bending radius, which is important in the tighter confines of a power block. Rubber cables were also expected to be long lived, and they have generally proven to be longer lived than XLPE cables. Over the years, numerous improvements to the design, materials, compounding of insulations, and manufacture of cables have occurred. For example, black EPR was replaced by brown, red, or pink EPR; high-molecular-weight polyethylene was supplanted by XLPE; and XLPE was replaced by tree-retardant XLPE (TR-XLPE). A history of these changes is provided in Section 6, Fundamentals of Cable Insulation Systems.

1.2 In-Plant Cables In-plant cables are generally located in dry conduits or trays. Dry cables tend to have a long life. When failures occur, they are generally related to an installation error, a manufacturing flaw, or physical or thermal damage. Even under such conditions, failure can take years to decades to occur. Thermal damage can occur from an external source such as an adjacent, uninsulated hot pipe or from ohmic heating on highly loaded cables, especially if there is a significant magnetic or resistive imbalance in the phase cables.

Introduction

1-3

1.3 Underground Cables Underground routing of power cables has long been desirable for a variety of reasons: the physical protection it provides; the low ambient temperature presented to the cable and the resultant high ampacities; the large natural heat sink provided by the soil and its favorable impact on transient loading capabilities; and the invisibility of the network once installed. Unfortunately, in most parts of the country, installation of cables underground exposes them to some degree of wetting or submergence. The extent and duration of that submergence are influenced by many variables. In systems provided with adequate and well-maintained drainage, short-term submergence consistent with post-storm runoff does little more than wet the surface of the cable, given the slow diffusion of moisture through common jacketing systems. In contrast, direct-burial installations or poorly sloped and drained raceways will ultimately expose the cables to extended submergence, which provides time for moisture to diffuse through most common polymeric jackets.

The earliest applications in the underground network of solid dielectric cables using natural rubber insulation were plagued with moisture-induced problems. The resultant degradation assumed two forms: electrical instability and physical instability. Electrical instability in water was characterized by ever-increasing leakage current versus time, with the risk of localized thermal runaway and failure of the circuit. Physical instability of the insulation system was characterized by leaching of compound constituents or significant swelling when exposed to water. Both modes of physical instability led to the development of large-scale voids and eventual failure.

The moisture-related failures had the greatest impact on distribution utilities, because their networks consist of numerous, long underground medium-voltage feeders serving a number of shorter low-voltage circuits. In contrast, underground cable systems typically consist of only a few long medium-voltage circuits. As a consequence, the bulk of the research to date has been directed toward understanding and resolving the moisture-driven issues that were of particular interest to the distribution market. The conclusions drawn from distribution-related research are not always directly transferable to the power industry. Assessing the applicability of distribution research for power applications remains a challenge for both the user and regulatory communities. Some of these issues are addressed here. This report provides a significant amount of information on rubber insulations that are used in conventional and nuclear plants but, until recently, were not often used in distribution systems. The report also provides some insights on design differences between plant cables and those used in distribution systems.

Water-related degradation became a concern to the nuclear industry in 2006 and to the United States Nuclear Regulatory Commission (NRC), as indicated by Generic Letter 2007-01, Inaccessible or Underground Power Cable Failures that Disable Accident Mitigation Systems or Cause Plant Transients [4]. Actions taken include implementation of routine inspections of manholes for submergence of cable.

The EPRI report Aging Management Program Guidance for Medium-Voltage Cable Systems for Nuclear Power Plants (1020805) [5] recommends limiting cables exposure to long-term wetting by manually or automatically pumping dry manholes, pits, and vaults to preclude long-term wetted or submerged conditions. In addition, it is recommended that cables that are shielded and are known or suspected of being wetted or submerged for longer than a few days at a time be tested to determine whether degradation has occurred.

Introduction

1-4

1.4 Abbreviations and Acronyms ac alternating current

ACLT accelerated cable life test

AEIC Association of Edison Illuminating Companies

AWG American Wire Gauge

CPE chlorinated polyethylene rubber

CSPE chlorosulfonated polyethylene rubber (Hypalon)

dc direct current

IEEE Institute of Electronic and Electrical Engineers

EPRI Electric Power Research Institute

EPR ethylene-propylene rubber

Hz hertz

ICEA Insulated Cable Engineers Association

IRC isothermal return current

kcmil thousands of circular mils, a unit of area for conductor size

kV kilovolts

LDPE low-density polyethylene

mil 1/1000th of an inch

NEI Nuclear Energy Institute

NEMA National Electrical Manufacturing Association

NRC Nuclear Regulatory Commission

OW oscillating wave

pC picocoulomb

PD partial discharge

PDEV partial discharge extinction voltage

PDIV partial discharge inception voltage

PE polyethylene

PVC polyvinyl chloride

RV return voltage

tan tangent (a loss factor of insulation)

Introduction

1-5

TDR time domain reflectometer

TR-XLPE tree-retardant cross-linked polyethylene

V0 phase-to-ground voltage (also referred to as U0 in the literature)

XLPE cross-linked polyethylene

1.5 Terminology Hydrophobic. A tendency of a polymer to reject and not to absorb or react with water.

Lossy. Referring to insulation having a somewhat higher leakage current. XLPE is a low-loss material, whereas EPR is lossy.

2-1

2 UNDERSTANDING THE DESIGN OF POWER PLANT CABLE SYSTEMS

Understanding the design of the cables and the cable system is important because it can influence the rate of degradation and the ability to detect that degradation. This section describes cable shielding, circuit grounding philosophy, cable ground insulation, and cable phase configuration choices that can influence or affect cable aging and testability. Protective relays and annunciation alarms, as well as actions that be taken in response to their actuation, are described.

2.1 Shielded and Nonshielded Cables Medium-voltage, 5-kV cables can be constructed with or without an insulation shielding system. Factors that influence this decision include the following:

Depending on the thickness of the insulation in the cable, nonshielded (grounded) cables can continue to function for a limited period with a single phase-to-ground fault should one occur under plant accident conditions. (Single phase-to-ground faults should be de-energized as soon as possible under normal conditions.)

Nonshielded cables are simpler to splice and terminate because there are no insulation shields.

Nonshielded cables have voltage on their exterior surface because the voltage distributes across any insulator, such as air, between the conductor and ground or the conductor and adjacent phases. The voltage at the surface of the cable can be as high as 90% of the conductor voltage.

When nonshielded cables touch or nearly touch grounded cabinets and conduits, high-voltage stress occurs in the air gap, which can lead to corona attach. Due to the high-voltage stress, the air in the gap can break down, causing streams of electrons to impact the cable surface. During each breakdown, the air gap is temporarily shorted, and the voltage is distributed across the insulation. At each discharge, a small increment of damage occurs to the polymer wall of the cable. Over long periods, the wall of the insulation can erode, leading to insulation failure.

Nonshielded cables are more difficult to protect. Relay settings must be less sensitive than for shielded cables. Manual intervention might be required for nonshielded cables to prevent a single-phase fault from developing into a phase-to-phase fault.

Understanding the Design of Power Plant Cable Systems

2-2

With a shielded cable, a multi-grounded metallic shield eliminates any voltage or tracking on the cables exterior surface. An additional safety advantage is developed because there is no surface voltage present, and workers can come into close proximity to energized cables, such as in manholes; however, contacting medium-voltage cables, whether shielded or nonshielded, is not recommended while they are energized.

Shielded cables are more complex to splice and terminate. Two shields exist in a majority of the medium-voltage cables used in power plants today (see Figure 2-1). The shielding material next to the conductor is called the conductor shield. Medium-voltage cables used in more recent plants have conductor shields. The conductor shield prevents discharges between the conductor and the insulation during operation. The shield that is of concern with regard to shielded and nonshielded cable is the insulation shield, which consists of a semiconducting layer in contact with the outer portion of the insulation, as well as a metallic portion on top of the semiconducting layer. The metallic portion must be grounded in at least one point in every circuit for the shield to contain the voltage stress within the insulation system. If the shield is not grounded, the cable will function just like a nonshielded cable and will have up to 90% of conductor voltage on the cables outer surface.

Figure 2-1 Shielded Cable Components

2.2 Grounding Systems, Protection, and Alarms

2.2.1 Grounding Systems

Significant differences exist in cable design and applications between medium-voltage cables used in utility residential and industrial customer distribution systems and those used in power plants. These differences are related to the electrical system design practices for power plants, the need for more flexible cables to allow installation within the restricted confines of the power plant, and service continuity requirements. Understanding the designs of cable systems, especially with respect to grounding and fault clearing, is critical to responding properly when a ground is identified but the cable remains in service.

Understanding the Design of Power Plant Cable Systems

2-3

Medium-voltage utility distribution systems in North America are generally served by transformers that are connected in a wye configuration that is solidly grounded. The advantage of this arrangement is that when a single phase-to-ground fault occurs, voltages on the unfaulted phases remain relatively stable and do not subject customers to high phase-to-phase voltages during fault conditions. The disadvantage of this system is that the voltage on the faulted phase drops to near zero even for transient faults and, therefore, interrupts service served from that phase.

Medium-voltage power plant auxiliary systems are frequently either connected in a delta configuration or a resistance or impedance is placed in the ground connection. The advantage of these systems is that the voltage on the faulted phase stays high on single-phase faults. This means that motor loads can continue to function and operate the device without interruption and fault currents are restricted during single-phase faults.

The importance of such a connection is that a time limit is involved for the circuit to be cleared. There are basic assumptions in these designs concerning how quickly manual action will be taken should a ground fault occur. These assumptions are frequently lost and are not included in system operating instructions. Often, plant personnel believe that the systems are designed to function for long periods with a single-phase faultwhich is rarely true. An important consideration is the possibility of the single-phase fault progressing to a two- or three-phase fault that could lead to severe damage to interconnected equipment.

Three voltage levels and corresponding insulation thicknesses have been established for medium-voltage cable systems, with corresponding times for a single-phase fault to be cleared [6]. They are included in Association of Edison Illuminating Companies (AEIC) specifications such as CS1-90 [7], which states the following:

100 Percent Level: This insulation level is designated by the normal phase-to-phase system voltage. This is applicable only to systems where the normal voltage between the cable conductor and the insulation shielding tape or metal sheath will not exceed 58 percent of the phase-to-phase voltage. These cables may be applied when the system is provided with relay protection such that the ground faults will be cleared as rapidly as possible, but in any case within one minute.

These cables are applicable to the great majority of cable installations that are on grounded neutral systems, and they may be used also on other systems for which the application of cables is acceptable, providing the above clearing requirements are met in completely de-energizing the faulted section.

133 Percent Level: This insulation level corresponds to that formerly designated for ungrounded systems. Cables in this category may be applied when the clearing time requirements for the 100 Percent Level category cannot be met, and yet there is adequate assurance that the faulted section will be de-energized in a time not exceeding one hour. Also they may be used when additional strength over the 100 Percent Level is desirable.

173 Percent Level: Cables of this designation should be applied on systems when the time required to de-energize a grounded section is indefinite. Their use is recommended also for resonant grounded systems.

Understanding the Design of Power Plant Cable Systems

2-4

Faults on power plant cables are expected to clear immediately by automatic means or within a short, finite time. Care must be taken that operators understand that if a cable shows signs of a phase-to-ground fault, such as a ground alarm or popping noises from a manhole or cubicle, the cable is de-energized as rapidly as possible.

2.2.2 Phase-to-Phase Faults

The description for the 173% level when the time requiredis indefinite seems to indicate that if the cable is rated at 173% of the operating voltage there is no reason to clear the fault manually or automatically. This is not true for several reasons. If the fault persists, the heat and arcing from the fault will likely cause additional damage to the adjacent phases. Significant damage can occur rapidly. The other sobering thought is that the failure of the first phase of a cable should lead one to believe that all phases of the cable are deteriorated and hence the other phases might fail soon.

If the second failure occurs, the fault current is no longer limited by the resistor or impedance in the ground connection and the current can reach tens of thousands of amperes. Phase-to-phase faults cable faults can lead to weakening or failure of upstream transformers and switchgear. If the transformer does not failure during the event, it can be left susceptible to failure should another surge occur or it can be degraded enough that it fails a short time later.

Operators of power plants should take defensive actionthat is, de-energize the circuit as soon as reasonably possible if a single phase-to-ground fault condition is suspected to eliminate the possibility of a phase-to-phase fault.

2.2.3 Number of Grounds on a Cable Insulation Shield The metallic portion of the insulation shield must be grounded at least once for each phase of every individual circuit for the shield to perform its function. This ground connection must be of sufficient ampacity to ensure that the protective relaying function is activated in the event of a fault. Two different levels of fault current can be involved in any circuit. If the circuit is fed by a resistance or impedance type of ground connection and is promptly cleared in the event of a single-phase fault, then one #6 copper ground might be adequate. If the circuit is designed to not promptly be cleared by automatic protection, a much larger ground lead (cable shield and grounding connection) must be provided.

Single-point grounding is often used on circuits where the conductor is 1000 kcmil (507 mm2) copper or larger. The reason for this is that the second ground on that metallic shield produces a transformer action that causes current to circulate in the shield system. This results in additional I2 R losses that effectively reduce the ampacity of that circuit. Although this is a concern in long transmission circuits, it is often not a problem in most power plant runs. Design calculations (see Figure 2-2), should be performed to determine whether an I2R heating is acceptable when both ends of the insulation shield are grounded. If the heating is too high, only one end should be grounded.

Understanding the Design of Power Plant Cable Systems

2-5

Figure 2-2 Single-Point Grounding of a Shield, Showing Voltage of Shield to Ground as a Function of Distance

However, with single-point grounding, a voltage builds up on the shield as a function of the current in the central conductor and the distance from the grounding point (see Figure 2-3).

Figure 2-3 Shield Losses and Voltages for Single-Conductor Cables [8]

Understanding the Design of Power Plant Cable Systems

2-6

This voltage can be as much as 100 V on a heavily loaded circuit and becomes a concern for the integrity of the jacket as well as a concern for workers that are not aware of the voltages and current that might be present on the shield. When current flows in the conductor of the cable, that current produces electromagnetic flux in the metallic shield, if present, or in any parallel conductor. This becomes a one-turn transformer when the shield is grounded two or more times, because a circuit is formed and current flows when a person touches the cable surface at the ungrounded end of the circuit. The cable circuits that should be considered for single-point grounding are systems with conductors of 1000 kcmil (507 mm2) and larger and anticipated loads of more than 500 A. The EPRI report Power Plant Electrical Reference Series, Volume 4: Wire and Cable (EL-5036) should be reviewed before designing a single-point grounding scheme [8].

In addition, if a discontinuity occurs in the metallic portion of the insulation shield that is grounded at only one end, a discharge site is created that can lead to carbonization and tracking at the point of the break leading to insulation failure. One such example is described in Section A.2.3.

If the shield is grounded two or more times or otherwise completes a circuit, the magnetic flux produces a current flow in the shield. The amount of current in the shield is inversely proportional to the resistance of the shield. The voltage on the jacket surface stays at zero if two or more grounds are connected but current is being carried in the shield. This is the reason that it is not safe to cut a shield in half while the cable is energized, because the two open ends become energized.

The distance between the grounds has no effect on the magnitude of the current. Whether the grounds are 1 ft or 1000 ft (0.3 m or 305 m) apart, the current is the samedepending on the current in the central conductor and the resistance and impedance of the shield. In the case of multiple cables, the spatial relationship of the cables is also a factor. Figure 2-2 provides equations for calculating shield currents and induced voltages.

2.3 Multiple Cables per Phase and Balanced Magnetic Fields The large currents that are involved in power plant auxiliary circuits often require two or more cables per phase to carry the load. (For this portion of the report, large current is defined as more than 200 A on a single cable.) Because copper conductors larger than 1000 kcmil (507 mm2) are generally not economically effective, there is a need to properly design and install parallel cables. These rules apply to low-voltage as well as medium-voltage cables.

The most obvious fact is that parallel cables must have the same length, conductor size and type, and similar connector types so that the resistance is equal on all cables in that phase. This is extremely important if the parallel cables are quite shortless than 20 ft.

To obtain sufficient ampacity, limit voltage drop, and attain load sharing between two or more cables, the position of each cable must be correct. There are three basic areas of concern when designing the installation of single conductor cables: 1) orientation or configuration of the phases, 2) eddy currents, and 3) circulating currents [9, 10].

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2.3.1 Configuration

When a single-conductor cable carries current, it generates a magnetic field that radiates out and alters the impedance of adjacent cables. If the configuration is not correct, a serious impedance imbalance and, therefore, an imbalance in the current of parallel cables can result. If is the configuration is done correctly, each cable making up one phase has an equal magnetic field acting on it.

Parallel cables must have the same conductor size and metal, the same length, the same insulation type, and the same connector type, and they should be larger than #1/0 (~0.08 in2 [~53.5 mm2]). Figures 2-4 through 2-6 show possible configurations for balanced magnetic fields.

Figure 2-4 Acceptable Configurations: Two Cables per Phase

Figure 2-5 Acceptable Configurations: Three Cables per Phase

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Figure 2-6 Acceptable Configurations: Four Cables per Phase

2.3.2 Eddy Currents

Eddy currents are generated in any ferrous metal (iron or steel) surrounding an individual, single-conductor cable carrying alternating current because of the varying magnetic field. Where a single conductor cable passes through steel plates on enclosures, structural steel, or steel conduitsto name a feweddy currents will flow. Heat is generated in the surrounding steel. As long as the current is less than about 200 A, the small amount of heat generated is usually not a significant factor.

The preferred practice is to use only nonferrous platessuch as aluminummounted over one common hole. The single-conductor cables plus the neutral enter through this one plate. No eddy currents are generated by this method because the individual magnetic fields from the three phases cancel one another.

This magnetic field cancellation effect also applies where cables pass through structural steel, are run in steel cable trays supported by steel, or are run on steel struts that are supported by steel. As long as the three phases are run through the same steel window, the net magnetic field and eddy current are almost zero.

When single-conductor cables run through reinforcing bar that might be embedded in concrete floors or walls, there is much less of a problem from heating because any heat that is produced by eddy currents will be dissipated by the large heat sink of the concrete. Direct contact between the cables and rebar should be avoided.

2.3.3 Circulating Currents

Circulating currents are generated in a metallic layer of single-conductor cables when the conductor is carrying currentregardless of the type of metaland when the metallic layer has more than one ground. The magnitude of this circulating current depends on the amount of current in the conductor, the resistance and impedance of the loop formed by this layer, and the ground path. This path could be interlocked armor, shielding tapes, or metallic conduit.

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2.4 Protective Relay and Annunciation Alarm Systems Most medium-voltage cable circuits have protective relaying or annunciation alarm systems or both to alert the operator when a fault occurs on that circuit. Protective relaying might not be designed to automatically trip (de-energize) the circuit, but this is no reason to ignore the cause by simply acknowledging the annunciation alarm. There are many different design factors for power plant circuits and, therefore, there is no single rule regarding the proper action to be taken after an alarm. A generalized protocol should include the following steps:

Determine whether a fault actually exists and record the current on the phase or phases. If a phase has a target, do not reclose until troubleshooting determines the cause of t