Future Inspection and Monitoring of Underground               Transmission Lines                    1020168
Future Inspection and Monitoring of Underground               Transmission Lines                                          ...
DISCLAIMER OF WARRANTIES AND LIMITATION OF LIABILITIESTHIS DOCUMENT WAS PREPARED BY THE ORGANIZATION(S) NAMED BELOW AS AN ...
CITATIONSThis document was prepared byElectric Power Research Institute (EPRI)1300 West W.T. Harris Blvd.Charlotte, NC 282...
PRODUCT DESCRIPTIONUnderground transmission lines have performed reliably for the power transmission industry.Nonetheless,...
of on-line, real-time monitoring and sensor technology; its results are provided in Section 5.Finally, Section 6 describes...
ABSTRACTUnderground transmission lines have performed reliably for the power transmission industry.Nonetheless, there are ...
ACKNOWLEDGMENTSThe report is a companion report to the EPRI report Future Inspection of Overhead TransmissionLines (101692...
CONTENTS1 BACKGROUND AND INTRODUCTION ................................................................................1-12...
3.13.1 Applications of Capacitive/Inductive Coupling................................................3-14     3.14 Flow, Te...
5.2.17 USi (New York)...................................................................................................5-...
1BACKGROUND AND INTRODUCTIONUnderground transmission lines provide reliable performance. These transmission lines can beca...
The following EPRI reports are listed for reference:•   On-Line DGA in HPFF Cables—Feasibility Study (1019504)•   Future I...
2SYSTEM CONCEPTS2.1      IntroductionSystem concepts are described for instrumentation of underground transmission cable s...
Table 2-1Inspection and monitoring of underground transmission lines    Failure              Diagnostic         Applicable...
Table 2-1 (continued)Inspection and monitoring of underground transmission lines    Failure              Diagnostic       ...
Table 2-1 (continued)Inspection and monitoring of underground transmission lines    Failure               Diagnostic      ...
Table 2-1 (continued)Inspection and monitoring of underground transmission lines    Failure             Diagnostic        ...
Figure 2-1Inspection and monitoring of ED underground transmission lines                                                  ...
Figure 2-2Inspection and monitoring of HPFF and HPGF underground transmission lines                                       ...
Figure 2-3Inspection and monitoring of SCFF underground transmission lines                                                ...
The system scope is limited to underground transmission line applications (>46–500 kV), notlower distribution voltages. It...
The sensor type and geolocation may be associated with the ID and hard-coded in a database atthe central repository so tha...
require very little maintenance—preferably, none. However, hubs are, in general, expected to bemore complex, requiring pos...
A communication system range is influenced by several factors. The vaults under considerationare underground concrete stru...
daisy chaining and long-haul communications may be an effective compromise. For example,vaults 1–20 could operate as a dai...
2.4.1 Potential for Harvesting Power from Magnetic FieldPower to operate a sensor in a vault can be harvested from the mag...
3CANDIDATE SENSOR TECHNOLOGIES3.1      IntroductionThis report attempts to address and provide insight into some of the en...
3. The image is segmented to identify regions that correspond to physical objects.      Segmentation algorithms may be bas...
3.2.2 CamerasMass production of components for consumer digital cameras has resulted in improvedperformance and reduced co...
IR cameras are often classified as cooled or uncooled. High-end thermal IR cameras oftenprovide a peltier or compressor sy...
Commercially available accelerometers can be obtained in a variety of form factors and withwidely varying sensitivities an...
3.7    Ultrasonic SensingUltrasonic testing is based on time-varying deformations or vibrations in materials. In solids,so...
In practice, the transmitted coil and receiver coil are the same or at least colocated. The sensor isconfigured to apply a...
3.7.2 Applications of Ultrasonic SensingOne common application of ultrasonic sensing is to evaluate material thickness and...
3.8.1 Applications of EMATThe application of EMAT has been in nondestructive evaluation (NDE) applications, such asflaw de...
3.9.1 Applications of Eddy Current SensingEddy current is used in a wide range of applications for the power and aerospace...
3.12   Fiberoptic SensingFiberoptic sensing has been applied for many decades to detect various physical and chemicalparam...
short-range communication systems—for example, within office buildings. Single-mode opticalfibers are used only for very l...
EPRI began using this technology for underground cable systems in the mid-1990s with a YorkDTS-80 system (in 2003, the equ...
3.13   Capacitive/Inductive Coupling (PD)PD measurements are used to assess insulation condition of cables and accessories...
Figure 3-5HFCTs placed around the cable sheath bonding link for PD measurements3.14   Flow, Temperature, Pressure, Volume,...
Future Inspection of Underground Transmission Lines
Future Inspection of Underground Transmission Lines
Future Inspection of Underground Transmission Lines
Future Inspection of Underground Transmission Lines
Future Inspection of Underground Transmission Lines
Future Inspection of Underground Transmission Lines
Future Inspection of Underground Transmission Lines
Future Inspection of Underground Transmission Lines
Future Inspection of Underground Transmission Lines
Future Inspection of Underground Transmission Lines
Future Inspection of Underground Transmission Lines
Future Inspection of Underground Transmission Lines
Future Inspection of Underground Transmission Lines
Future Inspection of Underground Transmission Lines
Future Inspection of Underground Transmission Lines
Future Inspection of Underground Transmission Lines
Future Inspection of Underground Transmission Lines
Future Inspection of Underground Transmission Lines
Future Inspection of Underground Transmission Lines
Future Inspection of Underground Transmission Lines
Future Inspection of Underground Transmission Lines
Future Inspection of Underground Transmission Lines
Future Inspection of Underground Transmission Lines
Future Inspection of Underground Transmission Lines
Future Inspection of Underground Transmission Lines
Future Inspection of Underground Transmission Lines
Future Inspection of Underground Transmission Lines
Future Inspection of Underground Transmission Lines
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Future Inspection of Underground Transmission Lines

  1. 1. Future Inspection and Monitoring of Underground Transmission Lines 1020168
  2. 2. Future Inspection and Monitoring of Underground Transmission Lines 1020168 Technical Update, December 2009 EPRI Project Manager S. Eckroad 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
  3. 3. DISCLAIMER OF WARRANTIES AND LIMITATION OF LIABILITIESTHIS DOCUMENT WAS PREPARED BY THE ORGANIZATION(S) NAMED BELOW AS AN ACCOUNT OFWORK SPONSORED OR COSPONSORED BY THE ELECTRIC POWER RESEARCH INSTITUTE, INC. (EPRI).NEITHER EPRI, ANY MEMBER OF EPRI, ANY COSPONSOR, THE ORGANIZATION(S) BELOW, NOR ANYPERSON ACTING ON BEHALF OF ANY OF THEM:(A) MAKES ANY WARRANTY OR REPRESENTATION WHATSOEVER, EXPRESS OR IMPLIED, (I) WITHRESPECT TO THE USE OF ANY INFORMATION, APPARATUS, METHOD, PROCESS, OR SIMILAR ITEMDISCLOSED IN THIS DOCUMENT, INCLUDING MERCHANTABILITY AND FITNESS FOR A PARTICULARPURPOSE, OR (II) THAT SUCH USE DOES NOT INFRINGE ON OR INTERFERE WITH PRIVATELY OWNEDRIGHTS, INCLUDING ANY PARTYS INTELLECTUAL PROPERTY, OR (III) THAT THIS DOCUMENT ISSUITABLE TO ANY PARTICULAR USERS CIRCUMSTANCE; OR(B) ASSUMES RESPONSIBILITY FOR ANY DAMAGES OR OTHER LIABILITY WHATSOEVER (INCLUDINGANY CONSEQUENTIAL DAMAGES, EVEN IF EPRI OR ANY EPRI REPRESENTATIVE HAS BEEN ADVISEDOF THE POSSIBILITY OF SUCH DAMAGES) RESULTING FROM YOUR SELECTION OR USE OF THISDOCUMENT OR ANY INFORMATION, APPARATUS, METHOD, PROCESS, OR SIMILAR ITEM DISCLOSED INTHIS DOCUMENT.ORGANIZATION(S) THAT PREPARED THIS DOCUMENTElectric Power Research Institute (EPRI)This is an EPRI Technical Update report. A Technical Update report is intended as an informal report ofcontinuing research, a meeting, or a topical study. It is not a final EPRI technical report.NOTEFor further information about EPRI, call the EPRI Customer Assistance Center at 800.313.3774 ore-mail askepri@epri.com.Electric Power Research Institute, EPRI, and TOGETHER…SHAPING THE FUTURE OF ELECTRICITYare registered service marks of the Electric Power Research Institute, Inc.Copyright © 2009 Electric Power Research Institute, Inc. All rights reserved.
  4. 4. CITATIONSThis document was prepared byElectric Power Research Institute (EPRI)1300 West W.T. Harris Blvd.Charlotte, NC 28262Principal InvestigatorsT. ZhaoS. EckroadA. MacPhailThis report describes research sponsored by EPRI.The report is a corporate document that should be cited in the literature in the following manner:Future Inspection and Monitoring of Underground Transmission Lines. EPRI, Palo Alto, CA:2009. 1020168. iii
  5. 5. PRODUCT DESCRIPTIONUnderground transmission lines have performed reliably for the power transmission industry.Nonetheless, there are opportunities to improve on-line condition assessment of the undergroundcable systems. Some of these opportunities can be realized by incorporating improved sensors,more efficient power sources to the sensors, enhanced data collection systems, and betterintegration with utilities’ operations systems. This report describes technologies that can beapplied in future inspection and monitoring of underground transmission lines.The report is a companion to the Electric Power Research Institute (EPRI) report FutureInspection of Overhead Transmission Lines (1016921).Results and FindingsSystems for inspection and monitoring of underground transmission lines consist of sensors thatacquire diagnostic data from components of interest and communications that collect the sensordata and deliver them to a central repository. The information contained herein accomplishes thefollowing:• Describes system concepts, including specific sensor system needs• Addresses candidate technologies for sensor and communication systems, including areas for improvement• Provides demonstration scenarios for the inspection and monitoring of underground transmission linesChallenges and ObjectivesThe objectives of the work described in this report are to improve the quality of preventivemaintenance performed on underground transmission lines and to make the maintenance lessexpensive. By doing so, utilities can reduce the frequency of corrective maintenance on theirunderground lines, which leads to improved reliability and operations. To achieve these goals,enhanced inspection and monitoring of critical components must be deployed, using newlydeveloped technology in the areas of sensors, power harvesting, and telecommunicationssystems. As the requirements for transmission line reliability and availability become morestringent, technology becomes a major enabler.Applications, Value, and UseThe report is targeted at maintenance personnel and managers who are responsible for theupkeep of their company’s underground transmission lines. It will serve as a roadmap for thedevelopment and demonstration of inspection and monitoring technologies for these importantsystems.After a brief introduction, Section 2 of this report covers the concepts that characterizediscussions about the assessment and maintenance methods used for extruded dielectric andlaminar dielectric cables of underground systems. Section 3 presents detailed information aboutthe candidate technologies for sensors, and Section 4 does the same for communicationtechnologies. EPRI conducted an industry scan of 18 companies worldwide regarding their use v
  6. 6. of on-line, real-time monitoring and sensor technology; its results are provided in Section 5.Finally, Section 6 describes possible demonstration scenarios for condition monitoring ofunderground transmission cable systems.EPRI PerspectiveEPRI has conducted the research described in this report in order to advance the field ofinspection and monitoring technologies for underground transmission. For EPRI-memberutilities, the chief benefits of better inspection and monitoring methods will be a combination oflower costs in system assessment and maintenance and fewer circuit failures and outages.ApproachUtility staff familiar with underground transmission line inspection and monitoring, experts insensing and communicating technology, and transmission system researchers collaborated anddeveloped this report.KeywordsCommunication technologyInspectionMonitoringSensorTransmissionUnderground vi
  7. 7. ABSTRACTUnderground transmission lines have performed reliably for the power transmission industry.Nonetheless, there are opportunities to improve on-line condition assessment of the undergroundcable systems. Some of these opportunities can be realized by incorporating improved sensors,more efficient power sources to the sensors, enhanced data collection systems, and betterintegration with utilities’ operations systems. This report, which is a companion to the ElectricPower Research Institute (EPRI) report Future Inspection of Overhead Transmission Lines(1016921), describes technologies that can be applied in future inspection and monitoring ofunderground transmission lines.Systems for inspection and monitoring of underground transmission lines consist of sensors thatacquire diagnostic data from components of interest and communications that collect the sensordata and deliver them to a central repository. This report describes system concepts, addressescandidate technologies for sensor and communication systems, and provides demonstrationscenarios for the inspection and monitoring of underground transmission lines. The objectives ofthe work described in this report are to improve the quality of preventive maintenance performedon underground transmission lines and to make the maintenance less expensive. By doing so,utilities can reduce the frequency of corrective maintenance on their underground lines, whichleads to improved reliability and operations.Utility staff familiar with underground transmission line inspection and monitoring, experts insensing and communicating technology, and transmission system researchers collaborated anddeveloped this report. vii
  8. 8. ACKNOWLEDGMENTSThe report is a companion report to the EPRI report Future Inspection of Overhead TransmissionLines (1016921). Special thanks to the Principal Investigators of Southwest Research Instituteand the Principal Investigator and Project Manager, Dr. Andrew Phillips of EPRI, whodeveloped that report. Technologies common to underground transmission are repeated orsummarized in this report for completeness.The participation of utility advisors in the report’s development is acknowledged andappreciated. ix
  9. 9. CONTENTS1 BACKGROUND AND INTRODUCTION ................................................................................1-12 SYSTEM CONCEPTS ............................................................................................................2-1 2.1 Introduction...............................................................................................................2-1 2.2 System Architecture .................................................................................................2-9 2.3 Communication Considerations .............................................................................2-11 2.4 Power Considerations ............................................................................................2-13 2.4.1 Potential for Harvesting Power from Magnetic Field ........................................2-14 2.4.2 Potential for Harvesting Power from Induced Voltage of Grounded Components.................................................................................................................2-14 2.4.3 Potential for Optical Power Transmission ........................................................2-14 2.4.4 Potential for Other Power Harvesting Methods ................................................2-143 CANDIDATE SENSOR TECHNOLOGIES .............................................................................3-1 3.1 Introduction...............................................................................................................3-1 3.2 Optical Image Sensing .............................................................................................3-1 3.2.1 Image Analysis ...................................................................................................3-1 3.2.2 Cameras.............................................................................................................3-3 3.2.3 Applications of Optical Imaging ..........................................................................3-3 3.3 IR Image Sensing.....................................................................................................3-3 3.3.1 Applications of IR Imaging..................................................................................3-4 3.4 Vibration Sensing .....................................................................................................3-4 3.4.1 Applications of Vibration Sensors.......................................................................3-5 3.5 Acoustic Sensing......................................................................................................3-5 3.6 Strain Sensing ..........................................................................................................3-5 3.6.1 Applications of Strain Sensors ...........................................................................3-5 3.7 Ultrasonic Sensing ...................................................................................................3-6 3.7.1 Magnetostrictive Sensing ...................................................................................3-6 3.7.2 Applications of Ultrasonic Sensing .....................................................................3-8 3.8 Electromagnetic-Acoustic Transducers....................................................................3-8 3.8.1 Applications of EMAT .........................................................................................3-9 3.9 Eddy Current Sensing ..............................................................................................3-9 3.9.1 Applications of Eddy Current Sensing..............................................................3-10 3.10 RF Interference Sensing ........................................................................................3-10 3.11 Fluid Dissolved Gas Sensing .................................................................................3-10 3.11.1 Applications of Fluid Dissolved Gas Sensing .................................................3-10 3.12 Fiberoptic Sensing..................................................................................................3-11 3.12.1 Applications of Fiberoptic Sensing .................................................................3-11 3.13 Capacitive/Inductive Coupling (PD)........................................................................3-14 xi
  10. 10. 3.13.1 Applications of Capacitive/Inductive Coupling................................................3-14 3.14 Flow, Temperature, Pressure, Volume, and Mass Sensing ...................................3-15 3.15 Voltage, Current, and Frequency Measurements ..................................................3-15 3.15.1 Dissipation Factor Measurement....................................................................3-15 3.15.2 Jacket Faults and SVL Failure Detection .......................................................3-154 CANDIDATE DATA COMMUNICATION TECHNOLOGIES..................................................4-1 4.1 Introduction...............................................................................................................4-1 4.2 RF Wireless LOS Transceiver..................................................................................4-1 4.2.1 IEEE 802 Standard Technologies ......................................................................4-2 4.2.2 Nonstandardized Technologies..........................................................................4-2 4.3 RF Wireless Backscatter ..........................................................................................4-3 4.4 RF Wireless OTH .....................................................................................................4-3 4.5 IR Wireless ...............................................................................................................4-4 4.6 Fiberoptic..................................................................................................................4-4 4.7 Free Space Optical Communication.........................................................................4-5 4.8 Data Communication over Power Cable Line ..........................................................4-5 4.9 Acoustic Signal Transmission Through Insulating Fluids .........................................4-6 4.10 Mobile Collection Platforms......................................................................................4-6 4.10.1 Manned Mobile Platforms.................................................................................4-6 4.10.2 Unmanned Mobile Platforms ............................................................................4-65 INDUSTRY SCAN ON SENSOR APPLICATIONS IN UNDERGROUND TRANSMISSIONCABLE SYSTEMS ....................................................................................................................5-1 5.1 Introduction...............................................................................................................5-1 5.2 List of Products/Services of Monitoring Transmission Cable Systems ....................5-1 5.2.1 Balfour Beatty Utility Solutions (United Kingdom) ..............................................5-1 5.2.2 BRUGG (Switzerland) ........................................................................................5-1 5.2.3 Genesys (Colorado) ...........................................................................................5-2 5.2.4 High Voltage Partial Discharge Ltd. (United Kingdom) ......................................5-2 5.2.5 KEMA (The Netherlands) ...................................................................................5-2 5.2.6 Kinectrics (Canada)............................................................................................5-2 5.2.7 LIOS Technology (Germany) .............................................................................5-3 5.2.8 LS Cable (South Korea) .....................................................................................5-3 5.2.9 Omicron (Austria) ...............................................................................................5-3 5.2.10 Sensornet (United Kingdom) ..............................................................................5-4 5.2.11 SensorTran (Texas) ...........................................................................................5-4 5.2.12 Schlumberger/Sensa (Houston/United Kingdom) ..............................................5-4 5.2.13 University of Southampton (United Kingdom) ....................................................5-5 5.2.14 Sumitomo/J-Power Systems (Japan) .................................................................5-5 5.2.15 TechImp (Italy) ...................................................................................................5-5 5.2.16 Tokyo Electric Power Company (Japan) ............................................................5-6 xii
  11. 11. 5.2.17 USi (New York)...................................................................................................5-6 5.2.18 UtilX/CableWise (Washington) ...........................................................................5-66 DEMONSTRATION SCENARIOS..........................................................................................6-1 6.1 Introduction...............................................................................................................6-1 6.2 Condition Monitoring of Underground Transmission Vaults .....................................6-1 6.3 Condition Monitoring for Underground Transmission XLPE Cables ........................6-1 6.4 Condition Monitoring for Underground Transmission Pipe-Type Cables .................6-27 REFERENCES .......................................................................................................................7-1 xiii
  12. 12. 1BACKGROUND AND INTRODUCTIONUnderground transmission lines provide reliable performance. These transmission lines can becategorized into two basic types—extruded dielectric (ED) cables and laminar dielectric cables.The insulation materials currently used in ED cables are cross-linked polyethylene (XLPE) and,to a lesser extent, ethylene propylene rubber. The laminar dielectric cables include high-pressurefluid-filled cables (HPFF), high-pressure gas-filled cables (HPGF), and self-contained fluid-filled cables (SCFF). There are opportunities for improvements in on-line condition assessmentof the cable systems, leading to enhanced reliability, operations, and maintenance. Some of theseopportunities can be realized by incorporating improved sensors, more efficient power sources tothe sensors, enhanced data collection systems, and better integration with utility operationsystems.Performance, by definition, must be measurable. The improved sensors, power sources, datacollection systems, and integration systems described in this report are all ultimately aimed atimproving the measurability of cable system performance. In the context of undergroundtransmission systems and this report, the components of performance are the following:• Reliability – Failure rate – Failure repair time• Operations – Planned outage frequency and duration – Unplanned outage frequency and duration – Loading flexibility• Maintenance – Preventive maintenance – Corrective maintenanceThe goals and objectives of the work described in this report are to improve the quality andlower the costs of preventive maintenance, and, in so doing, reduce the need for correctivemaintenance, which leads to improved reliability and operations. To achieve these goals andobjectives, enhanced inspection and monitoring of critical components must be deployed, usingnew technology developments in the areas of sensors, power harvesting, and telecommunicationssystems.Transmission line components are currently inspected and assessed, mainly using fieldpersonnel. The Electric Power Research Institute (EPRI) and others are currently investigatingand developing automated/unmanned inspection and monitoring technologies for undergroundtransmission lines. With transmission line security issues apparently growing in number, theneed for automated, unmanned, and continuous monitoring of underground transmission lines isincreasing. Technology advancements could enable an effective, comprehensive, automatedinspection and monitoring system for underground transmission lines. 1-1
  13. 13. The following EPRI reports are listed for reference:• On-Line DGA in HPFF Cables—Feasibility Study (1019504)• Future Inspection of Overhead Transmission Lines (1016921)• Low-Cost Sensors to Monitor Underground Distribution Systems (1013884)• Overhead Transmission Inspection and Assessment Guidelines (1012310)• Simplified Leak Detection System for HPFF Cable Systems (1010503)• Novel Applications of Fiber Optic Sensor Technology for Diagnostics of Underground Cables (1008712)• Application of Fiber-Optic Distributed Temperature Sensing to Power Transmission Cables at BC Hydro (1000443)• Condition and Power Transfer Assessment of CenterPoint Energy’s Polk-Garrott Pipe-type Cable Circuit (1007539)• Ampacity Evaluation and Distributed Fiber Optic Testing on Pipe-type Cables Under Bridgeport Harbor (1007534)• Application of Fiber-Optic Temperature Monitoring to Solid Dielectric Cable: DFOTS Installation at Con Edison (1000469)• Distributed Fiber-Optic Measurements on Distribution Cable Systems (TE-114897)• Distributed Fiber Optic Temperature Monitoring and Ampacity Analysis for XLPE Transmission Cables (TR-110630)• HPFF Cable Leak Location Using Perfluorocarbon Tracers (TR-109086)• Cable Oil Monitor and Tester (COMAT) (TR-109071)• DRUMS Leak Detection for HPFF Pipe-type Cable Systems (TR-105250)• Field Measurement of Cable Dissipation Factor (TR-102449)The objectives and outline of this report are as follows:• To document system concepts, including descriptions of specific sensor system needs• To address candidate technologies for sensor systems, including areas for improvement• To address possible demonstration examples and system implementation scenarios 1-2
  14. 14. 2SYSTEM CONCEPTS2.1 IntroductionSystem concepts are described for instrumentation of underground transmission cable systemswith sensor technology and communication systems. The purpose is to increase their efficiency,performance, reliability, safety, and security.The system concepts are fueled by a list of sensing needs. Table 2-1 lists inspection andmonitoring of underground transmission lines, grouped into the following four sections:• Presently available on-line, continuous monitoring methods• Presently available off-line maintenance inspection, with opportunities for continuous monitoring methods• Presently available off-line maintenance inspection based on laboratory tests, with opportunities for on-line continuous monitoring methods• Other desirable on-line, continuous inspection and monitoring methodsFigure 2-1, Figure 2-2, and Figure 2-3 show schematics of the inspection and monitoringapplications for ED, HPFF and HPGF, and SCFF transmission lines, respectively. 2-1
  15. 15. Table 2-1Inspection and monitoring of underground transmission lines Failure Diagnostic Applicable Cable Overall Monitoring Sensor Opportunity Comments forModes/Indicators Method Systems and Status Capability Future Research Auxiliary and Prioritization EquipmentPresently available on-line, continuous monitoringHot spots along Temperature ED, HPFF, HPGF, On-line Monitor through Distributed fiberoptic Commercial systemscables—limiting SCFF monitoring distributed temperature sensing available, EPRIfactor of loading available. fiberoptic and thermocouples tailored collaborationcapability and sensors and available. opportunity availableinsulation aging thermocouples.Hydraulic system Fluid or gas HPFF, HPGF, On-line Monitor at Pressure and other Commercial systemsmalfunction pressure, flow, SCFF monitoring pressurizing transducers available. available pumping plant available. systems. operation, reservoir fluid levels, piping damage, and leaksDeterioration of Partial discharge ED, HPFF, HPGF, On-line Monitor through Various sensors R&D on sensors,cable insulation (PD) detection, SCFF (limited monitoring capacitive and/or available (ultra-high sensitivity,and shield shield current effectiveness for available. inductive frequency [UHF], HF effectiveness,systems, localized measurement HPFF and HPGF) Expensive coupling or current transformers, integration, noisedefects especially and time- acoustic inductive and filtering, dataat joints, consuming emission capacitive couplers, processing, and soterminations, and inspection. sensors. Off-line acoustic emission). oninterfaces and on-line Optical fiber sensors maintenance under investigation. inspection. Distributed sensor development opportunities exist along cables. 2-2
  16. 16. Table 2-1 (continued)Inspection and monitoring of underground transmission lines Failure Diagnostic Applicable Cable Overall Monitoring Sensor Opportunity Comments forModes/Indicators Method Systems and Status Capability Future Research Auxiliary and Prioritization EquipmentBuried steel pipe Cathodic protection HPFF, HPGF On-line Monitor cathodic Potential and current Commercial systemscorrosion and system settings monitoring protection meters available. availablecoating damage and connections, available. systems at half-cell potential, substations, and aboveground vaults, or test survey stations.Metallic Cathodic protection SCFF On-line Monitor cathodic Potential and current Commercial systemssheath/shield system settings monitoring protection meters available. availablecorrosion and connections available. systems at substations.Fluid or gas leak Fluid pressure, HPFF, HPGF, On-line Monitor at Various transducers USi/EPRI system temperature, SCFF monitoring pressurizing available. available; circuit loading, available. systems and/or ConEd/EPRI and ambient condition, along cable Kinectrics/EPRI flow, and the like route. systems under investigationPresently available off-line maintenance inspection, with opportunities for on-line, continuous monitoringOverall insulation Dissipation factor HPFF, HPGF, In-field test Off-line Development EPRI in-field systemintegrity, such as SCFF with special maintenance opportunities exist for available for laminarmoisture, fluid equipment. inspection. on-line monitoring. dielectric cablescontaminationBonding and link Sheath current ED, SCFF In-person Off-line Sensors available but On-line monitoringbox corrosion, measurements inspection. maintenance need integration. desirableloose connection, inspection.insulation damageSheath voltage SVL current ED, SCFF In-person Off-line Sensors available but On-line monitoringlimiter (SVL) inspection. maintenance need integration. desirablefailure inspection. 2-3
  17. 17. Table 2-1 (continued)Inspection and monitoring of underground transmission lines Failure Diagnostic Applicable Cable Overall Monitoring Sensor Opportunity Comments forModes/Indicators Method Systems and Status Capability Future Research Auxiliary and Prioritization EquipmentVault hardware Optical image ED, SCFF Time- Off-line or on-line Sensor development On-line monitoringand component infrared image, consuming maintenance opportunities exist. desirable(ceiling, walls, vibration, acoustic inspection inspection. Some sensorspipe, clamps, sensing, and with safety available but needground wires, temperature concerns. integration.racks, pumping, indicating strips onand so on) components fordegradation, cracks, leaks,corrosion, corrosion, coatingoverheating, damage,flooding, safety- componentrelated gas damage, safety- related gas level, and so onInternal X-ray inspection ED, HPFF, HPGF, Expensive Off-line Portable X-ray On-line monitoringmovement, SCFF and time- maintenance equipment available. unlikelymisalignment, or consuming inspection.damage of cables inspection.and accessoriesFluid leak location Perfluorocarbon HPFF, SCFF Time- Off-line locating Sensors available. On-line monitoring tracers consuming after leak unlikely inspection. detected.Fault location Fault current ED, HPFF, HPGF, On-line Monitor fault Fiberoptic current Systems under SCFF monitoring current at each sensors developed. development by available. end of a cable Tokyo Electric Power section. Company for ED cables 2-4
  18. 18. Table 2-1 (continued)Inspection and monitoring of underground transmission lines Failure Diagnostic Applicable Cable Overall Monitoring Sensor Opportunity Comments forModes/Indicators Method Systems and Status Capability Future Research Auxiliary and Prioritization EquipmentPresently available off-line maintenance inspection based on laboratory tests, with opportunities for on-line, continuous monitoringAging/degradation Dissolved gas HPFF, HPGF, Laboratory Off-line Sensor development On-line monitoringof fluid or paper analysis (DGA), SCFF test with fluid maintenance opportunities exist. under investigationinsulation— dissipation factor, samples from inspection, fluid by EPRIindicator of hot direct current (dc) operating samples fromspots, PD, and resistance, equipment. operatingarcing alternating current equipment. (ac) resistance, moisture content, particle content, gas absorption capabilityAging of paper Degree of HPFF, HPGF, Laboratory Mechanical/ Sensor development Unlikely for on-lineinsulation polymerization SCFF test with electric strength opportunities exist. monitoring (DP), mechanical samples from versus DP strength, operating known. dissipation factor, equipment. furfuralOther desirable on-line, continuous inspection and monitoringThermo- Strain sensing, ED, HPFF, HPGF, New. On-line Sensor development On-line monitoringmechanical sidewall pressure SCFF monitoring opportunities exist. desirablebending sensing desirable.Moisture barrier Moisture level ED New. On-line Sensor development On-line monitoringdegradation monitoring opportunities exist. desirable desirable.Lead sheath Strain sensing SCFF New. On-line Sensor development On-line monitoringfatigue monitoring opportunities exist. desirable desirable. 2-5
  19. 19. Figure 2-1Inspection and monitoring of ED underground transmission lines 2-6
  20. 20. Figure 2-2Inspection and monitoring of HPFF and HPGF underground transmission lines 2-7
  21. 21. Figure 2-3Inspection and monitoring of SCFF underground transmission lines 2-8
  22. 22. The system scope is limited to underground transmission line applications (>46–500 kV), notlower distribution voltages. It was considered that the addition of electrical wiring tointerconnect distributed sensors is not viable because of electromagnetic susceptibility and otherconcerns. Consequently, sensor concepts at vault locations will mainly consider wireless and/orfiberoptic technology for communications, although other unique methods will be investigated,such as inductive coupling of signals onto cable conductors and shields, sheaths, or pipes.Some of the high-level concepts are as follows:• Sensors may be distributed in vaults and along cables.• Sensors might communicate immediately back to a central database.• Sensor information is collected, stored, and analyzed in a central database, which is a part of the utility’s current data management systems. The data can be collected/communicated from the sensors to the central database using one of the following methods: – Wirelessly back to the central database—for example, radio frequency (RF) directly, through satellite or cell phone network – Using a combination of fiberoptics and wireless – Using a vehicle traveling the length of the line. The data from the collection vehicle are transferred during or after the inspection. The vehicle may collect the data wirelessly from the sensors. – Using a combination of the preceding because some applications require an urgent response, suggesting real-time data availability at a control center2.2 System ArchitectureSystems for inspection and monitoring consist of sensors that acquire diagnostic data fromcomponents of interest and communications that collect the sensor data and deliver them to acentral repository.The sensors may be directly attached to the item being monitored or separately located, such asin the case of a camera in a vault. Communication devices may be mounted in or near vaults orlocated on a wide variety of remote, and possibly mobile, platforms. The sensors andcommunication devices may operate and be polled periodically (for instance, at intervals ofminutes, hours, or days) or continuously monitored (for example, in real time) depending on theapplications. In any case, sensors usually communicate their results to a central storage facility,such as using a supervisory control and data acquisition system (SCADA) and central energymanagement system computer with a PI server.An important feature of the system is flexibility and interoperability with a wide variety ofsensor types and communication methods. The information that is required for each sensorreading is the following:• Unique sensor identification (ID) (across all sensor types)• Raw data measurement or processed result• Date and time of the reading• Sensor type and geolocation 2-9
  23. 23. The sensor type and geolocation may be associated with the ID and hard-coded in a database atthe central repository so that this information does not need to be redundantly transmittedthrough the system for every reading. For remote sensors, the geolocation will need to becommunicated so that the system can associate the reading with a particular item (at a knowngeolocation) or area of interest.For flexibility, multiple protocols may be used for both short-range communication and long-haul communication between the sensors and the central repository. There may be applicationswhere relaying readings is an effective method to communicate data back to the centralrepository. Similarly, relaying readings between sensors is an acceptable communicationapproach.With regard to the handling of sensor data, there are system tradeoffs among processing power,communication bandwidth, and digital storage capacity. The system must be flexible to allowdifferent sensor applications to handle these tradeoffs differently. For example, in someapplications, it will be most efficient and optimal to process sensor data locally at the sensor andto report back the reading as a simple answer or alarm. In other applications, it may be desirableto have all the information communicated back to the central repository for archival and possiblyeven human interpretation. In the former case, the amount of data to be passed through thecommunication channel is very low (1 bit, maybe once a day), but the processing power requiredat the sensor may be high in order to make an intelligent decision with high confidence. In thelatter case, the amount of data passed through the communication channel is very high (maybe10 MB for a high-resolution image), with much greater potential for impact to system throughputand storage space. The latter approach may be merited when automated results are questionableand manual interpretation of the raw data is required.Hybrid sensing protocols or approaches may be advantageous and are supported by the systemarchitecture. For example, a flag sensor may simply indicate when a condition needs to befurther evaluated. Whether done remotely or while in the field, interacting with the sensor maybe desirable in order to control the amount of detailed data that is provided. The flag sensor mayconserve power by not communicating until there is a problem. One possibility is an intelligentsensor that monitors a system condition, and then, based on the sensed severity, applies acommensurate amount of on-board resources (power, processing, memory, and communicationbandwidth) in order to operate effectively and with high efficiency.Sensors typically require a source of power, a sensing mechanism, a controller to formatmeasurements into readings, and a short-range wireless data communication mechanism. Ifcommunication hubs are applied, they will have similar needs for power and controller functionsand will need wireless data communication mechanisms to collect sensor readings (short-range)and to relay sensor readings to the central repository (long-range). Communication hubs mayalso have local memory for storing readings, either to buffer data when communication links aredown or as a local repository for data archival/backup.Although there are functional differences between sensors and hubs, device implementation isflexible to combine features. In other words, hubs can also incorporate sensors and sensors canalso serve as hubs; it is not a requirement that they be separate devices. A distinguishing featureof a combinational device that is thought of as a sensor versus a hub may be its power source.Sensor devices are, in general, expected to harvest power from the environment, and thus, they 2-10
  24. 24. require very little maintenance—preferably, none. However, hubs are, in general, expected to bemore complex, requiring possibly significant power sources such as large batteries, and thus,they would require periodic maintenance.Conceptually, sensors use a short-range wireless, inductively coupled, or fiberoptic link to thehub, which uses a long-range wireless or leased line link to the central data repository. This isnot a requirement, but it is based on the vision that many low-cost, low-power, low-bandwidthsensors will be deployed at a vault site and that a local hub as described previously can help bycollecting these data, providing a local redundant data repository, and coordinating long-haulcommunications.Figure 2-4 shows a functional diagram for a sensor technology.Figure 2-4Sensor function2.3 Communication ConsiderationsA communication system provides a means for communicating sensor data at vaults and alongcables to a central data collection and processing facility. SCADA systems for wide-areamonitoring have long been in existence and offer reliability enhancements for electrical powertransmission systems. The system concept requires a customized implementation based on sensorpopulation, data rates, and ranges. The customized implementation can be interfaced into acentral facility SCADA system, or it can operate as a stand-alone system, running its ownSCADA. This report does not address the SCADA layer; it instead focuses on the hardware,making sure that the system is realizable with the proper protocols in place.Both the transmission line infrastructure and the sensors used for monitoring the infrastructuredefine the requirements for the operational characteristics of communication systems. Theprimary considerations are the distance over which the data need to be communicated (referredto as range) and the amount of data to be communicated in a period of time (referred to as datarate). 2-11
  25. 25. A communication system range is influenced by several factors. The vaults under considerationare underground concrete structures, separated by 500–4000 ft (150–1200 m) and installed overtens of miles and even longer, and the data that are generated locally need to be collected at acentral facility that may be tens to hundreds of miles away.A variety of sensor configurations are envisioned within the system concept. Some sensors willbe attached directly to cable circuit components—for example, splices or terminations. Othersensors will be mounted along the cables, pipes, and insulating fluids. The need for thesedifferent sensor configurations leads to a distributed sensing system. The communication systemwill need to coordinate the collection of data from many distributed sensors for transfer to acentral facility.The distributed location of sensors imposes several constraints on the sensor design. Sensors inthe vaults need to use limited power and to have a local power source with a limited power-producing capacity. The constraints of the sensor also apply to the technology selected forcommunicating the sensor data. Because low power consumption is the most restrictingconstraint for the sensors in vaults, the communication technology is consequently relativelyshort range and infrequent to keep power consumption at a minimum. The opposingrequirements of low power consumption and short-range communication, versus needing tocollect data at a faraway central facility, influence the architecture of the communication system.The required data rate is defined by the type of sensor technology. The data rate influences thepower requirements for the communication technology.Because hubs will likely require much higher power consumption than sensors in order tosupport long-range communication and greater bandwidth, it may be beneficial to incorporate alarge battery at the hub. This would dictate additional logistics and periodic maintenance, but thetradeoff may be worthwhile. On the other hand, it would not be desirable to do that for a largepopulation of sensors.Data from each sensor cannot be directly transmitted to the central facility due to range andpower consumption trades. Thus, the communication system requires data communicationrelays. A number of architectural options for the communication system are available, includingthe following:• Sensor to passing mobile platform to central facility.• Sensor to sensor, daisy chained to central facility (for example, a mesh network).• Sensor to over-the-horizon (OTH) platform (such as a balloon or a satellite) to central facility.• Sensor to hub on a nearby pole.• Sensor to hub in a nearby vault. The hub has the similar options of hub to passing platform, hub to hub, and hub to OTH platform for passing data to a central facility, except that the hub can possibly be longer range with higher transmitted (and consumed) power.Daisy chaining sensors and/or hubs results in an additive effect on the quantity of data to becommunicated. However, the very low duty cycle and data rate of many of the sensors makedaisy chaining possible for certain sensor technologies. Higher data rate sensors may requiremore restrictions on the number of devices sharing a communication channel. A combination of 2-12
  26. 26. daisy chaining and long-haul communications may be an effective compromise. For example,vaults 1–20 could operate as a daisy chain, with vault 20 transmitting back to the central facility.The next 20 vaults could be configured the same way.Range and data rate affect the communication system architecture, and a number of architecturaloptions should be considered during the evolution of the system concept.Figure 2-5 shows a concept for communications networking.Figure 2-5Communication networking (Sensor-to-sensor, daisy chained to a central facility.)2.4 Power ConsiderationsSensors and communication hubs will require power for operation. Although batteries may beconvenient to test and demonstrate the system, they are seen as a maintenance problem in thesystem concept. The goal is to use renewable power sources in lieu of batteries. This is a difficultchallenge, especially for wide-range, high-bandwidth data communication requirements. Withpresent technology, it is not really possible to implement a batteryless system, except for verylimited and simple scenarios. Even over the next 20 years, without significant breakthroughs, thiswill remain a difficult challenge, albeit a worthy one, to keep in mind as new technologies areintroduced.Alternatives to batteries include solar, thermoelectric, the electric and magnetic fields that aregenerated from the power lines, and simply running a supply in from a local distribution system.There are significant limitations with each of these alternatives, but in the right applications, theymay be effective. The use of a rechargeable battery coupled with power harvesting will havestrong merit. 2-13
  27. 27. 2.4.1 Potential for Harvesting Power from Magnetic FieldPower to operate a sensor in a vault can be harvested from the magnetic field that is generatedfrom the current flowing through the cable or cable pipe. A short coil on a ferrite rod or a currenttransformer coil placed around cables or cable pipes would be used along with rectification,conversion, and regulation circuitry. This arrangement is effective for the high currents that flowin transmission cables, and it may be possible for the lower currents flowing in cable sheaths andthe zero-sequence currents flowing in cable pipes. Detailed investigations would be needed toprove the abilities to operate effectively under very low and very high cable currents (such asfault currents) and to withstand switching surges and transient overvoltages.2.4.2 Potential for Harvesting Power from Induced Voltage of GroundedComponentsED and SCFF cable systems often employ a ground continuity conductor (GCC) with speciallybonded systems. Designs usually try to minimize the induced current, but some still inevitablyflows. Inductive power supplies could harvest some of the energy flowing through the GCCs.With all inductive power supply options, the harvestable energy would be proportional to the lineload. Rechargeable batteries would provide power during low loads or outages. Detailedinvestigations would be needed to prove effective performance under abnormal operatingconditions, such as faults, resulting in induction or high through-currents in the GCCs.2.4.3 Potential for Optical Power TransmissionNonconducting fiberoptics can be used to transmit small amounts of power, although theefficiency is low. The system consists of an optical source (light-emitting diode [LED] orlaser diode) coupled to a fiberoptic cable that delivers the light to a photovoltaic junction.Assuming a 1-watt laser diode or super-bright LED source, rough calculations indicate that10–30 mW of power can be generated at a photovoltaic junction (solar cell). This is based on50% efficiency coupling to and from the fiberoptic and 4%–8% photovoltaic conversionefficiency. This example of energy conversion efficiencies is only a guide; more accuratecalculations with specific components and laboratory confirmation should be done if this is tobe considered as a viable power option.Although this efficiency of 1%–3% is very low, there are cases where this method may be usefulfor powering a remote sensor. For example, if a solar panel and battery are located above a vault,a sensor in the vault could be operated by a two-fiber cable. One fiber would carry power, andthe other would be used to transmit control and data signals. For micropower sensors that areoperated only a few minutes a day, the low efficiency may not be a factor.2.4.4 Potential for Other Power Harvesting MethodsThere is good potential for other power harvesting methods, although a technical review of thesetechnologies is not a focus of this report. For example, in close proximity to an undergroundtransmission line system, the high magnetic fields can be harvested. 2-14
  28. 28. 3CANDIDATE SENSOR TECHNOLOGIES3.1 IntroductionThis report attempts to address and provide insight into some of the enabling sensor and datacommunication technologies that appear to be suited for the application.In addition to the common technologies for overhead transmission applications, some specificsensor improvements to underground applications are described, such as the following:• Strain sensing for cable bending and movement• Insulating fluid dissolved gas and quality sensing• Distributed sensing using fiberoptic technology along cable circuits• Sheath and SVL current sensing3.2 Optical Image SensingOptical imaging includes methods in which an image provided by a camera is interpreted bycomputer analysis to identify or detect specific conditions. Different camera systems can provideimage representations in visible, infrared (IR), or ultraviolet (UV) spectral bands, and each ofthese bands has advantages for detecting different conditions or defects. There is also a variety ofmethods for positioning or deploying imaging cameras, with some choices more suitable fordetecting certain types of defects. Optical imaging is the automated analog of current visualinspection methods and has potential application for a high percentage of the transmission cablecomponents in vaults and substations.3.2.1 Image AnalysisComputer analysis of images to detect specific conditions or abnormalities is widely used inmanufacturing and other well-structured areas where images are obtained with consistentlighting, viewpoint, magnification, and other factors. Analysis of images with wide variations inillumination is more complex, but adaptive methods are available to compensate for changingconditions. Statistical methods are used to normalize image intensity and minimize the effects ofslowly changing artifacts.Computer analysis typically consists of the following steps: 1. Image capture using monochrome, color, IR, or UV cameras. The image is converted to a digital representation either internally in a digital camera or by a frame grabber if an analog camera is used. 2. A filtering step is usually included to remove image noise, normalize illumination, or enhance image contrast. 3-1
  29. 29. 3. The image is segmented to identify regions that correspond to physical objects. Segmentation algorithms may be based on finding edges, corners, or other shapes. Segmentation may also be based on color differences or difference in image texture or other patterns. 4. Each object identified in the segmented image is characterized by describing a set of features. These feature sets include measurements of intensity, area, perimeter, shape, color, and connections to other objects. 5. Feature sets are matched against a database to identify specific types of objects. 6. Analysis of each object is done by comparing specific characteristics of the observed object with conditions specified in the database. 7. If certain conditions are met or not met, the computer system would signal to an operator for corrective action.Certain conditions in vaults or substations change slowly, and there can be a relatively low levelof activity, such as pipe corrosion or ED cable movement. This may make the processing ofimages more feasible. However, many of the conditions that are being inspected for are hiddenfrom clear view or require multiple lines of sight. With this in mind, there are three primaryapproaches to camera deployment and image processing, as follows: • Fixed cameras. Image analysis is simplified when cameras are mounted at fixed locations with fixed orientations. This facilitates storing a reference image for comparison with the current image to determine if anything has changed. If image analysis detects any new object in the current image, this would be interpreted as encroachment. A similar approach could be taken to evaluate component degradation. • Pan/tilt mounts with zoom lenses. The fixed-camera approach simplifies image analysis but would require more cameras than a method that uses cameras with azimuth and elevation (pan and tilt) control and possibly a zoom lens. Such a camera could be controlled to execute a repeated observation of a cable/splice span within the vault, using a raster scan with the zoom lens increasing image magnification for more distant views. Image analysis software would have to include inputs of the azimuth positions to determine the location of the image frame. This would be used to access a database listing the types of objects expected in each frame for comparison with the objects found in the current image. • Movable cameras. Additional flexibility can be introduced by mounting the camera with pan/tilt/zoom positioning on a platform that can move along the cable/splice within the vault. In this case, image analysis and comparison would include the camera location to determine the location of the image frame. The inspection strategy would most likely involve moving the sensors to specified coordinates and then capturing a sequence of images. Objects identified in each frame would be compared to objects in a database for all frames of view along the cable. The imaging system could perform a complete video tour and analysis from one location, and the sensor would then move to the next inspection location along the span. 3-2
  30. 30. 3.2.2 CamerasMass production of components for consumer digital cameras has resulted in improvedperformance and reduced cost for cameras intended for automated computer image analysis. Alarge number of monochrome and color cameras with resolutions ranging from 640 x 480 pixelsto 2K x 2K pixels are available, and image resolutions are expected to increase in the comingyears. Signal interfaces range from the conventional RS-170 analog signals to standard digitalinterfaces including USB, IEEE 1394 (Firewire), CameraLink, and GigabitEthernet as well aswireless modes. In the future, we can expect to see fewer analog cameras and more high-speeddigital transmission, especially wireless. Many cameras include electronic shutter control,allowing extended exposure times for low-light operation.Several manufacturers supply cameras with image processing computers built into the case. Allstandard image analysis routines can be programmed in these “smart cameras,” eliminating theneed for a separate image analysis computer. In addition to standard video output, these camerasystems include USB and wireless interfaces so that the results of image analysis can be reportedover a low bandwidth channel. They also provide the capability of transmitting compressedimages at low data rates when it is desirable for an operator to see a scene to verify a conclusionor decide on a course of action. Some of these smart camera computers can accept other inputsignals; they could potentially provide all of the computational functions of a sensor node.3.2.3 Applications of Optical ImagingComputer analysis of camera images can be used for automated detection of a wide range ofdefects that are currently found by visual observation. Encroachment (damage, water penetration,or foreign objects) into a vault or substation can be identified by detecting objects in locationsthat should be clear. The condition of structural components can be evaluated. The surfacepatterns of vault structures, cable clamping members, and terminations would also be analyzed todetect patterns that would indicate rust, corrosion, or other surface damage.3.3 IR Image SensingIR cameras are more sensitive to longer wavelengths than conventional color cameras. The mostuseful IR band is long-wave or thermal IR, from 8 to 14 microns in wavelength. Early thermal IRcameras used a single detector with a scanner to build up an image, but current systems usemicrobolometer arrays and quantum well devices fabricated with typical resolution of 320 x 240pixels. Many IR camera systems today are designed for operators to conduct thermal surveys,using image enhancement software and a viewing screen. Most that are intended for use at fairlyshort range and long focal length lenses (made from germanium) are expensive. Radiometriccameras are calibrated so that an accurate surface temperature can be read from the thermalimage. Nonradiometric cameras provide an indication of relative temperature but not absolutetemperature.The amount of IR radiation from a source depends on the temperature of the surface and theemissivity of the source. Very smooth or shiny surfaces emit a smaller amount of radiation thanrough or dull surfaces. Accurate temperature measurements require knowledge or assumptions ofthe surface emissivity. 3-3
  31. 31. IR cameras are often classified as cooled or uncooled. High-end thermal IR cameras oftenprovide a peltier or compressor system to cool the detector to reduce the effect of thermal noise.Uncooled cameras are typically less expensive, are smaller, and use less power, but they are lesssensitive and have more image noise.Some IR cameras, such as the Indigo OEM Photon from Infrared Systems or the CantronicThermal Ranger, are intended for integration into automated surveillance or inspection systems.Compared with handheld systems intended for operator use, these cameras are small, arecompact, have low power requirements, and are suitable for an automated inspection stationwhen used with custom image analysis software.In the underground transmission inspection systems, thermal IR cameras can be used to identifyhot spots caused by overheating splices in vaults. One alternative to a complete IR camerasystem is to include an IR thermometer, which is a single IR detector with optics to focusradiation from a small area on the detector (essentially a 1 x 1 pixel camera). The IRthermometer would be mounted and bore-sighted to a conventional camera on a pan/tilt mount.Image analysis would be used to aim the thermometer at locations in the image where elevatedtemperatures might indicate failing components. Slight variations in the orientation could beused to build up a thermal image of a component. This process would be very slow comparedwith that of an array IR camera but might be a useful low-cost alternative for a camera station.3.3.1 Applications of IR ImagingIR imaging can be used to detect excessive heat generated by failing components, such as asplice in a vault and a termination in a substation or on a transition tower. With appropriateimage analysis, it could be used for automated detection.3.4 Vibration SensingVibration sensors measure various quantities related to vibration, including displacement,velocity, and acceleration. The most commonly used vibration transducer is the accelerometer.Most commercially available accelerometers are piezoelectric transducers. They use aprepolarized piece of piezoelectric material that produces a charge proportional to forces actingon it. A piezoelectric accelerometer typically employs a mass (either in a shear or a compressionconfiguration) that produces a force on the piezoelectric element that is proportional to theacceleration experienced by the mass. Many piezoelectric accelerometers contain integralelectronics that convert the charge produced by the piezoelectric material to a voltage or current.With the advent of microelectromechanical systems (MEMS) devices, a new class ofaccelerometers is now commercially available. MEMS accelerometers are typically capacitivedevices that employ parallel plates or interdigitated fingers whose capacitance changes as afunction of applied acceleration. MEMS accelerometers are increasingly being used in manycommercial applications, such as airbag deployment sensors. Such devices can be produced withextremely small form factors, requiring very little power. Unlike piezoelectric accelerometers,capacitive MEMS accelerometers can respond to dc accelerations, making them appropriate foruse as tilt sensors as well as vibration sensors. 3-4
  32. 32. Commercially available accelerometers can be obtained in a variety of form factors and withwidely varying sensitivities and frequency responses. Piezoelectric accelerometers can be usedfor sensing vibration with frequencies as low as 0.1 Hz or less, and up to 10 kHz or more.Capacitive accelerometers are available that respond in a frequency range from dc up to a fewkHz. Transducers are available that are capable of measuring vibration levels ranging from a fewmicro-Gs to several thousand Gs.3.4.1 Applications of Vibration SensorsVibration data can be used to identify a wide variety of phenomena, from transient effects tonondestructive damage identification. For high-voltage transmission applications, vibrationtransducers could be used to identify heavy construction equipment near vaults and detect someforms of foundation damage.3.5 Acoustic SensingMeasurements of the acoustic signal and analysis of the results may be able to determine if thereis any PD in the cable systems or fluid leaks from the steel pipes. These might be more effectivewith cables terminating in gas-insulated switchgear, where acoustic emissions originating withinthe epoxy barrier or on the gas side would be less attenuated than emissions within cables, joints,or terminations.It can be possible to use the acoustic emission technology for fluid leak detection and locationbecause leaks may produce noises over a wide range of frequencies and the noises propagatethrough the pipe structure and can be detected. The typical equipment used for this techniqueincludes listening devices, such as piezoelectric elements, to sense sound or vibration.3.6 Strain SensingStrain measurements are typically made on structural components to determine the forces actingon them, whether the yield strength of the material has been exceeded or periodic vibrations orcyclic movements are severe enough to cause fatigue problems in the material. Strainmeasurement can also be accomplished with fiber Bragg grating sensors, which make the strainmeasurement attractive to transmission cable applications. But the devices are still very costlyand have limited availability.3.6.1 Applications of Strain SensorsStrain measurements could be used to identify deformation of structural members caused byexcessive mechanical loading. Typical examples include thermal-mechanical bending of powertransmission cables and deformation of underground vault structure components, such as cablesupport racks and clamps. Strain measurement sensors would need to be applied directly to thestructural members being measured. 3-5
  33. 33. 3.7 Ultrasonic SensingUltrasonic testing is based on time-varying deformations or vibrations in materials. In solids,sound waves can propagate in four principal modes: longitudinal waves, shear waves, surfacewaves, and in thin materials as plate waves, based on the way the particles oscillate.Compression waves can be generated in liquids, as well as solids, because the energy travelsthrough the atomic structure by a series of comparison and expansion (rarefaction) movements.Longitudinal and shear waves are most widely used. Guided waves can also be generated. Thewaves are controlled by the geometry of the object. These waves include plate waves, Lambwaves, and others. Plate waves can be generated only in thin metal plates. Lamb waves are themost commonly used plate waves in nondestructive testing. Lamb waves are complex vibrationwaves that travel through the entire thickness of a material. Propagation of Lamb waves dependson the density and the elastic material properties of the object. Lamb waves are affected by thetest frequency and material thickness.Ultrasonic waves are most often generated with piezoelectric transducers made frompiezoelectric ceramics. The conversion of electrical pulses to mechanical vibrations and theconversion of returned mechanical vibrations back into electrical energy is the basis forultrasonic testing. A number of variables will affect the ability of ultrasound to locate defects.These include the pulse length, type and voltage applied to the crystal, properties of the crystal,backing material, transducer diameter, and the receiver circuitry of the instrument.3.7.1 Magnetostrictive SensingMagnetostrictive sensor (MsS) technology is a method of generating ultrasonic guided wavesinto a material that can travel over a long range to detect changes in material cross section.Guided waves refer to mechanical (or elastic) waves in ultrasonic and sonic frequencies thatpropagate in a bounded medium (such as a pipe, plate, or rod) parallel to the plane of itsboundary. The wave is termed guided because it travels along the medium guided by thegeometric boundaries of the medium.Because the wave is guided by the geometric boundaries of the medium, the geometry has astrong influence on the behavior of the wave. In contrast to ultrasonic waves used inconventional ultrasonic inspections that propagate with a constant velocity, the velocity ofguided waves varies significantly with wave frequency and geometry of the medium. In addition,at a given wave frequency, guided waves can propagate in different wave modes and orders.Although the properties of guided waves are complex, with judicious selection and propercontrol of wave mode and frequency, guided waves can be used to achieve 100% volumetricinspection of a large area of a structure from a single sensor location.The MsS, developed and patented by Southwest Research Institute, is a sensor that generates anddetects guided waves electromagnetically in the material under testing. For wave generation, itrelies on the magnetostrictive (or Joule) effect: the manifestation of a small change in thephysical dimensions of ferromagnetic materials—on the order of several parts per million incarbon steel—caused by an externally applied magnetic field. For wave detection, it relies on theinverse-magnetostrictive (or Villari) effect: the change in the magnetic induction offerromagnetic material is caused by mechanical stress (or strain). Because the probe relies on themagnetostrictive effects, it is called a magnetostrictive sensor. 3-6
  34. 34. In practice, the transmitted coil and receiver coil are the same or at least colocated. The sensor isconfigured to apply a time-varying magnetic field to the material under testing and to pick upmagnetic induction changes in the material caused by the guided wave. For ferromagneticcylindrical objects (such as rods, tubes, or pipes), the MsS is ring-shaped and uses a coil thatencircles the object. For plate-like objects, the MsS is rectangular-shaped and uses either a coilwound on a U-shaped core or a flat coil. If the component is not ferromagnetic, a thinferromagnetic strip can be bonded to the part, and the guided wave is then generated in theferromagnetic strip, which is then coupled into the part being inspected.In practical inspection applications, the guided wave generation and detection are controlled towork primarily in one direction so that the area of the structure on either side of the sensor can beinspected separately. The wave direction control is achieved by employing two sensors and thephased-array principle of the MsS instrument.For operation, the MsS requires that the ferromagnetic material under testing be in a magnetizedstate. This is achieved by applying a dc bias magnetic field to the material using either apermanent magnet, electromagnet, or residual magnetization induced in the material. The dc biasmagnetization is necessary to enhance the transduction efficiency of the sensor (from electricalto mechanical and vice versa) and to make the frequencies of the electrical signals and guidedwaves the same.Technical features of the MsS include electromagnetic guided wave generation and detection.These features require no couplant, are capable of operating with a substantial gap to the materialsurface, and have good sensitivity in frequencies up to a few hundred kHz, which is ideal forlong-range guided wave inspection applications.The MsS is directly operable on structures made of ferrous materials, such as carbon steel oralloyed steel. The MsS is also operable on structures made of nonferrous materials, such asaluminum, by bonding a thin layer of ferromagnetic material (typically nickel) to the structureunder testing or inspection and placing the MsS over the layer. In the latter case, guided wavesare generated in the ferromagnetic layer and coupled to the nonferrous structure. Detection isachieved through the reverse process. This technology is applicable for monitoring structures.In long-range guided wave inspection and monitoring, a short pulse of guided waves in relativelylow frequencies (up to a few hundred kHz) is launched along the structure under inspection, andsignals reflected from geometric irregularities in the structure—such as welds and defects—aredetected in the pulse-echo mode. From the time to the defect signal and the signal amplitude, theaxial location and severity of the defect are determined.The typically achievable inspection range from one sensor location is more than 98.4 ft (30 m) inbare pipe and more than 32.8 ft (10 m) in bare plate. Within the inspection range, the cross-sectional area of detectable defect size using the MsS is typically 2%–3% of the total pipe-wallcross section in pipe and rod diameter in rod. In plates, it is typically 5% of the guided wavebeam size or larger. Because of the long inspection range and good sensitivity to defects, guided-wave inspection technology, such as MsS, is very useful for quickly surveying a large areastructure for defects, including areas that are difficult to access from a remotely accessiblelocation. 3-7
  35. 35. 3.7.2 Applications of Ultrasonic SensingOne common application of ultrasonic sensing is to evaluate material thickness and then detectloss of material caused by corrosion, to inspect cracks near the location of the transducers (usingangle beam), and to detect defects over a long range using guided waves. One major drawback toultrasonic sensing is the requirement to have the transducer coupled to the part. An ultrasonicguided wave technique has been evaluated for the detection of corrosion under coated pipes andcoating delamination [1]. Potential applications include fault location and leak location alongsteel pipes.MsS technology has been applied to inspection of suspender ropes on highway suspensionbridges and piping and heat exchanger tubes in refineries and chemical plants as well asdetection of corrosion in steel poles and transmission tower anchor rods in the powertransmission industry. Recent developments include monitoring of long lengths of continuousmetal with bolt holes and detection of loosened bolts, monitoring of the lattice structure buried inconcrete, and monitoring of ACSR conductors.3.8 Electromagnetic-Acoustic TransducersElectromagnetic-acoustic transducers (EMATs) generate ultrasonic waves in materials throughtotally different physical principles than piezoelectric transducers and do not need any couplingmaterials. When a wire is placed near the surface of an electrically conducting object and isdriven by a current at the desired ultrasonic frequency, eddy current will be induced in a nearsurface region of the object. If a static magnetic field is also present, these eddy currents willexperience Lorentz forces of the form F=J×Bwhere F is the body force per unit volume, J is the induced dynamic current density, and B is thestatic magnetic induction.Couplant-free transduction allows operation without contact at elevated temperatures and inremote locations. The coil and magnet structure can also be designed to excite complex wavepatterns and polarizations that would be difficult to realize with fluid-coupled piezoelectricprobes.Practical EMAT designs are relatively narrowband and require strong magnetic fields and largecurrents to produce ultrasound that is often weaker than that produced by piezoelectrictransducers. Rare-earth materials such as samarium-cobalt and neodymium-iron-boron are oftenused to produce sufficiently strong magnetic fields, which may also be generated by pulsedelectromagnets.EMAT offers many advantages based on its couplant-free operation. These advantages includethe abilities to operate in remote environments at elevated speeds and temperatures, to excitepolarizations not easily excited by fluid-coupled piezoelectrics, and to produce highly consistentmeasurements. These advantages are tempered by low efficiencies, and careful electronic designis essential to applications. EMAT is also more expensive than piezoelectric transducers. 3-8
  36. 36. 3.8.1 Applications of EMATThe application of EMAT has been in nondestructive evaluation (NDE) applications, such asflaw detection or material property characterization. EMAT is often used in high-temperatureapplications of ultrasonics or where no couplant is allowed for wall thickness and angle beaminspection for cracks. EMAT can also be used to generate guided waves in plate structures suchas lattice towers. There do not appear to be EMAT applications for long-range monitoring ofpiping, tubing, or rods, although the possibility of further development exists.3.9 Eddy Current SensingEddy current inspection is one of several NDE methods that use the principle ofelectromagnetism as the basis for conducting examinations. Several other methods, such asremote field testing, flux leakage, and Barkhausen noise, use this principle.Eddy currents are created through a process called electromagnetic induction. When alternatingcurrent is applied to the conductor, such as a copper wire, a magnetic field develops in andaround the conductor. This magnetic field expands as the alternating current rises to maximumand collapses as the current is reduced to zero. If another electrical conductor is brought intoclose proximity to this changing magnetic field, current will be induced in this second conductor.One of the major advantages of eddy current as an NDE tool is the variety of inspections andmeasurements that can be performed. In the proper circumstances, eddy currents can be used forthe following:• Crack detection• Material thickness measurements• Coating thickness measurements• Conductivity measurements for the following: – Material identification – Heat damage detection – Case depth determination – Heat treatment monitoringSome of the advantages of eddy current inspection are its sensitivity to small cracks and otherdefects, detection of surface and near-surface defects, immediate results, portable equipment,minimum part preparation, noncontact test probe, and the ability to inspect complex shapes andsizes of conductive materials.Some of the limitations of eddy current inspection are that only conductive materials can beinspected, the surface must be accessible to the probe, the skill and training required are moreextensive than for other techniques, surface finish and roughness may interfere, referencestandards are needed for setup, depth of penetration is limited, and flaws such as delaminationsthat lie parallel to the probe coil winding and probe scan direction are undetectable. Usually, theeddy current probe has to be moved over the part or placed over a part that is changing with time. 3-9
  37. 37. 3.9.1 Applications of Eddy Current SensingEddy current is used in a wide range of applications for the power and aerospace industries fordetection of cracks and corrosion. Present eddy current sensing technology could be used tomeasure corrosion depth and detect/size cracking. A specific application would be to analyze theextent of sheath fatigue in lead-alloy-sheathed SCFF or ED cables.3.10 RF Interference SensingPD in high-voltage system components produces RF interference that is detectable usingelectronic radio signal receivers. PD emissions at RFs (in the MHz range) can be demodulated tothe audio band and heard as distinctive bursts of crackling. Handheld devices—and devicesattached to the end of a live working tool—with a simple bar meter display, audio speaker, andgain control have been used in live line evaluation of distribution splices, elbows, and junctionmodules.EPRI has an ongoing project to locate PD in substations using multiple antennas and a wide-bandwidth multichannel oscilloscope to capture emissions and then signal processing algorithmsto analyze the data, correlate PD events, and estimate PD location based on the time of signalarrival from the different known antenna locations.3.11 Fluid Dissolved Gas SensingDGA is increasingly applied to both transformer and cable diagnostics. DGA can be usedthrough periodic sampling and measurement or continuous monitoring that can develop trending.EPRI is developing on-line DGA monitoring systems for use on transformers. One technology isthe metal-insulator-semiconductor (MIS) chemical sensor that is a solid-state device detectingmolecules from multi-gases such as hydrogen and acetylene. EPRI also funded a study infiberoptic sensors for on-line detection of hydrogen and acetylene inside power transformers.Novel holey fibers were recently developed to detect hydrogen, and optical microphone-basedlaser photoacoustic spectroscopy was proposed for acetylene detection.3.11.1 Applications of Fluid Dissolved Gas SensingEPRI has performed a feasibility study for on-line DGA for HPFF cables. This study examinedthe feasibility of the use of on-line DGA monitoring equipment on static, oscillating, andcirculating HPFF pipe-type cable systems and addressed the added complexity of the highpressure under which the cable operates. Several commercially available on-line gas monitoringsystems primarily used for transformers are available, such as the multi-gas analyzers fromServeron and Kelman and the single gas analyzers from GE (HYDRAN 1 ) and Morgan Schaeffer.The EPRI feasibility study recommended performing a laboratory study to investigate theeffectiveness of these analyzers in monitoring HPFF cables.The monitoring device using the MIS technology and fiberoptic methods for detecting dissolvedgases would be attractive for fluid monitoring of HPFF or SCFF cable systems.1 HYDRAN is a registered trademark of GE Energy. 3-10
  38. 38. 3.12 Fiberoptic SensingFiberoptic sensing has been applied for many decades to detect various physical and chemicalparameters. The characteristics of the fibers and the way light interacts with the fiber and fibercoating or environment around the fiber are the basis for various sensor technologies. Fiberopticsensors have many advantages over conventional sensors, including the following:• Are immune to electromagnetic interference• Can be configured as a distributed sensor as well as a point sensor• Can operate at high electrical potential• Are resistant to humidity and corrosion• Can be made small in size and light in weightIn remote sensing applications, a segment of the fiber is used as a sensor gauge while a longlength of the same or another fiber is used to convey the sensed information to a remote station.There is no electrical power supply needed at the sensor locations. A distributed sensor can beconstructed by multiplexing various point sensors along the length. Signal processing devices(for example, splitter, combiner, multiplexer, filter, or delay line) can also be made of fiberelements.Knowledge of the following parameters is of great value for the underground transmissionindustry:• Temperature• Electromagnetic field, current, voltage, and frequency• Pressure, strain, displacement, vibration, and acoustic emission• Chemical composition3.12.1 Applications of Fiberoptic Sensing3.12.1.1 Temperature SensingBoth point sensors and distributed sensors are used for measuring temperatures. Point sensorsuse a phosphorescent material at the end of the fiber. The temperature of transmission cablesplices, for example, can be monitored using the point sensors.Distributed temperature sensors (DTSs) realize the technology of laser injection into the opticalfiber. A fraction of the laser pulses is absorbed in the fiber and is backscattered as Ramansignals. The local temperature determines the intensity of the Raman signals. The intensity isused to calculate the temperature at that location. The time of flight of the laser light, opto-electronics, and a computer are used to determine location of the specific backscattered Ramanlight. Multimode or single-mode fibers are used for distributed temperature sensors. Inmultimode systems (1.8°F [1°C] accuracy), about 3.3 ft (1 m) of fiber length is needed to createa significant backscatter signal, whereas 13.1–32.8 ft (4–10 m) are needed for the single-modefiber (4.5°F [2.5°C) accuracy). These requirements designate the spatial resolution of themultimode and single-mode fibers. Multimode optical fibers are suitable for most DTSapplications, with a maximum range of 4.97–6.21 mi (8–10 km). They are typically used for 3-11
  39. 39. short-range communication systems—for example, within office buildings. Single-mode opticalfibers are used only for very long-range DTS applications with a maximum range of 18.64–24.85 mi (30–40 km). They are commonly used for long-distance communication systems.The sensors can be integrated in the cable or arranged separately near the cable. The sensorsintegrated in the cable lead to faster thermal response to the conductor and more accurateconductor temperature measurements. The sensors can also be installed in a spare duct or aseparate duct designed specifically for the purpose. Both installations can be used for hotspotmanagement, overload detection, and real-time dynamic thermal circuit ratings. Figure 3-1shows an example of distributed temperature sensing optical fibers incorporated into cablebedding tapes. Figure 3-2 shows distributed temperature sensing optical fibers in a 3-in. (76-mm)PVC conduit adjacent to a pipe-type cable pipe.Figure 3-1Distributed temperature-sensing optical fibers incorporated into cable bedding tapes(Water sensing can be constructed in a similar way under water blocking tapes.)Figure 3-2Distributed temperature sensing optical fibers in a 3-in. (76-mm) PVC conduit adjacent to a pipe-type cable pipe 3-12
  40. 40. EPRI began using this technology for underground cable systems in the mid-1990s with a YorkDTS-80 system (in 2003, the equipment was updated to a Sensa DTS-800) for measuringdistributed temperatures along underground cable routes. In addition to dynamic thermal ratingand hot spot identification, applications of optical fiber temperature sensing could be expandedto fault location, fire detection, and the like.3.12.1.2 Electromagnetic Field, Current, Voltage, and FrequencyElectromagnetic field, current, voltage, and frequency can be measured by fiberoptic sensors.The high sensitivity and wide range of frequency response, combined with other features offiberoptic sensing (such as distributed and point sensing), make the technology attractive forremote detection of PD and determination of fault location, corrosion, or insulation condition.3.12.1.3 Pressure, Strain, Vibration, and Acoustic EmissionPressure, strain, vibration, and acoustic sensors rely on application of a pressure to the sensorhead or grating in order to register an effect on the transmitted light. Distributed pressure sensingis not yet commercial although there are strain sensors in a single-mode fiber. Hydrostaticpressure monitoring tends to be at discrete points in most systems, such as for HPFF and SCFFcables and terminations.The sensors discussed could be used in a pigtail fashion and coupled to a distributed temperaturesensor for simultaneous pressure and temperature monitoring at joints and in joint casings forHPFF and SCFF cable systems.For pipe-type cables, the temperature and pressure information could be input into hydrauliccalculation programs to determine the size and location of possible leak areas along the pipelength. Optical fiber pressure sensing could be applied for monitoring thermal-mechanicalbehavior of cables, hydraulic systems, leaks, and corrosion. The acoustic measurement using afiberoptic sensor was developed as a PD sensor for transformers. Future studies can be carriedout to apply the fiberoptic sensors to monitor HPFF cables.3.12.1.4 Chemical CompositionFiberoptic sensing can be used to measure chemicals or component species of chemicals. Forexample, distributed hydrocarbon fiberoptic sensors are being used for fluid leak monitoringof large chemical storage facilities. The sensor consists of a length (usually less than 1.6 mi[2.5 km)) of fiberoptic cable. Hydrocarbons in contact with the fiberoptic cable induce a localpower loss that can be detected and located. The fiberoptic cables can be designed for thedetection of almost any petroleum derivative plus many synthetic organic liquids. Point sensorscan be used by a utility to monitor for gas chemicals in manholes and then pigtail the chemicalsensors to the distributed communication fiber to transfer the sensed information to a centralfacility. This type of chemical sensing could be used for detecting dissolved gases in the cableinsulation fluid, soil condition, and corrosion monitoring, provided that the changes to fibercharacteristics are temporary and can be restored to original conditions once the abnormality haspassed. 3-13
  41. 41. 3.13 Capacitive/Inductive Coupling (PD)PD measurements are used to assess insulation condition of cables and accessories. They can beused to verify proper installation of a cable circuit and assess insulation aging or degradation ifapplied continuously or at certain intervals.3.13.1 Applications of Capacitive/Inductive CouplingBoth capacitive and inductive couplers are used in underground transmission cable PD detection.The capacitive couplers can be integrated into the splices or joints by splice manufacturers (seeFigure 3-3) or installed in the field. Inductive couplers can be in the form of high-frequencycurrent transformers (HFCTs) placed around cable bonding lead (see Figure 3-4) or cable sheathbonding links (see Figure 3-5). Molded Insulation Molded Semicon Metal Casing Tinned Copper Braid (Sensor) Coaxial Cable Cable Insulation Shield Cable Metallic Shield Cable InsulationFigure 3-3Integral capacitive PD sensor on a pre-molded cable jointFigure 3-4High-frequency current transformers placed around cable bonding lead for PD measurements 3-14
  42. 42. Figure 3-5HFCTs placed around the cable sheath bonding link for PD measurements3.14 Flow, Temperature, Pressure, Volume, and Mass SensingSystem parameters, such as temperature, pressure, volume, or mass, can be used for hydraulicsystem monitoring of a pipe-type cable circuit.EPRI is investigating a leak detection system using artificial intelligence technology. The systemmeasures circuit load current, cable oil pressure, cable oil temperature, soil ambient temperature,and status changes in operating conditions (for example, in the pumping plant) and can beimplemented in a configuration networked with a user’s data acquisition system or as a stand-alone system.Mass flow meters are also used for pipe leak detection based on the fact that liquid mass willbalance between two ends of the pipe.3.15 Voltage, Current, and Frequency Measurements3.15.1 Dissipation Factor MeasurementDissipation factor measurement gives an indication of the average condition of the cableinsulation for the entire cable length with splices. It does not address the individual discretecomponents, such as splices, terminations, and any isolated defects. The method developed byEPRI in the 1990s [2] requires specialized field equipment and temporary line outages to install.On-line dissipation factor measurement has been discussed to develop trending through themeasurement, starting by comparing the measured dissipation factor value to the original factoryvalue. However, implementation would be difficult without permanent installation of a largereference capacitor.3.15.2 Jacket Faults and SVL Failure DetectionFor ED and SCFF cable systems, one of the most expensive maintenance activities is theperiodic testing of cable jackets to guard against corrosion. Corrosion damage could result inwater ingress in the case of ED cables and fluid leaks in the case of SCFF cables. Jacket faultscould also cause electrical safety hazards as sheath currents are injected into the ground, possibly 3-15

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