Future Inspection of Underground Transmission Lines

Future Inspection and Monitoring of Underground
Transmission Lines

1020168

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

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

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

(B) ASSUMES RESPONSIBILITY FOR ANY DAMAGES OR OTHER LIABILITY WHATSOEVER (INCLUDING
ANY CONSEQUENTIAL DAMAGES, EVEN IF EPRI OR ANY EPRI REPRESENTATIVE HAS BEEN ADVISED
OF THE POSSIBILITY OF SUCH DAMAGES) RESULTING FROM YOUR SELECTION OR USE OF THIS
DOCUMENT OR ANY INFORMATION, APPARATUS, METHOD, PROCESS, OR SIMILAR ITEM DISCLOSED IN
THIS DOCUMENT.

ORGANIZATION(S) THAT PREPARED THIS DOCUMENT

Electric Power Research Institute (EPRI)

This is an EPRI Technical Update report. A Technical Update report is intended as an informal report of
continuing research, a meeting, or a topical study. It is not a final EPRI technical report.

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

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

Copyright © 2009 Electric Power Research Institute, Inc. All rights reserved.

CITATIONS
This document was prepared by
Electric Power Research Institute (EPRI)
1300 West W.T. Harris Blvd.
Charlotte, NC 28262
Principal Investigators
T. Zhao
S. Eckroad
A. MacPhail
This 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

PRODUCT DESCRIPTION
Underground transmission lines have performed reliably for the power transmission industry.
Nonetheless, there are opportunities to improve on-line condition assessment of the underground
cable systems. Some of these opportunities can be realized by incorporating improved sensors,
more efficient power sources to the sensors, enhanced data collection systems, and better
integration with utilities’ operations systems. This report describes technologies that can be
applied in future inspection and monitoring of underground transmission lines.
The report is a companion to the Electric Power Research Institute (EPRI) report Future
Inspection of Overhead Transmission Lines (1016921).

Results and Findings
Systems for inspection and monitoring of underground transmission lines consist of sensors that
acquire diagnostic data from components of interest and communications that collect the sensor
data and deliver them to a central repository. The information contained herein accomplishes the
following:
• 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 lines

Challenges and Objectives
The objectives of the work described in this report are to improve the quality of preventive
maintenance performed on 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, which leads to improved reliability and operations. To achieve these goals,
enhanced inspection and monitoring of critical components must be deployed, using newly
developed technology in the areas of sensors, power harvesting, and telecommunications
systems. As the requirements for transmission line reliability and availability become more
stringent, technology becomes a major enabler.

Applications, Value, and Use
The report is targeted at maintenance personnel and managers who are responsible for the
upkeep of their company’s underground transmission lines. It will serve as a roadmap for the
development and demonstration of inspection and monitoring technologies for these important
systems.
After a brief introduction, Section 2 of this report covers the concepts that characterize
discussions about the assessment and maintenance methods used for extruded dielectric and
laminar dielectric cables of underground systems. Section 3 presents detailed information about
the candidate technologies for sensors, and Section 4 does the same for communication
technologies. EPRI conducted an industry scan of 18 companies worldwide regarding their use

v

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 of
underground transmission cable systems.

EPRI Perspective
EPRI has conducted the research described in this report in order to advance the field of
inspection and monitoring technologies for underground transmission. For EPRI-member
utilities, the chief benefits of better inspection and monitoring methods will be a combination of
lower costs in system assessment and maintenance and fewer circuit failures and outages.

Approach
Utility staff familiar with underground transmission line inspection and monitoring, experts in
sensing and communicating technology, and transmission system researchers collaborated and
developed this report.

Keywords
Communication technology
Inspection
Monitoring
Sensor
Transmission
Underground

vi

ABSTRACT

Underground transmission lines have performed reliably for the power transmission industry.
Nonetheless, there are opportunities to improve on-line condition assessment of the underground
cable systems. Some of these opportunities can be realized by incorporating improved sensors,
more efficient power sources to the sensors, enhanced data collection systems, and better
integration with utilities’ operations systems. This report, which is a companion to the Electric
Power Research Institute (EPRI) report Future Inspection of Overhead Transmission Lines
(1016921), describes technologies that can be applied in future inspection and monitoring of
underground transmission lines.
Systems for inspection and monitoring of underground transmission lines consist of sensors that
acquire diagnostic data from components of interest and communications that collect the sensor
data and deliver them to a central repository. This report describes system concepts, addresses
candidate technologies for sensor and communication systems, and provides demonstration
scenarios for the inspection and monitoring of underground transmission lines. The objectives of
the work described in this report are to improve the quality of preventive maintenance performed
on 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, which
leads to improved reliability and operations.
Utility staff familiar with underground transmission line inspection and monitoring, experts in
sensing and communicating technology, and transmission system researchers collaborated and
developed this report.

vii

ACKNOWLEDGMENTS
The report is a companion report to the EPRI report Future Inspection of Overhead Transmission
Lines (1016921). Special thanks to the Principal Investigators of Southwest Research Institute
and the Principal Investigator and Project Manager, Dr. Andrew Phillips of EPRI, who
developed that report. Technologies common to underground transmission are repeated or
summarized in this report for completeness.
The participation of utility advisors in the report’s development is acknowledged and
appreciated.

ix

CONTENTS
1 BACKGROUND AND INTRODUCTION ................................................................................1-1

2 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 ................................................2-14

3 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

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 .......................................................3-15

4 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 ............................................................................4-6

5 INDUSTRY SCAN ON SENSOR APPLICATIONS IN UNDERGROUND TRANSMISSION
CABLE 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

5.2.17 USi (New York)...................................................................................................5-6
5.2.18 UtilX/CableWise (Washington) ...........................................................................5-6

6 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 .................6-2

7 REFERENCES .......................................................................................................................7-1

xiii

1
BACKGROUND AND INTRODUCTION
Underground transmission lines provide reliable performance. These transmission lines can be
categorized 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-pressure
fluid-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 assessment
of the cable systems, leading to enhanced reliability, operations, and maintenance. Some of these
opportunities can be realized by incorporating improved sensors, more efficient power sources to
the sensors, enhanced data collection systems, and better integration with utility operation
systems.
Performance, by definition, must be measurable. The improved sensors, power sources, data
collection systems, and integration systems described in this report are all ultimately aimed at
improving the measurability of cable system performance. In the context of underground
transmission 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 maintenance

The goals and objectives of the work described in this report are to improve the quality and
lower the costs of preventive maintenance, and, in so doing, reduce the need for corrective
maintenance, which leads to improved reliability and operations. To achieve these goals and
objectives, enhanced inspection and monitoring of critical components must be deployed, using
new technology developments in the areas of sensors, power harvesting, and telecommunications
systems.
Transmission line components are currently inspected and assessed, mainly using field
personnel. The Electric Power Research Institute (EPRI) and others are currently investigating
and developing automated/unmanned inspection and monitoring technologies for underground
transmission lines. With transmission line security issues apparently growing in number, the
need for automated, unmanned, and continuous monitoring of underground transmission lines is
increasing. Technology advancements could enable an effective, comprehensive, automated
inspection and monitoring system for underground transmission lines.

1-1

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

2
SYSTEM CONCEPTS
2.1 Introduction
System concepts are described for instrumentation of underground transmission cable systems
with 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 and
monitoring 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 methods

Figure 2-1, Figure 2-2, and Figure 2-3 show schematics of the inspection and monitoring
applications for ED, HPFF and HPGF, and SCFF transmission lines, respectively.

2-1

Table 2-1
Inspection and monitoring of underground transmission lines

Failure Diagnostic Applicable Cable Overall Monitoring Sensor Opportunity Comments for
Modes/Indicators Method Systems and Status Capability Future Research
Auxiliary and Prioritization
Equipment
Presently available on-line, continuous monitoring
Hot spots along Temperature ED, HPFF, HPGF, On-line Monitor through Distributed fiberoptic Commercial systems
cables—limiting SCFF monitoring distributed temperature sensing available, EPRI
factor of loading available. fiberoptic and thermocouples tailored collaboration
capability and sensors and available. opportunity available
insulation aging thermocouples.
Hydraulic system Fluid or gas HPFF, HPGF, On-line Monitor at Pressure and other Commercial systems
malfunction pressure, flow, SCFF monitoring pressurizing transducers available. available
pumping plant available. systems.
operation, reservoir
fluid levels, piping
damage, and leaks
Deterioration 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, noise
defects especially and time- acoustic inductive and filtering, data
at joints, consuming emission capacitive couplers, processing, and so
terminations, and inspection. sensors. Off-line acoustic emission). on
interfaces and on-line Optical fiber sensors
maintenance under investigation.
inspection. Distributed sensor
development
opportunities exist
along cables.

2-2

Table 2-1 (continued)

Equipment
Buried steel pipe Cathodic protection HPFF, HPGF On-line Monitor cathodic Potential and current Commercial systems
corrosion and system settings monitoring protection meters available. available
coating 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 systems
sheath/shield system settings monitoring protection meters available. available
corrosion 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
investigation
Presently available off-line maintenance inspection, with opportunities for on-line, continuous monitoring
Overall insulation Dissipation factor HPFF, HPGF, In-field test Off-line Development EPRI in-field system
integrity, such as SCFF with special maintenance opportunities exist for available for laminar
moisture, fluid equipment. inspection. on-line monitoring. dielectric cables
contamination
Bonding and link Sheath current ED, SCFF In-person Off-line Sensors available but On-line monitoring
box corrosion, measurements inspection. maintenance need integration. desirable
loose connection, inspection.
insulation damage
Sheath voltage SVL current ED, SCFF In-person Off-line Sensors available but On-line monitoring
limiter (SVL) inspection. maintenance need integration. desirable
failure inspection.

2-3


Equipment
Vault hardware Optical image ED, SCFF Time- Off-line or on-line Sensor development On-line monitoring
and component infrared image, consuming maintenance opportunities exist. desirable
(ceiling, walls, vibration, acoustic inspection inspection. Some sensors
pipe, clamps, sensing, and with safety available but need
ground wires, temperature concerns. integration.
racks, pumping, indicating strips on
and so on) components for
degradation, cracks, leaks,
corrosion, corrosion, coating
overheating, damage,
flooding, safety- component
related gas damage, safety-
related gas level,
and so on
Internal X-ray inspection ED, HPFF, HPGF, Expensive Off-line Portable X-ray On-line monitoring
movement, SCFF and time- maintenance equipment available. unlikely
misalignment, or consuming inspection.
damage of cables inspection.
and accessories
Fluid 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


Equipment
Presently available off-line maintenance inspection based on laboratory tests, with opportunities for on-line, continuous monitoring
Aging/degradation Dissolved gas HPFF, HPGF, Laboratory Off-line Sensor development On-line monitoring
of fluid or paper analysis (DGA), SCFF test with fluid maintenance opportunities exist. under investigation
insulation— dissipation factor, samples from inspection, fluid by EPRI
indicator of hot direct current (dc) operating samples from
spots, PD, and resistance, equipment. operating
arcing alternating current equipment.
(ac) resistance,
moisture content,
particle content,
gas absorption
capability
Aging of paper Degree of HPFF, HPGF, Laboratory Mechanical/ Sensor development Unlikely for on-line
insulation polymerization SCFF test with electric strength opportunities exist. monitoring
(DP), mechanical samples from versus DP
strength, operating known.
dissipation factor, equipment.
furfural
Other desirable on-line, continuous inspection and monitoring
Thermo- Strain sensing, ED, HPFF, HPGF, New. On-line Sensor development On-line monitoring
mechanical sidewall pressure SCFF monitoring opportunities exist. desirable
bending sensing desirable.
Moisture barrier Moisture level ED New. On-line Sensor development On-line monitoring
degradation monitoring opportunities exist. desirable
desirable.
Lead sheath Strain sensing SCFF New. On-line Sensor development On-line monitoring
fatigue monitoring opportunities exist. desirable
desirable.

2-5

Figure 2-1
Inspection and monitoring of ED underground transmission lines

2-6

Figure 2-2
Inspection and monitoring of HPFF and HPGF underground transmission lines

2-7

Figure 2-3
Inspection and monitoring of SCFF underground transmission lines

2-8

The system scope is limited to underground transmission line applications (>46–500 kV), not
lower distribution voltages. It was considered that the addition of electrical wiring to
interconnect distributed sensors is not viable because of electromagnetic susceptibility and other
concerns. Consequently, sensor concepts at vault locations will mainly consider wireless and/or
fiberoptic 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 center

2.2 System Architecture
Systems for inspection and monitoring consist of sensors that acquire diagnostic data from
components of interest and communications that collect the sensor data and deliver them to a
central repository.
The sensors may be directly attached to the item being monitored or separately located, such as
in the case of a camera in a vault. Communication devices may be mounted in or near vaults or
located on a wide variety of remote, and possibly mobile, platforms. The sensors and
communication devices may operate and be polled periodically (for instance, at intervals of
minutes, hours, or days) or continuously monitored (for example, in real time) depending on the
applications. 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 energy
management system computer with a PI server.
An important feature of the system is flexibility and interoperability with a wide variety of
sensor types and communication methods. The information that is required for each sensor
reading 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

The sensor type and geolocation may be associated with the ID and hard-coded in a database at
the central repository so that this information does not need to be redundantly transmitted
through the system for every reading. For remote sensors, the geolocation will need to be
communicated so that the system can associate the reading with a particular item (at a known
geolocation) 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 applications
where relaying readings is an effective method to communicate data back to the central
repository. Similarly, relaying readings between sensors is an acceptable communication
approach.
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 allow
different sensor applications to handle these tradeoffs differently. For example, in some
applications, it will be most efficient and optimal to process sensor data locally at the sensor and
to report back the reading as a simple answer or alarm. In other applications, it may be desirable
to have all the information communicated back to the central repository for archival and possibly
even human interpretation. In the former case, the amount of data to be passed through the
communication channel is very low (1 bit, maybe once a day), but the processing power required
at the sensor may be high in order to make an intelligent decision with high confidence. In the
latter case, the amount of data passed through the communication channel is very high (maybe
10 MB for a high-resolution image), with much greater potential for impact to system throughput
and storage space. The latter approach may be merited when automated results are questionable
and manual interpretation of the raw data is required.
Hybrid sensing protocols or approaches may be advantageous and are supported by the system
architecture. For example, a flag sensor may simply indicate when a condition needs to be
further evaluated. Whether done remotely or while in the field, interacting with the sensor may
be desirable in order to control the amount of detailed data that is provided. The flag sensor may
conserve power by not communicating until there is a problem. One possibility is an intelligent
sensor that monitors a system condition, and then, based on the sensed severity, applies a
commensurate amount of on-board resources (power, processing, memory, and communication
bandwidth) in order to operate effectively and with high efficiency.
Sensors typically require a source of power, a sensing mechanism, a controller to format
measurements into readings, and a short-range wireless data communication mechanism. If
communication hubs are applied, they will have similar needs for power and controller functions
and 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 may
also have local memory for storing readings, either to buffer data when communication links are
down or as a local repository for data archival/backup.
Although there are functional differences between sensors and hubs, device implementation is
flexible to combine features. In other words, hubs can also incorporate sensors and sensors can
also serve as hubs; it is not a requirement that they be separate devices. A distinguishing feature
of 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

require very little maintenance—preferably, none. However, hubs are, in general, expected to be
more 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 the
hub, which uses a long-range wireless or leased line link to the central data repository. This is
not a requirement, but it is based on the vision that many low-cost, low-power, low-bandwidth
sensors will be deployed at a vault site and that a local hub as described previously can help by
collecting these data, providing a local redundant data repository, and coordinating long-haul
communications.
Figure 2-4 shows a functional diagram for a sensor technology.

Figure 2-4
Sensor function

2.3 Communication Considerations
A communication system provides a means for communicating sensor data at vaults and along
cables to a central data collection and processing facility. SCADA systems for wide-area
monitoring have long been in existence and offer reliability enhancements for electrical power
transmission systems. The system concept requires a customized implementation based on sensor
population, data rates, and ranges. The customized implementation can be interfaced into a
central facility SCADA system, or it can operate as a stand-alone system, running its own
SCADA. 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 infrastructure
define the requirements for the operational characteristics of communication systems. The
primary considerations are the distance over which the data need to be communicated (referred
to as range) and the amount of data to be communicated in a period of time (referred to as data
rate).

2-11

A communication system range is influenced by several factors. The vaults under consideration
are underground concrete structures, separated by 500–4000 ft (150–1200 m) and installed over
tens of miles and even longer, and the data that are generated locally need to be collected at a
central facility that may be tens to hundreds of miles away.
A variety of sensor configurations are envisioned within the system concept. Some sensors will
be attached directly to cable circuit components—for example, splices or terminations. Other
sensors will be mounted along the cables, pipes, and insulating fluids. The need for these
different sensor configurations leads to a distributed sensing system. The communication system
will need to coordinate the collection of data from many distributed sensors for transfer to a
central facility.
The distributed location of sensors imposes several constraints on the sensor design. Sensors in
the 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 for
communicating the sensor data. Because low power consumption is the most restricting
constraint for the sensors in vaults, the communication technology is consequently relatively
short range and infrequent to keep power consumption at a minimum. The opposing
requirements of low power consumption and short-range communication, versus needing to
collect 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 the
power requirements for the communication technology.
Because hubs will likely require much higher power consumption than sensors in order to
support long-range communication and greater bandwidth, it may be beneficial to incorporate a
large battery at the hub. This would dictate additional logistics and periodic maintenance, but the
tradeoff may be worthwhile. On the other hand, it would not be desirable to do that for a large
population of sensors.
Data from each sensor cannot be directly transmitted to the central facility due to range and
power consumption trades. Thus, the communication system requires data communication
relays. A number of architectural options for the communication system are available, including
the 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 be
communicated. However, the very low duty cycle and data rate of many of the sensors make
daisy chaining possible for certain sensor technologies. Higher data rate sensors may require
more restrictions on the number of devices sharing a communication channel. A combination of

2-12

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 architectural
options should be considered during the evolution of the system concept.
Figure 2-5 shows a concept for communications networking.

Figure 2-5
Communication networking (Sensor-to-sensor, daisy chained to a central facility.)

2.4 Power Considerations
Sensors and communication hubs will require power for operation. Although batteries may be
convenient to test and demonstrate the system, they are seen as a maintenance problem in the
system concept. The goal is to use renewable power sources in lieu of batteries. This is a difficult
challenge, especially for wide-range, high-bandwidth data communication requirements. With
present technology, it is not really possible to implement a batteryless system, except for very
limited and simple scenarios. Even over the next 20 years, without significant breakthroughs, this
will remain a difficult challenge, albeit a worthy one, to keep in mind as new technologies are
introduced.
Alternatives to batteries include solar, thermoelectric, the electric and magnetic fields that are
generated 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, they
may be effective. The use of a rechargeable battery coupled with power harvesting will have
strong merit.

2-13

2.4.1 Potential for Harvesting Power from Magnetic Field
Power to operate a sensor in a vault can be harvested from the magnetic field that is generated
from the current flowing through the cable or cable pipe. A short coil on a ferrite rod or a current
transformer 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 flow
in transmission cables, and it may be possible for the lower currents flowing in cable sheaths and
the zero-sequence currents flowing in cable pipes. Detailed investigations would be needed to
prove the abilities to operate effectively under very low and very high cable currents (such as
fault currents) and to withstand switching surges and transient overvoltages.

2.4.2 Potential for Harvesting Power from Induced Voltage of Grounded
Components
ED and SCFF cable systems often employ a ground continuity conductor (GCC) with specially
bonded systems. Designs usually try to minimize the induced current, but some still inevitably
flows. 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 line
load. Rechargeable batteries would provide power during low loads or outages. Detailed
investigations would be needed to prove effective performance under abnormal operating
conditions, such as faults, resulting in induction or high through-currents in the GCCs.

2.4.3 Potential for Optical Power Transmission
Nonconducting fiberoptics can be used to transmit small amounts of power, although the
efficiency is low. The system consists of an optical source (light-emitting diode [LED] or
laser 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 that
10–30 mW of power can be generated at a photovoltaic junction (solar cell). This is based on
50% efficiency coupling to and from the fiberoptic and 4%–8% photovoltaic conversion
efficiency. This example of energy conversion efficiencies is only a guide; more accurate
calculations with specific components and laboratory confirmation should be done if this is to
be considered as a viable power option.
Although this efficiency of 1%–3% is very low, there are cases where this method may be useful
for 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, and
the other would be used to transmit control and data signals. For micropower sensors that are
operated only a few minutes a day, the low efficiency may not be a factor.

2.4.4 Potential for Other Power Harvesting Methods
There is good potential for other power harvesting methods, although a technical review of these
technologies is not a focus of this report. For example, in close proximity to an underground
transmission line system, the high magnetic fields can be harvested.

2-14

3
CANDIDATE SENSOR TECHNOLOGIES
3.1 Introduction
This report attempts to address and provide insight into some of the enabling sensor and data
communication technologies that appear to be suited for the application.
In addition to the common technologies for overhead transmission applications, some specific
sensor 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 sensing

3.2 Optical Image Sensing
Optical imaging includes methods in which an image provided by a camera is interpreted by
computer analysis to identify or detect specific conditions. Different camera systems can provide
image representations in visible, infrared (IR), or ultraviolet (UV) spectral bands, and each of
these bands has advantages for detecting different conditions or defects. There is also a variety of
methods for positioning or deploying imaging cameras, with some choices more suitable for
detecting certain types of defects. Optical imaging is the automated analog of current visual
inspection methods and has potential application for a high percentage of the transmission cable
components in vaults and substations.

3.2.1 Image Analysis
Computer analysis of images to detect specific conditions or abnormalities is widely used in
manufacturing and other well-structured areas where images are obtained with consistent
lighting, viewpoint, magnification, and other factors. Analysis of images with wide variations in
illumination is more complex, but adaptive methods are available to compensate for changing
conditions. Statistical methods are used to normalize image intensity and minimize the effects of
slowly 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

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 level
of activity, such as pipe corrosion or ED cable movement. This may make the processing of
images more feasible. However, many of the conditions that are being inspected for are hidden
from clear view or require multiple lines of sight. With this in mind, there are three primary
approaches 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

3.2.2 Cameras
Mass production of components for consumer digital cameras has resulted in improved
performance and reduced cost for cameras intended for automated computer image analysis. A
large number of monochrome and color cameras with resolutions ranging from 640 x 480 pixels
to 2K x 2K pixels are available, and image resolutions are expected to increase in the coming
years. Signal interfaces range from the conventional RS-170 analog signals to standard digital
interfaces including USB, IEEE 1394 (Firewire), CameraLink, and GigabitEthernet as well as
wireless modes. In the future, we can expect to see fewer analog cameras and more high-speed
digital 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. All
standard image analysis routines can be programmed in these “smart cameras,” eliminating the
need for a separate image analysis computer. In addition to standard video output, these camera
systems include USB and wireless interfaces so that the results of image analysis can be reported
over a low bandwidth channel. They also provide the capability of transmitting compressed
images at low data rates when it is desirable for an operator to see a scene to verify a conclusion
or decide on a course of action. Some of these smart camera computers can accept other input
signals; they could potentially provide all of the computational functions of a sensor node.

3.2.3 Applications of Optical Imaging
Computer analysis of camera images can be used for automated detection of a wide range of
defects 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 locations
that should be clear. The condition of structural components can be evaluated. The surface
patterns of vault structures, cable clamping members, and terminations would also be analyzed to
detect patterns that would indicate rust, corrosion, or other surface damage.

3.3 IR Image Sensing
IR cameras are more sensitive to longer wavelengths than conventional color cameras. The most
useful IR band is long-wave or thermal IR, from 8 to 14 microns in wavelength. Early thermal IR
cameras used a single detector with a scanner to build up an image, but current systems use
microbolometer arrays and quantum well devices fabricated with typical resolution of 320 x 240
pixels. 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 fairly
short range and long focal length lenses (made from germanium) are expensive. Radiometric
cameras are calibrated so that an accurate surface temperature can be read from the thermal
image. Nonradiometric cameras provide an indication of relative temperature but not absolute
temperature.
The amount of IR radiation from a source depends on the temperature of the surface and the
emissivity of the source. Very smooth or shiny surfaces emit a smaller amount of radiation than
rough or dull surfaces. Accurate temperature measurements require knowledge or assumptions of
the surface emissivity.

3-3

IR cameras are often classified as cooled or uncooled. High-end thermal IR cameras often
provide 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 less
sensitive and have more image noise.
Some IR cameras, such as the Indigo OEM Photon from Infrared Systems or the Cantronic
Thermal Ranger, are intended for integration into automated surveillance or inspection systems.
Compared with handheld systems intended for operator use, these cameras are small, are
compact, have low power requirements, and are suitable for an automated inspection station
when used with custom image analysis software.
In the underground transmission inspection systems, thermal IR cameras can be used to identify
hot spots caused by overheating splices in vaults. One alternative to a complete IR camera
system is to include an IR thermometer, which is a single IR detector with optics to focus
radiation from a small area on the detector (essentially a 1 x 1 pixel camera). The IR
thermometer 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 elevated
temperatures might indicate failing components. Slight variations in the orientation could be
used to build up a thermal image of a component. This process would be very slow compared
with that of an array IR camera but might be a useful low-cost alternative for a camera station.

3.3.1 Applications of IR Imaging
IR imaging can be used to detect excessive heat generated by failing components, such as a
splice in a vault and a termination in a substation or on a transition tower. With appropriate
image analysis, it could be used for automated detection.

3.4 Vibration Sensing
Vibration 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 a
prepolarized piece of piezoelectric material that produces a charge proportional to forces acting
on it. A piezoelectric accelerometer typically employs a mass (either in a shear or a compression
configuration) that produces a force on the piezoelectric element that is proportional to the
acceleration experienced by the mass. Many piezoelectric accelerometers contain integral
electronics 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 of
accelerometers is now commercially available. MEMS accelerometers are typically capacitive
devices that employ parallel plates or interdigitated fingers whose capacitance changes as a
function of applied acceleration. MEMS accelerometers are increasingly being used in many
commercial applications, such as airbag deployment sensors. Such devices can be produced with
extremely small form factors, requiring very little power. Unlike piezoelectric accelerometers,
capacitive MEMS accelerometers can respond to dc accelerations, making them appropriate for
use as tilt sensors as well as vibration sensors.

3-4

Commercially available accelerometers can be obtained in a variety of form factors and with
widely varying sensitivities and frequency responses. Piezoelectric accelerometers can be used
for 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 few
kHz. Transducers are available that are capable of measuring vibration levels ranging from a few
micro-Gs to several thousand Gs.

3.4.1 Applications of Vibration Sensors
Vibration data can be used to identify a wide variety of phenomena, from transient effects to
nondestructive damage identification. For high-voltage transmission applications, vibration
transducers could be used to identify heavy construction equipment near vaults and detect some
forms of foundation damage.

3.5 Acoustic Sensing
Measurements of the acoustic signal and analysis of the results may be able to determine if there
is any PD in the cable systems or fluid leaks from the steel pipes. These might be more effective
with cables terminating in gas-insulated switchgear, where acoustic emissions originating within
the 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 location
because leaks may produce noises over a wide range of frequencies and the noises propagate
through the pipe structure and can be detected. The typical equipment used for this technique
includes listening devices, such as piezoelectric elements, to sense sound or vibration.

3.6 Strain Sensing
Strain measurements are typically made on structural components to determine the forces acting
on them, whether the yield strength of the material has been exceeded or periodic vibrations or
cyclic movements are severe enough to cause fatigue problems in the material. Strain
measurement can also be accomplished with fiber Bragg grating sensors, which make the strain
measurement attractive to transmission cable applications. But the devices are still very costly
and have limited availability.

3.6.1 Applications of Strain Sensors
Strain measurements could be used to identify deformation of structural members caused by
excessive mechanical loading. Typical examples include thermal-mechanical bending of power
transmission cables and deformation of underground vault structure components, such as cable
support racks and clamps. Strain measurement sensors would need to be applied directly to the
structural members being measured.

3-5

3.7 Ultrasonic Sensing
Ultrasonic 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, surface
waves, 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 travels
through 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. The
waves are controlled by the geometry of the object. These waves include plate waves, Lamb
waves, and others. Plate waves can be generated only in thin metal plates. Lamb waves are the
most commonly used plate waves in nondestructive testing. Lamb waves are complex vibration
waves that travel through the entire thickness of a material. Propagation of Lamb waves depends
on the density and the elastic material properties of the object. Lamb waves are affected by the
test frequency and material thickness.
Ultrasonic waves are most often generated with piezoelectric transducers made from
piezoelectric ceramics. The conversion of electrical pulses to mechanical vibrations and the
conversion of returned mechanical vibrations back into electrical energy is the basis for
ultrasonic 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 Sensing
Magnetostrictive sensor (MsS) technology is a method of generating ultrasonic guided waves
into 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 that
propagate in a bounded medium (such as a pipe, plate, or rod) parallel to the plane of its
boundary. The wave is termed guided because it travels along the medium guided by the
geometric boundaries of the medium.
Because the wave is guided by the geometric boundaries of the medium, the geometry has a
strong influence on the behavior of the wave. In contrast to ultrasonic waves used in
conventional ultrasonic inspections that propagate with a constant velocity, the velocity of
guided 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 proper
control of wave mode and frequency, guided waves can be used to achieve 100% volumetric
inspection 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 and
detects guided waves electromagnetically in the material under testing. For wave generation, it
relies on the magnetostrictive (or Joule) effect: the manifestation of a small change in the
physical dimensions of ferromagnetic materials—on the order of several parts per million in
carbon steel—caused by an externally applied magnetic field. For wave detection, it relies on the
inverse-magnetostrictive (or Villari) effect: the change in the magnetic induction of
ferromagnetic material is caused by mechanical stress (or strain). Because the probe relies on the
magnetostrictive effects, it is called a magnetostrictive sensor.

3-6

In practice, the transmitted coil and receiver coil are the same or at least colocated. The sensor is
configured to apply a time-varying magnetic field to the material under testing and to pick up
magnetic induction changes in the material caused by the guided wave. For ferromagnetic
cylindrical objects (such as rods, tubes, or pipes), the MsS is ring-shaped and uses a coil that
encircles the object. For plate-like objects, the MsS is rectangular-shaped and uses either a coil
wound on a U-shaped core or a flat coil. If the component is not ferromagnetic, a thin
ferromagnetic strip can be bonded to the part, and the guided wave is then generated in the
ferromagnetic strip, which is then coupled into the part being inspected.
In practical inspection applications, the guided wave generation and detection are controlled to
work primarily in one direction so that the area of the structure on either side of the sensor can be
inspected separately. The wave direction control is achieved by employing two sensors and the
phased-array principle of the MsS instrument.
For operation, the MsS requires that the ferromagnetic material under testing be in a magnetized
state. This is achieved by applying a dc bias magnetic field to the material using either a
permanent magnet, electromagnet, or residual magnetization induced in the material. The dc bias
magnetization is necessary to enhance the transduction efficiency of the sensor (from electrical
to mechanical and vice versa) and to make the frequencies of the electrical signals and guided
waves 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 material
surface, and have good sensitivity in frequencies up to a few hundred kHz, which is ideal for
long-range guided wave inspection applications.
The MsS is directly operable on structures made of ferrous materials, such as carbon steel or
alloyed steel. The MsS is also operable on structures made of nonferrous materials, such as
aluminum, by bonding a thin layer of ferromagnetic material (typically nickel) to the structure
under testing or inspection and placing the MsS over the layer. In the latter case, guided waves
are generated in the ferromagnetic layer and coupled to the nonferrous structure. Detection is
achieved 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 relatively
low frequencies (up to a few hundred kHz) is launched along the structure under inspection, and
signals reflected from geometric irregularities in the structure—such as welds and defects—are
detected in the pulse-echo mode. From the time to the defect signal and the signal amplitude, the
axial 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) in
bare 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-wall
cross section in pipe and rod diameter in rod. In plates, it is typically 5% of the guided wave
beam 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 area
structure for defects, including areas that are difficult to access from a remotely accessible
location.

3-7

3.7.2 Applications of Ultrasonic Sensing
One common application of ultrasonic sensing is to evaluate material thickness and then detect
loss of material caused by corrosion, to inspect cracks near the location of the transducers (using
angle beam), and to detect defects over a long range using guided waves. One major drawback to
ultrasonic sensing is the requirement to have the transducer coupled to the part. An ultrasonic
guided wave technique has been evaluated for the detection of corrosion under coated pipes and
coating delamination [1]. Potential applications include fault location and leak location along
steel pipes.
MsS technology has been applied to inspection of suspender ropes on highway suspension
bridges and piping and heat exchanger tubes in refineries and chemical plants as well as
detection of corrosion in steel poles and transmission tower anchor rods in the power
transmission industry. Recent developments include monitoring of long lengths of continuous
metal with bolt holes and detection of loosened bolts, monitoring of the lattice structure buried in
concrete, and monitoring of ACSR conductors.

3.8 Electromagnetic-Acoustic Transducers
Electromagnetic-acoustic transducers (EMATs) generate ultrasonic waves in materials through
totally different physical principles than piezoelectric transducers and do not need any coupling
materials. When a wire is placed near the surface of an electrically conducting object and is
driven by a current at the desired ultrasonic frequency, eddy current will be induced in a near
surface region of the object. If a static magnetic field is also present, these eddy currents will
experience Lorentz forces of the form
F=J×B
where F is the body force per unit volume, J is the induced dynamic current density, and B is the
static magnetic induction.
Couplant-free transduction allows operation without contact at elevated temperatures and in
remote locations. The coil and magnet structure can also be designed to excite complex wave
patterns and polarizations that would be difficult to realize with fluid-coupled piezoelectric
probes.
Practical EMAT designs are relatively narrowband and require strong magnetic fields and large
currents to produce ultrasound that is often weaker than that produced by piezoelectric
transducers. Rare-earth materials such as samarium-cobalt and neodymium-iron-boron are often
used to produce sufficiently strong magnetic fields, which may also be generated by pulsed
electromagnets.
EMAT offers many advantages based on its couplant-free operation. These advantages include
the abilities to operate in remote environments at elevated speeds and temperatures, to excite
polarizations not easily excited by fluid-coupled piezoelectrics, and to produce highly consistent
measurements. These advantages are tempered by low efficiencies, and careful electronic design
is essential to applications. EMAT is also more expensive than piezoelectric transducers.

3-8

3.8.1 Applications of EMAT
The application of EMAT has been in nondestructive evaluation (NDE) applications, such as
flaw detection or material property characterization. EMAT is often used in high-temperature
applications of ultrasonics or where no couplant is allowed for wall thickness and angle beam
inspection for cracks. EMAT can also be used to generate guided waves in plate structures such
as lattice towers. There do not appear to be EMAT applications for long-range monitoring of
piping, tubing, or rods, although the possibility of further development exists.

3.9 Eddy Current Sensing
Eddy current inspection is one of several NDE methods that use the principle of
electromagnetism as the basis for conducting examinations. Several other methods, such as
remote field testing, flux leakage, and Barkhausen noise, use this principle.
Eddy currents are created through a process called electromagnetic induction. When alternating
current is applied to the conductor, such as a copper wire, a magnetic field develops in and
around the conductor. This magnetic field expands as the alternating current rises to maximum
and collapses as the current is reduced to zero. If another electrical conductor is brought into
close 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 and
measurements that can be performed. In the proper circumstances, eddy currents can be used for
the following:
• Crack detection
• Material thickness measurements
• Coating thickness measurements
• Conductivity measurements for the following:
– Material identification
– Heat damage detection
– Case depth determination
– Heat treatment monitoring

Some of the advantages of eddy current inspection are its sensitivity to small cracks and other
defects, detection of surface and near-surface defects, immediate results, portable equipment,
minimum part preparation, noncontact test probe, and the ability to inspect complex shapes and
sizes of conductive materials.
Some of the limitations of eddy current inspection are that only conductive materials can be
inspected, the surface must be accessible to the probe, the skill and training required are more
extensive than for other techniques, surface finish and roughness may interfere, reference
standards are needed for setup, depth of penetration is limited, and flaws such as delaminations
that lie parallel to the probe coil winding and probe scan direction are undetectable. Usually, the
eddy current probe has to be moved over the part or placed over a part that is changing with time.

3-9

3.9.1 Applications of Eddy Current Sensing
Eddy current is used in a wide range of applications for the power and aerospace industries for
detection of cracks and corrosion. Present eddy current sensing technology could be used to
measure corrosion depth and detect/size cracking. A specific application would be to analyze the
extent of sheath fatigue in lead-alloy-sheathed SCFF or ED cables.

3.10 RF Interference Sensing
PD in high-voltage system components produces RF interference that is detectable using
electronic radio signal receivers. PD emissions at RFs (in the MHz range) can be demodulated to
the audio band and heard as distinctive bursts of crackling. Handheld devices—and devices
attached to the end of a live working tool—with a simple bar meter display, audio speaker, and
gain control have been used in live line evaluation of distribution splices, elbows, and junction
modules.
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 algorithms
to analyze the data, correlate PD events, and estimate PD location based on the time of signal
arrival from the different known antenna locations.

3.11 Fluid Dissolved Gas Sensing
DGA is increasingly applied to both transformer and cable diagnostics. DGA can be used
through 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 is
the metal-insulator-semiconductor (MIS) chemical sensor that is a solid-state device detecting
molecules from multi-gases such as hydrogen and acetylene. EPRI also funded a study in
fiberoptic sensors for on-line detection of hydrogen and acetylene inside power transformers.
Novel holey fibers were recently developed to detect hydrogen, and optical microphone-based
laser photoacoustic spectroscopy was proposed for acetylene detection.

3.11.1 Applications of Fluid Dissolved Gas Sensing
EPRI has performed a feasibility study for on-line DGA for HPFF cables. This study examined
the feasibility of the use of on-line DGA monitoring equipment on static, oscillating, and
circulating HPFF pipe-type cable systems and addressed the added complexity of the high
pressure under which the cable operates. Several commercially available on-line gas monitoring
systems primarily used for transformers are available, such as the multi-gas analyzers from
Serveron 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 the
effectiveness of these analyzers in monitoring HPFF cables.
The monitoring device using the MIS technology and fiberoptic methods for detecting dissolved
gases would be attractive for fluid monitoring of HPFF or SCFF cable systems.

1
HYDRAN is a registered trademark of GE Energy.

3-10

3.12 Fiberoptic Sensing
Fiberoptic sensing has been applied for many decades to detect various physical and chemical
parameters. The characteristics of the fibers and the way light interacts with the fiber and fiber
coating or environment around the fiber are the basis for various sensor technologies. Fiberoptic
sensors 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 weight
In remote sensing applications, a segment of the fiber is used as a sensor gauge while a long
length 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 be
constructed 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 fiber
elements.
Knowledge of the following parameters is of great value for the underground transmission
industry:
• Temperature
• Electromagnetic field, current, voltage, and frequency
• Pressure, strain, displacement, vibration, and acoustic emission
• Chemical composition

3.12.1 Applications of Fiberoptic Sensing

3.12.1.1 Temperature Sensing
Both point sensors and distributed sensors are used for measuring temperatures. Point sensors
use a phosphorescent material at the end of the fiber. The temperature of transmission cable
splices, for example, can be monitored using the point sensors.
Distributed temperature sensors (DTSs) realize the technology of laser injection into the optical
fiber. A fraction of the laser pulses is absorbed in the fiber and is backscattered as Raman
signals. The local temperature determines the intensity of the Raman signals. The intensity is
used 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 Raman
light. Multimode or single-mode fibers are used for distributed temperature sensors. In
multimode systems (1.8°F [1°C] accuracy), about 3.3 ft (1 m) of fiber length is needed to create
a significant backscatter signal, whereas 13.1–32.8 ft (4–10 m) are needed for the single-mode
fiber (4.5°F [2.5°C) accuracy). These requirements designate the spatial resolution of the
multimode and single-mode fibers. Multimode optical fibers are suitable for most DTS
applications, with a maximum range of 4.97–6.21 mi (8–10 km). They are typically used for

3-11

short-range communication systems—for example, within office buildings. Single-mode optical
fibers 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 sensors
integrated in the cable lead to faster thermal response to the conductor and more accurate
conductor temperature measurements. The sensors can also be installed in a spare duct or a
separate duct designed specifically for the purpose. Both installations can be used for hotspot
management, overload detection, and real-time dynamic thermal circuit ratings. Figure 3-1
shows an example of distributed temperature sensing optical fibers incorporated into cable
bedding 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 3-1
Distributed temperature-sensing optical fibers incorporated into cable bedding tapes
(Water sensing can be constructed in a similar way under water blocking tapes.)

Figure 3-2
Distributed temperature sensing optical fibers in a 3-in. (76-mm) PVC conduit adjacent to a pipe-
type cable pipe

3-12

EPRI began using this technology for underground cable systems in the mid-1990s with a York
DTS-80 system (in 2003, the equipment was updated to a Sensa DTS-800) for measuring
distributed temperatures along underground cable routes. In addition to dynamic thermal rating
and hot spot identification, applications of optical fiber temperature sensing could be expanded
to fault location, fire detection, and the like.

3.12.1.2 Electromagnetic Field, Current, Voltage, and Frequency
Electromagnetic field, current, voltage, and frequency can be measured by fiberoptic sensors.
The high sensitivity and wide range of frequency response, combined with other features of
fiberoptic sensing (such as distributed and point sensing), make the technology attractive for
remote detection of PD and determination of fault location, corrosion, or insulation condition.

3.12.1.3 Pressure, Strain, Vibration, and Acoustic Emission
Pressure, strain, vibration, and acoustic sensors rely on application of a pressure to the sensor
head or grating in order to register an effect on the transmitted light. Distributed pressure sensing
is not yet commercial although there are strain sensors in a single-mode fiber. Hydrostatic
pressure monitoring tends to be at discrete points in most systems, such as for HPFF and SCFF
cables and terminations.
The sensors discussed could be used in a pigtail fashion and coupled to a distributed temperature
sensor for simultaneous pressure and temperature monitoring at joints and in joint casings for
HPFF and SCFF cable systems.
For pipe-type cables, the temperature and pressure information could be input into hydraulic
calculation programs to determine the size and location of possible leak areas along the pipe
length. Optical fiber pressure sensing could be applied for monitoring thermal-mechanical
behavior of cables, hydraulic systems, leaks, and corrosion. The acoustic measurement using a
fiberoptic sensor was developed as a PD sensor for transformers. Future studies can be carried
out to apply the fiberoptic sensors to monitor HPFF cables.

3.12.1.4 Chemical Composition
Fiberoptic sensing can be used to measure chemicals or component species of chemicals. For
example, distributed hydrocarbon fiberoptic sensors are being used for fluid leak monitoring
of 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 local
power loss that can be detected and located. The fiberoptic cables can be designed for the
detection of almost any petroleum derivative plus many synthetic organic liquids. Point sensors
can be used by a utility to monitor for gas chemicals in manholes and then pigtail the chemical
sensors to the distributed communication fiber to transfer the sensed information to a central
facility. This type of chemical sensing could be used for detecting dissolved gases in the cable
insulation fluid, soil condition, and corrosion monitoring, provided that the changes to fiber
characteristics are temporary and can be restored to original conditions once the abnormality has
passed.

3-13

3.13 Capacitive/Inductive Coupling (PD)
PD measurements are used to assess insulation condition of cables and accessories. They can be
used to verify proper installation of a cable circuit and assess insulation aging or degradation if
applied continuously or at certain intervals.

3.13.1 Applications of Capacitive/Inductive Coupling
Both 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 (see
Figure 3-3) or installed in the field. Inductive couplers can be in the form of high-frequency
current transformers (HFCTs) placed around cable bonding lead (see Figure 3-4) or cable sheath
bonding links (see Figure 3-5).
Molded Insulation
Molded Semicon
Metal Casing
Tinned Copper Braid (Sensor)
Coaxial Cable

Cable Insulation Shield
Cable Metallic Shield

Cable Insulation

Figure 3-3
Integral capacitive PD sensor on a pre-molded cable joint

Figure 3-4
High-frequency current transformers placed around cable bonding lead for PD measurements

3-14

Figure 3-5
HFCTs placed around the cable sheath bonding link for PD measurements

3.14 Flow, Temperature, Pressure, Volume, and Mass Sensing
System parameters, such as temperature, pressure, volume, or mass, can be used for hydraulic
system monitoring of a pipe-type cable circuit.
EPRI is investigating a leak detection system using artificial intelligence technology. The system
measures 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 be
implemented 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 will
balance between two ends of the pipe.

3.15 Voltage, Current, and Frequency Measurements

3.15.1 Dissipation Factor Measurement
Dissipation factor measurement gives an indication of the average condition of the cable
insulation for the entire cable length with splices. It does not address the individual discrete
components, such as splices, terminations, and any isolated defects. The method developed by
EPRI 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 the
measurement, starting by comparing the measured dissipation factor value to the original factory
value. However, implementation would be difficult without permanent installation of a large
reference capacitor.

3.15.2 Jacket Faults and SVL Failure Detection
For ED and SCFF cable systems, one of the most expensive maintenance activities is the
periodic testing of cable jackets to guard against corrosion. Corrosion damage could result in
water ingress in the case of ED cables and fluid leaks in the case of SCFF cables. Jacket faults
could also cause electrical safety hazards as sheath currents are injected into the ground, possibly

3-15

Future Inspection of Underground Transmission Lines

Future Inspection of Underground Transmission Lines

Empfohlen

Empfohlen

Weitere ähnliche Inhalte

Was ist angesagt?

Was ist angesagt? (20)

Andere mochten auch

Andere mochten auch (20)

Ähnlich wie Future Inspection of Underground Transmission Lines

Ähnlich wie Future Inspection of Underground Transmission Lines (20)

Mehr von Corporación Eléctrica del Ecuador, CELEC EP

Mehr von Corporación Eléctrica del Ecuador, CELEC EP (20)

Kürzlich hochgeladen

Kürzlich hochgeladen (20)

Future Inspection of Underground Transmission Lines