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Future Inspection of Underground Transmission Lines
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Future Inspection and Monitoring of Underground Transmission Lines 1020168
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Transcript of Future Inspection of Underground Transmission Lines
Future Inspection and Monitoring of Underground Transmission Lines
1020168
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 ▪ [email protected] ▪ www.epri.com
Future Inspection and Monitoring of Underground Transmission Lines
1020168
Technical Update, December 2009
EPRI Project Manager
S. Eckroad
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 [email protected].
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
vii
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.
ix
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.
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
xiii
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
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
1-2
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
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
Tab
le 2
-1
Insp
ecti
on
an
d m
on
ito
rin
g o
f u
nd
erg
rou
nd
tra
nsm
issi
on
lin
es
Fai
lure
M
od
es/In
dic
ato
rs
Dia
gn
ost
ic
Met
ho
d
Ap
plic
able
Cab
le
Sys
tem
s an
d
Au
xilia
ry
Eq
uip
men
t
Ove
rall
Sta
tus
Mo
nit
ori
ng
C
apab
ility
S
enso
r O
pp
ort
un
ity
Co
mm
ents
fo
r F
utu
re R
esea
rch
an
d P
rio
riti
zati
on
Pre
sen
tly
avai
lab
le o
n-l
ine,
co
nti
nu
ou
s m
on
ito
rin
g
Hot
spo
ts a
long
ca
bles
—lim
iting
fa
ctor
of l
oadi
ng
capa
bilit
y an
d in
sula
tion
agin
g
Tem
pera
ture
E
D, H
PF
F, H
PG
F,
SC
FF
O
n-lin
e m
onito
ring
avai
labl
e.
Mon
itor
thro
ugh
dist
ribut
ed
fiber
optic
se
nsor
s an
d th
erm
ocou
ples
.
Dis
trib
uted
fibe
ropt
ic
tem
pera
ture
sen
sing
an
d th
erm
ocou
ples
av
aila
ble.
Com
mer
cial
sys
tem
s av
aila
ble,
EP
RI
tailo
red
colla
bora
tion
oppo
rtun
ity a
vaila
ble
Hyd
raul
ic s
yste
m
mal
func
tion
Flu
id o
r ga
s pr
essu
re, f
low
, pu
mpi
ng p
lant
op
erat
ion,
res
ervo
ir flu
id le
vels
, pip
ing
dam
age,
and
leak
s
HP
FF
, HP
GF
, S
CF
F
On-
line
mon
itorin
g av
aila
ble.
Mon
itor
at
pres
suriz
ing
syst
ems.
Pre
ssur
e an
d ot
her
tran
sduc
ers
avai
labl
e.
Com
mer
cial
sys
tem
s av
aila
ble
Det
erio
ratio
n of
ca
ble
insu
latio
n an
d sh
ield
sy
stem
s, lo
caliz
ed
defe
cts
espe
cial
ly
at jo
ints
, te
rmin
atio
ns, a
nd
inte
rfac
es
Par
tial d
isch
arge
(P
D)
dete
ctio
n,
shie
ld c
urre
nt
mea
sure
men
t
ED
, HP
FF
, HP
GF
, S
CF
F (
limite
d ef
fect
iven
ess
for
HP
FF
and
HP
GF
)
On-
line
mon
itorin
g av
aila
ble.
E
xpen
sive
an
d tim
e-co
nsum
ing
insp
ectio
n.
Mon
itor
thro
ugh
capa
citiv
e an
d/or
in
duct
ive
coup
ling
or
acou
stic
em
issi
on
sens
ors.
Off-
line
and
on-li
ne
mai
nten
ance
in
spec
tion.
Var
ious
sen
sors
av
aila
ble
(ultr
a-hi
gh
freq
uenc
y [U
HF
], H
F
curr
ent t
rans
form
ers,
in
duct
ive
and
capa
citiv
e co
uple
rs,
acou
stic
em
issi
on).
O
ptic
al fi
ber
sens
ors
unde
r in
vest
igat
ion.
D
istr
ibut
ed s
enso
r de
velo
pmen
t op
port
uniti
es e
xist
al
ong
cabl
es.
R&
D o
n se
nsor
s,
sens
itivi
ty,
effe
ctiv
enes
s,
inte
grat
ion,
noi
se
filte
ring,
dat
a pr
oces
sing
, and
so
on
2-2
Tab
le 2
-1 (
con
tin
ued
) In
spec
tio
n a
nd
mo
nit
ori
ng
of
un
der
gro
un
d t
ran
smis
sio
n li
nes
Fai
lure
M
od
es/In
dic
ato
rs
Dia
gn
ost
ic
Met
ho
d
Ap
plic
able
Cab
le
Sys
tem
s an
d
Au
xilia
ry
Eq
uip
men
t
Ove
rall
Sta
tus
Mo
nit
ori
ng
C
apab
ility
S
enso
r O
pp
ort
un
ity
Co
mm
ents
fo
r F
utu
re R
esea
rch
an
d P
rio
riti
zati
on
Bur
ied
stee
l pip
e co
rros
ion
and
coat
ing
dam
age
Cat
hodi
c pr
otec
tion
syst
em s
ettin
gs
and
conn
ectio
ns,
half-
cell
pote
ntia
l, an
d ab
oveg
roun
d su
rvey
HP
FF
, HP
GF
O
n-lin
e m
onito
ring
avai
labl
e.
Mon
itor
cath
odic
pr
otec
tion
syst
ems
at
subs
tatio
ns,
vaul
ts, o
r te
st
stat
ions
.
Pot
entia
l and
cur
rent
m
eter
s av
aila
ble.
C
omm
erci
al s
yste
ms
avai
labl
e
Met
allic
sh
eath
/shi
eld
corr
osio
n
Cat
hodi
c pr
otec
tion
syst
em s
ettin
gs
and
conn
ectio
ns
SC
FF
O
n-lin
e m
onito
ring
avai
labl
e.
Mon
itor
cath
odic
pr
otec
tion
syst
ems
at
subs
tatio
ns.
Pot
entia
l and
cur
rent
m
eter
s av
aila
ble.
C
omm
erci
al s
yste
ms
avai
labl
e
Flu
id o
r ga
s le
ak
Flu
id p
ress
ure,
te
mpe
ratu
re,
circ
uit l
oadi
ng,
ambi
ent c
ondi
tion,
flo
w, a
nd th
e lik
e
HP
FF
, HP
GF
, S
CF
F
On-
line
mon
itorin
g av
aila
ble.
Mon
itor
at
pres
suriz
ing
syst
ems
and/
or
alon
g ca
ble
rout
e.
Var
ious
tran
sduc
ers
avai
labl
e.
US
i/EP
RI s
yste
m
avai
labl
e;
Con
Ed/
EP
RI a
nd
Kin
ectr
ics/
EP
RI
syst
ems
unde
r in
vest
igat
ion
Pre
sen
tly
avai
lab
le o
ff-l
ine
mai
nte
nan
ce in
spec
tio
n, w
ith
op
po
rtu
nit
ies
for
on
-lin
e, c
on
tin
uo
us
mo
nit
ori
ng
Ove
rall
insu
latio
n in
tegr
ity, s
uch
as
moi
stur
e, fl
uid
cont
amin
atio
n
Dis
sipa
tion
fact
or
HP
FF
, HP
GF
, S
CF
F
In-f
ield
test
w
ith s
peci
al
equi
pmen
t.
Off-
line
mai
nten
ance
in
spec
tion.
Dev
elop
men
t op
port
uniti
es e
xist
for
on-li
ne m
onito
ring.
EP
RI i
n-fie
ld s
yste
m
avai
labl
e fo
r la
min
ar
diel
ectr
ic c
able
s
Bon
ding
and
link
bo
x co
rros
ion,
lo
ose
conn
ectio
n,
insu
latio
n da
mag
e
She
ath
curr
ent
mea
sure
men
ts
ED
, SC
FF
In
-per
son
insp
ectio
n.
Off-
line
mai
nten
ance
in
spec
tion.
Sen
sors
ava
ilabl
e bu
t ne
ed in
tegr
atio
n.
On-
line
mon
itorin
g de
sira
ble
She
ath
volta
ge
limite
r (S
VL)
fa
ilure
SV
L cu
rren
t E
D, S
CF
F
In-p
erso
n in
spec
tion.
O
ff-lin
e m
aint
enan
ce
insp
ectio
n.
Sen
sors
ava
ilabl
e bu
t ne
ed in
tegr
atio
n.
On-
line
mon
itorin
g de
sira
ble
2-3
Tab
le 2
-1 (
con
tin
ued
) In
spec
tio
n a
nd
mo
nit
ori
ng
of
un
der
gro
un
d t
ran
smis
sio
n li
nes
Fai
lure
M
od
es/In
dic
ato
rs
Dia
gn
ost
ic
Met
ho
d
Ap
plic
able
Cab
le
Sys
tem
s an
d
Au
xilia
ry
Eq
uip
men
t
Ove
rall
Sta
tus
Mo
nit
ori
ng
C
apab
ility
S
enso
r O
pp
ort
un
ity
Co
mm
ents
fo
r F
utu
re R
esea
rch
an
d P
rio
riti
zati
on
Vau
lt ha
rdw
are
and
com
pone
nt
(cei
ling,
wal
ls,
pipe
, cla
mps
, gr
ound
wire
s,
rack
s, p
umpi
ng,
and
so o
n)
degr
adat
ion,
co
rros
ion,
ov
erhe
atin
g,
flood
ing,
saf
ety-
rela
ted
gas
Opt
ical
imag
e in
frar
ed im
age,
vi
brat
ion,
aco
ustic
se
nsin
g, a
nd
tem
pera
ture
in
dica
ting
strip
s on
co
mpo
nent
s fo
r cr
acks
, lea
ks,
corr
osio
n, c
oatin
g da
mag
e,
com
pone
nt
dam
age,
saf
ety-
rela
ted
gas
leve
l, an
d so
on
ED
, SC
FF
T
ime-
co
nsum
ing
insp
ectio
n w
ith s
afet
y co
ncer
ns.
Off-
line
or o
n-lin
e m
aint
enan
ce
insp
ectio
n.
Sen
sor
deve
lopm
ent
oppo
rtun
ities
exi
st.
Som
e se
nsor
s av
aila
ble
but n
eed
inte
grat
ion.
On-
line
mon
itorin
g de
sira
ble
Inte
rnal
m
ovem
ent,
mis
alig
nmen
t, or
da
mag
e of
cab
les
and
acce
ssor
ies
X-r
ay in
spec
tion
ED
, HP
FF
, HP
GF
, S
CF
F
Exp
ensi
ve
and
time-
co
nsum
ing
insp
ectio
n.
Off-
line
mai
nten
ance
in
spec
tion.
Por
tabl
e X
-ray
eq
uipm
ent a
vaila
ble.
O
n-lin
e m
onito
ring
unlik
ely
Flu
id le
ak lo
catio
n P
erflu
oroc
arbo
n tr
acer
s H
PF
F, S
CF
F
Tim
e-
cons
umin
g in
spec
tion.
Off-
line
loca
ting
afte
r le
ak
dete
cted
.
Sen
sors
ava
ilabl
e.
On-
line
mon
itorin
g un
likel
y
Fau
lt lo
catio
n F
ault
curr
ent
ED
, HP
FF
, HP
GF
, S
CF
F
On-
line
mon
itorin
g av
aila
ble.
Mon
itor
faul
t cu
rren
t at e
ach
end
of a
cab
le
sect
ion.
Fib
erop
tic c
urre
nt
sens
ors
deve
lope
d.
Sys
tem
s un
der
deve
lopm
ent b
y T
okyo
Ele
ctric
Pow
er
Com
pany
for
ED
ca
bles
2-4
Tab
le 2
-1 (
con
tin
ued
) In
spec
tio
n a
nd
mo
nit
ori
ng
of
un
der
gro
un
d t
ran
smis
sio
n li
nes
Fai
lure
M
od
es/In
dic
ato
rs
Dia
gn
ost
ic
Met
ho
d
Ap
plic
able
Cab
le
Sys
tem
s an
d
Au
xilia
ry
Eq
uip
men
t
Ove
rall
Sta
tus
Mo
nit
ori
ng
C
apab
ility
S
enso
r O
pp
ort
un
ity
Co
mm
ents
fo
r F
utu
re R
esea
rch
an
d P
rio
riti
zati
on
Pre
sen
tly
avai
lab
le o
ff-l
ine
mai
nte
nan
ce in
spec
tio
n b
ased
on
lab
ora
tory
tes
ts, w
ith
op
po
rtu
nit
ies
for
on
-lin
e, c
on
tin
uo
us
mo
nit
ori
ng
Agi
ng/d
egra
datio
n of
flui
d or
pap
er
insu
latio
n—in
dica
tor
of h
ot
spot
s, P
D, a
nd
arci
ng
Dis
solv
ed g
as
anal
ysis
(D
GA
),
diss
ipat
ion
fact
or,
dire
ct c
urre
nt (
dc)
resi
stan
ce,
alte
rnat
ing
curr
ent
(ac)
res
ista
nce,
m
oist
ure
cont
ent,
part
icle
con
tent
, ga
s ab
sorp
tion
capa
bilit
y
HP
FF
, HP
GF
, S
CF
F
Labo
rato
ry
test
with
flui
d sa
mpl
es fr
om
oper
atin
g eq
uipm
ent.
Off-
line
mai
nten
ance
in
spec
tion,
flui
d sa
mpl
es fr
om
oper
atin
g eq
uipm
ent.
Sen
sor
deve
lopm
ent
oppo
rtun
ities
exi
st.
On-
line
mon
itorin
g un
der
inve
stig
atio
n by
EP
RI
Agi
ng o
f pap
er
insu
latio
n D
egre
e of
po
lym
eriz
atio
n (D
P),
mec
hani
cal
stre
ngth
, di
ssip
atio
n fa
ctor
, fu
rfur
al
HP
FF
, HP
GF
, S
CF
F
Labo
rato
ry
test
with
sa
mpl
es fr
om
oper
atin
g eq
uipm
ent.
Mec
hani
cal/
elec
tric
str
engt
h ve
rsus
DP
kn
own.
Sen
sor
deve
lopm
ent
oppo
rtun
ities
exi
st.
Unl
ikel
y fo
r on
-line
m
onito
ring
Oth
er d
esir
able
on
-lin
e, c
on
tin
uo
us
insp
ecti
on
an
d m
on
ito
rin
g
The
rmo-
mec
hani
cal
bend
ing
Str
ain
sens
ing,
si
dew
all p
ress
ure
sens
ing
ED
, HP
FF
, HP
GF
, S
CF
F
New
. O
n-lin
e m
onito
ring
desi
rabl
e.
Sen
sor
deve
lopm
ent
oppo
rtun
ities
exi
st.
On-
line
mon
itorin
g de
sira
ble
Moi
stur
e ba
rrie
r de
grad
atio
n M
oist
ure
leve
l E
D
New
. O
n-lin
e m
onito
ring
desi
rabl
e.
Sen
sor
deve
lopm
ent
oppo
rtun
ities
exi
st.
On-
line
mon
itorin
g de
sira
ble
Lead
she
ath
fatig
ue
Str
ain
sens
ing
SC
FF
N
ew.
On-
line
mon
itorin
g de
sira
ble.
Sen
sor
deve
lopm
ent
oppo
rtun
ities
exi
st.
On-
line
mon
itorin
g de
sira
ble
2-5
Fig
ure
2-1
In
spec
tio
n a
nd
mo
nit
ori
ng
of
ED
un
der
gro
un
d t
ran
smis
sio
n li
nes
2-6
Fig
ure
2-2
In
spec
tio
n a
nd
mo
nit
ori
ng
of
HP
FF
an
d H
PG
F u
nd
erg
rou
nd
tra
nsm
issi
on
lin
es
2-7
2-8
Fig
ure
2-3
In
spec
tio
n a
nd
mo
nit
ori
ng
of
SC
FF
un
der
gro
un
d t
ran
smis
sio
n li
nes
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
2-14
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.
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.
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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.
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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.
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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.
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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.
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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.
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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.
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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 (HYDRAN1) 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.
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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
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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
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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.
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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).
Metal CasingTinned Copper Braid (Sensor)
Coaxial Cable
Molded InsulationMolded Semicon
Cable Insulation ShieldCable 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
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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
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causing high local ground potential rises and step and touch potentials. Because the effectiveness of the special bonding systems to improve ampacity would be lost, cables could also be overheated. Industry practice is to apply a dc test voltage between the sheath/shield and ground, to verify integrity. Depending on utility practice, this is usually done about every three to five years. It is very expensive and disruptive because line outages are needed, all manholes must be entered to change and restore sheath bonding connections, and in the case of double circuits sharing a duct bank, safety hazards can exist due to induction.
There would be many benefits if such maintenance could be eliminated (or the interval increased) and if jacket conditions could be measured remotely, perhaps in real time. Such methods would comprise placing small current transformers around each sheath bonding connection and each SVL connection to check for irregular readings indicative of a jacket fault or a SVL failure. Currents and phase angles would be relayed to a hub and then to a control center for action, if needed. Alternatively, alarm levels could be established so that only alarm signals would be telemetered to a control center. Of course, the sheath currents vary proportionately to load currents, so conductor currents would also need to be considered, as would the effects of cross-bonding interconnections.
With the addition of this system and on-line PD systems for accessories, ED cable systems could almost be considered maintenance-free.
4 CANDIDATE DATA COMMUNICATION TECHNOLOGIES
4.1 Introduction
Sensor readings or derived alarm conditions must be communicated to a central location so that maintenance actions can be ascertained and prioritized and so that results can be archived for future reference. A number of electronic, optical, and acoustic data communication technologies are considered for this application. Technologies are covered by frequency spectrum, RF, then optical because of their vastly differing implementations. The dividing line in the spectrum between RF and optical is typically considered about 300 GHz. RF and optical technologies are in widespread use for data communications.
Candidate data communication technologies are addressed, organized as follows:
• RF wireless line-of-sight (LOS) transceiver • RF wireless backscatter • RF wireless OTH • IR wireless • Fiberoptic • Free space optical • Data communications over power cable lines • Mobile collection platforms • Acoustic signal transmission through insulating fluids for HPFF and SCFF cable systems
The purpose of this report is not to select a single communication technology, but to provide insight into the tradeoffs that exist among the different choices. The system in theory can support many different communication technologies as long as there is a common data language/format/ protocol in the end.
4.2 RF Wireless LOS Transceiver
RF wireless LOS technology encompasses a vast range of technologies and commercially available products. Commercially available RF LOS radios directed at the consumer data communication markets use RF waves in the UHF (300 MHz to 3 GHz) to low super-high frequency (3–30 GHz) bands. This discussion focuses on commercial products below 6 GHz because higher frequency microwave devices are highly directional, are expensive, and target broadcasting and telecommunication markets. Further, this discussion omits cellular mobile phone technology because it is not yet targeting low-cost, networked transfer for raw data, although the trend toward broadband Internet might have applications in the future.
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RF LOS radios can be generally considered in two categories: those supported by an industry standard and those that are not. The Institute of Electrical and Electronics Engineers (IEEE) defines many of these standards in the family of IEEE 802 standards for local area networks (LANs) and metropolitan area networks. A number of nonstandardized communication technologies are suitable for SCADA and monitoring/control applications.
4.2.1 IEEE 802 Standard Technologies
Standardized communication technologies under consideration include the 802.11 series, commonly referred to as WiFi; the 802.15 series, which includes Bluetooth and ZigBee; the 802.16 series, commonly referred to as WiMAX; and the 802.20 series, which is known as mobile broadband wireless access or mobile-fi. Off-the-shelf modules and chipsets are available for most. However, integration of chipsets into a custom solution typically requires additional levels of effort because of the complicated communication protocols.
WiFi, Bluetooth, and ZigBee are candidates for sensor-to-vault or intravault communication. WiFi technology supports higher data rate sensors at the cost of higher power consumption and larger size. Range extenders for WiFi exist, making it possible for vault-to-vault communication. Integration of a WiFi network, however, entails integration of complicated network architecture. Bluetooth is suitable for communication of low data rate sensor data. The range of Bluetooth is severely limited, and although longer range (300 ft [91 m]) Bluetooth Class 1 modules exist, the trend in Bluetooth is toward the shorter-range (tens of feet) Class 2 devices. Bluetooth modules have an advantage of being small and low power, but integration into a system is complicated by its master-slave topology. ZigBee technology was developed for sensor monitoring and control applications and is thus well suited to sensor communication. The devices are relatively small and low power, and the range is suitable for sensor-to-vault communication. As with WiFi and Bluetooth, ZigBee’s networking protocol adds to the complexity of system integration.
WiMAX and mobile-fi may eventually be applicable for vault-to-central-facility communication, which requires transferring potentially large amounts of data over long distance. Both technologies are still early in their development, require large amounts of power, and have yet to establish a firm market.
4.2.2 Nonstandardized Technologies
Nonstandardized RF LOS radios are available in off-the-shelf modules and chipsets. These have their networking and modulation schemes. Custom implementations can be tailored to the application, although modules can be more quickly integrated.
A number of products were scanned to develop the general descriptions of performance characteristics. These products include offerings from Cirronet, Aerocomm, Laipac, MaxStream, Nordic Semiconductor, Holy Stone, FreeWave, and HAC. Point-to-point and point-to-multipoint commercial transceiver modules offer a wide range of modules and chipsets that are appropriate for sensor-to-vault, sensor-to-mobile platform, and vault-to-vault communication. Integration of such devices into an instrumented vault system does not typically suffer from the complexities often associated with the network features of an IEEE 802 radio. Communication system capabilities can be tailored by a custom implementation based on a commercially available chipset, but modules based on these chipsets are available and enable rapid, low-risk development with competitive unit cost at lower production volumes.
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4.3 RF Wireless Backscatter
RF backscatter devices can be designed with only a few electronic components in order to achieve high reliability and multidecade, maintenance-free service life. RF backscatter devices can be designed to operate passively or with minuscule power consumption, which enables opportunities for environmental power harvesting. The power requirement of radio transceivers or transponders is much greater than RF backscatter devices and therefore significantly limits the potential for batteryless operation and environmental power harvesting.
In an RF backscatter system, an interrogator radio beam is used to stimulate remote sensors and to read the reflected signal energy that is modulated with sensor data. In theory, the sensor could be a purely passive device. However, a small amount of sensor power is desirable in practice in order to increase the read range, to provide “smarts” to allow operation when several sensors are within range, and to provide flexibility in sensor format.
RF backscatter technology is applicable to short-range, low-data-rate sensor data. This includes sensor-to-vault, intravault, and sensor-to-mobile communication platforms. Sensors can be automatically interrogated from a reader unit in the vault or a mobile platform passing by the vault/sensor.
In 2007, EPRI reported a series of tests performed at a Lenox, Massachusetts, facility [3]. The first test was to investigate the wireless communication approach between a backscatter sensor located in an underground concrete vault with different manhole covers and the interrogator system from ground level above the manhole. In the test, the sensor was positioned under the cover in the 14-ft-deep (4-m-deep) vault, facing up. The interrogator antenna was positioned facing down and a few inches above the manhole cover. The sensor and interrogator antennas were oriented to obtain the maximum output.
In the second test, the interrogator system was positioned in the vault with different manhole covers, and the RF signal level was measured by a portable spectrum analyzer from ground level above the manhole. An antenna identical to the interrogator antenna was used for the spectrum analyzer measurements.
The measurement results indicate that it was technically feasible to communicate from a sensor located inside an underground concrete vault using the RF backscatter technology and an effective design solution could be engineered with considerations of sensor position relative to manhole covers and sensor/antenna orientations.
4.4 RF Wireless OTH
RF wireless OTH technology uses a sky- or space-based repeater to achieve beyond LOS data transfer. A balloon- or aircraft-borne repeater is a common concept and uses a repeater for a particular radio technology. This discussion focuses on commercially available space-based repeaters using satellite communication (SATCOM).
There are many commercial SATCOM technologies available for data transfer. Example systems include Iridium, Globalstar, Orbcomm, and Inmarsat services. Iridium is a continuously available global satellite system consisting of 66 low earth-orbiting satellites. Voice and data are communicated using L-band transceivers, and data are available through a ground station.
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Globalstar is a partially global satellite system consisting of 48 low-earth orbiting satellites. Globalstar uses bent-pipe architecture for communicating voice and data from a transceiver to ground station, and it uses both S- and L-bands. Orbcomm is a global satellite system consisting of 36 low earth-orbiting satellites. Orbcomm uses a store and forward architecture operating in the very high frequency (VHF) band and features low-cost low-power miniature duplex transceivers with data capability. Inmarsat D+ provides near-global communication using 10 Inmarsat geostationary orbit satellites. It features low-cost messaging implementing low-speed, limited-length packet messages on L-band channels. Inmarsat Mini-M provides near-global communication using geostationary orbit satellites for voice and 6000 bits/second data within the Inmarsat spot beams. It is intended for fixed asset communication dealing primarily with voice, fax, and limited data, and it requires use of a directional antenna.
SATCOM technology is a candidate technology for long-range communication from a vault-based hub to the central facility for low data rate applications. SATCOM does not support video, and most architecture is based on noncontinuous burst transmissions of relatively small data packets. Although some SATCOM technologies advertise low power, these claims are made relative to other SATCOM products and not to low-power LOS radios. All SATCOM options charge for air time, access to services, numbers or messages/bytes/bits, or a flat fee for data transfer. Thus, usage charges for SATCOM services apply over the entire lifetime of the communication system.
4.5 IR Wireless
IR wireless communication uses electromagnetic radiation with a much higher frequency than radio waves but lower frequency than visible light to communicate data wirelessly. IR communication is commonly applied to remote control of electronic equipment. These devices are typically very low in power consumption and very short in range.
Remote control IR technology uses an IR LED as the transmitter and a photodiode as the receiver. The LED emits an IR signal that is modulated with data, and a lens focuses the modulated signal into a narrow beam. A photodiode at the receiver converts the IR signal to an electric current.
Although IR technology could support low data rate, short-range sensor-to-sensor and sensor-to- hub communication, several characteristics of IR technology limit its usefulness for this application. Communication over IR is limited to very short distances, is highly directional in nature, and is susceptible to minor path obstructions due to its strict LOS requirement.
4.6 Fiberoptic
Fiberoptic technology communicates data by sending light through an optical fiber. Modulated light from an LED or laser is transmitted into a glass or plastic optical fiber. The light reflects off the cladding of the fiber as it travels through the bending fiber. Because there is very little loss due to the reflections, long distances can be traveled. Optical regenerators can be used along the path to regenerate signals so that even longer distance can be covered. Optical receivers use a photocell or photodiode to detect the light.
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Fiberoptic communication is a candidate for the vault communication infrastructure because it possesses several advantageous characteristics. Its common application is for high-bandwidth, long-distance data communication, such as wired LAN, so it is well suited to the vault-based hub to central facility link. It is also used in applications where immunity to electromagnetic interference is important or in near-high-voltage potentials, because no electricity passes through the fiber. The technology can typically be used to transmit the data measured from remote vaults to a central database.
The fiberoptic technology is extremely attractive to underground transmission cable industry because the fibers have been integrated in many standard cable products offered by cable manufacturers. The disadvantages of this technology are the cost and difficulty associated with running the fiber if the optical fibers are not installed along the underground cable circuits or not integrated in the cables.
4.7 Free Space Optical Communication
Free space optical communication uses a beam of visible or IR light to wirelessly communicate data LOS. Although similar to fiberoptic communication, free space optics transmits signals through space rather than being guided through an optical cable and, unlike wireless RF systems, it does not require spectrum licensing. Although relatively unaffected by rain and snow, free space optical communication systems can be severely affected by fog and atmospheric turbulence. Free space optical communication targets high-data-rate applications, such as LAN-to-LAN communication between buildings.
Free space optical technology has a number of disadvantages when compared with other candidates for the vault communication infrastructure. Although it is of high data rate, its range supports only vault-to-vault (hub-to-hub) communication. Its high power consumption and large size make it impractical for use on distributed sensor, and its strict LOS requirements make it unsuitable for hub-to-mobile platform communications.
4.8 Data Communication over Power Cable Line
Transmission of data over power cables seems to fit the system concept well in that existing power cables could be used for the communication infrastructure. Broadband over power line (BPL) technology is a fast data communication concept for consumer Internet access. It typically launches data waveforms in the 10–30 MHz range onto medium-voltage power lines. BPL avoids high-voltage power lines because the amount of noise on these lines creates too much interference for reliable communication of data. The high inductance of a transformer acts as a low-pass filter and filters the BPL signals. New developments in surface wave propagation promise higher speeds, full duplex communication, and symmetric data rates. One of the challenges of BPL for this application would be coupling the signal onto the energized conductors. Similar to BPL, transmission of data over HPFF cable pipes also seems a good fit. Using special electronic polarization cells with band-pass filters and other modern techniques, it appears that certain baud rates could be possible for many monitoring applications.
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4.9 Acoustic Signal Transmission Through Insulating Fluids
Retrofitting existing HPFF and SCFF cable systems with communications fibers is challenging, especially if there are no spare pipes or ducts. However, the fluid provides a medium for acoustic signal transmission, albeit relatively slow. However, some sensor data do not change quickly, so fast data rates are not always needed. Many improvements have been made in the last decade to improve acoustic data transmission underwater, particularly for development of autonomous underwater vehicles and military applications. It appears that these new methods could be applied to at least daisy-chain some sensor data between HPFF cable manholes to terminals at the ends, where conventional methods could be used to connect to SCADA.
4.10 Mobile Collection Platforms
Mobile platforms could provide an effective and efficient means to deploy sensor and/or communication hub resources in order to cover the wide extents of a power transmission system.
4.10.1 Manned Mobile Platforms
Traditional manned mobile inspection platforms still fit within the system concept. In this case, the human provides common-sense inspection capabilities to supplement automatic methods. Thorough inspections can be made when time is afforded to get into each vault. In the future envisioned system, the operator or vehicle will carry a communication hub to collect the local sensor data for eventual relay back to a central location. Sensors (such as cameras) may also be carried on the same platform for remote sensing. Aside from fundamental functional and performance requirements, additional design factors—such as size, weight, configuration, battery life, and ergonomics—must be considered when designing and packaging sensors and hubs for specific manned mobile platforms. In addition to collecting sensor readings, manned platforms could contain their own sensors and/or collect information for sensors deployed on the transmission line.
4.10.2 Unmanned Mobile Platforms
Unmanned mobile inspection platforms play into an automated, instrumented vault system concept, especially for urban areas. In this case, crews are not required for routine data collection and can be deployed with a focus on critical maintenance activity.
Four unmanned mobile platform types are identified for consideration: unmanned aerial vehicle (UAV), balloon, satellite, and robotic crawler. These platforms all have the same requirement for a generic “black box” payload. By definition, a user interface (display, controls, switches, and so on) is not required for unmanned applications; however, test provisions are still likely desired and necessary. In general, the smaller the size, weight, and power consumption of the black box, the more opportunities there will be to integrate it onto mobile platforms. Just like manned sensor platforms, in addition to collecting sensor readings, unmanned platforms, such as UAVs, could contain their own sensors and/or collect information for sensors deployed on the transmission line.
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4.10.2.1 Unmanned Aerial Vehicles
UAVs have been used primarily for military applications where it is dangerous to send personnel. EPRI has researched the potential for using UAVs for transmission line inspections. The primary negative with respect to this notion is the liability concern in the event of a catastrophic crash. Also, Federal Aviation Administration line-of-sight requirements are restrictive. In the future, it is expected that these concerns will be mitigated as related successful ventures are demonstrated, and confidence is gained in the technology. Once these barriers are broken, UAVs could become effective mobile inspection platforms carrying sensors and hubs over established transmission line routes.
4.10.2.2 Balloons
Stratospheric balloons or airships can be used for communication relay applications where satellites cannot be afforded. They have strong merit because they provide the benefits of a dedicated satellite at a fraction of the price. Similar to UAV technology, as other industry applications prove this technology, the utility industry will gain the confidence needed to adopt it. The difference between UAVs and balloons is that whereas UAVs would fly the traditional inspection route, the balloon would be stationed high in the stratosphere, serving as a communication relay with coverage of hundreds of miles. The UAV mission would be a matter of hours, whereas the balloon would be left in the stratosphere as long as possible, possibly for months. An advantage of operating in the stratosphere is that the air space is uncontrolled with respect to air traffic control operations. In addition to collecting wireless sensor readings, the balloon could collect and process optical data—visual, IR, UV, and so forth.
4.10.2.3 Satellites
There are many space satellites orbiting the earth, both government- and commercially sponsored, for wide-ranging applications using advanced sensors and data communication equipment. Although it is not reasonable to propose dedicated satellites for the instrumented vault concept, leasing existing satellite services for imagery and data communications is possible now, and more options will surely be available in the future.
4.10.2.4 Robotic Crawler Platform
The development of a robotic crawler for vault inspection is another area for development. Technical areas such as software and electronics issues are important but will likely benefit from ongoing robotics research.
5 INDUSTRY SCAN ON SENSOR APPLICATIONS IN UNDERGROUND TRANSMISSION CABLE SYSTEMS
5.1 Introduction
An industry scan of on-line, real-time monitoring and sensor technology was carried out. The following information, where available, was provided for each product and/or service: company, product/service, system to monitor, component to monitor, parameter to measure, sensor technology, communication technology, web site, and description.
5.2 List of Products/Services of Monitoring Transmission Cable Systems
5.2.1 Balfour Beatty Utility Solutions (United Kingdom)
Balfour Beatty’s products and services are described as follows:
• Product/service: PD detection. • System to monitor: cable systems. • Components to monitor: cables, joints, terminations. • Parameter to measure: PD. • Web site: www.bbusl.com. • Description: Balfour Beatty Utility Solutions provides on-line PD testing on short lengths of
transmission cables.
5.2.2 BRUGG (Switzerland)
BRUGG’s products and services are described as follows:
• Product/service: PD and fiberoptic systems for distributed temperature sensing. • System to monitor: cable systems. • Components to monitor: cables, joints, terminations. • Parameters to measure: PD and temperature. • Sensor technology: UHF PD sensor and fiberoptic cables. • Communication technology: wire and fiberoptic cables. • Web site: www.bruggcables.com. • Description: BRUGG offers fiberoptic sensing cables and system solutions as well as PD
monitoring on terminations.
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5.2.3 Genesys (Colorado)
BRUGG’s products and services are described as follows (many other companies supply similar systems):
• Product/service: cathodic protection system monitoring. • System to monitor: pipe-type cable systems. • Components to monitor: cathodic protection systems. • Parameters to measure: current, voltage. • Sensor technology: meters. • Communication technology: RF wireless, satellite. • Web site: www.genesysinst.com. • Description: The system consists of monitor devices and a centralized database with
software.
5.2.4 High Voltage Partial Discharge Ltd. (United Kingdom)
High Voltage Partial Discharge’s (HVPD’s) products and services are described as follows:
• Product/service: PD detection. • System to monitor: cable systems. • Components to monitor: cables, joints, terminations. • Parameter to measure: PD. • Sensor technology: inductive/capacitive couplers. • Web site: www.hvpd.co.uk. • Description: HVPD Ltd provides on-line testing services on transmission cable circuits.
5.2.5 KEMA (The Netherlands)
KEMA’s products and services are described as follows:
• Product/service: PD detection. • System to monitor: cable systems. • Components to monitor: cables, joints, terminations. • Parameter to measure: PD. • Sensor technology: inductive/capacitive couplers. • Web site: www.kema.com. • Description: KEMA has experience with on-line PD monitoring, though to a lesser extent.
5.2.6 Kinectrics (Canada)
Kinectric’ products and services are described as follows:
• Product/service: PD and buried pipe leak detection. • System to monitor: cable systems.
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• Components to monitor: cables, joints, terminations, buried pipes. • Parameters to measure: PD, temperature, pressure, flow, valve operation, tank fluid levels. • Sensor technology: inductive/capacitive couplers, thermal couples and meters. • Communication technology: wired and wireless. • Web site: www.kinectrics.com. • Description: Kinectrics has experience with on-line PD testing and leak detection.
5.2.7 LIOS Technology (Germany)
LIOS Technology’s products and services are described as follows:
• Product/service: dynamic cable rating. • System to monitor: cable systems. • Components to monitor: cables, joints, terminations. • Parameter to measure: temperature. • Sensor technology: distributed fiberoptic cables. • Communication technology: fiberoptic cables. • Web site: www.lios-tech.com. • Description: LIOS Technology develops and supplies frequency domain-based DTS systems.
The systems use real-time, multimode fiber-optic-based linear temperature measuring devices. LIOS also offers real-time thermal rating services. LIOS Technology and TechImp recently announced a partnership in providing combined DTS and PD detection systems.
5.2.8 LS Cable (South Korea)
LS Cable’s products and services are described as follows:
• Product/service: on-line systems for DTS and PD measurement. • System to monitor: cable systems. • Components to monitor: cables, joints, terminations. • Parameters to measure: temperature and PD. • Sensor technology: fiberoptic cables, capacitive/inductive couplers. • Web site: www.lscable.com. • Description: LS Cable offers fiberoptic sensing cables and systems and PD measurement.
5.2.9 Omicron (Austria)
Omnicron’s products and services are described as follows:
• Product/service: cable system condition monitoring. • System to monitor: cable systems. • Components to monitor: cables, joints, terminations. • Parameter to measure: PD. • Sensor technology: capacitive/inductive couplers.
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• Communication technology: wireless, optical fiber link. • Web site: www.omicron.com. • Description: Omicron offers real-time, on-line diagnostic and condition monitoring services
using PD detection.
5.2.10 Sensornet (United Kingdom)
Sensornet’s products and services are described as follows:
• Product/service: fiberoptic systems for DTS. • System to monitor: cable systems. • Components to monitor: cables, joints, terminations. • Parameter to measure: temperature. • Sensor technology: distributed fiberoptic cables. • Communication technology: fiberoptic cables. • Web site: www.sensornet.co.uk. • Description: Sensornet offers fiberoptic temperature sensors that are used to continuously
monitor the temperature of buried cables.
5.2.11 SensorTran (Texas)
SensorTran’s products and services are described as follows:
• Product/service: fiberoptic systems for DTS. • System to monitor: cable systems. • Components to monitor: cables, joints, terminations. • Sensor technology: distributed fiberoptic cables. • Communication technology: fiberoptic cables. • Web site: www.sensortran.com. • Description: SensorTran offers fiberoptic temperature sensors that are used to continuously
monitor temperature of buried cables.
5.2.12 Schlumberger/Sensa (Houston/United Kingdom)
Schlumberger/Sensa’s products and services are described as follows:
• Product/service: fiberoptic systems for distributed temperature sensing and cable fire detection.
• System to monitor: cable systems. • Components to monitor: cables, joints, terminations. • Parameter to measure: temperature. • Sensor technology: distributed fiberoptic cables. • Communication technology: fiberoptic cables.
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• Web site: www.sensa.org. • Description: Sensa offers fiberoptic temperature sensors that are used to continuously
monitor temperature of buried cables or cables in ducts and tunnels. This monitoring provides temperature information that can also detect fires.
5.2.13 University of Southampton (United Kingdom)
The University of Southampton’s products and services are described as follows:
• Product/service: cable system insulation condition and defect location. • System to monitor: cable systems. • Components to monitor: cables, joints, terminations. • Parameter to measure: PD. • Sensor technology: Fiberoptic cables. • Web site: www.ecs.soton.ac.uk. • Description: Several technical papers were found in this area. The concepts were in the
research stage.
5.2.14 Sumitomo/J-Power Systems (Japan)
J-Power Systems’ products and services are described as follows:
• Product/service: fiberoptic systems for DTS. • System to monitor: cable systems. • Components to monitor: cables, joints, terminations. • Parameter to measure: temperature. • Sensor technology: distributed fiberoptic cables. • Communication technology: fiberoptic cables. • Web site: www.sumitomoelectricusa.com. • Description: Sumitomo offers fiberoptic temperature sensors that are used to continuously
measure real-time temperature profile along cables.
5.2.15 TechImp (Italy)
TechImp’s products and services are described as follows:
• Product/service: cable system insulation condition and defect location. • System to monitor: cable systems. • Components to monitor: cables, joints, terminations. • Parameters to measure: PD, sheath temperature, and current. • Sensor technology: capacitive/inductive couplers. • Communication technology: wireless, optical fiber link.
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• Web site: www.techimp.com. • Description: TechImp offers real-time, on-line diagnostic and condition monitoring services
using PD detection. Applications have been found on various circuits of extruded and pipe-type cables and accessories. LIOS Technology and TechImp recently announced a partnership in providing combined DTS and PD detection systems.
5.2.16 Tokyo Electric Power Company (Japan)
Tokyo Electric’s products and services are described as follows:
• Product/service: cable system fault location. • System to monitor: cable systems. • Components to monitor: cable, joint, and termination faults. • Parameter to measure: surge current. • Sensor technology: fiberoptic cables. • Communication technology: wireless, GPS coordination. • Web site: www.tepso.co.jp. • Description: Current sensors are installed at each end of a cable section to detect zero-phase
fault currents, decide if the fault is within or outside the protected section, and then activate a locking mechanism of the reclosing circuit breaker.
5.2.17 USi (New York)
USi’s products and services are described as follows:
• Product/service: thermal monitoring and ratings, leak detection. • Systems to monitor: cables, hydraulic systems, buried pipes. • Components to monitor: cables, pipes, joints, terminations. • Parameters to measure: temperature, pressure, flow, valve operation, tank fluid levels. • Sensor technology: thermocouples, meters, and gauges. • Communication technology: wired and wireless. • Web site: www.usi-power.com. • Description: The system uses temperature measurements and thermal models to provide
dynamic ratings and detect leaks.
5.2.18 UtilX/CableWise (Washington)
UtilX/CableWise’s products and services are described as follows:
• Product/service: cable diagnostic. • System to monitor: cable systems. • Components to monitor: cables, joints, terminations. • Parameter to measure: PD. • Sensor technology: inductive/capacitive couplers.
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• Web site: www.utilx.com. • Description: CableWise offers an on-line diagnostic system that measures pre-discharge and
discharge signals..
6 DEMONSTRATION SCENARIOS
6.1 Introduction
This report describes technologies that can be applied for inspection and monitoring of underground transmission lines in a multidecade time frame. This report serves as a roadmap and provides a vision for the inspection and monitoring technology development and demonstration on underground transmission systems. Utility staff familiar with underground transmission line inspection, experts in sensing and communicating technology, and transmission system researchers collaborated to develop this document.
Possibilities exist to demonstrate some described technologies at host utilities on both XLPE and HPFF or SCFF cable circuits. If the outcome of the demonstrations shows issues in the methods, further research and development can be established to enhance the technologies. The following three sections list some scenarios for the technology demonstrations.
6.2 Condition Monitoring of Underground Transmission Vaults
The fundamental features of the system concept can be demonstrated with several sensor types installed in one to two vaults, a fixed communication hub located at the vaults, and a mobile platform carrying a communication hub. The sensors can be used to monitor vault conditions, such as component or environment temperature, hardware corrosion, safety-related gas levels, and flooding. Different power harvesting schemes can also be demonstrated, including harvesting power from induced cable conductor, sheath, or pipe current. Different communication techniques should be used between sensors and hubs. At least one of the sensors should be monitored in real time as an alarm for a simulated catastrophic condition, such as a system outage. The rest of the sensors can be periodically polled at modest intervals adequate for detection of gradual condition degradation that is typical of power transmission components. Different scenarios can be simulated with these key elements to demonstrate a variety of system features and benefits.
6.3 Condition Monitoring for Underground Transmission XLPE Cables
PD sensors and current measurement devices can be installed at the joints and link boxes along the circuits and terminations. A distributed temperature sensing system can be installed and the temperature monitored. Fiberoptic cables can be used to transmit the measurement data to a central location for processing. Current sensors can also be applied for circuit load monitoring. Various communication schemes as described in Section 6.2 can be applied for the demonstration.
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6.4 Condition Monitoring for Underground Transmission Pipe-Type Cables
On-line DGA sensors and their variations can be applied at joints within vaults and terminations at a substation. A leak detection system can be installed that includes monitoring of hydraulic systems, pipe pressure and temperature, and possibly mass flows. A DTS system can be installed in the duct adjacent to the pipe to monitor the temperature of the cable circuits. A remote cathodic protection monitoring system can be applied. Various communication schemes as described in Section 6.2 can be applied for the demonstration.
7 REFERENCES
1. Using Guided Waves to Detect and Locate Corrosion in Coated Piping. EPRI, Palo Alto, CA: 2004. 1008715.
2. Field Measurement of Cable Dissipation Factor. EPRI, Palo Alto, CA: 1993. TR-102449.
3. Low-Cost Sensors to Monitor Underground Distribution Systems. EPRI, Palo Alto, CA: 2007. 1013884.
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