Development of a High-Strength Conductor, Accessories andStringing Method for an Over-Water Crossing of
the Osaki Thermal Power Lineby Hideki Kamiyama *, Satoshi Hiratsuka *2, Nozomu Fukuda *3, Toru Kojima *4,
Shoji Aotani *4, Osamu Asayama *5, Koichi Hanada *5 and Masanao Miyashita *5

The Osaki Thermal Power Line, which traverses Japan's Inland Sea area, is one of

ABSTRACT the world's largest overhead power transmission lines having two water crossings.
To assure adequate over-water clearance for these crossings, a 690-mm2 internally corrosion-proofed
KTACSR/EST conductor has been developed, combining high strength and corrosion resistance. It
is one of the world's largest conductors providing both high strength and large current-carrying
capacity. Cross-wire dampers were also developed to suppress the aeolian vibration that can occur
on long spans, together with dead-end clamps for high-strength conductors. Systems for the supervision of helicopter stringing and for over-water supervision were also developed to assure thesmooth progress of the conductor stringing work. New aluminum come-alongs were also developed to replace the steel type used previously, reducing mass by approximately 40% and improving
stringing efficiency. The stringing method adopted was the semi-prefabrication method, in which thelength of conductor to be cut is determined on the basis of measurements of sag made after
temporary tensioning.
The overhead water crossings were strung using the conductor, accessories and stringing method
described here, and the work was successfully completed in October, 1997.

1. INTRODUCTION
The Osaki Thermal Power Line is a 220-kV transmission
line running from Chugoku Electric Power's Osaki Power
Station on the island of Nagashima in the Inland Sea tothe Kurose Substation in the town of Kurose, Hiroshima
Prefecture. The route involves two water crossings--1,603
m from Nagashima to Usujima, and 2,145 m from Usujima
to the mainland, and is one of the largest such overheadlines in the world.
Furukawa Electric was in charge of the high-strength
conductor, accessories and stringing method for the 1,603m Nagashima-Usujima sector, and completed the work
successfully in October, 1997.
The conductor used was a high-strength type for improved sag characteristics, and new stringing techniques
were introduced in consideration of safety and working* Engineering Dept., Bare Wire & Cable Div. Power CableGr.
* 2 Research Sec., Development Dept., Bare Wire & Cable
Div. Power Cable Gr.
* 3 Design Sec., Conductor Accessories Dept., Bare Wire
& Cable Div. Power Cable Gr.
* 4 Overhead Transmission Line Engineering Div. Power
Cable Gr.
* 5 The Chugoku Electric Power Co., Inc.

Town of Akitsu

City of Takehara

Yoshina sector
Town of Higashino

UsujimaInland Sea(Yanagi-no-Seto)

Nagashima sector

Nagashima
Osaki Thermal Power Station
Town of
Osaki

Figure 1 Route of the Osaki Thermal Power Line showing
over-water sectors

37

convenience.
This paper describes in detail the development of thehigh-strength conductor, accessories and stringing methods used on this over-water portion, and describe thestringing work.

2. CONSTRUCTION OVERVIEW
The line runs from Nagashima via Usujima to Yoshina in
Takehara, and comprises two water crossings--theNagashima sector (towers No. 8 through No. 11) and the
Yoshina sector (towers No. 12 through No. 15). Figure 1
shows a map of the route.

3. DESIGN
3.1 Conductor
Since the route of the line crosses shipping lanes, it was
decided to provide sufficient clearance--28.4 m on the
Nagashima sector and 44.4 m on the Yoshina sector--so
as not to interfere with shipping. It was also considereddesirable, from both the economic and the environmental
points of view, that the towers on the Nagashima sector
be as low as possible, and to effect this improvement insag characteristics required a special conductor that could
be used under extremely high tensile strain. It was alsoessential that the conductor be capable of carrying a large
current--1,360 A or more--and be able to withstand seTable 1 Specifications of 690-mm2 KTACSR/EST Conductor
Item
Type of wire

Unit

Characteristic-

Internally corrosion-proofed
KTACSR/EST

Nominal cross-sectional area

mm2

Cross sectional view

Stranding
structure

Crosssectional
area
Mass

-

KTAL

strands/mm

58/3.9

EST

strands/mm

61/3.3kN {kgf}

979.6 {99,680}

Min. tensile load
Outsidediameter

690

KTAL

mm

45.3

EST

mm

29.7

KTAL

mm2

693.1

EST

mm2

521.7

Total

mm2

1,215

KTAL

kg/km

1,928

EST

kg/km

4,130

Grease

kg/km

231

Total

kg/km

6,289

Electrical resistance

W/km

0.0465

Coefficient of elasticity

GPa{kgf/mm2}

124.7 {12,720}/˚C

14.9×10-6

Coefficient oflinear expansion
Current- Continuous (150˚C)carrying
capacity Short term (180˚C)

38

A

1,969

A

2,266

vere salt corrosion. After considering conductors of 7 different structures, a 690-mm2 stranded aluminum conductor with an internally corrosion-proofed special steel core
(KTAl 58 strands/3.9 mm + Est 61 strands/3.3mm). Table
1 shows conductor specifications.
This conductor is made by multilayer stranding of 61
1,770-MPa (180 kgf/mm2) class 3.3-mm diameter extrastrong galvanized steel wires, stranded together with 58
3.9-mm high-strength heat-resistant aluminum alloy wires,with an aluminum-to-steel ratio of 1.33 and a tensile loadof 979.6 kN (99,680 kgf). As a measure against salt corrosion, the conductor is "internally corrosion-proofed," i.e.,
packed with corrosion-proofing grease down to the insideof the outer layer, and to prevent rotation of the conductor
during stringing, it is right-hand laid to match the directionof the stringing wire.
In the past there have been few instances in which thehigh-strength conductors used in over-water crossings also
provided large current-carrying capacity. The conductor
here used for the Osaki Thermal Power Line can be regarded as the world's largest high-strength high-current
conductor.
3.2 Vibration Dampers
The maximum tensions applied to this conductor are extremely high, reaching 287.3 kN (29,000 kgf), and thenormal tension is relatively high, at 25% of the tensile
strength of the conductor. It is therefore necessary, in designing the vibration dampers, to take particular note of
conductor damage resulting from aeolian vibration. Studies were therefore undertaken to design a cross-wiredamper (CWD) based on existing wind data and on theexperience previously gained in designing the Matsushima
Thermal Line.
Mounting the CWD on the conductor by applying crosswires of appropriate length at an angle of 45° to the line
causes the cross wires to resonate when aeolian vibration is produced in the conductor, so that the energy iscanceled out by the elastic hysteresis loss of the crosswires, vibration interference and torsional movement, andconductor vibration is suppressed.
On the one hand, the large amount of energy input when
the span is long requires that the CWDs also be large,
while on the other, ease of installation and the problem ofstrain on the towers necessitate reduced size and weight.
Accordingly, with a view to easier installation, the CWDs
were designed as combinations of lengths of 2 - 5 m. And
although it is normal practice to make the cross wires ofthe same material as the line conductor, in this case con-Figure 2 Configuration of the CWD8 8-loop cross-wiredamperFurukawa Review, No. 18. 1999

ductor stock was used only for the 5-m loop, and the otherloops were made of 980-mm2 ACSR. It was thus possible
to decrease weight while suppressing a broader range of
frequencies. Figure 2 shows the mounting configuration.
3.3 Dead-End Clamps
The dead-end clamps, like those for (T) ACSR in the JEC
standards, were of compressive-type construction, comprising a steel clamp for holding the steel core, an aluminum clamp for holding the aluminum wire, and a jumper
socket for holding the jumper wire. Because of their size,
however, there was fear that with the ordinary method ofclamping, unraveling of the conductor might occur. Reverse compression was therefore adopted for the aluminum clamp. The following factors were considered in determining clamp structure:
1) A draw-out plate was used in the aluminum compression mechanism, to the mount of which was welded acast iron socket. By this means differences in inner diameter were reduced and it was possible to maintain avirtually constant amount of elongation during compression.
2) To absorb differences in elongation at compression, an
aluminum ring was mounted between the aluminum
clamp and the steel clamp.
3) Since the ratio of tensile load borne by the steel core islarge, the outer diameter of the compressing mechanism of the steel clamp was made larger than that of
the stranded wire. This means that the aluminum wire
compressing mechanism requires a collar, and to facilitate the compressing work, a unitized structure wasadopted in which this collar was welded to the innersurface of the aluminum clamp.
Figure 3 shows the clamp.
3.4 Lightweight Come-Alongs
Because of the high tensile load on the conductor, comealongs of conventional design would have been too large
and massive. Accordingly a new and lighter design wasdeveloped made of METACS--a compound material con-sisting of aluminum alloy and ceramic--with a hanger madeof the AL7000 alloy used in aircraft. This resulted in a unitthat had the same strength as conventional types but only
about 40% of the weight.

Table 2 Evaluation Tests
Test subject
Stranded
wire

Test item
Current
capacity
test

ACresistancePurposeTo confirm influence ofskin effect, iron loss etc.

Temperature To compare and verify calculated
rise
and measured values
Stresselongation

Creepcharacteristics

Coefficientof elasticity

To compare and verify conventional and measured values

Distributed
stress foraluminum

To compare and verify calculated
and measured values

At room
To take account of tensioning
temperature temperature
At elevatedtemperature

Vibration test
Corrosion

Vibration
dampers

Dead-end
clampsTo confirm fatigue limiting stress
Salt spray

To confirm corrosion resistanceGreasesoftness

To verify heat-resistanceof grease

Absorbed energy
characteristics

To verify vibration damping
performanceStrength of holding
mechanism

To confirm the strength of theholding mechanism fittings

Strength of compressive
mechanism

To verify the strength of thecompressive mechanism of theclamp

Holding force

To verify the conductor holdingforce

Anti-fatigue strength

To confirm loosening of bolts
during vibration, etc.

Heat cycling

To verify strength after heat
cycling

Tension test
Lightweight Holding force test
come-alongs

Cross-section ofaluminum clamp

Cross-section ofsteel clamp

Cross-section of
jumper socket

BeforeAftercompres- compression
sion

BeforeAftercompres- compression
sion

BeforeAftercompres- compression
sion

No.

To verify breaking strength
To verify wire holding force at
actual tension

Name
Material
Aluminum clamp A1070TD
Lug plate
JIS H 2110
Steel clamp
S15CK
Jumper socket
JIS H 2110
Aluminum ring
JIS H 2110
Bolt
SS400

Figure 3 Compressive-type dead-end clamp for 690-mm2
KTACSR/EST conductor

39

5. STRINGING METHOD

In addition to general performance tests, the conductor
and accessories were subjected to the performance evaluation tests in Table 2. Satisfactory results were obtained.
The results of tests to confirm the vibration performanceof the CWDs showed that for conductor alone, a distortionof ±110µm was produced at frequencies of 30 Hz andbelow, whereas with the vibration damper mounted thiswas reduced to ±30µm (see Figure 4).

Among methods that can be used for stringing the pilotrope for over-water crossings are by float, by crane barge
and by helicopter. The committee responsible for the safe
installation of line decided to adopt a helicopter methodthat used loop stringing, which is considered the most reliable for installing a conductor on a water crossing. Figure
5 shows the main machinery and equipment used.

Strain ( ±µm)

4. CHARACTERISTIC CONFIRMATION TESTS

5.1 Preparation
The machinery and equipment used was refurbished andimproved from that used to install Chugoku Electric Power's
South Hiroshima connector, which was completed in June,
1981. A series of verification tests were carried out to confirm working convenience and safety. Table 3 shows theverification tests. And to assure safety and working convenience at the drum engine area (tower No. 8) and thereturn pulley area (tower No. 11), large inclined stages
made of steel and scaffolding pipe were erected (see Photo1).

Without CWD

Permissible conductor strain

With CWD

Frequency (Hz)Figure 4 Maximum anticipated strain on 690-mm2 KTACSR/
EST conductor with and without CWD8 damper

Photo 1 No. 8 tower, drum yard and large inclined stages

2.0-m diam.
tower-mounted pulley

2.0-m diam.
tower-mounted pulley

20 m or more

(30 m or more at night
and on holidays)

Plan view
1.5-m-diam. presser pulley
2.0-m-diam.
presser pulleyTake-up/pay-out machine

2.5-m-diam. return pulley

2-wheel loop roller tensioner

10-tf stringing engine
Working stage

Working stage
Nagashima Is.

Figure 5

40

Usujima Is.

Equipment setup for stringing on the Nagashima-Usujima sector

Furukawa Review, No. 18. 1999

Table 3 Verification Tests
Test item

PurposeLoop roller/tensioner loadtest

To confirm temperature and strain in mainroller/tensioner components under continuous
operation.

Roller/tensioner
pass-through test
(temporary connection
section)To confirm pass-through characteristics oftemporary connection section

Movement characteristics
test (loop roller/tensioner,
take-up and pay-out machines and control equipment)

To verify movement characteristics ofequipment and techniques of operation

Tensioning in anti-tension
devices

To subject all tools and methods subjected
to individual evaluation tests to tensioning in
the anti-tension to confirm whether there are
any problems with the methods

5-wheel block strength test

Breakdown test of the 5-wheel block used intensioning

Conductor car test

To evaluate the suitability of the conductor car
used in mounting the dampers using afull-scale model.

Reverse T ladder test

To evaluate the suitability of the ladder usedin mounting the accessories using a full-scalemodel.

Helicopter stringing flighttest

To conduct helicopter stringing under conditions as close as possible to actual conditions,and confirm the condition of the roller/tensioner, helicopter and stringing rope.

5.2 Helicopter Stringing
It was decided to develop roller/tensioner machines using
high-performance brakes, etc. and to conduct verification
tests of helicopter stringing techniques in mountainous
terrain using spans of the same length as the crossing.
Based on a survey of ship traffic in the area, it was
decided to carry out the helicopter stringing operation between 9:00 and 11:00 a.m. on June 5, 1997. The opera-tion was completed safely and effectively, using a warning
vessel, marine monitoring station, radar, and so on. The
helicopter used was an SA330J Puma from Aerospaciale,using a 12-mm-diam. pilot rope made of strong, lightweight
Kevlar fiber and keeping the rope 45 m or more above thesurface of the water. The stringing took approximately 30
min per rope, enabling both ropes to be strung within theallotted time.
5.3 Conductor Stringing (Loop Stringing)Due to the high stringing tension on the conductor (approximately 180 kN or 18 tf), the need to maintain the wire
at an over-water clearance of 20 m or more (or 30 m or
more at night and on holidays when no monitoring personnel are present), and the need to increase conductor
height temporarily during the passage of large ships, it
was decided to use the loop stringing method, which allows wire and conductor sag to be easily adjusted during
the stringing process.
The loop stringing method involves providing a loop roller
tensioner and a return pulley on opposite sides of the water,and starting from the Kevlar rope strung by the helicopter,
gradually increasing rope diameter up to 18 mm, at which
point a loop is formed and wire diameter is increased from18 to 24 mm, from 24 to 32 mm, and finally from 32-mm
wire to the conductor. Figure 6 shows the basics of themethod.
The height of the wire or conductor can then be easilyadjusted during stringing by moving the return pulley, whichis positioned on a rail.
A method was also adopted to minimize the increasein tension when temporary joints passed through the loop
roller/tensioner and to prevent damage to the groove of
the roller/tensioner.

18-mm diam. wire to 24-mm diam. wire
18-mm diam. wire
Return pulley

Height above water:
20 m

24-mm diam. wire

24-mm diam. wire to 32-mm diam. wire
24-mm diam. wire

Loop
stringing

Return pulley

Height above water:
20 m

32-mm diam. wire

32-mm diam. wire to conductor
32-mm diam. wire
Return pulley

Height above water:
20 m

Helicopter stringing of 12-mm diam. Kevlar rope
12-mm diam. Kevlar rope to 14-mm diam. Kevlar rope
14-mm diam. Kevlar rope to 14-mm diam. wire
Restringing of 14-mm diam. wire from single to triple pulley
14-mm diam. wire to 18-mm diam. wire
18-mm diam. wire to two strands of 18-mm diam. wire
Restringing of 18-mm diam. wire

Conductor

18-mm diam. wire to 24-mm diam. wire
24-mm diam. wire to 32-mm diam. wire
32-mm diam. wire to conductor
18-mm diam. wire to two strands of 18-mm diam. wire
Restringing of 18-mm diam. wire
Thereafter repeat steps throughFigure 6 Method of loop stringing

41

5.4 Monitoring Systems
5.4.1 Monitoring System for Helicopter Stringing
A computer was installed in the helicopter cockpit and in
the operating room at tower No. 8, and information sentfrom the global positioning system (GPS), air-pressure
altimeter, load cell, etc. were used to effect real-time monitoring of the sag of the stringing rope on the computer
screens. This stabilized the stringing operation, greatly
contributing to the safety of the work (see Figure 7).
5.4.2 Marine Monitoring System
Since it was possible that ships with masts 20 m or morein height might pass under the site of the stringing operation, efforts were made to ensure the safety of shipping by
setting up a marine system to provide ordinary monitoringby human operators and a warning vessel, together withradar observation of shipping. This would enable early identification of those of the many ships passing through thatwould pose a danger of striking the stringing wire or the
conductor. Radar was used to locate ships within a radiusof 6 km from the work site, so that the mast-top of a likely
vessel could be sighted through a transit, its mast heightand compass heading calculated by computer and thevessel followed by closed-circuit TV. In this way information on the passage of large ships can be obtained early.

1) Based on the results of precise optical-wave measurements between the support points on each tower andbetween the anchor at tower No. 8 and the support pointof tower No. 9, calculations of sag and tension were
performed using a mainframe computer to find the actual length of conductor required at completion.
2) After attaching the conductor at tower No. 11, the otherend was fastened to the dead-end clamp of the anchor
beneath tower No. 8 and tensioned to the target sag(approximately 80% of the standard sag) to find theactual length under temporary tensioning.
3) A correction factor was found to compensate for suchfactors as steel clamp length, clamp elongation at com-

Conductor length required at completion (neglecting elongation) L0

Standard sag

Conductor length under temporary tension (neglecting elongation) L1

Target sag
Anchor hole

6. TENSIONING
6.1 Semi-Prefabrication Construction Method
Because of the high tension on the conductor (approximately 20 tf/conductor) and the difficulty of setting the compressive clamp on the tower, and to increase the working
convenience, a semi-prefabrication method was used. In
this method, the actual conductor length at completion issubtracted from the actual length when temporarily
tensioned, and the conductor is cut on the ground, clamped
and strung (see Figure 8).

L' = (L1 – L0)
L1 = Conductor length at temporary tension (neglecting elongation)
From base anchor at tower No. 8 to conductor ending in dead-end clamp
at tower No. 11
L0 = Conductor length required at completion (neglecting elongation)
From conductor ending in dead-end clamp at tower No. 8 to conductor
ending in dead-end clamp at tower No. 11
L' = Cutting compensation length
From base anchor at tower No. 8 to point at which conductor is cut

Figure 8

Method of semi-prefabricated stringing

Helicopter-borne system
Wireless

GPS antenna
Notebook
PC

Serial card
Altimeter

A/D port
Load cell

Antenna
Millimeter wave

Notebook
PC

Optical fiber
cable
Wireless

A/D port

Control
panel

Control
panel

Control panel

Ground-based system
Drum area

Figure 7 Monitoring system for helicopter stringing

42

Furukawa Review, No. 18. 1999

pression, elongation of ganged anti-tension insulators,
tower flexure, etc., to correct the actual conductor length
at completion and under temporary tensioning.
The corrected length cut was then determined from theabove calculations and the temperature of the conductor
during temporary tensioning.
6.2 Tensioning
When loop stringing of the conductor was completed, theconductor loops and presser pulleys were disassembled
using 5-wheel and 4-wheel prestretch blocks, the conductor was moved to the 25-tf anchor at the foot of tower No.
11, cut at the designated position, and fastened to thedead-end clamp.
After completion of clamping, the 4-wheel prestretch
blocks on the tower and on the ground were set, and the
conductor was mounted to the anti-tension device bymeans of a prestretch block method that dispenses withthe come-along.
After mounting to tower No. 11, the conductor was temporarily tensioned on the ground at tower No. 8 using thesemi-prefabrication method, the dead-end clamp was fastened and mounted on the anti-tension device by a wiretype method without come-along, thereby completing thetensioning operation. Note that to prevent twisting of thetower, the tensioning and clamping operations were performed simultaneously for both circuits (see Photo 2).

Photo 2 Tensioning at tower No. 8

competed on time and without problems. It was an excellent example of coordination among the power utility, equipment manufacturer and construction personnel. The conductor, accessories and basic design of this project weredeveloped around 1980, and its successful completion isa tribute, not only to those actually engaged in the manufacture, development and construction, but also to those
who participated in the original design and development.
The authors would like to express their deep appreciation
to all those involved.
Manuscript received on November 25, 1998.

7. CONCLUSION
The stringing of the Osaki Thermal Power Line--the last
and largest line in this century with a water crossing--was43