Cold Shrinkable Joint for 66-kV and 110-kV XLPE Cables
Applied to Practical Transmission Lines
by Hiroaki Kurihara *, Shozo Kobayashi *2, Masami Kitaya *3, Yoshihiro Arashitani *4,
Hiroshi Oda *5, Toru Serizawa *6 and Toyoaki Tashiro *In the overseas market of power cable accessories for 110-kV rating or over, prefabricated joints (PJs) of on-site assembly type are very often designated in thepurchasing specifications due to their simple installation and high reliability. In particular, onepiece rubber block joints (RBJs) featuring simple structure and easy installation are growing in
popularity thereby replacing epoxy-unit type PJs comprised of epoxy unit and rubber stress cone.
In the domestic market for 66-kV rating also, the RBJ is known for its high quality, low cost, andshort installation time so that it is going to supplant tape-wrapping joints (TJs) now dominating themarket.
In view of these market needs, the authors have successfully developed a cold shrinkable joint(CSJ) as a class of RBJ, in which one-piece molded rubber block is expanded at the factory inadvance followed by installation on the cable at the construction site 1). This is the world's firstachievement of factory expansion, cold shrinkable joints with ultra-high voltage rating.
This paper reports on the first application of this factory expansion, cold shrinkable joint topractical transmission lines.

ABSTRACT1.

INTRODUCTION

Among various means to upgrade the quality of cable
joints, the use of prefabricated joints (PJs) goes a longway, in which joint components are fabricated in the factory in advance under consistent quality control followed byassembly at the construction site. These PJs are going toprevail in the domestic as well as overseas market of
power cable accessories due to their high reliability. In
particular, rubber block joints (RBJs) using one-piecemolded rubber block for the main insulator are overwhelming the market, armed with simple structure and excellent
installation performance.
In the domestic market where efficiency and avoiding
waste are earnestly required due to the harsh economic
conditions, the RBJ is expected to enable short-time
installation and cost reduction, while upgrading the quality
and reliability of installation.
Domestically, tape-wrapping type joint (TJ) in which eth-

* Power Cable Accessories Production Dept., Power Cables
Div.
*2 Energy Transmission Research Dept., Ecology and Energy
Lab.
*3 Power Cable Engineering Dept., Power Cables Div.
*4 Environmental Technology Development Dept., Ecology andEnergy Lab.
*5 Construction Engineering Dept., Power Cables Div.
*6 Equipment Quality Assurance Dept., Power Cables Div.

ylene-propylene rubber tape is wrapped using a taping
machine perpendicular to the electric field gradient for
insulation is typical of cable joints for 66-kV CV cables,
and the joint has a track record of more than 10,000 phases. While the TJ can carry out installation at a pace of
three phases per three days, the RBJ that permits installation in shorter time is coming in as a new approach to cost
reduction.
When it comes to overseas market, the construction ofelectric cable networks is being carried out flourishingly in
East Asia especially; and exploding growth in the demandfor electric power accompanied by economic development
sometimes results in a huge construction project that
includes a total cable length in excess of 50 km. In these
construction projects, demand focuses on prefabricatedjoints for 110-kV rating which permit easy installation without the need for dedicated apparatuses. Heretofore, themarket of prefabricated joints has been dominated by the
epoxy-unit PJs that combine epoxy unit and rubber stressrelief cone --a design based on the concept of termination
joint. Recently, however, RBJs have inevitably risen in thisfield since they have fewer components and they are able
to finish the assembly work of the main insulating portion
simply by mounting the molded rubber block onto thecable insulator. RBJs are simple in structure and permit
easy assembly, thus enabling total cost reduction throughthe cost merit of the reduction of installation time.
We have developed a state-of-the-art RBJ called cold
shrinkable joint (CSJ), which is based on a factory-expan-

47

Silicone insulating layer
Stress-relief cone layer
Inner semiconductive layer

Cable insulator

Figure 2

Figure 1

110-kV expanded rubber unit with carrier pipe

sion, room temperature shrinking scheme 1). In this innovative jointing technology, silicone rubber is used for the
one-piece molded rubber block serving as main insulator;
and taking advantage of low mechanical stress relaxation,i.e., small permanent set of the material, the rubber blockis expanded to a diameter greater than that of the cablesheath using a carrier pipe called spiral core at the factoryin advance (See Figure 1); and it is installed on the cableat the site simply by pulling out the spiral core. Thus, customers can elude purchasing the expansion apparatus
including hydraulic machinery thereby lessening the burden of initial investment. This fact helps considerably inpromoting our sales campaigns advantageously.
We have achieved the world's first supply, in the field of
ultra-high voltage cables, of one-piece rubber block jointsof factory-expansion type using silicone rubber. This
paper reports on the technological characteristics of thesejoints together with their application to practical transmission lines.

2.

FACTORY-EXPANSION, COLD-SHRINKABLE, SILICONE RUBBER JOINT

2.1 Characteristics of Cold Shrinkable Joint(1) Factory Expansion and Low Permanent Set
While cold shrinking technology has been widely used inmany applications besides power cable jointing, we havebeen successful in improving its working efficiency significantly by using silicone rubber.
In the case of one-piece rubber block using ethylenepropylene rubber, the expansion ratio can not be as great
as that of silicone rubber due to the limitations of stressrelaxation characteristics of the material. Moreover, it isindispensable to use an expansion apparatus including
hydraulic machinery at the site --hence, it is called siteexpansion type-- since the period of time allowed formaintaining sufficient strain is as short as a few hours. By
selecting silicone rubber with excellent stress relaxation
characteristics in contrast, we have achieved a factoryexpansion type in which the rubber unit is expanded at thefactory in advance and, at the site, installation work iscompleted simply by pulling out the end of a spiral core48

Calculated electric field distribution of rubber unit

string that is formed into a cylinder, so that the unit shrinkage is effected at room temperature without using any
special tool. This technology can ensure a sufficiently long
period of the unit expansion --to a size larger than that of
the cable sheath diameter-- even for exported products
where product maintenance is extremely difficult after
delivery.
(2) Structure Simplification
Simplification of the product structure leads to cost reduction through both material cost and installation cost. In thisregard, the CSJ is an integrated rubber unit comprised ofinner semi-conductive layer, outer semi-conductive layer,
stress-relief cone layer, and silicone insulating layer, thus
enabling a great deal of components unification in comparison to epoxy-unit PJs in particular.
In addition, unlike epoxy-unit PJs, one size of CSJ canbe assigned to a plurality of cable sizes resulting in a cost
reduction. This is because, so long as the inner diameterof a CSJ rubber unit remains within a certain range smaller than the outer diameter of a cable insulator, the unit
adheres with sufficient strength to the cable, whereby theunit is expanded and subsequently fitted onto the cable.
2.1.1 Key Design Factors
Key factors in designing a combination of different insulators are to ensure an appropriate interfacial pressure at
the interface between the insulators as well as to control
an optimized distribution of electric fields. CSJs maintainthe interfacial performance between the surface of thecable insulator by making use of their intrinsic contraction
force without using any pressurizing apparatus based on
spring. If stress relaxation occurs, i.e., rubber slackens
due to expansion hysteresis, the interfacial electric performance degrades considerably. Therefore, the material
characteristics have to be carefully selected so as to prevent this degradation.
In terms of electric field distribution, it is optimized bymeans of conductive silicone rubber that is molded integrally with the insulating silicone material. See Figure 2.
As can be seen, these materials together with theadvanced molding techniques support the foundation of
the CSJ technology, based on which the CSJ rubber unit
never separates nor tears even when the unit significantly
changes its geometry during expansion and contraction.
The carrier pipe or the spiral core poses another important task in the application of factory-expansion to ultrahigh voltage cables. Through careful investigations, wehave optimized the geometry and molding technique ofthe spiral core string that offers a sufficient strength to

Furukawa Review, No. 20

2001

CSJ rubber unit

Carrier pipe (Spiral core)(a) Expanded CSJ rubber unit
Stepped cable end
with insulation removed

Cable sheath(b) Temporary insertion onto the cable

Conductor sleeve

Figure 4110-kV rubber unit inserted onto cable

Figure 5

Assembled CSJ for 110-kV XLPE cable

Figure 6

66-kV rubber unit inserted onto cable(c) Conductor jointing(d) Removal of carrier pipe (Assembly of insulator)

Figure 3

Assembly procedures of CSJ

support the great contraction force associated with expansion as well as an excellent property for core withdrawal.
2.2 Assembly Method of CSJ
The simplicity of assembling the rubber unit to form theinsulator is no doubt the most distinguished advantage offactory-expansion type CSJs using silicone rubber. See
Figure 3.
Generally speaking, the most critical operation in jointing work is the assembly of insulator. PJs are commonly
considered to be stabilized in quality necessitating fewer
quality control measures on site, since they are assembled on site using insulators electrical characteristics
thereof are verified at the factory before shipping. Among
PJs, CSJs with one-piece rubber block insulation, in particular, are believed to be extremely free from interference
by foreign inclusions and external injuries to cause electrical degradation, since they have fewer components andthe assembly is completed in short time. Because theexpansion work is finished at the factory before shipment,
only the interface between the cable insulator and theinsulator unit has to be taken care of not to be interferedby unexpected elements at the time of assembly on site.
See Figures 4, 5, and 6. It can be said that the CSJ technology has approached an ideal state of consistent quality
control encompassing the factory and the site. See Table1.
2.3 Installation Process
The one-piece insulator enables the reduction of installation hours in addition to the simplification of constituting
components thus leading to the material cost down. For
example, whereas epoxy-unit type PJs of 110-kV rating
for overseas specifications take about eleven days per

49

Table 1
Item

Comparison of PJ, TJ, and CSJ in structure and installation method
Evaluation of CSJ

66~110-kV CSJ

66-kV TJ

110-kV epoxy-unit PJ
Epoxy-unit with metallic electrode

Structure

EP rubber stress relief cone

(insulation)

EP rubber tape

One-piece silicone rubber unit
• Simple structure and easy

Compression device
Assembling
methodRubber stress relief cone is

Shrinkage of rubber unit at room

• Unlikely to contain foreign

taping machinetemperature and spiral core removal

inclusions and external injuries

Compression force of spring

Tape wrapping tension

Compression force of rubber

11 days/3 phases

3 days/3 phases

Many components

Taping machine needed

compression device

Interfacepressurizing
Installation
hours

installation
Rubber tape wrapping bypressed against epoxy unit by

66 kV(domestic): 2 days/3 phases
110 kV(overseas): 3~4 days/3 phases• Short-time installation• Cost reduction in both material

Cost

Table 2Comparison of 110-kV PJ and CSJ in installation
process

2
3
4
5

6
7
8

9

Comparison of 66-kV TJ and CSJ in size and installation days

CSJ
Cable cutting; shaving outer

Size in mmstraightening

semi-conductive layer(entire length x outer diameter)

Cable straightening

Assembly; waterproofing

Installation days *1

Assembly for phase 1
Assembly and waterproofing forphase 1
Shaving outer semi-conductive layer;
cable processing for phase 2
Assembly for phase 2
Assembly and waterproofing forphase 2
Shaving outer semi-conductive layer;
cable processing for phase 3

Assembly and waterproofing forphase 3TJ

CSJ

L 1190 × φ135

L 800 × φ135

3 days / 3 phases 2 days / 3 phases

Note *1: 66-kV CV-T cable is exemplified. Likely to change depending
on installation environments.

cable processing for phase 1

10 Assembly for phase 3
11

Table 3

Epoxy-unit PJ

Shaving outer semi-conductive layer; Assembly; waterproofing

and work

hours; special apparatus not needed

Cable cutting; preparation for cableDays
1

Few components; short installation(Assembly; waterproofing)---

Note: 110-kV CAZV cable is exemplified. Likely to change depending

Figure 7

Assembled CSJ for 66-kV XLPE cableon installation conditions.

three phases for installation, CSJs take three to four days
per three phases. See Table 2.
In the case of domestic 66-kV joints, while conventional
TJs used to be assembled at a pace of three days per
three phases, it was further shortened to two days per
three phases through the adoption of the one-piece rubberblock structure and the optimization of the work process.
Moreover, as shown in Table 3, the length of the outercasing is shortened by 400 mm in comparison to TJs, sothat increased applications to manholes are expected
where the offset dimensions are under tight limitations.
See Figures 7 and 8.
Figure 9 shows the structure of a 110-kV CSJ for comparison.

50

2.4 Electric Performance of CSJ
Tests are carried out in accordance with customers' specifications at the time of delivery. In terms of 110-kV specifications, CSJs have passed the type tests in conformity to
the IEC standards under the presence of the third-party
agency. See Table 4 1). As for 66-kV specifications, they
have passed the test items stipulated in accordance with
JEC-3408 standard including a long-term test for one
month as shown in Figure 10. See Table 5.

3.

CONCLUSION

A factory-expansion, cold shrinkable joint (CSJ) of onepiece rubber unit type has been developed for 66-kV and
110-kV ratings as a class or prefabricated joints, success-Furukawa Review, No. 20

2001

4

3

1

2

6

φ135

±10

5

Figure 85800 ±60

1

3

4

6
5
4
3
2
1
No.

Bonding wire
Anti-corrosion layer
Outer casing
Waterproof compound
Rubber unit
Conductor sleeve
Name of parts

6

1400 ±100

Table 4Anti-corrosion layer
Grounding wire lug
Outer casing
Waterproof compound
Rubber unit
Conductor sleeve
Name of parts

Structure of CSJ for 66-kV XLPE cable2

Figure 9

6
5
4
3
2
1
No.

Structure of CSJ for 110-kV XLPE cable

Electrical Characteristics of 110-kV CSJ in accordance with IEC 60840
Item

Partialdischargetan δ (hightemperature)

Characteristics
When loaded with AC 112 kV for 10 seconds followed by gradual lowering down to 96 kV, no partial
discharge should be observed at room temperature
Loading voltage: AC 64 kV
Conductor temperature: 95~100°C
Specified value: 0.001 or less
Loading voltage: 128 kV

AC heatcycle test

Temperature rise: Room temperature to 95~100°C(Heater: 8hr ON / 16 hr OFF)
After 20 times, partial discharge should be measured at room- and high-temperatures
Loading voltage: ±550 kV, 10 times eachLightning

Conductor temperature: 95~100°C

impulse test

After loading, no breakdown should occur when AC

Figure 10 Long-term test of 66-kV CSJ

160 kV is applied for 15 min
Water depth: At least 1 m from joint upper end towater surface
Water immersion
heat cycle test

Table 5

Water temperature: Room temperature ±10°C to95~100°C

Electrical Characteristics of 66-kV CSJ in accordancewith JEC 3408-1997
Item

High temperature duration: At least 5 hrHeat cycle: 20 times

Conductor temp. of

Post water imersion

Long-term loading or commercial

test
DC withstand test

Loading voltage: 20 kV

frequency withstand voltage

Loading time: 1 min

Lightning impulse Between parts: ±75 kV, 10 times eachwithstand test

Characteristics
65 kV (8 hr ON /16 hr OFF cycle)

Each part to earth: ±37.5 kV, 10 times eachroom temp.~105°C: 5 times
or, 130 kV.1hr at room temp. forconductor

Lightning impulse withstand±485 kV, 1 hr at room temperature

voltagefor conductor

Lightning impulse withstandfully passing the tests in accordance with JEC-3408 andIEC standards.
With this new joint using silicone rubber of excellent

room temp.~90°C: 25 times

voltage for anti-corrosion layer
Watertightness-40 kV, 3 times
98 kPa, 1hr

51

Table 6
Destination
Domestic electric
power companies

Overseas electric
power companies

Track record of 66-kV and 110-kV CSJ
Cable size

Quantity (phase)

66 kV 325 mm2

18

66 kV 325 mm2

9110 kV 800 mm2

6

110 kV 400 mm2

12

110 kV 400 mm2

18

110 kV 630 mm2

12

110 kV 300 mm230

110 kV 400 mm2

9110 kV 400 mm2

6stress relaxation property, the following has been realized.
(1) Simplification of installation equipment, improvementof installation efficiency, and shortening of installation
hours at the site(2) Downsizing of joints(3) Cost down in both material and installation
Using the advantages of easy and short time installation, we have received orders of more than 100 phases in
the first half of 2000 as shown in Table 4. In the future,applications are expected to expand to high-voltage
areas.

REFERENCES
1) Suzuki et al.: Development of factory-expansion, cold shrinkableintermediate joint for 110-kV CV cables, Furukawa Electric
Review, No.104, July 1999 (in Japanese)2) Suzuki et al.: Development of factory-expansion, cold shrinkableintermediate joint for high-voltage CV cables, Proceedings,
Electric Power and Energy, IEEJ, 348, 1999 (in Japanese)3) Kurihara et al.: Development of cold shrinkable joint for 66-kV CV
cables, Electrical Review, No.416, March 2000 (in Japanese)

Manuscript received on November 17, 2000.

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Furukawa Review, No. 20

2001