Development of Cold-Shrinkable Straight-Through Joints
for 22-kV XLPE Cables
by Hideo Takei *, Kazutoshi Mochizuki *, Koichi Yokosuka *, Kouzou Takahashi *2,
Isao Takaoka *2, Toyoaki Tashiro *3, Hideo Kawahara *4 and Yoshinari Hane *2

In the electric power distribution field of these days, cable joints are required to
be low in cost, easier to install, take less time to install, and applicable to more
space-limited places. Against this background, the authors have developed cold-shrinkablestraight-through joints for use specifically with 22-kV XLPE (cross-linked polyethylene) cables.
The developed joints and accessories proved, through the initial and long-term evaluation, tohave satisfactory performance, offering outstanding ease of installation.

ABSTRACT1.

INTRODUCTION
Table 1

Straight-through joints for 22-kV XLPE cables now used inelectric power companies come generally in three types -slip-on type, prefabricated-composite type, and taping
type; and they are specified in Electric Utility Standard
1998. The current situation of the electric power distribution field requires that cable joints be low in cost, easier toinstall, take less time to install, and applicable to more
space-limited places, resulting in an urgent need fordeveloping a new type of joint.
In response to such a need, Furukawa Electric has
been developing, jointly with Tokyo Electric Power Co.,
cold-shrinkable straight-through joints that would meet theabove-mentioned requirements. This paper presents an
overview of the process of developing the new joints,
which not only meet such requirements but also have satisfactory performance including ease of installation.

Target specifications

Item

Target performance

Partial discharge at commercial
frequency voltage
Long-term withstand voltage atcommercial frequency

Normal temperature-230 kV/3 times

High temperature±165 kV/3 times

DC withstand voltage-64 kV/1 hr.

Temperature cycling

105°C/3 hrs x 3 times

Short-circuit current loading

24 kA/2 sec.

Waterproofness

98 kPa at outer pressure x 1 hr.
27 kV at 90°C conductor temp.

In air8 hrs. ON, 16 hrs. OFF/30 cycles
Same as for "in air" with joints

In water

DESIGN OF NEW STRAIGHT-THROUGH
JOINT

57 kV/3 hrs.

Lightning impulse withstand voltage

Load cycling

2.

10 pC or less at 17 kV

immersed in water

Installation work

Reduced skill level and ease of
installation

2.1 Target Specifications
Table 1 shows target specifications. Because the development program had started before the relevant standards -High Voltage Tests on Cross-linked Polyethylene
Insulated Cables and their Accessories for Rated Voltages
from 11 kV up to 275 kV (JEC-3408-1977) and Electric
680~700*

Tokyo Electric Power Co.
Silicone Products Development Team, Power Cable
Accessories Div.
*3 Engineering Sec., Production Dept., Power Cable
Accessories Div.
*4 2nd Engineering Sec., Distribution Cables & Accessories
Engineering Dept., Power Cable Accessories Div.
*2

1
2
3
4

Connector
Insulation layer of splice body
Shielding layer
Tin-plated soft copper wire

Figure 15678Sealing mastic
Waterproof tube (Small)
Waterproof tube (Large)
Adhesive cross-linked
polyethylene tape

Structure of cold-shrinkable straight-through joint

103

Utility Standard 1998-- were published or amended, thedesign intended to meet the old standards --JEC-209 andElectric Utility Standard 1979. However, it is also applicable to the new standards mentioned above.
The design adopted cold-shrinkable method working onsite to make the installation work more efficient and lessskill-oriented, so that the site work is improved in terms of
elimination of flame tools and adjustment of insertion
force. The structure of the developed product is shown in
Figure 1.
2.2 Design
2.2.1 Installation Method
Components of cold-shrinkable joints are expanded in factory and shipped as expanded fitted on inner core, that is
expansion support. Then on the site, shrinking and installation is completed simply by pulling out the inner core
without using heat or insertion force, thereby eliminating
any special tools for expansion or shrinking.
2.2.2 Basic Concept of Cold-shrinking
Cold-shrinkable products have to meet not only electricalbut also mechanical requirements when installed by theon-site shrinking method mentioned above. Figure 2 illustrates diameter changes during cold-shrinking.
Interference rate for installation on the object is one of
the most important elements that control the product performance, since this induces interference pressure. In
addition, sufficient clearance with respect to the object isneeded from the standpoint of ease of installation. Thus,
proper selection of cold-shrinkable material properties isessential to make these two elements compatible witheach other. Suitable combinations of these elements willlead to unification of applicable cable sizes and usable
cold-shrinkable splice bodies as well as elimination of
reducer, thereby contributing a great deal to cost down
due to components number reduction.

2.2.3 Materials(1) Splice bodyIn the cold-shrinkable technology, stress relaxation characteristics including permanent elongation set and theirtemperature dependence are major design parameters to
be considered from the standpoint of quality assurance interms of diameter expansion period and long-term performance after installation. Although ethylene-propylene rubber (EP rubber) is mainly used for jointing materials nowadays, silicone rubber is superior to EP rubber in thesemechanical properties. See Table 2. We decided, therefore, to use silicone rubber for the splice body, a major
insulating component, based on our evaluation that thematerial is suitable for cold-shrinkable products.
(2) Waterproof protective cover
It is required to assume that the cold-shrinkable joints areused fully immersed in water. Since silicone rubber is inferior to EP rubber in the water-imperviousness at full water
immersion irrespective of its high hydrophobic properties,
we decided to use EP rubber for the waterproof cover ofcold-shrinkable joints.
2.3 Detailed Design of Splice Body
2.3.1 Electric Field Design
The electrical stress of every portion of joint shown in
Figure 3 was calculated by electric field analysis, anddimensions of the splice body were determined by optimizing such stresses as follows.
Table 2

Material properties of silicone and EP rubbers
Item

Silicone rubber

Ultimate elongation

790%

750%

Tensile strength

10 MPa

9.3 MPa

Hardness (JIS-A)
Tear strength (JIS-A)

34

60

21.5 N/mm

11.8 N/mm

Permanent elongation set at 100%
Dielectric breakdown strength

ObjectEP rubber

τ1

After installation

2.6%

32.4%

20 kV/mm

20 kV/mm

τ2

τ4

Cable sheathτ3
φdInner semiconductive layer
φd2Outer semiconductive layer

φd1Interference rate

After expansion

Figure 3

Cable
insulation

Insulating layer

Cable semiconductive shieldStress relief cone

Connector

Structure of splice body and its electrical design

Original stateφd: Outside diameter of object
Outside diameter of cable insulation or finished cable
φd1: Inner diameter of original cold-shrinkable tube
Inner diameter of original cold-shrinkable tube to ensure
required interference pressure with respect to the object
φd2: Inner diameter of expanded cold-shrinkable tube
Inner diameter with a clearance for passing over the object

Figure 2

104

Diameter changes during cold-shrinking

Figure 4

Result of electric field analysis for designing splice
body

Furukawa Review, No. 19. 2000

τ1: Stress at the level portion of the inner semiconductive layer
τ2: Stress at the end portion of the inner semiconductivelayer
τ3: Stress along the XLPE/rubber interface
τ4: Stress at the rising portion of the outer semiconductive layer
Figure 4 shows the result of electric field analysis overthe dimensioned splice body, which optimizes the stressbalance.
2.3.2 Interference Pressure Design
The inner pressure of cable is calculated as 3.7x104 Pa,
or 4.9x104 Pa allowing for a margin of safety, using BoyleCharles law and neglecting expansion of cable for a maximum cable temperature difference of 105°C (-15°C~90°C). This inner pressure is to be counteracted by theinterference pressure of splice body due to interference,
so that sufficient interference pressures are maintained tosuppress interface gap even after an elapse of 30 years.
The interference pressure of splice body is calculatedby the following equation, in which the body is considered
as a circular cylinder under inner pressure with its bothends open.

P=δ(b 2 -a 2 )×(1- ν )a 2 +(1- ν )b 2
afor its material, which have appropriate strength to maintain expanded products as well as recyclability after use.
2.4.2 Strength of Inner Core
The inner core has to be designed to ensure a sufficientmechanical strength so as to withstand the pressure due
to products expanded to the required diameter. Hence, we
calculated the hoop stress of the expanded product which
generates the product's inner pressure, and determined
the supporting strength of the inner core to withstand thispressure.
Calculation of hoop stressσθ = modulus x elongation due to expansion
Calculation of the inner pressure of product generated
by expansion

P a=

k=b e /a ewhere, ae is the inside diameter after expansion, be is the outside diameter after expansion,and Pa is the generated inner pressure. Further,ae and be are calculated assuming that the crosssection of cold-shrinkable tube remain the same
before and after expansion.

E

where, P is the interference pressure, a and b
are the inside and outside diameter of originaltube, ν is Poisson's ratio, δ is the diameter interference, and E is Young's modulus.

(k 2 -1)× σθ(k 2 +1)

Calculation of the supporting strength of the inner coreP b=
2.4 Design of Inner Core
The splice body and the waterproof tube are maintained
and stored mounted on the inner core, enabling easy
installation by cold-shrinking on site without using any
special tool. Thus, the inner core plays an important roll in
implementing cold-shrinking.

2×t× σ pp /S
D

2.4.1 Material
Although the inner core is an important component that
influences expansion-holding and cold-shrinking, it eventually turns into waste when installation is finished.
Accordingly, we selected polypropylene and polyethylene
Photo 1

Splice body (right) and waterproof tube (left) before
expansion

Photo 2

Splice body (right) and waterproof tube (left) after
expansion

φD

t

W

L

Figure 5

Structure of inner core and cross section of core ribbon

105where, t is the thickness of inner core ribbon, σpp is theyield stress of inner core ribbon, S is a safety factor, andD is the inside diameter of inner core.
2.4.3 Structure of Inner Core
The inner core comprises a core ribbon having L-shapededges, which is formed into a pipe under optimal welding
conditions such that the core keeps expanded products
properly but collapses easily at on-site shrinking without
using any special tool. See Figure 5.

3.

EXPANSION TECHNIQUE

An expansion apparatus which expands cold-shrinkableproducts and transfers them onto inner cores has beendeveloped, enabling easy expansion without causingslightest damages thanks to its low friction resistance. See
Photos 1 and 2.

Table 3

4.

EVALUATION OF PRODUCT PERFORMANCE

4.1 Initial Performance
Table 3 gives the test results of the developed product for
the initial performance, showing that every test item meetsthe target specifications. In particular, the result of lightning impulse withstand voltage test at high temperatures,
which was specified in consideration of the IEC specifications, was found to be equivalent to that of at normal temperatures.
Moreover, long-term withstand voltage test at commercial frequency and lightning impulse withstand voltage test
were conducted on the specimens that underwent temperature rise test and short-circuit current loading test, andtheir breakdown voltages were equivalent to those of initial state, thus confirming excellent performance.
4.2 Long-term Performance
In addition to load cycling test in air with 30 cycles, test inwater with 30 cycles as well as test in hot-water with 180
cycles for 6 months were also conducted, and satisfactory

Results of initial performance tests
Item

Results

Partial discharge at commercial frequency voltage

No occurrenceLong-term withstand voltage at commercial frequency

Good

Lightning impulse withstand voltage
Normal temperature

Good

High temperatureGood

DC withstand voltage

Good

Temperature cycling

Good

Short-circuit current loading

Good

Waterproofness

Good

Shieldingcopper tape

C-shaped
fitting

Groundingwire

Groundingwire

Figure 6
Table 5

Shieldingcopper tape

C-shaped
fitting
Coil spring

Structure of solder-less fittings

Evaluation items of solder-less grounding fitting

Evaluation item / Test item

Test results

Basic performance
Contact resistance

To be measured every 25 cycles ofnormal mode test

Table 4

Results of load cycling tests
Item

Temperature risenormal mode test

Results

In airGood

In water (normal temperature)Good

In hot water at 60°C

Good

30 cycle test
180 cycle test

To be measured every 25 cycles ofNormal mode performance
Heat cycle and sheathcurrent loading

Conditions of conductor heat cycle
Conductor temperature: 90°C
4 hrs. ON - 4 hrs. OFF/ cycles
Conditions of sheath current loading
Sheath circulating current: 16 A
1 hrs. ON - 1 hrs. OFF/ 500 cycles

Failure mode performance
Large current loading

Loading current: 650 A
2 sec ON - 1 min. OFF/ 3 cycles

Table 6
Evaluation item
Normal mode
performance

Photo 3106

Load cycling test

Test procedures for fittings
Test item
Basic performance→Normal mode performance

Failure mode

Basic performance→Failure mode performance

performance→ Normal mode performance

Furukawa Review, No. 19. 2000

results were obtained. Residual performance was evaluated subsequently for withstand voltage at commercial frequency and lightning withstand voltage, and it was confirmed that the performance was comparable to that of initial state. See Table 4 and Photo 3.

resistance changes during the normal mode and failuremode tests are seen to be small compared with those of
the bare cable shielding material.
When it comes to installed cables, it should be notedthat the same tests as for the new cables have to be carried out to confirm their reliability.

5.

5.2 Fixing Fitting for Joint
The length of the fixing fitting for joint was set to be thesame as for the existing tray, considering the interval of
supporting fittings in the manhole.
Joints were clamped by the three-core bundling method,since the joint diameters were diversified according to thecable size due to the jointing method based on coldshrinking.
Furthermore, plastic covers were provided to protect thejoints. See Figure 9.

Development of Accessories

5.1 Grounding Fitting
5.1.1 Structure and Evaluation Method of Solder-less
Grounding Fitting
The structure of grounding fitting was studied to make itsolder-less and the installation work easy using no flame
tools. Figure 6 shows the structures of two types of the fittings thus studied. Tables 5 and 6 show the items of theevaluation test and the test procedures, respectively.
5.1.2 Evaluation Results
Two types of solder-less fittings and a conventional soldering samples were evaluated in addition to a bare cableshielding material, using new cables. Satisfactory results
were obtained as shown in Figures 7 and 8, in which

5.3 Verification of Improved Installation
Compared with the existing slip-on type joints that are
most frequently used, the developed product reduces theinstallation time by 17-28%.
Moreover, the working cable length required was
reduced to 80% of that of slip-on straight joints due to theResistance (µΩ)

2500

Table 7

2000

Required time and cable length for installation

Hose band

1500

Coil spring

Type of joint

Installation time

Cable length

Slip-on straight joint Cable length180-210 min

2500 mm

150 min2000 mm

Soldering

1000
Bare shielding
material

Cold-shrinkable straight joint

500

0
25
50
75
100
125
150
175
200
225
250
275
300
325
350
375
400
425
450
475
500

0

Number of cycles

Figure 7

Results of normal mode test

2000
Hose band
Coil spring

1500

Soldering

1000
Bare shielding
material

500
0
0
25
50
75
100
125
150
175
200
225
250
275
300
325
350
375
400
425
450
475
500

Resistance (µΩ)

2500

Number of cycles

Figure 8

Results of failure mode test

405~645
1350

1
2
3
4

Supporter
Tray
Fitting for tray
Aluminum cleat

Figure 9

230

5
6
7

Rubber pad
Fastening belt
Protective coverFixing fitting and cover for the joint

Photo 4

Installation of cold-shrinkable joint107

reduction in parts number as well as upgrading of installation techniques and procedures. See Table 7 and Photo 4.

6.

CONCLUSION

In an effort to develop a new type of straight-through joint
for 22-kV XLPE cables, the authors developed coldshrinkable straight-through joints based on factory expansion, which not only meet the target specifications but also
improve installation work significantly. The following
improvements have been achieved.
1) Upgrading of ease of installation
2) Sufficient and stabilized electrical performance3) Reduction of installation time4) Reduction of working cable length required5) Cost reduction

108

REFERENCES

1) Mochizuki et al.: Development of a New Type of Straightthrough Joint for 22-kV XLPE Cables, 344, Electric Power andEnergy Div., IEEJ Conference, 1999. (in Japanese)

Notes

1) Manuscript received on November 22, 1999.
2) Printed on January 21, 2000.
3) To be published in Furukawa Review No.19, April 2000.

Furukawa Review, No. 19. 2000