Development of Cold-Shrinkable Joint for 110-kV XLPE Cables
by Hiroaki Suzuki *, Shozo Kobayashi *, Tsutomu Ono *, Hiroyuki Nomura *2,
Hiroaki Kurihara *3, Hiroyuki Iizuka *3, Daisuke Muto * and Satoru Tanaka *One-piece, pre-molded joints have been used all over the world for (extra) highvoltage ((E)HV) XLPE cables. For those pre-molded joints, ethylene-propylene
rubber (EPR) has mainly been applied for many years because of its favorable electrical andmechanical properties.
With research on mechanical and electrical properties, we have succeeded to apply a new
cold-shrink technology on those pre-molded joints using silicone rubber which has lower elasticity, better mechanical properties, such as lower permanent set, and so on. With this cold-shrink
technology, pre-molded sleeves can be shipped expanded onto the carrier pipes. So the customer needs no tools for assembly at the jointing site and just needs to pull out the carrier pipes.
All the type tests required on IEC standards 840 were carried out including water immersion
and heat cycling tests on our newly developed 110-kV cold-shrinkable joints (CSJ) and haveshown good performance in all the tests, which means we became the first manufacturer in theworld to put cold-shrinkable joints using silicone rubber for (E)HV XLPE cables on the market.

ABSTRACT1.

INTRODUCTION

Accessories for (E)HV XLPE cables, such as normal andinsulation joints have a significant role on reliability of all
the cable systems which are now going up in voltage to ashigh as 500 kV. For those joints, there are various kinds ofstructures like extrusion-molded type (EMJ), pre-fabricated type (PJ), tape wrapping type (TJ) and so on. With significant improvements in molding, one-piece, pre-molded
joints has been widely used in this decade. Those premolded joints have an advantage of quality control
because most of their parts are manufactured, tested andcomposed at the factory site. For pre-molded joints, ethylene-propylene rubber (EPR) has mainly been applied.
On the other hand, there have been achieved significant
improvements in mechanical properties of silicone rubber
this decade. This enables us not only to apply silicone
rubber for cable accessories but gives an added dimension with its lower elasticity, lower permanent set thanEPR.
We have taken advantage of those performances in silicone rubber and have developed cold-shrinkable joints for(extra) high voltage XLPE cables up to above 110 kV forthe first time in the world. We also have developed a newtechnology in which those rubber-molded sleeves are
expanded and fitted onto carrier-pipes --which can be

Rubber-molded sleeve

Carrier pipe
(Spiral core)(a) Expanded rubber-molded sleeve

Cable sheath(b) Temporary insertion into cableEnd of cable with
the insulation
removedConductor joint(c) Conductor jointing

Pull

End of the spiral core*

Power Cable Equipment Sec., Chiba Research Lab., R&D
Div.
*2 2nd Development Sec., Hiratsuka Research Lab., R&D Div.
*3 2 nd Engineering Sec., Power Cable Engineering Dept.,
Power Cables Div.

(d) Removal of carrier pipe and installation of reinforcing insulation

Figure 1

Assembly of cold shrinkable joints

109

"pulled out" easily during assembly-- at the factory site aswell as the pipes themselves.

2.

COLD-SHRINKABLE JOINTS FOR (E)HVXLPE CABLES 1)

Cold-shrinkable joints which should be shipped from thefactory site with its rubber-molded sleeves expanded havesignificant advantages over any other pre-molded joints.
Assembly of the rubber sleeves which is shown in Figure
1 should be easier because nothing except "pulling-out"
the carrier-pipes is required. Moreover, the inner diameterof carrier-pipes which is larger than the outer diameter ofthe cable jackets enables it to remove the sheath at minimum length. It should give minimum size in joints including protection boxes in every case.
Previous technology which expands rubber molded
sleeves at the assembly site generally requires expansion/shrinking equipment at the site. This contributed not
only to the assembly cost including the equipment, but tothe chances of imparting defects onto the interface
between the rubber molded sleeve and the cable insulation.
We are the first manufacturer in the world who has introduced cold-shrinkable joints for (E)HV XLPE cables.

3.

MATERIAL RESEARCH

3.1 Mechanical Properties
Silicone rubber had a limited applicability for cable accessories because of its low tear strength for many years
since it was developed. But recent technology enables us
to produce one which has high enough tear strength.

Table 1

Material properties of silicone rubber

Item

Silicone rubber

EPR

Elongation at break

790%

750%

Tensile strength

10 MPa

9.3 MPa

Stiffness (JIS-A)

34

60

Tear strength (JIS-A)

21.5 N/mm

11.8 N/mm

Permanent set at 100%

2.6%

32.4%

Silicone rubber also has a significant advantage in stability over EPR in its properties at much higher temperatures, lower elasticity and permanent set, especially. So,
we have decided to take advantage of its performanceand chosen silicone rubber for our new accessories.
On the other hand, it should be excellent in its properties of elongation and permanent set when applied on
cold-shrinkable joints. See Table 1.
3.2 Electrical Properties
3.2.1 Test Setup
To estimate its dielectric strength, we modified recessed
specimens with a surface area of 153 mm2 and tested
them with an electrode which has a round shape and isembedded into epoxy as shown in Figure 2.
The electrical tests were also carried out in oil on morethan 5 specimens for each condition. After the breakdown,
thickness of the insulation layer was measured under amicroscope and dielectric strength was calculated.
3.2.2 AC Voltage Breakdown Test
Test condition
Initial voltage: 6 kV/10 min
Step up voltage: 1 kV/10 min
Temperature:
ambient, 90°C, 105°C
Test results
Figure 3 shows no temperature dependence of dielectric strength on AC voltage observed up to 105°C.
3.2.3 Impulse Voltage Breakdown Test
Impulse tests were carried out for each of the negativeand positive pulses on different specimens.
Test condition
Initial voltage: + or -14 kV/3 times
Step up voltage: + or -1 kV/3 times
Temperature:
ambient, 90°C, 105°C
Test results
No effect of polarity on lightning impulse voltage
was observed. But temperature dependence wasobserved for each of the positive and negative impulses(Figure 4). The temperature coefficient on dielectricstrength for impulse voltage was 1.1 (ambient/105°C)
which is lower than other materials such as XLPE.

Oil

Epoxy

Insulation rubber

Grounding electrode

Semiconductive
rubberAC dielectric strength (kV/mm)

80
70
60
50
40
30
20
0

150-200 µm

110Electrical test setup with recessed specimen

100

Temperature (°C)

Figure 3
Figure 250

Temperature dependence of AC dielectric strengthfor silicone rubber

Furukawa Review, No. 19. 2000

3.2.4 Long Term TestLong term test has been carried out under certain AC voltage conditions for more than a year and is still continuing.
Life index "n" for this silicone rubber is more than 17,
which is higher than other materials such as EPR andepoxy. See Figure 5.
With all the tests, we found out that the silicone rubber
we have chosen has good enough electrical properties forcable accessories from short term point of view to long
term.

4.

DESIGN BASIS OF CSJ

4.1 Electrical Design
Electric field in the rubber-molded sleeve of CSJ was optimized by means of a computer. Stress-relief configuration
is so well designed that the size of the rubber-molded
sleeve is minimized. The designing features of CSJ are
limited to four major points (τ 1-τ 4) as shown in Figure 6
because of its simple composure.
Figure 7 shows the result of electric field calculation for
optimized CSJ which is set onto the cable.

4.2 Design of Interfacial Pressure
For any cable joints, interfacial design is the most important part. It is known that the stability in interfacial dielectric performance for pre-molded joints is mainly affectedby interfacial pressure 2).
We established the design concept of interfacial pressure for our CSJ including its reduction in pressure during
expansion state and operation period for about 30 years
as shown in Figure 8. With applying silicone rubber which
has better performance in permanent set for CSJ, thereduction in the interfacial pressure is suppressed to minimum. To estimate the reliability on designed interfacialpressure for CSJ, we used the models for each τ 4 asshown in Figure 9 and τ 3 as shown in Figure 10.
We also modified the rubber molded sleeve for (E)HV
itself as shown in Figures 11 and 12 for testing at lower
interfacial pressures. The cables which has a smallerouter diameter were used to maintain that lower pressure.
In a modified rubber sleeve as shown in Figure 11, we
Edge of stress-relief
cone ( τ 4)

Surface of stress-relief
layer ( τ 1)

Interface between
XLPE/rubber ( τ 3)

τ 1: surface of the inner stress relief layerτ 2: tip of the inner stress relief layerτ 3: interface between XLPE/rubber molded sleeveτ 4: edge of the stress relief coneStress-relief
cone

Stress-relief
layer

Tip of stress-relief
layer ( τ 2)

Insulation
Semiconductive
layer of cable layer of cable

Impulse dielectric strength (kV/mm)

Conductor sleeve
Insulation layerof rubber sleeve

180

Figure 6160

Insulation shielding layer

120

Insulation rubber
Positive
Negative

80
0Stress-relief cone

50

Figure 4

Inner stress-relief layer

100

Cable insulation

Temperature (°C)

Cable conductor

Temperature dependence of lightning impulse dielectric strength for silicone rubber

Figure 7

Result of electric field calculation for designing rubber-molded sleeve

102
70
60
50
40

Interfacial pressure (MPa)Applied electric stress (kV/mm)

Electrical design of rubber sleeve of CSJ

140

100

Cable sheath

Semiconductive
layer of rubber sleeve

n=17

30
Broken down
Still continuing

20

101
10-2

10-1

100

10,

102

103

104

105

106

Time to breakdown (min)Figure 5

V-t characteristics of silicone rubber

107

Reduction of interfacial pressureduring expanded state

2

Reduction of interfacial pressureduring 30 years of operation
1
Design level
0.5

0Expanded
state

Figure 8

Operation periodof 30 years

Reduction of interfacial pressure between rubber
molded sleeve and XLPE cable in CSJ

111

Pressure

H.V. electrode
Semiconductive
silicone rubber

Pressure

Epoxy
SiliconerubberXPLE

Figure 9

Grounding electrode

Sample setup which models electrical stress distribution at τ 4
Photo 1Pressure

Pressure

Assembly of rubber molded sleeve for CSJ onto
bonded cable

H.V. electrode

Epoxy
XPLE
Insulating
silicone rubber
Groundingelectrode

Semiconductive
silicone rubber

Figure 10 Sample setup which models electrical stress distribution at τ 3Stress-relief cone

Photo 2

Rubber-molded unit shrunk onto the cable

Photo 3

Appearance of the completed 110-kV CSJ

Silicone insulation layer

Figure 11 Modified sample to evaluate the dielectric strength at
the edge of the stress-relief coneStress-relief coneStress-relief layer

Silicone insulation layer

Figure 12 Modified sample to evaluate the dielectric strength ofinterface between XLPE and silicone rubber

could measure the effect of interfacial pressure on dielectric strength of the interface between rubber insulation and
XLPE (τ 3).
As a result, we found out that even with 0.04 MPa which
is well below the designated level of 0.05 MPa, our CSJ
can offer excellent dielectric strength at the interface.
Thus, we confirmed the designated interfacial pressurelevel of 0.05 MPa at minimum.

This carrier-pipe not only keeps the rubber-molded
sleeve expanded at any state from expansion to shrinking
but is pulled out easily by hand during shrinking state. To
apply these "pull-out" carrier-pipes onto rubber molded
sleeves for (E)HV joints which usually impart much biggerpressure onto the joints because of their thickness, it wasnecessary to optimize the shape of the string as well as
bonded strength of its L-shaped edge.
5.2

5.

EXPANSION TECHNOLOGY FOR CSJ

5.1 Design of "Pull-Out" Carrier-Pipes
To ease the assembly, rubber molded sleeves of newly
developed CSJ should be shipped expanded onto the"pull-out" carrier pipes which are composed only of a plastic string as shown in Figure 1.

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Development of Special Equipment for
Expansion
We also have developed special equipment to expand therubber-molded sleeves onto carrier-pipes at the factorysite. With newly developed equipment which has lower
friction between the expansion rod and rubber molded
sleeves, it becomes possible to expand quite easily thesleeves without any damage onto them in a short time.

Furukawa Review, No. 19. 2000

φD±50L±100

Conductive sleeve

Rubber molded sleeve

Waterproof compound

Conductor sizeDL

240-2000 mm2

250 mm1300 mmProtection box

Bonding wire

Figure 13 Structure of CSJ for 110-kV XLPE cable

Table 2

Summary of the cable

Voltage class

110 kV

Conductor

Cu 400 mm2
Thickness

17.5 mm

Outer diameter

61 mm

AC 128 kV
H.V. Shield
H.V. TransBus bar

XLPE insulation
Aluminum

Metallic sheath
Material

Termination

Polyethylene

Outer jacket
Outer diameter

CT

89 mm

Ground110-kV CSJ

Table 3

Type test program and results
Reference joint

No.

Contents

Results

Conductor temperature
measuring point

1. Partial discharge test at 96 kV and ambient temperature after
112 kV x 10 sec loading was conducted.

Figure 14 Test circuit for heating cycle test

PassedRequirement: less than 5 pC
2. tan δ measurement at 64 kV and 95-100°C
Requirement: less than 10-3

Passed

3. Heating cycle voltage test
20 cycles comprising 8 hrs heating to a conductor temperature of 95-100°C with the temperature maintained at least

Passedduring the last 2 hrs followed by 16 hrs cooling. A test voltageof 128 kV was applied during the above heat test.
4. Partial discharge test at 96 kV and ambient temperature and95-100°C after 112 kV for 10 sec was conducted.

PassedRequirement: less than 5 pC
5. Lightning impulse test
10 impulses 5/59 µs with ±550 kV of each polarity at a con-6.

IEC TYPE TEST

Tests in accordance with IEC standard 840 and IEC committee draft SC20A were performed in the presence of arepresentative of the third party inspection.
All the tests were carried out successfully and provedthat newly developed CSJ has quite high reliability for
110-kV XLPE cables.

Passedductor temperature of 95-100°C
6. AC voltage test with 160 kV for 15 min.

7.

PassedASSEMBLY INSPECTION

Expanded rubber molded sleeve for (E)HV CSJ was fitted
onto the sheath of the prepared cable end. After splicing
the connector, preset rubber-molded sleeve was shrunkby pulling out the string of carrier-pipe as shown in Photo
1 through Photo 3. It took less than 10 minutes to shrink
the rubber sleeve onto the cable. Figure 13 shows the
structure of CSJ for 110-kV XLPE cables.

7.1 Heating Cycle Voltage Test
We applied our CSJ onto the cable which is summarizedin Table 2 and carried out each required test in the ordershown in Table 3. Figure 14 and Photo 4 show the test circuit for heating cycle test which includes reference joint.
After all those tests, the protection box and waterproof
compound were removed and all the parts including rubber-molded sleeve were found to have no damage.
7.2 Water Immersion and Heat Cycling Tests
7.2.1 Test Condition
Water immersion and heat cycling test on the cold-shrinkable insulation joints (IJ) were carried out under the regulated condition of IEC 840 committee draft SC20A.

113

Table 4

Summary of water immersion tests

Heating More than 3 cycles comprising 8 hrs heating to a conductor
cycle

temperature of 95-100°C with the temperature maintained forin airat least during the last 2 hrs followed by 16 hrs cooling

Heating 20 cycles comprising heating to a water temperature of 70-75
cycle°C with the temperature maintained for at least 5 hrs followedin water by cooling to within 10°C of ambient temperature

Table 5

Electrical test results after water immersion and heat
cycling
Test

D.C. withstand(Tolerance)

Photo 4

Test circuit for heating cycle test
Impulse withstand7.2.2 Test Results
After the water immersion and heat cycling test summarized in Table 4, the electrical tests shown in Table 5 werecarried out.
No significant changes inside the protection including
the joint and the cable were detected.

8.

CONCLUSION

Requirement

Results

20 kV for 1 min

Passed±75 kV/10 times between parts

Passed±37.5 kV/10 times to earth

PassedREFERENCE

1) Nozawa et al.; Development of intermediate joint for 66-kV CV
cable with improved workability, '98 Annual meeting of IEEJ.
p.460. (in Japanese)
2) Dang et al.; CEIDP, p.518, (1994).
Manuscript received on September 17, 1999.

We have successfully developed cold-shrinkable joints for(E)HV XLPE cables.
With its cold shrinking technology, we can guarantee
the easiest assembly, the smallest body size as well as
best performance for (extra) high voltage class in bestprice.

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Furukawa Review, No. 19. 2000