Development of Cold-Shrinkable Terminationsfor 6600-V XLPE Cable
by Koichi Sakai *, Hiroshi Munakata *, Hiroshi Sakurai *2, Atsushi Kimura *3,
Shun'ichi Asakura *4, Kiyoshi Yamada *3, Toru Serizawa *3 and Yoshinari Hane *5

Slip-on terminations used heretofore on 6600-V XLPE cables come in two types

ABSTRACT depending on the level of pollution in the area where they are to be used--the potheadtype for areas of heavy pollution and the outdoor type for ordinary areas. Pothead terminations havethe disadvantages of being heavy and difficult to handle and when EP rubber out-door-type terminations were incorrectly substituted in areas of heavy pollution, instances of insulation breakdown
occurred. And since both are of the slip-on type, incidents of breakdown have also occurred, though
infrequently, due to insufficient insertion of the cable. Against this background the authors havedeveloped a new type of termination made of silicone rubber for 6600-V XLPE cables which is easy
to assemble, guards against problems of improper installation on the cable, and can be used inboth ordinary and heavily polluted areas.

1.

INTRODUCTION

Slip-on terminations used heretofore on 6600-V XLPE
cables come in two types--a pothead type made of porcelain and outdoor type made of EP rubber. Figure 1 shows
an example of slip-on termination of the type currently inuse. Normally the pothead type is used in areas of heavy
pollution and the outdoor type is used in other areas. Because of its porcelain construction the pothead terminations offer outstanding contamination performance, but areheavy and difficult to handle. Outdoor type terminations,being made of EP rubber, suffer dielectric breakdown ifthey are incorrectly selected and used in areas of heavy
pollution. And since both are of the slip-on type, incidents
of breakdown still occur, though infrequently, due to insufficient insertion of the cable.
The authors have therefore developed a new type oftermination made of silicone rubber for 6600-V XLPE
cables which is easy to assemble, guards against problems of improper installation on the cable, and can be usedin both ordinary and heavily polluted areas.
This paper gives an overview of the process of developing the new terminations.

* East Japan Railway Co.
* 2 Engineering & Development Dept., Power Cable Accessories Div.
* 3 Engineering Sec., Engineering & Development Dept.,
Power Cable Accessories Div.
* 4 2nd Development Sec., Hiratsuka R&D Lab., R&D Div.
5
* Distribution Cables & Accessories Engineering Dept.,
Power Cables Gr.

2.

DESIGNING THE NEW TERMINATIONS

2.1 Target Specifications
Table 1 shows the target specifications. The objective wasa termination that could be used without distinction, in bothStress relief cone
Terminal lug

Termination body(porcelain bushing)

Figure 1

Insulation tape
Groundingfitting

Pothead type slip-on termination

Table 1

Target Specifications

Item
Initial performance
AC partial discharge voltage

Specification

Max 10 pC at 6.9 kV

AC withstand voltage

35 kV for 1 hr

Impulse withstand voltage-95 kV, 3 times

Watertightness

49 kPa for 24 hr

Contamination performance
Wet AC flashover voltage
Contaminated flashover voltage

Ease of installation

17 kV or more
More than 7.2 kV at ESDD
0.35mg/cm2
No special skills required
Suppression of installation errors

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ordinary and heavily polluted areas, so all performance
specifications were set with reference to the current
pothead type. With respect to installation operations, theobjective was to improve procedures requiring a high degree of skill, and to suppress installation errors due to insufficient cable insertion. Photo 1 shows the new termination developed to satisfy these target specifications.
Terminal lug

Photo 1Bell-shaped shed

Termination body

Newly-developed cold-shrinkable termination
Rib

Bell-shaped shed

2.2 Details of the Design
2.2.1 Material
For the new termination to be used in areas of heavy pollution, it was necessary for it to overcome the disadvantages of the existing pothead termination in terms of weightand difficulty in handling. It was therefore decided to makeit using silicone rubber, which has excellent contamination performance, as the main insulating material. This had
the effect of reducing its weight to about 1 kg, versus about
5 kg for the pothead termination.
2.2.2 Structure
Because of the decision to use insulating rubber in areasof heavy pollution, it was necessary to assure contamination performance by means of the shape of the shed aswell as by proper material selection. Accordingly a bellshaped shed was adopted in addition to the horizontal
shed (Figure 2). Dimensions were also reduced, and a rib
was provided on the inside of the bell-shaped shed so as
to obtain the requisite creepage distance.
By means of these measures relating to the material
and the structure, it was possible to achieve contamination performance equivalent to those of the existing
pothead terminations (see Table 2).
2.2.3 Method of Installation
The installation method adopted was cold shrinkable typeutilizing the inherent elasticity of rubber, in which the cable
is attached by keeping the inner diameter of the terminaInsulating layer
Stress-control layer

Figure 2

Shed shape

Termination body
Support core

Figure 3

Method for controlling electrical stress

Terminal lug

Termination body

Sealing material
Support core

Photo 2

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Expanded state

Figure 4

Separating terminal lug from termination body

Furukawa Review, No. 18. 1999

tion several times larger than the outer diameter of thecable and then releasing it at the designated point of attachment (Photo 2). This method made it possible to eliminate the insertion force of the slip-on termination to thecable.
2.2.4 Method for Controlling Electrical Stress
Because the adoption of the cold shrinkable type requiresthat the termination body be expanded, the body should
be as thin as possible. However the conventional geometric method of stress control using stress relief cones requires at least that part to be thickened. In the termination
developed here, stress control was provided by a stresscontrol layer of high permittivity (see Figure 3). This madeit possible to reduce the thickness of the termination body.
2.2.5 Elimination of Insufficient Cable Insertion
As was stated above, one problem with the slip-on terminations now in use is the continuing occurrence of breakdown faults, though infrequently, resulting from insufficient
insertion of the cable. One reason for this is that the termiTerminal lug

Sealing material

Sealing material

Figure 5

Grounding fitting

Method of grounding and waterproofing

nal lug of the conventional slip-on is molded integral withthe termination body so that the terminal lug must be
crimped on the assumption that the cable is fully inserted
into the termination body. Accordingly it was decided in
the newly developed termination to separate the body andthe terminal lug (Figure 4), thereby making it possible toconfirm that the conductor was completely inserted into
the lug and crimped before going on to the next operation,and to eliminate incomplete insertion as a cause of breakdowns.
2.2.6 Common Sizes
In addition to separating the terminal lug and the termination body, the adoption of the previously described method
whereby the inner diameter of the body was first expanded
and held to several times that of the cable insulation madeit possible to use a single type of termination body for morethan one size of cable. In the case of the terminations
developed here, each type serves two sizes of cable incommon: 22 and 38 mm2, and 60 and 100 mm2.
2.2.7 Improving Ease of Installation
Grounding is accomplished using a coil spring (Figure 5),rendering soldering unnecessary. Watertightness is accomplished by applying sealing material between the bodyand the cable sheath and between the body and the terminal lug, eliminating the taping used previously.

3.

EVALUATION

Since the termination developed in this project was intended for use in heavily polluted areas, it was necessary
to evaluate not only performance in the completed state,
but also the weathering and tracking resistance of the material of which it was made.

Contact angle (˚)a) Original

3.1 Evaluation of Material
All terminations, including this one, that are to be used
outdoors are subject to the effects of ozone and sunshine,
as well as wind and rain. If the surface becomes wet, thewell-known phenomenon of dry band arcing occurs. Accordingly the silicone rubber of which these terminations
were to be made was evaluated for resistance to weathering and tracking in accordance with the accelerated tests
described below.

b) After 500 hr

Photo 3Specimen surface before and after ozone
resistance test

Time (hr)Figure 6

Change in contact angle to sample surface bysunshine weathering test

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3.1.1 Ozone Resistance
Normally ozone is present in the air in concentrations ranging from 1 to 5 pphm1). Under polluted conditions, however, dry band arcing generates even more. In these tests,
therefore, ozone concentration was set high at 150 ppm;
otherwise, in accordance with JIS K 6301, test samples
were elongated by 50% at 40°C, and the surface was examined for cracking, etc.
Photo 3 shows the condition of the specimen surface

Table 2
Test
voltage(kV)

Results of Tracking-Resistance Test (IEC587)
Length of test (hr)
0

100

1000

2000

3000

3.5

Passed Passed Passed Passed---

Passed Passed

4.5

Passed---Passed Passed

---

500

---

Passed5000

Target specification:
All 5 test samples to withstand 6 hr without breakdown
Criterion:
The end point is reached when the value of the current in the highvoltage circuit through the specimen exceeds 60 mA, or when the
track reaches a mark on the specimen surface 25 mm from thelower electrode.

Table 3Result of Initial and Contamination Performance
Tests
Item

Evaluation

Initial performance
AC partial discharge

None

AC withstand voltage

PassedImpulse withstand voltage

PassedWatertightness

PassedContamination performance
Wet flashover voltage
Contaminated flashover voltage
Ease of installation

Photo 458

Outdoor exposure test
(heavy pollution area)

PassedPassedbefore the start of the ozone resistance test, and after theelapse of 500 hr. At 500 hr, no particular cracks or otherabnormalities were observed.
3.1.2

Sunshine Weathering and Tracking Resistance
Tests
Tests using the weatherometer are to determine weatherability--primarily ultraviolet light. Around the light source isprovided a rotating frame to which the test samples aremounted so that they are rotated.
Previous research has already demonstrated the superior contamination performance of silicone rubber: the hydrophobic properties of its surface are maintained for extended periods, and even if lost due to dry band arcing,are regained rapidly2). The contact angle method is normally used to determine hydrophobicity. Accordingly using a maximum irradiation period of 5000 hr, several
samples were extracted from time to time and the contact
angle to their surfaces was measured. Tracking resistance
was measured using the inclined plane test of IEC 587.
Figure 6 shows the change in contact angle, and Table
2 shows the results of the tracking-resistance tests. There
was no great change in contact angle in the course of thetest, and the fact that the value was not much different
from that for original samples suggests that the hydrophobicity of the material was fully maintained. Tracking-resistance was also fully maintained, and even samples that
had been irradiated for 5000 hr showed values similar to
those for original samples.
3.2 Evaluation of the Installed Termination
Evaluations were also made of the initial performance andcontamination performance of terminations that were installed under conditions similar to those of actual use. And
to confirm long-term performance under conditions of actual use, samples were mounted in distribution systems
actually in service in areas of heavy, medium and light
pollution and subjected to outdoor exposure tests for aperiod of approximately one year.

PassedPhoto 5

Outdoor exposure test
(medium pollution area)

Photo 6

Outdoor exposure test
(light pollution area)

Furukawa Review, No. 18. 1999

Table 4

Results of outdoor exposure tests

Item

Evaluation

Appearance

No abnormality

AC partial discharge

None

AC withstand voltage

PassedImpulse withstand voltage

PassedLeakage current(10 kV DC applied)

Contact angle (˚)

Photo 7

Sample after exposure for 1 year in area of medium
pollution

Light pollution; upper sideof horizontal shed
Medium pollution; upperside of horizontal shed
Heavy pollution; upperside of horizontal shed
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Jan Feb Mar'96 '96 '96 '96 '96 '96 '96 '96 '96 '96 '96 '96 '97 '97 '97

Time of measurement

Figure 7

Changes in contact angle to sample surface at eachtest site

3.2.1 Initial and Contamination Performance
It was confirmed that the initial performance of the terminations developed in this project satisfied the performancetargets for all common-use sizes. No abnormalities were
found in the new waterproofing structure, in which taping
was eliminated in favor of the use of sealing material. In
terms of contamination performance as well, performancewas equal to or better than that of the conventional potheadterminations. Table 3 shows the results obtained.
3.2.2 Long-term Characteristics
The sites selected for the tests were: an area located on
the sea about 10 m distant from the water and subject toheavy pollution from sea water and salt-laden winds; anarea of medium pollution where conditions were extremely
severe owing to the presence of diesel train exhaust, dust
and iron powder from brakes, without the cleansing effectof rain; and an area of light pollution located in a railway
station parking lot subject to the effects of exhaust gases,
dust and dirt. The three sites are pictured in Photos 4
through 6. Four samples were exposed at each site, andat regular intervals one was removed for evaluation.
Photo 7 shows a sample which was removed after one
year's exposure at the area of medium pollution. No abnormalities due to erosion or tracking were found. The otherDry

Dry 0.1 mA or lessWet

Wet 0.1 mA or lesssamples were also found to be free of observable abnormalities during the course of the exposure tests, irrespective of the environment in which they were placed.
After removal, all samples were subjected to the tests
shown in Table 4, and were confirmed to be without problems.
When removed, the samples were also examined forchanges in the contact angle as a sign of change in hydrophobicity. Figure 7 shows the results. There was no
major change in contact angle, irrespective of the environment, suggesting that the hydrophobicity of the surface of terminations made of silicone rubber was retained
throughout the test period.
3.2.3 Verification of Ease of Installation
The working time required to install the newly developed
terminations on the cable was 23% shorter than that forthe conventional type, reflecting the elimination of thewaterproof taping process and other skilled procedures.
In addition to eliminating the skill requirement, installation
was simplified by the fact that no insertion force was required for slip-on, making it possible to confirm that the
conductor was firmly inserted in the terminal before goingon to the next operation, and to eliminate incomplete insertion as a cause of breakdowns.

4.

CONCLUSION

A new type of termination has been developed for 6600-V
XLPE cable which offers outstanding ease of installation,
prevents breakdowns due to incorrect work procedures
and can be used in both ordinary and heavily polluted
areas. The new terminations went into commercial service in 1998.

REFERENCES

1) Nishizawa; Journ. Japan Institute of Rubber, Vol.49,
No.4(1976), 327. (in Japanese)2) R.S.Gorur et al.; IEEE Trans. on Power Delivery, Vol.5,
No.4(1990), 1923.
Manuscript received on November 10, 1998.

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