Development of 22-kV XLPE Cable and Joint
with Reduced Insulation Thickness
Having Water-Impervious Aluminum Layer
by Hiroyuki Mabuchi *, Satoru Koizumi *, Haruki Nozawa *, Yoshihiro Maeda *2,
Hideo Kawahara *3, Junji Hirano *3 and Shigeru Fujii *3

Electric power supply using 22-kV systems is expanding aiming at reduction ofelectric power distribution costs through upgrading the efficiency of facilities. It
was thus required to develop a cable with joint that enabled maintaining the reliability as well as
suppressing the cost of electric power supply. In response to this requirement, the authors havedeveloped a new cable having a water-impervious layer of aluminum laminated tape together
with its joint, whereby the thickness of the insulating layer was reduced to 4.5 mm --including theinner semiconductive layer-- by devising a novel method of terminating the end of the outer semiconductive layer. Moreover, a cold shrinkable straight joint has been developed thus improving
the working efficiency at the site. This paper reports on the structure, performance, and working
efficiency of this cable and the joint.

ABSTRACT1.

INTRODUCTION

Electric power supply using 22-kV systems is expanding
aiming at reduction of electric power distribution costs
through upgrading the efficiency of facilities. In responseto the requirement of developing a cable with joint thatenables suppressing the cost while maintaining the reliability of electric power supply, we have developed a new
cable with reduced insulation thickness having a waterimpervious layer together with its joint.
By using an aluminum laminated tape for the cable andjoint, we have been successful in maintaining the reliability
while reducing the insulation thickness. This paper reportson the structure, characteristics, and working efficiency ofthis cable and the joint.

2.

DESIGN OF 22-KV CV CABLE WITH
REDUCED INSULATION THICKNESSHAVING WATER-IMPERVIOUS LAYER

2.1 Target Characteristics and Structure
Table 1 shows target characteristics. Whereas the electriccharacteristics are in compliance with JEC-3408-1997 1) ,the lightning impulse test voltage is ±135 kV since light-ning impulse withstand voltage (LIWV) is given as 95 kV.
As for the water-imperviousness of the cable, it was
fixed at a value equivalent to that of water-impervious leadlayer (WILL) having an established track record in the 66kV CV cables currently in use.
2.2 Design Details
2.2.1 Investigation of insulation thickness
The cable insulation thickness was determined by adopting the greatest among those obtained by the electric field
intensity of cable design (EL) as well as the characteristics
at the rise-up portion of the stress cone for joint and partial
discharge characteristics. Required insulation thicknesses
based the design factors mentioned above are described
below.
(1) Insulation thickness based on the cable design
The insulation thickness based on the required AC withstand voltage is 2.3 mm including inner semiconductive
layer (ISCL), given the AC design voltage of 35 kV/mm 2).
Table 1

Target characteristics of cableTarget characteristics

Item
Partial discharge

5 pC or less at 30 kV.10 min

AC withstand voltage

45 kV.1 hr

Lightning impulse voltage ±135 kV.3 repetitions (LIWV = 95 kV)* Tokyo Electric Power Co.
*2 Engineering & Development Dept., Insulated Conductors &Components Div.
*3 Distribution Cable Accessories Sec., Engineering Dept.,
Power Cables Div.

Long-term loading

20 kV loading; Conductor temperature: 90°C

(in air)

8 hr ON / 16 hr OFF, 180 repetitions

Water imperviousness

Averaged water permeation:
1 x 10-7 g.(cm/cm2).day.mmHg or less59

Meanwhile, the thickness based on the required lightning
withstand voltage is 2.8 mm including ISCL, given thelightning withstand design voltage of 75 kV/mm 2). Thus,
the insulation thickness based on the cable design wasdetermined as 3.0 mm.
(2) Insulation thickness based on the characteristics at
the rise-up portion of the stress cone for joint
Since reduction of the cable insulator thickness increases
the electric field intensity on the cable insulator, withstandvoltage characteristics of the rise-up portion of the stressrelief cone must be considered. We calculated these characteristics based on the minimum breakdown voltage
denoted by Gmin that was derived from breakdown test
data of joints.
Table 2 shows typical breakdown test data of 22-kV
joints together with the calculated results, indicating thatthe cable insulation thickness can be reduced down to
4.0~4.5 mm including ISCL.
(3) Insulation thickness based on partial discharge characteristics at the edge of outer semiconductive layer
Since reduction of the cable insulator thickness has an
influence on partial discharge characteristics at the edgeof the outer semiconductive layer (OSCL), various termination methods were studied for their partial discharge
characteristics. As a result, an insulation thickness of 4.0
mm including ISCL was identified to result in a partial discharge start voltage of 18.2 kV even for the greatest conTable 2
Joint type

Breakdown test data of joint

Minimum breakdown

Gmin

Insulation thickness(kV/mm) including ISCL (mm)voltage (kV)Slip-on straightjoint

AC 130

17.4

4.1

Imp 380 *

51.0

4.2

Prefabricatedjoint

AC 16719.8

3.7

Imp 380 *

45.1

4.5

ductor size of 500 mm2, thus meeting the specification forthe joints of 17 kV•10 pC or less.
Taking into consideration, however, such variations asdata spread and possible degradation due to working
environment at the site, the insulation thickness should be
4.5 mm or more.
Based on the results of the foregoing investigation it
was decided to adopt the greatest value of 4.5 mm including ISCL for the cable insulation thickness.
2.2.2 Water-Impervious Layer
The water-impervious layer uses aluminum and employs adouble laminated structure considering mechanicalstrength and corrosion resistance. The layer is placed
underneath the cable sheath lengthwise and they are
bonded together so as to improve the mechanical characteristics of the cable allowing them to integrally follow
bending, expansion, and contraction of the cable.
2.2.3 Detailed Structure of the Cable
Figure 1 shows the structure of the CVT cable with waterimpervious aluminum layer (WIAL) developed at this time.

3.

CABLE EVALUATION

3.1 Initial and Long-Term Characteristics
Table 3 shows the results of the initial and long-term electrical characteristics tests of the developed cable. All test
items are seen to meet the target characteristics.
Long-term loading tests of 180-day duration were carried out to confirm the long-term performance of the cable.
As for the test conditions, not only in-air test conditions but
also in-water test conditions were investigated simulating
submerged conditions to be encountered in practicalNotes(1) The * mark corresponds to the data outside the rise-up portion of

Table 3stress relief cone.
(2) Insulation thickness is calculated assuming that the required withstandvoltage equals to the standard value multiplied by a sufficient margin--because of small numbers of measured data-- of 1.3.
AC: 45 kV x 1.3 = 58.5 kV
Imp: 135 kV x 1.3 = 175.5 kV

Cross-section of water-impervious
aluminum layer

Adhesive layer

Electrical test results of 325-sq cable
Test results

Test item
Partial discharge test

No occurrence (30 kV.10 min)Lightning impulse withstand voltage±135 kV.3 repetitions: GoodLightning impulse breakdown-650 kV.1 repetition

AC long-term withstand voltage

45 kV.1 hr: Good

AC breakdown

325 kV.6 minLong-term loading test

Plastic layer

No abnormalities(in air and in hot water)

Aluminum foil
Conductive plastic layer
WIAL

Partial discharge No occurrence

Screening copper tape Conductive binder tape(more than 30 kV)

test

SheathResidual performance after long-term AC breakdown *1

ISCL

loading test

In air: 115~165 kV *2
In hot water: 135~145 kV *2

Lightning impulse In air: -345 kV *2
breakdown *1

Conductor

In hot water: -345 kV *2

Notes

Insulator*1: 60°C hot water long-term tests were carried out at the Central
OSCL

Research Institute of Electric Power Industry.

Figure 160

22-kV CVT cable with WIAL

*2: All breakdowns occurred at joints.

Furukawa Review, No. 20

2001installation environments. But if the water temperature
was set to a normal temperature while raising the in-water
cable conductor temperature to 90°C, the temperature of
the in-air cable conductor --the test system is controlled
based on this temperature-- would exceed the allowabletemperature of the cable. Thus, the water temperature
was set to 60°C enabling the test temperature of 90°C to
be applied for both the in-air and in-water cables.
No abnormalities such as breakdown were found during
this test. Furthermore, residual performance tests including AC partial discharge test, AC breakdown test, andlightning impulse breakdown test were carried out after
this test, and it was confirmed, as shown in Table 3, thatthe performance remained unchanged from the initialcharacteristics.
3.2 Water-Imperviousness
The following tests were carried out to evaluate the performance of the water-impervious structure applied to thedeveloped cable, ultimately confirming that the WIAL wasequivalent to the WILL of established track record and thatthe cable would withstand 30-years of use.
3.2.1 Water Permeation Measurement
Water permeation was measured simulating the permeation of water from the outside, in which a cable sheathalone was submerged into hot water followed by measuring the amount of permeated water to obtain water permeation.
Figure 2 and Table 5 show the results of measurement
together with the water permeation.

4.

DESIGN OF JOINTS FOR 22-KV XLPECABLE WITH REDUCED INSULATIONTHICKNESS HAVING WIAL

4.1 Target Characteristics and Structure
Development objectives are straight joint, Y-branch joint,
and terminating joint and their target characteristics areshown in Table 6. Whereas the electric characteristics arein compliance with JEC-3408-1997 1 ) , the lightning
impulse test voltage is ±135 kV since the LIWV is given as
95 kV.
As for the water-imperviousness of the joints, since
there are no standards for joints, it was fixed at a value
equivalent to that of heat shrinkable WILL which has anestablished track record in the water-impervious terminations for metal-sheathed cables of 66-kV rating or over.
4.2 Design Details
Figure 4 shows the structure of the developed straightjoint.
Table 5

10

Water permeation (g)3.2.2 Thermo-Mechanical Characteristics Test
The cable was tested to see whether cracks occur in the
WIAL by expansion and contraction due to 30 years ofheat cycle. The expansion of the cable caused by temperature rise due to current loading was measured, and it
was investigated whether cracks occur when a strain corresponding to this expansion was applied to the WIAL,
while the S-N characteristic of the WIAL was obtained inadvance to support this investigation.
Table 5 and Figure 3 show the results of the strain measurement and the S-N characteristic curve.

1

Estimated strains of the sheath due to temperaturechangeCategory

Accumulated strain cycle

Strain10,950

0.38%

30

0.61%

30

1.05%

Diurnal temp. change(once a day)

0.1

Short-duration temp.
change (once a year)

0.01

Annual temp. change(once a year)

0.001
0

10

20

30

40

Time (day)Without water-impervious layer

10With WILL

Results of water permeation measurement of cable

Table 4

Measured water permeation of cable
Present 22-kV CV
cable with no impervious layer

Averaged water
permeation

Developed cable

Reference cablewith WIAL

with WILL

Strain (%)

Figure 2

With WIAL

1

Strain due to short-duration
temperature change
Strain due to diurnal
temperature change

0.1
1.0E+00

1.32×10-6

Sheet sample of WIAL

Strain due to annual
temperature change

8.73×10-9

1.93×10-9

Water permeation unit: g.(cm/cm2).day.mmHg

1.0E+01

1.0E+02

1.0E+03

1.0E+04

1.0E+05

1.0E+06

1.0E+07

Number of strain cycle until crack occurrence (count)

Figure 3

Results of S-N characteristic test

61

Table 6

Aluminum foil

Target characteristics of joints
Aluminum foil

Item

Polyolefin layer

Target characteristics
10 pC or less at 17 kV

Partial discharge

Polyolefin layer(in conformity to JEC-209)

AC withstand voltage

45 kV.1 hr

Lightning impulse voltage±135 kV.3 repetitions (LIWV = 95 kV)Long-term loading (in air)

20 kV loading; Conductor temperature: 90°C
8 hr ON / 16 hr OFF, 180 repetitions

Adhesive layer

Figure 6

Cross-section of a shrinkable with aluminum waterimpervious layer

26.2 kA.2 sec,

Current loading

No abnormalities should be observed
External pressure: 98 kPA.1 hr, No water

Water-tightness

permeation should be observed

Water imperviousness

Equivalent to heat shrinkable WILL

232
1025

Approx. 750
4
3
2
1
No.

Figure 4

Water-impervious shrinking tube
Grounding terminal
Cold shrinkable splice housing
Conductor sleeve
Name

7
6
5
No.

approx. 1290

7
6
5
4
3
2
1
No.

Waterproofing tape
ACP tape
Semiconductive tape
Name

Straight joint for 22-kV XLPE cable with reduced
insulation thickness having WIAL
Figure 7
Semiconductive tape

Heat-shrinkable tube is heated using a hair drier,smoothing the edge by the heat and the tube's contraction force

New processing method for the edge of OSCL

Considering cost reduction by eliminating spacer sleeve
covers together with easy installation at the site, a coldshrinkable type was employed for insulation, in which a
cold-shrinkable splicing housing was expanded at the factory in advance and was subsequently installed by on-site
shrinking. As for the material of the splicing housing, silicone rubber was employed for its excellent electrical aswell as permanent set characteristics. While permanent
set is one of the most important key factors in the application of cold-shrinkable technology with factory expansion,
the use of silicone rubber with excellent permanent set
performance enables to suppress degradation of interfacial pressures satisfactorily during the entire service period of 30 years.
Additionally, whereas the reduction of the insulation
thickness increased the electric field intensity on the insulator surface, the conventional method for processing the
OSCL of 22-kV class, i.e., ACP tape wrapping on the
OSCL became unable to meet the target performancerequirement in terms of partial discharge characteristics.
Therefore, a new OSCL processing method of "conductive
paint coating plus semiconductive tape wrapping" wasdeveloped as illustrated in Figure 5.

62

14
13
12
11
10
9
8
No.

Sealing tape
ACP tape
Conductive paintSelf-bonding semiconductive tape
Terminal lug
Grounding wire
Water-impervious shrink tube
Name

Y-branch joint for 22-kV XLPE cable with reduced
insulation thickness having WIALACP tape

OSCL of cable
Conductive paintFigure 5

Insulating cover
Splice housing (B)SpacerSplice housing (A)Terminal lug
Metal housing
Main bodyName

In this method, semiconductive paint that had been successfully applied in the class of 66-kV rating or over wascoated over the edge of the cable OSCL to eliminate triangular voids thereof, thus improving the partial discharge
characteristics. Furthermore, semiconductive tape waswrapped for protection against possible peeling due toclamping wedge in the case of pre-molded type joint, followed by a smoothing process in which a heat-shrinkabletube --to be removed after the process-- and a hair drier
were used to make the edge of the tape level with the surface of the cable insulator, thereby enabling suppressionof the voids while making this structure applicable to alltypes of such joints. By applying this method, a partial discharge start voltage of 30 kV or more was realized evenwith the cable insulation thickness of 4.5 mm including
ISCL.
A shrinkable tube with aluminum water-impervious layer
was employed for the water-impervious structure. The
tube consists of polyolefin shrinkable tube and waterimpervious aluminum layer sandwiched therein, and thestructure is such that each layer is firmly bonded together
by an adhesive layer, enabling control of water permeation by adjusting the lapping width of the water-impervious layer. By shrinking the tube on its entire length, thestraight joint can be made as impervious to water as thecable.
Figure 6 shows the cross-section of a shrinkable tube
with aluminum water-impervious layer.
Figure 7 shows the structure of a Y-branch joint developed this time. The joint employed, attaching much impor-

Furukawa Review, No. 20

2001Results of the initial and breakdown characteristic
tests of 325-sq joints
Test item

Test results
No occurrence (Occurrence voltage

Partial discharge testof 30 kV or more was confirmed)

Lightning impulse withstand voltage±135 kV.3 repetitions: GoodLightning impulse breakdown(10 kV.3 step-ups)

S-joint: -395~-425 kV

AC withstand voltage

45 kV.1 hr: Good

AC breakdown(10 kV.1 hr step-ups)

S-joint: 145~185 kV

0.1

0.01

0

Good

Test item

20

30

40

Figure 8

Results of water permeation measurement of joints

Table 9Results of water permeation measurement of joints

Test results

Long-term loading test

No abnormalities(in air and in hot water)

Test item

Test conditions

Partial discharge

No occurrence (Occurrence voltage

Inner-pressure

testof 30 kV or more was confirmed)

tightness

No air-leak should be observed
at 49 kPa.24 hr in 60°C water

In air: 115~165 kV

Water

Water permeation after 10, 20, and 30

AC break-In hot water: 135~145 kV

permeation

days of immersion in 60°C water to bedown

In air: 85~95 kV

measurement

measured and compared with WILL

S-joint

acteristics after

Y-joint

long-term tests
Lightning

50

WILL
Water-tight tube without water-impervious layer
Power-rule calculation for WIAL
Power-rule calculation for water-tight tube without water-impervious layer
WIAL
WILL with 10 test pinholes of φ2Power-rule calculation for WILL

the same as for the initial)

Results of long-term tests of 325-sq joints

10

Time (day)Good (Residual performance was

Water-tightness

Residual char-1

Y-joint: 75~95 kVCurrent loading

Table 8Y-joint: -205~-215 kV

10Water permeation (g)Table 7

S-joint

impulseIn hot water: 85~95 kV

Good

Good

No crack should occur when following

In air: -345 kV

Thermo-strains are appliedIn hot water: -345 kVmechanical

Diurnal strain: 10,950 countcharacteristics

Short-duration strain: 30 countbreakdown Y-joint In air: -335 kV
In hot water: -275 kV

Test results

Good

Annual strain: 30 countNote
60°C hot water long-term tests were carried out at the Central ResearchInstitute of Electric Power Industry.

tance to cost reduction, a three-layered rubber mold structure with outer casing of metal, terminal lug jointing for
conductor joints, and the "splice housing plus spacer"
structure for insulation. In terms of OSCL edge processing
and the water-impervious structure, the same method andstructure as for the straight joint were applied.

5.

EVALUATION OF JOINTS

5.1 Initial Characteristics
It was confirmed that the initial characteristics of the joint
satisfactorily met every requirement of target characteristics. Table 7 shows the results of the initial characteristics
tests together with the breakdown tests that were conducted to know the ultimate performance.
5.2 Long-Term Test
Long-term test of 180-days duration was carried out toconfirm the long-term performance of the developed
joints, and Table 8 shows the results.
In this test, no abnormalities such as breakdown were

found. Furthermore, the residual performance tests after
the long-term test were conducted in terms of partial discharge, AC breakdown, and lightning impulse, verifyingthat the performance was the same as for the initial characteristics as shown in Figure 8.
5.3 Water-Imperviousness
The following tests were carried out to evaluate the performance of the water-impervious structure applied to thedeveloped joints, confirming that the water-imperviousness of the shrinkable tube with WIAL was equivalent to
the WILL of established track record.
Table 9 and Figure 8 show the results of the tests.
5.3.1 Inner-Pressure Air-Tightness Test
The water-impervious structure was tested whether it
could withstand the pressure increase within the joints.
5.3.2 Water Permeation Measurement
The water permeation of WIAL and WILL was measured
and compared in accordance with the water permeation
measuring methods of CV cables with 66-kV rating or
over.

63

Table 10

Comparison of installation work

[Straight joint]
Item

Joint currently in use

Splice housing Dedicated tool ismounting

Joint developed
Neither skill nor tools required

used since manual

thanks to cold-shrinkable spliceinsertion is difficult

housing

Shrinkable tube Heat source required The same as for the waterproof
tube in present use

mounting
Finished
dimensions

φ 130 x 1150 (mm)Downsized to about 60 % of
current joint

Photo 1

Appearance of installed straight joint

Photo 2

Appearance of installed Y-branch joint[Y-branch joint]
Dedicated tools re-Bolt connection using terminal

Conductor

quired for wedge

lug outside the Y-branch body,splicing

splicing within theensuring secure connection

Y-branch bodywithout using dedicated tools

Finished

W150 x H292 x L1260 Downsized to about 90 % ofdimensions(mm)current joint except for theequivalent total length

5.3.3 Thermo-Mechanical Characteristics Test
Thermo-mechanical characteristics were evaluated basedon a method that is basically the same as that describedin Clause 3.2.2.
5.4 Installation Work
Ease of installation of the joints was evaluated and it wasverified the developed joints could be installed in a shortertime than the conventional slip-on type straight joint or
prefabricated type Y-branch joint. Table 10 shows thecomparison, and Photos 1 and 2 show the installedstraight joint and Y-branch joint.

6.

CONCLUSIONREFERENCES
1) JEC-3408, "High-Voltage Testing Method for Ultra-High Voltage -11 kV to 275 kV-- Cross-Linked Polyethylene Cables and Joints",
1997 (in Japanese)
2) "High-Voltage Testing Method for CV Cables and Joints",
Electrical Cooperative Research, Vol. 51, No. 1 (1995) (inJapanese)

Manuscript received on December 26, 2000.

We have developed a new cable with reduced insulation
thickness having water-impervious layer in conjunction
with its joints for 22-kV power supply systems in which
future application is expected to expand. The development program was aimed at cost reduction through insulation thickness reduction while maintaining reliability andimproved environment-friendliness.
Last but not least, we would like to thank Mr. Takeda,
Chief Researcher of the Central Research Institute ofElectric Power Industry for his great help in carrying out
the long-term loading test in hot water.

64

Furukawa Review, No. 20

2001