Special Issue

Technologies supporting the achievement of Furukawa Electric Group
Medium-term Management Plan 2022 - 2025
Contribution to the achievement of SDGs
by Furukawa Electric Group

Development of a Cross-Linked Polyethylene Insulation Material
for DC Cables
Takahiro Kanaya*1, Sakurako Tomii *2, Naoto Shigemori *2,
Kazuyoshi Akizuki *1, Yutaka Suzuki *1
We have developed an insulating material whose main component is crosslinked polyethylene (XLPE) that can be applied to extra-high voltage DC cables.
The actual DC cable was manufactured with the material by utilizing the supply chain built after
optimizing the stable supply and handling of raw materials. As a result of conducting various inhouse and outside evaluations on the cable, it was confirmed that all the characteristics weresatisfied. We have already completed a long-term energization test of a DC 525 kV class cablesystem, and we believe that we can make a major contribution to the expansion of renewableenergy in the future.

ABSTRACT1.

INTRODUCTION

DC power transmission technology is useful for longdistance, large capacity power transmission because it
has less power loss in current paths and higher systemstability than AC power transmission. However, since AC
to DC and DC to AC converters require more space thantransformer equipment between AC, they are mainly
applied to projects with sufficiently large transmission
distances and transmission capacities.
Renewable energies such as solar and wind power,
which are expected to become more widespread in the
future, often require large installation areas for facilities. In
the case of generating power on a large scale, it isthought that deployment will take place in areas with relatively low population densities and in offshore areas.
Therefore, it is expected that the distance between thelocation of renewable energy power sources and demandareas will become longer. At the same time, it is thoughtthat the interconnection between electric power companies and between regional power grids will need to be
more and more dense in the future. In order to realize theabove, the introduction of large-capacity grid connectionlines is being considered. As a result, it is thought that theusefulness of DC power transmission technology, which
excels at long-distance power transmission, will continue
to attract attention in the future.
We have been providing oil impregnated paper insulated (OF) cables for DC for some time, but it is becoming
difficult to maintain this business from the perspective ofElectrical Power Material Development Department,
Telecommunications & Energy Laboratory*2
Cable System R&D Department, Power Cable Division*1business continuity, including environmental characteristics and technology transfer. The production of the OF
cable has been withdrawn several years ago. On theother hand, as DC cables, types that use solid insulating
materials such as XLPE has become the main stream atpresent, and we have been working hard to develop
XLPE insulated cables for DC. And this time, we havedetermined the final composition of the solid insulating
material that can be used for DC applications, optimized
the supply chain and conditions, etc., and finalized it. The
details of the process and the performance of the completed DC cable are reported below.

2.
2

XLPE IMPROVEMENT /MODIFICATION POLICY AND METHOD

Insulating materials for DC cables were developed basedon the following policies.
• The base material is XLPE, which has many achievements in conventional AC cables.
• The material will be processed by cable manufacturing equipment that can be adjusted from equipmentthat has conventionally manufactured AC cables.
• Allowable conductor operating temperature is 90°C.
• In order to avoid problems such as poor dispersion in
the base material, the modification for the DC conversion does not use fillers or low molecular-weight components, but modifies and denatures the XLPE itself.
• Specifically, a polar group is introduced to function asa charge trapping site.
• We are conscious of the stability of the supply chain
and emphasize the availability and handling of raw
materials.
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Development of a Cross-Linked Polyethylene Insulation Material for DC Cables

3

ELECTRICAL PROPERTIES REQUIREDFOR DC INSULATING MATERIALS AND
THEIR EVALUATION RESULTS

Insulating materials used for DC cables are required tohave different properties from those for AC cables. In particular, space charge characteristics can be mentioned as
one of the required characteristics unique to direct current systems. In this report, we focus on this spacecharge characteristics, introduce two evaluation methods,
and report on the performance of the developed material
in each method.
3.1 Current Integration Charge (Q(t)) Method
When an insulator is sandwiched between electrodes andheld in direct current electric field, charges (electrons,
holes) are three-dimensionally injected into the insulator
from the electrodes. These injected charges are called
space charges. If these space charges move or accumulate in the insulator along over a charging time, their distribution and density changes, causing local changes inthe electric field strength inside the insulator, and in theworst case, dielectric breakdown may be occurring in an
abnormal amplified electric field. Therefore, it is importantfor direct-current insulating materials not to accumulate
space charges, not to vary with time, and as a result not
to favor a local electric field amplification.
This report introduces two methods for measuring thespace charge characteristic. The first method is the Q(t)method.
Figure 1 is a conceptual diagram of Q(t) measurement.
This is a simple measurement method in which a step
type DC voltage shown in Figure 2 is applied between
electrodes sandwiching the sample, and the amount ofcharge injected into the sample over time is measured.

Electrode

Current
Integrator

SampleElectrode

DC Power
SupplyApplication voltage / kV/mm

40
30
20
10
0

0

Figure 2

500

1,000
Time / s

1,500

2,000

DC electric field applied on a sample.

Figure 3 and 4 show the measurement results of Q(t) at
the maximum cable operating temperature of 90°C for
sheet samples of the XLPE that we use for normal AC
cables and the developed DC XLPE. It can be seen thatthe integration charge on the vertical axis increases as
the application time increases and the applied electricfield strength increases. Also, in the case of AC-XLPE,
significant accumulation of integration charge is observed
from around 20 kV/mm of electric field. On the other
hand, the developed DC-XLPE does not show significant
accumulation of integration charge even at 30 kV/mm,
and has relatively good characteristics.
4

Appliedelectric ﬁeld

3.5

Integration charge Q (t)(C×10 -6)3.

50

5 kV/mm3

10 kV/mm

2.5

20 kV/mm

230 kV/mm

1.5

40 kV/mm

1
0.5
0

0

50 100 150 200 250 300 350

Time / sFigure 3

Q(t) measurement results for an AC-XLPE sample at
90°C.
4Appliedelectric ﬁeld

3.5

Integration charge Q(t)(C×10 -6)

As a method for modifying and denaturing XLPE, we
decided to investigate the polar group modification technology of polyethylene. While predicting the electricalperformance of the modified material using the latest
quantum chemical calculation method, we conducted
experiments on numerous compounds of chemical components and repeated material evaluation tests such as
the required properties as a DC insulation material
described from chapter 3 onward. As a result, we wereable to obtain a good insulating material for the DC cable.
The evaluation results are shown below.

5 kV/mm3

10 kV/mm

2.5

20 kV/mm

230 kV/mm

1.5

40 kV/mm

1
0.5
0

0

50

100 150 200 250 300 350

Time / sFigure 1

Conceptual diagram of a Q(t) measurement.

Figure 4

Q(t) measurement results for a DC-XLPE sample at
90°C.

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Development of a Cross-Linked Polyethylene Insulation Material for DC Cables

In order to express the easiness of the charge accumulation, the ratio of electric charge immediately after charging and 300 seconds after charging is defined and usedas the electric charge ratio. It can be said that the smaller
the electric charge ratio, the better the DC performance.
Figure 5 plots the relationship between the electriccharge ratio and the DC electric field. It can be seen thatthe developed DC-XLPE sample has less space charge
accumulation than the AC-XLPE sample in the electricfield region, that is, the developed product has improved
DC characteristics.

Pulse power
supplyCR

DC Power
supplyElectrode
Semiconductive
layer
SampleAmpliﬁcation
ampliﬁer

Electrode5

Figure 6

Conceptual diagram of a PEA measurement.

3.5
3

DC-XLPE

2.5

AC-XLPE

2

1

0

10

20

30

40

50

Application Voltage Vdc / kV/mm
Relationship between the DC electric field and theelectric charge ratio.

3.2 Pulsed Electro-Acoustic (PEA) Method
3.2.1 Measurement results for sheet samples
The second evaluation method for space charge characteristics is the PEA method. This is also the same as theQ(t) method with respect that a DC electric field is applied
to the samples sandwiched between the electrodes, but it
is different with respect that a pulse voltage is applied
between the electrodes and the pressure wave generated
inside the sample is detected at that time. When thespace charge accumulates in the sample, the location
receives force from the pulsed electric field and vibrates,
generating a pressure wave. By measuring this pressure
wave and measuring its delay and magnitude, it is possible to capture the position of the space charges in the
thickness direction and the amount of accumulation of
the space charges. Compared to Q(t) method, this method is superior because it can provide information on theposition and the amount of space charges in the thickness direction of the sample and the charge distribution
in the sample.
Figure 6 shows a conceptual diagram of a PEA measurement.

Using a sheet-shaped sample, PEA measurement was
performed under the following conditions.
• Sample: Cross-linked sheet formed to a thickness of
0.1 to 0.5 mm• Temperature: Heating with a heater (90°C)• Applied electric field: –30kV/mm
As part of results, here we introduce the measurement
results at 90°C, which is the maximum operating temperature of the developed product. In addition, the groundelectrode is placed at the position where the sample
thickness is 0 mm%, and the high voltage electrode isplaced at the position where the sample thickness is 100
mm%.
Figure 7 and 8 show the PEA measurement results ofan AC-XLPE sheet. In spite of the short time of only 7hours, it is observed that the accumulation of positive
charges gradually progresses in the vicinity of the electrode over time. This injected charge is a space charge,
and if the measurement is continued for a long time, it isexpected to accumulate and develop into a disturbancein the local electric field distribution (Figure 8 shows thatthe measurement time is too short for the electric field
distribution to be significantly affected yet).
50

Charge density / C/m3

Electric charge ratio

4

1.5

Figure 5

Piezoelectric
sheet

Oscilloscope

4.5

30

0s

1h

2h

3h

4h

5h

6h

7h

10
-10
-30
-50
-20 -10 0

10 20 30 40 50 60 70 80 90 100 110 120

Sample thickness / mm%Figure 7

PEA measurement results of AC-XLPE sheets:
Distribution of charge density along the thickness
direction at 90°C.

Furukawa Electric Review, No. 54 2023

14

50

50

40

40

Electrical ﬁeld / kV/mm

Electrical ﬁeld / kV/mm

Development of a Cross-Linked Polyethylene Insulation Material for DC Cables

30
20
10

0s

1h

2h

3h

4h

5h

6h

7h

0
-10

-20 -10 0

10 20 30 40 50 60 70 80 90 100 110 120

PEA measurement results of AC-XLPE sheets:
Distribution of electric field along the thickness
direction at 90°C.

On the other hand, the DC-XLPE sheet in Figure 9 does
not seem to change from 1 hour to 5hours.
50

Charge density / C/m3

0s

1h

2h

3h

1d

2d

3d

4d

5d

6d

7d

30
20
10
0
-10
-20 -10 0

10 20 30 40 50 60 70 80 90 100 110 120

Sample thickness / mm%

Sample thickness / mm%Figure 8

0s

4h

5h

30

Figure 11 PEA measurement results of DC-XLPE sheets:
Distribution of electric field along the thickness
direction at 90°C (long time).

3.2.2 Measurement results for a miniature cablemanufactured with the developed DC-XLPE
As the next step, we decided to manufacture a miniature
cable (a small prototype cable) using the developed
DC-XLPE as an insulation layer and conduct PEA measurements (Figure 12).

10
-10
-30
-50
-20 -10 0

10 20 30 40 50 60 70 80 90 100 110 120

Sample thickness / mm%Figure 9

PEA measurement results of DC-XLPE sheets:
Distribution of charge density along the thickness
direction at 90°C (short time).

Therefore, we decided to extend the measurement timeand observe the amount of space charge accumulated in
the DC-XLPE sheet over a longer period of time.
Figure 10 and 11 are the results. The developed product did not show any remarkable change in charge density distribution even after 7 days of continuous DC application, and we believe that we were able to confirm its
excellent DC characteristics.

Charge density / C/m3

50
30

0s

1d

2d

3d

4d

5d

6d

7d

10
-10
-30
-50
-20 -10 0

Figure 12 Cross-section of a miniature cable (the copper
strands in the center are falling out).

Test condition:• Cable outer diameter: 5 to 10 mm• Structure: Stranded copper wire / conductor screen /insulation layer / insulation screen• Insulation layer thickness: Around 1 mm
• Temperature: C ombing conductor energization andheater heating (90°C)• Applied voltage: – 27kV
(high voltage side: conductor screen,
Ground side: insulation screen)
From Figure 13 and 14, it was observed that a small
negative charge was accumulated near the ground electrode in the early stage, but it disappeared with time, andafter 730 hours (approximately one month), a very welldefined curve was obtained.

10 20 30 40 50 60 70 80 90 100 110 120

Sample thickness / mm%Figure 10 PEA measurement results of DC-XLPE sheets:
Distribution of charge density along the thickness
direction at 90°C (long time).

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Development of a Cross-Linked Polyethylene Insulation Material for DC Cables

Charge density / C/mHigh voltage side
0 min
12 h
72 h
480 h

10
5

30 min
24 h
84 h
640 h

1h
36 h
96 h
730 h

5h
48 h
108 h

10 h
60 h
320 h

0
-5
-10

2.7 2.6 2.5 2.4 2.3 2.2 2.1

2

1.9 1.8 1.7 1.6

Position / mmFigure 13 PEA measurement results of a miniature cable usinga DC-XLPE insulation: Distribution of the charge
density along the thickness direction at 90°C.

Ground side

Electric Field / kV/mm

60

High voltage side

50
40
30
20

0 min
12 h
72 h
480 h

10
0
-10

30 min
24 h
84 h
640 h

1h
36 h
96 h
730 h

2.7 2.6 2.5 2.4 2.3 2.2 2.1 2
Position / mm

5h
48 h
108 h

10 h
60 h
320 h

1.9 1.8 1.7 1.6

Figure 14 PEA measurement results of miniature cable using a
DC-XLPE insulation: Distribution of the electric fieldalong the thickness direction at 90°C.

Electric ﬁeld enhancement factor

Figure 15, shown below, plots the value obtained by
dividing the maximum local electric field in the entire
thickness for each measurement time by the initial maximum electric field value which is called the electric fieldenhancement factor. If the electric enhancement factor
exceeds 1 and gradually increases, it means that a position where more electric field stress is applied to theinside of the cable as it is used, it can be said that thecable cannot be relieved even if it withstands the initialtest after construction.
1.50
1.40
1.30
1.20
1.10
1.00
0.90
0.80
0.70
0.60
0.50

Looking at Figure 15, it can be seen that the electricfield enhancement factor of the cable using the developed DC-XLPE settles down in a short period of time andbecomes almost flat after 500 hours. From these results,
we believe that we have confirmed that the DC-XLPE
insulation developed this time has excellent long-term DC
characteristics.

4.
4

OTHER IMPORTANT ELECTRICALPERFORMANCE RESULTS

In addition to this, we would like to introduce some evaluation tests for the material that should be noted for DC
cables.
4.1 Volume Resistivity
The measurement results of the volume resistivity at hightemperature in accordance with the following measurement condition are shown below.
• Sample structure: A miniature cable with an insulation
thickness of about 1 mm
• Measurement electric field: –10 to –40kV/mm• Measurement temperature: 90°C(heating with a heater)
As for DC-XLPE insulation, there are several existingproducts on the market from manufacturers of insulating
and semiconductive materials for power cables. Three ofthem were obtained and compared (commercial products
1, 2, 3).
As a result, our DC-XLPE insulation is superior to our
AC-XLPE insulation and the commercially available
DC-XLPE insulation in the entire measured electrical field
range, and has an excellent volume resistivity within thepractical electric field design range for power cables(Figure 16).
Commercial product 1
Commercial product 2
Commercial product 3

DC-XLPE
AC-XLPE
1.0E+17
Volume resistivity / Ω・cm

Ground side

15

1.0E+16
1.0E+15
1.0E+141.0E+13

0

10

20

30

40

50

Electrical ﬁeld / kV/mm

0

200

400
Time / h600

800

Figure 15 Time variation of the electric field enhancement
factor of a miniature cable using DC-XLPE insulation
at 90°C.

Figure 16 Comparison of the volume resistivity between
the developed DC-XLPE insulation and the othercommercially available products.

4.2 Dielectric Breakdown Characteristic
Regarding dielectric breakdown characteristics, we compared the performance of the developed product withthat of other companies’ commercial DC-XLPE insulations under two conditions, DC and impulse. The test
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Development of a Cross-Linked Polyethylene Insulation Material for DC Cables

DC breakdown strength
kV/mm

200
150

35
30

50

Developed
Product

Commercial Commercial Commercial
product 1
product 2
product 3

Developed
Product

Commercial Commercial Commercial
product 1
product 2
product 3

300
250
200
150
100
50
0

Figure 17 Summary of results from the top: DC breakdown test
and the bottom: impulse breakdown test.

4.3 Mechanical Strength (Tensile Strength Test)
If the cable is too rigid, it will adversely affect the installation and connection workability. Therefore, as a representative evaluation of mechanical properties, we will also
compare the developed DC-XLPE insulating material andthe conventional AC-XLPE insulating material in the tensile strength test. The test conditions are as follows.
• Sample structure: 1 mm thick sheet was formed andpunched according to JIS No.3
dumbbell shape.
• Tensile speed: 200 mm/min
As shown below, the mechanical properties of the con-AC-XLPE

20
15
10
5
0
-100

0

100 200 300 400 500 600 700 800
Tensile elongation / ％Figure 18 Comparison of stress-strain curves at room
temperature.

In addition, after going through various customerrequested tests, in-house tests, and reference tests, wewere able to complete a high-performance DC insulationmaterial that satisfies all of these requirements.

5.

100

DC-XLPE25

50

Imp. breakdown strength
kV/mm

ventional AC-XLPE insulating material and the developed
DC-XLPE insulating material are almost the same, and it
is possible to obtain the same handleability as the conventional AC cable (Figure 18).

Tensile stress / MPa

conditions are as follows.
DC dielectric breakdown test:• Sample structure: A miniature cable with an insulation thickness of about 1 mm
• Voltage boost method: –70 kV/min−> –5 kV/min step boost• Measurement temperature: 90°C
Impulse dielectric breakdown test:• Sample structure: A miniature cable with an insulation thickness of about 1 mm
• Voltage boost method: –100 kV × three times−> – 5 kV/min × step up threetimes• Measurement temperature: 90°C
Due to variations in measured values, it is difficult todetermine superiority or inferiority, however, the productdeveloped by our company is considered to be at least
as good as the commercially available product. Therefore,
it is considered that the sufficient performance of ourdeveloped product has been confirmed (Figure 17).

LONG-TERM ENERGIZATION TEST OF
DC 525 kV CLASS CABLE SYSTEM

Although the content overlaps with the news release fromour company on April 30, 2021, we will introduce it again.
An actual cable was manufactured using the DC insulation material developed in this report, and a test line
including a cable system consisting of various components such as connection parts was constructed andsubjected to the following long term energization test.
We conducted a pre-qualification test of the DC 525 kV
cable system in accordance with testing recommendations based on CIGRE Technical Brochure No.496. As an
extra-high voltage submarine cable, the test was carriedout in the “the Next-Generation Offshore DC Power
Transmission System development project” of the New
Energy and Industrial Technology Development
Organization (NEDO) from years 2015 to 2019, while land
cable was conducted by a third party. The tests werecompleted without any problems. We believe that theseresults have demonstrated the high reliability of our DC
cables (Figure 19) 2).

Polyethylene sheath

Copper conductor

Bedding tape

Conductor screen

Polyethylene rod

DC-XLPE insulation

Steel wire armor

Insulation screen

Polypropylene yarn

Lead sheath

2

Figure 19 DC 525 kV 1800 mm submarine cable tested for longterm energization.

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Development of a Cross-Linked Polyethylene Insulation Material for DC Cables

6

6.

CONCLUSION

We have developed the following insulating material thatcan be applied to extra-high voltage DC cables.
• It was confirmed by two evaluation methods (Q(t)
method and PEA method) that the DC characteristics
were excellent.
• Compared with the commercially available DC-XLPE
insulations, the developed product was equal orsuperior in all properties, especially in volume resistivity.
• Consideration has also been given to the availabilityof raw materials, and a stable supply chain has
already been established.
• A power cable was manufactured using the developed insulation material, and a long-term energization
test was conducted on a DC 525 kV class cable system, and the test was successfully completed.

REFERENCES
1) Satoru Maruyama, Noboru Ishii, Michihiro Shimada, Shinji
Kojima, Hideo Tanaka, Mitsumasa Asano, Tetsuya Yamanaka,
and Shin’ichi Kawakami: “Development of a 500-kV DC XLPE
Cable System” Furukawa Electric Review No.25 (2004), 47.
2) Ryosuke Kuwabara, Satoru Maruyama, Naoto Shigemori, Yosuke
Otake, Masaki Matsuura, Masakuni Echigo, Ryo Matsuumi:“Technologiac Trends, Development Tasks and Initiatives for
Submarine Power Cable Systems ~Contribution to the Achievementof SDGs~” Furukawa Electric Review No.53 (2022), 39.

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