Contribution to the achievement of SDGs
by Furukawa Electric Group

Technological Trends, Development Tasks and Initiatives for
Submarine Power Cable Systems~ Contribution to the Achievement of SDGs ~Ryosuke Kuwabara *1, Satoru Maruyama *1, Naoto Shigemori *1, Yosuke Otake *1,
Masaki Matsuura *1, Masakuni Echigo * 2, Ryo Matsuumi * 3
In recent years, initiatives for the achievement of the Sustainable Development
Goals (SDGs) and efforts toward the carbon neutrality in 2050 have become
active, and attention has been focused on reforming the energy structure. Among them,Japanese government has set a target of 30 to 45 GW of offshore wind power generation in
2040, and the demand of the submarine power cable is increasing accordingly.
In addition, since offshore wind power generation is planned to be intensively introduced in
Hokkaido, Tohoku and Kyushu areas where the wind conditions are good, a long- distancesubmarine transmission grid for transmitting electricity to the metropolitan area and Kansai area,
which are power load areas. The importance of submarine power cable is increasing more thanever. Furukawa Electric (FEC) is developing technologies from a broad set of perspectives tomeet the demands, and this paper describes the recent development status of submarine power
cables and future technological trends.

ABSTRACTINTRODUCTION
1.1.INTRODUCTION
Natural disasters caused by global warming will increasein the 21st century worldwide. As a result, initiatives forachievement of SDGs or efforts toward the carbon
neutrality in 2050 have become active and an energy
structure reform are getting a lot of attention. The recent
power supply composition in Japan and the forecast for
FY 2030 upheld in the Strategic Energy Plan by the
Ministry of Economy, Trade and Industry (METI) is shown in
Figure 1. The ratio of power generation using fossil fuelswith high CO2 emission is high and its renewable energy
share remains 18% (in the case of discounting hydropower is 10%), however the rate will increase to 37%
according to the plan. Since the trend of decarbonizationin energy supply is a main stream approach in the worldand each country is proceeding with solutions, Japan isrequired to develop its own rapid solution.
Offshore wind power generation is expected as main
power source in Japan which is an island country surroundedby the sea, using its advantageous geographical position,
is developing offshore wind power generation as a lastresort as the main power source in energy policy1). In April
2019, the “Act on Promoting the Utilization of Sea Areasfor the Development of Marine Renewable Energy Power
Generation Facilities” was put into a law in force and thelegal basis were laid for promoting large scale offshore
wind power generation. At the “Public-Private Council on
Cable System R&D Department, Power Cable Division
Cable System Engineering Department, Power Cable Division*3
New Energy Engineering Department, Power Cable Division*1*2

Enhancement of Industrial Competitiveness for Offshore
Wind Power Generation” held in December 2020, theintroduction target of 10 GW in 2030 and 30 to 45 GW in
2040 was agreed, and the public invitation of four
locations in Japan and a total 1.5 GW has already started,and further increase in offshore wind power generation is
expected. Furthermore, the regions with high potential for
introducing offshore wind power generation are Hokkaido,
Tohoku and Kyushu regions, which are far from the areas
with high electricity demand. Therefore, a nationwide
submarine DC power transmission grid is planned 2), andthe demand of submarine power cable is expected toincrease significantly much more than the current level.
We are developing submarine power cables from a
broad set of perspectives to respond to the above
expectations and to contribute to the achievement ofSDGs and the implementation of carbon neutralization by
2050, and this paper describes the current development
status and future technological trends.
2%
FY2030(Forecast)19%

1%
20～22%

21%

Fossil 41％

Non-fossil 59％
Fossil 76％

FY201932%(Actual)36～38%

7%

Non-fossil 24％

37%

18%
6%

Oil
Coal
LNG
Renewable energy

Nuclear power
Hydrogen or ammonia

Figure 1 Comparison of power supply composition between
3)
FY2019 (actual) and FY2030 (forecast).

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39

Technological Trends, Development Tasks and Initiatives for Submarine Power Cable Systems ~ Contribution to the Achievement of SDGs ~Fixed wind turbine(Gravity)(Jacket)Floating wind turbine

Substation

Inter-array cable
Export cable

Dynamic cable

Figure 2 Image of offshore wind power generation.

SUBMARINE
POWER
CABLE
FIXED
2.2.SUBMARINE
POWER
CABLE
FORFORFIXEDOFFOFFSHORE
POWER
GENERATIONSHORE
WIND WIND
POWER
GENERATION
Figure 2 shows an image of offshore wind power generation.
Currently, most of the offshore wind power generationcommercialized worldwide is a system having a foundation
built on the seabed that is called fixed offshore wind
power. It is being built in a relatively shallow sea area witha water depth of less than 60 m. The cables used areroughly divided into two types, inter-array cables forconnecting between wind turbines or between windturbines and the offshore substation, and an export cablefor connecting from the offshore substation to land. Andin either cable, in case of the cable for the fixed offshore
wind power generation, it does not need to consider
fatigue or the like because most part of the cables arelaid in a fixed state such as buried laying. However, since
the cable is exposed in some part around the foundation,a Cable Protection System (CPS) is used generally for
anti-fatigue.
The fixed offshore wind power generation will start in afull-scale introduction around 2023 in Japan. We are
steadily proceeding with the development by utilizing theknowhow cultivated in the development of submarine
power cables so far according to the above schedule.
2.1 Inter-array Cable
In the early stage of offshore wind power generations, it was
about 3 to 5 MW for one port area and the transmission
voltage was AC 22/33 kV class. On the other hand, inrecent years, due to the expansion of the power generation
scale, the study has shifted to the use of wind turbines ofover 8 to 12 MW class, and the transmission voltage has
typically become AC 66 kV class, and the cable isincreasing in size. In addition, as the number of installedwind turbines in wind farms is increasing, thenumber of inter-array cables installed is increasing, andworkability becomes very important. In inter-array cables,4)

development for weight reduction is becoming more
important.
The conventional inter-array cable has been the
cross-linked polyethylene insulated, lead sheathed, iron
wire armored submarine power cable which is used as
islands power links. The cable has a lead sheath forpreventing the water tree generation, but the use of leadincreases the weight of cable.
Therefore, based on the recent technological trends ininter-array cables for fixed offshore power generation
facilities, we are studying a low-cost and lighter submarine
cables with a semi-dry structure. A cross section ofsubmarine power cable with a semi-dry structure isshown in Figure 3. The cable has two characteristics, theapplication of tree retardant XLPE (TR-XLPE) and theelimination of lead sheath. The TR-XLPE has a function ofwater tree prevention and has already been utilized inoverseas distribution underground cables with voltages
below AC 33 kV.
Conductor
Conductor screen
TR-XLPE Insulation
Insulation screen
Wire shield
Aluminum-foil
PE sheath
FillerBinder & Bedding
Wire armor
Polypropylene yarn
Optical ﬁber

Figure 3 Cross section of semi-dry structured submarine
power cable.

By applying of TR-XLPE, it is possible to apply thesimple water shielding, such as aluminum laminate film tothe water shielding layer instead of the complete water
shielding like in the conventional lead sheath, and it can
Furukawa Electric Review, No. 53 2022

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Technological Trends, Development Tasks and Initiatives for Submarine Power Cable Systems ~ Contribution to the Achievement of SDGs ~make cable weight lighter. In addition, since lead isdesignated as an environmentally hazardous substance,
it is contributing to the achievement of the SDGs by
reducing the amount of used lead.
2.2 Export Cable
Export cable has the same structure as the conventionallarge submarine power cable which is used for islands
power links (Figure 4). As mentioned previously, thepower generated by each wind turbine is generallycollected at the offshore substation and transmission
voltage is increased in order to reduce transmission loss
and transferred to the land. The present export cable ismainly AC 154 kV class, but the installation location of theoffshore wind power generation is gradually expanding
offshore to reach favorable wind conditions, and theamount of power generation is also increasing due to theincrease in installed wind turbines, it is expected thathigher voltage of AC 220 to 275 kV class cable will be thestandard instead of the one of the present cable.

Polyethylene sheath

Copper conductor

Optical ﬁber

Conductor screen

Formed ﬁller

XLPE insulation

Steel wire armor

Insulation screen

Polypropylene yarn

Lead sheath

2

Figure 4 AC 275 kV 1400 mm submarine power cable.

We are approaching the task from both sides of thetechnical development and the production equipment forhigher voltage cable. We are proceeding with thedevelopment of a factory joint (FJ) for manufacturing thelong submarine cable and a repair joint (RJ) for getting
the certification from the technical point of view. From theproduction equipment point of view, a large stranding
machine is applied to strand a larger cable than before.

POWER
CABLE
FORFLOATINGFLOATING OFF3.3.POWER
CABLE
FOR
OFFSHORE
WIND
POWER
GENERATION
SHORE
WIND
POWER
GENERATION
Floating offshore wind power generation is a power
generation system in which a wind turbine is built on a
floating body (Figure 2). Since the floating offshore wind
power generation generally has a cost advantage in thesea area with more than approximately 60 m of depth
comparing with the fixed wind power generation, thefloating system is more advantageous in Japan because
shallow water area is limited. In addition, since we have a
lot of strong wind condition area in Japan shown in Figure
5, potential generation with floating offshore generation isestimated at around 424 GW and the floating system hasa tremendous potential (around 128 GW for the fixedsystem) 5).

Average wind speed
3

4

5

6

7

8

9

Figure 5 Wind conditions in the sea around Japan.

10 11 m/s
6)

Currently, floating offshore wind power generation has
a little track record, but considering the high potential andthe possibility of overcoming issues, it is expected thatthe number of floating large-scale wind farms will
increase from around 2030, we are also developing a
power cable for floating offshore wind power generation
to contribute to it.
3.1 Dynamic Cable
Figure 6 shows the structure of a dynamic cable. A power
cable using for floating wind turbines is called in general adynamic cable, and since the cable receives a dynamic
behavior repeatedly, for instance curvature fluctuationand tension fluctuation due to the wave sway of floating
body and fluid force caused by tidal current, durability(fatigue resistance) against tension and bending isrequired for the cable. Therefore, the design is such as aflexible stranded conductor, a wire strand, a metallic
sheath with excellent performance against bending
fatigue, a double stranded armor, etc.

Polyethylene sheath

Copper conductor

Optical ﬁber

Conductor screenFillerXLPE insulation

Binder & bedding

Insulation screen

Steel wire armor

Wire shield
Steel corrugated sheath

Polyethylene sheath
2

Figure 6 AC 66 kV 100 mm dynamic cable.

FEC participated in the Fukushima Forward Project 7) of
METI commissioned project started in 2013. The project
aims to create new industries and jobs focusing on
renewable energy for recovery from the Great East Japan
Earthquake disaster, and three kinds of floating windturbines of 2 MW, 5 MW and 7 MW were installed andFurukawa Electric Review, No. 53 2022

41

Technological Trends, Development Tasks and Initiatives for Submarine Power Cable Systems ~ Contribution to the Achievement of SDGs ~operated off the coast of Fukushima prefecture as a field
verification test. FEC provided with AC 22 kV and 66 kV
dynamic cables for the floating wind turbines and afloating offshore substation, and 66 kV submarine export
cable to landing point. The objective was to realize theworld’s first floating offshore wind turbine and its safety,
as well as reliability and economy.
Furthermore, FEC technology had a great reputation
based on the Fukushima Forward Project and got anorder of a technical development task for an extra highvoltage dynamic cable from Carbon Trust of the United
Kingdom 8) and we carried out structural design, installation
design, etc. Currently, power cables for offshore wind
power generation, led by Carbon Trust, are underconsideration to increase the voltage class, and export ofdynamic cables is assumed in requirements for higher
voltage cables (AC over 66 kV to 275 kV class) in
future. We are proceeding world-leading research anddevelopment for the above needs.
3.2 Technology Development for Cost Reduction
The example of a floating wind turbine is shown in Figure 7.
The mooring method for the present floating offshore
wind power generation is in general a catenary laying
system, for instance spar-buoy type, semi-submersible
type, etc. However, the method has the following problems
that the floating body is greatly shaken and the cost of
the system is increased by mitigations against fatigue,
and that the area occupied by the sea area is large andthe influence on fishing and ship operation is important.
To solve the problems, a floating and mooring system ofTension Leg Platform (TLP) is being studied now. Since
the TLP has a good sway performance, the cable system
can be simplified and also occupied sea area by mooringline is significantly reduced. The TLP can contribute to
the realization of the floating offshore power generation
with highly socially acceptable level and low cost.

Spar-buoy

Semi-submersible

Tension leg platform

Figure 7 Example of a floating wind turbine.

9)

SUBMARINE
POWER
FOR WIDE
4.4.Submarine
power
cableCABLE
FOR WIDE
AREA
AREA INTERCONNECTION
INTERCONNECTIONGRID GRID
As mentioned at the beginning, domestic suitable regions
for offshore wind power generation are Hokkaido,
Tohoku, Kyushu area and the areas are far from highdemand area of electricity. Since insufficient free space inthe power transmission system becomes a big issue forthe transportation of generated power, the developmentof DC submarine power grid is planned.
Figure 8 shows a conceptual diagram comparing cost
between the DC and AC transmission systems. The DC
transmission system is suitable for large capacity andlong distance where the cross AC/DC break-even point,
generally approx. 50 km for submarine power cable, inthe case of above construction of transmission power
grid, the DC system is more advantageous.

Cost
Break-even point

AC system

DC system

Loss

Cable cost

Equipment costLine length

Figure 8 Cost comparison between AC and DC systems.

In addition, power supply between regions becomes
possible by a multi-terminal grid between multiple windfarms and land power grid and it leads to stronger
resilience.
4.1 Development of Insulation Material for DC
SystemThe conventional crosslinked polyethylene used for AC(AC-XLPE) decreases the insulation property under DC
voltage due to the accumulation of the space charge in
the insulation 10). In order to prevent the phenomena, the
XLPE developed for DC (DC-XLPE) is applied for extra
high voltage DC transmission system.
Figure 9 shows the result of space charge measurements
for AC-XLPE and DC-XLPE. The space charge distributionchanged significantly by polarity reversal for the
AC-XLPE, but the DC-XLPE had very stable space charge
distribution11). The performance of insulation material
for DC cable is important not only for the space charge
property but for the volume resistivity and insulation
property for long term, and we have been working todevelop new insulation material with different property
from conventional AC material.

Furukawa Electric Review, No. 53 2022

42

Positionχ / µm＋

1st cycle－

Anode (SC)140＋

Cathode (SC)

2nd cycle－

Anode (SC)Cathode (SC)

80
40
0

0 Cathode (Al)0

Anode (Al)Cathode (Al)Anode (Al)10 20 30 40 50 60 70 80 90 100 110
110120120

Charge densityρ(x,t) / C/m3

Technological Trends, Development Tasks and Initiatives for Submarine Power Cable Systems ~ Contribution to the Achievement of SDGs ~

-40
-80

Time t / min

Positionχ / µm＋

1st cycle

Anode (SC)140

2nd cycle－－＋

Cathode (SC)Anode (SC)Cathode (SC)

80
40
0

0 Cathode (Al)0

Anode (Al)Cathode (Al)Anode (Al)10 20 30 40 50 60 70 80 90 100 110 120

Charge densityρ(x,t) / C/m3

AC-XLPE-40-80

Time t / min

DC-XLPE
Figure 9 Comparison of space charge characteristics between
AC and DC materials.

4.2 Optimization of the DC Cable System
As submarine power cables for wide area interconnection
grid will be extra long-distance transmission over several
hundred kilometers, the basic temperature varies greatly
depending on the surrounding environment. In general,
the thermal condition in the land area tends to be stricter
than in the under-sea area. Since using of the mostthermally severe part as a reference in cable design(selection of conductor size), larger conductor size leads
to high cost.
Therefore, the conductor size of the part that issusceptible to thermal restrictions and the part that is not
susceptible to thermal restrictions (for instance, under-seapart) are appropriately changed and the cost of the entire
system can be reduced. We are developing a DC submarine
power cable transmission system that includes cable factory
joint with different conductor size (different diameter FJ)
and succeeded in developing of a DC 500 kV class 12).
Figure 10 shows the cable used for 525 kV submarine
power cable system development, and as a long-term
performance test, we conducted a one-year long-term
test based on CIGRE TB496 (VSC condition) 13), andwe passed all the test listed including the residual
performance test and obtained good results.

Polyethylene sheath

Copper conductor

Bedding tape

Conductor screen

Polyethylene rod

DC-XLPE insulation

Steel wire armor

Insulation screen

Polypropylene yarn

The conventional submarine power cable has a limitationon the water depth at which it can be installed, and is up
to about 300 m. Therefore, when installing an extra-longsubmarine power cable for wide area interconnection
grid, it is necessary to detour deep sea area and set aroute along the shallow sea area, it leads to high cost andlong installation period. Therefore, we are proceedingwith the design and manufacturing technology of DC 500
kV class cable that can be installed in the deep sea with amaximum depth of 1500 m, and the development of an
installing technology at the same cost level as before.

POWER
CABLE
INSTALLATIONTECHNOLOGY
5.5.POWER
CABLE
INSTALLINGTECHNOLOGYFOROFFSHOREOFFSHORE WINDFOR
WINDGENERATIONGENERATION
We have more than 100 years of experience in submarine
power cables for islands power link and a very large
number of deliveries (about 300 cases, 200 km or more).
The offshore wind power generation business has also
been developing since the dawn of offshore wind power
generation, such as installing the first cables in Japan.
Table 1 shows the past track record and the state of thecable installation barge of the Fukushima Forward Project
shown in Figure 11.
Table 1

Installation results of cables for offshore wind power.

Name

Year

Turbine type

Remarks

Setana

2003

Fixed

The first projectin JapanHibikinada (NEDO)

2012

Fixed

-

Fukushima

2013

Floating

AC66 kV
dynamic cable

Fukushima

2014

Floating-

Fukushima

2015

Floating-

Fukushima

2016

Floating-

Choshi (NEDO)

2015 (Repairing)

Fixed

-

Lead sheath

2

Figure 10 DC 525 kV 1800 mm submarine power cable.

Figure 11 Cable installation barge “KAIYO”.

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Technological Trends, Development Tasks and Initiatives for Submarine Power Cable Systems ~ Contribution to the Achievement of SDGs ~Submarine Power Cable Installation Technology
for Fixed Offshore Wind Power Generation
An example of the procedure for installing submarine
power cable for fixed offshore wind power generation isshown in Figure 12. A protection tube for the hem of thewind turbine called cable protection system (CPS) is
attached toward the end of the submarine power cableon the installation barge. The submarine power cable issent out of the installation barge and is pulled in the windturbine foundation. After that, the submarine power cable
is installed toward the next wind turbine position, and thesubmarine power cable is cut at the required length at the
specified position, and the CPS is attached toward the endof the cable. While hanging the cable with a semi-circularsheave, the submarine power cable is pulled out into thewind turbine, the sheave is pulled out, and the installationis completed.

Oﬀshore substation

5.1

Cable Installation barge
Buoy (A)BS
Dynamic cable (A)Dynamic cable (substation end) pull-in

Submarine
power cable
Dynamic
cable (A)

Joint box (A)Buoy (A)Protection
tube (A)

Dynamic cable (substation end) laying, joint box (A) setting
TurbinePull-in equipment
Cable installation barge

CPS

Submarine power cable 1st pull-in

SemicircularsheaveCPS
Submarine power cable 2nd pull-in
Figure 12 Example of a submarine power cable installation
procedure for fixed wind power generation.

Power Cable Installation Technology for Floating
Offshore Wind Power Generation
An installation procedure of the cable for floating offshore
wind power generation is shown in Figure 13. A dynamic
cable with a bend stiffer, an extreme bend preventing
material, attached to the end is sent out from the installation
barge and pulled into the offshore substation. The cable
is installed while attaching accessories such as distributed
buoys and jointed to the submarine power cable at the
specified position, and the joint box is immersed and laid
down at a specified location.
Next, the submarine power cable is installed toward thewind turbine, and the end of the cable is temporarily
placed on the seabed at a specified position. The dynamic
cable is pulled into the wind turbine and installed in thesame procedure as on the offshore substation side, andthe temporarily placed submarine power cable is pulled
up on the installation barge for jointing work. After that,the joint box is sunk and the installation is completed.
5.2

Submarine power cable end

Submarine power cable laying,submarine power cable end temporary placing

Buoy (B) Dynamic cable (B)Protection
tube(B)

Submarine
power cable
Joint box (B)Joint box (B) setting

Figure 13 Example of dynamic cable installation procedure.

As mentioned before, the introduction of floating offshore
wind generation is expected to increase after 2030. We
will keep developing to contribute to the introduction not
only from the view point of the cable manufacturing but
also from the installation technology perspective.

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Technological Trends, Development Tasks and Initiatives for Submarine Power Cable Systems ~ Contribution to the Achievement of SDGs ~OPERATION
AND
MAINTENANCEOF
OF
6.6.OPERATIONAND
MAINTENANCE
SUBMARINEPOWER
POWER CABLE
CABLE
SUBMARINE
Submarine power cables are more difficult to maintain thanunderground power transmission cables. As mentioned
above, the installation environment differs greatly in the
track, such as in the sea or in the air, so it is necessary todevise cable design and operation. As shown in the
Figure 14, operation and maintenance account for 36.2%of the cost of the offshore wind industry, and how to keep
this low is the key to reducing the cost of the offshore wind
power generation.
2.9%
Total cost

23.8%

6.7%

7.2%

7.7%
15.5%

developed an algorithm for estimating the conductor
temperature. This temperature response function should
be obtained ideally from an actual installed cable system,
but it can also be obtained by analysis. We think that theanalysis is an effective method for offshore power
generation cable systems installed in different ambientenvironments.
Figure 15 shows a model in which a load control
experiment of a DC single core submarine power cable
was performed using a DRS Server that implemented theestimation algorithm adopted in this development andFigure 16 is an example of the model result. From the
Figure 16 (1), the result shows that the current value iscontrolled below the allowable conductor temperature(90˚C in this experiment). Since the error of the conductor

36.2%
DRS-Server
signal6.1 Dynamic Rating System
When constructing a conventional submarine power
cable transmission system, the transmission capacity isdetermined by the allowable temperature of the conductor,
it is designed by selecting a conductor cross-sectional
area that enables constant transmission of rated current
in the most severe part of the temperature environment
over the entire length. However, in the offshore wind
power generation, since generated power fluctuates, thesize tends to be excessive when applying the conventional
conductor cross-sectional area design. Therefore, if thecable conductor temperature is estimated from theamount of power generation and the transmission current
is controlled so that the cable can be operated with the
optimum conductor size, the transmission system can beconstructed efficiently and at low cost. A system that
controls the load by estimating the conductor temperature
is called a Dynamic Rating System (DRS), and is partially
put into practical use in the field of underground powertransmission. Cables used for the offshore wind power
generation are placed in an installation environment,
where the ambient temperature changes drastically,
therefore an algorithm that can predict the temperatureappropriately according to the environment is required.
The method for estimating the cable conductor
temperature is generally to define the thermal resistanceand heat capacity of the constituent material and theambient environment, formulate the heat balance
between each layer in a one-dimensional equation, andcalculate it by unsteady transient analysis. Then, there isa problem that a strict thermal resistance/heat capacity
identification is required and it takes a long time to calculate.
Therefore, we obtained the temperature response functionof the cable according to the installation environment andWeather DevicePowercontroller

Date logger

DRS-Gateway
ThermocoupleConductor
Distribution
Temperature Optical ﬁber
SystemTemperature
measureDC power
supplyConductor
Insulation

ThermocoupleSheathWire armor
Optical ﬁber

DC-XLPE composite submarine power cable(Optical ﬁber integrated)

Figure 15 DRS control experiment model.

100

Estimeted conductor temperature
Measured conductor temperature
Generated current
Direct buried section
Applied current

90

2800

during load control

80

3200
2400

70

2000

60

1600

50

1200

40

800

30

400

20
70

80

90

100

110

Time / h

120

130

140

Current / A

Figure 14 Cost structure of the offshore wind industry (Fixed
14)wind power generation).

Temperature / ℃

Removal0150(1) Conductor temperature of cable and applied current.
Diﬀerence between measured and estimated temparature

4

ΔT / ℃

R&D
Wind turbines
Foundation
Electric system
Installation
O&M

2
0
-2
-4

70

80

90

100

110

Time / h

120

130

140

150

(2) Error of estimated conductor temperature.
Figure 16 Example of current control experiment results used
DRS.

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Technological Trends, Development Tasks and Initiatives for Submarine Power Cable Systems ~ Contribution to the Achievement of SDGs ~temperature estimated from the figure 16 (2) is within
about 2˚C, it is considered to be a practically sufficientaccuracy.
We believe that the use of such DRS can contribute tothe optimization of the cable system used for the offshore
wind power generation.
6.2 Maintenance System
Since submarine power cables are installed and operated
on the seabed, it is difficult to carry out regular inspections
and maintenance in the same way as underground cables
that are directly buried or installed in ducts. In the case ofsubmarine power cables, it is technically possible to checkthe cable installing and burying status using a remotely
operated vehicle, but it is not an easy maintenance method
because it is costly. Therefore, as constant maintenance,a system for temperature monitoring using the composite
optical fiber and a system for detecting cable damage
caused by anchors and fishing gear of ships by combining
resistance wires have been put into practical use.
However, the technology for diagnosing the degree ofdeterioration of the cables has not been established atpresent. The insulation deterioration diagnosis method
used for underground power transmission cables targets
water tree deterioration occurring in cable insulation
without a moisture barrier and the insulation deterioration
mechanism for submarine power cable with moisture barrier
has not yet been clarified. Maintenance and operation
related to such insulation deterioration is a future
development issue.
Therefore, measures for early restoration of submarine
power cables and accident prevention measures due todamage are currently under consideration 15). Repairing asubmarine power cable generally requires a lot of timeand cost to secure a ship and workers because the cableis generally cut at the seabed, pulled up to a work ship on
the sea, and then the cables is repaired. In future, joint
ownership of cable maintenance and repair vessels solvethese problems. In addition, the development of amanagement system that utilizes the location informationof ships and cables to reduce the risk of accidents on
submarine cables and repair methods on the seabed arebeing considered.

CONCLUSION
7.7.CONCLUSION

Power Transmission Systems” and “Research andDevelopment of a Multi-Purpose and Multi-Terminal High
Voltage DC Transmission System”.

REFERENCES
1) https://www.meti.go.jp/english/press/2021/1022_002.html
		 “Cabinet Decision on the Sixth Strategic Energy Plan”
2) Related material of reference 1) “Energy supply and demand
outlook for 2030”. (in Japanese)3) https://www.enecho.meti.go.jp/en/category/others/basic_plan/
pdf/6th_outline.pdf
		 “Outline of Strategic Energy Plan”
4) Aker Solutions HP: https://www.akersolutions.com/what-we-do/
renewable-energy-solutions/offshore-wind-solutions/5) Japan wind Power Association, “Opinions on the formulation of abasic energy plan for the realization of carbon neutrality in 2050”(2021). (in Japanese)
6) New Energy and Industrial Technology Development, “Offshore
Wind Map” NeoWhttps://appwdc1.infoc.nedo.go.jp/Nedo_
Webgis/top.htmlins (nedo.go.jp). (in Japanese)7) Floating offshore wind power generation system demonstration
research project off the coast of Fukushima General Committee,“2018 Floating offshore wind turbine system off the coast of
Fukushima demonstration research project General Committee
Report” (2018). (in Japanese)
8) Furukawa Electric Co., “Received an order for extra high voltage
dynamic submarine cable development work for floating offshore
wind turbines”
		 https://www.furukawa.co.jp/release/2019/ene_190604.html (inJapanese)9) New Energy and Industrial Technology Development, “Floating
Of fshore Wind Turbine Technology Guidebook Appendix 7.
Floating offshore wind power generation project examples inJapan and Overseas” (2018).
		 https://www.nedo.go.jp/librar y/fuur yoku_guidebook.html (inJapanese)
10) H. Fukagawa et al.: “Insulation property of DC 250 kV XLPE
cable” Report of the Central Research Institute of Electric Power
Industry, No. 177010 (1982). (in Japanese)
11) S. Kikuchi, H. Mori, T. Sakurai, Y. Yagi: “Performance of optimized XLPE material for controlling space charge in HVDC
transmission cable system” CEIDP 2017, 8B-6 (2017).
12) N. Shigemori, M. Yagi, T. Tabuchi, H. Mori, “Development of 350
kV and 525 kV HVDC extruded cable system”, Jicable 19, A9.5
(2019).
13) CIGRE TB490, “Recommendations for Testing of long AC
Submarine Cables with Extruded Insulation for system voltage
above 30(36) to 500(550) kV” (2012).
14) The 1st Public0Private Council for Strengthen the Industrial
Competitiveness of Of fshore Wind Power” Appendix 3 “To
strengthen the industrial competitiveness of offshore wind power(Secretariat materials) (2020). (in Japanese)
15) 2018 CIGRE SESSION, “Innovative Fault Location and Repair ofSubmarine Power Cables on the seabed”, B1-305 (2018).

In this article, we described the technological trends ofsubmarine power cables and results of our efforts. As
mentioned earlier, the expansion of the offshore wind
power generation is indispensable for achieving the
scenario of the Japanese government’s green growthstrategy. We will continue to push forward with thedevelopment of submarine power cables and contributeto the achievement of SDGs and the realization of carbon
neutrality by 2050.
The development related to section 3.2, 4.1 and 6.2 isthe result of the New Energy and Industrial Technology
Development Organization “Next-generation Offshore DC
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