luigi.colla@prysmiangroup.com

Submarine projects in the Mediterranean Sea. Technology developments and future challenges.

Luigi Colla, Luisa Vittoria Lombardo, Nikola Kuljaka, Ernesto Zaccone Prysmian Group

Italy

SUMMARYThe Mediterranean Sea is bordered by three continents and 23 Countries (two only partially recognised). There are six large islands: Majorca, Corsica, Sardinia, Sicily, Crete and Cyprus, plus a total of around 3300 smaller islands. At the time of writing within the Mediterranean Sea there are 12 HV links in operation, 4 of which are HVDC. Some new links are under construction while several ones are either planned or at feasibility study stage. In recent years there has been a growing interest in new submarine interconnection projects in the Mediterranean area to promote the development of renewable energies, the security of supply and the integration of electricity markets. The Mediterranean Sea has an average depth of 1500 m and the deepest recorded point is 5267 m in the Calypso Deep in the Ionian Sea. Sea depth represents a key issue in submarine cables design and installation. The paper deals with the technology and system challenges typical of submarine cable links in the Mediterranean Sea. HVAC and HVDC cable technologies available today are described for the realization of submarine cable connections and shows typical applications and limitations of the technology. Due to great importance and financial implications the evolution of power transmission submarine cables is a slow process: a very high reliability is required, and, before being adopted on a commercial basis, a new type of cable shall pass through a tough development and prequalification process requiring years or decades before being adopted with a sufficient level of confidence. In the last decades, a significant increase of submarine transmission circuits, both in Alternate Current (AC) up to the voltage of 550 kV and Direct Current (DC) up to the voltage of 600 kV have been realized worldwide. Future developments and applications are outlined, with a specific focus on deep water and high power transmission over long distances.

KEYWORDS Submarine Cable, Cable technology, High Voltage, Interconnectors, HVDC, HVAC, P-Laser, XLPE, EPR, MI, PPL, SCFF.

First South East European Regional CIGRÉ Conference, Portoroz 2016 SEERC

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1. GENERAL

The growing interest in international interconnection lines as well as offshore-located energy generation has contributed to the evolution of high voltage submarine transmission technology. The Mediterranean Sea is bordered by three continents and 23 Countries (two only partially recognised). There are six large islands: Majorca, Corsica, Sardinia, Sicily, Crete and Cyprus, plus a total of around 3300 smaller islands. The total electricity consumption was approximately 1800 TWh in year 2015. At the time of writing within the Mediterranean Sea there are 13 HV links in operation, 4 of which are HVDC. Some new links are under construction while several ones are either planned or at feasibility study stage. In recent years there has been a growing interest in new submarine interconnection projects in the Mediterranean area to promote the development of renewable energies, the security of supply and the integration of electricity markets. The Mediterranean Sea has an average depth of 1500 m and the deepest recorded point is 5267 m in the Calypso Deep in the Ionian Sea. Sea depth represents a key issue in submarine cables design and installation. Due to strategic importance and financial implications the evolution of power transmission submarine cables is a slow process: a very high reliability is required, and, before being adopted on a commercial basis, a new type of cable shall pass through a tough development and prequalification process requiring years or decades before being adopted with a sufficient level of confidence. Thanks to the efforts of the industry in the recent years, extruded dielectric transmission cables, having the XLPE (Cross Linked Polyethylene), EPR (Ethylene Propylene Rubber) or the recent HPTE (High Performance Thermoplastic Elastomer) based insulation, are now commercially available. The extruded insulation technology is simpler than the traditional impregnated lapped paper insulation, and requires a lower level of workmanship and maintenance, especially due to the fact that prefabricated accessories (joints and terminations) are available. In the last decades, thanks to cable industry developments, a significant increase of submarine transmission circuits, both in Alternate Current (AC) up to the voltage of 420 kV and Direct Current (DC) up to the voltage of 600 kV have been realized worldwide.

2. SUBMARINE CABLE TECHNOLOGIES

Adequate cable technologies are a pre-requisite for reliable submarine cable connections. For shorter transmission distances HVAC submarine cables have generally been used. Long distances generally require use of HVDC links.

2.1. AC Submarine cables

AC cables can be divided into a few large families, based on design and insulation material. The most commonly designs used nowadays are shown in Figure 1. With respect to design, three-core and single-core cables are to be distinguished. For HV submarine applications both three single-core cables or three-core cables can be used. In the case of the high transmission capacity is required; single-core cables offer a preferable solution because of their larger conductor size available and better heat dissipation. The better thermal behaviour, obtained by large cable spacing, has the drawback of a higher installation cost - because there would be three laying campaigns for each circuit. In order to reduce the losses in the armour of single cores submarine cables, non-magnetic and high conductivity armours have to be used. Three-core design reduces the cable circuit manufacturing cost since copper armour is not required and the installation cost is also generally reduced. Applicability of 3-core design is still limited in terms of maximum rated power per circuit if compared with the single-core cable design. Maximum water depth may also represent limiting factor for this kind of cables.

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CHARACTERISTICS

MV THREE CORE HV THREE CORE HV SINGLE CORE

Insulation EPR or XLPE XLPE XLPE Self Contained Fluid

Filled

Maximum voltage 72.5 kV 245 kV 420 kV 525 kV

Maximum power per circuit

90 MVA 400 MVA 1000÷1200 MVA 1200 MVA

Maximum operating

temperature90 °C 90 °C 90 °C 90 °C

Maximum lengthLimited by voltage drop and/or power losses:

~ 150 km

Limited by power losses and/or capacitive current:

~ 100-150 km

Limited by capacitive current: ~ 100 km

~ 60 km due to hydraulic system limits

Figure 1 AC submarine cable types

In terms of the insulation material, it is worth noting the difference between Paper Insulated Self-Contained Fluid-Filled (SCFF) cables and Extruded Insulation Cross-Linked Polyethylene (XLPE) or EPR cables: • Extruded insulation Cables. During the last 40 years, extruded dielectric (XLPE and EPR)

insulation cables have been developed and have gradually replaced impregnated paper cables. Due to the possibility to adopt prefabricated components, extruded insulation cable systems contributed to the development and to widespread application of underground and submarine power transmission systems both in AC and DC. EPR insulated cables have been used in wet-design submarine cables up to 72.5 kV and up to 150 kV with dry design. XLPE insulatied submarine cable systems are currently used up to 420 kV.

• Self-Contained Fluid-Filled (SCFF) Cables. These cables have been used for underground and submarine power transmission for more than 70 years. Since 1924, SCFF cables have been used for important connections, mainly inside cities and some submarine crossings. Today, the adoption of SCFF cables for land applications is practically abandoned, but some submarine applications are still relevant. To keep the dielectric losses of paper-filled cables low and the permeability for the fluid high the paper used is usually low density. The fluid is of a low viscosity and pressurised during operation. In subsea cables, the pressure of the fluid is maintained from shore stations and must allow for thermal expansion and contraction. Electrical and thermal properties of the insulation have been enhanced nowadays by replacing the traditional Kraft paper with Polypropylene Paper Laminate (PPL). SCFF cables have been used up to 550 kV.

AC submarine cables active power transmission is limited by capacitive current. The maximum feasible length of AC submarine cables is depending on the power to be transmitted and has to be evaluated on a case by case basis. As a general indication some typical route length upper limits for AC submarine systems are reported in Figure 1. In Table 1 are reported some of the most important AC submarine cable projects realized up to the time of writing.

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Table 1 Some major HV-EHV Submarine AC cable projects

Country Project name

Voltage

Conductor Insulation

N° of circuits

and length km

Water depth

m Year

kV mm2 material

Spain Mallorca-Menorca 132 500 CU SCFF 1 x 42 90 1973

Canada Vancouver island 525 1600 CU SCFF 1 x 39 400 1984

USA Long Island Sound 345 2000 CU SCFF 1 x13 35 1991

Philippines Leyete – Cebu 230 630 CU SCFF 1 x 33 280 1995

Malaysia Penang isl. 275 800 CU SCFF 2 x 14 20 1996

Spain Spain Morocco 400 800 CU SCFF 1 x 26 630 1997

Egypt Gulf of Aqaba 400 1000 CU SCFF 1 x 13 840 1997

UK Isle of Man 90 3x300 CU XLPE 1 x 105 40 2000

USA Galveston Isl 138 3x630 CU XLPE 1 x 5 15 2001

Denmark Horns Rev wind farm 150 3 x630 AL XLPE 1 x 21 20 2002

Denmark Seas Roedsand wind farm 132 3x760 CU XLPE 1 x 22 20 2003

Italy Sardinia –Corse island 150 3x400 CU XLPE 1 x 15 75 2005

Norway Gossen isl. 400 1600 CU XLPE 1x3.2 200 2007

Italy Sicily - Malta 220 3x630 CU XLPE 1x97 160 2014

Italy Sicily - Mainland 400 1600 CU SCFF 2 x 38 350 2015

Turkey Çanakkale 400 1600 CU XLPE 2x4 90 2015

2.2. DC Submarine cables

DC cables can be divided into a few different types, based on design and insulation material. The most commonly designs used nowadays are shown in Figure 2.

CHARACTERISTICS

SINGLE CORE

Insulation XLPE P-Laser Self Contained

Fluid Filled MI paper MI-PPL

Maximum voltage ±525 kV ±525 kV ±600 kV ±525 kV ±600 kV

Maximum power per bipole

2000 MW 2200 MW 2500 MW 1800 MW 2400 MW

Maximum operating

temperature 70 °C 90 °C 90 °C 55 °C 80 °C

Maximum lengthNot limited by

cable technology Not limited by

cable technology

~ 60 km due to hydraulic system

limits

Not limited by cable technology

Not limited by cable technology

Compatible converter

VSC any voltage LCC up to 250 kV

Both LCC and VSC

Both LCC and VSCBoth LCC and

VSC Both LCC and VSC

Figure 2 DC submarine cable types

DC submarine cables can be divided into some types depending on insulation technology:

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• DC cable with extruded insulation are mostly used nowadays with DC XLPE as insulation, other innovative extruded materials have been recently developed and are now commercially available [2]. HVDC cables with extruded insulation have been in operation since 1998 at ±80 kV. Extensive developments from the second half of the 1990s onwards allowed for a rapid transition towards industrial use with confidence. During recent years, the use of extruded insulation in the HVDC cable links showed a large increase due to the relative simplicity of the technology required to produce extruded cables compared to that needed to produce mass impregnated or oil filled cables; as a result, the production costs are lower as well. In addition, extruded insulation can operate at higher temperatures than mass impregnated cables, which allows a higher transmissible power per cable pair using the same conductor cross section of the cables. The polymeric insulation compound used for HVDC cables is generally different from the one used for HVAC but on the other hand the manufacturing of the insulating system is identical. For DC the insulating material needs special formulation to reduce the accumulation of space charges. Uncontrolled space charges could otherwise increase the electric field within the cable insulation to a level which could result in insulation break down. In recent years, considerable progress for semi-conductive and insulation polymeric materials has been achieved and insulation technology has improved significantly alongside it. All the extruded HVDC cable systems currently in operation are operated with Voltage Source Converters (VSC), meaning that the cable system is not subjected to polarity reversals. Some projects with extruded insulated cables and LCC converters technology are also in progress. At the time of writing there are in service submarine DC cable systems with extruded-insulation submarine cables up to ±320 kV, but higher voltage cables are being developed and are their implementation is anticipated in the near future up to and beyond ±525 kV. Additionally, the broad application of VSC technology (which allows a reversal in the power flow without changing the polarity of the DC voltage) is encouraging the use of solid synthetic insulated cables.

• Mass Impregnated (MI) has been used for HVDC for more than 100 years, reaching a voltage of ±75 kV as early as 1906. A MI submarine cable at ±100 kV was first used in 1954 for a Sweden – Gotland connection. Since then this type of cable has represented the major share of the HVDC cable installations up to ±525 kV even in extreme environmental conditions. Cables basically consist of a conductor lapped with semiconducting and insulating tapes which are then impregnated with a suitable viscous compound. Paper lapping is performed in a controlled environment to ensure high levels of cleanliness. The impregnation is made in large vessels and takes a long time, thus limiting the production capacity for this kind of cable. The insulation is then enclosed within an extruded lead alloy sheath, protected with a polyethylene jacket and reinforced with metallic tapes.

• Paper Polypropylene Laminate (MI-PPL) improves the electrical and thermal performances of the Mass Impregnated Cables. This makes this technology particularly suitable for the highest power transmission range. Currently a ±600 kV submarine cable project based on this technology is under construction.

• Self-Contained Fluid-Filled Paper Insulated Cables (SCFF) have the peculiarity that they can be designed to operate in AC and DC. This characteristic has been adopted in some links that were planned to start operation in AC with the intention of conversion to DC at a later stage, in order to increase the power transmitted by the link. SCFF cables can be competitive for relatively short lengths and very high power links. Because of their very good thermal performances, SCFF cables have sometimes been used in conjunction with MI cables to overcome thermal hot spots, as done in the land portions of the ±400 kV submarine link between Italy and Greece. Due to the hydraulic circuit and the need to maintain the necessary oil pressure the SCFF cables have a route length limitation up to approximately 50 km.

In the last decade there has been an impressive worldwide increase in HVDC submarine cable projects, these projects are mostly based on MI and XLPE insulation, as shown in Figure 3.

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Figure 3 HVDC Cables in operation worldwide

In Table 2 are reported some of the most important DC submarine cable projects realized up to the time of writing.

Table 2 List of some of the world’s Major DC Submarine Cable Links

Name of the project Year Voltage

(kV) Power (MW)

Length (km)

Max. Water Depth (m)

Type

Gotland 1 1956 100 20 100 160 MI

Italy – Sardinia 1965 200 100 2 x 118 450 MI

Konti-Skan 1 1965 285 300 64 80 MI

Vancouver Is. 1 1969 300 156 3 x 27 200 MI

Skaggerak 1,2 1976 263 250 2 x 125 600 MI

Vancouver Is. 2 1976 300 185 2 x 35 200 MI

Hokkaido/Honshu 1980 250 150 2 x 42 290 SCFF

Gotland 2,3 1983 150 160 2 x 100 160 MI

Cross-Channel 2 1986 270 250 8 x 50 55 MI

Konti-Skan 2,3 1988 285 300 2 x 64 80 MI

Fenno-Skan 1989 400 500 200 117 MI

Cook Strait 2 1991 350 500 3 x 40 300 MI

Skagerrak 3 1993 350 500 125 500 MI

Cheju (Korea) 1993 180 150 2 x 96 160 MI

Baltic Cable 1994 450 600 250 60 MI

Sweden – Poland 1999 450 600 253 90 MI

KII Channel Japan 2000 500 2800 4 x 49 70 SCFF

Italy – Greece 2001 400 500 1 x 160 1000 MI

Moyle (UK) 2001 250 500 2 x 55 100 MI

Cross Sound (USA) 2002 150 330 2 x 42 40 XLPE

Bass Link (Aus) 2005 400 500 1 x 290 75 MI

Norway-Netherlands 2007 450 700 1 x 580 410 MI

Sardinia-Italy 2008

2010 500

500

500

1 x 420

1 x 420 1650 MI

Trans Bay S. Francisco 2009 200 400 2 x 83 30 XLPE

Borwin 2 2015 300 900 2x200 40 XLPE

Helwin 1 2015 250 576 2x135 40 XLPE

Sylwin 1 2015 320 864 2x205 40 XLPE

Helwin 2 2015 320 690 2x135 40 XLPE

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3. SPECIFIC FEATURES OF SUBMARINE CABLES

With respect to the land cables the submarine cables have some major differences and enhanced performances that can be summarized as follows:

o Submarine cables have to be manufactured in long continuous lengths by minimizing the number of joints, this request that the cable is to coiled or wound in large platforms both during manufacturing and installation.

o High level of reliability with practical absence of expected faults, the rate of internal failures of submarine cables if very low, as reported in [1].

o Robustness. In submarine cables mechanical aspects are at least as important as the electrical ones. Significant mechanical stresses occur during installation, and recovery (if any damage ever occurs)

o The armour. A single layer of round or flat wires is generally used for the installation in shallow water, for instance up to few hundred meters depth. A double layer of round or flat wires armour is generally used for deep water cables, up to more than a thousand meters. These two layers have to be applied in the reverse direction in order to make the cable suitably torque balanced to avoid uncontrolled twisting during installation. Recent developments of specially designed cables ensure the feasibility of submarine cables to be installed at depth of 3000 m and beyond, for HVDC transmission with power in the range of 1000 MW per bipole. Armour wires have to be resistant to corrosion and are therefore generally made of either with galvanized steel (three-core AC or DC cables) or with copper (single core AC cables). Round wires are cheaper, flat wires provide better coverage and cable compactness allowing longer shipping lengths.

o To simplify installation and handling procedures and in general to minimize the cable tension especially in deep water, it is necessary to keep the cable as light and compact as possible. Particular design have been used and in some cases aluminium conductors have been used

o Flexible joints, with no or minimal diameter variation, which are practically a reconstruction the original cable are fundamental for the submarine cable technology. These joints must be included in cable samples for mechanical and electrical type testing. Repair joints to be used on site in case of cable damages or variation of the cable routes are also very important and shall be included in the type testing program.

4. INSTALLATION OF SUBMARINE CABLES

Installation of submarine cables is a complex and critical operation, specific ships and skilled operators are necessary. The knowledge of environmental conditions is also fundamental for the success of the operations. Figure 4 shows the Giulio Verne cable laying ship that is one of the most advanced worldwide.

Figure 4 Submarine cables laying ship

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Typical characteristics for a cable ship to install deep water cables are the availability of a laying machine capable to withstand high pulling force and dynamic positioning system for navigation. The availability of a rotating platform is also generally required for the storage of cables designed for installation in deep waters. The Giulio Verne ship shown in the previous picture has a capstan able to withstand a braking force of 55 tons in dynamic conditions

As reported in [1] the major cause of failure of submarine cables is due to third parties mechanical damages, for this reason it is recommended to protect the cable especially when installed in relatively shallow water till to some hundreds of meters that are the depths that can be reached by the ship anchors and by the fishing operation, deep water cables are less subjected to external damages.

The cables are in general protected by the installation in trenches excavated in the sea bottom, different trenching methods and techniques are available depending on the nature of the sea soil.

Figure 5 Submarine cables protection tools

In case of presence of very hard soil or presence of other services where the trenching is not possible the cable has to be protected by the use of mattresses or rock dumping Particular aspects need to be addressed to the installation of cables in deep water applications that can be summarized as follows:

- Pulling forces, due to the long suspended weights of cable during installation and recovery - Bending under tension, on the installation vessel sheave - External water pressure

These factors will determine the actual cable design and vessel specifications. Figure 6 shows a plot with the maximum installation depth reached on different projects worldwide.

Figure 6 Maximum installation depth reached on different projects worldwide

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5. SOME EXISTING SUBMARINE LINKS IN THE MEDITERRANEAN SEA

5.1. SA.PE.I (Sardegna - PEnisola Italiana)

SA.PE.I link is a bi-polar system with ±500 kV rated voltage and a rated power of 1000 MW, which allows energy to flow in both directions between Sardinia and Italy Mainland. The link is based on Line Commutated Converters (LCC) and is conceived to operate with an emergency sea return in case of outage of one cable or converter pole. This system is in operation since 2008. The submarine portion is approximately 425 km long and develops for about 55% in very deep waters, up to the maximum depth of 1650 m, a depth never reached before by any high-voltage submarine cable. [4]

Submarine cable route

Submarine cable route profile

HVDC cable system main features

• Rated Power: 1000 MW

• Rated Voltage: 500 kV DC

• Submarine length: 2x425 km

• Maximum water depth: 1650 m (world record)

• Land length: 2x15 km

• Bipolar system, can be operated in monopolar

operation with sea return (electrodes)

• Submarine cables designs

5.2. GR.ITA. (GReece - ITAly)

GRITA is a ±400 kV mono-polar link with sea return, with 500 MW rated power which can flow in both directions. The link is based on LCC converters and is already conceived for a possible 1000 MW bipolar extension. GRITA submarine cables reached 1000 m water depth of 1000 m, which was the deepest one at that time for a high voltage submarine cable. [9]

Submarine cable route

Submarine cable route profile

HVDC cable system main features

• Rated voltage: ±400 kV DC

• Rated Power: 500 MW

• Scheme: monopolar configuration with sea return

• Submarine length: 160 km

• Maximum water depth: 1000 m

• Land length: 43 km

• Submarine cable design:

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5.3. Mallorca-Ibiza

The Mallorca-Ibiza link here described is the second circuit of the interconnection between the Balearic Islands of Mallorca and Ibiza. The project consists of a 132 kV AC cable system which is designed to transmit 118 MVA, The three core cable includes a fibre optic and its route is more than 123 km long (115 km submarine and 8.6 km land). This link will enable the integration of Ibiza with the Spanish Mainland network through the existing “Romulo” (Iberian Peninsula-Mallorca) HVDC cable system.

Submarine cable route

Submarine cable route profile

HVAC cable system main features

• Rated Voltage: 132 kV

• Rated power: 118 MVA

• Submarine length: 115 km

• Maximum water depth: 800 m

• Land length: 8 km

• Submarine cables designs

- XLPE 3x500mm2 Cu SWA (shallow water)

- XLPE 3x300mm2 Cu DWA (depth >100m)

5.4. Sorgente-Rizziconi

This 400 kV AC double circuit system, currently under construction, will strengthen the link between Sicily and Italy Mainland currently connected by a 400 kV AC 1000 MW link which was installed in 1984 by the same company, then known as Pirelli Cables [9]. It consists of Oil filled PPL insulated single-core submarine cables designed to transmit 2000 MW along approximately 38 km submarine route.[6]

Submarine cable route HVAC cable system main features

• Rated power: 2x1000 MW

• Rated Voltage: 400 kV

• Submarine section: 38 km

• Sicily land part: 2km

• Italy land part: 3km

• Submarine cable design

1500 mm2 Cu Oil filled-PPL insulated

• Land cable design: 2500 mm2 Cu XLPE insulated

800 m

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5.5. Lapseki – Sutluce

In the Dardanelles Strait, Turkey the project named Lapseki-Sutluce is in operation at the time of writing. It is a 400 kV AC double circuit (with one single core cable as spare for redundancy) designed to transmit 2x1000 MW. In addition n.2 fibre optic cables with n.48 fibres each have been laid. The construction of the cable system has been justified for the transmission to Istanbul, via the Western corridor (Trakya), of a large part of the power generated by the power plants located along the Southern coast of the Marmara Sea. The submarine XLPE insulated cables have 1600 mm2 copper conductor cross section designed to minimize power losses and are among the very few XLPE submarine systems at 400 kV AC installed and in service worldwide.

Submarine cable route

Submarine cable route profile

HVAC cable system main features

• Rated voltage: 400 kV

• Rated power: 2x1000 MVA

• Route length: 6 km

• Maximum water depth: 100 m

• Number of cables: 7 (2x3 phase system + 1

spare redundancy)

• Submarine cable design:

1600 mm2 Cu conductor XLPE insulated

6. CONCLUSION

After more than one century of application the submarine cables are today a mature technology that can offer a viable support to the implementation of high voltage power transmission systems. In the last years, submarine transmission cable technologies have benefited from the introduction of numerous innovative solutions, due to the strategically importance of the transmission system these innovations have been considered with extreme care and subjected to long and severe prequalification protocols. The major innovations on submarine cable systems and relevant standards are in the direction of increasing the performances, reliability, safety, availability, and feasibility of the projects. As reported by the CIGRE survey [1], a high intrinsic level of reliability has been reached for submarine connections. The same survey evidenced a certain rate of failures due to the external damages generally of mechanical nature and caused by third party activities. The precaution and rules adopted during the last years for the installation of submarine cables will likely reduce strongly these kinds of failures. It is expected that a considerable increase of DC submarine cable connections will happen in the next years that will contribute to develop a well meshed European and Mediterranean transmission system enabling an optimal deployment of renewable power resources. Recent technology developments ensure the feasibility of submarine cables to be installed at depth of 3000 m and beyond, for HVDC transmission of power in the range of 1000 MW per bipole. These developments will make possible the installation in the Mediterranean Sea of ultra-deep water high-power transmission cables.

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REFERENCES

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April 2009

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stable insulation system” Jicable Conference, Versaille 2015

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[4] R. Rendina, A. Gualano, M. R. Guarniere, G. Pazienza, E. Colombo, S. Malgarotti, F. Bocchi, A.

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CIGRE 2008 Session, Paris

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system electrical design studies” CIGRE 2010 Session, Paris

[6] L. Colla, F. Iliceto, M. Rebolini “400 kV AC new submarine cable links between Sicily and the

Italian mainland. Outline of project and special electrical studies” CIGRE 2008 Session, Paris

[7] L. Colla, M. Marelli, S. Lauria, M. Schembari, F. Palone, M. Rebolini “Mediterranean high

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Mondello 2013

[8] A.Orini, M.Marelli, E.Zaccone “Collegamenti in cavo HVDC stato dell’arte e prospettive future”

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K.G.Danielsson, H.Jonasson, A.Orini, R.Grampa “The Italy-Greece HVDC Link” CIGRE 2002

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[10] L. Rebuffat, G.M. Lanfranconi, F. Magnani, U. Arnaud “Installation of submarine power

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