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Three-Core Cables of Submarine Power Cables

Modern medium-voltage submarine cables (52kV) are usually designed as three-core (3C)  cables with XLPE insulation. The cable cores are similar to those of land cables. Many different design alternatives are used for this class of submarine cables. Many cable constructive layers can be applied either to each single core, or to the three cores in common. Furthermore, many different screen/sheath configurations can be found.

Depending on the specifications submarine medium-voltage cables can be produced with or without a metallic sheath. The low electric stress in extruded medium-voltage cables allows for a cable design without total lock-out of water. An extruded plastic sheath provides a secure water barrier while water-absorbing agents inside the plastic sheath can take care of the minute amount of water vapour, which can diffuse through the extruded plastic sheath.

Another common system is the aluminium laminate for water protection combined with a copper wire screen. The aluminium laminate constitutes a radial water barrier, while the copper wire screen is designed to carry fault currents.

The non-metallic sheath of three-core submarine cables can be produced from semi-conducting polymeric materials. The capacitive currents of the three phases can balance each other, which can increase the cable ampacity. Many different combinations of sheath, screen, and corrosion protection are possible. Manufacturers have their own “recipes” to meet the very different conditions and requirements of submarine power links.

High-voltage cables are available in 3C design up to and including 170 kV. The first 245 kV 3C submarine cable has been installed in a moderate length of 7.8 km in Canada in 2008. This system voltage is possible for short cables without the requirement of flexible joints, which are not available yet for higher voltages. Almost all modern high-voltage 3C cables are produced with XLPE insulation.

Once the cable cores are manufactured and tested, they are laid-up to provide the flexibility of the cable. Beside standard S and Z lay-ups, there is also a S-Z-lay-up for MV cables, where the cable cores are twisted right-hand and left-hand in sequence. After lay-up, the diameter of the 3C cable over the core binder is 2.16 times the diameter of the individual cores. Four interstices are generated during the lay-up, three in the periphery and one central. To provide a stable circular base for the armoring, the outer interstices are normally filled with fillers. Ropes of different sizes or tailor-made extruded polymeric profiles are used to fill the space. Extruded profiles from PE or PVC provide an excellent support for the armoring wires and can accommodate small optical cables. PE requires more complicated extruding dies but lower material costs per unit length of the cable, compared to PVC. For very long submarine cables, the use of PE is more economic, and for short length, PVC is the cheaper alternative. Even lead profiles have been used as filler profiles, when the cable needs additional weight for an installation in streaming water.

Filler ropes can be made from recycled polymeric material of any kind. A combination of three ropes of two sizes can fill the interstices reasonably well.

The central space of the cable usually does not need a filler for stability. The three cable cores support each other so they do not collapse into the central space. It is not suitable to include an optical cable into the central space because it might be exposed to tensional forces as the central line is not the neutral line in cable bending. After lay-up, or sometimes in the same operation, the cable is fed into the armoring machine. In 3C a.c. cables, the magnetic fields from the conductor currents cancel each other to a large extent. Therefore, the armoring can be made from steel wires without excessive magnetic losses. 3C cables for high voltage can easily reach a diameter of 200 mm and more. As the number of spools in the armoring machine is limiting the number of armoring wires on the cable, sometimes very thick steel wires must be used to cover the entire circumference of the cable. For a cable with 200 mm diameter (under the armoring) and an armoring machine with a capacity for 84 wires, the combined steel cross section of a single armoring layer would be about 3700 mm2. This massive armoring would be sufficient for more than 300 m laying depth. Flat armoring wires can offer sufficient steel cross section for shallow waters and still cover the complete cable circumference. However, the protective effect against external aggression of armoring wires goes down with the square of the wire diameter. Many cable operators think that the better protection of round wires justify the increased material costs compared to flat wires.

3C cables have been produced also with oil-filled insulation for up to at least 170 kV. These cables are always equipped with a lead sheath, either a common lead sheath for all three cores, or individual lead sheathes for each core. In a common lead sheath, the interstices between the cable cores provide conduits for the oil transport to and from the “breathing” cable under the influence of load changes. An oil conduit in the centre of the conductor is not necessary. Filler ropes in the interstices give some support to the extruded lead sheath. However, a common lead sheath over three cable cores plus some filler ropes is not a very stable design. These cables are sensitive to fatigue, and external forces or movements can damage the lead sheath. Also, these cables can be manufactured only in a short length and require many joints for long installation lengths. The 150 kV Java-Madura 3C oil-filled cable (commissioned 1987) has a common lead sheath and was produced in short drum lengths. These were transported to a jointing facility at the port where the cable pieces were jointed and loaded on-board the vessel.

3C-cables with individual lead sheath (denominated “S.L.” for “single lead”) have been used since many decades ago as submarine cables. Especially 3C cables with extruded insulation are almost always made with individual lead sheaths.

The individual lead concept has some important advantages over the common-lead concept:

larger flexibility in the factory to joint individually sheathed cable cores

better stability of lead sheaths because they are smaller in diameter and have a circular core underneath

better stability during installation.

The advantage of a 3C cable with common lead sheath is the large oil channel provided by interstices in-between the individual cores. But for all practical applications these systems can be replaced by XLPE systems today without the need of oil channels or oil feeding.

Oil-filled submarine cables are still being produced for the EHV level – 500 kV and higher. Cables at this voltage levels are so massive that they cannot be produced as 3C cables.

As 3C oil-filled submarine cables are not produced anymore, this subject is not further elaborated here.
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