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Anti-seawater Cable And Its Technical Difficulties
Jan 30, 2018

XLPE Insulated Cable (XLPE)

Cross-linked polyethylene insulated (XLPE) submarine cable developed in the 80 's, most of the 220kV and below voltage levels [1], its manufacturing and operating experience is far less than the oil-filled submarine cable. Up to the present, the highest voltage level of the XLPE AC submarine cable is the NEXANS (Ormenlange) company is located in the Norwegian Sea, a large gas field installed in the 2.2km long 420kv4 root single-core submarine cable. 500kV AC long-distance submarine cable, currently used only oil-filled cable.

Compared with the oil-filled cable, XLPE cable has the following advantages:

①xlpe cable is solid insulation, no complex oil-filled system, no need to detect oil level, control hydraulic pressure, low operating costs;

②xlpe Cable has no lead sheath, small bending radius, light quality, can be produced, laying longer length, and in the installation and transport than oil-filled cable simple;

The electrical and mechanical properties of ③XLPE submarine cables are superior to those of oil-filled cables. Because of this, XLPE insulated submarine cable development has a broader prospect, but there are many technical problems still need to be solved. The normal Xlpe cable in the DC voltage, the cable insulation in the space charge will be concentrated in place, resulting in the local field strength is too high to be penetrated. The use of additives in insulating materials can slow the accumulation of space charge in cable insulation, so that xlpe cables can be used for DC high-voltage power supply. 2002, the first extruded single core DC submarine cable (Light direct current cable, Swedish ABB), voltage ±150kv, length 40km, capacity 330MW, used to connect New York Long I. and Connecticut in the United States. This kind of direct current submarine cable uses 3 layers of polymeric materials to squeeze into a single polar cable, the internal and external shielding layer and insulation layer at the same time squeeze, with high strength, environmental protection and easy to bury the advantages, suitable for the deep sea and other harsh environment.

XLPE Insulated DC submarine cable current maximum voltage up to 320kV AC cable insulation equivalent capacitance increases with the length of the cable, in the energy transfer process, the equivalent capacitance and power supply between the constant charging discharge, the charging current can reach a maximum value and affect the normal active load transmission, So the AC submarine cable has a theoretical limit.

Transmission distance, a number of sea-crossing engineering shows that the distance is about 40km[3], over this distance, the use of AC transmission of electricity is not cost-effective. While the DC cable length is not limited by the charging current, no reactive compensation device, easy fabrication and installation, dielectric loss and conductor loss, has a good market prospects. But high voltage direct current submarine cable is also like space charge accumulation mechanism and restrain method, the insulation aging mechanism under DC voltage, the long-term stability of newly developed insulating material, the influence of partial discharge and so on have to be solved. "

General EHV AC submarine cables are Dancen, but because 3 heart AC submarine cable can save the cost of production and laying, so large section,

High voltage level of 3 Xlpe AC submarine cable is also gradually promoted. In 2008, Nike founded the world's first 3 XLPE insulated submarine cable with a voltage of up to 245kV. Polyethylene (PE) insulated cable and EPR (ethylene-propylene rubber) Insulated cable ethylene-propylene rubber cable and Xlpe cable (tgδ≤0.0005), dielectric loss tangent tgδ, and the dielectric constant ε are relatively large, but compared with polyethylene cable to prevent branch and partial discharge, generally only for medium-voltage submarine cables. To date, the highest grade ethylene-propylene rubber submarine cable was installed in 2001 in Venice, Italy-Murano-Mestre (venezia-murano-mestre) 150kV submarine cable.

Air-filled cable

Inflatable submarine cable in the structure and oil-filled cable is very similar to the use of prepreg tape to do insulation, and then filled with pressure nitrogen, with pressure gas filled with the gap between the paper tape, improve the breakdown voltage. The inflatable submarine cable can be used for AC/DC transmission, which is more suitable for the long submarine cable network than the oil-filled cable. However, because of the need for high pressure operation in deep water, the difficulty of designing cables and their accessories is increased, which is generally limited to a depth of 300m.

Technical problems related to submarine cable

Waterproof of submarine cable

When mechanical stress or external forces cause cable sheath and insulation damage, joint damage, moisture or moisture will be along the cable longitudinal and radial clearance immersion, reduce the electrical strength of insulation, so most high-voltage submarine cables have

There are longitudinal and radial waterproof measures to prevent water intrusion. The radial measures are mainly in the insulation shielding and metal shielding layer outside the wrapped half conductive water expansion belt, outside the metal shielding layer to add metal waterproof layer, that is, metal sheath, medium-voltage cable electric field strength is relatively low, the general use of aluminum-plastic composite sheath, there are only polymer sheath, high-voltage cable is the use of lead, aluminum, stainless steel metal seals. Polymer sheath has water resistance, but there is a certain rate of absorption, this is because its structure is mainly composed of crystalline and amorphous phase of the semi-crystalline polymer. The crystal phase structure is compact, the molecular arrangement in the amorphous phase is loose, and there is a large gap between the molecules. Under the action of alternating, polar water molecules are constantly flipped back and forth, which can penetrate into the insulating material through gaps and grain boundary defects. When using polymer sheath, the sheath should be added with water absorbent agent.

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