Anti-Saltwater Cables and Technical Difficulties

500kV AC long-distance submarine cable, the only oil-filled cable currently used.

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

XLPE cable is a solid insulation, without complicated oil-filled system, do not need to detect the oil level, control the hydraulic pressure, low operating costs.

XLPE cable without lead sheath, bending radius is small, light weight, production, laying longer, and in the laying of the installation and transportation than the oil-filled cable is simple.

XLPE submarine cable electrical performance and mechanical properties are also better than oil-filled cable. Because of this.

XLPE insulated submarine cable has a broader prospect of development, but there are many technical problems still need to be resolved. 

Ordinary cross-linked polyethylene cable under DC voltage, the space charge in the cable insulation will be concentrated somewhere, resulting in the local field strength is too high to be punctured. 

The use of additives in insulating materials can slow the accumulation of charge in the space of the cable insulation so that the XLPE cable can be used for DC high voltage power supply. 

In 2002, the first extruded single heart DC submarine cable (light DC cable, ABB, Sweden) with a voltage of ± 150 kV, a length of 40 km and a capacity of 330 MW was used to connect Long Island, New York to Connecticut in the United States. 

The DC submarine cable is extruded with 3 layers of polymeric material into unipolar cables. 

The inner and outer shielding layers and insulating layers are extruded at the same time, which has the advantages of high strength, environmental protection and easy burying, and is suitable for harsh environments such as the deep sea. 

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XLPE insulated direct current submarine cable is up to a maximum voltage of 320kV AC cable insulation capacitance increases with the length of the cable increases in the energy transfer process.

The equivalent capacitor and power supply kept charging and discharging, the charging current Can reach the maximum and affect the transmission of normal active load, so AC submarine cable has a theoretical limit.

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Transmission distance, multiple cross-sea projects show that the distance is about 40 km [3], beyond this distance, the use of AC transmission of electricity is not economical. 

The DC cable length from the charging current limit, without reactive power compensation device, manufacturing and installation is easy, dielectric loss and conductor loss, has a good market prospects. 

However, there are still many problems to be solved in the HVDC submarine cable, such as the mechanism of space charge accumulation and its suppression.

The mechanism of insulation aging under DC voltage, the long-term stability of newly developed insulating materials and the influence of partial discharge.

General EHV AC submarine cables are single heart, but because of the three-core AC submarine cable can save the cost of production and laying, so the large cross-section.

High-voltage 3-core XLPE AC submarine cable is also gradually promoted. 

In 2008, Nexans laid the world's first 3-core XLPE insulated submarine cable to 245 kV in Canada. 

Polyethylene (PE) insulated cables and EPR (ethylene-propylene rubber) insulated cables Ethylene propylene rubber cables XLPE cables (tgδ ≤ 0.0005) compared with the dielectric loss tangent tgδ.

And the dielectric constant ε are relatively large, but with polyethylene Compared to the cable to prevent branch and partial discharge, generally only for medium voltage submarine cable. 

Up to now, the highest grade of EPR submarine cable is the 150kV submarine cable installed in Venezia-Murano-Mestre, Italy in 2001.

Inflatable Cable

The inflatable submarine cable is structurally similar to the oil filled cable and is also insulated with pre-impregnated paper tape, filled with pressurized nitrogen gas, and the pressurized gas fills the gap between the paper tapes and increases the breakdown voltage . 

Inflatable submarine cable can be used for AC and DC power transmission, it is more suitable for oil-filled cable in the longer submarine cable network. However, the use of high pressure operation in deep water increases the difficulty of designing cables and their accessories, which are generally limited to water depths of up to 300 m.

Submarine cable related technical problems

Submarine cable waterproof

When mechanical stress or external forces cause cable sheathing and insulation damage, when the connector is damaged, moisture or moisture can dip in the longitudinal and radial gaps of the cable, reducing the electrical strength of the insulation. 

Therefore, most high-voltage subsea cables have.

Longitudinal, radial waterproofing to prevent water intrusion. 

Radial measures are mainly in the insulation shield and metal shielding around the outer semi-conductive water blocking tape expansion.

The metal shield to add a metal waterproof layer that is metal sheath, medium voltage cable electric field strength is relatively low.

The general use of aluminum-plastic composite Sheaths, also have only polymer sheath, high-voltage cables are used lead, aluminum, stainless steel metal gland. 

The polymer jacket is water-repellent but has a certain degree of water absorption since its structure is mainly a semi-crystalline polymer consisting of a crystalline phase and an amorphous phase. 

The crystal phase structure is compact, the molecules in the amorphous phase are loosely arranged, and there is a large gap between the molecules. 

In the role of alternating electric field, the polar water molecules continue to flip back and forth, through gaps and grain boundary defects penetrate into the insulating material. 

When using polymer sheath, the sheath should add water blocking agent.

Longitudinal water blocking mainly adoptscompression type thread center.

Add water-blocking material between the wires and the cable core shielding area to block the diffusion channel of moisture in the cable core. 

Longitudinal water blocking with water-proof powder filling effect is good.

It's water absorption of several times its own times or even thousands of times, water absorption intensity, high expansion rate, after the rapid expansion of water to form a gel-like material, block water seepage channels, stop the water And the further proliferation of moisture and extension, to minimize the length of damp cables.