How can submarine cables be fully waterproofed?
Submarine cables are cables wrapped with insulating materials, mainly by laying cables on the seabed to complete the transmission of telecommunication. XLPE insulated submarine cables were developed in the 1980s, most of which are used for voltage levels of 220kV and below, and their manufacturing and operating experience is far inferior to that of oil-filled submarine cables. Up to now, the XLPE AC submarine cable with the highest voltage level is a 2.2km long 420kV 4 single-core submarine cable that NEXANS is installing for the large OrmenLange gas field in the Norwegian Sea.
XLPE insulated submarine cables have the following advantages:
1.XLPE insulated cables are solid insulation, no complicated oil filling system, oil level detection, oil pressure control, low operating costs;
2.XLPE insulation has no lead sheath, has a small bending radius and light weight, can be produced and laid longer, and is easier to lay, install and transport than oil-filled cables;
3. The electrical and mechanical properties of XLPE insulated submarine cables are also better than those of oil-filled cables. Under the action of DC voltage in ordinary XLPE cables, the space charge in the cable insulation will concentrate in a certain place, which will cause the local field strength to be too high and be broken down. The use of additives in insulating materials can slow down the accumulation of space charges in cable insulation, making XLPE cables suitable for DC high voltage power supply. The DC submarine cable is extruded into a unipolar cable with 3 layers of polymer materials, and the inner and outer shielding layers are extruded at the same time as the insulating layer. It has the advantages of high strength, environmental protection and easy burial, and is suitable for harsh environments such as deep sea.
The highest voltage of XLPE insulated DC submarine cables can reach 320kV. The equivalent capacitance in the AC cable insulation increases with the length of the cable. During the energy transmission process, the equivalent capacitance and the power supply are continuously charged and discharged. Its charging current can reach a maximum value and affect the transmission of normal active loads. Therefore, AC submarine cables have a theoretical limit transmission distance. Multiple cross-sea projects have shown that the distance is about 40km. Beyond this distance, AC transmission power is used. It is no longer economical. The length of the DC cable is not limited by the charging current, no reactive power compensation device is required, the manufacture and installation is simple, the loss is small, and the market prospect is good.
Generally, ultra-high-voltage AC submarine cables are single-core, but since 3-core AC submarine cables can save production and laying costs, 3-core XLPE AC submarine cables with large cross-sections and high voltage levels are gradually being promoted. the
Waterproof Technical Problems of Submarine Cables
When mechanical stress or external force causes cable sheath, insulation damage, and joint damage, moisture or water will penetrate along the longitudinal and radial gaps of the cable, reducing the electrical strength of the insulation. Therefore, most high-voltage submarine cables have longitudinal and radial waterproof measures to prevent moisture intrusion.
The radial water blocking measures are mainly to wrap the semiconductive resistance water expansion tape outside the insulating shield and the metal shielding layer, and add a metal waterproof layer, that is, a metal sheath, outside the metal shielding layer. The electric field strength of the medium voltage cable is relatively low, and aluminum There are plastic composite sheaths, and some only use polymer sheaths, and high-voltage cables use lead, aluminum, and stainless steel metal sealing sleeves. The polymer sheath is waterproof but has a certain degree of water absorption because its structure is mainly a semi-crystalline high polymer composed of a crystalline phase and an amorphous phase. The structure of the crystalline phase is compact, and the molecular arrangement in the amorphous phase is loose, and there are large gaps between the molecules. Under the action of an alternating electric field, the polar water molecules flip back and forth continuously, and can penetrate into the insulating material through gaps and grain boundary defects. When using a polymer sheath, a water blocking agent with water absorption should be added to the sheath.
Longitudinal water blocking mainly adopts ①compressed core; ②adds water blocking material between the wires and the shielding area of the cable core to block the diffusion channel of moisture in the cable core. Longitudinal water-blocking is filled with water-blocking powder. Its water absorption is dozens or even thousands of times that of itself. Water-blocking powder has a high expansion rate and good water absorption performance. After absorbing water, it can expand rapidly to form a gel-like substance that blocks water seepage , Terminate the further diffusion and extension of moisture and moisture, and minimize the length of damp cables.