Structure of DC cable
Sep 10, 2022
1. Conductive core
Copper wire is generally used as conductor core material, and its section is selected according to rated current, allowable voltage drop, short circuit capacity and other factors. When selecting the core wire structure, the problem of seawater penetration after failure shall be taken into account. Generally, water blocking measures such as condensation, welding, and coating of water sealing materials can be taken.
2. Insulation layer
The insulation layer thickness of DC cable shall meet four requirements at the same time: ① For the rated DC voltage, the field strength on the conductor surface shall be below the allowable value when there is no load; ② For the rated DC voltage, the field strength at the outer sheath shall be below the allowable value under full load; ③ Be able to withstand the impulse test voltage; ④ Under rated current, the temperature of conductor shall be below the allowable value. Generally, several insulation thicknesses can be assumed for calculation, and then the most appropriate insulation thickness can be selected.
3. Outer sheath
DC cable has no induced voltage on metal sheath and armor, so there is no sheath loss problem. The structure of the protective layer mainly considers the mechanical protection and corrosion prevention, especially for submarine cables.
(1) Metal sheath. In order to ensure the reliability and flexibility of the metal sheath, so far, DC cables have adopted lead sheath, the thickness of the lead sheath is generally 2.5~3.0 mm. For internal pressure oil filled or gas filled cables, it must also be corresponding to the pressure used, and several layers of reinforcing layers made of metal tape are added.
(2) Anti corrosion layer. During the normal operation of submarine cable, due to the leakage current and the current effect of seawater as the loop, the metal sheath and reinforcement layer will be subject to electrolytic corrosion. Generally, only a sheath made of plastic is added to prevent electric corrosion. However, once the plastic anti-corrosion layer is damaged, local rapid corrosion will occur. Therefore, some cables are coated with paper tape impregnated with asphalt insulation adhesive to form a light anti-corrosion layer, or asphalt insulation adhesive is applied between several layers of rubber tape. In recent years, most of them use extruded layers of polyethylene or neoprene as the anti-corrosion layer, because polyethylene or rubber has a large elastic modulus, which can partially absorb the mechanical stress acting on the lead package and make the stress distribution more reasonable. At the same time, it has good waterproof performance and forms a double waterproof seal together with the lead package. Moreover, its insulation performance is also good, and it is suitable for withstanding transient over-voltage of metal sheath.
(3) Armor. In order to prevent external mechanical damage, steel strip or steel wire armouring can be added outside the anti-corrosion layer according to specific conditions. Submarine cables are generally armored with steel wire. When submarine cables are laid or salvaged, due to the self weight of the cables, they are subject to great mechanical stress. At the same time, in a complex marine environment, the cables will be eroded by seawater, marine organisms, etc.
