Conductive core
The conductive core material is generally copper wire, and its cross section is selected according to factors such as rated current, allowable voltage drop, and short-circuit capacity. When selecting the core wire structure, the problem of seawater penetration after a failure should be considered. Generally, water blocking measures such as pressure aggregation, welding, and water-coated sealing materials can be used.
Insulation
The thickness of the insulation layer of the DC cable should meet the four requirements at the same time: ①For the rated DC voltage, the field strength at the surface of the conductor should be below the allowable value when there is no load; ②For the rated DC voltage, the field strength at the outer sheath at full load It should be below the allowable value; ③ can withstand the impulse test voltage; ④ at the rated current, the temperature of the conductor should be below the allowable value. Generally, you can first assume several insulation thicknesses for calculation, and then select the most appropriate insulation layer thickness.
Outer sheath
Since there will be no induced voltage on the metal sheath and armor of the DC cable, there is no problem of sheath loss. The structure of the sheath mainly considers 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 sheaths. The thickness of the lead sheath is generally 2.5~3.0mm. For internal pressure oil-filled or gas-filled cables, the pressure must be Correspondingly, several reinforcing layers made of metal belts are added.
(2) Anti-corrosion layer. During the normal operation of the submarine cable, the metal sheath and reinforcement layer will be subjected to electrolytic corrosion due to the leakage current and the current of the seawater as a loop. Usually, just add a layer of plastic sheath to prevent galvanic corrosion. But once the plastic anticorrosion layer is damaged, local rapid corrosion will occur. Therefore, some cables use paper tape impregnated with asphalt insulating glue to form a light anti-corrosion layer, or apply asphalt insulating glue between several layers of rubber tape. In recent years, most of the extruded layers of polyethylene or neoprene are used as the anti-corrosion layer, because the elastic modulus of polyethylene or rubber is large, which can partially absorb the mechanical stress acting on the lead bag and make the stress distribution It's more reasonable. At the same time, it has good waterproof performance and forms a double waterproof seal together with the lead bag. And its insulation performance is also good, suitable for withstanding the transient overvoltage of the metal sheath.
(3) Armouring. In order to prevent external mechanical damage, steel tape or steel wire armor can be added to the outside of the anti-corrosion layer according to the specific situation. Submarine cables generally use steel wire armor. When laying or salvaging a submarine cable, the cable is subject to great mechanical stress due to its own weight. At the same time, in a complex marine environment, the cable will be eroded by sea water and marine organisms.
