The DC transmission and distribution system usually runs on two wires (positive and negative), so the DC cables are generally single-core cables. Because the conductor of the DC cable has no skin effect and proximity effect, even if a large current is transported, there is no need to adopt a complicated split conductor structure. The insulation of DC cables can be the same as that of oil-impregnated paper insulated cables, solid extruded polymer cables, self-contained oil-filled cables, and gas-filled cables, and the armor layer is not affected by the conductor current, so the armor layer does not need to be considered. Loss and measures to reduce the resistivity of the armor layer.
The maximum field strength of an AC cable is always on the surface of the conductor close to the insulation layer, while the electric field strength of a DC cable is distributed in proportion to the resistivity of the insulation. The resistivity of the insulation changes with temperature. The maximum electric field strength is not only related to the applied voltage, but also to the load. When there is no load, the maximum electric field strength is on the surface of the cable conductor. When the load increases, the insulation resistance coefficient decreases due to the higher temperature near the conductor, and the electric field intensity on the conductor surface decreases, while the electric field intensity on the insulating surface gradually increases. Therefore, the maximum allowable load of the DC cable should not cause the electric field strength of the insulating surface to exceed its allowable value, that is, not only the maximum operating temperature of the cable must be considered, but also the temperature distribution of the insulating layer.
Another characteristic of DC cables is that the insulation must be able to withstand rapid polarity reversals. In the case of a load, the polarity conversion will cause an increase in the electric field strength in the insulation, usually by 50% to 70%.
