Oil-impregnated paper solid cable: This type of DC cable is the earliest and most used. This type of cable has a simple structure, is easy to manufacture and maintain, and is low in price, but its working electric field strength can only reach about 25kV/mm. This limitation determines that the voltage of this cable can only be manufactured to 250~300kV. It is suitable for long-distance seabed laying because it does not require oil supply, and the good cooling effect of seawater can avoid the loss of impregnant. This kind of cable is not suitable for large-drop laying, because when operating under this condition, the impregnant will move to the lower part of the cable, which will cause the insulation at high places to dry out and reduce the insulation strength.
Oil-filled cables: When the rated voltage exceeds 250kV, oil-filled cables are mostly used. When it is used on land, it has superior technical performance than other types of cables. In recent years, it can also be used as a submarine cable because it has solved the technical problem of long-distance fuel supply.
Inflatable cable: Its medium is usually composed of high-density impregnated paper and then filled with compressed gas (such as nitrogen). It has high insulation strength and its working electric field strength can reach 25kV/mm or more. It is suitable for long-distance submarine cable laying And large drop laying, such as the Cook Strait DC transmission project in New Zealand used this kind of cable. However, because the compressed gas in the cable puts high requirements on the sealing and mechanical strength of the cable and its accessories, it has not been widely adopted.
Extruded polyethylene cable: This kind of cable has a simple and sturdy structure. It is more suitable for use as a submarine cable, but according to its DC withstand voltage capability, the working voltage can only reach about 250kV. At present, most of the DC cables actually used are glue-impregnated solid cables and oil-filled cables, but there is no metal sheath, only steel wire armored polyethylene cables.
