Type of DC cable

Since the early power supply in the world was based on DC, DC cables were used as early as the early 20th century. But later, with the rapid development of AC transmission technology, AC cables have been widely used. DC cables have absorbed the mature experience of AC cables in their development, so they have many similarities with AC cables in structure. There are the following types of high-voltage DC cables currently in use.

1. Oil-impregnated paper solid cable

This kind of DC cable is the earliest and the most used. This kind of cable has simple structure, convenient manufacture and maintenance, and low 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. This cable is used in projects such as Gotland Island, the British-French Channel, Sardinia, Vancouver Island, and Kanti-Scandinavian. It is suitable for long-distance subsea laying, because it does not require oil supply, and the good cooling effect of sea water can avoid the loss of impregnating agent. This kind of cable is not suitable for laying with a large drop, because under this condition, the impregnating agent will move to the lower part of the cable, which will cause the insulation at high places to dry up and reduce the insulation strength.

2. Oil-filled cable

When the rated voltage exceeds 250kV, oil-filled cables are mostly used. When used on land, it has superior technical performance over other types of cables. In recent years, it can also be used as a submarine cable because the technical problem of long-distance oil supply has been solved. In foreign countries, this oil-filled cable has been used in DC projects such as Kingsnorth, Kanti-Scan, and Shikoku-Inchu.

3. Inflatable cable

Its medium is usually composed of high-density impregnated paper and then filled with compressed gas (such as nitrogen). It has high dielectric strength, and its working electric field strength can reach more than 25kV/mm. It is suitable for long-distance submarine cable laying and large drop. Laying, such as New Zealand's Cook Strait DC transmission project uses this kind of cable. However, because the compressed gas in the cable puts forward high requirements on the sealing and mechanical strength of the cable and its accessories, it is not widely used.

4. Extruded polyethylene cable

This kind of cable has a simple and sturdy structure and is 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. Due to certain advantages in some aspects, they have been widely used recently. focus on.