Rubber-plastic insulated cables began to take the lead over oil-paper insulated cables in 1975. The proportion of paper-piezoelectric cables in rubber-plastic insulation in the United States has reached 99%, of which 95% are KV PE and XLPE, 2% are ethylene-propylene, and less than 1% are oil-paper insulated cables. German 1 kV plastic cable accounts for 90%, 10 kV XLPE for 5%, PE for 8%, PVC for 12%, oil paper for 72%, 25-35 kV XLPE for 34%, oil paper for 42%, oil paper for 20% in 1980s, XLPE for 56%. In Britain, PVC and XLPE account for 67% of 1 kV and below, and rose to 75% in the 1980s. In the late 1980s and early 1990s, XLPE insulated cables will slightly exceed oil-paper insulated cables, especially in new projects. Oil-paper insulated cables will be eliminated. In 20-30 kV, XLPE insulated cables plus other rubber-plastic cables account for 80% or more. In the field of high voltage cables, XLPE insulated cables will be eliminated. The cable has also reached the share of oil-paper insulated cable. Although XLPE insulated cables with 765 kV voltage can not compete with oil-filled cables at ultra-high voltage level, it is possible for XLPE insulated cables to catch up with or surpass oil-filled power cables in the near future from the manufacturing level of 550 kV XLPE insulated cables developed now, mainly because XLPE insulated cables have high performance. The operating temperature increases the current carrying capacity of the cable; XLPE insulated cable also has the advantages of smaller bending radius, light weight, no need for oil supply system, easy maintenance and installation.
In the manufacturing technology of heat-shrinkable pipes, the initial XLPE used steam as the pressure and heating medium of chemical reaction, so this method is called wet cross-linking. It is generally believed that XLPE insulation contains micron-sized micropore, and water vapor crosslinked by wet method is easy to penetrate into molten PE under high humidity and high pressure. Therefore, this method can increase the number of micropore and increase the size of micropore in XLPE. In the early 1970s, many countries introduced dry cross-linking, which reduced the microvoids and moisture in XLPE and improved the reliability of XLPE insulated cables. At the end of 1970s, XLPE manufacturing has made greater progress. In addition to improving the good physical and electrical properties of XLPE itself, new semi-conductive shielding materials and super-clean insulating materials have emerged, which further reduce the impurity content in insulators. Multilayer co-extrusion method has been introduced in the process, which reduces the interlayer interface and makes XLPE absolute. The partial discharge of edge cables is greatly reduced, which lays a foundation for the development of UHV cables.
In the field of semi-conductive shielding, Graphite-Coated tape was initially used in XLPE insulated cables to wrap around the insulation. This method is based on the interface problem, which makes the partial discharge of the cables very large. Once the water enters the cables, the water directly contacts with the insulation, which easily leads to water trees and electric trees. Therefore, it has been eliminated in the 1970s abroad. In China, this technology was phased out only in the 1980s. Later, the semi-conductive shield was extruded by three layers at the same time. XLPE was used as the material. Waterproof tree agent and anti-electronic transmitter were added to the material, which made the performance of the cable more excellent.
In terms of insulation manufacturing, since the 1990s, in order to reduce the problem of X LPE insulation retraction, the Finnish company's manufacturing stress relief device has been adopted, which improves the problem of cable retraction.