Cross-Linked Polyethylene Materials In Medium And High Voltage Cables

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 As a substitute for paper-insulated cables, cross-linked polyethylene insulated (XLPE) cables have entered the field of power cables quite prevalent internationally. Paper-insulated cables once played a leading role in the power transmission stage. Therefore, after having an impact on its use in the medium-voltage cable category, the stage seems to be shifting to XLPE cables in the high-voltage and ultra-high-voltage range. Especially in the higher voltage field, the monopoly of paper-insulated cables. Such as oil-filled cables or inflatable or compressed cables are gradually disintegrating to coexist with cross-linked polyethylene cables. It has great advantages and economic reasons, such as higher allowable temperature levels, easy connection and maintenance technology, and the omission of equipment inherent in pressurized cables.

The insulation material of various types of XLPE cables is low-density (LD) polyethylene. PE is due to its excellent electrical and mechanical properties, light weight, low temperature flexibility, good moisture resistance, chemical resistance, ozone resistance and other characteristics, as well as relatively low prices. It has been widely used as cable insulation and sheath material for a long time.

However, low-density polyethylene has characteristics that limit its use as a cable insulation material. As a thermoplastic, its softening temperature is 105-II5°C. Another disadvantage is that stress cracking easily occurs when in contact with certain surfactants. Through a process similar to rubber vulcanization, PE molecules can be cross-linked, thereby greatly improving the thermal and mechanical properties of the material, while its electrical properties remain basically unchanged. Therefore, this cross-linked polyethylene (XLPE) product is no longer a thermoplastic. It has an elastic, rubber-like consistency, which remains unchanged even when the temperature rises further. The tendency of stress cracking disappeared completely, and the material also obtained very good anti-aging ability in hot air. PE is composed of long molecular chains, by cross-linking these chains to form a strong bond network, PE is converted into cross-linked polyethylene, XLPE.


Characteristics of XLPE

THERMAL PERFORMANCE: XLPE is a very heat-resistant material due to cross-linking. It cannot melt like polyethylene, but it will decompose and carbonize if exposed to temperatures above 300°C for a long time. Therefore, the allowable conductor temperature is set to 250°C when short-circuited for 1 second. Under continuous load, conductors insulated with XLPE may have a temperature of 90°C. These temperatures are specified by international standards. In emergencies and limited time, XLPE can withstand 130°C. Like PE, XLPE maintains its low-temperature flexibility at temperatures as low as -40°C, which means it has a huge advantage when laying cables.

MECHANICAL PROPERTIES: Polyethylene has good mechanical properties. Interestingly, PE can resist local stress better than PVC at room temperature. In this regard, XLPE has the same advantageous characteristics as PE, in certain grades. For example, filled XLPE insulation material is also more resistant to wear than polyethylene.

CHEMICAL PROPERTIES: Due to the cross-linking of molecules, XLPE is more resistant to most chemicals (such as common acids, alkalis and oils) than PE.

ENVIRONMENTAL POLLUTION AND CABLES: From an environmental point of view, PVC and oil-impregnated paper insulated cables have obvious shortcomings. When PVC cables burn, corrosive gases are released, and oil-filled cables that leak oil may cause serious damage to the environment. XLPE will burn, but the products of burning carbon dioxide and water will not cause damage. Filled XLPE for low-voltage cables can also be made to resist flame propagation, and the compound does not produce halogen.

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