Repsol YPF

Power Cables

Unlike telecommunication cables, power cables have special material requirements that make up their structure as they must resist high temperatures. This factor requires specific technology for the manufacture of these cables that is based on the crosslinking process.

Crosslinking

Crosslinking transforms a thermoplastic into a thermally stable product as a result of the bridges between its polymer chains. The crosslinking process may be carried out by chemical or physical means.

Physical process: Crosslinking by irradiation

Crosslinking by irradiation is not very often used in the cable industry since it requires the use of an electron accelerator or a radioactive source. It is an expensive process and its handling is very inconvenient. The use of these plants is common in the U.S., but not in Europe. 

Chemical process: Chemical crosslinking

The chemical crosslinking process is widespread in the cable industry. There are several types of processes depending on the crosslinking agent used. Basically, crosslinking of an energy cable is done with peroxides or silanes.

Peroxides:

Crosslinking with organic peroxides is based on the liberation of free radicals, which in turn generate radicals in the polymer chains to finally produce the C-C interchain bond. Crosslinking in the industrial process is done once the cable has been manufactured. A crosslinking tube is incorporated into the extrusion lines, and the cable is subjected to high temperature to produce the aforementioned phenomenon. Mainly steam and N2 lines are used.

Silanes:

Silane crosslinking is based on the interchained bond through the siloxane bridges Si-O-Si. Unlike crosslinking with peroxides, this process does not depend on temperature, but it depends on H2O presence. During cable manufacturing, silane crosslinking can be done by either a one- or a two-stage system. The two-stage system consists on two extrusions, the first in which the silane is grafted into the polymer and a second phase in which the catalyst is added and the cable is finally extruded. The one-stage system is a simplification of the previous method and consists of the incorporation of both the silane and the catalyst in a single step or extrusion. The formation of the siloxane bridge in both processes takes place once the cable has been extruded in the presence of water.

The design of an energy cable, depends mainly on the working voltage. There are three different types of cable:

• Low-voltage cable
• Medium-voltage cable
• High-voltage cable