EuroWire May 2017

Technical article

Inspection and analysis of XLPE and HPTE material at medium- and high-voltage cable production By Harry Prunk, Sikora AG, Bremen, Germany

Abstract Plastics that are used for the insulation of medium- high- and extra-high- voltage cables have to comply with the highest purity standards. Most commonly XLPE (cross-linkable polyethylene) material is used for the insulation of cable. For cross-linking of the XLPE the cable passes through a CV tube (continuous vulcanisation tube) where it is exposed to high temperatures for the cross-linking. Typically at the manufacturing of these types of cables the CV tube is filled with nitrogen and operated at a pressure of 10 bars. Before the cable can be further processed, such as the application of a screen as well as the outer sheaths, a degassing is necessary, which may take up to ten days. Commonly, the line is built as a CCV or VCV line (Catenary Continuous Vulcanisation or Vertical Continuous Vulcanisation). The technology is well proven as it has been used for decades, but it involves high costs for the building and the production line. In addition, there is another method on the advance where high performance thermoplastic elastomer (HPTE), based on polypropylene, is used as insulation material. This new technology has been further invented by Prysmian and is published as the so called ‘P-Laser’ technology. [1] The material shows good electrical and thermo-mechanical properties, comparable and in some aspects even superior to those of XLPE.

XLPE and HPTE insulated medium- and high-voltage cable Today, insulation material that is mainly in use for MV and HV cables is cross-linked polyethylene (XLPE). Cross-linked polyethylene is produced from polyethylene under high pressure with organic peroxides as additives. Under heat and pressure the cross-linking takes place. The individual molecular chains are linked with one another, and this results in a material change from a thermoplastic to an elastic material. An advantage of XLPE as insulation for medium- and high-voltage cables is their low dielectric loss and their excellent electrical and physical properties for power transmission. As XLPE is resistant to thermal deformation and to ageing, an XLPE cable allows the carrying of large currents. Another feature is the easy installation of XLPE cable. It withstands small radius bending and is light in weight. [2] Some years ago the company Prysmian invented a new type of cable where instead of XLPE an HPTE (high performance elastomer) is used. This material has equivalent characteristics to XLPE. The difference is that there is no cross- linking and accordingly no by-products. HPTE is re-usable, allows shorter production time and reduced facility area, and is compatible with the existing network components.

The HPTE does not require a cross-linking section and no degassing treatments. That means that the complete cable can be manufactured in an in-line production process, where even the screening and the outer sheath could be applied in one production step. No matter which approach is applied, the demands for dielectric strength of XLPE and HPTE remains on the same high level and needs to be assured. Important in this regard is the cleanliness of the insulating material. This is the point where the advanced purity inspection and sorting system comes into focus. This paper will introduce an online inspection and sorting system that detects contamination inside and on the pellets such as metallic or organic contamination, colour variations, agglomerates, cross- contamination and foreign pellets. The system presented includes the unique combination of X-ray technology and an optical camera technology for highest detection probability for all sorts of contamination. Thus, the quality of XLPE or HPTE material and accordingly the production process are optimised and costs reduced. Furthermore, the paper will briefly refer to an innovative offline system, which inspects and analyses small batches of pellets. This is further of importance to check pellets that have been sorted out by the online inspection and sorting system.

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May 2017