Background Image
Table of Contents Table of Contents
Previous Page  48 / 84 Next Page
Information
Show Menu
Previous Page 48 / 84 Next Page
Page Background

Technical article

May 2017

46

www.read-eurowire.com

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.

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.

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.