WCA July 2017

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Wire & Cable ASIA – July/August 2017
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 a HPTE (high
performance elastomer) is used. This material has
equivalent characteristics to XLPE.
Inspection and analysis
of XLPE and HPTE
material at medium-
and high-voltage cable
production
By Harry Prunk, Sikora AG, Bremen, Germany