EuroWire May 2017

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Technical article
May 2017 46
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.