EuroWire July 2015

32 / 80

EuroWire – January 2012
Flaw density in check
Technologynews
July 2015
When extruding highly expensive
submarine length (XLPE) MV/HV power
cables, it is important for good run
economics to continuously be aware of
the status of extrusion quality monitoring
(EQM) systems, including the level of
contaminants, protrusions and voids
(CPV) which affect post-production
electrical stress testing.
A lot of work has already been under-
taken into keeping process cleanliness
factors high in an attempt to ensure that
no contaminants find their way into the
cable, or are formed during the molten
process.
This helps to keep the process running
continuously for as long as possible
without stopping to clean down and
eliminate any build-up of excess product.
But how do they know when to finally
stop the extrusion before problems
build up?
At present, experiential factors are used
as there are very few on-line methods to
ensure the internal quality of the cable
over its entire length of production,
since it is, of course, designed to be a
continuous process lasting several days
or weeks – you simply cannot cut into the
cable to find out how good it is currently!
So a non-contact ‘scanning’ method
of monitoring the cable as it is being
produced is required, and rapidly
scanning ultrasonics is the only solution,
according to Acuity Products.
The UltraScreen system has feature
detectors that automatically look for
geometric anomalies at the inner and
outer semicon interfaces with the insu-
lation layer (typically linked to features
like protrusions and fall-ins) and for the
presence of unexpected artefacts within
the insulation layer (typically linked to
features like contaminants and voids).
These features are automatically detected
when their level of geometry anomaly
and/or presence within the insulation
layer exceeds certain predefined
threshold values. But in looking for
positive indication of a stopping point,
the cable maker does not need to know
where each of the defects lie, just to
know if their density has increased.
Acuity Products has included flaw density
measures in the UltraScreen system with
an associated alarm threshold.
From recent assessment of several long
production runs in which the UltraScreen
system has been employed, Acuity has
established that there are small changes
in the layers of the extrusion and their
form which give rise to new indicators
about the current effectiveness of
production.
Using a combination of time and
frequency domain analysis, the data
sets analysed support a proposition that
the quality of the extrusion of the inner
screen changes over the length of the run.
These gradual changes can be considered
as degradation as they increase the
measured variability.
Acuity Products – UK
Website:
▲ ▲ The UltraScreen system from Acuity Products
30