Discussion of the link between 'normal' core structures and the inner screen features found in HV cables

1. UltraScreen TM
Core and Inner Screen Structures
June 2015
Author: Dr Gareth Humphreys-Jones
gareth.h-jones@acuityproducts.co.uk
www.acuityproducts.co.uk

2. Introduction to UltraScreen Operation and B Scan Display Format
Each of the following exemplar single channel B Scan displays represents about 15s of
production run with typical line speeds in the range 1–2m/min, So each display presents a
cable length of typically 25-50cm. Twin Channel displays present about half of that, so
typically 12.5-25cm per channel. Radially the system measures to an accuracy of ~10 micron
and on one of the displays radial cursor measurements are included to provide a sense of
scale. As presented in this note, the cable is running through the machine from the bottom to
the top of the displays.

3. Overview of Core and Inner Screen Structures
Fig 1 presents a sketch of a cable core showing two generic features:
1. The tape wrapped around the cable core – with the overlaps indicated by the blue lines on
the sketch.
2. The ‘valleys’ at the boundaries between the four sectors in the core – indicated by the red
lines in the sketch.
The large black arrow overlaid on the sketch shows an illustration of the path that an individual
transducer channel on UltraScreen would scan along as the cable passes through the machine on
the production line.
Fig 1 – Sketch of Cable Core Structures
Fig 2 presents these structures as seen in an UltraScreen ultrasonic image.
Fig 2 – Structures as seen in an Ultrasonic Image
On this figure, four different types of structure have been marked up:
A This periodic structure at the core /inner screen interface is the tape overlapping structure,
i.e. the green pattern in Fig 1, as the longitudinal periodicity seems to match well with the declared
tape widths. (Note: that when ‘Tape Ruckles’ are mentioned this refers to specific situations when
a mis-winding of this tape has caused a much larger, but very localised, disturbance at this
interface). From Acuity’s observations, the pattern above is the ‘normal’ pattern for this interface.

4. B This structure at the inner screen / insulation interface is also the ‘normal’ pattern for this
interface. And the matching of the periodicities of structures A and B, suggests that structure B is
caused as a ramification of structure A, i.e. perhaps that a variation in extrusion pressure caused
by ‘tape overlap’ structure A causes the extrusion perturbation, structure B.
C This structure at the core /inner screen interface that occurs at intervals throughout the run
is, as will be shown in this note, the valley structure, i.e. the red pattern on Fig 1. (Note that such
valleys appear more extended on an ultrasonic image than on an a ‘cable slice’ image due to the
fact that the ultrasonic image is obtained by scanning longitudinally over this structure at a
relatively acute ‘incident’ angle – between the red lines and the black arrow). Nevertheless, this
structure shows clearly that the ‘tape’ surface is clearly depressed below its normal level either
side of the valley and, as such, has fallen into the valley between the core sectors.
D This structure at the inner screen / insulation interface is what Acuity terms a Protrusion,
and/or a Fall-in, depending upon its exact structure, and is believed to be the ramification of
structure C (just as structure B is a ramification of structure A). For that reason it is also believed
to have been caused by, perhaps, a more significant variation in extrusion pressure resulting from
structure C – which is why structure D is more pronounced than structure B.
The time sequence of the occurrence of structures C and D is considered in the following section
to tie them definitively to the red line, valley structure shown in Fig 1.
‘Valley’ Structure Time Sequence
As already noted, the sequencing of the data displayed in Fig 2 is best understood by noting that
the display presents a section of time with the earliest time at the top of the display and the latest
time at the bottom of the display. Thus, moving down the display is the same as moving along the
cable. And as the individual UltraScreen channels are positioned 22.5o
apart, then the fact that a
feature is slowly winding around the cable will be seen from the fact that the same feature will be
seen in adjacent channels with a time delay between its appearance in the channels.
Fig 3 presents a composite B Scan re-presenting the UltraScreen Channel 13 image in the upper
screen, along with the Channel 14 image for the same time period, in the lower screen.
Fig 3 – Channels 14 & 13
Looking at the top of the lower screen, the same feature can be seen in Channel 14 as can be
seen a bit later on in the upper screen, Channel 13. Now the detail of the feature is not identical as

5. the Channel 14 image is taking a different longitudinal ‘slice’ through the feature than that taken in
Channel 13 and, as will be discussed later, the fact this feature itself is not constant ‘channel-to-
channel’ has important ramifications.
As the feature appeared first in Channel 14, and then in Channel 13, it would be logical to expect
that it would next appear in Channel 12. And this fact is demonstrated in Fig 4 which shows the
composite B Scan for Channels 13 & 12.
Fig 4 – Channels 13 & 12
And again in this figure the same feature can be seen at the top of the lower screen, Channel 13,
and a bit later on at the bottom of the upper screen, Channel 12. Logically then the feature would
next be expected to appear in Channel 11, then Channel 10, and then Channel 9, and so on.
Figs 5, 6 & 7 show this progression which is as far as this progression can be taken within this
individual file of data. And again the difference in the appearance of the feature across these five
channels should be noted.
Fig 5 – Channels 12 & 11

6. Fig 6 – Channels 11 & 10
Fig 7 – Channels 10 & 9
Time Sequence Summary
The schema below attempts to summarise the time sequence that has been presented in the
figures above.
From this it may be seen that the time delay between the appearance of the feature in successive,
adjacent channels on the B Scan image reflects the fact that the feature itself is winding around
the cable as the cable moves through the UltraScreen Scanning.
Also, as in this trial the cable had valleys at 90o
separation around the cable, it may be expected
that the adjacent valleys should also be seen 90o
away, i.e. four Channels away, from the valley
highlighted in black in the schema above, i.e. in a positions highlighted in red!
And this fact is confirmed in Fig 8 below, which shows the adjacent valley (and protrusions)
appearing in Channels 10 & 9 at the start of this data set, and in Fig 9 below, which shows the
other adjacent valley (and protrusions) appearing in Channels 14 & 13 at the end of this data set.

7. Time Sequence v Channel Number Schema
Fig 8 – Channels 10 & 9 at the start of the data set.

8. Fig 9 – Channels 14 & 13 at the end of the data set.
Observations
 What is clearly shown in this note is the link between features at the core, e.g. tape
overlaps and valleys between adjacent core sectors, and ‘instability’ features at the inner
screen / insulation interface.
 However, what is also clear is that the same core feature does not always produce the
same instability feature. Consider the inner screen / insulation interface towards the bottom
of Fig 2, where a small fall-in can be clearly seen in response to a core ‘tape overlap’
feature that looks no different to the others on the figure that have produced no such level
of response. Consider also that all of the protrusion / fall-in features presented in Figs 3 – 7
have been produced in response to the same valley in the core as it has wound its way
around the core. However, what is clear is the structure of the protrusion / fall-in features,
which occur in the different channels, is not constant, and that it can be seen from these
exemplars that there is a variation in the magnitude of the protrusion / fall-in produced by
different instances of the same valley.
 The observation that this suggests is that the link between core and ’instability’ features is
not ‘deterministic’, i.e. where the same demand always generates the same response. And,
perhaps, such a non-deterministic response is what should be expected to a perturbation of
what is essentially a ‘fluid’ screen extrusion process.
 And the non-deterministic, or ‘chaotic’, nature of this link is important because what it
implies is that if steps are taken to control link, e.g. to try and reduce such protrusions, then
it is likely to only be controllable in a ‘probabilistic’ manner, i.e. the probability of a large
protrusion arising can be reduced, but this should not be taken as implying that large
protrusions still can’t arise – as they can – it’s just less likely.
 Thus improving the quality of extrusion does not remove the need to monitor the cable for
the presence of such protrusion / fall-in features!
Note: None of the above, should not be interpreted as saying that the only cause of protrusion /
fall-in features at the inner screen are core structures, other extrusion effects, e.g. hang up, etc.,
can produce inner screen features, and it is also thought that ‘pips’ in the semicon material can
also produce smaller features.