TE STING
papers is attached under the upper
electrode and wetted by a
conductive electrolyte.
Tracking on Insulator Surfaces in Presence of
Conductive Defects within Housing to Core Interface
The electrolyte is made to flow
constantly between the electrodes
along the sample’s surface and its
rate is regulated depending on
specified test voltage (e.g. at 4.5 kV
the flow rate is 0.6 ml/min).
Duration of the test is limited to
maximum of 6 hours for cases where
tracking does not occur. There are a
number of different failure criteria in
the standards. Among these is a
maximum level of surface leakage
current (60 mA) or a specified
length of the tracking path (e.g. 2.5
mm depending on standard) which,
when reached, terminates the test.
This article is the third in a series looking at the development of certain types of ageing phenomena
that could arise in composite insulators in the presence of interfacial defects. The research so far has
concentrated on defects that might appear at the housing-core interface within large hollow core
composite insulators, either as a result of improper handling or through poor manufacturing practice.
In such cases, the cost involved in insulator replacement would probably be very high. However, in
regard to the physico-chemical processes involved, the considerations presented up to now apply as
well for all types of composite insulators, including those used on overhead lines.
An article published in INMR (Number 3, 2008) described past investigation of adhesion defects,
where the presence of partial discharges (PDs) and moisture ingress could initiate degradation
processes affecting long-term behaviour. These defects, for example, could grow in size over time. At
the same time, development of PD activity as well as the appearance of water and degradation byproducts could lead to a localized increase in the material’s conductivity. Such conductive spots can
also be introduced during manufacturing.
Independent of their origin, semi-conductive or conductive spots within interfacial areas in composite
insulators will locally distort electric field distribution and also enhance it. The aim of the project
described in this article was therefore to establish whether the presence of conductive defects at the
housing-core interface might adversely affect the external properties of insulators, especially the
resistance of their housing to tracking. This was to be assessed by testing resistance to surface
tracking on specially-prepared samples of silicone rubber (SIR) bonded onto fibre reinforced (FRP)
epoxy substrates containing metallic defects. Tracking performance of these samples was then
compared with the performance of identical samples without defects.
This work was overseen by INMR Columnist, Professor Stanislaw Gubanski and performed jointly by
Chalmers University of Technology in Gothenburg, the ABB Corporate Research centre in Västerås,
Sweden and the Instituto de Investigaciones Eléctricas (IIE) in Cuernavaca, Mexico. PhD student Johan
Andersson conducted the experiments while Dr. Henrik Hillborg of ABB assisted in the preparation of
test objects and Dr. Ruben Saldivar at IIE helped to organize the tracking tests. Schneider Electric’s
Memorial Fund to honor Erik Feuk (Sweden) provided additional financial support.
IIE is one of Mexico’s main research centers dedicated to the electrical and oil industries. In this
regard, it performs a range of standardized electrical tests on dielectric materials for different
polymeric insulation systems used in suspension insulators, surge arresters, bushings and
encapsulated components. The IIE also has a specialist group working on new polymer composites
and nano-composites.
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PREVIEW
Modern IPT arrangements are
equipped with data acquisition
systems (DAQ) and temperature
sensors to record both leakage
currents and surface temperature
changes. Moreover, the tests are
usually performed in a sealed and
ventilated chamber to control
humidity. The particular chamber used
in this study is shown in Figure 1.
Figure 1: Test chamber used in the study.
Background on Surface Tracking Tests
Leakage currents and arcing that
appear on insulator surfaces under
contaminated conditions can
degrade and decompose polymeric
housing materials. This process
might even develop into tracking,
which is the formation of conductive
carbonaceous paths. Once such
tracking starts, insulation properties
in the area around it are greatly
reduced and proper function of the
insulator is at risk.
The Inclined Plane Test or IPT
(described in IEC 60587 and ASTM
2303) specifies the experimental
conditions to test tracking resistance
of electrical insulation materials by
simulating severe surface
contamination. In this test, a
rectangular-shaped material sample
is mounted at an inclination of 45°
between specially shaped electrodes
separated by a distance of 5 cm and
energized from a high voltage
source. In addition, a layer of filter
TRACKING
ON INSULATOR
The DAQ system used in this study
allowed surface leakage currents to
be monitored continuously and also
the temperature of a selected small
spot on the sample to be measured
every second. Three samples could
be tested simultaneously.
Figure 2: View of test sample
containing conductive defect.
SURFACES
IN
PRESENCE
OF
To properly investigate tracking
behaviour in the presence of
conductive interfacial defects, the
test samples had to fully simulate
the real structure of a composite
insulator, containing both the core
and housing material. This was
achieved by joining rectangular
pieces of FRP epoxy substrate with
an ATH-filled HTV silicone rubber
housing of 6 mm thickness. The
interfacial defect (in the form of a
circular piece of copper foil of 12
mm diameter and 70 µm thickness)
was glued on the epoxy before
bonding with the HTV SIR.
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INMR ® 7 7
the defect edges (from ~ 2 kV/mm
to ~ 5 kV/mm).
The centre of the defect was placed
exactly in the middle between the
lower and upper electrodes (2.5 cm
above the lower electrode, as in
Figure 3).
Yet another interesting finding of the
simulations was that the presence of
the defect introduced an overall
increased electric field stress not
only localized in its vicinity but also
in the larger surrounding volume.
Figure 6 illustrates a field solution
inside the SIR layer for the model
including both the defect and the
restive electrolyte layer.
In this study, 9 test samples were
prepared with internal defects and 9
without defects. Both types were
then tested at 4.5 kV as shown in
Figure 4. The pyrometers seen in
Figure 1 were directed to measure
the surface temperature 2.5 cm
above the lower electrode, exactly
over the centre of the defects.
Figure 3: Sample under test that started to track.
Findings & Results
To explain how such conductive
defects distort local electric field
distribution, finite element
numerical simulations were
performed. Geometrical models were
thus obtained for different cases,
e.g. with and without the defect.
Solutions were also obtained for a
case when a conductive layer of
electrolyte was added on the
housing surface to imitate the
constant flow of the electrolyte
during the IPT.
Since the solution was obtained for
an applied voltage of 1 V, the
numerical value of 100 V/m in the
colour bar legend of Figure 6
corresponds to field strength of 0.45
kV/mm for 4.5 kV test voltage. The
electric field inside the silicone
rubber housing material between the
defect and the contamination layer
varied between 0.3-0.5 kV/mm
which was almost ten times higher
than for the reference model. This
field strength continued to also be
greater closer to the lower electrode
than in the reference model. Such
an increased electric field strength
could hypothetically have a strong
effect on initiation of surface arcing
and thereby could also accelerate
the degradation of the material.
Figure 5: Initiation of tracking process.
These solutions revealed that the
electric field distribution became
significantly distorted by the
presence of the defect when
comparing it to the reference case.
Even stronger field distortion
appeared for the model having both
the defect and the electrolyte layer.
The additional interaction between
the resistive layer on the sample
surface and the defect at the epoxySIR interface more than doubled the
value of local field strength around
Figure 4: Simulated electric
field intensity in the middle
of the SIR housing (i.e. 3 mm
above the defect and 3 mm
below the SIR surface) with
present contamination layer.
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The photo in Figure 7 illustrates a
sample when tracking has just
started close to the lower electrode.
While this tracking is located
initially on the surface, it penetrates
into the material over time until the
6 mm thick layer of silicone become
damaged and the epoxy-SIR
interface is exposed. In total, 5
reference samples and 6 samples
with defects started to track within
6 hours of testing (see photos in
Figures 8 and 9).
Figure 6: Appearance of tracking on samples without defects.
TRACKING
ON INSULATOR
SURFACES
IN
PRESENCE
OF
On 5 of the 6 tracked samples with
defects and on 4 of the 5 tracked
reference samples, the tracking
paths were longer than 2.5 cm.
When the samples from these
Figures were compared to each
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INMR ® 7 9
other, there were clear differences in
the appearance of the tracking
paths. Instead of track development
along the line between the
electrodes, as on the reference
samples, the tracking direction
made a turn around the inserted
copper foil on the defective samples.
This changed tracking direction
could also be followed when the
samples were inspected from below,
where heat released during arcing
darkened the epoxy substrate, as
illustrated in Figure 10.
Visual inspection of the inserted
defects after tracking revealed that
the copper foils remained relatively
intact since most of the tracking
occurred around them. The heat also
separated the epoxy and silicone
materials from each other, close to
the tracking paths. The reason
behind the changed direction of
tracking could be related to the
different electric field distribution
close to the defect.
Figure 7: Appearance of tracking on samples with defects
(defect position indicated).
On average, the time to track the
distance of 2.5 cm from the lower
electrode appeared earlier on the
samples with defects than on the
reference samples, i.e. it decreased
by 93 minutes, from 316 min
(reference samples) to 223 min
(samples with defect). Details are
presented in Table 1.
TABLE 1: COMPARISON OF TIME TO
TRACK FOR TESTED SAMPLES
Reference
Time to track 316 ± 55 min
Discussion & Conclusions
The results of this study effectively
demonstrate that the tracking
behaviour of an insulator’s housing
material can be seriously affected by
the presence of interfacial
conductive defects.
Figure 9: Variation of temperature and current during IPT on a
reference sample.
A change in tracking direction was
not the only difference found
between the two types of samples
tested. When analyzing the data
recorded by the pyrometers (which
measure surface temperature at the
spots 2.5 cm above the lower
electrodes), a significant increase in
temperatures was registered. Figure
11 presents typical current and
temperature graphs for both types of
samples.
Figure 8: Appearance of back side of tracked samples indicating
visible changes on surface of epoxy substrates.
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Before the tracking started, the
surface temperatures and leakage
currents remained quite stable, at ~
60-80°C and ~ 5 mA respectively.
But after reaching a tracking length
of 2.5 cm, the local temperature
jumped to ~ 700°C and most likely
the temperature at the centre of the
arc reached several thousand
degrees. The current increased
steadily as tracking proceeded and,
at the same time as tracking
reached the target areas for the
pyrometers, rose to ~ 10-20 mA.
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PREVIEW
With defect
223 ± 44 min
Figure 10: Temperature and current graphs of a sample with defect.
TRACKING
ON INSULATOR
SURFACES
IN
PRESENCE
OF
The simulations carried out as part
of the research generated results
which reveal that the presence of a
conductive defect at the housingcore interface in combination with a
layer of surface contamination lead
to increased electric field strength in
the vicinity of the defective
interface. This then increases its
detrimental effect on the resistance
of the material to tracking by
reducing the time necessary for its
development.
The question still to be investigated
is the exact mechanism behind such
observed behaviour. One possible
explanation is that an enhanced
localized electric field provides
favourable conditions for
concentrating the partial arcing at
one spot on the housing and also
maintaining this for a longer period
of time. In any case, the results of
this work clearly indicate that any
conductive defects at the housingcore interface will adversely affect
performance of a composite
insulator. ⌧
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