1
CHAPTER 1
INTRODUCTION
1.1 OVERVIEW
Power transmission at high voltages has gained a considerable
importance recently. Glass and ceramics have been preferred for the
manufacturing of insulators, bushings, cable terminations and surge arrestors
for many years. Presently, polymeric insulators are increasingly used both in
the distribution and transmission systems and steadily capture a wider share
of the market because of their better dielectric properties, low weight, easy
handling, vandal resistance, and cost effectiveness (Gorur et al 1999). With
the advancement in power transmission capability, it has become more
important to design and develop compact, cost effective and reliable
insulating structures. Outdoor insulation is simultaneously subjected to
various stresses such as electrical, thermal and mechanical stresses which
cause degradation or aging of the insulating material. The electrical and
mechanical strengths of the insulator become diminishing and it results in
material deteriorations. A great problem yet to be overcome is the tracking
and erosion of outdoor polymer insulators. Tracking is a peculiar
phenomenon which occurs on the surface of the insulator due to the creepage
distance resulting from surface contamination. The researchers all over the
world are making an effort to reduce the tracking and erosion effects in
outdoor insulators through their research works.
2
After tracking occurs, the insulation property of the material is
totally lost and there are no ways to improve the insulation property. The
tracking phenomenon is investigated worldwide to improve the reliability and
performance of insulating materials thereby solving the problems of surface
degradation and adverse effects of tracking and erosion of the materials. The
micro sized inorganic fillers are incorporated into the polymer materials so as
to reduce the above mentioned problems and the results have been discussed
by Meyer et al (2004a). The selection of appropriate filler is one of the most
important aspects in the formulation of silicone composite and is based on the
filler properties such as particle size, surface area, thermal and electrical
conductivity. El-Hag et al (2006) reported that the micro filler with 30-65%
filler content are required to get the desired electrical insulation properties for
outdoor applications.
The recent increase in demand for high quality outdoor insulating
materials tends to focus on polymer nano-composite materials. Nano
composites are in the range of nanometers in size, different with three orders
of magnitude in length compared to micro composites. This would mean a
difference of approximately nine orders in their number density. Therefore,
the distance between the neighboring fillers are much smaller in nano
composites than in micro composites. In terms of specific surface area, nano
composites have high specific surface area of fillers (about three orders larger
than micro composites). With this, the interaction of polymer’s matrices with
fillers is expected to be much more in nano composites and this will enhance
the electrical and mechanical properties of the insulating materials. Research
findings on nano-sized particle filled dielectrics show similar property
improvements for considerably reduced filler additions compared to a higher
amount of micro-sized fillers.
3
Considering this fact, in the present work, due emphasis is given to
analyse and understand the tracking and erosion resistances of the micro and
nano sized alumina (Al2O3), nano sized silica (SiO2) and nano sized
aluminium hydroxide Al(OH)3)filled silicone rubber materials by conducting
experiments according to (International Electro Technical Commission)
IEC –60587 under AC and DC Voltage with ammonium chloride as a
contaminant. The characteristics variations in the fundamental and harmonic
components of leakage current signals of the filled silicone rubber of both
micro and nano sized alumina composite materials are investigated
thoroughly. The effect of leakage current in initial stage, dry band formation,
surface discharge and during severe is also analysed in micro and nano Al2O3
filled silicone rubber (SIR). Moving average fundamental component of the
initial and final stage of leakage current (LC) signals of the filled silicone
rubber of nano sized silica is also investigated. To understand the electrical
performance of the polymeric insulating materials under certain
environmentally polluted conditions, the artificially aged nano sized Al(OH)3
and SiO2 filled silicone rubber have been analysed. The Thermo
Gravimetry- Derivative Thermo Gravimetric (TG-DTG), Scanning Electron
Microscope (SEM) with Energy Dispersive X-ray analysis (EDAX) and
Fourier Transform Infra-red Spectroscopy (FTIR) were used to characterise
the thermal, physical and chemical properties of the silicone rubber
specimens.
1.2 OVERHEAD LINE INSULATORS AND MATERIALS
There are two main types of insulator materials: Inorganic materials
and polymeric materials. The inorganic materials consist of ceramic and glass,
whereas the polymeric material consists of composite insulators and resin
insulators. These materials act as the dielectric of the insulator, when it is
attached to the terminal or end fitting. Ceramic or porcelain is a very stable
4
material and thus very immune to degradation through electrical discharge
activities, ultra violet (UV) radiation and other environmental factors. The
basic components used to make the ceramic insulator are clay, fine sand,
quartz and feldspar. Alumina and cristobalite are usually added as filler.
Glazing is used to smooth the insulator surface, to improve hydrophobicity,
and also to increase the mechanical strength. The cement that is used to attach
the ceramic insulator shed to the metal cap and pin is believed to cause
electrical puncture on the ceramic insulator in some cases (Gorur et al 1999).
Ceramic is a brittle material, and cracks and breaks are common
problems. However, it is not easily shattered. The glass insulators consist of a
mixture of silica, limestone, dolomite, feldspar and soda ash. They are made
by annealed glass. It is used for high voltages, which require heavier
conductors and higher strength insulators. The problem with glass is that it is
a brittle material and easily fractures due to mechanical stress on the surface.
Ceramics are first used and then followed by glass. Ceramics and glass have
similar general shapes. The higher the voltages, the bigger are the number of
units attached. It means that the insulators are heavier for higher line voltages.
This has made ceramics and glass insulators difficult to use with the increase
in system voltage requiring longer insulators. Considering the polymeric
insulators, it consists of composite and resin insulators. The composite
insulators are composed from more than one insulator material whereas the
resin insulators are made only one insulator material. Resin insulators are
made from various types of heavily filled polymeric resins, including
polyester, polyurethane and cycloaliphatic. The fillers are added in order to
improve the tracking and erosion resistance of the polymers, since their
formulations include a fair amount of carbon. However, the high amounts of
filler make it more difficult to cast the insulator shape.
5
The interest in very high voltage power transmissions has
encouraged the industry to produce a lightweight insulator with better
electrical and mechanical properties compared to ceramic and glass. General
Electric introduced the non-ceramic non-glass insulator called composite
insulator. Composite literally means something which is made of different
parts. This type of insulator is called composite because it consists of three
parts: (i) a core made of glass fiber, (ii) external weather sheds made of
polymer, and (iii) metal fittings made of aluminium for attachment. Their
designs are also significantly different from ceramic and glass types. The
physical description, service environmental factors affecting and advantages
of composite insulators (polymeric) are clearly discussed in the following
sections.
1.2.1 Composite Insulator Development History
The composite insulators were first developed in the 1960s and
installed in the 1970s. The initial designs of the composite insulators had a
rod coated with silicone rubber and shed that were separately mounted on
them. These designs failed in short periods while they were put into the
service. It was because of the slipping of sheds from rod or opening damage
of rod shed interface. It was then followed by designs improved by mounting
shed on the rod directly and then encapsulating it with silicone rubber.
However, these designs also failed shortly. It was found that the mechanical
failures were the main cause of the failures. With the advancement of
modeling and fabrication techniques, new designs were introduced that have a
rod covered with silicone rubber having sheds in it’s own mould as one unit.
This whole outer portion was called sheath. This modern structure proved to
be very successful and is implemented till today. Also the composite
insulators have not yet attained the same level of experience or
6
standardization as glass or porcelain, and their weaknesses were still being
discovered.
1.2.2 Physical Description of Composite Insulators
Figure 1.1 shows the physical description of composite insulator.
Composite insulators have three main components namely, a fiberglass core,
end fitting hardware connected to the core, and a protective housing with
weather sheds for the core. The fiberglass rod is the most important part of a
composite insulator because it is the mechanical load bearing part. The
hardware allows the insulator to be attached to an overhead tower and to an
energized conductor. The end attachment hardware is typically forged steel,
ductile cast iron, malleable iron or aluminum and is selected for mechanical
strength. The core by itself is inappropriate for use outdoors, since moisture
and voltage cause electrical tracking on the core and result in mechanical
failure. Therefore, housing is mounded onto the core, protecting the core from
electrical tracking and the design of the housing provides the electrical
strength under wet and polluted conditions. The materials for housing (sheds
and sheath) are required to have excellent aging resistance under multiple
environmental stresses. The possible materials of the weather sheds include
epoxy resins, Ethylene-Propylene Diene Monomer (EPDM), Ethylene-
Propylene Rubber (EPR) and Silicone Rubber (SIR). The SIR is preferred due
to its characteristic hydrophobicity and its good performance in polluted
environments.
7
Figure 1.1 Cross Section Diagram of Composite Insulators
1.2.3 Common Failure Modes of Insulating Materials
The insulating materials of various types of insulators are subjected
to failure when they are used in outdoor insulation applications. Some of the
modes are discussed below.
1.2.3.1 Mechanical failure
Porcelain and Glass insulators are mostly affected in this type of
failure due to shattering. Shattering is caused by impurities gathered up during
the manufacturing process. These impurities cause stress concentrations
which can result in the insulator shattering suddenly. The shattering or
breaking of the insulators also takes place due to abrupt temperature changes
or uneven heating due to power arcs.
1.2.3.2 Thermal runway
The electrical conduction through glass insulators is mainly
accomplished by ionic conduction. The ionic migration within the material
can be aggravated by applying the voltage, depending on the resistivity and
temperature of the insulating materials. The temperature of the insulating
materials are increased when the electrical current flowing through them and
8
the resistivity of the materials are decreased. This decreased resistivity causes
the increased electrical current flow and temperature. If this process can
repeat itself, until the thermal capacity of the material reached, will lead to the
failure of the insulating materials.
1.2.3.3 Electrical punctures
These types of failures mostly take place due to the steep front
electrical pulses, which are caused by the external processes of lightning
strikes or switching rather than the normal system operating voltage. The
porcelain and resin type insulators are mostly affected in this type of failures.
1.3 SERVICE ENVIRONMENTAL OF POLYMERIC
INSULATORS
The polymeric insulators are very effectively used in outdoor
services. The outdoor service environment consists of moisture in the form of
rain, fog, dew, direct spray, pollutants from the sea and roads that are salted
during winter months in cold climates, and chemicals from industry. The
polymeric insulators are very much suitable to be used in the above
environmental conditions. These insulators have also been used in coastal
areas, industrial areas, desert areas and in regions with a high incidence of
vandalism. The reduced number of metallic parts in this kind of insulator
makes it suitable for saline areas and the corrosion of the hardware is a
significant issue.
1.3.1 Advantages of Polymeric Insulators
The polymeric insulators have many advantages when compared
with the conventional insulators. Some of the important advantages are listed
below:
9
90% weight reduction
Reduced breakage
Lower installation costs
Improved resistance to vandalism
Improved handling of shock loads
Improved power frequency insulation
Improved contamination performance
Figure 1.2 shows the exact reasons for the polymeric insulators
used as outdoor insulators when compared with conventional insulators. It is
clear that the polymeric insulators are the right choice to be used in the
outdoor insulators. Eventhough the polymeric insulators have many
advantages, the housing material of the insulators are subjected to ultraviolet
radiation, temperature extremes, overvoltage due to switching and lightning
surges, and mechanical loads due to heavy wind flow and ice coating. In order
to overcome the above effects, the silicone rubber (SIR) is used as an
insulating material of the polymeric insulators. The reason for the SIR
material used in the polymeric insulators is discussed in Section 1.5.
10
Figure 1.2 Reasons for the Polymeric Insulator used for Outdoor
Insulators (Elizabeth da Silva, University of Manchester
2008)
1.4 FACTORS AFFECTING THE POLYMERIC INSULATORS
Factors affecting the polymeric insulators are categorized by
mechanical factors, environmental conditions, power system design and
operation. The mechanical factor includes polymer base quality, core quality,
and end fitting gap attachment method and damage during installation. The
environmental condition includes Ultraviolet radiation (UV), wind, rain, fog,
ozone, temperature, pressure, humidity, and organic or inorganic pollution.
Power system design and operation includes electric field stress and leakage
current. Due to the above factors, the polymeric insulator surface can be
degraded. The degradation of insulator is called aging. Due to the above
factors, the electric field stress and environmental pollution become very
important factors to be investigated thoroughly.
11
1.4.1 Factors Affecting Polymeric Insulator Material Aging
Aging of insulators describe the deterioration of the insulator
material over time or it is the effect produced on it in field after a specified
period of service. Aging of insulators is mainly concerned with deterioration
of outer sheath/shed. Deterioration of insulator material is concerned with the
breakdown of macromolecules causing reduction in molecular weight. This
slow break down of the material is induced by the external factors. It starts at
the insulator surface and then proceeds deeper into the material. This aging
decreases the electrical and mechanical performance of the insulator. Outdoor
weathering is a natural phenomenon which ages the insulator materials to
some extent. The following are some of the major factors responsible for the
aging of insulating materials:
1.4.1.1 Pollution
It is a broad term used for any particle that may accumulate on
the surface of the insulator. There are two types of pollution
Pre- deposited pollution
It accumulates over a long period of time. It can be either
active, which means that it can form a conducting electrolyte
when wetted. Sources for this type of pollution include salt,
chemical water products, bird droppings and many more.
Instantaneous pollution
It occurs when the insulator surface becomes covered in a
highly conductive pollutant. Sources are salt fog, acid fog, or
bird streams.
12
1.4.1.2 Humidity and rain
Humidity is one of the pollutions to form the conducting electrolyte
on the insulator surface. Once it occurs, leakage currents can flow along the
length of the insulators, leading to dry band formation and electrical leakage
current discharges take place. This can damage the insulating materials of the
insulators. The humidity is normally drawn from the surrounding air or from
the rain and fog. Rain is not the only a source of humidity. The acid rain can
increase the pollution layer’s conductivity and cause the tracking and erosion
of the surface of the insulator material. Erosion or degradation is irreversible
and the non-conduction of the surface of the insulator occurs by a major loss
of material (that is more than 1 mm). It significantly reduces the thickness of
the polymer sheath that ingresses to the core rod. It is slower from material
loss and normally does not lead to failure unless it is so severe that it reaches
up to the rod. Tracking or carbonizing is caused by leakage current activity.
This is an irreversible deterioration by the formation of paths starting and
developing on the surface of insulators. These tracks have the appearance of
carbon tracks which cannot be easily removed and are conductive even under
dry conditions. Figure 1.3 shows the tracking and erosion effects that have
taken place on the polymeric insulator surface.
Figure 1.3 Tracking and Erosion Effects on the Polymeric Insulator
Surface
13
1.4.1.3 Solar radiation
It is the one of the major factors responsible for degradation of
insulator materials. The energy from the sunlight destructive to insulator
materials are between 320 and 270 nm. This destructive energy constitutes
less than five percent of the total radiation reaching the surface of the planet.
The absorption of this UV radiation results in mechanical and chemical
degradation in the insulating materials of the insulators. The rate at which the
degradation occurs depends on the intensity and wavelength of the radiation.
These factors vary with season and elevation. The effects of solar radiation
include: chalking, crazing or cracking on the surface. Also the UV-B photons
can cause in some degradation insulator materials. Chalking is the appearance
of a rough and whitish powdery surface giving the insulator a chalky
appearance. The factors which are responsible for chalking are UV radiation.
When a small quantity of rubber is removed from a surface because of these
factors, the filler material is exposed on the surface. This filler material is a
white powdery substance, giving the insulator a chalky appearance. It allows
more accumulation of water and contamination on the surface of the insulator
which leads to degradation of the surface. Crazing is the appearance of
shallow cracks on the insulator surface. Depth of these micro fractures is less
than 0.1 mm. The reason is UV radiation and electrical stress. Figure 1.4
illustrates the chalking and crazing effects on the polymeric insulator surface.
14
Figure 1.4 Chalking and Crazing Effects on the Silicone Rubber
Insulator Surface
1.4.1.4 Bird droppings and streams
Nitrogen content is affluent in bird droppings. In the presence of
water, this may lead to the formation of nitric acids which can damage the
insulator surface material. Moreover, it is highly conductive in nature and it
increases the leakage current discharges. Bird streams, which are long strings
of bird excrement with high conductivity can lead to immediate flashover.
1.4.1.5 Damage through vandalism and animals
Eventhough these factors do not actively cause aging, the damage
caused by animals and /or vandalism can create areas on the surface of the
insulator which deteriorates at a faster rate than the undamaged material and
thus promote aging of the insulator. Birds, rodents and termites are usually
responsible for animal damage to silicone rubber insulators. Figure 1.5 shows
15
the sources of damage and damaged silicone rubber insulators. The bird
attacks eat part of the insulator and render it useless.
Figure 1.5 Sources of Damage and Damaged Polymeric Insulator
1.5 HOUSING MATERIAL FOR POLYMERIC INSULATORS
The behavior of a polymeric insulator is closely linked with the
properties of the housing material, as its lifetime depends mainly on the
persistency of the housing. As suggested by Ansorge et al (2010), the most
important property of the housing material is its resistance against tracking
and erosion followed by the hydrophobicity. In the world wide development
of polymeric insulators, a lot of housing materials are tested, such as epoxy
resin, ethylene propylene rubber (EPR) and silicone rubber. However, it
turned out quite soon that for mechanical reasons the housing must be rubber-
like. So the most common material to be used nowadays for polymeric
insulators is silicone rubber. Silicone rubber is not a unique material, but
consists basically of a base polymer, inorganic fillers and a cross linking
agent. Additionally to their mechanical reinforcing function, fillers that
improve the tracking and erosion property might be added. The erosion of the
material in the tracking and erosion test was mainly due to the thermal impact
of dry band arcing (Kumagai and Yoshimura 2003a). Figure 1.6 shows the
benefits of silicone rubber while used as housing material in polymeric
insulators for outdoor insulation applications.
16
Figure 1.6 Benefits of Silicone Rubber for Outdoor Insulations
SIR is the only housing material able to transfer its water-repellent
property to a pollution layer on the surface. Therefore, leakage currents are
suppressed and the risk of flashover is reduced. Moreover, polymeric
insulators with silicone rubber housing do not require cleaning. It has proven
its outstanding suitability for outdoor applications for more than 30 years
even under severe environmental conditions.
1.6 SILICONE RUBBER CLASSIFICATION FOR HIGH
VOLTAGE INSULATION
The ASTM D1418-05 standard denotes the main classes of silicone
rubbers. The Q class represents the silicon and oxygen in the polymer chain.
In MQ class, M preceding Q indicates that methyl is one of the substituent
groups on the polymer chain. In VMQ class, MQ is preceded by V when vinyl
side groups are present. Type VMQ is commonly used for insulators. The
most common base polymer for the housing in outdoor insulation is VMQ,
composed of organic methyl groups (-CH3), vinyl groups ( CH=CH2), and a
17
linear silicon-oxygen backbone. Various fillers such as fumed silica, alumina
and aluminium hydroxide are added to the base material and mixed, thus
forming a compound for injection molding of insulator housings.
Figure 1.7 Structure of Silicone Rubber Molecule
Silicone rubber consists of a polymer fused together with a filler
material by a process called vulcanization. Silicone and oxygen are the
backbone of the polymer, which is bonded in an alternating pattern to form
either long molecular chains. The length of the chain and the organic groups
attached to the silicon atoms will decide the viscosity of the compound.
Figure1.7 shows the structure of a typical silicone rubber molecule. It
illustrates a silicone–oxygen (Si-O) backbone with two methyl groups (CH3)
attached to the silicone. The use of silicone as a base molecule (in conjunction
with oxygen) offers many advantages. The advantages are: 1.The silicone–
oxygen bond is very strong, which offers thermal stability for the final
molecule over a wide range of temperatures. Also it offers resistance to
weathering, corona discharge, and oxidation by ozone. 2. The resistance to
natural attack, good dielectric properties, fire resistance and it being a
relatively harmless substance.
18
1.7 CHEMICAL REACTIONS OF SILICONE RUBBER
MATERIALS DURING DRY BAND ARCING
The discharges induce changes in the insulation material through
chemical reactions occurring at the locations of the electrical discharges. The
breaking or dissociation of the bond between the molecules requires sufficient
amount of energy. Accordingly, such processes usually occur in the insulation
material at the location of the dry band arcing, due to frequency of high
temperature observed areas. However, heat can also be transmitted along the
surface and through the bulk of the material to regions adjacent to the
locations of electrical discharge activity. This results in a gradual heating of
these areas until the accumulated heat energy is large enough to cause gradual
chemical changes in these regions as well.
The chemical reaction taking place in the insulating material fully
depends on the relative bond strengths of the molecules presents in the
insulating materials. Bond dissociation energy is one of the measures of the
bond strength which is the energy required to break the bonds for one mole of
a given molecule in its gaseous form. When the heat energy is being supplied
to the insulating material, those molecules showing the lowest bond strength
are separated first. The Si-O bond has a high bond energy, which offers the
high thermal resistance of the silicone rubber materials. The Si-C and H-C
bonds show weaker bond strengths. The chemical changes in silicone rubber
thus usually occur in the methyl groups rather than in the silicone backbone.
The types of elements present in the insulating material and the surrounding
condition decide the type of chemical reaction that takes place. The chemical
changes in a silicone rubber is generally caused by the following three
processes
1. Scission and interchange of either bonds or chains.
2. Hydrolysis of siloxane bonds and hydrocarbon groups
19
3. Oxidation of hydrocarbon groups and crosslinking of siloxane
bonds
In the scission process, the heat energy liberated by the arcing
process causes scissions in either the methyl groups or even silicone backbone
and it creates free radicals of form O, CH3, or Si with the dot designating
the atom as a free radical. These free radicals are molecular fragments with a
reactive nature, due to the free radicals one or more unpaired electrons in the
outer electron cell. Because of their relative nature, they usually exist only in
a transitional state before combining with other molecules to form new
substances. Figure 1.8 shows the formation of the free radical from the
silicone rubber structure.
Figure 1.8 Scission of Silicone Rubber Chains and Formation of Free
Radicals(Heger 2009)
After the formation of the free radical, interchange reactions can
occur. This means that two neighbored chains combine after having been
broken into smaller chains, with the shorter chains fusing with their
counterparts in the neighboring chain according to the type of radicals
formed. This process is shown in Figure 1.9
long chain backbone
heat fromdry band arcing
CH3
SiO
CH3
O Si
CH3
CH3
O
heat fromdry band arcing
O Si
CH3
CH3
O Si
CH3
OO + CH3
SiO
CH3
CH3
+ O
CH3
Si
CH3
O
20
Figure 1.9 Interchanging Reactions between Two Split Polymer
Chains(Heger 2009)
The hydrolysis processes usually take place in the presence of
water (H2O), like rain or fog, deposited on the material surfaces. If such a
condition exists, hydrolysis of the silicone rubber structure will occur due to
the water being split into –OH and –H groups through electrical discharge
processes. Hydrolysis reaction can be responsible for a larger mass loss due to
random scission occurring in the polymer chains. The –OH and –H groups
formed during the disassociation of water combine with the free radicals in
the scission process to form chains with silanol (SiOH) as side and end groups
with possible liberation of methane gas (CH4) depending on the free radicals
present. Figure 1.10 shows typical hydrolysis reactions in the presence of
water, indicating the formation of gaseous (CH4) compound.
Figure 1.10 Hydrolytic Reactions in the Presence of Water(Heger 2009)
Hydrolytic scission
heat fromdry band
arcing
heat fromdry band
arcing
CH3
SO
OH
S
CH3
CH3
OO + CH4
O
CH3
SO
CH3
S
CH3
CH3
OH + OH
+ H2O
CH3
S OO S
CH3
CH3
O
CH3
SO
CH3
+ H2O+ S
CH3
CH3
OO
CH3
S OO
CH3
+ S
CH3
CH3
O
CH3
SO
CH3
+ S
CH3
CH3
OO
heat fromdry band arcing
CH3
SO
CH3
S
CH3
CH3
O
CH3
SO
CH3
S
CH3
CH3
O
OO
after chain scission Short interchanging polymerbackbones
21
Finally, the oxidation process follows the hydrolysis of the siloxanebonds and hydrocarbon groups. The crosslinking process can link two parallelpolymer chains, create a branching from one polymer chain to several othersor form several shorter polymer chains. All these different processes arepresented in Figure 1.11. However, the hydrolysis reactions do not occur onmaterials in well ventilated surroundings due to the low water vapor contentin the atmosphere at the material’s surface. Instead, further crosslinkingreactions occur between the silicone rubber chains through oxidation of themethyl groups, which cause the material to become brittle if exposed toextended periods of high temperatures. Due to the dry band arcing process,the chemical changes on the silicone rubber surface take place when thesurface temperature is raised to a level higher than 400 ºC. Since tracking anderosion in silicone rubber materials have only been observed to occur if thehotspots of a temperature above are 400 ºC.
Figure 1.11 Possible Forms of Crosslinking between Polymer
Chains(Heger 2009)
CH3
Si OO
OH
Si
CH3
OH
O
CH3
SiO
CH3
Si
CH3
CH3
OOH + OH
+ 2H2O
CH3
SiO Si
CH3
O
SiO
CH3
Si
CH3
O
CH3
OO
CH3
CH3CH3
CH3
Si OO
OH
Si
CH3
OH
O
OH
SiO
CH3
Si
OH3
CH3
OO
heat fromdry band
arcing
formation of short chains
CH3
Si OH + OHO
CH3
Si
CH3
CH3
O
CH3
SiO
CH3
Si
CH3
CH3
OOH + OH
heat fromdry band
arcing
heat fromdry band
arcing
CH3
SiO Si
CH3
O
SiO
CH3
Si
CH3
O
branch formation
OO + 2H2O
O
OH+OH
Crosslinking betweenpolymer chains
CH3
SiO Si
CH3
O
SiO
CH3
Si
CH3
OO
O
OO + H2O
22
1.8 ROLE OF FILLERS IN SILICONE RUBBER INSULATION
Inorganic fillers are essential in the formulation of SIR outdoor
insulation. Their inclusion improves tracking and erosion degradation
resistance. The desirable and undesirable are the types of effects due to the
fillers in silicone rubber. Some of the desirable effects of fillers are as
follows: a) Improved thermal conductivity of the compound, so that the heat
dissipation is considerably improving in the composites. b) Reduced organic
material exposure to heat from dry band arcing, thus decreasing the weight
loss of the compound subsequent to aging. Considering the undesirable effect,
the fillers act as a “diffusion barrier” for the Low Molecular Weight (LMW)
fluid and slow down the recovery process. If the filler content is increased
then the recovery process will be faster. Thus the quantity and type of filler to
be included in the formulation is a critical task. The micro fillers are used to
improve the physical properties of silicone compositions through molecular
bonding with the silicone polymer. The micro silica filled SIR has been
studied extensively in outdoor insulation applications.
1.9 INTRODUCTION TO NANO FILLERS
The SIR as the base material with the addition of micro fillers is
used in the manufacturing of outdoor insulation. Many researchers have been
done in relation to the use of micro fillers in SIR. The fillers used in these
materials are micro-sized, with a particle size of 5-10 µm. Currently, the
industries are using 30% to 65%, by weight (hereafter referred to as t % wt),
of micro fillers to achieve the required electrical properties for outdoor
insulation applications. The nano fillers of nano technology have been
introduced to enhance the electrical and mechanical properties of the
insulating materials. The great advantage of the nano sized fillers is that they
have larger specific surface area compared to that of the micro sized fillers.
The particle sizes between 1 and 100 nm are used in the field of electrical
23
insulating materials (Tanaka 2005). The use of nano particles in the matrix of
polymeric material can improve the mechanical and electrical properties of
polymeric composites. The various studies have been reported that the
performance of nano and micro sized particle filled silicone rubber and these
studies are discussed in detail the literature review section.
1.10 LITERATURE REVIEW
Gorur et al (1992) had presented experimentally, the aging
produced by dry band arcing in silicone rubber material. Aging indicated by
permanent changes in the silicone rubber surfaces was recognized using
analytical techniques of FTIR, EDX, X-ray Diffraction and surface roughness
measurement. In this work, they have reported that the permanent changes
occurring in the material lead to progressive degradation in the long run even
though there can be a complete recovery of surface hydrophobicity in short
time. Guoxiang Xu et al (1996) had presented the climatic aging experiments
conducted on silicone rubber and ethylene propylene diene monomer (EPDM)
outdoor polymer insulators by using a programmable weather-ometer. The
accelerated aging stresses comprised of ultraviolet radiations, elevated
temperature, temperature cycling, thermal shock and high humidity. The
effects of aging on the insulator surface conditions and electrical performance
were examined through visual inspection and SEM studies, contact angle
measurements, TGA and EDAX analysis. It was found that a significant
degradation on the EPDM insulator surface was caused by aging. The work
also reported that the silicone rubber have superior resistance to climatic
aging stresses over the EPDM. Gorur et al (1997) had conducted the
laboratory test to evaluate and compare the tracking and erosion resistance of
High Voltage (HV) outdoor polymeric insulating materials. The test was
based on combining some features of the ASTM D-2132 Dust and Fog test,
and ASTM D-2303 Inclined Plane test. The experimental conditions
24
employed could be related to actual field conditions and hence the results
obtained provided a more realistic evaluation of materials’ tracking and
erosion performance. The developed test method was capable of ranking
materials in a timely manner, and also demonstrates the wide variations in the
tracking and erosion resistance among different material families and
formulations within the same family. It was found that the magnitude and
harmonic content of the leakage current and discharge duration were
significantly different during the portion of the test when there was no visible
degradation, compared to their values at the onset of visible degradation.
Torbjorn et al (1998) had presented two silicone rubber based
polymeric materials for outdoor insulation under field conditions for about
nine years and also evaluated them in the laboratory. The field test was
conducted in an environment of light coastal pollution and the materials were
non -energized as well as energized with both HVAC and HVDC. The long
term study focused on the hydrophobicity and the material changes of the
insulator surfaces combined with leakage current performance of the
insulators. The results showed that the surface aging of the two types of
silicone rubbers was relatively moderate but the chemical changes appeared to
be greater in the HVDC exposed insulators. The laboratory studies revealed
that the different insulators had dissimilar leakage current under clean fog
conditions and this indicates that the hydrophobicity was not a good indicator
of the surface resistance of the insulator material. Homma et al (1998) had
used the thermo gravimetric analysis to evaluate the surface degradation of
polymer insulators used in outdoor high voltage applications. The TGA
measurements of silicone samples with various amount of ATH filler was
performed with two different atmospheres. It was found that the
decomposition of siloxane matrix was affected by oxygen and promoted
chemical reactions on the surface.
25
Xinsheng Wang and Noboru Yoshimura (1999) had studied the
tracking resistance of high temperature vulcanized silicone rubber under
various types of precipitation. It was evaluated by the incline-plane method,
using artificial rain water as a contaminant. The influences of ion
concentration, acidity, and conductivity of rainwater on the energized silicone
rubber material were studied. Experimental results were reported that the
degradation of the energized materials increased with ion concentration of the
rainwater, the discharge current and increase in erosion of material. The
conductive ions in rainwater could induce more tracking and discharge on the
energized material surface, and acidic rain water could accelerate the
electrolyte process in the materials and dissolve the inorganic filler in silicone
rubber. Although the silicone rubber degrades under the process of
accelerated aging using artificial rainwater, the concentration, acidity and
conductivity of actual precipitation was insufficient to exert a significant
degradation effect on silicone rubber. It was found that the silicone rubber can
resist the erosion of even the most corrosive water and does not fail and in
addition, has a strong resistance to various types of precipitation.
Moreno and Gorur (1999) had experimentally investigated the
performance of several polymer outdoor insulator formulations under
alternating current (AC) and direct current (DC) stresses. The performance of
the polymers was evaluated in terms of their charge accumulation and decay
characteristics as a function of humidity, and their tracking and erosion
resistance. They obtained results which showed that the significant
differences exist in surface charging phenomena between AC and DC. It was
also proved that the DC polarization processes led to hydrophobicity loss
when the materials were subjected to pollution and DC electrical stresses. In
addition, it was found that the substantial reduction in the tracking and
erosion resistance of the polymeric materials with DC stresses compared to
alternation current (AC) stresses. Furthermore, the poorer electrical
26
performance of the materials was found due to higher magnitudes and longer
durations of the dc voltage discharge current. Kumagai and Yoshimura (1999)
had studied the single and multiple effects of UV, corona, thermal, water
absorption and acid rain stresses on the tracking and erosion characteristics of
room temperature vulcanized silicone rubber (RTV). The test results showed
that corona stress and water absorption stress had decreased the tracking and
erosion resistance of RTV, while thermal stress and UV stress improved it.
Also, the RTV subjected to simultaneous multiple stresses was evaluated and
the obtained results showed significant variations in the tracking and erosion
resistance. The chemical and morphological analysis for assessing the aging
level was carried out by SEM-EDX, ATR-FTIR and Differential Scanning
Calorimetry (DSC). It was found that the DSC played an important role to
detect boiling and combustion temperatures of byproducts affecting tracking
and erosion.
Li Xuguang et al (2000) had investigated the influence of alumina
trihydrate (ATH) and magnesium hydroxide on the tracking and erosion
resistance of silicon rubber. The research showed that the tracking and erosion
resistance of silicon rubber was improved with the increase of the filler
concentration of ATH and magnesium hydroxide, but adding excessive filler
had decreased the performance of silicon rubber. The experiment results also
showed that the tracking and erosion resistance of compound hydroxide filler
was superior to single hydroxide filler. It was found that the effect of
hydroxide filler level played an important role on vulcanization characteristics
of silicon rubber. Also, the experimentation indicated that lack or over
vulcanization that decreased the tracking and erosion resistance capability of
silicon rubber. Sarathi and Uma Maheshwar Rao (2001) explored the
tracking phenomena study with ethylene propylene diene monomer (EPDM)
material under AC and DC. It was reported that the tracking time depended on
the conductivity and flow rate of the contaminant. They also carried out the
27
physico-chemical analyses using the wide angle X-ray diffraction (WAXD),
thermo-gravimetric differential thermal analysis (TG-DTA) and the
differential scanning calorimetry (DSC) studies. Finally, they confirmed that
the tracking process was a surface degradation process and the tracking time
was different for AC. and DC voltages. Sarathi et al (2002) explored the
tracking phenomenon for silicone rubber material under AC and DC voltages
with ammonium chloride as the contaminant. They made a detailed discussion
on the tracking phenomenon with leakage current measurement. The
physicochemical analysis was also carried out using the thermo gravimetric
differential thermal analysis and differential scanning calorimetry. They
concluded that the tracing was due to surface degradation and the tracking
time was different for AC and DC voltages. The tracking time depended on
the conductivity and flow rate of the contaminant further reported. Meyer et al
(2002) explored the work of thermal behavior of silicone rubber filled with
alumina trihydrate (ATH) and silica at different filler concentrations. Three
particle sizes of 1.5µm, 5µm, and 10µm were employed in sample
preparation. The infra-red laser was used to heat the samples and they found
that the concentration of the filler played a major role on improving the
thermal conductivity when compared to the particle size of the filler.
Omranipour et al (2002) studied the effect of filler particle size and
concentration on the tracking and erosion resistance of silicone rubber loaded
with silica and alumina tri-hydrate as fillers. The samples with different filler
particle size and concentrations were tested in an inclined plane tracking and
erosion resistance test apparatus. They explored that the loss of material was
linked with the power dissipated by the fundamental and harmonic
components, due do dry band arcing. Also their results showed that the
smaller particle sizes and higher percentages of fillers, up to 50%, improved
the tracking and erosion resistance of silicone rubber. Meyer et al (2002)
investigated the influence of particle size and concentration of fillers used in
28
silicone rubber compounds for outdoor insulation. The alumina tri-hydrate
and silica with mean particle sizes of 5 and 10 pm were used as fillers with
concentrations of 10, 30 and 50 % by weight in a two-part room temperature
vulcanized silicone rubber base polymer. The obtained results from inclined
plane tests showed that the samples with high concentrations of filler and/or
smaller particle size have better tracking and erosion resistance than samples
with lower concentration and/or larger particle size. Afendi et al (2003)
described the design and development of a leakage current monitoring system
associated with surface tracking and erosion resistance test set-up of IEC 587.
LabVIEW software package was used to develop a measurement program for
recording and analysing leakage current signals. Experimental works have
shown the capability of the developed system in detecting the performance of
insulating materials as well as identifying the characteristics of the surface
discharges.
El Hag et al (2004) had studied the physicochemical properties of
silica filled silicone rubber nano composites. They have discussed the erosion
resistance of silicone composites with silica fillers with laser based method,
used as a source of heat to treat the filled and unfilled silicone rubber. The
thermal and chemical bonding behaviors have been analyzed using the atomic
force microscopy (AFM), thermo-gravimetric analysis (TGA) and infrared
microscopy (FTIR). It was concluded that the physical and chemical
properties significantly improved the nano filled SIR than the unfilled SIR.
El.Hag et al (2004) had studied the influence of nano size and micro size
fillers in silicone rubber using the inclined plane tracking and erosion test.
The low frequency components of leakage current and eroded volume were
used to evaluate the performance of the composites. They found that the
fundamental component of leakage current did not correlate with the erosion
and the third harmonic component of the leakage current showed good
correlation to the erosion in terms of volume. It was reported that 10% weight
29
of the nano size filler in silicone rubber had given a performance that was
similar to that obtained with 50% by weight of micro size filler. Dengke et al
(2004) had reported that the addition of a small amount (2 to 5 %wt) of
inorganic nano fillers to polymers should be sufficient for mechanical and
thermal stability and performance improvement. Although the hydrophobicity
of all the composites decreased after corona aging and the hydrophobicity was
recovered after a few hours. Meyer et al (2004) had investigated the effect of
filler particle size and concentration on the tracking and erosion resistance of
silicone rubber loaded with silica and alumina tri-hydrate as fillers. The filler
particle size and concentrations were tested with the inclined plane tracking
and erosion resistance test apparatus. It was found that the loss of material is
linked with the power dissipated by the fundamental and harmonic
components due to dry band arcing. They also reported that the higher
percentages of fillers improved the tracking and erosion resistance of silicone
rubber.
Meyer et al (2004) had reported how alumina tri-hydrate and silica
fillers improved the erosion resistance of silicone rubber during dry band
arcing. It was found that the thermal conductivity of the composite material is
dependent on the concentration, particle size, and bonding of the filler
particles to the silicone matrix. It was also found that the ATH filler imported
better erosion resistance than silica in silicone rubber. Rajini et al (2004) had
investigated the surface tracking phenomena in different polymeric insulating
materials such as SIR, Ethylene-Propylene Diene Monomer (EPDM), and
High density polyethylene (HDPE). The tracking test was conducted as per
IEC (587) standard under AC and DC voltages. It was found that the silicone
rubber tracking performance was superior compared with other materials.
Ratzke et al (2005) had studied how nano fillers and micro fillers in an HTV
(High Temperature Vulcanizing) silicone elastomer affect the resistance to
arcing. In this work the best dispersion was obtained for nano silica. On the
30
other hand, large agglomerates were found to be formed by nano alumina.
The results of the arcing tests demonstrated longer test time duration with
increased filler concentrations of silica and alumina. The authors found that
the thermal conductivity increased in an approximately linear fashion with the
filler concentration. Enhanced resistance to arcing with nano silica was
achieved only at a high concentration of filler, approximately40% wt. It was
also found that the strong interfacial bonding and small inter-filler spacing of
the nano dielectrics restrict material degradation. Chernery et al (2005) had
studied the inorganic fillers used in silicone rubber to enhance the properties
of thermal conductivity, relative permittivity, and electrical conductivity for
outdoor insulation application. They used barium titanate fillers to contribute
for increasing the relative permittivity and the fillers with antimony doped
zinc oxide contributed towards electrical conductivity and preventing the
partial discharge and corona discharge on the surface of the insulations. They
also used silica and aluminium trihydrate to improve the thermal conductivity
of the composites and increased the resistance to erosion due to heat produced
by dry band arcing. It was reported that the relative permittivity of the
composites predicted with greater accuracy if the relative permittivity of the
individual filler particles was known and the predicted thermal conductivity
should be possible if the gas layer is thin between the filler and polymer
matrix.
Roy et al (2005) had studied the voltage endurance behavior of
cross-linked polyethylene (XLPE) significantly improved with the inclusion
of treated nano particles (aminosilane- treated nano silica, vinylsilane-treated
nano silica). It was found that all the nano scale fillers had significantly
improved breakdown strength and endurance over the base resin. Yong Zhu
et al (2005) had explored experimentally to examine the suppression effect of
alumina trihydrate (ATH) filler on the erosion of filled SIR exposed dry band
arc discharge. In this study, one simulated electrolyte electrode system was
31
used to generate dry band arc discharge and eroded mass of SIR filled with 0-
50% wt ATH was measured. The TG-DTA was applied to study the thermal
characteristics of filled SIR and clarify the suppression mechanism of ATH
on the heat erosion. The physicochemical analyses on the degraded specimens
were carried out by using FTIR and SEM. They demonstrated that the erosion
of filled SIR due to dry band discharge can be well suppressed with the
increase of ATH filler content. Ehsani et al (2005) reported that the
experimental investigation concerned with electrical and surface properties of
a silicone modified polymer in comparison with silicone rubber ethylene-
propylene-dine monomer and alloy of SIR-EPDM. The loss of
hydrophobocity of polymeric materials induced by ultraviolet radiation (UV),
saltspary and water salinity aging was examined in this work. The ATR-FTR
was used to study the surface degradation of polymers occurring during UV
aging. Tracking and erosion induced by high electrical stress reduced the
lifetime of polymeric materials. The results of standard tests showed that the
SIR suffered from deterioration of tracking resistance caused by the loss of
hydrophobicity from the action of water salinity stresses. The silicone
modified polymer showed good hydrobobicity behavior in environmental
conditions and excellent tracking and erosion resistance compared to SIR,
EPDM and alloy of SIR-EPDM.
Chandrasekar et al (2006) investigated the tracking phenomenon in
silicone rubber material under AC and DC voltage. The leakage current
during the tracking studies was measured and analysed by using moving
average technique. They reported that the tracking was more severe and the
tracking time was less under DC voltage. They also concluded that the
leakage current magnitude was high in thermally aged specimens compared to
the virgin sample. They extended their research to measure the surface
condition of the insulation materials with water aged specimen and the
diffusion co efficient of the material was calculated. The reduction in contact
32
angle of the specimen had directly influencing the reduction of tracking time
were found. The tensile test and dynamic mechanical analysis (DMA) are
used to understand the mechanical properties of the material.
Sarathi et al (2006) research explored the tracking phenomenon of
silicone rubber material under AC voltages. They analysed the characteristic
changes in the tracking time of the aged specimens through the leakage
current during the tracking using the moving average current technique and
finally concluded that the material with high leakage current magnitude
allows tracking to occur easy. El-Hag et al (2006) had reported that the
erosion resistance of silicone rubber filled with micro and nano sized fillers.
They compared the tracking and erosion resistance of micro and nano filled
SIR materials using Inclined Plane Tracking and Erosion Test (IPT). It was
found that the tracking resistances were improved when filler percentage
concentration was increased. It was also found that the 10 % wt of nano filled
SIR gave a performance that was similar to that obtained with 50% wt of
micro filled SIR. This indicated that the nono filled SIR composites depicted
significant improvement in weight loss and tracking resistance. The low
frequency component of leakage current were used to evaluate the relative
erosion resistance of the composites and the third harmonic component of the
leakage current showed good correlation of the Inclined Plane Tracking and
Erosion Test. They concluded that the nano filled SIR had improved in
erosion and tracking resistance due to the virtue of their size.
Meyer et al (2006) made an attempt to use nano silica and micro
silica in silicone rubber for ceramic insulators in the coastal area. The nano
and micro filled room temperature vulcanized (RTV) silicone rubber were
compared with respect to erosion damage, surface roughness and contact
angle. The results showed that the nano silica filled room temperature
vulcanized silicone rubber has higher erosion resistance, lower
33
hydrophobicity and low surface roughness than the micro filled silicone
rubber. They also found that less contamination accumulation took place in
nano filled RTV SIR than the micro filled RTV SIR. It indicated that the
nano filled RTV SR was better suited for the contaminated environments than
micro filled RTV SIR. Imai et al (2006) had studied the epoxy resin included
in the mixing of nano, micro, and the combination of nano and micro
composites (NMMC). It was evident that micro filled epoxy and NMMC
maintained a far smaller erosion depth than the base epoxy resin. For the
lifetime tests, the base epoxy resin required less time to break down than the
nano filled formulation, followed by micro filled epoxy, and finally,
significantly longer time for the NMMC formulation. IEC60587 Publications
(2007) reported that the detailed procedures of electrical insulating materials
were used under severe ambient conditions and test methods for evaluating
resistance to tracking and erosion.
Kumagai et al (2007) had evaluated the leakage current suppression
and the resistance to tracking and erosion of high-temperature vulcanized
silicone rubbers (HTV-SIR) containing different levels of silicone plasticizer.
Slab HTV-SIRs unfilled and filled with ATH (50 wt %) and with added linear
silicone plasticizer (0–6.0 wt %), were used in this study. Evaluation using the
IEC 60587 inclined-plane method indicated that the plasticizer had no
influence on the tracking and erosion behavior. Salt-fog test results indicated
that a higher level of plasticizer in the ATH-filled HTV-SIR showed smaller
leakage current. Gas chromatography and mass spectrometry suggested that
the enhanced suppression of the leakage current for ATH-filled HTV-SIR can
be attributed to the migration of linear silicone plasticizer onto the surface
contaminants. It was concluded that the addition of silicone plasticizer was
useful for improving the leakage current suppression ability of HTV-SIR
without reducing its tracking and erosion resistance in the above laboratory
tests.
34
Isaias Ramirez et al (2007) had implemented the eroded mass
measurements for nano filled silicone rubber by infrared laser heating on wet
and polluted composite insulators. They also demonstrated the improvement
of filler dispersion by surfactants and calcination techniques. The result of the
surface treatment with a surfactant and calcinations was improved in filler
dispersion and reduced the eroded mass significantly in silicone rubber with
nano fillers than the micro fillers. Naderian Jahromi et al (2008) had reported
that the four RTV silicone rubber high voltage insulator coatings aimed to
achieving information on the relative performance of the coatings with respect
to aging. Venkatesulu and Joy Thomas (2008) had studied the nano – sized
magnesium dihydroxide (MDH) and micron-sized ATH fillers as flame
retardants in RTV silicone rubber to enhance the tracking and erosion
resistance. It was concluded that the MDH filled composites performed much
better than the ATH composites in terms of eroded mass, depth of erosion,
width and length of erosion. Also, they found that the eroded mass of MDH
composite was less than that of ATH composite, which was due to the high
surface area and higher thermal stability of the MDH nano fillers.
Hass et al (2009) had investigated the aging phenomena in
polymeric insulating materials using Differential Scanning Calorimetry
(DSC) and Thermo Gravimetrical Analysis (TGA). The samples were aged by
thermal aging and voltage stress subsequently. They reported that the
measured curves of new and aged samples gave information about
modifications in the polymer due to aging and the parameters such as glass
transition, melting temperature and degree of cross linking and crystallation.
Congzhen XIE et al (2009) explored the evaluation of composite insulator of
various degrees by using the FTIR. The results showed that there was a close
relation between absorption areas of the functional group of Si-CH3 and Si-O-
Si of silicone rubber shed and housing aging. Along with the severity of aging
in the appearance of composite insulator, the absorption areas of the
35
functional group of Si-CH3 and Si-O-Si were decreased. Raetzke et al (2009)
had studied the silicone rubber with two different kinds of nano scale (20 nm
and 20 to 30 nm) SiO2 filler particles were tested with respect to the resistance
to high voltage arcing and the resistance to tracking and erosion. They
reported the high improvement of the resistance to both stresses for one filler
type at very low filler contents. Furthermore, a maximum resistance to high
voltage arcing was observed for these nano composites at a filler content of 5
% wt. Such a maximum was not observed for the materials with the other
filler type, where the improvement is not as high as for the first type. The
effects were explained by the ratio of interphase to bulk material. Based on
the interphase, which was formed around each filler particle, a possible mode
of action was illustrated and influencing factors were identified. Ranjini et al
(2009) had investigated the aging of silicone rubber by gamma radiation. The
silicone rubber aged by gamma irradiation with three different doses and the
resistance to tracking on applying was examined with AC and DC voltages.
They reported that the resistance to tracking under DC voltages was less than
that of their AC values and the DC stressed samples showed a higher surface
degradation compared to those stressed by AC voltages. The erosion depth
and contact angle affected by radiation have been studied and the aging of the
surface was assessed by FTIR and SEM with EDAX analysis.
Mansab Ali et al (2009) reported the hydrophobicity of HTV
silicone rubber after immersion in saline solutions of different conductivities
and different temperatures. They estimated the surface properties during the
above period with the help of Scanning Electron Microscopy and Fourier
Transform Infrared Spectroscopy. Venkatesulu et al (2010) had presented the
experimental studies on the erosion resistance of low weight percentage
alumina filled nano composites and highly filled ATH filled micro
composites. The obtained result indicated that the performance of 4% by
weight nano composite was comparable to that of the 30% by weight ATH
36
composite. The thermo gravimetric analysis showed that the thermal stability
of the nano composite was better than the micro composite with low filler
concentration. Schmidt Lars et al (2010) investigated using only silica filler
and increasing flame retardancy and resistance to tracking and erosion by
adding a phosphate or nitrogen based flame-retardant. The results suggested
that a tracking on erosion resistant silicone rubber could be obtained without
the addition of ATH. They formulated a HTV silicone rubber with good
tracking and erosion resistance was a balance between high enough filler
content, maintaining a good dispersion of the particles in the polymer matrix,
as well as good bonding between particles and matrix. Isaias Ramirez et al
(2010) had explored how the addition of inorganic nano filler and micro filler
to silicone rubber can impart resistance to erosion for overhead insulation.
The effect of the surfactant on the surface wettability of the composites was
analysed with contact angle measurement. The salt-fog, inclined plane and
laser ablation tests were conducted to evaluate the erosion resistance of the
micro and nano filled composites. They concluded that the combination of
micro and nano fillers with surfactant resulted in composites with improved
erosion resistance to dry band arcing with the exception of the case where
calcination was used in the formulation.
Heger et al (2010) had studied the inclined plane test method to
determine the performance of RTV, HTV silicone rubber and EPDM rubber
material. They used the constant voltage method which was employed to
evaluate the performance of the above samples, when energized by AC and
DC (both polarities) voltages. It was reported that the EPDM rubber
performed better under DC voltage than silicone rubber based materials and
also they confirmed that the dc test was more severe than the AC test for all
materials. Bruce and Rowland (2010) had investigated the DC inclined plane
tracking and erosion test with three formulations of silicone rubber. The
materials were tested under three voltage levels (2.3, 2.7 and 3.2 kV) with
37
both polarities. They reported that the positive DC tests have the highest
average and peak leakage current and exhibited a higher degree of surface
damage. Moreover, the surface degradation pattern was heavily dependent on
polarity was found. The higher level of erosion in the higher voltage positive
cases was also observed. Thong-om et al (2011) explored the experimental
analysis of salt fog aging test of silicone rubber housing material for outdoor
polymer insulator. The different amount of ATH filled HTV silicone rubber
sheet tested continuously in salt fog chamber and the chemical changes on the
surface were analysed using FTIR technique. The results were compared with
the new specimen and they concluded that the surface was changed due to
aging.
Joseph Vimal Vas et al (2012) had presented the tracking and
erosion of silicone rubber nano composites under DC voltages. The micro
sized alumina trihydrate and nano sized alumina fillers were added to silicone
rubber matrix to improve the resistance to tracking and erosion. The leakage
current and eroded mass at the end of the tests were monitored. Scanning
Electron Microscopy (SEM) and Energy dispersive X-ray (EDX) studies were
conducted to understand the filler dispersion and changes in surface
morphology in both the composites. They suggested that the nano composites
performed better the than micro composites even for small filler loading for
both the positive and negative DC stresses. They also suggested that the
tracking and erosion performance of SIR was better under negative DC when
compared to positive DC voltages
1.11 MOTIVATION FOR RESEARCH
Researchers have completed different range of studies to
investigate the problem of surface degradation and to reduce the effects of
tracking and erosion under various environmental conditions. The above
mentioned investigations have been reported in the literature but still there is a
38
need for some more investigation to understand the surface degradation and to
reduce the effects of tracking and erosion under the aging condition. The
required investigations under aging are: (i) Performance of micro and nano
fillers to improve the properties of the dielectrics against the surface
degradation and reduce the effects of tracking and erosion under AC and DC
voltage. (ii) How the nano fillers help to improve the properties of the
artificially aged dielectrics against the surface degradation and reduce the
effects of tracking and erosion under AC and DC voltage. Most of the
researches are focused on SIR materials with micro or nano sized alumina tri-
hydrate and few of the researches on SIR materials with micro and nano silica
(SiO2), alumina (Al2O3) are performed individually to improve the properties
of the dielectrics against surface degradation and reduce the effects of
tracking and erosion. They did not carry out the evaluation of surface
degradation and improvement of tracking and erosion using different nano
fillers with different concentration in artificially aged silicone rubber under
one roof.
This research mainly focuses on the investigation of the
performance of the silicone rubber insulating materials against the surface
degradation and effects of tracking and erosion problem when it is used in AC
and DC Transmission lines. Because the polymeric insulating materials are
subjected to various stresses such as electrical, thermal stresses which cause
degradation of the insulating material, tracking and erosion problem, which
are mainly due to the electrical discharges across dry bands in the presence of
the wet contaminant layer on the insulator surface. In order to solve the
problem of surface degradation and to reduce the effects of tracking and
erosion of the material under normal and polluted condition, the experimental
studies have been proposed and the above practical issues are investigated in
the silicone rubber material under normal and artificially aged conditions.
These investigations are carried out in the silicone rubber specimens filled
39
with various fillers of different % weight concentrations. Also, the study is
focused on improving the dry band arcing erosion resistance of nano filled
silicone dielectrics, with the objective of increasing the service life of polymer
insulators under polluted outdoor service environments. In addition to the
study of nano filled silicone composites for outdoor high voltage insulation,
the mechanisms in nano filled dielectrics can provide a better understanding
of degradation, so that the design of insulating materials can be improved.
1.12 OBJECTIVES OF THE RESEARCH
The primary objective of this research is to investigate the problem
of surface degradation and to reduce the effects of tracking and erosion of the
nano filled silicone rubber material under normal and artificially aged
condition. For the efficient use of the silicone rubber insulating material in
the power transmission line, the following research objectives are set:
To analyse the characteristic variations in the trackingresistance and eroded masses of micro and nano sizedalumina, nano aluminium hydroxide and nano silica filledsilicone rubber specimens through AC and DC leakage currentmeasurements.
To analyse the fundamental, third and fifth harmoniccomponent of the AC leakage current and the fundamentalharmonic component of the DC leakage current variationduring the tracking study using FFT and moving averagecurrent technique in both the micro and nano Al2O3, nanoSiO2 filled SIR
To carry out the material characterization studies in micro,nano alumina filled virgin SIR and artificially aged nanoaluminium hydroxide, nano silica filled SIR using TG-DTG,SEM, EDAX and FTIR for thermal, physical and chemicalproperty analysis.
40
1.13 ORGANIZATION OF THESIS
Chapter 2 describes the materials used, sample preparation and
experimental procedure of this research work. The leakage current
measurement method, aging mechanism of materials and material
characterization techniques are also discussed.
Chapter 3 illustrates the theoretical studies of leakage current
analysis using Fast Fourier Transform and moving average technique.
Chapter 4 deals with the experimental results and its discussion
obtained from IPT, TG-DTG (Thermal property), SEM with EDAX (Physical
property) and FTIR (Chemical property) in virgin silicone rubber materials.
Chapter 5 expresses the experimental results and its discussion
obtained from IPT,TG-DTG (Thermal property), SEM with EDAX (Physical
property) and FTIR (Chemical property) in artificially aged silicone rubber
materials.
Chapter 6 presents the experimental results and the discussion of
leakage current using moving average technique in virgin silicone rubber
materials.
Chapter 7 presents the summary of the conclusions and scope for
further researches is provided.