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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.

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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.

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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

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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.

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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

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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.

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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

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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:

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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.

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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.

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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.

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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

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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.

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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

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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.

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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

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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.

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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

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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

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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

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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

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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

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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.