Vbr notes-ms-02

Material Science – Insulation Material
Prepared by, Professor Vijay Balu Raskar
(vijayraskar2003@yahoo.co.in / vijay.raskar1986@gmail.com)

MATERIAL SCIENCE – NOTES - 02
Electrical Engineering – Pune

 Properties of Good Insulating Materials:-

1) Mechanical Properties:-
 It should have high tensile strength, stiffness, elasticity, Plasticity,
ductility, toughness, hardness, brittle, malleability etc.
2) Thermal Properties:-
 It should have high thermal conductivity.
 It should withstand high temperature for short and continuous time.
 It should be non inflammable.
 It should have good mechanical strength to withstand vibrations.
3) Electrical Properties:-
 It should have high insulation resistance & Leakage current is less.
 It should have low dielectric loss.
 It should have low permittivity.
4) Magnetic Properties:-
 It should have high Permeability and coercive force.
5) Physical Properties:-
 Dimensions, Density, porosity and structure should be well manner.
6) Chemical Properties:-
 Corrosion, Resistance, acidity, composition and oxidation should be
high. Etc.
7) Optical Properties:-
 Color, light transmission, Light reflection etc
8) Acoustical Properties:-
 Sound Transmission, sound reflection etc
______________________________________________________________________

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 Classification of Solid insulating material:-

Fibrous Material:-

 It consists of elongated particles called fibres.
 Many particles are derived from cellulose which is main constituent of vegitable
plants.
 The fibres are mechanically strong and cheaper.
 They are hygroscopic. So, they are impregnated.
 Impregnation process of treating fibrous material with insulating material like
varnishesh, resigns, oil etc.
 Examples of fibrous insulating materials are wood, paper, pressboard, Asbestos,
Cotton, Silk, etc.
(1) Wood:-
 Cheaper and easily available.
 Good insulating material.
 It is hygroscopic.
 Dielectric constant varies from 2.5 to 7.7.
 Used for low voltage installation only.
 Used for making switch board, terminal boxes, spacer between HV – LV
winding in transformer.
(2) Paper:-
 It has good mechanical strength.
 Ability to withstand high temperature.
 Low dielectric loss.
 It is made up of glass or cellulose.
 Easily available and cheaper.
 Easily wrapped around conductor.
 It is used as spacer In transformer.
 Used in capacitor, cables and slot wedges.
(3) Press Board:-
 It is similar to paper but it is more thick and denser than paper.
 It is less flexible as compared to paper.
 Its insulation resistance is 10 7 m .
 Its dielectric strength is 50kV/mm.
Application:-
 Used to make slot lining.
 Separator in transformer winding.
 Making of wedges for stator and rotor core stacks.
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a) Pressphan:-

 It has good mechanical and dielectric properties.
 It is hygroscopic. So, need impregnation.
 It with stands maximum voltage of 600V.
b) Leatheroid:-

 It is thicker than pressphan.
 It is strong, tough and flexible.
 It is hygroscopic. So, need impregnation.
 It is used for low voltage only.
(4) Ceramics:-

 It is used to fabricate insulators, components and circuit boards.
 They are clay product.
 They are fused at high temperature of about 100 0 C
 Ceramics are hard, strong and dense.
 They possess excellent dielectric properties.
 Not affected by chemical action except by strong acid and alkalies.
 They good electrical insulating properties are complemented by the high thermal
conductivity.
 They cannot be machined but are to be moulded.
Application:-
 Ceramics are used in electronics circuits.
 It can be operated at high frequency and can be made in any shape and size.
 Permittivity is greater than 12 are used as dielectric in capacitor.
 Permittivity is less than 12 are used as insulator in transmission line and bushing.

a) Porcelain:-

 Its raw material is clay, filler agent like quartz etc

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 It dielectric strength 10 to 30kV / cm 2
 Its permittivity is 6 to 7
 It is used as insulation in manufacturing all types of insulators which is used in
transmission and distribution lines.
 It is used for transmission bushing.
 It is used for low and high voltage insulation purpose.
 Used for fuse base and carrier, plugs and sockets, etc.
b) Steatile:-

 It is hard, tough and mechanically strong.
 Absorbs less moisture.
 Having excellent compressive strength.
 Absorbs less moisture.
 It is used for low voltage and high frequency applications like variable capacitors,
switches etc.
c) Alumina:-

 It is made from fine crystals of Aluminium oxide.
 It has high mechanical and dielectric strength.
 It has low dielectric loss.
 It is chemically stable.
 It is used for circuit breakers, spark plugs, power transistors, furnaces.
d) Titanite:-

 It is made from metal oxide and titanium dioxide berrium titanate.
 It has high dielectric constant.
 It has positive temperature coefficient of resistance.
 It is used in capacitor design.
______________________________________________________________________
 Factors Affecting on Ceramics:-
a. Temperature:-
 The electrical resistance of ceramic is inversely proportional to the
temperature.

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b. Moisture:-
 They are not affected more by moisture.
 They are glazed to reduce penetration of moisture.
 Glazing also helps to prevent dust and dirt formation.
______________________________________________________________________
 MICA:-
 Mica is inorganic mineral material.
 It has excellent insulation properties.
 Best natural insulating material.
 Easily available.
 It releases water when heated.
 80% of world requirement of mica furnished by India.

a) Muscovite Mica:-

 It is strong, tough and less flexible.
 It is found in India, Brazil and USA.
 Dielectric constant is 6 to 7.
 Dielectric strength is 40 to 150kV/mm.
 White and ruby colored varieties are superior to green type.
 It is non hygroscopic.
 It is not affected by chemicals.
 It stands a maximum temperature of 500 degree C.
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 Loses its water at high temperature.
 It is used in radio and radio circuit capacitor.
 It is used in commutator segment insulation.
b) Phlogopite Mica:-

 This is dark in color.
 It is also called as magnesium mica.
 Its dielectric strength is 60Kv/mm.
 It has greater thermal stability.
 It is more resistant to alkalies than acids.
 It is lighter.
 It is used in domastic application.
 It is more suitable for commutators segment insulation.
 Insulating properties are poorer than Muscovite Mica.
 It is used where greater need of thermal stability is required.
 It is used in domestic applications.
______________________________________________________________________
 Insulating Resins OR Plastics:-

 These are material of high molecular weight.
 Capable to form into desired shape.
 There are two types as per figure shown.
 1) Natural Resins and 2) Synthetic Resins
1) NATURAL RESINS:-
 These are derived from plant and resources.
 Simple purification or a little chemical modification is made in natural resins for
use as an electrical insulation.
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 They have three types:- 1) Amber , 2) Wood Resin and 3) Sheliac
 Amber:-
 Used to make electrical insulating components in measuring instruments.
 It is light yellow in colour.
 it has high electrical resistance.
 Wood Resin:-
 Sticky and gummy like substance.
 Dielectric Constant 2.5 to 3.
 Dielectric Strength 10 to 16kV/mm
 Preparing for Paints and varnisheh, Insulating Oils.
 Shellac:-
 It is na animal resin.
 Poor resistance to heat, moisture and solvents.
 Less mechanical strength.
 Dielectric constant is 3.5
 Dielectric strength 14 to 50kV/mm
 Used for impregnation and manufacture of micanite.
2) SYNTHETIC RESIN:-
 These are organic substances which resemble to natural resins in properties like
plasticity and heat resistance.
 Chemical composition is differ from widely differs from natural resins.
 More than 50% insulating material used from this category..
 There are two types:- 1) Thermoplastic and 2) Thermosetting Resins.
Thermoplastic Resins:-

 They are soft and melt on heating and again solidify when cooled.
 The heating cycle can be repeated without any change in the properties of
material.
 By polymerization process in which molecules of a compound react together to
form new compounds.
 Thermoplastics types are:- 1) Perspex, 2) Polyethylene and 3) PVC
 1)Perspex:-
 It is obtained by controlled hydrolysis of tri-acetate solution.
 It has good electrical properties, mouldability, toughness, low softening
temperature, low inflammability.
 Used in model making and decoration purpose.
 2) Polyethylene:-
 It is obtained by polymerization of ethylene.
 Less hygroscopic.
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 It has good electrical and mechanical properties.
 Not dissolved at ordinary temperature but dissolved at high temperature in
petroleum solvents.
 Uses for wires, high frequency cables, communication cable etc
 PVC:-
 Used as cable insulation, insulation of wires, conduit pipes.
 It is obtained by combination of acetylene and hydrogen chloride in the
presence of a catalyst at a temperature of 50 degree C.
 PVC can be produced by polymerization of ethylene dichloride and
sodium hydroxide in the presence of crystal.
 Properties of PVC can be improved by adding Stabilizers, Fillers,
Plasticizers, Additives etc.
 Characteristics of PVC:-
i. More flexible.
ii. Low joining cost
iii. Low weight and reduced size.
iv. High resistance to chemical action.
v. Resistance to ultra violet rays and ozone.
vi. Immunity to corrosion, abrasion and moisture.
vii. Fire Retarding.
 PVC having good electrical and mechanical properties.
 PVC is used for PVC film tapes, Sheets, conduit pipes, insulation for dry
batteries.
Thermosetting Resins:-
 They cannot be soften and cannot be re-shaped.by heating.
 The moulding temperature is higher than thermoplastic materials for materials
 They undergo chemical changes when moulded.
 There are three different types 1) Bakelite, 2) Silicon and 3) Epoxy
 Bakelite:-
 It is hard dark coloured.
 It is available in the form of films, roda, sheets OR moulding compounds.
 It is used to manufactured lamp holders, switches , plug sockets and bases, and
small panel boards.
 Silicon Resins:-
 These are organic compounds of silicon.
 They are produced by hydrolysis and condensation from organochlorosilane.
 They posses good insulating properties and heat resistance.
 They are chemically inert and resistant to weathering effect.
 It is used for cooling and impregnating liquids for capacitors and transformers.

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 They are used to make silicon rubber.
 EPOXY:-
 Epoxy resins have good mechanical strength, less shrinkage, excellent
dimensional stability after casting, good adhesive metal etc.
 It is good adhesive metal.
 Used to manufacture of aminated insulating boards and insulating varnishes.
 They are hardened by the use of hardeners like organic acids.
____________________________________________________________________
 TRANSFORMER OIL or MINERAL OIL:-
 This is obtained by crude petroleum by distillation of the various by product
obtained.
 Insulating oil is one among them,
The properties of good insulating oil are:-
 Low Specific Gravity,
 Low power factor.
 Low Viscosity.
 Low Specific heat.
 High Resistivity.
 High Dielectric Strength.
 High Flash Point
 High Thermal Conductivity.
 Arc Extinguishing Property.
 The dielectric strength decreases with reduction in temperature.
 The oil should be free from even a drop of water as it will reduce the insulating
property.
 Mineral oil is used in transformer for two purposes:-
1) For cooling purposes and 2) maintain the insulation of windings.
 The insulation value of transformer oil is further reduced due to the formation of
sludge as a result of oxidation due to air and temperature.
 To minimize oxidation, the oil used in transformers should not be exposed to air.
 Sludge is also formed due to the pressure of acids and alkies.
 Sludge formation :- 1) Reduce the rate of heat transfer, 2) Blocks the ducts and
3) Increases the operating temperature
 To prevent moisture from entering the oil, the whole apparatusd is made airlight
and calcium chloride, silicagel fillets are used.
 Permitivity is 2.1 to 2.5
 Breakdown voltage is 30-50 kV / 2.5 mm.
 Low viscosity oil is used in high tension oil filled cables.
 Medium viscosity oil is used in switch gears and cables.
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 High Viscosity oil is used in gas filled cables and solid cables.
______________________________________________________________________
 INSULATING MATERIAL USED FOR POWER AND DISTRIBUTION
TRANSFORMER:-

 Power and Distribution Transformers:-
 PROPERTEIS:-
1) Good Thermal Stability and low aging effect.
2) Good mechanical property to withstand fabrication and handling.
3) Good electrical property to withstand over voltage, over frequency etc.
 Thick radial spacer or tubes made up of press board, paper, and glass fabric
between the windings.
 Turn to turn made up of paper, glass tape, varnish, Organic enamel.
 Stamping by varnish.
 Layer to layer or coil to coil by craft paper, press board etc
 Mineral oil or air for cooling and insulation.
 Bushing made by resins, Porcelain etc
 PVC sleeves for connections.
 Crape paper for connection of insulation of wires.
 Cork sheet is used to prevent leakage.
 For small rating, the coils are made of super-enameled copper wire.
 For layer to layer, coil to coil and coil to ground (iron core) craft paper is used.
 However, for large size transformers paper or glass tape is rapped on the
rectangular conductors whereas for coil to coil or coil to ground, insulation is
provided using thick radial spacers made of press board or glass fiber.
 In oil-filled transformers, the transformer oil is the main insulation.
 However between various layers of low voltage and high voltage winding oil-
impregnated press boards are placed.
 SF6 gas insulated power transformers make use of sheet Aluminium conductors
for windings and turn to turn insulation is provided by a polymer film.
 The transformer has annular cooling ducts through which SF6 gas circulates for
cooling the winding.
 SF6 gas provides insulations to all major gaps in the transformer.
 This transformer is used where oil filled transform is not suitable e.g., in cinema
halls, high rise buildings and some especial circumstances:
 The end turns of a large power transformer are provided with extra insulation to
avoid damage to coil when lighting or switching surges of high frequency are
incident on the transformer winding.

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 The terminal bushings of large size power transformer are made of condenser
type bushing.
 The terminal itself consists of a brass rod or tube which is wound with alternate
layers of treated paper and tin foil, so proportioned, as to length, that the series
of condensers formed by the tin foil cylinders and the intervening insulation have
equal capacitances, thereby the dielectric stress is distributed uniformly.
______________________________________________________________________
 Insulating Materials used for Rotating Machines:-

 PROPERTIES:-
1) Good mechanical characteristics.
2) High dielectric strength.
 Micanites is used for commutator sleeves, coiled frames or slot insulation in
machine.
 Pressboard for making slot wedges, cores, endliners etc.
 Wood is used for slots and wedges.
 For low voltage a.c. and d.c. machines, the winding wire are super enameled
wire and the other insulation used are vulcanized rubber and varnished cambric
and paper.
 For high voltage and large power capacity machines, the space limitations
demand the use of insulating materials having substantially greater dielectric
strength..
 Mica is considered to be a good choice not only due to space requirements but
because of its ability to withstand higher temperatures.
 However, the brittleness of mica makes it necessary to build up the required
thickness by using thin flakes cemented together by varnish or bakelite generally
with a backing of thin paper or cloth and then baking it under pressure.
 Epoxy resin bounded mica paper is widely used for both low and high voltage
machines.
 Multilayer slot insulation is made of press board and polyester film. However, for
machines with high operating temperatures kapton polymide is used for slot
insulation.
 Mica has always been used for stator insulations.
 In addition to mica, conducting non-woven polyesters are used for corona
protection both inside and at the edges of the slots. Glass fiber reinforced epoxy
wedge profiles are used to provide support between the winding bars, slots and
the core laminations.
 Glass fiber reinforces epoxy wedge is used for support between winding bars,
slots and core laminations.
 For rotor, single layer glass or hard fabric or glass fabric or nomex is used as
insulation.
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______________________________________________________________________
 Insulating material used for Power Cables:-
 The various insulating materials used are vulcanised rubber, PVC, Polyethylene
and impregnated papers.
 Vulcanised rubber, insulated cables are used for wiring of houses, buildings and
factories for low power work.
 PVC is inert to oxygen, oils, alkalies and acids and therefore, if the environmental
conditions are such that these things are present in the atmosphere, PVC is
more useful than rubber.
 Polyethylene is used for high frequency cables. This has been used to a limited
extent for power cables also.
 The thermal dissipation properties are better than those of impregnated proper.
 The maximum operating temperature of this cable under short circuits is 100°C.
 Non hygroscopic.
 Non inflammable.
 Chemically stable.
 High insulation resistance to avoid leakage current.
 High mechanical strength.
 Low dissipation factor.
 In case of impregnated paper, a suitable layer of the paper is lapped on the
conductor depending upon the operating voltage. It is then dried by the combined
application of heat and vacuum.
 The compound used in case of impregnated paper is semi fluid and when the
cables are laid on gradients the fluid tends to move from higher to lower
gradients which reduces the compound content at higher gradients and may
result in void formation at higher gradients.
 For this reason, impregnated paper cables are used up to 3.3 kV.
 Insulation liquids are also used as insulation in cable for coolant.
 XLPE (Cross Linked Poly Ethylene) cables are mostly used.
 XLPE ratings are 11kV, 22kV, 33kV, 66kV ratings.
 Above 66kV, solid type cables are unreliable because of danger of breakdown of
insulation.
______________________________________________________________________

 Insulating material used for Capacitor:-
Power Capacitors:-
 Capacitor can be made up of Paper, Mica, Silica Glass, Mineral Oil, Ceramic and
oil impregnated paper capacitor.
 Power capacitor uses tissue paper or polypropelene film as dielectric and it is
impregnated with mineral oil.
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 Ceramic capacitor uses ceramic and mica capacitor uses mica as dielectric.
 Capacitor is made up of conducting plates and a dielectric.
 Dielectric may be air, solid, liquid or combination of solid and liquid insulating
material.
 They should have high dielectric strength and low loss factor.
 Non hygoscopic.
 Oil filled capacitor are used where large capacitance required.
DIAGRAM:- (Refer Book)

 The most commonly used capacitor for the purpose is the impregnated paper
capacitor. This consists of a pair of aluminium foil electrodes separated by a
number of Kraft paper tissues which are impregnated with chlorinated diphenyl
and has a higher permittivity and results in reduction in the quantity of materials
required for a given capacitance and the cost.
 The working stress of an impregnated paper is 15 to 25 V/μ and papers of
thickness 6–12μ are available and hence depending upon the operating voltage
of the capacitor, a suitable thickness of the paper can be selected.
 Because of imperfection involved in the manufacturing process of the dielectric
paper it is desirable to use at least two layers of tissues between metal foils so
that the possibility of coincidence of weak spots is avoided.
 The effective relative permittivity depends upon the paper and the impregnant.
For chlorinated diphenyl impregnant the relative permittivity lies between 5 and 6.
______________________________________________________________________
Only for knowledge:-
The functions of a Capacitor are as follows:-

1. It blocks the flow of DC and permits the flow of AC.
2. It is used for coupling of the two sections.
3. It bypasses (grounds) the unwanted frequencies.
13


4. It feeds the desired signal to any section.
5. It is used for phase shifting.
6. It is also used for creating a delay in time.
7. It is also used for filtration especially ripples from rectified waveforms.
8. It is used to get tuned frequency.
9. It is used for motor starter.

____________________________________________________________________

 Impregnation Process:-
 It is the process of treating fibrous insulating material by other insulating material.
 It reduces hygroscopic nature in material.
 It also reduces chemical and thermal deterioration.
 It increases dielectric and mechanical strength of material.
 Fibrous materials have air pocket or voids inside them.They are presents during
manufacturing process.
 It reduces permeability and dielectric strength.
 The breakdown occurs in the air pockets making insulation worst.
 Due to breakdown, electric field intensity becomes non uniform in the material.
 By impregnation, air pockets are filled with oil or varnishes. Making them
electrically strong and producing uniform electric field intensity in the material.
 Insulation material should be dry for impregnation.
 Most of the insulating materials are impregnated before they are use in electrical
device.
 Resins, varnishes, bitumins and drying oils are normally used for impregnating
the insulating materials.
 Paper and cotton are impregnated in a double input vertical machine.

DIAGRAM:- (Ref Text book / notebook)

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 Dry Insulation material and varnish should be carefully observed.
 Sometimes thinner is also added in varnish for proper viscocity.
 For adjusting tension of the material time required can be adjusted.
 In the machine winding, after the winding they are dipped in a varnish bath at 110
degree C temperature.
 This winding should be dried for 5 to 10Hrs by putting into ovens.
 15 to 20 minutes in the varnish bath and brought out and allowed to dry for 10 to
18hrs in oven at 110 degree C. This process repeated twice.

______________________________________________________________________

 SF6 Gas:-
 SF6 Sulphur Hexa Flouride.
 SF6 Gas is an excellent insulating and arc-quenching medium.
 It has a non toxic, non flammable, high dielectric strength and arc quenching
capacity.
 SF6 gas insulated equipment, small footprint, no fire danger, improves power
system reliability.
 SF6 gas uses under high pressure.

1) Physical Properties:-

 Under standard conditions:-
 SF6 gas is colorless and odorless.
 It is in the form of liquid for transport and storage.
 Non inflammable nature
 Heat transfer ability is 2.5 times greater than air.
 Non toxic in nature.

2) Chemical Properties:-

 It is stable up to 500 Degree C.
 It is electro negative gas.
 Excellent arc quenching.
 Chemically stable.
 Chemically inert.

3) Dielectric Properties:-
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 Dielectric property of SF6 gas is 3 times more than air.
 Dielectric strength is higher than oil when Pressure of SF6 gas is more than
3kg/cm2.

Application of SF6 Gas:-

 It is used as GIS (Gas Insulated Substation).
 Used in electrical Equipments like CB, Switchgears, etc
 SF6 is used for protection.
 Used in power station.

______________________________________________________________________

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