APPLICATIONS OF INSULATING MATERIAL CHAPTER 5 1 All electrical - - PowerPoint PPT Presentation

APPLICATIONS OF INSULATING MATERIAL

CHAPTER 5

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• All electrical systems require some kind of

insulation to prevent short circuits and leaking currents.

• 3 forms of insulators: solid, liquid and gaseous
• Performance of these insulators depend on the

temperature

• Classification according to their temperature

rating:

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• Temperature Classification for Insulating Materials
• Application of Insulating Materials in electrical apparatus:
• In Power Transformer
• In Rotating Machines
• In Circuit Breaker
• In Power Capacitors
• In High Voltage bushing
• In Small Machines

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• Insulating material must be able to withstand electrical

stresses, in addition to it should be able to withstand certain

• ther stress namely during manufacture, storage and
• peration.
• The performance of the insulation depends on its operating
• temperature. The higher temperature, the higher will be the

rate of its chemical deterioration.

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The insulating materials are grouped into different classes with temperature limit : Impregnation: Letting the solid material absorb some liquid

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IEC (International Electrotechnical Commission)

APPLICATIONS IN POWER TRANSFORMER

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(a) conductor or turn-to-turn insulation, (b) coil-to-coil insulation, (c) low voltage coil-to-earth insulation, (d) high voltage coil-to-low voltage coil insulation, and (e) high voltage coil-to-ground insulation.

There are four principal areas where insulation must be applied

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APPLICATIONS IN POWER TRANSFORMERS

The Transformer insulation is divided into: a) Conductor or turn to turn insulation;

• using directly organic enamel on conductor for smaller

rating.

• Using paper or glass type wrapped on the rectangular

conductors. b) Coil to coil insulation,

• Kraft paper is used in smaller transformer
• Pressboard, glass fabric, or porcelain are used in the

case of higher rating transformer.

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c) Low voltage coil to earth insulation,

Normally consist of solid tubes combined with liquid or

gas filled spaces. The liquid or gas in the spaces help to remove the heat from the core and coil structure and also help to improve the insulation strengths d) Low voltage coil to low voltage coil insulation, Generally of the insulation system used in oil-filled transformers consist of oil impregnated pressboard. e) High voltage coil to ground insulation, Same at c.

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• The winding Insulation is normally made of paper.
• Cellulose board is also used as internal insulation in liquid-filled

transformers.

• Cellulose insulation is impregnable with the insulating fluid of

transformers.

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• Conductor insulations is usually paper, in the form

pressboard.

• Insulating board is made of wood or mixture of

wood and cotton.

• Cast resin also used as insulating system at lower

voltage rating.

• Higher temperature systems, employ synthetic fluid
• Gas Insulated Power Transformers use sheet

aluminum conductors for windings, a polymer film for turn-turn insulation.

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• provides the required dielectric strength and insulation
• cools the transformer by circulating itself through the core and the coil

structure.

• should be in the liquid state over the complete operating range of temperatures

between -40°C and+50°C.

• The oil gets oxidized when exposed to oxygen at high temperatures
• formation of peroxides, water, organic acids and sludge, which cause

chemical deterioration of the paper insulation and the metal parts of the transformer.

• sludge being heavy, reduces the heat transfer capabilities of the oil, and

also forms as a heat insulating layer on the

• coil structure, the core and the tank walls.
• the effects of oxidation are minimized by designing them such that access to
• xygen itself is limited.
• sealed transformers
• filling with nitrogen gas,
• providing oxygen absorbers like activated clay or alumina
• arc discharge inside a transformer decomposes the oil and causes explosions

Transformer Oil

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• Insulation of Dry type transformers is now using prepregs

(composite fibres where a material, such as epoxy is already present)

• manufactured using the following baking materials: glass

fibres, Nomex, Aramid paper etc.

• Good dielectric strength of the coil, easy production

method and could eliminate expensive vacuum impregnation of low voltage coil.

Dry type transformers

APPLICATIONS IN ROTATING MACHINES

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• 2 types of rotating machine:
• low voltage machines: up to 6,600 V , class E or F insulation
• high voltage machines: 6,600 V and up, Class F insulation
• machines above 22 kV rating are not built except under special

conditions.

• Insulation that has been used for machines are mica.
• Mica available in the form of very thin splitting.
• The selection of the right material depends on the power rating and the

conditions under which it operates.

• Polyester film with or without impregnation being used for slot insulation.
• A Particularly important of the machines are the insulation of rotors and

stators.

• The main insulation of the stators have always been mica based.
• For the support between the winding bars, slots and the core

lamination , glass fiber reinforced epoxy is used.

APPLICATIONS IN CIRCUIT BREAKER

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• A circuit breaker: a switch which automatically opens the circuit when a

critical current or voltage rating is exceeded.

• AC currents are considerably easier to interrupt than DC currents.
• AC current interruption sequence:

1) an arc for part of the metallic circuit 2) its deionization when the current goes through zero, so that the arc will not form again.

• Circuit breakers categories: the low voltage and high voltage types.
• Low voltage breakers
• use synthetic resin moldings to carry the metallic parts.
• for higher temperatures ceramic parts are used.
• if the arc is likely to come into contact with molded parts, melanine or

some special kind of alkyd resins are used because of their greater arc resistance.

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• High voltage breakers: air circuit breakers and oil circuit breakers.
• many insulating fluids are suitable for arc extinction
• the choice depends on the rating and type of the circuit breaker.
• commonly used insulating fluids:
• - atmospheric air,
• - compressed air,
• - high vacuum,
• - SF6 and
• - oil (transformer oil) (interrupts the arc)
• - Askarels produce large quantities of toxic and corrosive

products.

• The circuit breaker bushings of lower voltage ratings may consist of

solid cylinders of porcelain and shellac or resin treated paper wrapped

• n the current carrying electrode.
• High voltage bushings of voltages of 66 kV and above are filled with
• il. The constructional details vary widely

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Various Insulating Material For High Voltage Switchgear

• Epoxy Resins

Low pressure castings for bushings, Switchgear, bus-bars, instruments transformer.

• Epoxy Resins bonded

For components such as arc control devices, CB glassfibre

• Poly ester resins

Insulating lever for CB and phase barrier plate in switch board.

• Porcelain

Insulators and bushings of power transformer

• Synthetic resin bonded

Bushings, arc chambers, etc paper

• Nylon

Injection moldings for arc control devices in CB

• Silicone rubber

Filling for molded joint boxes in SF6 insulated CB

APPLICATIONS IN CABLES

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• Synthetic rubbers and plastics are used as cable

insulation.

• The insulator should have
• good elongation and tensile strength and toughness to

withstand handling during installation and

• service.
• low dielectric constant and power factor
• high dielectric strength and insulation resistance.
• excellent resistance to ageing at high temperatures.
• resistance to long exposure to sunlight and various

chemicals.

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• High voltage cables also give rise to ozone and the

insulation will deteriorate in its presence. This is particularly severe for the insulation near the conductors.

• Underwater applications require very low water absorption

for the insulator.

• At low temperature the insulation should not become stiff

and brittle.

• The partial discharges in the cable insulation should also

be kept as low as possible.

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The main type of insulations used in the cable industries are paper, rubber, plastic and compressed gas. Paper insulated lead sheathed cables are still used because of the reliability, high dielectric strength, low dielectric loss, and long life. Low and medium voltage (up to 3.3 kv), polyvinylchloride (PVC), Polyethylene and cross-linked Polyethylene (XLPE) is most used. PVC is not suitable for high voltage because High dielectric constant and high loss Polyethylene has low dielectric constant and low loss but high dielectric strength. The best material for high voltage and high temperature is teflon (PTFE) which can be employed up to 250 0C

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3.6/6 ~ 18/30kV XLPE/DSTA/PVC Power Cable

1-Conductor 2-Conductor shielding ( Semi-conducting layer ) 3-Insulation 4-Insulation shielding ( Semi-conducting layer ) 5-Metallic shielding 6-Fillers 7-Binder tape 8-Separation sheath 9-Steel tape armour 10-Outer sheath

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PVC INSULATED CABLES

APPLICATIONS IN POWER CAPACITOR

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• power capacitors are used for
• - voltage regulation of power transmission systems
• - the improvement of power factor of power distribution

networks

• - power factor correction in high frequency heaters and

induction furnaces

• also used in DC applications such as
• - impulse voltage generators
• - energy storage
• - welding
• - high intensity flash x-ray and light photography.

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Power capacitors are made of

• - several layers of insulation paper of adequate thickness
• - aluminium foil of 6 microns thickness as electrodes interleaved and

wound.

• - single units are connected in parallel to increase capacity
• - placed in containers hermetically sealed, thoroughly dried, and then

impregnated with insulating oil:

• - mineral oil (high cost)
• - chlorinated diphenyl oil (low cost, non-inflammable; hence,

preferred)

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• Properties required for the insulation paper:
• - high dielectric strength
• - low dielectric loss
• - high dielectric constant
• - uniform thickness, and
• - minimum conducting particles.
• Compared to paper, polypropylene film has considerable power dielectric loss

and higher operating voltage.

• However, paper is still widely used partly, mainly due to the reason that paper

after impregnation offers many desirable properties required for use at high voltages in addition to economy.

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The Concept of The Dielectric in paper and all paper film In Capacitor

APPLICATIONS IN HIGH VOLTAGE BUSHING

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• Insulation material used for high voltage bushing is paper that is treated
• 3 types for treated paper are
• Resin-bonded paper (RBP)
• Oil-impregnated paper (OIP)
• Resin-impregnated paper (RIP)
• The paper is wound onto a central tube and conducting layers are then

inserted to form a series of concentric capacitors between the tube and the mounting flange.

• Since the 1960s, resin materials have been used for all types of bushing up

to the highest voltages.

• Resin insulation has greater dielectric strength than paper and requires less

stress control at voltages below 25 kV

• Failure due to partial discharge caused by the difficulties of eliminating

micro voids in the resin around the screens during the casting process.

• The need to eliminate voids in resin becomes more sensitive as voltages

increases, and it is normal to revert back to resin impregnated, foiled paper insulation for bushings rated over 72.5 kV.

APPLICATIONS IN HORSE POWER MOTOR

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APPLICATIONS IN FRACTIONAL HORSE POWER MOTORS

Small Generator :

• The insulation system must guarantee reliability, combined with

good processing capability, mechanical strength an tolerance to severe short thermal stress while in operation.

• Kind of application the following materials have been found to have

excellent insulation properties, viz. presspaper reels, NMN multilayers (made of nomex and polyester film) as well as saturated DMD (made from non-woven polyester and polyester film).

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APPLICATIONS IN FRACTIONAL HORSE POWER MOTORS

Small size motors : Main insulation for small size motor is pressboard due to no melting board. Multilayer material made from pressboard and polyester film (used in electrical power tools) can withstand short-term thermal stresses. Many kinds of Polymers have been developed and are being widely used as insulating materials in small size motor.