APPLICATIONS OF INSULATING MATERIAL CHAPTER 5 1
• 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: 2
• 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 3
• Insulating material must be able to withstand electrical stresses, in addition to it should be able to withstand certain other stress namely during manufacture, storage and operation. • The performance of the insulation depends on its operating temperature. The higher temperature, the higher will be the rate of its chemical deterioration. 4
The insulating materials are grouped into different classes with temperature limit : Impregnation: Letting the solid material absorb some liquid 5
IEC (International Electrotechnical Commission ) 6
APPLICATIONS IN POWER TRANSFORMER 7
There are four principal areas where insulation must be applied (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. 8
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. 9
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. 10
• 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. 11
• 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. 12
Transformer Oil • 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 oxygen itself is limited. • sealed transformers • filling with nitrogen gas, • providing oxygen absorbers like activated clay or alumina 13 • arc discharge inside a transformer decomposes the oil and causes explosions
Dry type transformers • 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. 14
APPLICATIONS IN ROTATING MACHINES 15
• 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. 16
APPLICATIONS IN CIRCUIT BREAKER 17
• 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. 18
• 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 on the current carrying electrode. • High voltage bushings of voltages of 66 kV and above are filled with oil. The constructional details vary widely 19
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 SF 6 insulated CB 20
APPLICATIONS IN CABLES 21
• 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. 22
• 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. 23
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 0 C 24
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 25
PVC INSULATED CABLES 26
APPLICATIONS IN POWER CAPACITOR 27
Recommend
More recommend
