PRESENTATION ON GAS INSULATED

TRANSMISSION LINES

Phenice johnVIII sem, EE

introduction

•Composed of pipes that houses conductors in SF6 •Invented in 1974•Both overhead & underground system•Most economic, environment friendly & maintenance free alternative

GIL

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Gas Insulated Transmission Lines (GIL) are a means of bulk electric power transmission at high and extra high voltage. GIL consists of tubular aluminium conductors encased in a metallic tube that is filled with a mixture of Nitrogen and Sulphur Hexafluoride gases for electrical insulation

Why gil• Installation expense is less

• Other transmission lines need extra cooling arrangement

• Allowed conductors to be larger in cross-section

• Construction of large capacity transmission link

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• LowlossesThe resistive losses of GIL are lower compared to cables and overhead lines. The dielectric losses of GIL are negligible. This reduces the operation costs and causes savings. Due to the large outer diameter, the heat dissipation is better than with cables and GIL normally do not require sophisticated cooling systems.

• No AgeingThe insulation system of GIL shows no ageing phenomena, neither electrical nor thermal. There is practically no maintenance needed on the GIL for up to 50 years or more.

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• HighSafetyIf an insulation failure would occur in a GIL, the fault arc would be safely enclosed within the outer housing. No influence would occur to the outside. The GIL are fire resistant and do not contribute to fire load. This means optimal protection of persons and environment.

• High ReliabilityGIL technology has proven its reliability since over 35 years in service without any failure up to now.

Sf6 & n2 gas SF6 mixtures are presently applied or considered

for application in two major fields, primarily to enable switchgear operation at low ambient temperatures and to save SF6 in large insulation systems. In both cases, the performance of the mixture is inferior to that of unmixed SF6.

SUPERIOR PROPERTIES OF SF6 GAS

• High dielectric strength at lower cost

• Regeneration capacity following a breakdown

• Low pressure-increase following a breakdown

• High load transfer capacity

INSTALLATION• Ground level• Elevated above ground• Below grade in an open or covered trench• Directly buried underground• Vertically in tunnels, shafts or towers• Suspended from existing substation structures

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Aboveground Installation GIL installation aboveground is a trouble free

option, even for extreme environmental conditions. GIL are unaffected by high ambient temperatures, intensive solar radiation or severe atmospheric pollution (such as dust, sand or moisture). Corrosion protection is not always necessary. Particularly high transmission power can be achieved with aboveground installation.

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Tunnel Installation Tunnels made up of prefabricated structural elements are

another quick and easy method of GIL installation. The tunnel elements are assembled in a trench, which is then backfilled to prevent any long-term disfiguring of the local landscape. The GIL is installed once the tunnel has been completed. With this method of installation the land above the tunnel can be fully restored to agricultural use. Only a negligible amount of heat is dissipated to the soil from the GIL. The system stays accessible for easy inspection and high transmission capacity is ensured.

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Vertical Installation. Gas-insulated tubular conductors can be installed without

a problem at any gradient, even vertically. This makes them a top solution especially for cavern hydropower plants, where large amounts of energy have to be transmitted from the underground machine transformer to the switchgear and overhead line on the surface. As GIL systems pose no fire risk, they can be installed in a tunnel or shaft that is accessible and can also be used for ventilation at the same time.

Direct burial

These systems are coated with a continuous polyethylene layer to safeguard the corrosion-resistant aluminium alloy of the enclosure, providing protection of the buried system for >40 years. As magnetic fields are marginal in the vicinity of all Siemens GIL applications, the land can be returned to agricultural use with very minor restrictions once the system is completed.

MAIN COMPONENTS

• Conductors & enclosures• Insulators & particle traps• Plug in contacts• Aluminum bellows• Enclosure joints

Conductors and Enclosures

Extruded aluminum alloys, which are widely used in aluminum pipe bus conductors at substations, were chosen as materials for conductors and enclosures. They satisfy conductivity, machinability and economic efficiency requirements. Considering the enclosures had to be transported to site by vehicles, the length of each section of the enclosure was 14 m (46 ft). Thus, some 1500 sections and joints are included in the GIL tunnel.

Insulators and Particle Traps

• Heat-resistant epoxy resin was selected as the material for insulators to meet thermal design requirements, with consideration to electrical and mechanical properties and casting workability of epoxy. Relatively inexpensive post-type and tri-post-type insulators were installed in the standard sections of GILs, and cone-type insulators were used in some parts of the lines to form gas sections.

• to prevent metal foreign objects from any sections from adhering to insulators, particle traps capable of catching these objects and making them harmless were fitted inside the enclosures where the insulators were installed.

Plug-in Contacts Two types of plug-in contacts were used: a

standard-type plug-in contact, used to connect conductors in the field; and a long-type plug-in contact, capable of absorbing the thermal expansion of conductors and accommodating a seismic displacement. The long-type plug-in contacts with a sliding stroke of ±130 mm (±5.1 inches) and standard-type plug-in contacts with a sliding stroke of ±30 mm (1.2 inches) were specially developed.

Aluminum Bellows

Bellows absorb the thermal expansion of enclosures and a seismic displacement To secure adequate current-carrying performance, aluminum bellows were developed .The aluminum bellows have an expansion stroke of ±120 mm.

Enclosure Joints

For enclosure joints, expanded pipe joints with a ±1 degree variable angle performance and with high levels of air tightness were adopted. As expanded pipe joints allowed the tip of 14-m (46-ft)-long enclosures to move by 30 cm (12 inches), an error in the installation of units on a support in the field could be easily corrected.

ADVANTAGES OF SF6-N2 MIXTURE

• Less harm to atmosphere

• Reduced cost for GIL

• Useful for long distance power transmission

• Insulating property is improved

ADVANTAGES• High current carrying capacity• Limited working space• High efficiency• Low resistive & capacitive loss• Good appearance• No interference with communication circuits• No extra cooling is required• High reliability

DISADVANTAGES

• Length of is each GIL section is limited. • Particle contamination lower the insulating

reliability of GIL.• Breakdown of insulator.• Earthquake resistant design must be considered.• SF6 gas is harmful to ozone.

APPLICATIONS• Installation in tunnels

• Ideal for environments that are sensitive to electro-magnetic fields.

• Suitable for metropolitan areas where high energy rate is required.

• Well suited for high power transmission.

CONCLUSION

One of the best transmission system in the worldSafe and reliable for more than 20 yearsResearches are still going on to improve efficiency

Thanks!!!