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Gas insulated

transmission line

(GITL)

GIL high-power

transmission

technology

Abstract:

Second-generation gas-

insulated lines for high power transmission

are the best option where environmental or

structuralConsiderations rule out the use of overhead

transmission lines.

The outstanding features of aGIL system are its high transmission

capacity, superior electromagnetic

compatibility (EMC)to any other

transmission system, low losses, high safety(no fire hazard) and flexible installation

options.

GIL can be laid above ground,installed in tunnels or buried directly in the

soil, depending on individual requirements.

INTRODUCTION:Gas Insulated Transmission line

(GIL) is best choice where High rating

power transmission is needed.

It is nothing but 2 Conductors,Gas mixture and supporters (Resistive

support). Gas Insulated Lines are

environmental friendly and technically easyto implement. Its operational features are

also excellent.

CONSTRUCTION:

GIL consists of 2 concentricAluminum tubes. The tubes are made of

corrosion resistive aluminum alloy.

Outer conductor is to cover inner

conductor and gas filled in it. Tubes are

welded by computerized process and tested

by ultrasonic device to inspect and remove

cracks and to ensure that system is gas-tight.Outer tube is taken as ground potential. A

particle trap is inserted in the tubes to

increase their life.Cast resin insulators are used tosupport inner conductor. These insulators

also helps to put inner Aluminum tube at

center.Mixture of gas is filled between inner and

outer tube. This increases insulating capacity

of Transmission line. Mixture of gas

consists of 80% Nitrogen and 20% SF6(sulphur hexafluoride) which provides

electrical insulation.

Length of GIL lies between 60km to100km. Because of the large cross section of

the conductor, electrical losses in GIL are

less when compared with other transmission

systems. So, transmission and operating costreduces.

GIL systems are based on the

successful SF6tubularconductor technology,

which has been around for several decades.GIL consist of a central aluminum conductor

with a typical electrical cross section of up

to5,300 mm2.

The conductor rests on cast resininsulators, which center it within the outer

enclosure. This enclosure is formed by a

sturdy aluminum tube, which provides a

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solid mechanical and electro technical

containment for the system. To meet up-to-

date environmental and technical aspects,GIL are filled with an insulating gas mixture

of mainly nitrogen and a smaller percentage

of SF6. For increased lifetime, theperformance product series has alongitudinal particle trap installed over the

entire horizontal route section. An

automated orbital welding procedure,accompanied by tailored ultrasonic

inspection techniques, ensures perfect gas

tightness of the aluminum tubes.

During service, the fully encapsulateddesign fully protects the GIL against

environmental influences. Thanks to the

technologically clear-cut, logical design andthe use of high-quality materials, an

absolutely maintenance-free product is

achieved which requires external inspection

only.And at the end of its service life, the

issue of de installation is solved. The GIL

tubular system with all its components andthe insulation gas mixture are100 percent

recyclable. These factors help to minimize

lifetime costs.

GIL system in Germanys Wehr power

plant:

A GIL in a tunnel in the Wehr pumped

storage power station in the Black Forest as

long ago as 1975. With a single-phase lengthof almost 4 km this installation is still a

significant reference among worldwide GIL

projects. Notwithstanding its service time,an inspection after 30 years showed that allcomponents were still in top condition, and

assured the customer that

GIL will provide many more years ofreliable operation.

Flexible grid connectionsoptimum grid

integration

GIL systems consist of a manageablenumber of modular elements which can be

combined according to defined technical

rules. Thus GIL systems are not limited intheir entire length. Moreover, they are suited

for almost any kind of routing, for instance

through built-up areas or road crossings, on

marshy ground, etc. To meet theserequirements the actual installation of GIL

makes use of sophisticated laying

techniques. These techniques are based onthe well-proven procedures of the pipeline

construction industry, adding some product-

specific modifications.

Consequently project implementationtime can be kept to a minimum. Due to their

outstanding design features Siemens GIL are

remarkably flexible for differentapplications. Thanks to their transmission

capacity and low losses GIL can be linked

directly, one-to-one to overhead lines,

continuing the lines underground. As aresult of the low electrical capacitance of

GIL, compensating reactors are generally

not required, even for very long GILsections of up to 70 km. The technical

particulars of Siemens GIL allow for an auto

reclosure pattern for the OHL, so that no

modification of the protection concept isneeded. For the same reasons, GIL are also

perfectly suitable for direct connection to

substations or transformers.

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Outstanding safety in operation:GIL systems live up to their reputation not

just with their technical specifications, but

also by providing excellent operational

safety. GIL systems are immuneto hazardsthat are inherent to other power transmission

systems.

They are safe to touch in operation, as

their housing is solidly grounded. They arefireproof and explosion-proof. The electrical

insulation system is not subject to aging

phenomena, which reduces the risk ofinternal failures to virtually zero. GIL are

constructed employing separate gastight

compartments of variable length,which further increases safety in case of

an external impact. GIL systems are gastightand sealed for their lifetime. Consequently

they retain their superior operating

properties throughout their service life

Excellent electromagnetic compatibility

enables

flexible

route planningThe construction of the GIL results in much

smaller electromagnetic fieldsas much as15 to 20 times smallerthan withconventional power transmission systems .

This makes GIL suitable for completely

new routings through populated areas (e.g.next to hospitals or residential areas, in the

vicinity of flight monitoring systems,etc.).

GIL can be laid in combined infrastructure

tunnels together with foreign elements (e.g.close to telecommunication equipment).

Thus, GIL provides maximum flexibility for

the planning of transmission networks inEMC-sensitive environments, where

magnetic fields have to be avoided. Siemens

GIL systems satisfy the most stringent

magnetic flux density requirements, forexample, the Swiss limit of 1 T

.

application

and laying methods:

Flexibility for your successDue to their unique properties GIL systems

have become well established in all parts of

the world, to solve difficult transmissiontasks in complex routings. GIL installations

have been realized in every conceivable

layout, with shafts mastering straight

vertical distances of 200 m, overcoming

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steeply inclined slopes, passing around

buildings both above- and belowground,

and smoothly following serpentine routingswithout angle units.

Aboveground installation

GIL installation aboveground is a trouble-free option, even for extreme environmental

conditions. GIL are unaffected by high

ambient temperatures, intensive solarradiationmoisture). Corrosion protection is

not always necessary.Particularly high

transmission power can be achieved with

aboveground installation.

Tunnel installation

Tunnels made up of prefabricated structuralelements are another quick and easy methodof 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 GILis installed once the tunnel has

been completed. With this method of

installation the land above the tunnel can befully 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 hightransmission capacity is ensured.

Vertical installation

Gas-insulated tubular conductors can beinstalled 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 tothe switchgear and overhead line on the

surface. As GIL systems pose no fire risk,

they can be installed in a tunnel or shaft thatis accessible and can also be used forventilation at the same time.

Direct burial

Siemens also offers GIL solutions designedfor direct burial. These systems are coated

with a continuous polyethylene layer to

safeguard the corrosion-resistant aluminum

alloy of the enclosure, providing protectionof the buried system for > 40 years. As

magnetic fields are marginal in the vicinity

of all Siemens GIL applications, the landcan be returned to agricultural use with very

minor restrictions once the system is

completed.

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Typical referencesIn the Limberg II pumped-storage power

station in Kaprun, Austria, built in 2010, a

GIL system was laid in a shaft with agradient of 42 . It connects the cavern

power plant with the 380 kV overhead line

at an altitude of about 1,600 meters. As GILsystems pose no fire risk, the GIL tunnel is

not only accessible but also used

forventilation purposes. This resulted insubstantial cost reduction by eliminating the

need for a second shaft in this project.

A typical example of low EMC values is thePALEXPO Project in Geneva, Switzerland.

A GIL system in a tunnel replaced 500meters of a former 300 kV double overhead

line which had to be moved for the raised

exhibition center building. The line owner

based his decision to opt fora GIL solution over a cable solution on the

GILs much bettervalues with respect to

EMC. Highly sensitive electronic equipmentcan now be exhibited and operated in the

newhall without any danger of interference fromthe 300 kVconnection located below it.

Minimum effortoptimum result

GIL was developed to meet a wide variety

of requirements for installation andoperation. A decisive factor in meeting this

demand was an installation process that

permits assembly of prefabricated modulesat the installation site, thus allowing

optimum adoption of the selected routing.This concept also has logistic advantages.All elements such as tubes, angles and

special modules are lightweight and small

enough to be transported by comparatively

light standard trucks. During installation amajor focus is providing gastight

connections for the components. To

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accomplish this requirement, Siemens

employs a computer-controlled automatic

welding process. A welding robot ensuresthe highest precision and reproducibility of

the welding seams. The quality of each seam

is verified to 100 percent with ultrasonictests to ensure perfect gas tightness andmechanical strength. As a result, no

replenishment of insulation gas

is needed during the entire service life of >50 years.

GIL TechnicaldataRated voltage

Typical ratedcurrent(higher values onrequest)Rated short-circuit current

Insulating gas

mixture

Typical systemlength

Impulsewithstand voltage

Capacitance

Overloadcapacity

depending ondesignand requirements

245 to 550 kV

up to 4,500 A

63 kA/3 s

N2 and SF6

100 m to 100km

1,050 to 1,675kV

55 nF/km

up to 100 %

Outerdiameter ~375

to 512 mm

50 kg/m

Weight per phase

Advantages:

Cooling system:Due to large outer tube surface GIL dont

require any sophisticated cooling system.

Reactive compensation:Gil have low capacitance. Dielectric

losses are also less, so, reactive

compensation is not needed.High safety:

Due to presence of SF6, GIL are capable

of extinguishing any arc produced in it at the

time of failure. GIL are fire resistant. Sosafety to person and environment is ensured.

Highest reliability:

GIL is being used all over world fromlast 35+ years without any case of failure.

Maintenance is less:

Electrical and thermal aging in case of

GIL is less hence very less maintenance isrequired. (Once in 50 years)

Low emission of magnetic fields:

The conductor current in the inner

aluminum tube induces a reverse current ofthe same size. Consequently the

electromagnetic field outside the GIL is

negligible. No special shielding is requiredeven in areas which are critical with respect

to EMC (e.g. airports or computer centres or

in populated areas).Landscape view:

GIL can be used underground without

any special shielding. It can be replaced by

high towers and landscape view ofsurrounding can be maintained. It also

reduces unwanted usage of land.

Arrangement:

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GIL can be used horizontally, vertically

or in any inclined arrangement.

Applications:GIL are commonly used for transmission

systems with 345-550 kV operative voltages

and 2500-5000 Amp operation current.Following are general sites of applications;1. Feeder connections in power plants

2. Transmission systems in tunnels beneath

cities3. Transmission of EHV (Extra High

Voltage) is required within restricted places.

Challenges now and

in the future:Our company founder, Werner von Siemens,had a passionfor inventing trendsettingtechnologies and placing them into the

service of mankind. Siemens GIL is very

much

in line with this philosophy, since itspioneering technological advantages

revolutionize energy transmission with

extra-high voltage and extra-high current.Continuously growing world population and

urbanization lead to a strongly increaseddemand for bulk power transmission atextra-

high voltage, right into the heart of cities. Atthe same time the available space for

transmission systems has been restricted

more and more, and environmental

requirements such as EMC and fireprotection have gained increased

importance. GIL fulfill these requirements

perfectly. Besides these demands intransmission, power generation is

undergoing a conceptual change. As naturalresources are limited, regenerative powergeneration systems are

becoming more important. Offshore wind

parks and solar power plants are being

installed, providing a huge amount of energyat remote places. Consequently,

transmission

systems are needed that can transport bulk

power with the utmost reliability and

minimal losses.

Company founderWerner von Siemens, 1887The answer to these challenges isGIL:At places where overhead lines cannot be

used, Siemens

gas-insulated transmission lines provide

numerous advantagesthat differentiate them from any other

transmissionsystem:

High power ratings (transmission

capacity up to 3,700 MVA per system)

High overload capability

Autoreclosure functionality

Suitable for long distances (70 km and

more without compensation of reactive

power)High short-circuit withstand capability

(including internal arc faults)

Possibility of direct connection to gas-insulated switchgear (GIS) and gas-insulated

arresters without cable entrance fitting

Nonflammable; no fire risk in the event

of failure Lowest electromagnetic field

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Conclution:

GIL will undoubtedly also be thebackbone of the demanding

transmission projects of the future

whether for the transmission ofgigawatts of power from very large

offshore wind farms through underseatunnels, as is being considered for the

North Sea in Europe; or for a

maximally reliable connection of

important power stations to outgoing

lines, as for projects like Desert- Tecetc.; or for the transfer of bulk power

underground right into the megacities

of the future, for a continuallyimproving quality of life for mankind.

Applications:GIL are commonly used for

transmission systems with 345-550

kV operative voltages and 2500-5000Amp operation current. Following are

general sites of applications;1. Feeder connections in power plants

2. Transmission systems in tunnels

beneath cities3. Transmission of EHV (Extra High

Voltage) is required within restrictedplaces.

Some technical information:Typical Technical voltage : 220-500

kVMaximum operation voltage: 245-550

kVNominal frequency: 50 / 60 Hz (as per

system design)

Housing and conductor material:

Aluminium alloyInsulation gas: Gas-mixture (80% N2

and 20% SF6)

Nominal pressure (absolute): 0.7 MPa

Environmental temperatures: -30Cup to +50C

Weight per phase: 50-70 kg/m

(Approx.)

Capacitance: 55 nF/km

Inductance: 0.205 mH/km

Surge impedance: approx. 60

Ohm

Electromagnetic field: negligible

T

Total length single-phase >

90,000 mAboveground installation >62,000 mTunnel installation > 23,000 mDirectly buried installation >5,000 m