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GAS INSULATED SUBSTATION


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A

project

report on

GAS INSULATED SUBSTAION (GIS)

Submitted in partial fullfillment of

requirements for the award of degree of

DIPLOMA ENGINEERING

( ELECTRICAL ENGINEERING )

To

GTU

Under the Guidence of Prof. Mitesh Tadvi

Department of Electrical Engineering

SHRI K.J. POLYTECHNIC, BHOLAV

BHARUCH, GUJARAT , INDIA


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SHRI K.J. POLYTECHNIC, BHARUCH

CERTIFICATE

This is to certify that the work presented in the project report on

GAS INSULATED SUBSTATION

Has been carried out

By

MITAL K. SHAH 096450309001

PARYANAK G. DAVE 096450309008

ROHIT D. WAGH 096450309010

BHAVIN S. OZA 096450309023

DARSHIT R. SHAH 096450309034

JIGAR M. SOLANKI 096450309035

NACHIKET M. BHAGAT 096450309040

SACHIN C. PARMAR 096450309073

DATTESH V. VASAVA 096450309109 RAJ N. JETHWA 096450309

In a manner sufficiently satisfactory to warrant its acceptance

As partical fullfillment of the requirement for the award of the degree of the

DIPLOMA IN ELECTRICAL ENGINEERING

PROF. MITESH TADVI

(GUIDE)

Head of the department Pricipal

(Mrs. Bina Pandya ) (prof. )

ACKNOWLEDGEMENT


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On completion of my seminar, it gives us feeling of great pleasure andsatisfaction. Hence we would like to take this opportunity to express our gratitude towardsthose who made this possible.

Words cannot express the zeal of ecstasy while depicting our deep sense of,gratitude to oue esteemed knowledge guid Prof. MITESH TADVI who inspires us toundertake this seminar. His dynamic guidence and continous interest kept us going in spiteof all the difficulties. His technical guidence, knowleageble and dynamic approch is thecorner stone. This dissertation is impossible to complete without his most valuable suggesionand guidence .

We also thankful for all staf member of electical department for their kind

support at various stages of the work .

THANK YOU.

MITAL K. SHAH

PARYANAK G. DAVE

ROHIT D. WAGH

BHAVIN S. OZA

DARSHIT R. SHAH

JIGAR M. SOLANKI

NACHIKET M. BHAGAT

SACHIN C. PARMAR

DATTESH V. VASAVA

RAJ N. JETHWA


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ABSTRACT

Conventional substations requires, small installation size,protection against atmospheric pollution and moisture, noiselessoperation, nonexplosive and flame resistant, reducedmaintenance, minimal radio interference, but totally enclosedsubstations using SF6 gas as insulation that are also known asGIS is now in widespread use in the electrical power industry.

The first GISs were put in operation in 1967 in Switzerland andGermany. The GIS in Germany is still in operation, whereas theGIS in Switzerland were decommissioned after 35 years of operation without major fault or gas leak.


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1.Introduction

Gas Insulated Substations (GIS) is a compact,multicomponentassembly enclosed in a ground metallic housing in which the

primary insulating medium is compressed Sulphur hexafluoride(SF6)gas.

Conventional substations requires, small installation size,protection against atmospheric pollution and moisture, noiselessoperation, non explosive and flame resistant,

reduced maintenance, minimal radio interference, but totallyenclosed substations using SF6 gas as insulation that are alsoknown as GIS is now in widespread use in the electrical powerindustry.

Gas insulated substations are particularly

suitable for underground construction beneathpublic buildings.Gas insulated substation mainly

used for a power transmission system or

a sub-station system, of which outgoing

bus bar is shortened to reduce consumption

of the outgoing bus bar.


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1.1 Sulfur Hexafluoride (SF6) Gas:

A colorless, odorless, non-toxic, liquefied gas. Shipped as a liquidunder its own vapor pressure. Sulfur Hexafluoride gas, which is itself inert.

It disassociates in the presence of an RF field to form reactivefluorine ions.

SF6 acts as insulation between live parts & the earthed metal closure.

The dielectric strength of SF6 gas at atmospheric

pressure is approximately three times thatof air.pace requirement is only 10 to 25percent of what is required is aconventional substation.

SF6 can be prepared from the elements through exposure of S8 to F2.

It does not react with molten sodium, butreacts exothermically with lithium.Of the 8,000 tons of SF6 produced per year, most (6,000 tons) is usedas a gaseous dielectric medium in the electrical industry.

It has a much higher dielectric strength than air or dry nitrogen. Thisproperty makes it possible to significantly reduce the size of electricalgear.


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1.2 Properties of SF6:


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1.3 SF6 gas as insulating and arc-quenchingmedium:

Sulphur hexafluoride gas (SF6) is employed as insulation in allparts of the installation, and in the circuit-breaker also for arc-

quenching. SF6 is an electronegative gas, its dielectric strength atatmospheric pressure is approximately three times that of air. It isincombustible, non-toxic, odourless, chemically inert with arc-quenching properties 3 to 4 times better than air at the same pressure.

Commercially available SF6 is not dangerous, and so is not subjectto the Hazardous Substances Order or Technical Regulations onHazardous Substances (TRGS). New SF6 gas must comply with IEC60376 (VDE 0373 Part 1). Gas returned from SF6 installations andapparatus is dealt with in IEC 60480 (VDE 0373 Part 2). SF6 releasedinto the atmosphere is considered a greenhouse gas. With itscontribution to the greenhouse effect below 0.1%, the proportion of SF6is low compared to that of the better known greenhouse gases (carbondioxide, methane, nitrous oxide etc.).

The gas pressure is monitored in the individually sealed gascompartments and in the circuit-breaker housing. The low gas losses(below 1 % per year) are taken into account with the first gas filling.Automatic make-up facilities are not necessary.

The isolating gas pressure is generally 350 to 450 kPa at 20C.Insome cases this can be up to 600 kPa. The quenching gas pressure is

600 to 700 kPa. Outdoor apparatus exposed to arctic conditions containsa mixture of SF6and N2, to prevent the gas from liquefyin


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DESIGN OF GIS:


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SPECIFICATION OF GIS:

The range of application of SF6 gas-insulated switchgear extends fromvoltage ratings of 72.5 up to 800 kV with breaking currents of up to 63kA, and in special cases up to 80 kA. Both small transformer substations

and large load-centre substations can be designed with GIS technology.

GIS equipment is usually of modular construction. All componentssuch as bus bars, disconnectors, circuit-breakers, instrumenttransformers, cable terminations and joints are contained in earthedenclosures filled with sulphur hexafluoride gas (SF6).

Up to ratings of 170 kV, the three phases of GIS are generally in acommon enclosure; at higher voltages the phases are segregated. Theencapsulation consists of non- magnetic and corrosion-resistant castaluminum or welded aluminum sheet.

The weight and size of the GIS equipment do not change appreciablywith the voltage class as the bulk of the current carrying components

and enclosures have identical dimensions for similar thermal and shorttime current. The additional insulation required for the next voltageclass is achieved by increased gas density. Owing to these flexibilities, afew manufacturers offer the same equipment for two voltage classes(like 170/145 kV).Even when the GIS equipment is designed for anindividual voltage class, the dimensions and weights of the equipment

differ marginally Station arrangement


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GAS INSULATED TRANSFORMER

Use SF6 Gas as the insulating and cooling medium instead of insulating oil.

First units produced in 1967.

Transformer applications: Distribution class units up to 400 MVA, 345 kV.

Primarily used in substations located in urban areas (including inside buildings, underground)due to safety benefits.

Worlds largest Gas Insulated Transformer(GIT)

MERITS OF GIT OVER OIT

Nonflammability Gas insulated transformers , using incombustible SF6 gas as insulation andcooling medium, enable to remove a fire fighting equipment from transformer room.

Non Tank explosion -Pressure tank enables to withstand the pressure rise in case of internalfault.

Compactness Since conservator or pressure relief equipment is not necessary, height of transformer room can be reduced approximately 2 2.5 meters.

Easy installation oil or liquid purifying process is not necessary in case of gas-insulatedtransformer.

Easy inspection and maintenance work -Only SF6 gas pressure shall be basically monitoredduring periodically inspection.


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BASIC MODULE OF GIS STATION:

TYPES OF MODULES IN A GIS Bus Bar Module : This module is equivalent

to the Bus Bar arrangements in normal AIS station,and tappings are taken at different points.

Line Module: In this module as shown in thesecond figure it has one Isolator, CB, One CT andOne LA all built inside the closed chamber.

Transformer Module : In this module as shown in the third figure it has one Isolator,Circuit Breaker and One CT all built inside the closed chamber.

PT Module: For the sake of measuring the Bus Bar voltage at any time, the PT isconnected to the Bus Bar through PT Module .

Bus Coupler Module : In case if it is a two Bus Bar arrangement, for interconnectingthose two buses Bus Coupler module is used.

Bus

module line module transformer module PT module


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2. Components of GIS:

A typical GIS arrangement consists of 1.

Bus Bar 2. Disconnector3. Main Earthing Switch 4. Current Transformer 5. CircuitBreaker 6. Current Transformer 7. Main EarthingSwitch 8. Disconnector 9. Earthing Switch10. Voltage Transformer 11. Bushing & Cable and boxes

Gas sections are used as spacers in order to minimize the range of trouble, allow for prompt repair, and monitor the gas effectively.


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Circuit Breaker:

Under short circuit conditions, however, the current may reach tens of thousands of amperes at a power factor as low as 0.1. It is duty of acircuit breaker to interrupt such currents as soon as possible toavoid equipment damage.

Current Transformer:

Current transformer (CT) issued for measurement of electric current.Current transformers are also known as instrument transformers.Current transformers are commonly used in metering and protectiverelays in the electrical power industry .

Voltage Transformer:

Voltage transformers (VTs), also referred to as Potential transformers"

(PTs), are used in high-voltage circuits.They are designed to present a negligible load to the supply beingmeasured, to allow protective relay equipment to be operated at lowervoltages, and to have a precise winding ratio for accurate metering.


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Disconnector:

Disconnectors or isolators are used for electrical isolation of circuitparts. They are slow acting and operating at off load Disconnectors mustbe carefully designed and tested to be able to break small chargingcurrent without generating too-high over voltage,

Bus Bar:

According to the rating current Specified, the conductorsAre made of copper or aluminum and the enclosures are made of mildor stainless steel, Or aluminum.

Earthing Switch: Every earthing switch has a short-time current endurance function. Theyare divided into two types with each onehaving its own particular functions. The first isthe maintenance earthing switch, and it ismotor driven. The second, the high-speedearthing switch, can be motor drive-spring

charged. Manual operation is possible for alltypes. High-speed earthing switch has ashort circuit making current capability. The earthing switches can beused as primary injection terminals for checking voltages and currents .


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Comparison between GIS & AIS:


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Advantages Of GIS: Compactness: The space occupied by sf6 installation is only 10% of that of conventional

outdoor substation. High cost is compensated by the savings in the cost of space. Reduced Installation Time Protection from pollution Increased safety Superior Arc Interruption Explosion proof and Fire proof installation They generate no noise & have no radio interference It offer solutions including -Industrial areas where space & pollution problems -Mountain areas where ice & snow are major problem SF6 has been identified as a greenhouse gas, safety regulations are being introduced in

order to prevent its release into atmosphere.

Disadvantages of GIS:

High cost compared to conventional outdoor substation Excessive damage in case of internal fault. Long outage periods as required to repair Requirement of cleanliness are very stringent This substations are generally indoor and needs separate building Procurement of gas and supply of the gas to the site is problematic VFTO (very fast transient over voltage) during switching operations or earth faults and

transient enclosure voltages and particle contamination The life of GIS is affected by certain factors such as: conductive particles, partial

discharges and contamination (decomposition products, water, etc.) However conducting contamination (i.e. aluminum, copper and silver particles) could

seriously reduce the dielectric strength of gas-insulated system.

.


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Disadvantages of AIS:

a) It requires huge amount of area .

b) Each and every component of substation is exposed to air andpollution.

c) Particularly in coastal area all the insulators are exposed to airand saline contamination.

d) Frequent flashovers and breakdown occurs.

e) Maintenance cost is more.

f) Installation time is also more.

g) More concrete work is necessary.

CONCLUSION

GIS are necessary for EHV& UHV and some important areas to bestudied include more conservative designs better particle control &


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improved gas handling & decomposition product management GIShas small ground space requirements. techniques

Achieving & maintaining high levels of availability requires a more

integrated approach to quality control by both users andmanufactures

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