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MODULE GIL BASICS

GAS INSULATED TRANSMISSION LINES - GIL

A. Introduction

GIL bus has evolved differently than bus used in Gas Insu-lated Substations. GIS stations are a compact arrangement ofcircuit breakers, disconnects, earthing switches, voltage andcurrent transformers etc. Usually connections between theseactive elements are only a few meters in length. Since the re-quired electrical shapes for enclosures are often complicated,castings are used extensively for both active components andfor the flanges used on interconnecting bus. A typical GISbus section and elbow is shown in figure II-1.

Fig. II-1. GIS Bus section. Bolted contacts and cast elbow.

The spacing between support insulators is determined bythe maximum allowable sag on the conductor and the maxi-mum length handable in GIS factories. This spacing is about6 meters and relatively independent of the voltage class.Since the insulators are designed to be captured betweenflanges there will be a flange set every 6 meters as well.

Each bus section will have an insulator on each end. Con-ductor contacts are bolted to either side of the insulator.When the insulator is required to allow free passage of theinsulating gas holes are cast into the insulator surface.Changes in direction are limited to 90 degrees.

Particle traps are not normally used in GIS bus since thesections are short and easily inspected for particle contamina-tion and the active elements have natural low field regions,which work as effective traps.

This arrangement provides an economical solution whenthe required bus section length is less than 6 meters. Whenthe bus length exceeds 6 meters designs specificallydeveloped for GIL can often provide a better solution.

Two concepts for GIL design are shown in figures II-2 andII-3. The first concept is factory assembled. The second ex-ample is designed for site assembly. Since the GIL designsare optimized for longer circuits, you will notice severaldifference between GIL and GIS bus. For GIL bus:

1. Insulators are completely contained inside theenclosure.

2. Particle traps are used to mitigate the effects of freeconducting particles.

3. Changes in direction can be done at any anglebetween about 85 degrees and 178 degrees.

4. Field connections either can be done with wroughtflanges or with field welded joints. Castings arerarely used.

5. Two types of insulators are available. An open typewhen free passage of gas is required and cone ordisk insulators where one gas compartment isisolated from another.

6. GIL bus is much more flexible than GIS bus. Sincethe insulators are totally enclosed and mechanicallydecoupled from the enclosure flanges, the bus can bend to accommodate changes in direction orsupport misalignment.

Fig. II-2. GIL design factory assembled sections. Particle traps used exten-sively with two types of insulators. Field welded or bolted joints are used forsection assembly. Elbows are an integral part of the shipping assembly.

The GIL design shown in figure II-2 illustrates some ofthese differences. While the conductor is still supportedevery 6 meters or so, the length of the shipping section is

only limited by the restrictions of transportation to site. In North America, 18 meter sections can be shipped withoutspecial permitting.

The insulators do not have bolted connections and do notpenetrate the enclosure envelope. Therefore a conductor con-tact is only required at each end of the 18 meter section. Thecontacts are an integral part of the conductor. Bolted jointsare not used.

Particle traps are used at each insulator assembly and atnatural low points in the system. Thus, field assembly proce-dures do not require the same degree of cleanliness control asfor GIS bus.

Flanges are much simpler since there is no insulator to

support. Single or double o-rings can be used to seal theflanges. If a double o-ring is used it is possible to leak checkthe joint without first filling the system with gas. Typicalcircuit lengths are longer for GIL connections. The longerlengths are usually done in one or more large gascompartments rather than several smaller compartments.Pressure relief devices are not required. In the unlikely eventof an internal fault, pressure rise will be minimal.

The wrought flanges offer better corrosion resistance thancast flanges. Surface protection and prevention of moistureingress is less critical.

Simply omitting the flange and using a welded joint can provide both a hermetic seal immune to environmental

effects and increased flexibility for alignment during fieldassembly.

Figure II-3 illustrates a design for longer GIL lengths.Both this design and the one shown in figure II-2 share thesame features of totally enclosed dielectric systems and openinsulator designs. The major difference is that the designshown in Figure II-3 is optimized for long circuit lengths andsite assembly.

Fig. II-3. GIL field assembled system for long circuit lengths.

1

Barrier Insulator Particle trap

Open Insulator18 meters

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When circuit lengths is longer (about 500 meters) it isoften more economical to pre-assemble the components at

site then install in longer sections. Mixtures of SF6 andNitrogen can also be used to reduce the overall SF6 content ofthe system.

The GIL is shipped to site in 12-18 meter lengths. Typi-cally, 10 sections are assembled using welded joints for boththe conductor and enclosure connections. Pre-fabricated 180meter sections of up to 500 m are then installed in their finalposition. Conductor plug and socket contacts are used only asnecessary to compensate for thermal expansion.

The welding is done using an automatic welding machine.This guarantees good weld quality and a leak tight seals.

B. Design

The dielectric design of GIL is based on a simple formulafor the electric field in a coaxial geometry. The electricalstress (E) at any given radial point (r) in the system isdetermined by:

22

1

ln

UE

rr

r

=

Where U is the applied voltage, r2and r1are the enclosureand conductor radii respectively.

The most economical use of materials occur when theconductor stress is minimized. This occurs when ln(r2/r1)=1.

Most GIL designs in use today are based on log ratios

close to unity. Determining the overall dimensions for aspecific voltage class is simply a matter of determining theallowable voltage stress on the conductor.

Allowable voltage stress is determined by the insulatinggas used and the gas density. Either pure SF6 or a mixture ofSF6 and Nitrogen are the usual choices.

Since the conductor stress is higher than the enclosurestress, the negative impulse voltage (BIL) is normally used todetermine system size.

The curves below show how these design principles mightbe applied to a typical GIL design. The curves are for pureSF6 at about four atmospheres absolute.

100

200

300

400500

600

700

50 100 150 200 250

Conductor (mm)

Enclosure(mm)

Fig. II-4. Relationship between enclosure and conductor diameters for

ln(r2/r1)= 1 (best use of the dielectric space)

Based on the required BIL performance of the system youfirst select the required conductor diameter. Required enclo-sure diameter is then selected based on the log ratio of 1.

0

500

1000

1500

2000

2500

50 100 150 200 250

Conductor diameter (mm)

BIL(kV)

Fig. II-5. BIL performance for various conductor diameters. (Assumes themost economical ratio of conductor to enclosure diameter)

Figure II-5 covers the entire range of transmission systemsfrom 115 kV through 800 kV.

The choice of insulating gas or gas mixture will determinethe allowable system dielectric stress. Small amounts of SF6in a Nitrogen base has very beneficial effects on thedielectric performance. For long circuit lengths, SF6/N2mixtures offer an environmentally friendly and economical

approach. Figure II-6 shows that a modest 15% SF6 contentwill double the power frequency breakdown voltage overpure Nitrogen at the same gas pressure.

While pure SF6 still offers the best dielectric performanceat a given gas density, 10-20% SF6/N2 systems operating at amodest increases of pressure can perform with equaldielectric ratings [II-21, II-22, II-23, II-24, II-25, II-27, II-28].

2

up to 500 m

12 - 18 meter lengths

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0

100

200

300

400

500

600

700

0 100 200 300 400 500 600 700 800

Pressure (kPa)

60HzBreakdown

kV

30 % SF6/N2

15 % SF6/N2

10 % SF6/N2

5 % SF6/N2

Pure N2

Fig. II-6. 60 Hz breakdown results for a 145 kV Coaxial Geometry withEnclosure/Conductor Dimensions of 241.3/88.9 mm.

Since GIL systems are usually designed based on the re-quired dielectric performance, each system will have aninherent current carrying capacity. The current carryingcapacity exceeds that of a single conventional cable circuitand can be matched to the capacities of overhead lines.

Figure II-7 shows typical current ratings based on enclosurediameters.

0

2

4

6

8

100 200 300 400 500 600 700

Enclosure diameter (mm)

(kA)

Fig. II-7. Ampacity- open air system without solar radiation as a function ofenclosure diameter.

C. History of GIL design

The first Gas Insulated Transmission line installed in theUS was at the PSEG Hudson Generating station in NewJersey in 1972. The 242 kV, 1600 amp system is rated 900kV BIL. The GIL is direct buried connecting the AISsubstation equipment to a transformer located remote fromthe substation. This installation is still energized and in usetoday.

Fig. II-8. 242 kV GIL being installed in the PSEG Hudson generating station.New Jersey. 1972. View looking back from the SF6-Air bushings.

The First Gas Insulated Transmission Line installed inEurope was in 1974 to connect the electrical generator of ahydro pump storage plant in Schluchsee, Germany.

Fig. II-9 shows a view into the tunnel in the Black Forestmountain with two systems of 420 kV to be connected to theoverhead line on top of the mountain. The rated current is2500 A.. The GIL went into service in 1975 and has been inservice without interruption ever since, to deliver peakenergy into the southwestern 420 kV network in Germany.

With 700 meters of system length running through atunnel in the mountain, this GIL is still the longest appli-cation at this voltage level in the world. Today at voltagelevels from 135 to 800 kV about 200 kilometers are installedworldwide. The applications vary from inside high voltagesubstations, power generation plants, areas with severe envi-ronmental conditions and to solve specific routing, right ofway or access problems.

Fig. II-9. Schluchsee Hydro Pump Storage Plant.

Typical applications of GIL are links within power plantsto connect the high voltage transformer with the high voltageswitchgear, within cavern power plants to connect highvoltage transformers in the cavern with the overhead line onthe outside, to connect gas-insulated substations (GIS) withoverhead lines or transformers, as a bus connectingcomponents within gas-insulated substations or as standalone installations of GIL for resolving specificenvironmental, routing, access or right of way issues. Circuitlengths can vary from 10 meters to kilometers in length. The

applications are carried out under all different climateconditions from the low temperature in Canada to the highambient temperatures in Saudi Arabia or Singapore, or severeconditions in Europe or in South Africa. The GILtransmission system is independent from environmentalconditions because the complete high voltage systemincluding insulators is completely sealed inside the aluminumenclosure.

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The GIL technology had proven its technical reliability inmore than 3000 km years of operation without majorfailures [II-3, II-5]. This high system reliability has beenachieved because of the simplicity of such a transmissionsystem where only aluminum tubes for conductor andenclosure are used and the insulating medium is a gasexhibiting no aging.

Up to now the advantages of GIL have been used for spe-cial applications. However, with the introduction of site as-sembly, standardization of components and improved designdetails, GIL is economical for the application over long dis-tances.

The breakthrough in cost reduction could be reached bydeveloping highly standardized GIL units, automated orbitalwelding machines, and pipeline laying methods. This allowsa much faster laying of the GIL. Since the insulators aretotally enclosed, by using the elastic bending of thealuminum pipes to follow the contours of the landscape orthe tunnel, angle units can be avoided.

This along with the use of N2/SF6 gas mixture now offers aeconomic transmission system for power transmission overlong distances, especially if high power ratings are needed.

The SF6/N2 GIL systems began their development in 1994as right of way approvals for conventional overhead lines be-came increasingly difficult. Several designs were developedand tested in France and Germany using mixtures of Nitrogenand SF6 gas as the dielectric medium. See Fig. II-10 and II-11.

Fig. II-10. 420 kV, 3150 Amp SF6/N2 GIL test loop under construction at EDFlaboratory in France.

Fig. II-11. 420 kV, 3150 A, SF6/N2 GIL test loop under construction at the IPHLaboratory in Berlin, Germany.

The SF6/N2 420 kV systems underwent significant testingduring their development. The test loops were instrumentedwith strain gauges and thermocouples then tested for 6000hours at 1.5 X line to ground voltage. Current was injectedfrom 0 to 1.5 X rated current to produce maximum thermaland mechanical stresses. Once the 6000 hour test was com-pleted a mock repair was done to verify that the system couldbe easily repaired should a fault ever occur. A bus segmentwas cut out and replaced then the system was tested again toverify that the ratings were not affected [II-1, II-2, II-4].

The first commercial site-assembled GIL of this type hasbeen built for eos (energie ouest suisse) at PALEXPO closeto the Geneva Airport in Switzerland. Since January 2001

this GIL is in operation as part of the overhead lineconnecting France with Switzerland. In this project it hasbeen successfully shown, that the new laying techniques aresuitable to build also very long GIL transmission links ofseveral tens of kilometers within a short and suitable timeschedule.

D. Component Detail

To illustrate the various standard components used in atypical GIL, the design suitable for assembly at site will bereviewed. Since there are normally no active components in aGIL system only straight bus, elbows or angle units and teesare needed to construct a system. As an example a straight

unit combined with an angle unit is shown in Fig. II-12. Thestraight unit consists of a single phase enclosure made of alu-minum alloy.

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Fig. II-12. Typical bus section

In the enclosure (1) the inner conductor (2) is fixed by aconical insulator (4) and rests on support insulators (5). Thethermal expansion of the conductor towards the enclosurewill be adjusted by the sliding contact system (3a, 3b). Onestraight unit has a length of up to 500 m made by single pipesections jointly welded together by orbital welding machines.If a directional change is needed that exceeds the elastic bending allowed by the enclosure then an angle element

shown will be added. The angle element covers angles from4 to 90.Under normal laying conditions, no angle units are

needed, since the elastic bending of the system with abending radius of 400 m is sufficient to follow the landscapecontour in most cases.

At distances of 1200 to 1500 meters, disconnecting unitsare placed in underground shafts. Disconnecting units areused to separate gas compartments and to connect highvoltage testing equipment for the commissioning of the GIL.

The compensator unit (figure II-17)is used to take care ofthe thermal expansion of the enclosure. This compensatorwill be used in tunnel laid GIL as well as in shafts ortrenches.

The enclosure of the directly buried GIL is coated in thefactory with a multilayer polymer sheath as passiveprotection against corrosion. On-site, after completion of theorbital weld, a final covering for corrosion protection isapplied on the joint area.

Fig. II-15. Application of corrosion protection to field welded joint.

Since the GIL is a totally enclosed and grounded system itis protected from direct lightning strikes. Therefore, it is pos-sible to reduce the lightning impulse voltage level by usingsurge arresters at the end of the GIL. The integrated surgearrester concept allows reduction of high frequency overvolt-ages by connecting the surge arresters to the GIL in the gascompartment [6].

Secondary equipment is installed for the measurement ofthe gas pressure and the gas temperature (gas density) of theGIL. These are the same elements used in Gas Insulated Sub-stations (GIS). For commissioning, partial dischargemeasurements are used with sensitive UHF measuringmethods.

An electrical measurement system to detect arc locationcan be implemented at the ends of the GIL. Electrical signalsare measured and the position of a very unlikely case of aninternal fault will be calculated by the arc location system(ALS) with an accuracy of ten meters.

If a directional change cannot be met with elastic bendingof the enclosure, an angle element covering angles of 4 to90 can be added as a second type of basic module as shownin figure II-16. Orbital welding with the straight unit

connects the angle element.

Fig. II-16. Angle unit

The third component is the enclosure compensator. In thetunnel-laid version or in an underground shaft or trench theenclosure of the GIL is not fixed, so it will expand with thethermal heat-up during operation. The thermal expansion ofthe enclosure will be absorbed by the compensation unit.

If theGIL is directly buried in the soil, the compensationunit is not needed. The weight of the soil and the friction atthe surface of the enclosure anchors the GIL in the soil.

Fig. II-17. Expansion compensator unit

The fourth and last basic module used is the disconnectingunit or tee shown in figure II-18. This will be used every 1.2to 1.5 km to separate the GIL in gas compartments. Also thedisconnecting unit is used to carry out sectional high voltagecommissioning testing.

5

1 enclosure2 inner conductor3a male sliding contact3b female sliding contact4 conical insulator5 support insulator

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Fig. II-18. Disconnecting unit

An assembly of all these elements as a typical set-up isshown in figure II-19

The figure shows a section of a GIL between two shafts(1). The underground shafts are housing the disconnectingand compensator unit (2). The distance between the shafts is between 1200 to 1500 m and represents one single gascompartment. Also in the middle the directly buried angle (3)unit is shown as an example. Each angle unit has a fix point,where the conductor is fixed to the enclosure.

Fig. II-19. Gas zone with disconnectlinks. 1.2-1.4 km long gas compartmentwith angle unit. Direct buried systemshown.

E. Site assembly and installation of direct buried and tunnelGIL.

The GIL can be laid above ground on structures, in opentrenches, in a tunnel or directly buried into the soil like an oilor gas pipeline. The overall costs for the directly buried ver-sion of the GIL, in most cases, is the least expensive versionof GIL laying when circuit lengths are long. Usually, for thislaying method, a certain accessibility for working on site isnecessary, so that this directly buried laying will generally beused in open landscape crossing the countryside, similar to

overhead lines, but invisible.

The most economical and fastest method of laying crosscountry is the directly buried GIL. Similar to pipeline layingthe GIL will be continuously laid with an open trench with anearby preassembly site to reduce the transportation of GILunits. With the elastic bending of the metallic enclosure theGIL will flexibly adapt to the contours of the landscape. Inthe soil the GIL is continuous anchored, so that no additionalcompensation elements are needed [8, 9].

Fig. II-20. Typical direct buried GIL installation methods.

The graphic in Figure II-20 shows a building close to the

trench used for prefabrication of the individual 12 - 18 metersegments. The tent structure next to the trench is where GILunits of up to 500 meters length are preassembled and pre-pared for laying.

They will be transported by cranes close to the trench andthen laid into the trench. In the trench within a clean housingtent the connection to the already laid section will be done.

Fig. II-21. 120 meter section being lowered into the trench.

The clean housing tent will be moved to the next joint andthe trench will be backfilled. The moment of laying the GILinto the trench is shown in Figure II-21 while Figure II-22shows the bent tube and backfilling of the trench.

6

1: underground shaft2: disconnecting and

compensator unit3: angle unit

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Fig. II-22. Backfill being added to the trench. Note the natural curvature of theGIL following the landscape contour.

If the desired location of the GIL is much deeper than the2 meters used for direct burial or if not enough space is avail-able, laying the GIL into a tunnel will be the mostappropriate method. This tunnel laying method is used incities or metropolitan areas as well as for crossing a river orinterconnecting islands. A new way of application is nowavailable for GIL by using traffic or railway tunnels. Forexample in the mountains existing or newly built traffic orrailway tunnels can be used to insert the GIL since it offersthe highest degree of safety for the surroundings.

Modern tunneling techniques have been developed in thepast few years with improvements in drilling speed and accu-

racy. Today tunnels of a diameter of about 3 m areeconomical solutions in cases when directly buried GIL isnot possible, for example, in urban areas, or in mountaincrossings, or in connecting islands under the sea.

Tunnels are usually the shortest connection between two points and, therefore, reduce the cost of transmissionsystems. After commissioning the system will be easilyaccessible.

In Fig. II-23 a view of two circuits of GIL as realized atPALEXPO at Geneva Airport in Switzerland is shown. Thetunnel dimensioning in this case was 2.4 m wide and 2.6 mhigh. The transmission capability of this GIL is 2250 MVA at420/550 kV rated voltage with rated currents of up to3150 kA

In both laying methods the elastic bending of the GIL isused as can be seen in figure II-22 for the directly buried ver-sion and in figure II-23 for the tunnel laid GIL. Theminimum acceptable bending radius is 400 m for 420 kVGIL and will vary depending on the system voltage andenclosure dimensions.

Fig. II-23. 2 GIL Circuits 420 kV 3150 Amps. Geneva Airport in Switzerland.

The principles of the installation procedure used in Geneva isshown in Figure II-24.

Fig. II-24. Cross section view showing the installation and testing procedures.

GIL-units of 12 to 18 meters length are brought into a tun-nel by access shafts and then connected to the GIL transmis-sion line in the tunnel. In cases of horizontal accessibility,like in a traffic tunnel for trains or vehicles the GIL units can be much longer, 20 to 30 meters if done by railtransportation. This increase in length will reduce theassembly work and time and allows major cost reductions.For the assembly site a special working place for mountingand welding is installed [10].

The delivery and supply of prefabricated elements (1) isbrought to the shaft or tunnel entrance (see Fig. II-24). After

the GIL elements are brought into the shaft to the assemblyand welding area (2) the elements will be jointed together byan orbital welding machine. The GIL section will then be brought into the tunnel (3). If a section is ready a highvoltage test will be carried out (4) to validate each section.

F. Installation of above ground and trench mounted GIL.

Above ground and trench mounted GIL circuits areusually less than 500 meters in length. Whether they are usedto connect within substations, crossing environmentallysensitive areas or as exits from AIS or GIS stations, theassembly methods are similar. Supporting steel is of simple

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construction with support spans from 12-18 meters dependingon seismic requirements. The flexibility of the GIL andelbows is used to minimize the use of compensator elements.Often very long circuits of GIL can be installed without theneed for any compensators. The GIL is allowed to bend andflex to accommodate thermal expansion [II-10, II-26].

Usually the GIL is shipped factory assembled and testedsince there are fewer field joints to make up. Assembly canbe done using either bolted or welded enclosure connections.

Figures II-25 and II-26 show two examples of aboveground installations.

Fig. II-25. trench mounted 550 kV GIL. Field welded construction. GILtransitions from a trench with steel grating covers to a concrete road crossing.

Figure II-25 is an open trench in a power plant. Site accessrequirements required the use of trenches to allow access tothe balance of the plant equipment. The GIL passes underseveral road ways. Since heights are restricted the cross bonding connections are fabricated to allow personnel towalk between the phases.

Fig. II-26: Above ground installation on steel structure overpassing streets

In Fig. II-26 an above ground installation is showninstalled in Saudi Arabia. The GIL is elevated so that streetscan be overpassed.

Fig. II-27. 550 kV GIL crossing under two 242 kV transmission lines.

Figure II-27 shows a 550 kV GIL installed on concretepedestals. The GIL crosses a flood plain where water risesabove the base of the steel structures every spring.

GIL sections are shipped to site on extended bed trucks.Handling on site can be done by crane or forklift trucks. An18 meter bus section is shown in figure II-28 as it is being

unloaded from the truck.

Fig. II-28. 18 meter bus section being unloaded at site.

The support structures are placed in position first androughly leveled. Perfect alignment is not necessary since theGIL is flexible.

The GIL shipping sections are moved into place and con-nected together as shown in figure II-29. A special clean tentis not normally required except for heavy dust conditions.

Flanged or welded joints are done in a similar fashion.The Conductor connection is made up using a plug andsocket connection. It will engage automatically as the twoGIL sections are brought together. Then the enclosure joint ismade.

If double o-rings are used on the flanges, the joint can beleak checked immediately after assembly by bagging the areaaround the joint with plastic, pressurizing the area betweenthe two o-rings and checking for leaks with a hand held leakdetector. Using this technique for long circuit lengths insuresthat each joint is complete and leak tight without firstevacuating and filling the system with gas.

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Fig. II-29. Final assembly of 550 kV field joint. Locating pins are used toalign the flanges. Conductor connection is a plug and socket connection andself aligning

G. On Site Testing of GIL

Once assembled the GIL circuit goes through site commis-

sioning tests similar to that for GIS [II-7, II-8, II-9, II-16].

1) Resistance of current carrying parts.GIS, with multiple bolted conductor contacts usually goes

through a test for conductor resistance. This test is normallyboth difficult and unnecessary for GIL. Since the conductorcontacts are an integral part of the conductor, factory assem-bled and tested and do not include bolted joints there is noreason to verify the integrity of field assembled conductor joints. The length of the GIL circuits and installation ofenclosure cross bonding makes use of the enclosure as areturn current path for circuit resistance difficult, if notimpossible. Connection of a micro-Ohm meter between each

end of the circuit can be tricky and requires long lengths oflow resistance cables [II-18].

2) Evacuation filling and Leakage tests.The GIL gas compartments are evacuated and filled with

gas. Once the system has stabilized the moisture and gaspurity are checked. Since individual flanged connects havealready been leak checked during the assembly process thereare only a few other points like gas monitors and fill valvesrequiring a detailed leak check. If welded joints are usedsimply bagging the joint makes it easy to do a very sensitiveleak check [II-11, II-12, II-13, II-14, II-15].

3) Mechanical and instrument checks.The system should be checked to verify groundingintegrity. Density monitor units are either factory calibratedor can be checked prior to installation for accuracy. A wiringcheck for the density monitors completes these tests.

4) Dielectric acceptance testsThe final test required is the application of a high voltage

test to verify the system has been assembled cleanly andwithout damage. The most accepted field test for GIL is a power frequency test applied at 80% of the factory testvoltage. A conditioning series of voltage steps is normally

used to allow any particles to be moved to the particle traps.Partial discharge testing can be used during the test sequencefor further verification.

One of the easiest way to apply voltage for the sitecommissioning tests is to use a variable frequency resonancetest set. Variable frequency sets use a frequency converter totune the fixed inductive reactance of the test set to thecapacitance of the GIL. Unlike series resonant test systems,which have complex moving parts in the reactors the variablefrequency sets use fixed inductors with no moving parts.Sizes, weights and power requirements are lower sinceimpedances are higher for lower frequencies. A three phaseinput power supply is used rather than the single phaserequirements of series resonant sets, making it easier tosupply test power on site. A 650 kV variable frequency testset is shown in figure II-30.

Fig. II-30. Variable frequency test set. 650 kV, 9 amps. 30-300 Hz.

The test set shown in figure II-30 can test a 1200 meter

phase of 550 kV GIL . Current IEEE C37.122 standardsallow for test frequencies between 45 and 200 Hz.

H. Maintenance

One of the advantages of GIL is the low maintenance re-quirements. Standards require leakage rates to be less than0.5%/year. With many manufacturers guaranteeing less than0.1%/year the GIL will never require additional gas. In caseof welded design the GIL is practical gas tight and needs norefill for the 50 years life time.

Elastomer seals have 30 year or more design life and, ifnot disturbed, will normally last much longer. The onlyactive component is the density monitor which should bechecked for proper operation periodically.

I. advantages of GIL

1) System OperationsThe GIL is a transmission system for the transmission of

high power levels over long distances, to solve specificrouting or access problems, or as connections in power plants, and substations. Current ratings of 4000 Amps ormore per circuit and distances of several kilometers are possible. As a gas-insulated system, the GIL has theadvantage of having electrical behavior similar to anoverhead line. This is of importance to system operation. GIL

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capacitance is very low allowing long lines to be constructedwithout reactive compensation. Further more, the dielectricgases are non-aging so that there is almost no limitation tolifetime. This is a big cost advantage because of the high in-vestment costs of long power transmission systems.

2) Economics and Life Cycle CostsThe GIL has, because of the large cross section of the con-

ductor, the lowest electrical losses of all availabletransmission systems (overhead lines and cables).

This reduces the operating costs and is a contribution toreducing the global warm-up because less power needs to begenerated. A comparison between the losses of an overheadline and GIL is shown in table II-1.

TABLE II-1 COSTOFLOSSESFOROHL AND GIL. (ENERGYCOSTS: $.10/KWH X 8,600H X 12,800 KW)

OHL GIL

Transmitted Power mW 1400 1400Losses per circuit meter W/m 580 180Losses per 32 circuit

meters (20 miles)

mW 18.56 5.76

Difference between GILand OHL

mW 12,800

Cost of additional losses of

the OHL per year (USD)

$10,908,000

Long lengths of GIL can be produced economically. Thechart shown in figure II-31 shows how the per meter cost ofGIL drops significantly as the installed length increases.

Fig. II-31. Normalized equipment cost of GIL as a function of circuit length.

3) SafetyThe personnel and environmental safety in surrounding a

GIL is very high because the metallic enclosure is reliableprotection from external magnetic fields and internal faults.

a) Cross bonded enclosures and magnetic fieldsGIL enclosures are cross bonded to allow return currents

of up to 99% or more to flow in the enclosure. Enclosures aregrounded frequently to minimize the touch potential duringsystem faults. This minimizes the external magnetic field andprovides safe approach and handling for personnel. FiguresII-32 and II-33 show a comparison of the magnetic fields forGIL and conventional cable. As you can see from the graphsthe GIL external magnetic fields are only 5% of the levels forconventional cable at ground level [II-19, II-20].

Fig. II-32. Magnetic fields for GIL in T

Fig. II-33. Magnetic fields for conventional cable in T

10

0.40

0.50

0.60

0.70

0.80

0.90

1.00

0 500 1000 1500 2000 2500 3000

3 phase Circuit length (meters)

Normalizedcost

550kV Flanged

550kV Welded

362kV Flanged

362kV Welded

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4) Internal FaultsGIL construction uses large volume gas compartments androbust aluminum tubes for enclosures. Compartment lengths20 or more meters in length have sufficient volume so thatinternal arcs do not increase the internal pressure enough tooperate a pressure relief device. Thus a pressure relief deviceis not necessary for GIL installations. In the remote case ofan internal fault the arc will be driven by magnetic forces andexternal rupture will not occur. In the 30 year history of GIL

there has never been an arc burn through.Figures II-34 and II-35 compare the effect of internal faultsfor GIL and solid dielectric cables.

Fig.II-34.

Results of an internal arc of 63kA for 300 ms on a 420 kV GIL. There are noexternal effects. The system retains gas pressure and all arc byproducts arecontained within the enclosure.

Fig. II-35. Result of an internal fault of 50 kA for 300 ms on an XLPE cable.There was a release of toxic gases and damage to adjacent cable circuits.

The inherent safety of GIL allows it to be brought intostreet or railway tunnels, and on to bridges No flammablematerials are used to build a GIL. This makes the use intunnels safer and more economical.

J. Conclusions

GIL has a long and reliable history. The flexibility of the de-signs allow it to be installed in a variety of locations.

Direct Buried In tunnels, horizontal, vertical and any slope in be-

tween. In open or closed trenches at ground level. At ground level Elevated above ground level to allow access below.

With robust aluminum enclosures, exclusion of weather,lightning strikes and magnetic fields and the complete con-tainment of to internal faults GIL can be installed in highway

or rail tunnels, across bridges, closer to the public than everbefore possible.

When life cycle costs are considered, the low losses, mini-mal maintenance costs, high reliability and long life of GILmake it an attractive solution to transmission lineinstallations.

Summary of installed length of GIL world wide.

K. References

[II-1] H. Koch, A. Schuette: Gas-insulated Transmission Lines for highpower transmission over long distances, EPSR, Hongkong, 12/97

[II-2] H. Koch: Underground gas-insulated cables show promise, ModernPower Systems, London, 05/97

[II-3] G. Baer, A. Diessner, G. Luxa: 420 kV SF6-Insulated Tubular Busfor the Wehr Pumped-Storage Plant, Electric Tests, IEEETransactions on Power Apparatus and Systems, Vol. PAS-95, No. 2March/April 1976

[II-4] Henningsen, C. G.; Kaul, G.; Koch, H.; Schuette, A.; Plath, R:Electrical and Mechanical Long-Time Behavior of Gas-InsulatedTransmission Lines, CIGRE 2000

[II-5] Koch, H. et al.: N2/SF6 gas-insulated line of a new GIL generation inservice, CIGRE Session 2002, Paris

[II-6] O. Vlcker, H. Koch: Insulation co-ordination for gas-insulatedtransmission lines (GIL), IEEE Transactions, to be published in 2000

[II-7] IEC 61640 "Rigid high-voltage, gas-insulated transmission lines forrated voltages of 72.5 kV and above"

[II-8] U. Schichler, J. Gorablenkow, A. Diessner: UHF PD Detection inGIS Substations During On-Site Testing, 8th Int. Conf. on DielectricMaterials, Measurements and Application, Edinburgh, 2000, pp. 139-144

[II-9] G. Schffner, W. Boeck, R. Graf, A. Diessner: Attenuation of UHFSignals in GIL, 12th Int. Symp. on High Voltage Eng., Bangalore,2001, No.4-57, pp. 453-456

[II-10] A. Schuette: Gas-Insulated Transmission Lines, Siemens, PowerEngineering Guide, 1997

[II-11] IEC 60480 "Guide to the checking of sulphur hexafluoride (SF6)taken from electrical equipment"

[II-12] IEC 61634 "High-voltage switchgear and controlgear - Use andhandling of sulphur hexafluoride (SF6) in high-voltage switchgearand controlgear"

[II-13] Cigre Working Group 23.02, Task Force 01: Guide for SF6 mixtures,Cigre Brochure 163, 2000

[II-14] L.G. Christophorou, L.R. van Brunt: SF6 - N2 Mixtures, IEEE Trans.Dielectr. Electr. Insul. 2 (5) 1995, pp. 952-1003

[II-15] H. Knobloch: The Comparison of Arc-Extinguishing Capability ofSulphur Hexafluoride (SF6) with Alternative Gases in High-VoltageCircuit Breakers, 8th Int. Symp. on Gaseous Dielectrics, VirginiaBeach, USA, 1998

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Enclosure

Conductor

Cumulated

Ur length

kV m

1200 420

800 1200

550 52,650

420 63,600

362 10,107

242/300 32,900

72/145/172 37,100

72 to 1200 198,000

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[II-16] A. Diener, H. Koch, E. Kynast, A. Schuette: Progress in HighVoltage Testing of Gas-insulated Transmission Lines, ISH Montreal,06/97

[II-17] H. Koch, A. Schuette: Gas-Insulated Transmission Lines (GIL) -Type tests and prequalification, Jicable, Versailles, 06/99

[II-18] IEC 60287, 1982: calculation of the continuous current rating ofcables (100 % load factor)

[II-19] Germany: Sechsundzwanzigste Verordnung zur Durchfhrung desBundesimmissionsschutzgesetzes (Verordnung berelektromagnetische Felder - 26. BImSchV), 16. Dezember 1996

[II-20] Switzerland: Verordnung ber den Schutz vor nicht ionisierenderStrahlung (NISV), 23. Dezember 1999

[II-21] A. Diessner, M. Finkel, A. Grund, E. Kynast: Dielectric Properties ofN2/SF6 Mixtures for Use in GIS or GIL, High Voltage EngineeringSymposium, Conf. Publ. No 487, IEE, 1999

[II-22] R. Graf, W. Boeck: Defect Sensibility of N2-SF6 Gas Mixtures withEqual Dielectric Strength,Annual Report CEIDP 2000, Victoria, Vol.I,

pp. 422-425[II-23] R. Graf, W. Boeck: Statistical Breakdown Behavior of N2-SF6 Gas

Mixtures under LI Stress, 11th Int. Symp. of High Voltage Eng.,London, 1999, No 3.96.S20

[II-24] M. Finkel, W. Boeck, L.-R. Jnicke, E. Kynast: ExperimentalStudies on the Statistical Breakdown Characteristic of SF6, AnnualReport CEIDP 2000, Victoria, Vol. I, pp. 405-408

[II-25] Task Force 15.03.07 of Working Group 15.03: Long Term Per-formance of SF6 Insulated Systems, CIGRE-Report 15-301, 2002,Paris

[II-26] T. Hillers, H. Koch: Gas Insulated Transmission Lines (GIL): A

solution for the power supply of metropolitan areas, CEPSI,Thailand, 07/98[II-27] H. I. Marsden , S. J. Dale*,M. D. Hopkins, C. R. Eck III: High

Voltage Performance of a Gas Insulated Cable with N2 and N2/SF6Mixtures 1998 Gas Dielectric Conference (GDC)

[II-28] H. I. Marsden, M. D. Hopkins, C. R. Eck III: Lightning ImpulseWithstand Performance of a Practical GIC With 5 and 10 PercentSF6/N2 Mixtures"

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