Gas Insulated Lines - Think Grid n°7.pdf

7/26/2019 Gas Insulated Lines - Think Grid n7.pdf

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Respectingthe environmentLow power losses and a lesser visual impact on the landscape make Gas-Insulated Lines (GIL) a

very environmentally friendly transmission solution compared to overhead lines. Alstom Grids

eco-design makes them even more so. Power electronics also have great environmental

credentials as distributed energy sources become more widespread. Meanwhile, ester oils haveproved their value in distribution transformers; research is now focusing on overcoming the

challenges to their potential for power transformers.

M AIN FEATURE CHAPTER I RESEARCH WITH AN EYE ON THE FUTURE

12 AlstomGrid///Summer 2010


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AlstomGrid///Summer 2010 1

Gas-Insulat ed Lin es ( GIL) :cho ices t oday and t om or r owThanks to their high transmission capacities and reliability, GIL have been in use

alternative to cables or overhead lines in a growing number of applications.

ing from a few meters to 900-1,000 meters.Directly laid on the soil or in trenches or tun- widen (and lengthen) to stand-alone instal-lations up to several kilometers for resolving right-of-way issues. GIL provide an effectivetechnological answer to the increasing energyneeds of our urban society and to the exten-sion and reinforcement of the present elec-tricity transmission and distribution networks.Transmitting bulk power over short and long

Gas-Insulated Lines are a transmissiontechnology developed in the late 1960s foruse in substations, hydro power plants,underground facilities, and so on. Their hightransmission capacity (up to 3,000 MVAthrough one GIL circuit) and multiple advantages for transmitting bulk electricpower in such complex sites. In substations,for instance, GIL serve as connectionsbetween GIS switchgear and power trans-formers or overhead lines, at distances rang-

distances in a safe and environmentallyfriendly way represents one of the majorchallenges of the coming decades; GIL is akey to solve this issue.

Electric pipelines

Encapsulated in a 400 to 600 mm diam-eter metallic case, Gas-Insulated Lines beara resemblance to pipelines. Originally devel-oped for GIS connections, GIL have similarbus sections and busbar arrangements. Theelectric pipeline consists mainly

Encapsulated in a 400 to 600 mm diametermetall ic case, Gas-insulated Linesbear a resemblance to pipelines.


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of a pair of tubular aluminum coax-ial conductors, the inner one being at thehigh voltage level and the outer one beingearthed. To ensure insulation, the spacebetween the enclosure and the conductor 6) or apressurized gas mixture of nitrogen andSF6. With their very high conductor cross-section, GIL are designed for high transmis-sion capacities with rated currents typicallyreaching 4,000 A at 550 kV, and a short-

circuit current capability up to 63 kA,explains Mathieu Bernard, GIL Project Man-ager. This offers multiple advantages: GILcombine low capacitance (and thereforevery low transmission losses), high reli-ability, and operational safety thanks to arobust casing and the fact that they are long lifetimeup to 50 yearswith virtuallyno material ageing. Moreover, thanks to route capabilities, GIL are adapted to solve

complex geographic installation issues likehydroelectric power plants in new or exist-ing underground conditions, Bernard adds.One drawback, how-ever, is their cost, whichis still relatively highwhen compared t o seven times moreexpensive). In fact, GILare not designed to

compete with overheadlines everywhere butrather as a means ofpower transmission where OHL cannot ormust not be applied.

High service continuity

ago, more than 250 km of three-phase GILsystems have been installed in the world(750 km of single-phase pipes). They oper-ate at different voltage levels between

72.5 kV and 800 kV (mainly at the 420 kVlevel), and demand has significantlyincreased over the past few years. At Alstom

Grid alone, over 50 kmof three-phase circuitshave been installedor ordered since thebeginning of GIS pro-duction, including30 km over the last12 years, showing

clear acceleration.Almost maintenance-free and with low

operating costs, GIL are now widely recog-nized as a well-proven technology with along service record and a high level of reli-ability. Even so, modern digital monitoringsystems help further improve their reliabilityand service continuity, says FranoisBiquez, Senior Expert R&D Manager for transmission enable the real-time monitor-

GIL combinelow capacitance,high reliabilityand operationalsafety.

The T155 Gas-Insulated Substation installedat Shuqaiq, Saudi Arabia, a major Alstom Gridreference demonstrating that the company can


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ing of parameters such as SF6or gas mixturedensity, SF6leakage trend analysis, tem-peratures in compartments, partial dis-charges, internal arc localization anddetection, etc. Abnormal conditions thatcould lead to potential failures are thereforerapidly detected. As a result, safety for per-

sonnel and switchgear is enhanced, as wellas security and quality of power supply,protection of the environment (against SF6leakages), asset management (maintenance

and outage planning, integration in a digi- due to energy deregulation requiring morerapid and accurate data management, andso on.On the other hand, todays computerizeddesign tools offer the ability to adapt GILto severe operating conditions. For eachparticular installation, calculations can bemade to select the right solution andensure optimal working conditions whileadhering to strict regulatory standards.The dimensioning of a GIL clearly depends

on its dielectric withstand performance andon its thermal capability to carry the ratedcurrent, explains Biquez. As the thermallimits of the product are affected by the waythe GIL is installed, computer simulationsare necessary to decide on the appropriatedesign. In the case of an undergroundhydro power plant, thermal simulationshave enabled Alstom Grids designers toevaluate the thermal behavior of the GILenclosure as a function of the tunnel height,or to compare the conductor and enclosuretemperature distribution depending on

whether the GIL is placed in a horizontaltunnel or a vertical shaft.

Opting for a GIL solution brings major envi- to a very low capacitance, the electrical resis-tance and power losses of GIL are extremelyreducedup to 70 percent lower than

TECHNICAL BENEFITSOF GIL COMPARED W ITHCABLES AT HAM S HALLThe 420 kV Hams Hall substation inthe West Midlands, England, requireda new connection to reinforce thenetwork. However, existing overheadlines and buried cables and pipesdemanded that the connection be intrenches at the one end and above

ground at the other end. Both GIL andcables offered suitable solutions.However, for both technical andeconomic reasons, the British utilityNational Grid adopted the Alstom GridGIL solution. A continuous currentrating of 3,190 A would have requiredtwo cables with two outdoorterminations per phase compared withone GIL, says Alain Girodet, SeniorExpert, Alstom Grid High VoltageTechnology and Materials Manager.Cable junctions would also have beenmore of an issue from a dielectricperspective, he adds. To minimizeenvironmental risks with GIL, aninsulating gas mixture made of 20percent SF6and 80 percent N2waschosen, as well as a totally weldedpipe design (which required specialcare in the assembly process to avoidweld slags seeping inside the GIL).Another advantage is that the N2/SF6mixture can be easily separated andrecycled after use.

M O R E

Alain Girodet

HIGH SERVICE CONTINUITY


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IN TERVIEW W ITHGERHARD SEYRLING,SENIORVICE-PRESIDENTINNOVATION& STRATEGY

What is AlstomGridsexperience with GIL?Alstom Grid has longexperience with GIL designand installation, especially forlarge power sites. One of ourmajor references is the PP9project in Saudi Arabia: withtotal circuit lengths ofapproximately 5.6 km, morethan 17 km of single-phaseenclosures were necessary .Alstom Grid has developedand masters a whole range of

or welded connections andvarious insulating gasmixtures to optimize cost,performance, reliability andenvironmental protection.

How can GIL compete againstoverhead lines and cables?GIL have proved their ability torespond to any installationconstraints as well as carryinghigh power ratings withoutrequiring multiple circuit overhead lines, the majoradvantage is better integrationin the urban environment andlandscapes. Compared withcables, GIL can transmitgreater quantities of energy:3,000 MVA can be transmitted

through one GIL circuit whiletwo circuits would be requiredfor the alternative HV cablesolution; this performance hasa direct impact on price,footprint and right-of-wayrequirements.

What are customerslooking for?Besides substations, answers to two types ofissues. One is carrying highrated currents in urban areaswhere OHL are prohibited;the other is to ensure regionaland international bulk powerconnections with GIL offering powerful solution throughtunnels, bridges, and so on.

M O R E

Gerhard Seyrling

those of overhead lines (OHL). Thismeans less waste of energy, and no need forreactive compensation for lengthy GIL, con-trary to long cable connections. Another

advantage is a lower visual impact on thelandscape, suggesting that GIL may be arealistic solution in sites where OHL wouldbe banned. A costly solution, indeedbutthe expense may be worth it, one chiefexecutive of a U.S. transmission company -nitely expensive if their construction isresolutely opposed by local communities.To minimize the environmental impact ofits GIL solutions, Alstom Grid has imple-mented an eco-design approach, as for anumber of its products. The latest designs

are nowadays completely optimized in asustainable development perspective. Thewhole life of the product is considered, fromcradle to grave. It starts with a life cycle encountered when manufacturing andusing the GIL, from raw material extractionand transportation to the disposal of theproduct at the end of its life. Up to 16 indi-cators are used, showing the impact of thevarious steps of the product life cycle onfeatures such as global warming, ozone

and a GIS, thus preventing the fire fromspreading and allowing the rest of the system

explosion-proof characteristics, GIL can beinstalled in shared infrastructure like roadand rail tunnels, open public areas, etc. Anillustration of an in-public area installationof a GIL is the Hams Hall project in the U.K.,which was energized in 2004 (see sidebar). radiation emission. Cross-bonding of single-phase enclosures allows the return currentto pass through the enclosures. This createsa screen effect that provides the GIL with alower magnetic field radiation compared

with alternative solutions such as cables andOHL. Computer calculations show that mag- phase GIL system (operating at 550 kV and4,000 A) are lower than 250 T, half theexposure limit for workers set by the Euro-pean Union directive. At a distance of 1 meterto the enclosure, the value drops to 10 T,less than one-tenth the emission of buriedXLPE cables 1 meter above ground level,and less than a quarter of overhead lines(below the lines at 1 meter above groundlevel). In fact, GIL enclosures can even be

safely touched.

toxicity, hazardous waste, amount of slag/ashes, etc. Each component of a GIL maythen be optimally designed to achieve

contributing to a moderate and responsibleuse of the needed resources, and ensuringa high recyclability rate. The latest devel-opments have led, for instance, to a moreenvironmentally friendly insulationmedium: the amount of greenhouse effectSF6gas produced has been dramaticallyreduced through the use of a g as mixturebased on 80-90 percent of nitrogen andthe rest of SF6, says Biquez. Alstom Gridwas also able to optimize the section of theconductors, enabling the GIL to exhibitlower heat losses compared with OHL and

HV cables.

Ensuring safety for people

As high voltage power transmission lines even in crowded public areas, the safety ofthe surroundings is essential. Thanks to thedesign of their aluminum enclosures, GILcompletely exclude the risk of explosion. materials (such as oil) makes the GIL abso- even in the case of an electrical fault; the

GIL will even play the role of a buffer or


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Alstom Grid has installed (or received orders for) more than50 kilometers of 3-phase GIL systems since the beginning of GIS success are the Jebel Ali power plant in the United Arab Emirates,Shuqaiq in Saudi Arabia, and the Hams Hall substation inthe West Midlands, U.K.

Gas Insulated Lines - Think Grid n°7.pdf

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