pc37.122.4-g.pdf

PC37.122.4/DG, December 2012 Draft Guide for Application and User Guide for Gas-insulated Transmission Lines (GIL), Rated 72.5 kV and Above

PC37.122.4™/DG 1

Draft Guide for Application and User 2

Guide for Gas-insulated Transmission 3

Lines (GIL), Rated 72.5 kV and Above 4

Sponsor 5 6 Substations Committee 7 of the 8 IEEE Power and Energy Society 9 10 11 Approved <Date Approved> 12 13 IEEE-SA Standards Board 14 15 Copyright © 2013 by the Institute of Electrical and Electronics Engineers, Inc. 16 Three Park Avenue 17 New York, New York 10016-5997, USA 18

All rights reserved. 19

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Abstract: This is similar to the Scope. 1 Keywords: Standard, IEEE draft template 2 3

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vi

Participants 1

At the time this draft guide was completed, the Gas Insulated Substations K5 Working Group had the 2 following membership: 3

Patrick Fitzgerald, Chair 4 <Vice-chair Name>, Vice Chair 5

6 Arun Arora 7 George Becker 8 Roberto Benato 9 Philip Bolin 10 Markus Etter 11

Arnaud Ficheux 12 Noboru Fujimoto 13 David Giegel 14 Jack Gustin 15 Charles Hand 16

Hermann Koch 17 Ahmet Oztepe 18 Devki Sharma 19 Dave Solhtalab 20 Ryan Stone21

22

The following members of the <individual/entity> balloting committee voted on this guide. Balloters may 23 have voted for approval, disapproval, or abstention. 24

[To be supplied by IEEE] 25

Balloter1 26 Balloter2 27 Balloter3 28



35

When the IEEE-SA Standards Board approved this guide on <Date Approved>, it had the following 36 membership: 37

[To be supplied by IEEE] 38

<Name>, Chair 39 <Name>, Vice Chair 40 <Name>, Past Chair 41 <Name>, Secretary 42

SBMember1 43 SBMember2 44 SBMember3 45



*Member Emeritus 52 53

Also included are the following nonvoting IEEE-SA Standards Board liaisons: 54

<Name>, DOE Representative 55 <Name>, NIST Representative 56

57 <Name> 58

IEEE Standards Program Manager, Document Development 59 60

<Name> 61 IEEE Standards Program Manager, Technical Program Development 62

63




vii

Introduction 1

This introduction is not part of PC37.122.4/DG, Draft Guide for Application and User Guide for Gas-insulated 2 Transmission Lines (GIL), Rated 72.5 kV and Above. 3

GIL has been manufactured starting in the early 1970s when the first gas-insulated substations were 4 introduced. IEEE has not had any standard to address any manufacturing of GIL. To address IEEE policy 5 that IEEE standards should be harmonized with international standards whenever possible a study was 6 conducted by a joint task force of the Substations Committee and IEC. This included creating an IEEE 7 Standard that generally aligned with the IEC GIL standards. 8




viii

Contents 1

2

1. Overview .................................................................................................................................................... 1 3 1.1 Scope ................................................................................................................................................... 1 4 1.2 Purpose ................................................................................................................................................ 2 5

2. Normative references .................................................................................................................................. 2 6

3. Definitions .................................................................................................................................................. 3 7

4. Technical Details of GIL ............................................................................................................................ 3 8 4.1 Description .......................................................................................................................................... 3 9 4.2 Typical Installations ............................................................................................................................ 4 10 4.3 GIL Application Information ............................................................................................................... 5 11 4.4 Influence of GIL on the Transmission Network .................................................................................. 5 12 4.5 Temperature Design Criteria ............................................................................................................... 6 13 4.6 Seismic Aspects ................................................................................................................................... 6 14 4.7 Terminations ........................................................................................................................................ 6 15 4.8 Installation Criteria .............................................................................................................................. 7 16 4.9 Testing Criteria .................................................................................................................................... 8 17 4.10 Environmental Aspects ...................................................................................................................... 8 18 4.11 Technical Data to be given in the Request for Proposal .................................................................... 8 19

5. Detailed Project Implementation and Service ............................................................................................ 9 20 5.1 Construction Aspects ........................................................................................................................... 9 21 5.2 Transportation and Storage ................................................................................................................ 10 22 5.3 GIL Installation .................................................................................................................................. 11 23 5.4 Testing and Commissioning .............................................................................................................. 12 24 5.5 Secondary Equipment ........................................................................................................................ 14 25 5.6 GIL Grounding .................................................................................................................................. 15 26 5.7 Seismic Requirements ....................................................................................................................... 16 27 5.8 Repair Techniques ............................................................................................................................. 17 28

6. Gas Handling of Gas Mixtures ................................................................................................................. 17 29 6.1 Pure SF6 and N2/SF6 Gas Mixture ................................................................................................... 17 30 6.2 Gas Handling Equipment ................................................................................................................... 17 31 6.3 Filling With SF6 or SF6/N2 Gas Mixture.......................................................................................... 18 32 6.4 Removal of SF6 or SF6/N2 Gas Mixture .......................................................................................... 18 33

7. Maintenance and Inspections .................................................................................................................... 18 34

8. Monitoring equipment shall be checked periodically as recommended by the manufacturer. Training .. 18 35

9. Decommissioning ..................................................................................................................................... 19 36 9.1 Removal of SF6 or SF6/N2 Gas Mixture .......................................................................................... 19 37 9.2 Dismantling of the GIL ...................................................................................................................... 19 38

Annex A (Informative) Typical Installations ............................................................................................... 20 39 A.1 Installations in a Tunnel .................................................................................................................... 20 40 A.2 Above Ground .................................................................................................................................. 24 41 A.3 Direct Burried ................................................................................................................................... 27 42




ix

Annex B (Informative) Comparison of GIL, OHL, and XLPE Cable .......................................................... 28 1

Annex C (Informative) Proposal Fill-in Data Table ..................................................................................... 30 2

Annex D (Informative) Execution of a GIL Project-Planning and Engineering Process ............................. 32 3 D.1 Initiation of a Project ........................................................................................................................ 33 4 D.2 Preliminary System Studies / Need Assessment ............................................................................... 33 5 D.3 Routing ............................................................................................................................................. 33 6 D.4 Pre-Proposal Stage ............................................................................................................................ 33 7 D.5 Further studies .................................................................................................................................. 33 8 D.6 Approval of the Right-of-way ........................................................................................................... 34 9 D.7 Preparation of a Request for Proposal .............................................................................................. 34 10 D.8 Evaluation of Proposals .................................................................................................................... 35 11 D.9 Project implementation ..................................................................................................................... 35 12

Annex E (Informative) Bibliography............................................................................................................ 37 13 14




1

Draft Guide for Application and User 1

Guide for Gas-insulated Transmission 2

Lines (GIL), Rated 72.5 kV and Above 3

IMPORTANT NOTICE: IEEE Standards documents are not intended to ensure safety, health, or 4 environmental protection, or ensure against interference with or from other devices or networks. 5 Implementers of IEEE Standards documents are responsible for determining and complying with all 6 appropriate safety, security, environmental, health, and interference protection practices and all 7 applicable laws and regulations. 8

This IEEE document is made available for use subject to important notices and legal disclaimers. 9 These notices and disclaimers appear in all publications containing this document and may 10 be found under the heading “Important Notice” or “Important Notices and Disclaimers 11 Concerning IEEE Documents.” They can also be obtained on request from IEEE or viewed at 12 http://standards.ieee.org/IPR/disclaimers.html. 13

1. Overview 14

The purpose of this guide is to provide technical assistance for the selection, application and project 15 management for a Gas Insulated Line (GIL) transmission project from initial planning through 16 decommissioning and retirement. It is not within the scope of this guide to address contractual or 17 commercial questions related to GIL projects. 18

The document included tables, flowcharts and other aids that maybe of use in a typical GIL project. While 19 not required, a certain minimum knowledge of GIL is required in order to utilize the benefits of this 20 technology. The relevant information for accurate system planning using GIL is also included in this 21 document along with operations and maintenance information. 22

This guide includes equipment with the following specifications or characteristics: rated voltage 72.5 kV 23 and above, alternating current, single-phase, rigid (solid aluminum) enclosure, installed above ground, in a 24 trench, directly buried or in tunnel, laid horizontally, vertically at any angle and using pure SF6 or a 25 mixture of SF6 with nitrogen as the insulating medium. 26

1.1 Scope 27

This project will develop a guide for the planning, permitting, design, equipment specification, installation, 28 commissioning, operation, and maintenance of gas insulated transmission lines. The guide will address 29




2

technical aspects only. Commercial and legal issues associated with gas insulated transmission lines are not 1 considered. This guide applies to AC transmission lines rated for maximum operating voltage of 72.5 kV 2 and higher. 3

1.2 Purpose 4

There is currently no complete guide covering planning, design, installation, and operation of gas insulated 5 transmission lines. The guide produced by this project will fill this void and become a useful reference for 6 electric power engineers considering the installation of gas insulated lines. 7 8

2. Normative references 9

The following referenced documents are indispensable for the application of this document (i.e., they must 10 be understood and used, so each referenced document is cited in text and its relationship to this document is 11 explained). For dated references, only the edition cited applies. For undated references, the latest edition of 12 the referenced document (including any amendments or corrigenda) applies. 13

IEEE Std C37.122 TM, IEEE Standard for Gas-Insulated Substations 14

IEEE C37.122.1TM. IEEE Guide for Gas Insulated Substation 15

IEEE Std C37.017 TM, IEEE Standard for Bushings for High-Voltage [over 1000 V (ac)] Circuit Breakers 16 and Gas-Insulated Switchgear 17

IEEE Std 80, IEEE Guide for Safety in Substation Grounding 18

IEEE Std 693, IEEE Recommended Practice of Seismic Design of Substations 19

IEEE Std C37.123TM, IEEE Specification for GIS Electric Power Substation Equipment 20

IEEE Std C37.122.5, Guide for Moisture Measurement and Control in SF6 Gas-Insulated Equipment 21

IEC 62271-1, Common clauses for high-voltage switchgear and controlgear standards 22

IEC 62271-204, HV gas-insulated transmission lines for rated voltages of 72,5 kV and above 23

IEC 60376, Specification of technical grade sulfur hexafluoride (SF6) for use in electrical equipment 24

IEC 60480, Guidelines for the checking and treatment of sulfur hexafluoride (SF6) taken from electrical 25 equipment and specification for its re-use 26

CIGRE 218, Gas-Insulated Transmission Line (GIL) (Brochure) 27

CIGRE 351, Application of Long High Capacity Gas-Insulated Lines in Structures (Brochure) 28

ASTM D2472, Standard Specification for Sulfur Hexafluoride 29

ASTM D1933, Standard specification for Nitrogen Gas as an Electrical Insulating Material 30

CIGRE 276, SF6Gas Handling Guide (Brochure) 31

IEEE Std C37.122.3TM, IEEE SF6Gas Handling Guide 32




3

CIGRE 360. Insulation Co-Ordination related to Internal Insulation of Gas Insulated Systems with SF6 and 1 N2/SF6 Gas Mixtures under AC Condition (Brochure) 2

IEEE Std C37.122.6TM, Recommended Practice for the Interface of New Gas-Insulated Equipment in 3 Existing Gas-Insulated Substations Rated Above 52kV 4

3. Definitions 5

For the purposes of this document, the following terms and definitions apply. The IEEE Standards 6 Dictionary Online should be consulted for terms not defined in this clause. 1 Users should reference IEEE 7 Std C37.122 for most definitions. 8

support insulators: Epoxy insulating material that is used to hold the conductor concentric to the 9 enclosure 10

4. Technical Details of GIL 11

CIGRE 218 provides comprehensive information about GIL. The following sections provide a short 12 overview of GIL features to familiarize the reader with the characteristics of GIL. 13

4.1 Description 14

The technology of GIL is similar to gas-insulated substations (GIS), where a high voltage conductor 15 supported by insulators is positioned within a grounded enclosure and the remaining space between is filled 16 with a pressurized gas for electrical insulation. The support insulators are composed of composite cast resin 17 material. The conductor of each phase is located within an aluminum enclosure. Sliding Contacts provide 18 compensation for thermal expansion of the conductor and where the enclosure expansion (i.e. in tunnel or 19 trench installations) is compensated by bellows assemblies. The GIL is divided along its length into 20 separate gas compartments. GIL dimensions are determined by the dielectric, thermal and mechanical 21 design requirements. Conductor and enclosure diameters and thicknesses, and gas composition and 22 pressure may be varied according to the application to provide an optimum solution. In typical standard 23 applications, dielectric considerations will determine the enclosure dimensions. For high current rated 24 circuits, thermal considerations may require larger dimensions in order to maintain temperatures within 25 acceptable limits. 26

1IEEE Standards Dictionary Online subscription is available at: http://www.ieee.org/portal/innovate/products/standard/standards_dictionary.html.




4

1 Figure 1 —General design of GIL 2

4.2 Typical Installations 3

The GIL installation is generally, defined by the transmission system or application requirements. The GIL 4 is usually installed in three different methods: in a tunnel, duct, or trench, on steel structures above ground 5 or directly buried in the ground. 6

The above ground installation is most commonly found in electrical switchyards, installed on steel 7 structures above ground and/or in concrete trenches or walls. In a switchyard application GIL connects the 8 GIS with other remote equipment like transformers, overhead lines, or other sections of an existing 9 substation. This application is also the most economical since minimal excavation is required for the 10 structural supports. An above ground installation can also be provided in public access areas; however 11 physical security should be considered including fences, climbing deterrents, and gas density monitor 12 covers etc. 13

Directly buried installations are used inside or outside substations where the GIL is used to connect remote 14 equipment or switchyards. In this case, GIS is typically covered with a minimum of 1 m soil and provided 15 with cathodic (corrosion) protection. This cathodic protection that can be used is either a traditional system, 16 or a coating that is applied to the enclosure. If a concealed installation is required for either security or 17 aesthetic purposes, a buried installation should also be considered. In any case, the GIL route should be 18 carefully investigated for interferences above and below grade. Buried installation may also require a 19 larger trench width “foot print” since the phases are usually arrangement in a flat configuration. 20

A tunnel is widely used for underground installations in cities or other areas with limited space and 21 obstacles along the route. These tunnels are either drilled very deep, typically 20 to 40 m below the surface, 22 or close to the surface built in an open trench. A tunnel of 3 m in diameter is sufficient for incorporating a 23 GIL double system. A GIL tunnel installation is often found in conjunction with hydroelectric or pump 24 storage power plants where the electric power generation is in a mountain cavern and the GIL is used to 25 transmit the electrical energy to the transmission grid at the surface 26

All grounding should be done in accordance with IEEE 80. 27

Examples of typical GIL installations are shown in Annex A. 28




5

4.3 GIL Application Information 1

GIL has multiple features that may help the application engineer address specific problems, these include: 2

a) Overall transmission losses are low compared to overhead lines, because of large conductor cross 3 sections 4

b) There are no significant dielectric losses 5

c) Ratings of 2000MVA and more at 420-550 kV are available with directly buried GIL without 6 auxiliary cooling 7

d) Significantly greater than 2000MVA are available for outdoor installations in air (dependent chiefly 8 on ambient temperature) 9

e) GIL capacitance is approximately four times greater than the capacitance of an equivalent overhead 10 line but approximately four times less than the capacitance of an XLPE cable 11

f) The high voltage portion of GIL is exposed to environmental conditions like snow, ice, wind, 12 pollution, contamination 13

g) GIL lacks any significant aging phenomena, as reported by CIGRE on Gas-Insulated equipment, 14 [B29] 15

h) Lengths of lines are possible without phase compensation, in the range of 60-80 km 16

i) Greater flexibility in routing the line, as any angle is possible 17

Some of the environmental features include: 18 a) Less visual impact than overhead lines 19

b) Low external electromagnetic field levels 20

c) No increased risk of fire as the enclosure, conductor, and insulation materials are made of materials 21 that do not readily burn 22

High degree of gas tightness, especially in welded design, Some Economic considerations include: 23 a) Lower transmission losses over the life of the GIL installation compared to overhead lines 24

b) Higher availability due to minimal maintenance and established reliability 25

26 Pure SF6 is used in switching elements of GIS and high voltage circuit breakers mainly because of its 27 excellent arc quenching characteristics. To meet only electrical insulation requirements (non-switching), 28 similar to a GIL, a gas mixture of 90-80% of Nitrogen (N2) and 10-20% by volume of SF6 is needed. This 29 reduces the amount of SF6 required. 30

Annex B provides a detailed comparison between OHL, Cable, and GIL. 31

4.4 Influence of GIL on the Transmission Network 32

Due to the special characteristics of GIL, the following influences on the transmission network might to 33 occur. The individual influence must be determined in network studies. See D.2 and D.5. 34

Due to its lower inductance, the GIL will tend to carry a greater share of the transmitted power when 35 connected parallel to existing OHL or cables. 36




6

A requirement for line compensation should be studied. However, for very long GIL ( greater than 60-80 1 km), reactive compensation may be needed, depending on the inductive power already available in the 2 network. See CIGRE Brochure 351 for further explanation. 3

The high transmission capability of GIL (2000 MVA per system) may provides the same capacity in a 4 direct buried installation as that of an overhead line with one circuit. 5

The GIL/Overhead line hybrid system allows auto-reclosure as the probability that the fault is in the GIL 6 section is very low. If the fault is in the GIL portion, the additional damage resulting from the reclosure 7 will not normally cause damage beyond the faulted section of GIL(the insulating gas is self-restoring) or 8 result in loss of gas. This allows the system to operate without major changes in operation and protection 9 schemes. 10

4.5 Temperature Design Criteria 11

IEEE Std C37.122 should be followed with regards to the temperature limits, with the following additions: 12

For tunnel installation, the maximum enclosure temperature should not exceed 70°C. Outdoor (open air) 13 installations, the maximal enclosure temperature should not exceed 80°C. Direct buried installations the 14 maximum enclosure temperature should be limited to avoid soil drying, typically between 50°C and 60°C. 15

4.6 Seismic Aspects 16

Seismic analysis should be performed as stated in IEEE Std 693. 17

The seismic behavior of GIL is similar to pipelines. Welded tubes are very flexible and usually withstand 18 earthquakes without any damage, as long as there is no significant settlement or gap in the ground. Special 19 attention has to be paid to external elements that the GIL may pass through that modify the support 20 characteristics of the GIL (e.g. crossing bridge abutments or expansion joints and building entries) as well 21 as special laying areas, with poor soil stability (e.g. sands and shale formations) 22

Seismic performance studies analyzing the line under the required seismic conditions shall be performed by 23 the manufacturer or an independent consultant and form part of the order documentation. The specific 24 requirements shall be defined in the request for proposal documents. 25

4.7 Terminations 26

4.7.1 SF6/Air Bushings 27

SF6 gas bushings are typically used to transition from GIL to the transmission system. The bushings can 28 be ceramic or composite by design. 29

All Bushings should be designed and tested in accordance with IEEE Std C37.017. 30




7

4.7.2 GIS Interfaces 1

A GIS/GIL connection typically happens inside a switchyard, when a hazard or obstacle presents itself that 2 no other alternative can be used. This connection, if done between different manufacturers or technologies, 3 should be done in accordance with IEEE Std C37.122.6 & C37.122.1. 4

4.8 Installation Criteria 5

As noted earlier, GIL can be installed in various arrangements: above ground, in a duct, in a tunnel or 6 directly buried, from horizontal or vertical arrangement, with tight bend radii and any required angle. With 7 this flexibility however, there are some limitations that should be considered. 8

4.8.1 Bending 9

Field bending of GIL is possible down to a bending radius of approximately 400 m without special elbow 10 sections for all voltage classes. Smaller bending radii are possible for lower voltage class GIL. This GIL 11 radius flexibility simplifies installations along most cross-country routes and in drilled tunnels. Smaller 12 bending radii are possible with special design of elbow elements to accommodate angles between 0 and 13 180 degrees. 14

4.8.2 Gas Compartmentalization 15

Long runs of GILs are segregated into multiple gas zones that simplify the gas management. The size of the 16 zones will impact the time required to process the gas zone, and complete leak tests, evacuation and filling. 17 Multiple gas zones also define a convenient gas volumes for handling purposes should the gas need to be 18 removed. In the event of a failure, smaller gas zones also limit the contamination and impacts on the effort 19 required to repair and return to service. In general, the size of the gas zones is a design parameter to be 20 considered in view of the owner’s operational and maintenance requirements. 21

Separation of gas compartments is done using a partition insulator and generally the position is marked by 22 external means. 23

The maximum volume of the gas compartments vary with the owners gas handling equipment capabilities 24 provided for evacuation, storage and filling. Typical gas compartments should be sized to allow removal of 25 the gas from any two adjacent compartments into the storage systems provided. In Figure 2, if work needs 26 to be done in Gas Zone 3, gas needs to be evacuated from Gas Zone 2, Gas Zone 3, and Gas Zone 4 for 27 safety reasons. The handling cart should have enough storage to accommodate this. 28

29 Figure 2 —Gas Zone Arrangement 30

The GIL gas monitoring system uses a temperature compensated gas density monitor mounted directly on 31 the bus enclosure. A gas density monitor is supplied on each single-phase gas compartment. The gas 32 density monitor is normally supplied with two alarm contacts, one at generally 5% below rated filling 33 pressure and the other at minimum functional pressure. Additional alarm contacts can be provided for 34 special applications on the same density monitor. These alarm contacts should be wired into a local control 35




8

cubicle for annunciation of the different alarm levels. Upon receiving an alarm, the system should be 1 scheduled for any required maintenance. 2

Each of the gas zones should be filled and evacuated through independent fill valves. 3

All gas work should be done in accordance with IEEE Std C37.122.3 & CIGRE 276. 4

4.8.3 Pre-Construction Material Storage 5

The installation is more efficient if the user provides sufficient space for storage, assembly, and installation 6 of the GIL. 7

4.8.4 Shipping Assemblies 8

Depending on the manufacturer’s design, there are two different types of shipping assemblies. One type 9 ships the enclosure, conductor, and insulators as separate components. The other type ships factory 10 assembled sections 11

4.9 Testing Criteria 12

IEC 62271-204 has recommendations regarding type-tests and site commissioning tests. 13

4.10 Environmental Aspects 14

4.10.1 Low Electromagnetic Fields 15

The conductor current induces in the enclosure a reverse current of approximately the same size, so that the 16 electromagnetic field outside the GIL is negligible. Values of below 10 µT can be reached at rated current 17 levels close to the GIL. Therefore no special shielding is required even in areas which are critical with 18 respect to EMF, e.g. airports or computer centers. In order to further understand the shielding effectiveness 19 of GIL, a GIL directly buried to a depth equal to 1.3 meters below the surface to the GIL axis and with a 20 current of 3150A will create a magnetic field of 2 µT at the GIL axis and a magnetic field of 0.5 µT at a 21 distance of 5 meters from the GIL axis. For the magnetic field produced by a GIL installed in a tunnel, 22 refer to [B28]. 23

4.10.2 Minimized Losses 24

The resistive losses are very low (See Table B.1— Electrical Characteristics for 400 kV GIL, Overhead 25 Lines, and Cable), and the dielectric losses are negligible. No reactive power compensation and 26 sophisticated cooling systems are needed for lengths less than 60-80 km. This reduces the operation costs 27 significantly and causes savings and contributes to environmental protection. 28

4.11 Technical Data to be given in the Request for Proposal 29

Annex C provides a detailed fill in form that has the minimum required information for a manufacturer to 30 choose a suitable GIL design. 31




9

4.11.1 Other Information Required 1

The following information will help to provide the user with the most effective solution of the GIL system: 2

a) Accurate plans of the route, showing all existing roads, buildings, other obstacles, all other 3 underground installations, available assembly areas including use of steel structures, existing 4 grounding system, dimensions of tunnels or shafts, approximate dimensions of existing right of 5 way or street trench 6

b) Vehicle or other access limitations to or on the site including temporary roadways and wetlands 7

c) Maximum transport dimensions and weights allowed for transport 8

d) Any building access limitations on site (doors, deck openings, hatches) 9

e) Weight limitations at site (floors, ramps, lifting gear, etc.) 10

f) Local working conditions and any restrictions that may apply (e.g., safety equipment, normal 11 working hours, union requirements for local erection crew coordination, etc.) 12

g) Health and Safety regulations that must be adhered to 13

h) Specific pressure vessel rules and procedures that may apply during erection and commissioning 14 tests 15

i) Other local regulations having any influence on the work to be performed at site (e.g. noise 16 limitations, traffic limitations, specific backfill requirements or waste/ground water handling, etc.) 17

j) In-service conditions or operating restrictions of other equipment close to the installation site that 18 must be respected 19

k) Detailed scope of work description including electrical one-line diagram, and GIL interconnection 20 types 21

5. Detailed Project Implementation and Service 22

5.1 Construction Aspects 23

The route should be investigated for environmental impacts including archeological, hazardous waste, and 24 endangered species habitat. If historical activity was likely, consideration should be given to having access 25 (on-call) to an archeologist during the construction. 26

If the GIL route involves public property permits including roadway openings, traffic control, bridge or 27 street weight limitations should be investigated as part of the preliminary engineering. Projects may be 28 required to use specific procedures and practices mandated by local or state regulations. Construction 29 work times, equipment placement along the route, trench excavation techniques (Shoring) may require 30 additional approvals. An initial meeting to review the route with the local governing authority is 31 recommended. 32

The route selection and evaluation process should also identify and examine interferences including major 33 highway or street crossings, stream or other water body crossings, public parks or recreation areas. Legal 34 easement requirements should also be identified as part of the preliminary engineering process. 35

Material storage or lay down areas, marshalling sites for construction equipment, should also be 36 incorporated in the initial planning. Dependent upon the projection location, consideration should be given 37 to physical security of materials and equipment (e.g. fencing, locked storage containers). If the project 38




10

requires excavation, removed soils may require removal from site. In urban or “brown field” sites, soils 1 should be tested before removal for contaminates. 2

All parties (owner, manufacturer, and installation company) in the project should participate in the schedule 3 development and provide regular updates. 4

5.2 Transportation and Storage 5

5.2.1 General 6

All the various parties involved in transport of the equipment between the manufacturer’s plant and the 7 final user’s site should be included in the project planning. These parties may include: Manufacturer(s), 8 general and subcontractors, freight/transportation companies, state or government transportation agencies, 9 insurance companies, and customs agent/brokers; all of whom have different requirements, limitations, 10 priorities, and specialties. Requirements from any of these parties could affect the material delivery 11 schedule and project in service date. 12

5.2.2 Supplier Transportation Responsibilities 13

The manufacturer's shipping methodology is generally finalized during the detailed design phase of the 14 project, dependent on the choice of sub-suppliers and logistics needs. The manufacturer will customize 15 each shipment to ensure the safe arrival of the materials at a user's site, taking into consideration: 16

a) The optimized transport size of the shipping assemblies, depending on system design, transport and 17 access limitations and with a consideration toward reducing the assembly work 18

b) Special handling or packaging requirements for sensitive components (e.g. insulators or monitor 19 equipment), to cover potential rough handling or off-road transport 20

c) Size of delivered lots limitations. The shipping assemblies and other hardware needed for a GIL 21 project is usually delivered in several lots. The size of the lots depends on storage facilities at or 22 near the site 23

d) The packing depends on transportation and storage facilities (outdoor or indoor). The shipping 24 assemblies could be delivered in bundled lots or protected in a more sophisticated manner against 25 contamination or ocean transport. 26

5.2.3 Shipping Insurance and Customs 27

A clear demarcation of responsibility associated with all shipments should be established during the 28 ordering process. International standards (e.g. INCOTERMS) offer clear definitions of all options to meet a 29 specific user requests. Overlapping or open areas of responsibility should be avoided and at each 30 responsibility interface, both parties concerned should check the condition of the hardware. All appropriate 31 documentation as stated in the contract (e.g. packing lists) required for clearing of the equipment through 32 customs must be exchanged properly and in sufficient time between all parties. 33

5.2.4 Storage 34

The storage method utilized (i.e. indoors, outdoors, covered, etc.) should be consistent with the storage 35 method specified by the user and designed by the manufacturer. The equipment should have periodic 36 inspections as recommended by the manufacturer. 37




11

The temporary storage of GIL on-site will be defined by the manufacturer depending on his installation 1 procedures. The actual GIL components are transported to and stored at the installation area in a logical 2 manner, conducive to the installation sequence. A site storage area of sufficient size helps to optimize the 3 installation process and reduce installation time. 4

5.3 GIL Installation 5

5.3.1 Laying Process 6

There are two methods used to install GIL: Prefabricated segments and onsite assembled. This guide will 7 focus on the on-site assembled since the prefabricated and tested sections are typical of GIS installations. 8

The onsite assembled GIL is built of minimal components which are delivered directly from the sub-9 supplier to site, where they are prepared for final assembly. This reduces the risk of transport damages, but 10 also requires more temporary/lay down space close to the installation site. The laying is usually done in 11 parallel with the civil works, i.e. GIL will be installed directly after a section of the tunnel or trench is 12 completed, while the civil works continues at the next section. With this procedure, the installation requires 13 continuous coordination between civil works and GIL installation crew. 14

5.3.2 Installation Crew 15

The installation of GIL requires specially trained skilled workers for material preparation, assembly, 16 welding and final installation. The specialists may be supported by local labor, either from the user or from 17 other local contractors. 18

The overall site management is defined by the contractual agreements between the parties; however the 19 manufacturer’s representative should provide the required direction and be continuously consulted during 20 the installation. 21

Before starting the work, each contractor, work crew and other pertinent individuals should be provided 22 training in the project health and safety requirements and the installation specific criteria (cleanliness 23 requirements). 24

5.3.3 Equipment and Tool Requirements 25

During installation, manufacturers use specialized welding tools and equipment (e.g. welding machines) for 26 jointing the tubes to maximize efficiencies and provide high quality and productivity. This equipment is 27 only required for the initial installation, and there is no requirement for the user to purchase it. If 28 specialized welding equipment for future maintenance access is required, the requirements should be 29 discussed with the manufacturer and may be addressed under a service agreement.. 30

Two sets of any other required tools should be supplied by the manufacturer once the installation is 31 complete. 32

5.3.4 Environmental Considerations 33

The project schedule should allocate time for adverse weather conditions that may influence the project 34 completion time and/or the costs. For example, an installation of a directly buried GIL in winter or during 35




12

the rainy season might require extra protection against water or frost. Postponement of the project to a more 1 favorable timeframe and season might be more effective. 2

5.3.5 Work Procedure 3

The installation of GIL involves the coordination of multiple parties and compliance with various rules and 4 regulations. Generally, the installation is often divided into three phases: prefabrication, assembly and 5 laying. 6

Prefabrication is where the components are unpacked and prepared for assembly; typically the tubes are 7 prepared for jointing and components are cleaned. 8

The assembly process includes the insertion of conductor tubes into the enclosures and aligning the 9 insulators. 10

Laying includes placing the GIL in its final position, and welding the joint to the previous section. 11

Dependent on the project situation and manufacturers design and practices, the work phases may vary. If 12 the user has not decided to run the project on a turn-key basis, the manufacturer should as part of the supply 13 contract describe the installation work procedure. This will enable the user to identify and resolve possible 14 conflicts in early stages of the project. 15

5.4 Testing and Commissioning 16

IEEE Std C37.122 and IEC 62271-204 define three types of tests for GIL: Type (design) Tests, Routine 17 tests, and Commissioning tests. 18

5.4.1 Type Tests 19

The type or design tests are for the purpose of proving the characteristics of the system. They are made on a 20 given design, to prove compliance with the various engineering standards. 21

The manufacturer must be able to demonstrate that all the type tests have been performed on subassemblies 22 of the same design supplied to the user. Type tests are not part of a quality assurance system applicable to 23 each supply consignment and are typically performed only once for a given design. 24

In addition to the type test of short GIL samples, a long-term test should be performed at a typical direct 25 buried GIL arrangement. Details of the test procedure are given in IEC 62271-204. The long term test goal 26 is to establish the reliability of all components and to test the on-site mounting procedure. These long term 27 tests performed at a voltage higher than the nominal system voltage and lasting thousands of hours are 28 intended to represent aging over the equipment life expectancy. The long term test provides a unique 29 opportunity to apply with a voltage stress in conjunction with other stresses affecting equipment life(e.g 30 thermal limits due to current cycles, mechanical stresses associated with the enclosure thermal expansion). 31 Once performed successfully on a GIL basic design, all GIL based on the same basic design, are expected 32 to present similar long term behaviors. 33

5.4.2 Routine Tests 34

Routine tests are a part of the quality assurance process. They are carried out during manufacturing on each 35 item of equipment, with the purpose of revealing faults in material or construction. When the GIL is 36




13

assembled on site, the routine tests are limited to prefabrication tests of some elements, like pressure tests 1 on cast housings or partial discharge tests on the insulators. On the factory assembled sub-sections where 2 the conductors and insulators are factory installed, all of these tests may be done at the factory before 3 shipping. 4

5.4.3 Commissioning Tests 5

For GIL, the tests during and after erection on site are the most important ones. They are carried out in 6 order to detect possible damage suffered during transportation, storage, exposure to the environment, or 7 final assembly. It is important to point out that on-site testing is not a repetition of the type tests or the 8 routine tests. The aim is to prove the integrity of the system before it is energized. It is the final step in the 9 process of quality control and quality assurance. 10

Recommendations as well as technical and practical considerations of site testing are given IEC 62271-204. 11

Particular attention must be paid to dielectric tests. Depending on the length of the line, the testing of GIL 12 in separate sections might be required. 13

5.4.3.1 Basic Requirements 14

The GIL may be completely assembled on-site with sub supplies delivered directly on-site. The on-site 15 testing and commissioning is performed on completely assembled sections. There may also be additional 16 tests in case of factory assembled elements that might also be necessary on-site. 17

The tests carried out after installation is completed are: 18

a) Pressure Test (When not completed in the factory) 19

b) High Voltage Test 20

c) Leak Check 21

d) Contact Resistance Test 22

e) Grounding and Bonding 23

f) Density Monitor Test 24

Acoustic Partial Discharge Test in IEC 62271-204, testing requirements are given. 25

5.4.3.2 Overpressure Testing 26

A field overpressure test is may be required for GIL welded at site. 27

5.4.3.3 High Voltage Testing 28

The on-site HV testing and commissioning of GIL installations is very similar to the procedures used for 29 GIS. The differences are related to the longer lengths of conductor, which has several implications: 30

a) Test equipment must be capable of high test currents for long test sections, In general, variable 31 inductance resonant test sets (available today) are capable of testing approx 10-20 nF of load 32 (~200-400m of 550 kV GIL) . Variable frequency test systems are capable of much higher loads 33




14

(up to ~1600 nF) but at much lower voltages (260 kV max) In some cases, combinations of 2 or 1 more test systems can be used to achieve higher test capability. 2

b) Larger test loads imply that more energy would be dissipated in the event of a testing flashover. 3

c) In order to achieve testability in a practical sense, a GIL may be sectionalized and intermediate test 4 points introduced. The logistics of testing needs to be considered at the design stage 5

The following tests are recommended: 6 a) AC conditioning of the line prior to testing with long time sequences for each voltage step, up to 7

the test voltage. 8

b) AC Power Frequency at 80% of the type test voltage. 9

c) Partial discharge test per IEEE Std C37.122 or IEC 62271-203. Discharge (PD) testing using 10 sensitive UHF method or acoustic detection. 11

5.4.3.4 PD-Testing and Acoustic Partial detection 12

The most common ‘defect’ found in GIL are metallic particles introduced during the assembly 13 process. The conditioning phase of the test will cause many of these particles to migrate to low-field 14 regions (“particle traps’) where they become harmless. However, some form of particle or partial 15 discharge detection is recommended to ensure that no particles remain in high stress areas. The 16 procedure for partial discharge detection and interpretation shall be provided by manufacturer and 17 agreed between user and manufacturer. In this way, the chances of detecting particles can be 18 maximized as some particles could be ‘activated’ by the high voltage. 19

Acoustic particle detection can be easily done using portable instruments. However, modern partial 20 discharge testing is also capable of detecting particles (moving particles will also exhibit electrical 21 discharges) Partial discharge testing may also detect other forms of defects at the expense of added 22 testing complexity. 23

Partial discharge testing is recommended during commissioning process. The experience with gas insulated 24 systems like GIL shows that after the system is commissioned, a continuous measurement of the partial 25 discharge intensity may not be necessary. 26

In summary, the antennas provided for PD sensing will remain in the GIL and may be used at any time 27 later. In the same way acoustic particle detection can be re-applied (at system voltage) whenever 28 required. 29

5.5 Secondary Equipment 30

5.5.1 Density Monitoring 31

The density of the insulated gas is important for the dielectric strength of the high voltage system. GIL is 32 usually divided into several gas compartments. The density in every gas compartment is monitored 33 individually. The monitoring can be done either by standard density monitors with alarm levels, indicating 34 if the density drops to critical values, or by modern microprocessor-controlled systems. Density monitors 35 are economical for short GIL installations, while microprocessor-controlled systems offer advantages for 36 longer GIL, especially if there are a significant number of sensors along the line reporting to a local and/or 37 remote monitoring center (PC or workstation). The microprocessor control system also can measure 38 temperature and pressure separately and calculates the density. 39




15

5.5.2 Arc Location System 1

If a flashover to ground occurs, its precise location may not be immediately apparent, because the internal 2 fault has no external impact. Sometimes, a flashover with low energy might even be self-restoring. 3 Although insulation faults on GIL are unlikely, some users and manufacturers equip GIL with fault location 4 devices to assist in the identification of the precise fault location to reduce the outage time. Such devices 5 can utilize various technologies like optical, electromagnetic, overpressure, acoustic and chemical sensors 6 or temperature-sensitive paint. 7

The accuracy of such a system should be in the range of +/- 10 m, independent of the GIL length, because 8 usually only some 10 m of GIL are impacted after an internal fault and need to be replaced. 9

5.5.3 Appropriate Auxiliary Wiring Practices 10

There are usually different kinds of auxiliary wires to be installed, such as fiber-optics, copper control 11 wires, auxiliary service power and telecommunication cables, etc. In a tunnel, these wires are usually laid 12 in an open or a closed cable tray. At a buried GIL, these wires should be installed in a separate ductwork 13 that has been installed either along with the GIL or after laying the main system. 14

The wiring installation generally depends on the individual project and manufacturer’s practices. 15

As wireless data transmission becomes more and more common, the data transmission of the GIL 16 monitoring system might in the future be handled by this technique and eliminates the installation of the 17 long auxiliary wiring. 18

5.5.4 Auxiliary Power Supply 19

For the auxiliary power supply along the transmission line different solutions can be chosen: 20

Direct supply from the local distribution system 21

Parallel aux. power cable installation (AC or DC, 400 V – 6 kV) 22

Solar panels with batteries 23

Wind generator with battery 24 The choice of the best auxiliary power supply is dependent on the Users requirements and site conditions 25 along the route. 26

5.5.5 Cathodic Protection 27

Cathodic protection needs to be carefully looked at for directly buried GIL systems only. At a minimum, a 28 protective coating should be applied to the GIL enclosure. Other cathodic protection schemes may be used 29 in addition to this. A complete study should be looked at when a direct buried GIL is installed. 30

5.6 GIL Grounding 31

GIL typically follows the practice of a solidly grounded system. At each accessible place, the GIL is 32 grounded, and at the ends, the GIL enclosure is connected to the ground grid of the substation. In the case 33 of a tunnel installation, it is worth remembering that the enclosures should be systematically bonded 34 together and bonded to the tunnel steel reinforcement at regular intervals. This would insure that the 35




16

voltage difference between the tunnel wall and the GIL enclosures is minimal and would constitute a good 1 distributed grounding system. In this way the touch potentials inside the gallery are within acceptable 2 parameters even during a single-phase fault to assure complete safety for operators employed in GIL 3 inspection or other persons inside the gallery at the moment of fault occurrence. 4

5.7 Seismic Requirements 5

5.7.1 Earthquake Evaluation Method 6

The seismic design of GIL requires evaluation of the mechanical stresses. The stresses are generated in 7 various parts of a GIL by deformation of the body of GIL, which is caused by earthquake vibrations, and 8 the stresses applied to the body of GIL by deformation of a tunnel. 9

Seismic analysis should be done in accordance with IEEE Std 693. 10

5.7.2 Evaluation of GIL Natural Frequencies 11

Based on the conductor support span (intervals between the support by spacers on the conductor) and the 12 GIL support span (intervals between the support by racks on GIL), analysis is made on the natural 13 frequencies of GIL. The number of dominant frequencies of an earthquake is around 0.5Hz through 10Hz. 14 To avoid resonance phenomenon, the number of natural frequencies of GIL needs to be outside the range of 15 such dominant frequencies. 16

5.7.3 Study of Seismic Force 17

Seismic waves used for evaluation are three sine waves, artificial earthquake waves and real earthquake 18 waves. To be on the safe side, the three sine waves are used for the calculation using the GIL resonance 19 frequencies. The evaluation by three sine waves is relatively easy to perform. Artificial earthquake waves 20 and real earthquake waves are used for the evaluation of several vibration modes. 21

5.7.4 Analytical Method 22

Static analysis and dynamic analysis are available. By the static analysis which requires simple techniques, 23 we obtain the value by multiplying the GIL weight by the amplification factor which has already been 24 found from the experiment on the full-scale machine. By using this value as the inertial force applied to 25 GIL during an earthquake, we evaluate the stress and displacement generated to GIL. By the dynamic 26 analysis, we evaluate and calculate on the computer the time-to-time changes in the stress and displacement 27 generated in various parts of GIL when seismic waves are applied to GIL. 28

5.7.5 Evaluation 29

The objective is to verify that the stress found in the preceding paragraph is smaller than the allowable 30 strength of the conductor, the sheath and the spacer. 31




17

5.7.6 Testing 1

The GIL as a whole is too large to be tested on a shaking table, but elements as sliding contacts and 2 insulators need to be approved. Representative set-ups with all the elements of the GIL can be used on 3 tables. 4

5.8 Repair Techniques 5

The supplier shall provide with the proposal a repair plan, the necessary training and the recommended 6 spare parts. Preferably the efficiency of this concept shall be proven by a simulated repair under site 7 conditions. 8

6. Gas Handling of Gas Mixtures 9

Gas handling standards and guides exist, IEC 62271-303, IEEE Std C37.122.3, and CIGRE Brochure 276, 10 and should be followed. 11

All gas should conform to ASTM D2472 & ASTM D1933. 12

13

6.1 Pure SF6 and N2/SF6 Gas Mixture 14

GIL may be operated with pure SF6 or a gas-mixture of SF6 and Nitrogen (N2). The advantage of using a 15 N2/SF6 gas mixture is the reduction of the amount of SF6. The GIL offers this opportunity because there 16 are no switching or breaking capability needed, only insulation, and therefore an N2 percentage of 80 % or 17 more is possible with the remaining being SF6 at an absolute pressure of the gas mixture of 0.8 MPa 18 (approximately 100 psig) as an example. 19

CIGRE 360 discusses the mixture of SF6 & N2 in more detail. 20

6.2 Gas Handling Equipment 21

Most of the gas handling equipment is the same as the gas handling equipment used for GIS. The following 22 devices are required: vacuum pump, mixing/filling device, filtration system, suction pump, pressure control 23 and monitoring equipment. All of these components are typically installed on a prefabricated “gas cart”. 24 As noted earlier, the storage of the gas cart should be selected based upon the largest gas zone plus 25 reducing pressure in the two adjacent zones. 26

CAUTION 27

Liquid filling is not recommended. Please consult with the equipment manufacturer if liquid filling is 28 desired. 29




18

6.3 Filling With SF6 or SF6/N2 Gas Mixture 1

Before filling the GIL with the insulation gas, the tube shall be evacuated. The vacuum shall be kept for a 2 certain period of time as specified by the manufacturer. This ensures that the humidity inside the GIL is 3 removed by using dry air and humidity filters. This dry air will be pumped in a cycle until the dry stage is 4 reached with a dew point of -20 °C. 5

The SF6 and N2 gas for the initial filling is usually supplied separately and has to be mixed in the required 6 ratio at site. It is recommended to use a special mixing device, which produces the gas mixture in a defined 7 mixture ratio prior to filling into the GIL. These high accuracy mixing devices work on a continuous basis 8 while filling the GIL. 9

Before checking the final mixture ratio and the final humidity, the gas should be in the GIL for minimum of 10 3 days to allow complete homogenization. Longer time may be needed with larger zones. The 11 manufacturer of the equipment will have a required time frame to perform the final check of the mixture. 12

These measurements should be done in accordance with C37.122.5 13

6.4 Removal of SF6 or SF6/N2 Gas Mixture 14

In the case of removal of the insulation gas, either for maintenance or a failure, the mixture should be 15 stored in its mixed condition in high pressure bottles. The filling pressure of the storage depends on the 16 design of the bottles. During removal and refilling, the gas should flow through particle and humidity filters 17 to remove any potential contamination or moisture from the storage bottles. 18

Before checking the final mixture ratio and the final humidity, the gas should be in the GIL for a minimum 19 of 3 days to allow complete re-mixture and homogenization. 20

IEC 60480 defines the standards for the reuse of SF6, and IEC 60376 defines the standard for new SF6. 21

7. Maintenance and Inspections 22

Once the GIL is filled with gas and energized, no maintenance is required to operate the GIL. The GIL has 23 a gas tightness which lasts 50 years or longer. 24

In case of directly buried GIL the corrosion protection needs to be checked periodically as it is done with 25 other types of directly buried cables. 26

In case of tunnel laid GIL visual checks should be carried out periodically to check the condition of the 27 tunnel, e. g. water leaks. 28

If moisture measurements are preformed, they should be in accordance with C37.122.5 29

8. Monitoring equipment shall be checked periodically as recommended by 30 the manufacturer. Training 31

The manufacturer’s initial training should cover configuration, operation, maintenance, SF6 gas handling 32 and monitoring, and trouble shooting. New members to a crew should also be provided the same training 33 along with periodic refresher sessions for the work force. 34




19

9. Decommissioning 1

The GIL is easy to disassemble into its component parts: gas or gas mixture, insulators, aluminum pipes of 2 conductor and enclosure and the auxiliary components can be fully recycled. 3

9.1 Removal of SF6 or SF6/N2 Gas Mixture 4

Refer to 6.4. 5

9.2 Dismantling of the GIL 6

After de-energizing and removal of the gas, the line is either cut into pieces and the components separated 7 for a welded system, or unbolted for a flanged system. The design of the GIL should permit all materials to 8 separate as easy as possible. 9

Recycling firms can then cut, divide or segregate the aluminum alloys (conductor, enclosures), cast epoxy 10 resin (all types of insulators) and electronic devices and control wiring (monitoring system) so all materials 11 are re-used. In case of a buried GIL, the anti-corrosion protection coating of the enclosures must be 12 stripped off. This is a standard procedure well defined from other pipeline projects. 13




20

Annex A 1

(Informative) 2

Typical Installations 3

A.1 Installations in a Tunnel 4

A.1.1 Schluchee, Gernamy 5

Design Ratings: 6

Ur 420 kV Ir 2500 A

UBIL 1640 kV Is 53 kA

5 4

3,5 m

2,8

7 1 600 MVA Transformer 2 Encapsulated Surge Arrestors 3 Transfer Switching units 4 GIL Connection 5 Open Air Surge Arrestor 6 Overhead Line

Features: 8

The installation connects a peak loading hydro power plant to the 420 kV transmission system. The GIL is 9 installed in a tunnel that was laid thru a mountain, and operated at rated currents during pumping and peak 10 load generation operations. The GIL has been in continuous, reliable operation since 1976. 11

12




21

A.1.2 Shinmeika-Tokai-Line, Japan 1

Design Ratings: 2

Ur 275 kV Ir 6300 A

UBIL 1050 kV Rated Gas Pressure 0.34 Mpa

Transmission Capacity 2,850 MW/circuit 3 Conductor (Aluminum) Outer Diameter 170 mm

Thickness 20 mm Enclosure (Aluminum) Outer Diameter 460 mm

Thickness 10 mm 4 5

6

7

8

9 10




22

A.1.3 PALEXPO, Switzerland 1

Design Ratings: 2

Ur 300 kV Ir 2000 A


3

In this application, two Circuits, totaling 3680 meters were installed using an SF6/N2 gas mixture. The 4 GIL has a bending radius of 700 meters, accomplished using no elbows, and connects to an overhead line at 5 the Geneva Airport. The GIL was installed in 2001, and remains in service today. This GIL was installed 6 using a prefabrication tent at site. 7

8

9

10

11

12

13

14

15

16

17




23

A.1.4 Laxiwa Hydro Electric Power Plant, China 1

Design Ratings: 2

Ur 800 kV Ir 4000 A


3

This application was installed inside a mountain for a hydroelectric power plant to reach the 800 kV 4 transmission system. It consisted of a 200 meter vertical shaft, as well as a 300 meter horizontal tunnel. 5

6

7 8

9

10

11

12

13

14

15




24

A.2 Above Ground 1

A.2.1 Baxter Wilson Power Plant, Mississippi, USA 2

Ur 550 kV Ir 4500 A


3 This application consists of a single Circuit Gil with SF6 to air bushing at each end of the line. Total length 4 is 1250 meters. This was installed in 2001, and was decommissioned in 2009. The bus was sent back to 5 the factory and reconditioned for use at a different site by the owner. 6

Several existing 550 kV and 242 kV lines crossed the required right of way for a new line. Elevating an air 7 insulated conductor and travelling over the existing 550 kV lines was not possible due to reliability 8 concerns. GIL below the existing lines was identified as an economical solution and helped meet the 9 continuous current duty of 4,000 amps at 500kV. 10

The application also included a portion of the circuit passing through a water retention area which floods to 11 several feet in the spring. GIL supported by pylons to keep it above the high water line addressed the need. 12 Expansion flexibility is achieved with mitered elbows and the bushings on the south end of the circuit are 13 allowed to slide +/- 6 inches on the fixed support structure, eliminating the need for bellows assemblies. 14

15

16




25

1

A.2.2 Power Plant #9, Saudi Arabia 2

Design Ratings: 3

Ur 420 kV Ir 1200 A at 55 °C


4 GIL was selected in this application for high reliability in a demanding environment, high power transfer 5 capability, low losses and simplified installation requirements. The GIL is about 17km and connects eight 6 separate lines between the step-up power transformers and the plant’s 420kV GIS. The GIL is installed at 7 heights between 7 and 9 meters on steel supports. 8

The GIL design features include: extruded aluminum alloy tubes with welded flanges for bolted 9 connections, conical insulators with long creepage distances for conductors support and compartments 10 partitioning, and rollers fixed on steel supports to allow the free movements of the enclosures when 11 exposed to thermal variations. An outdoor moveable tent allowed the enclosures to be assembled in the in 12 a manner and avoid airborne contamination in dusty and windy conditions 13

14

15 16




26

A.2.3 Hams Hall, England 1

Design Ratings: 2

Ur 420 kV Ir 4000 A


3

This GIL connection is through an existing AIS 420-275kV substation, and is partly above ground (within 4 the substation) in a vertical formation and partly in covered concrete trenches (outside the substation). 5

Features of this design include: Low wear contacts, use of particle traps around contacts and in the 6 enclosures, conical insulators with long creepage distance, connection between enclosures by welding on 7 site, gas monitoring by electronic system using digital serial connections measurement and UHF sensors for 8 partial discharge measurement. 9

10

11

12 13




27

A.3 Direct Burried 1

A.3.1 Hudson Switching Station 2

Design Requirements: 3

Ur 145 kV Ir 2000 A

UBIL 650 kV Is 63 kA/3 s

4

There are two GIL circuits totaling 1640 meters in length. Multiple elevation changes and odd angle bends 5 are used to follow the contours of the site. This allows optimized routings and minimized trenching 6 requirements. 7

Cathodic protection protects the aluminum enclosure from corrosion with polarization cells on each end of 8 the circuits to isolate the enclosure from ground and generate the correct corrosion potential voltage. Also, 9 the entire enclosure had a corrosion protection coating applied to the enclosure prior to leaving the factory. 10 After the welds joining the sections were completed, a corrosion protection tape was applied. This entire 11 coating was then checked in the field, and repairs were made prior to backfilling the GIL. 12

The GIL is solidly bonded at both ends to permit enclosure currents and minimal external magnetic fields. 13

Each shipping section was 18 meters in length to comply with roadway transportation requirements. Field 14 welded connections are used on all direct buried connections. 15

16




28

Annex B 1

(Informative) 2

Comparison of GIL, OHL, and XLPE Cable 3

The purpose of this Annex is to provide a comparison of the technical characteristics of GIL, overhead 4 lines and underground XLPE cable. Table B.1 shows typical values for the electrical characteristics of a 5 400 kV and 50 Hz GIL with a continuous thermal rating of 2000 MVA. Figure B.1 shows the differences 6 in losses between Overhead Lines, Cable, and GIL. 7

GIL OHL XLPE CABLE (2 PER PHASE)

Current Rating (A) 3000 3000 3000 Transmissible Power

(MVA) 2000 2000 2000

Resistive Losses (W/m) @ 3000A

180 540 166

Dielectric Losses (W/m)

- 2.4 15

Total Losses (W/m) 180 542.4 181 AC Resistance (μ

Ω/m) 6.7 20 6.0

Inductance (nH/m) 162 892 189 Capacitance (pF/m) 68.6 13 426

Surge impedance (Ω) 48.6 263 12

Table B.1— Electrical Characteristics for 400 kV GIL, Overhead Lines, and Cable 8

9

10

Figure B.1—Comparison of Losses of a 400 kV Overhead Line, XLPE Cable, and GIL 11

The single-phase GIL is solidly bonded at both ends and at intermediate points. The skin and proximity 12 effects are negligible. The basic insulation levels are according to IEEE Std C37.122 and IEC 62271-204. 13

0

100

200

300

400

500

600

500 1000 1500 2000 2500

OHL 4 Cables 240mm2 cross section With a 4/0 Steel Runner Per Cable

W/m

1800 14001000700 350

XLPE Cable 2x1600 mm2

GIL 6000 mm2

Comparison of GIL, Overhead Line and Cable

Amps MVA




29

B.1.1 OHL, Cable and GIL Technical Points 1

Any planning study comparing the three technologies must first identify the basic requirements i.e. rated 2 voltage and current for the new line, reactive power compensation for cable installations and other points 3 identified below as the needs of the new application dictate. 4

Are there any limitations in the Right-of-way? OHL need a larger Right-of-way, than GIL or 5 cables. GIL and cables may require street opening permits and sigificant traffic congestions. 6

Which environmental aspects are important (aesthetic, EMF, severe external influences like storms, 7 ice, and vandalism)? GIL and cables are protected against most kinds of external influences. 8 Furthermore GIL has the lowest external magnetic field of any other kind of transmission line due 9 to the strong shielding effect of enclosure opposing current phasors. GIL may be an acceptable 10 solution when there is strong local opposition to a new transmission line since GIL significantly 11 reduces the environmental and magnetic impacts and hence it permits the transmission line to pass 12 through or near protected sites such as schools or through congested urban areas. 13

Which approval processes are necessary for the different systems and how long is the expected 14 durration for approval. A long approval process often causes higher project costs. 15

Are there any operational aspects to be considered, e.g. a underground section in line with OHL 16 requires auto-reclosure operation. 17

Depending on the installed length and the available short circuit current, a cable may require 18 reactive power compensation equipment [25] 19

Especially in tunnel installations and in other areas close to public access safety aspects can be of 20 importance, e.g. the risk of fire or explosion in case of a failure. In fact, there are many GIL 21 features which make GIL compatible with tunnel installations [24] 22

Electrodynamic forces during phase-to-enclosure faults are self-centering inside and negligible 23 outside the enclosure, hence any harmful effects to personnel or structures near the GIL are greatly 24 minimized. 25

Enclosure shielding during phase-to-enclosure faults is very effective, thus electromagnetic 26 interference with neighboring metallic structures or communications, signaling, or power supply 27 systems (e.g. railway supply systems) is avoided. 28

GIL can be considered incombustible because it is a closed gas system which has exclusively 29 metallic external surfaces. 30

GIL is designed to minimize external damage caused by high fault currents 31

How are soil conditions along the right of way? 32




30

Annex C 1

(Informative) 2

Proposal Fill-in Data Table 3

The following table includes “fill-in” information that a user should supply to a manufacturer as part of the 4 initial bid. A Sample Specification may be obtained from IEEE C37.123. 5

Electrical Ratings User Request Manufacturer Supplied Units Maximum Continuous Voltage kV

Frequency Hz Rated Insulation Levels (BIL/SIL) kV

Rated Continuous Current A Rated Momentary Withstand Current kA

Rated Short Circuit Current and Duration kA/S Rated Control Voltage VAC/VDC Ambient Conditions

Ambient Temperature Range (high/low) F (C) Solar Radiations (peak/average) W/ft2 (W/m2)

Elevation Ft (m) Maximum Wind Load MPH (KPH) Maximum Ice Load Inch (mm)

Maximum Seismic Load G

Table C.1—Minumum Required Information for all Applications 6

Tunnels & Shafts User Request Manufacturer Supplied Units Required Tunnel Cross Section (>3m

diameter) Ft (m)

Ventilation Shaft size and location Ft (m) Air Temp at inlet (peak, daily average,

seasonal average) F (C)

Maximum Seismic Load G Maximum Elevation change Ft (m)

Civil Conditions of the shaft Size of Opening Ft (m)

Access and lift Support Systems

Table C.2—Required Information for Tunnel and Shaft Installations 7

Direct Buried User Request Manufacturer Supplied Units Soil Temperature (seasonal Average) F (C)

Required minimum laying depth Ft (m) Thermal Soil Resistivity m-C/Watt

Core Boring Data Ft (m) Depth of Water Table Ft (m)

Kind of Soil (sand, rock) Mechanical soil Characteristics

Required loads after laying (agricultural/traffic)

Lbs (N)

Maximum Seismic Load G

Table C.3—Required Information for Direct Buried Installations 8




31

Trench User Request Manufacturer Supplied Units Required Trench Dimensions Inches (mm)

Kind of Ventilation (Natural, Forced) Air Temperature at inlet (peak, daily

average, seasonal average) F (C)

Maximum Seismic load G Maximum Elevation Change Ft (m)

Civil Conditions Drainage

Access and life support systems

Table C.4—Required Information for Trench Installations 1




32

Annex D 1

(Informative) 2

Execution of a GIL Project-Planning and Engineering Process 3

The following flow-chart shows the different phases of a typical GIL project. Depending on the individual 4 project conditions variations in the different phases may occur. However, it is generally recommended to 5 contact the GIL manufacturers as early as possible in order to discuss the project and learn as much as 6 possible on potential solutions. It is generally more economic to discuss the work and technologies before 7 fixing the final route, since routing significantly influences the project costs. 8

Iniation of a Project

Preliminary System StudiesNeeds Assessment

Routing

Pre Proposal Stage

Further Studies

Approval of Right of Way

Preperation for a Request for Proposal

Evaluation of Proposal

Project Implimentation

Detailed Design Phase With ApprovalManufacturing Phase

Installation PhaseTesting & Commissioning

Turnover to User 9

Figure D.1— Flowchart: Execution of GIL-Projects 10




33

D.1 Initiation of a Project 1

The main reason for a project is the requirement for a new transmission line, e.g. the connection of new 2 load centers or new power plants to the grid or a new interconnection link due to load flow requirements. 3

Other reasons becoming more common today are a GIL substitution of an existing overhead line (OHL) 4 because of environmental considerations and the substitution of GIL an existing cable to upgrade the 5 existing connection while still using the existing corridor or right-of-way. In this initiation phase many 6 basic decisions are made. New power transmission lines approaches like using railroad tunnels, or traffic 7 tunnels with GIL should be explored at this time. In Europe, for instance, there are several proposals to 8 integrate GIL and railway or highway projects [21, 22, and 23] or to integrate GIL with natural features 9 [24] within the same corridor to optimize the rights-of-way needed for public services. 10

D.2 Preliminary System Studies / Need Assessment 11

The initial project concept is usually followed by a transmission network study. This study should indicate 12 the basic requirements of the new line. Important technical data will be finalized in this step including 13 ratings for transmission voltage, normal and maximum current, impedance of the line and its influence to 14 the existing network, short circuit rating and availability. Based on the actual load flow with and without 15 the new line under consideration, stability aspects, and the long term forecast, the basic requirements for 16 the new line should be determined 17

D.3 Routing 18

The general routing is given by the location of the new line. However, when discussing the detailed 19 routing, it is required to know the type of system to be installed because each type requires special 20 considerations, e.g. an OHL needs a large right of way, a cable needs a large bending radius, and a GIL 21 needs accessibility. A preliminary route should thus be chosen under consideration of the special 22 requirements of the chosen system. 23

D.4 Pre-Proposal Stage 24

Manufacturers can provide information that could help optimize the routing and the installation procedure. 25 The routing has a strong influence on the price, and often some minor changes in the route can significantly 26 decrease or increase the price. A pre-proposal enquiry to different manufacturers with submission of all 27 information known at this stage enables the manufacturers to comment before decisions are made which 28 could result in unnecessarily cost increases. 29

D.5 Further studies 30

Once the preliminary decision for the type of transmission system is made, the approximate route is fixed, 31 and the comments of different manufacturers have been received, then all of this project information is used 32 to finalize the studies. 33

With knowledge of the actual line length and line parameters, detailed transmission system studies of load 34 flow, compensation requirements and transient network behavior/insulation co-ordination can be 35 performed. These studies give the rating requirements for the new line. 36

It is important to know that the requirement of higher ratings (current and voltage) does not increase the 37 system costs of a GIL significantly. So, with respect to the future use of the GIL with its lifetime of 50 38




34

years or more, it is recommended to consider the expected long-term load growth for the definition of the 1 required transmission capability. 2

Other studies or investigations that may be required include: 3

EMF requirements 4

Grounding parameters (soil resistance etc.) 5

Soil parameters of the route (Civil and Thermal) 6

Other important civil engineering aspects 7

Seismic studies 8

Enviornmental Impact Studies 9

D.6 Approval of the Right-of-way 10

In addition to the studies, the permit process to acquire the necessary Right-of-way should proceed. It is 11 recommended that the various manufacturers involved in the project confirm the route is satisfactory and 12 suitable for the installation and operation of their system. 13

D.7 Preparation of a Request for Proposal 14

When all necessary preliminary studies and pre-proposal discussions have been performed and all basic 15 data are conclusive, this information will form the foundation of the technical part of the request for 16 proposal. Details of information to be given with the proposal are given in Annex C and Section 4.4. 17

At this stage the question of the project implementation should be addressed. The project organization 18 depends on whether the user has an experienced engineering staff, a consulting company may be involved, 19 and a GIL manufacturer turn-key installation or a contracting company is the best approach. In any of the 20 approaches close cooperation between civil works and electrical works is essential for an economical and 21 efficient GIL project; so a turn-key installation including the civil works offers certain advantages. 22

When sending out the final request for proposal, all information and requirements (electrical, mechanical, 23 thermal and EMF) necessary for submission of firm quotations should be defined. This objective is reached 24 by conducting preliminary investigations, including pre-proposal discussions with manufacturers. The final 25 proposal should be limited to one technique (GIL, cable or OHL) to optimize civil works. If the user 26 decides to open the proposal for Cable and GIL, all relevant cost factors, technical advantages, and 27 environmental impacts should be considered during evaluation. 28

The commercial part of the request for proposal should include the evaluation parameters for all relevant 29 cost factors, not only the investment costs for the line. This is especially important when the aim is for an 30 economical comparison between different types of transmission technologies. An accurate economic 31 assessment must consider the overall costs and not only the initial investment. All expected operational 32 costs over the life span of the line should be taken into account [B26] [B27]. 33

The following costs should be considered: 34

Initial costs: Transmission system including installation and commissioning, civil works, real estate and 35 cost for the right-of-way, additional equipment for cooling, reactive power compensation, fire fighting, 36 ventilation, EMF shielding, monitoring equipment 37

Operational costs: should evaluate cost of losses for life time, life time (Maintenance) cost of equipment, 38 risk of failures, and calculated expense for outages / repairs depending on expected availability figures. 39




35

D.8 Evaluation of Proposals 1

The potential manufacturers have now presented their proposals on how to meet the requirements specified 2 by the user and they have provided their cost proposals. If the addition of relevant additional costs, results 3 in an overall project price increase further negotiations are in order. 4

Due to the nature of GIL there should be detailed discussions between user and manufacturers to ensure 5 that each manufacturer fully understands the user's requirements and at the end of such discussions and 6 final evaluations the user will invite manufacturers for final contract negotiations ending up with the 7 signing of a contract. 8

The contract must include the following items: 9

a detailed description of the scope of supplies and services 10

a detailed description of all interfaces to others (civil works, secondary equipment, other equipment 11 suppliers, etc.) with a clear definition of the related responsibilities 12

an outline description of the installation requirements and procedure (available space, accessibility, 13 etc...), because these factors are of major importance for the installation costs and time. 14

a project time schedule 15

a detailed description of all required regulatory approvals and a clear definition as to which party is 16 responsible for obtaining each regulatory approval. 17

The clear definition of the responsibilities and the time schedule is of major importance because of the 18 strong dependence of GIL installation on progress and quality of the civil works. 19

D.9 Project implementation 20

D.9.1 Detailed design phase with approval procedure 21

When the order is placed and other possible contract conditions have been fulfilled, the manufacturer starts 22 the detailed engineering of the installation. 23

During this activity all related parties (electrical, civil, consultants etc.) should perform one or more 24 "Design Reviews", to ensure that all items are covered and addressed according to the original 25 requirements. At this stage an organization chart incorporating all responsible parties and identifying all 26 individuals working on the project should be published. 27

The final design is to be approved by the user. The user and manufacturer should ensure continuity of 28 technical agreements reached before and during contract negotiations. 29

So as not to delay the progress of the project, it is important to also establish efficient approval procedures 30 including firm deadlines for submission and approval of information subject to acceptance by the user. 31

The design work concerning arrangement of equipment and grounding systems should be completed before 32 the commencement of civil works. 33

D.9.2 Manufacturing Phase 34

GIL consists mainly of inner conductor, outer enclosure and insulators and usually only a few other special 35 elements. The manufacturing for the tubes is limited to preparation for assembly, so this work is often done 36




36

directly at site in parallel with the assembly. An alternative approach to site assembly is factory assembly 1 and testing of complete GIL sections. This alternative approach is often used for shorter GIL’s. 2 Environmental protection (e.g. ISO 14000 series) and quality assurance at the site is of major importance 3 for the quality of the installation. An inspection and test plan according to a Quality Assurance Plan (e. g. 4 ISO 9000 series), showing all the checks and tests of various parts performed on the GIL should therefore 5 be agreed upon by the user. 6

D.9.3 Installation Phase 7

In order to reduce the overall project time, the installation of GIL usually starts before the civil works are 8 ended. Close co-operation between civil works and GIL installation at site is essential in this step. The 9 better the project schedule and responsibilities are defined during contract phase and followed by the 10 involved parties, the better the project will proceed. 11

The GIL installation is typically completed by the manufacturer because experience and special skills are 12 necessary. The manufacturer often has special tools available (e.g. mechanized welding equipment), which 13 helps reduce the installation time to a minimum. However, if the user wants to reduce the installation costs 14 his personnel under the guidance or a manufacturer’s representative is another approach. 15

D.9.4 Testing and commissioning 16

Recommendations for testing and commissioning are given in IEC 62271-204. However, depending on 17 manufacturers experience and practice, equivalent tests might be suggested and performed. 18

D.9.5 Operation 19

With formal acceptance, the user takes responsibility for the installation. Before formal acceptance the final 20 “as-built” documentation shall be provided to the user. 21

The documentation shall consist: An operation and maintenance manual, a set of as-built-drawings of the 22 line and the QA-documentation (test certificates and confirmations) including gas tests and high voltage 23 test results. The supplier shall also provide a detailed description of the warranty including any negotiated 24 extended provisions. 25




37

Annex E 1

(Informative) 2

Bibliography 3

Bibliographical references are resources that provide additional or helpful material but do not need to be 4 understood or used to implement this standard. Reference to these resources is made for informational use 5 only. 6

[B1] H. Koch: "Underground gas insulated cables show promise", Siemens AG Power Transmission & 7 Distribution Group, MPS REVIEW, May 1997, pp.21-24. 8

[B2] IEC 61640 “Rigid high-voltage, gas-insulated transmission lines for rated voltages of 72,5 kV and 9 above" 10

[B3] CIGRE WG 23.02, Task Force 01, "Guide for SF6 gas mixtures (application and handling in 11 electrical power equipment)", CIGRE Brochure 163, 2000. 12

[B4] D. Feldmann, Y. Maugain, M. Bourdet, M. Hopkins, P.M. Lanquetin: "Development of a directly 13 buried 400 kV Gas Insulated Line technology", CIGRE Session 2000, Report 21/23/33-02 14

[B5] CIGRE WG 23.02, Task Force 01, "Guide for SF6 gas mixtures (application and handling in 15 electrical power equipment)", Cigre Brochure 163, 2000. 16

[B6] P. O'Connell and all (CIGRE WG23.02), "SF6 in the Electric Industry, Status 2000", Electra n°200, 17 February 2002, pp16-25 18

[B7] H. Koch, A. Schütte: “Review of gas insulated transmission lines for high power transmission over 19 long distances”, IEEE Summer Power Meeting, Berlin, 06/97 20

[B8] A. Chakir, H. Koch: “Numerical solution for a turbulent natural convection in cylindrical horizontal 21 annuli”, ASME-Journal 2000 22

[B9] H. Koch, “Optimized pipeline for electricity,” Power Today, pp. 6–9,2001 23

[B10] O. Völcker, H. Koch: “Insulation co-ordination for gas-insulated transmission lines (GIL)”, IEEE 24 Transactions on Power Delivery, Vol. 16, No. 1, January 2001, PE-102 PRD 25

[B11] C. Henningsen, G. Kaul, H. Koch, A. Schütte, R. Plath: “Electrical and Mechanical Long-Time 26 Behaviour of Gas-Insulated Transmission Lines”, CIGRE Session 2000, Paris 27

[B12] A. Chakir, H. Koch: “Thermal Calculation for Buried Gas-Insulated Transmission Lines (GIL) and 28 XLPE-Cable”, IEEE Winter Power Meeting 2001, Columbus 29

[B13] A. Chakir, H. Koch: “Long Term Test of Buried Gas Insulated Transmission Lines (GIL)”, IEEE 30 WPM 2002, New York 31

[B14] A. Chakir, H. Koch: “Turbulent Natural Convection and Thermal Behaviour of Cylindrical Gas-32 Insulated Transmission Lines (GIL)”, IEEE PES Summer Meeting, Vancouver 2001 33

[B15] A. Chakir, H. Koch: “Corrosion Protection for Gas-Insulated Transmission Lines”, IEEE Summer 34 Meeting, Chicago, 2002 35

[B16] J. Alter, M. Ammann, W. Boeck, W. Degen, A. Diessner, H. Koch, F. Renaud, S. Pöhler: “N2/SF6 36 gas-insulated line of a new GIL generation in service”, CIGRE Session 2002, Paris 37

[B17] H. Koch, G. Schoeffner: “Gas-Insulated Transmission Line - To Solve Transmission Tasks of the 38 Future”, IPEC Conference 2003, Singapore 39

[B18] H. Koch: “Gas-Insulated Transmission Line (GIL)” , IEEE General Meeting 2003, Toronto 40




38

[B19] R. Benato, E. M. Carlini, C. Di Mario, L. Fellin, A. Paolucci, R. Turri: "Gas Insulated Transmission 1 Lines in Railway Galleries", IEEE Trans. on Power Delivery, Vol. 20, Issue 2, April 2005, pp. 704-709. 2

[B20] R. Benato, E. M. Carlini, C. Di Mario, L. Fellin, G. Knollseisen, M. Laußegger, M. Muhr, H. Wörle, 3 R. Woschitz: "Gas Insulated Transmission Lines in Railway Galleries – Part II", Proceedings of IEEE St. 4 Petersburg Power Tech'05 Conference, 27-30 June 2005, S. Petersburgh, Russia. 5

[B21] R. Benato, P. Brunello, E.M. Carlini, C. Di Mario, L. Fellin, G. Knollseisen, M. Laußegger, M. 6 Muhr, A. Paolucci, W. Stroppa, H. Wörle, R. Woschitz: Italy-Austria GIL in the new planned railway 7 galleries Fortezza-Innsbruck under Brenner Pass, CIGRE Session 2006, PAPER B1-304, Paris. 8

[B22] R. Benato, C. Di Mario, H. Koch: "High capability applications of Long Gas Insulated Lines in 9 Structures", Proceedings of IEEE Transmission and Distribution Conference, May 2006, Dallas; also 10 accepted for publication in IEEE. Trans. on Power Delivery. 11

[B23] R. Benato, A. Paolucci: Operating Capability of Long AC EHV Transmission Cables, Electric 12 Power Systems Research, Vol. 75/1, July 2005, pp. 17-27. 13

[B24] R. Benato, D. Capra, R. Conti, M. Gatto, A. Lorenzoni, M. Marazzi, G. Paris, F. Sala: 14 Methodologies to assess the interaction of network, environment and territory in planning transmission 15 lines, CIGRE Session 2006, PAPER C3-208, Paris. 16

[B25] R. Benato, M. Del Brenna, C. Di Mario, A. Lorenzoni, E. Zaccone: A New Procedure to Compare 17 the Social Costs of EHV-HV Overhead Lines and Underground XLPE Cables, CIGRE Session 2006, 18 PAPER B1-301, Paris. 19

[B26] R. Benato, F. Dughiero, M. Forzan, A. Paolucci: "Proximity Effect and Magnetic Field Calculation 20 in GIL and in Isolated Phase Bus Ducts", IEEE Transactions on Magnetics, Vol.38, No2, Mar. 2002, pp. 21 781–784. 22

[B27] R. Benato, F. Dughiero: "Solution of Coupled Electromagnetic and Thermal Problems in Gas 23 Insulated Transmission Lines", IEEE Transactions on Magnetics, Vol.39, No3, May 2003, pp. 1741 – 1744. 24

[B28] R. Benato, L. Fellin: “Magnetic field computation for gas insulated lines installed in gallery", 25 Proceedings of 39th Universities' Power Engineering Conference – UPEC 2004. Bristol, UK, September 6-26 8, 2004; Vol. I pp.6-10. 27

[B29] CIGRE Brochure 150 “Report on the 2nd international survey on high voltage gas insulated 28 substations (GIS) service experience.” 29