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Gas insulated switch gear : study,reports,and evaluations

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Preface The GIS offers economic solutions for complex switchgear applications, e.g. in densely populated areas, for aesthetic town planning or under severe environmental impacts. Precisely speaking GIS leads to an effective use of a limited space. Due to the inherent safety and compact layout of a Gas insulated substation, it is the best solution for substations which are to be built in restricted underground spaces. Since all live parts of GIS are contained in a metal enclosure, they are fully protected against environmental effects, such as salt deposits in coastal regions, sand storms, humidity in the atmosphere, etc.; insulator cleaning is eliminated and, thus, high reliability is achieved. As the Live parts (e.g. buses and connections) are contained within earthed enclosures of GIS and are inaccessible, as such it ensures maximum operator safety and reduced maintenance. Besides that GIS meets with recent requirements for aesthetic compatibility with surroundings. More over adoption of GIS offers an excellent solution for environmental protection since it helps in protecting the system from hazards. GIS is considered trouble free and maintenance free installation. However in case of any fault the services of manufactures representatives are required to attend and repair the fault since all the modules of GIS comes directly under assembled form and in sealed condition from factory since no technical expertise is available indigenously for repair and maintenance of GIS. A manual on GIS will certainly help the users to understand a brief of GIS regarding the details of inside components of GIS, its installation process and operation and maintenance of GIS. The preparation of manual on GIS is an attempt in this direction it will prove be a guide/recommendatory document for the power station utilities in the country.

CHAPTER-1 1.0 INTENT The manual is a maiden attempt to consolidate information in a scientifically organized way, so as to engineer a document which should be available to user utilities as a Manual on gas insulated switchgear and gas handling aspects. Document incorporates broadly:

Check application of GIS at feasibility stage. Propose complete data and rationalized specifications with techno economic provisions on spares and effective execution. Develop a GIS station which should give reliable service deliberating specifically with reference to standard schemes. User interface in engineering, quality assurance and testing Consolidate gas handling, maintenance and testing related equipments/norms. Considerations and decisions to be taken by users considering overall implications High Frequency transients and their controlling measures Provide list of updated standards and codes. Emerging technologies.

The manual shall also touch the options of hybrid switchgear available to users as a technoeconomical solution under specific cases. Besides Technical Aspects, the manual shall also cover safety and training aspects. The manual has been prepared taking into consideration the availability of information from manufacturers, testing organizations, and users etc. This document is to be considered by the user as a recommendatory document and customarization with respect to project specific requirement is essentially required taking into account prevailing standards at the time of application. The gas used in the switchgear needs proper handling on account of environmental considerations. Normally the pressure inside the GIS is kept above four bars and liquefaction of the gas at this pressure could happen on temperatures below (-) 30oC. The application therefore should be carefully done in case the ambient temperature happens to go below this temperature. GIS internationally is getting acceptance on account of technical merits. It has excellent adaptability in meeting most difficult terrain or space constraints. The land area required for a GIS is in the order of 10% to 20% of that for an AIS 9(Air Insulated Switchgear) substation considering the switchgear bay, the saving in overall land area depends very much on the specific voltage level and the connection to transformers, reactors and incoming and outgoing lines. If the substation is connected to overhead lines, then space will have to be allocated for tower and droppers which might reduce the total land saving, . Indoor and underground GIS is possible even in urban and highly populated areas which will allow building of the substation at the point of consumption which will bring about significant cost savings in the distribution network.. It is upto 220 kV level, transportation conditions permit factory assembled bay which is most attractive and favorable feature for indoor & outdoor application alike due to very short erection and commissioning time. Reduced requirement of maintenance make them suitable option for most reliable operation. In fact quality consciousness of equipment alone makes GIS as favored equipment choice for remote operation. All these factors in totality make GIS one of the attractive equipment options to adopt. GIS is also considered for severe environment condition, where saline pollution near coastal areas of industrial pollution requiring regular cleaning of insulators and corrosion of metallic components and electrical joints. GIS is also adopted when substation is to be installed at very high altitudes or very low temperatures or seismic considerations and hydro station.

CHAPTER- 2 TECHNO ECONOMIC FEASIBILITY Initial equipment cost price of GIS is usually higher than that of AIS. There are many other merits/ considerations which need to be justified/ evaluated including the higher initial equipment expenditure. Therefore, the choice of GIS with respect to AIS is a technoeconomic decision to be made taking into account technical, economical and environmental factors. The main advantages of GIS are as under: Compactness/ Land requirements Suitable for Mountainous zones Environmental conditions Replacement/up gradation of AIS due to growing power demand Reliability Safety Applicability to strategic locations. Less area of land requirement

2.1 Compactness / Land Requirement GIS compact dimensions and design make it possible to install substations up to 550kV right in the middle of load centers of urban or industrial areas. This compactness of GIS has a direct influence on land requirement and land cost. Compact modular designs offer many possibilities in layout design and allow specific site requirements to be met as compared to AIS. The land requirement for GIS substation (considering switchgear only) is of the order of 8 to 20 percent of that for an AIS substation. The savings in land area depends upon the voltage level and the associated equipments like transformers, reactors and incoming/outgoing lines. The savings could also be maximized by using cable connections and short length GIS trunking applications. In the case of underground hydroelectric power plants, GIS can be erected close to transformers or near the outdoor yard. A study of cost has to be made considering the space availability, cavern dimension in considerations in terms of height, width and

number of cables required for power evacuation system from the HEP. This is due to the fact that in EHV cables of small current carrying ratings cost is largely governed by insulation requirements. Hence economy has to be studied on case to case basis considering a wide spectrum of parameters.

2.2 Mountainous Zones For AIS the site preferably should be fairly flat land or maximum it could be in steps. The requirement increases as the altitude increases as compared to plains due to air density correction factor. In mountainous zones, it is difficult to get piece of flat land & it has to be obtained in different terraces. This entails high preparation cost whereas for GIS the space requirement is low and can be housed in a building (leveling of site can be minimized).Precaution is however required to be taken to ensure that natural calamity or collapse of rock etc should not damage the GIS and it continues to perform as envisaged. The stability of land & adjoining rock is required to be kept in view before finalizing switchyard locations. 2.3 Environmental Conditions Severe environmental considerations, listed below, lead to very high maintenance costs for the AIS. Coastal site where saline pollution deposits may be heavy, Substantial industrial pollution may require frequent/regular (possibly monthly or annual) insulator cleaning/ hot line washing. Industrial pollution may cause severe corrosion of metallic components, flanges, electrical joints etc. Where substations are installed at very high altitudes the effects of low air density, snow loading & sub zero temperature need be considered for all equipments of AIS (could require the provision of additional costly insulation) where as for GIS installation only SF6/air bushing need only be considered Extreme climatic conditions would also require additional measures to be considered for bushings and GIS parts outside the building, Seismic considerations may dictate extensive mechanical support and bracing of AIS in order to meet specified requirements. The physical design of GIS allows seismic criteria to be more readily achieved at a lower overall cost. GIS enclosed within the building would be immune from these effects except for the SF6/air bushings. GIS therefore becomes an environmental friendly option for both rural and urban areas. 2.4 Replacement/ Up gradation of AIS due to growing Power Demand Most of the power systems operate with equipment of different ages. Optimal moment for replacement of the equipment is determined by technical, economical, environmental and/or strategic reasons. GIS with higher rating can be used for replacement of AIS in the event of growing power demand/ replacement at the end of the existing equipments lifetime or of the need for a higher transmission voltage without the additional land requirements. For example an 8 to 10 bay 132kV AIS could easily be replaced by a 14 to 18 bay 220kV GIS. GIS should be extendable to meet the requirement of additional bays in future for which the required space should be made available in the GIs room. 2.5 Availability and Reliability Each bay is factory assembled and includes the full completeness of circuit breaker, isolator switches, grounding switches, instrument transformers, control and protection equipment, interlocking and monitoring facilities. A high quality standard is ensured by the fact that all the equipment is produced, routine and site tested by one manufacturer. Further in the case of integrated or prefabricated control cubicles and cabling, all circuits and functions may be factory-tested, which reduces the failure probability on site and the time for erection and commissioning significantly. Modern GIS are expected to perform satisfactorily in service for many years with minimal or even no maintenance due to the fact that deterioration due to weathering is eliminated totally. Unless GIS is subjected to regular and onerous switching duties, maintenance of CBs, disconnectors and earth switches may not be required for many years. Therefore it is expected that the failure rates of the equipments/ components would be very low, GIS would have very high

reliability and availability (low failure frequency rate and long maintenance intervals). Due to enclosed bus bar, bus faults are minimal.

Fig 2 : Reliability of GIS in a country 2.6 Safety The encapsulation of GIS provides a high degree of safety for operators and other personnel due to the fact that it is impossible to touch any live part of the switchgear inadvertently. Protection against vermin or vandalism is also provided. The consequences of an internal arc are normally contained within the enclosure through rapid operation of the protection. Even under worst-case conditions it is limited to the operation of a pressure relief device or burn through after some time. No GIS part will explode, and the damaged region is limited due to the gas compartmentalization. 2.7 Strategic Locations The use of indoor GIS is absolutely necessary for strategic locations as it can be installed in a new building, existing building, or even in an underground cavern, or a simple hall. All components such as Busbars.Disconnectors, Circuit Breakers, Instrument Transformers, cable terminations, and joints are contained in earthed enclosures filled with SF6 gas. The enclosures are non -magnetic metallic containers. In indoor stations there is no other requirement whereas in outdoor installations effect of emissivity is required to be considered due to solar radiations. GIS installations are assembled from variety of standard modules which are assembled together with flanged connections and plug in type of contacts on the conductors, so as to easily disassemble individual components subsequently. The gas tight enclosure surrounds all the live parts, which are supported on spacer (insulators) and insulated from the enclosure by SF6 gas. The barrier insulators divide the bay into separate gas compartments sealed off from each other. This minimizes effect on other components during plant extensions and also enables inspection and maintenance. The flanged joints contain non-ageing gaskets. Leakage of gas can pass only to atmosphere, but not between the compartments. 2.8 Cost (Life Cycle Cost) Factors affecting life cycle cost :

GIS/AIS Primary hardware The capital cost of GIS/AIS including installation cost, land, building, main equipment cost etc and the cost of all secondary control, protection and monitoring equipment. Maintenance The cost of all preventive maintenance, predictive maintenance and repair maintenance. Operation Costs -It includes operation & maintenance cost and cost of inventory & spares. Outage Costs This includes outage costs like power/energy interruption and repairing & capital maintenance. Overall LCC would be the sum of above. If cost analysis is done considering LCC, GIS will become obvious cost effective solution.

2.9

Other Considerations GIS finds favorable acceptance in case of urban areas , underground power stations, places where environmental considerations are given added favour . It may be important even in case of rural and other areas where space is not a constraint but due to the following factors: (a) Price of land (b) Cost of acquisition of land (c) Cost of environmental disturbance including crop/tree considerations (d) Developmental cost of land (e) Associated deployment / R&R of affected people (f) Cost of civil works for AIS versus GIS this could include but not limited to indoor/outdoor GIS and building requirements for associated auxiliaries (g) Cable - supply ,erection and maintenance including selection of higher sizes on account of voltage drop considerations, long lengths in AIS as compared to GIS (h) Operating cost -this could include but not limited to maintaining crushed rock, illumination and lightening protection requirements (i) Maintenance requirements of CBs , CTs , dis-connectors, and bus PTs (j) Probability of bus faults /conductor faults due to over heating. 2.10 Cost Comparison between AIS and GIS

In order to compare the economic viability of a Gas Insulating Sub station Vs the Air Insulated Sub Stations the following three cases have been studied which clearly establish the superiority of GIS over AIS: 420 kV, 6 bay Grid Sub Station i) Double Bus Bar Vs I type scheme having 4 diameters ii) Double Bus Bar Vs D type scheme having 4 diameters 220 kV, 6 bay Grid Sub Station

The working details of the above three cases are given in Table 1, 2 and 3 respectively TABLE 1 220 KV Cost Comparison Between GIS and AIS Assumptions:1. No. of Bays 6 , 220 KV Grid Substation 2. Double Bus Bar Scheme 3. Expected normal Life 25 years (taken same as for AIS for the purpose of comparison) 4. Area Required FOR GIS 250 Sqm For AIS 8260 sqm. Initial Cost Rupees GIS AIS 1. Land Cost @ Rs. 250/sqm 62500 2065000 2. Development Cost @5% 3125 103250 3. Environmental Considerations @ 3% 1875 61950 4. Land Acquisition Cost @ 2% 1250 41300 5. Oustees Compensation @ 8% 5000 165200 Total Land Cost in Rural Area 6. Main Equipment Cost 7. Erection Cost 8. Cabling Cost 9. Earthling Cost 10. Illumination Requirements Fixed Cost (Total 1-10) 11. Operation & Maintenance expenses in 25 yrs. of life Span 12. Inventory & Spares Variable Cost (11+12) 13. Repairing & Capital Maintenance 73750 180000000 0 360000 360000 360000 181153750 22500000 2700000 25200000 5400000 2436700 108000000 8640000 1080000 1080000 1080000 122316700 81000000 3240000 84240000 10800000

14. Total Cost 189253750 217356700 While Comparing the costs, control Room and pothead yard requirements have been excluded as they are same in both the cases. Values shown in the tables are indicative only to bring out the comparison. Capitalization will depend more on application and scheme selection. In case of generating stations the variants could be hydro, thermal etc. However, in grid where system redundancy can take care of outages to avoid interruption the capital loss on this account shall not be significant. Similarly radial feed environmental consideration also can contribute towards power/energy. 1. Since various modules of circuit breakers instrument transformers, bus bar etc. come under factory assembled module so GIS takes very little time in erection, testing & commissioning. 2. GIS is either practically of low maintenance or very little maintenance so once erected does not pose any maintenance problem for years together except some filling of gas in the modules to maintain the required pressure inside the module.

TABLE 2 420 KV Cost Comparison Between GIS and AIS Assumptions:1. No. of Bays 6 , 420 KV Grid Substation 2. Double Bus Bar Scheme Vs I Type layout having 4 diameter for 6 bays 3. Expected normal Life 25 years (taken same as for AIS the purpose of comparison) 4. Area Required For GIS 640 Sqm Initial Cost GIS 1. Land Cost @ Rs. 250/sqm 2. Development Cost @5% 3. Environmental Considerations @ 3% 4. Land Acquisition Cost @ 2% 5. Oustees Compensation @ 8% Total Land Cost in Rural Area 6. Main Equipment Cost 7. Erection Cost 8. Cabling Cost 9. Earthing Cost 10. Illumination Requirements Fixed Cost (Total 1-10) 11. Operation & Maintenance expenses in 25 yrs. of life Span 12. Inventory & Spares Variable Cost (11+12) 13. Repairing & Capital Maintenance 10800000 18000000 45000000 5400000 135000000 5400000

For AIS 26400 sqm. Rupees AIS 160000 6600000 8000 330000 4800 198000 3200 132000 12800 528000 188800 7788000 180000000 14400000 1800000 1800000 1800000

360000000 0 720000 720000 720000

14. Total Cost 378548800 365988000 While Comparing the costs, control Room and pothead yard requirements have been excluded as they are same in both the cases. Values shown in the tables are indicative only to bring out the comparison. Capitalization will depend more on application and scheme selection. In case of generating stations the variants could be hydro, thermal etc. However, in grid where system redundancy can take care of outages to avoid interruption the capital loss on this account shall not be significant. Similarly radial feed environmental consideration also can contribute towards power/energy. 1.Since various modules of circuit breakers instrument transformers, bus bar etc. come under factory assembled module so GIS takes very little time in erection, testing & commissioning. 2. GIS is either practically of low maintenance or very little maintenance, so once erected does not pose any maintenance problem for years together except some filling of gas in the modules to maintain the required pressure inside the module.

TABLE 3 420 KV Cost Comparison Between GIS and AIS Assumptions:1. No. of Bays 6, 420 KV Grid Substation 2. Double Bus Bar Scheme Vs D Type scheme having 4 Diameter 3. Expected normal Life 25 years (taken same as for AIS for the purpose of comparison) 4. Area Required For AIS For GIS 640 Sqm 48720 sqm. Initial Cost Rupees GIS AIS 1. Land Cost @ Rs. 250/sqm 160000 12180000 2. Development Cost @5% 8000 609000 3. Environmental Considerations @ 3% 4800 365400 4. Land Acquisition Cost @ 2% 3200 243600 5. Oustees Compensation @ 8% 12800 974400 Total Land Cost in Rural Area 6. Main Equipment Cost 7. Erection Cost 8. Cabling Cost 9. Earthling Cost 10. Illumination Requirements Fixed Cost (Total 1-10) 11. Operation & Maintenance expenses in 25 yrs. of life Span 12. Inventory & Spares Variable Cost (11+12) 13. Repairing & Capital Maintenance 10800000 18000000 45000000 5400000 135000000 5400000 188800 360000000 0 720000 720000 720000 14372400 180000000 14400000 1800000 1800000 1800000

14. Total Cost 378548800 372572400 While Comparing the costs, control Room and pothead yard requirements have been excluded as they are same in both the cases. Values shown in the tables are indicative only to bring out the comparison. Capitalization will depend more on application and scheme selection. In case of generating stations the variants could be hydro, thermal etc. However, in grid where system redundancy can take care of outages to avoid interruption the capital loss on this account shall not be significant. Similarly radial feed environmental consideration also can contribute towards power/energy. 1.Since various modules of circuit breakers instrument transformers, bus bar etc. come under factory assembled module so GIS takes very little time in erection, testing & commissioning. 2. GIS is either practically of low maintenance or very little maintenance, so once erected does not pose any maintenance problem for years together except some filling of gas in the modules to maintain the required pressure inside the module.

2.11

Hybrid Options Comparison of AIS, GIS and comparison of AIS, GIS and Hybrid Options is broadly given below along with single phase enclosure of GIS: Comparison table Criteria Space requirement Equipment cost Environmental Influence Maintenance requirements AIS High Low High High GIS Low High Low Low Hybrid Middle Middle Middle Low

The modular construction of SF6 switchgear has an added advantage to build GIS for all type of layout schemes of EHV substation.

Busbar

Disconnector Current Transformer Circuit Breaker

Cable Connection

Disconnector Current Transformer Circuit Breaker

Fig 3 : Single Phase Enclosure of GIS SF6 gas is a superior insulation medium, having dielectric strength 3 times that of air at atmospheric pressure. Up to 72.5 kV, SF6 pressure is about 3.5 to 4.5 bar and may go as much as 6-7 bars for high voltage & EHV CBs.

CHAPTER- 3 SWITCHING SCHEME OPTIONS AND THEIR SINGLE LINE DIAGRAMS Switching scheme for high and medium voltage GIS are governed by consideration of its operation. Selection of Bus bar numbers, i.e , single or multiple depends upon how the system is to be operated and also keeping in view the need for sectionalisation. Other factors like the need to isolate parts of installation for maintenance purpose, future extensions etc are also taken into account. Following are the major aspects considered while selecting switching schemes: Number of incoming and outgoing circuits and their effects on outage in terms of consequential loss. Quantum of revenue loss on account of outage of number of circuits Amount of power to be handled and level of security for each circuit Number of duplicate circuits and alternate supplies towards next Substations Higher degree of flexibility Scope of future expansion without shut-down or with limited shut down Reliability of supply during various possible faults and maintenance. Cost: The selection of the most suitable switching scheme for particular substation is done after evaluation of technical aspects. Simplicity of operation may not give adequate reliability, whereas very highly reliable scheme will require large number of CBs and likely to cost more. Maintenance of any desired part of substation without major shut down Selection of scheme depends on the size and importance of the substation.

3.1 Categorization of GIS Schemes Keeping above aspects into consideration, schemes are normally selected in the following categories: 3.1.1 Distribution GIS - Up to 33 KV

Single Bus bar System : Where outages can be allowed for all the circuits and can be planned where there is a space constraint. Fig-1a

Single Sectionalized Bus bar : Where outages in the important feeders are permitted as per planned shutdown for maintenance. Fig. 1 b

Double Bus: Where outages in the important feeders, are permitted as per planned shutdown for maintenance as well as built in reliability OF CB/ Bus bar failure. Fig. 2

3.1.2

Medium size GIS- 66KV to 145KV Single Bus bar System: Importance of power supply to be ascertained by utility where security & flexibility are of a little concern. Double bus: Where maximum security and flexibility is desired to connect feeders in any bus. Maintenance of any bus bar is possible. Main and Transfer bus: Where outages in the important feeders are permitted as per planned shutdown for maintenance. Here bus bar maintenance is possible by transferring the important load on transfer bus. Fig-4

Fig 4: Typical Ring for 4 bay Arrangements 3.1.3. Large EHV GIS, both for Generating Stations and Transmission Network- 245 kV and above Double bus bars with bus coupler: Suitable for Substation in a highly inter connected network, in which there are many incoming and outgoing feeders with a possibility of expansion . Bus bar maintenance is possible. Fig-3

Double main & transfer bus with transfer breaker scheme: This is adopted in those Substations where quantum of power transfer per feeder is high. This scheme provides switching flexibility, high security against bus bar faults and minimum outage during maintenance. Bus bar maintenance and outage of each CB is possible without interruption of flow of power. This scheme is more proffered scheme for AIS, but the scheme is considerably costly .Fig 5

Breaker and a half scheme: This is adopted in those Substations where quantum of power transfer per feeder is high. This scheme provides enough switching flexibility, high security against bus bar faults, breaker fault. The scheme offers minimum outage during maintenance. Cost is moderately higher. Fig 6

Ring bus/ Inter connected mesh: This is adopted for those substations, where there are up to 5 number of circuits carrying high power. It provides maximum security against bus bar faults and and power flow continuity is maintained. This scheme requires less number of CBs than in breaker and a half scheme. However, it has lesser flexibility in Switching operation.fig-6

Most acceptable switching scheme: Normally double busbar with bus coupler configuration would be most acceptable proposition and is widely used in the substation. However user may opt different schemes depending upon their technical requirement and also on cost consideration. 3.2 Single Line Diagrams ( SLDs) General details of the single Line Diagrams of GIS are given above. Typical single Line Diagram of the GIS stations of 220 kV Chamera -II H.E. Project and 400 kV Chamera-I level installed on some of the underground Hydro Power Stations in operation are enclosed Chamera-I and typical single line diagram and section diagram of 220 KV power station.

KV

CHAPTER- 4 STUDIES REQUIRED BEFORE APPLICATIONOF GIS (245 KV & ABOVE) Once techno-economic viability is established and it is decided to use GIS tentative layout is required to be developed. The layout should necessarily consider connection of incoming and outgoing bays for the GIS. The method of connection i.e. cable or overhead connection or a bus duct also has an important bearing on the cost and reliability of the GIS installation. Capacitance of these components varies and adequate care is required to be taken with the help of studies to take precautions for possible resonance or over voltage. It is recommended to get the following studies done as a part of planning first and then based on specific equipment information, adequate protective measures are to be adopted in time to avoid failures. Normally following studies are recommended 4.1 Insulation Co-ordination Studies These studies would reflect requirements in terms of electrical parameters for the planned system and it will be exposed to power frequency, impulse and switching over voltages. Each study will need system data in terms of immediate interconnecting transmission lines. The outcome would determine the location and parameters of lightening arresters required to be installed on the incoming bays /outgoing bays and the bus bars. In the event of systems having reactors at the ends of the line, the study would also determine neutral reactor arrestor ratings. 4.2 Transient Recovery Voltage Studies Switching in GIS also generates high frequency transients which get enlarged and are sometimes detrimental to adjoining equipments. Ferro-resonance phenomenon and effects of higher frequency transients & Very Fast Transient Over Voltages (VFTO ) are also required to be studied before system engineering is completed to ensure that reflected transients do not become a cause of Ferro resonance. Users have normally found it difficult to carry out these studies on account of availability of tools and data required for the studies. Normally EMTP group users should be able to manage the studies and data for the studies have to be furnished by the user in respect of system and prospective suppliers of GIS. 4.3 Ferro-Resonance Phenomenon: Ferro-resonance is basically a low frequency phenomenon which is non-linear in nature and may cause sudden change of the state of the system resulting in excessive switching over voltages and/or currents. Abnormal levels of harmonics, over voltages/ currents, either as stable oscillation or as transients caused by Ferro-resonance often represent risk for electrical equipment. Ferro-resonance is a complex phenomenon that can occur in electric circuits having a nonlinear inductance, capacitance and a voltage source. Power system networks are made up of large number of inductances (power transformers, voltage transformers, shunt reactors) as well as capacitors (cables, long lines, CVTs, capacitor banks) etc. thus presenting situations under which Ferro-resonance can occur. Switching transients, energizing or de-energizing transformers or loads, occurrence or removal of faults may initiate Ferro-resonance.

4.3.1

Studies for predicting Ferro-resonance

Ferro resonance problems can be studied by simulation techniques involving the modeling of power systems to ensure that the over voltages due to Ferro-resonance are lower than the rated power frequency withstand voltage levels . Time Domain digital simulation in transient state : based on a reduced three-phase model representation of the power system with correct parameter values and initial condition a long time simulation study is to be carried out with the help of transient simulation packages. 4.4 Seismic Studies: Probable failures that could happen during an earthquake are: GIS modules can fall from their elevated platforms and can get severely damaged due to inadequately anchored rail-supports. Damage/Leakage in GIS bushings. Rigid connectors can transfer force to damage studs/connectors of the bushing Current transformers, capacitive coupled voltage transformers, surge arrestors and line traps can get damaged;

4.4.1

One of the main difficulties when substation equipment is damaged is that there is limited number of spare parts or spare replacement equipment available. Repair and replacement of damaged equipment is a time-consuming and labour-intensive task hence seismic considerations are to be kept in mind while engineering GIS application. IEEE Standard 693, recommended Practice for Seismic Design of Substations, clearly defines qualification seismic levels, qualification procedures, and acceptance criteria. This standard recommends that sites with projected ground motions above 0.1g should have their equipment seismically qualified. Thus power utilities with service areas in seismic Zone III,IV and V (as per BIS 1893:2000) should have their substation equipment seismically qualified. Following are some of the considerations utilities must evaluate when deciding seismic level: Expected magnitude of an earthquake at the substation; The criticality of the substation with respect to the utility's total system; The speed at which equipment can be replaced; Safety considerations; The possibility and acceptability of bypassing the equipment should the equipment fail; The overall reliability of the system.

4.4.2

For these reasons, zone maps are provided in BIS 1893 as an aid to guide the utility in selecting the appropriate level, not as a requirement. The utility must evaluate the site and all the other considerations to determine which level is appropriate.

Fig 5 : Seismic Test set up 4.4.3 Seismic Analysis

Before starting seismic calculations, static calculations can be carried out taking into account the static effects on the structure and the foundations. The analysis can be carried out with two different load cases: 1. Static Load Case: Weight Thermal Expansion Wind Load--is considered only in case the GIS is planed to be located in outdoor.

2. Dynamic or seismic load case This analysis is to be performed in two orthogonal directions : Seismic specification in the X direction Seismic specification in the Y direction

Equipment and supporting structures of substations located in seismically sensitive regions / zones have to be designed to withstand possible earthquakes. Procedure used to verify the seismic design of equipment includes simulations based on the finite element method combined with either response spectrum or time history analysis and shake table testing. One of the most effective ways of reducing earthquake damage for new installations is to use equipment that has been seismically qualified. Central Power Research Institute (CPRI), Bangalore can help both develop the system performance criteria and evaluate equipment seismic withstand capability. CPRI can carry out structural analysis on civil structures and electrical equipment to determine seismic withstand capability. Time history response and spectrum analyses of equipment are carried out using the Finite Element Method. CPRI is equipped with the state-of-the-art facilities for model or real size testing of structures, components and electrical equipment using as seismic shake-table.

CHAPTER - 5 STANDARD LAYOUT EQUIPMENT ARRANGEMENT & SIZE OF BUILDING 5.0 Strategic Locations The use of indoor GIS is absolutely necessary for strategic locations as it can be installed in a new building, existing building, an underground cavern, or a simple hall. All components such as Busbar, Disconnector, Circuit Breaker, Instrument Transformers, Cable terminations and joints are contained in earth enclosure filled with SF6 gas at a pressure above 3kg/sq.cm. The enclosure are of non-magnetic and corrosion resistant cast aluminium or weld aluminium sheet or stainless steel sheet. The GIS installations are assembled from variety of standard modules which are Joined together by flange connections and plug contacts on the conductors, so as to easily dissemble individual component subsequently. The gas tight enclosure of high grade aluminum and surrounds all the live parts, which are supported on spacer(insulator) and insulated from the enclosure by SF6 gas. The barrier insulators divide the bay in to separate gas compartments sealed off from each other. This minimizes effect of other components during plant extensions and also enables inspection and maintenance. The flanged joints contain non-ageing gaskets. Leakage of gas can pass only to the outside, but not between the compartments. 5.1 Layout Of GIS Modular SF6 GIS can be tailor made to suit the particular site requirement. GIS could be suitably erected on any floor or basement and ducts could be taken through walls via SF6 gas insulated busbar /cable. GIS is suitably located and accommodated in various types of applications including following: In switchgear room of Hydro Electric power station near turbine-generator hall or at a distance where pothead yard could be made . Basement of multistoried building. terrace/top floor of multi-storied building under ground substation In a building to receive outdoor transmission line termination. Irrespective of space consideration,the location of GIS and associated components should be so placed that any change in adjoining hills etc. should not have impact on the GIS. This will avoid prolonged outages due to limited availability of spares in the country and equipment generally is tailor made. In GIS installation, all basic substation Busbar schemes as explained in earlier chapter of Switching scheme options can be realized with the help of standard modules. The modular construction offers the following advantages: i. ii. iii. iv. v. vi. vii. Quick Installation Simple stocking of spares Easy transportation No risk of pollution and corrosion Occupies less space No threat of security Easy extension Standardization of components

viii.

The bus bars are laid longitudinally in indoor hall to which incoming and outgoing bays are connected. The bays are connected to bus bars cross-wise. Bus bars are partitioned at each bay with an objective to isolate Busbar compartments for the purpose of extension and at the same time avoid damage to adjacent bays in the event of fault.

i. ii.iii.

The enclosure of GIS may be of aluminum alloy or stainless steel. The selection of material largely depends on temperature rise consideration and permissible limit depending on emissivity (solar radiation) and / or temperature rise of conductor. It is found economical to adopt 3 phase enclosures up to 145KV system voltage. For system voltage above 145KV single - phase enclosure design are preferred The GIS component like circuit breaker, load break switches, earthing switches, isolators, voltage transformers, current transformers, surge arrestors and connectors are functionally separate modules of a standardized modular system

GIS could be in common enclosure or phase segregated (single phase) depending upon customer requirement and cost consideration. For higher voltages i.e. 220 KV and above, phase segregated construction is adopted. 5.1.1 Building Space Requirement Layout of GIS and consequently requirement of building space depends upon following factors: 5.1.1.1 External termination with overhead lines: Overhead line exit require spreading of GIS bus arrangement to meet phase to phase clearances in air. In case external termination of lines are only at one side of building, these are usually at intervals of 3 to 4 bays to maintain required clearances between bushings. In case external termination of lines are on both sides of building or are taken some distance by means of SF6 bus connections, the respective bays can be next to each other. Connection of overhead line with GIS rigid busbar is made through outdoor condenser bushings 5.1.1.2 Location and type of connection with Transformer: Unlike overhead line connection, wherein conductor is taken through bushing, the GIS conductor directly terminates at transformer bushing terminal with the help of lateral mounting unit and barrier insulator. A flexible bellow takes care of thermal expansion & erection tolerances and prevents transfer of tank vibration to switchgear enclosure. In case, there is difficulty in connecting GIS directly on to transformer bushing terminal, we can adopt external termination method by connecting transformer bushing terminal to outdoor condenser bushing. In such cases, Surge Arrester is accommodated below outdoor condenser bushing.

5.1.1.3 Termination of feeders through high voltage Cable: The connection between GIS and high voltage cable is done through cable termination / cable sealing end. Plug in cable sealing ends for XLPE/Oil Filled cables could be through SF6/XLPE, SF6/Oil cable termination also be a good option. They consist of gas tight plug in sockets, which are installed in the switchgear, and prefabricated plugs with grading elements of silicone rubber. A separate cable basement is provided for cable entry, its distribution and installation.

5.1.1.4 Location of Surge Arrestor to suit connection with Line and Transformer: The arrangement as discussed above for termination of line and transformer decide the location of Surge Arrester inside the GIS building or outside. Location of Surge arrester depends upon distance between GIS and incoming termination, particularly as it depends upon the calculated distance as per system studies.

5.1.1.5

Future expansion with a view to have minimum downtime, it should be necessary on the part of user to specifically mention the possibility and requirement of expansion clearly. Erection and Maintenance aspects- Gangways of sufficient dimension must allow unhindered access to the components for erection and maintenance. To achieve maximum flexibility for break down and routine maintenance, additional area at the end of installation is preferred for storing SF6 gas handling plant and vital T&P and spare numbers of SF6 cylinders to be provided. Height of building is governed by the maintenance clearances required for the assembly/module of GIS to be lifted by crane . However the opinion of manufacturer may be sought. The clearance between breaker poles Mounting of Circuit Breaker- Vertical or horizontal

5.1.1.6

5.1.1.7

5.1.1.8. 5.1.1.9

Considering high reliability of GIS components, simplified arrangement with less redundancy could be given preference to reduce cost. Various Equipment Modules of GIS are described as under. 5.2 Busbar Modules Main conductors are Aluminium or Copper tubes and its dimension depends upon mechanical strength corresponding to short circuit forces. The conductors are provided with silver plated finger contact assembly mounted on support Insulator. These sliding contacts allow tubular conductors to expand axially with temperature rise without additional stress on support insulators. The shape of support insulator for tubular conductor normally ensures that field distribution is uniform For easy assembly and any type of design configuration, busbar modules are standardized in various forms, like L-unit(90 degree junction), four way junction, angle unit(120-180 degree junction), T-unit with flange for earthing switch. 5.3 Circuit Breaker Module The circuit breakers shall be phase segregated & have duplicate trip coils for 220 KV and above. In 132 KV and below three phase configuration of CB can be with one trip coil These shall be trip free, and anti-pumping with either or both of the duplicate trip circuits connected. A manual emergency trip facility could add to flexibility to meet contingency needs to trip the CB. Pole discrepancy tripping shall be provided. Each 220 KV level or above circuit breaker shall be equipped with electrically separate two shunt trip systems per mechanism. Two trip coils are arranged to share a common magnetic circuit, the operation of either coil shall be independent of other, i.e., if one of the trip coils has been damaged or destroyed, it shall not affect the operation of the other. Facilities shall be ensured so as to enable timing tests of the circuit breaker to be carried out after switchgear has been energized with SF6 gas. It should not be necessary to open up any gas section to make test connections to the circuit breaker primary terminals for this test. 5.3.1 Operating Mechanism Circuit breakers shall be power operated either by a motor charged spring operated mechanism or by hydraulic mechanism. Main poles of the breaker shall be such that the design shall ensure a close pole spread with a maximum of 5 ms closing.

Optimally user can opt for two type of mechanisms one suitable for A/R and one for generator/transformer switching. Operating mechanism of first type shall be suitable for high speed enclosing. It shall be anti-pumping electrically and mechanically under every method of closing (except during manual closing of a breaker for maintenance). A latch checking switch shall be provided on mechanically trip free mechanisms to prevent reclosure before the breaker latches have reset. Main poles shall operate simultaneously. There shall be no objectionable rebound and the mechanism shall not require any critical adjustment. It shall be strong, rigid, positive and fast in operation. A closing release shall operate correctly at all values of control voltage between 80% and 110% of the rated voltage. A shunt trip shall operate correctly under all operating conditions of the circuit breaker up to the rated breaking capacity of the circuit breaker and at all values of control supply voltage between 70% and 110% of rated voltage. Working parts of the mechanism shall be of corrosion resisting material. Bearings which require grease shall be equipped with pressure type grease fittings. Bearing pin, bolts, nuts and other parts shall be adequately pinned or locked to prevent loosening or changing of adjustment with repeated operation of the breaker. Self lubricating dry type bearing should be accepted Provision shall be ensured for attaching an operation analyser to perform speed tests after installation of the CB at site. 5..3.2 Spring Operated Mechanism Spring operated mechanism shall be complete with motor, opening spring, closing spring with limit switch for automatic charging and all necessary accessories to make the mechanism a complete operating unit. As long as power is available to the motor, a continuous sequence of closing and opening operations shall be possible. After failure of power supply to the motor, at least two close-open (C-O) operations of the circuit breaker shall be possible. Breaker operation shall be independent of the motor which shall be used solely for compressing the closing spring. Motor rating shall be such that it requires only about 30 seconds for fully charging the closing spring. Closing action of the circuit breaker shall compress the opening spring ready for tripping. When closing springs are discharged after closing a breaker, closing springs shall automatically be charged for the next operation.

Spring operated mechanisms have very low operating energy, simplified drives, enhanced operation reliability ,easy erection at site ,simple in principle construction, ease of handling with minimal maintenance. It gives a high accuracy along with very high mechanical endurance. 5.3.2.1 Hydraulic Operated Mechanism Hydraulic operated mechanism shall comprise self contained operating unit with power cylinder, control valves, high and low pressure reservoir, motor, etc. A portable pump set shall also be provided for emergency operation. The oil pressure controlling the oil pump and pressure in the high pressure reservoir shall be continuously monitored. Necessary hardware to achieve this, including the loose pressure gauge, instruments and interconnecting piping etc shall form integral part of this mechanism The mechanism shall be suitable for at least two close-open operations after failure of AC supply to the motor.

It is up to the user to specify both above mechanisms or select any one depending upon merits and requirements of operation of GIS. 5.3.2.2 Operating Mechanism and accessories Housing

Common marshalling box with necessary tubing and interconnecting cables are required for trouble free operation. A light point with door switch and one 3 pin 240V AC 15A socket outlet can be provided in the housing. 5.3.3 Duty Cycle of Operating Mechanism Normally o-c-o cycle would be an acceptable proposition. irrespective of the type of operating mechanism the breaker has even after the failure of auxiliary supply. 5.3.4 Circuit Breaker Performances The maximum interrupting time at the minimum operating pressure of the mechanism shall be specified by the user utility. This is normally governed by stability requirements of fault clearance total time. 5.3.5 Auto Reclosing Equipment This will govern number of operating mechanisms per CB. High speed, single shot auto reclosing could be single or three phase. All relays, timers, controls and interlocks required for auto reclosing along with a selector switch for single pole and three pole auto reclosure and all the associated hardware shall be required . The duty cycle of the auto reclosing breakers shall be O-t-CO-t'-CO, where the dead time interval (t) shall be adjustable. User shall clearly state the minimum dead time interval (t) & (t) that can be used and the range of adjustment for the same. IEC 62271 part-100 could be referred for selection . Auto reclosing equipment shall be suitable for operation on the DC control voltage specified. Control circuitry shall be such that the reclosing sequence shall not come into operation if the breaker is opened by hand (healthy trip) and also if the breaker is closed on to a short circuit. 5.4 Disconnector (Isolators) and Earth Switches

5.4.1 Disconnector : They are either on-load break switch or no-current break type (for isolation only). Disconnector for isolation contains following features: Mechanism which operates with or without remote control Necessary inter-locks feature with CB, dis-connectors, PTs and earth switches. Mechanically coupled position indicator Some of the users specify a viewing glass for physically inspecting the position of disconnector. It is considered not to be a technical requirement but more of a statutory consideration. In fact providing a glass window adds to possibility of gas leakage and weak points prone to damage during transportation handling. Some times due to flash overs/ arching, it has been noticed that impurity deposited on the window lead to defeating the function 5.4.2 Earthling Switch They are normally mounted or integrated in the isolator housing. They can be hand or motor operated with necessary interlocks/ capacitive tap to operate only when high voltage part is dead. Fast acting earth switches for overhead lines must be capable of interrupting capacitive and inductive currents. Normally due to standardization the values in this regard are

adopted in accordance with IEC. Transformer connected to long cable should also have Fast acting Earthling switch. 5.4.3 Construction Features The isolator and earthling switch shall be provided with high current carrying contacts on the hinge and jaw ends and all contact surfaces shall be of silver faced copper. Each isolator shall have an individual gas compartment. Associated earth switch should be in the gas compartment of the isolator except for bus bar side isolator earth switch which will have a separate and independent gas compartment to avoid shut down to both bus bars in the event of failure of junction gas barrier. Motor operated high speed earthling switch shall be designed in a manner to prevent transmitting of impact to earth switch bushing. Provision shall be made to padlock the hand operated earth switch in both the open and closed positions. Earthling switches on the line-side of incoming feeders shall have make proof contacts and stored energy high speed operating mechanisms that make them suitable to close on a fault. These motor operated earthing switches on incoming lines shall be of high speed closing (quick make action) type. All earth switches shall be insulated from the enclosure and connected to the enclosure by a removable link to facilitate current injection High speed earthing switches on the line/cable entrance side shall be capable of interrupting line capacitive currents upon opening and in worst conditions of closing. 5.4.4 Operating Mechanism and Controls The isolator shall be provided with positive continuous control throughout the entire cycle of operation. The operating pipes and rods shall be sufficiently rigid to maintain positive control under most adverse conditions and when operated in tension or compression for isolator closing. They shall also be capable of withstanding all torsional and bending stresses due to operation of the isolator. It shall not be possible, after final adjustment has been made, for any part of the mechanism to be displaced at any point in the travel sufficiently to allow improper functioning of the isolator when the isolator is opened or closed at any speed. All holes in cracks, linkages, etc., having pins shall be drilled to accurate fit so as to maintain the minimum amount of slack and lost motion in the entire mechanism. The operating mechanism design shall be such that during the operation of the isolator (especially manual operation), once the moving blades reach the sparking distance, springs shall take over to give a quick, snap action closing so that the isolator closing is independent of manual efforts. Similarly, the springs must assist during the opening operation to give quick breaking feature. Disconnector and high speed motor operated earthing switch mechanisms shall be provided with a mechanism with stored energy to always assure completed operations. 5.4.5 Interlocks Interlocking devices must provide absolute and positive protection against potentially harmful maloperations of the switchgear. The following functions interlocks are to be provided: To ensure that safe and logic sequence to actuate breakers, switches, isolators and grounding switches.

Checking the actual fully closed or fully open position of all switching elements before and after each move. Providing the logical checks and issuing the resulting PERMISSIVE or BLOCKED signals for the switchgear. Indicating positively the absolute condition/position of the supervised equipment. Local manual and remote electrical operation of all essential functions. Local emergency unlocking facilities via safety-key switches under the full responsibility of the operator.

Intrabay and interbay interlocking should normally be provided by the GIS manufacturer until unless specifically excluded from the scope. Electrical interlocking for circuit breaker and isolator shall be provided and it shall be fail safe type. Mechanical interlocks for isolator & Earthing Switch shall be fail-safe type. 5.4.6 Short Circuit Requirements

The rated peak short-circuit current or the rated short time current carried by an isolator or earthing switch for the rated maximum duration of short circuit shall not cause: 5..5 5..5.1 Mechanical damage to any part of the isolator or earthing switch. Separation of the contacts or contact welding. A temperature rise likely to damage insulation.

Current and Voltage Transformers Current Transformers module They are toroidal-core type and arranged before or after CBs depending upon measuring and protection concept of the user. The numbers of cores is not a limitation as the length can be suitably increased to adopt required no. of CTs.

5..5.2

Voltage Transformers Voltage transformers shall be of the metal enclosed, gas-insulated inductive type, mounted directly on the high voltage enclosure. Secondary terminals must be located in accessible grounded terminal boxes on the PT enclosure itself. The secondary connections must be wired to the terminal strip in the respective bay marshalling cubicle. PTs should be in segregated compartment and not forming a part of bus bar.

5..5.3

General Requirements Secondary terminals of each voltage and current transformers should be brought out in a weatherproof terminal box. Short circuiting and earthing the CT secondary at the terminal box should be ensured as a good practice. The star point whenever required shall be formed at the terminal box only. Terminal and polarity marks should be indelibly marked on each VT & CT on the associated terminals and these marks shall be in accordance with relevant standards. Each VT & CT shall be provided with a rating plate showing the particulars as required by the relevant standard. Each CT shall, when called for in specific requirements be equipped with an over voltage protective device to limit the voltage developed across the secondary terminals to a safe value not exceeding 3kV.

The secondary terminal box for the voltage transformers shall also include necessary HRC fuses for protecting the secondary circuit. Further, for the purpose of fuse supervision on remote panel both terminals of fuse shall be brought out to the terminal box. Whenever a VT secondary winding is used for both measurement and protection application, it shall have accuracy class of 0.2 /0.5/3.0 as required . All CT cores in this specification shall be of low reactance type. No turns compensation shall be used in case of 'Class-PX CTs. In case of multi ratio CTs, the minimum specified requirements in respect of VA, accuracy and knee point Voltage (KPV) and maximum secondary resistance specified shall be met at all taps. Voltage transformers shall be of electromagnetic type. Voltage and current transformers shall be given tropicalised treatment for satisfactory operation in hot and humid climate. 5.6 Surge arrester Gap-less ZnO arresters are provided either outdoor or indoor depending upon distance between GIS and incoming termination from overhead line/transformer. Normally isolable outdoor surge arrestors on the bus bar should be used. This will facilitate quick isolation and coupling whenever bus bars are required to be exposed to high voltage test. Surge arresters shall be of the hermetically sealed, Gapless (Metal Oxide), suitable for use with gas insulated switchgear. They shall have adequate thermal discharge capacity for severe switching surges, long duration surges and multiple strokes. The surge arresters when provided with pressure relief devices shall be capable of withstanding the internal pressures developed during the above discharges without operation of the pressure relief devices. 5.6.1 Discharge Counter Self contained discharge counter shall be provided for each single pole unit. A leakage current detector as an integral part of the discharge counter shall be supplied. The counter along with the detector shall be so arranged that it will be possible to read the leakage current values from outside the cubicle. The value of leakage current beyond which the operation is abnormal shall be clearly marked in red colour on the detector. 5.6.2 Control Cabinets

5.6.2.1 The elements for control, indication and alarms are contained in local control cabinets mounted close to bays. The elements normally mounted in the control cabinets consist of the following: (i) Mimic diagram with control switches for electrically operated breakers, load break switches, disconnects and earthing switches and indicators for all components provided with auxiliary switches. (ii) Local/Remote Selector Switches. (iii) Alarm facia with indicating lamps for monitoring operating system, gas density and auxiliary supplies. (iv) Contactors, timing relays etc. (v) Terminal blocks. (vi) Interior lighting, heater, cable glands. (vii) Lockable bypass switches for defeating the interlocks to facilitate maintenance work.

5.7 5.7.1

SAFETY LOCKS Safety locks for locking the disconnects and earthing switches in the positions Operation or Maintenance are also provided. In the Maintenance position these locks interrupt the control circuits of motor drives for disconnects and earhing switches. In the manually operated earthing switches, these locks in the Operation position do not permit engagement of manual operating handle with the earthing switches operating shaft. INTERLOCKS GIS control cabinet includes electrical interlocks to prevent incorrect switching sequence and ensure correct operation of isolators, circuit breakers and earthing switches from local control cabinet or from the control room. SF6 GAS DENSITY Density of SF6 affects the dielectric strength of GIS and breaking strength of SF6 circuit breaker. Since the gas pressure is influenced by temperature, it is the density of gas which is monitored. For this purpose temperature compensated gas density monitors are provided in the bus bar, circuit breaker and isolator compartments formed by the provision of barrier insulators. The compartments are fitted with non-return valve connections for installing density monitors, evacuation and for filling gas. SF6 is five times as dense as air. It is used in GIS on pressure from 3.5-7 bars absolute. The pressure is so selected such that gas will not condense into liquid at the lowest temperature, the equipment could experience. The is about 100 times superior than air in terms of interrupting arc. SUPPORTING STRUCTURES

5.8 5.8.1

5.9 5.9.1

5.9.2

5.10

5.10.1 Depending on the design of installation, the GIS can be self supporting or erected on steel supporting structures of simple design anchored to the substation floor. 5.11 GIS Terminations GIS terminations could be any of the following: SF6 to air bushings SF6 to cable termination SF6 to oil bushings for direct connection to transformer SF6 bus duct

All termination modules are commonly used to connect the GIS with transformer. Overhead lines could be connected to GIS either though cables or through SF6 to air bushings. Type of terminations has also bearing on the size of substations. If cable or SF6 bus ducts are used, substation can be kept quite compact. SF6 to air bushings, on the other hand, requires minimum clearance in air and thus requires more space and in addition, they are subject to environmental conditions. Especially in cities/ industrial areas where space is both restricted & expensive and the surrounding environment has impact on type of termination, preference should be for cable termination or S6 bus duct. Selection of cable termination will have to be judiciously done keeping in view the specific requirement.

CHAPTER-6 EARTHING OF GIS All enclosures of all the GIS should be grounded at several points so that there should be grounded gauge all live parts. All conduits and cables sheaths should be connected to the ground bus, to be provided, in the control cubicles and the marshalling boxes. The three enclosures of single phase GIS are required to be bonded to each other at the ends of GIS to ensure flow of circulating currents. These circulating currents cancel the magnetic field that would otherwise exist outside the enclosure. GIS does not have circulating currents but does have eddy currents in the enclosure and should also be multi point grounded. Although multi point grounding leads to some losses in the enclosure due to circulating current multi point grounding results in many parallel paths for the current from an internal path to flow to the switchyard ground grid. All steel structures should be grounded. 1 All wirings to GIS should be shielded and grounded. 2 Subassembly to subassembly ground conductors should be provided to assure safe voltage gradients. The recommendations of manufacturers and multi point grounding concept normally ensures touch and step potentials within safe levels prescribed by IEEE 80-2000. The GIS should be extendable to meet the requirement of addition of bays in future. The side on which the extension should be provided with suitable extension bellows /flanges with blanking plates. The building that is to house the GIS should have space provision for future extension. 6.1 Definitions: Most important terms relating to earthing systems are summarized below: Earth refers both to earth as a place and to earth as a substance e.g. humus, clay, sand, gravel, rock Reference earth is that part of earth, particularly the ground surface outside the sphere of influence of an earth electrode or earthing system, between two random points in which there occur no perceptible voltages resulting from the earthing current. Earth electrode is a conductor which is embedded in the ground and electrically connected to it, or a conductor embedded in concrete which is in contact with the earth over a large area (e.g. Foundation earth) Earthing conductor is a conductor connecting a piece of equipment to an earthing electrode provided it is not in the ground, or in the ground but insulated. Main earthing conductor is an earthing conductor to which a number of earthing conductors are connected. Earthing system is the located delimited sum total of conductivity interconnected earth electrodes or metal parts acting in the same manner and earthing conductors. Earth resistivity is the specific electrical resistivity of the earth. Dissipation resistance of an earth electrode is the resistance of the earth between the electrode and the reference earth. Earthing impedance is the a.c. impedance between an earthing system and the reference earth at operating frequency. Impulse earthing resistance is the resistance presented to the passage of lighting / currents between a point of an earthing system and the reference earth. Protective earthing is the earthing of a conductive component not forming part of the normal electrical circuit in order to protect people from unacceptable touch voltages. System earthing is the earthing of a point in the normal electrical circuit in order that apparatus or systems can be maintained properly. Lightening protection earthing is the earthing of a conductive part not forming part of the normal electrical circuit in order to avoid as far as possible flashover to the normally live conductors as a result of lightening strokes (back flashovers) Earthing voltage is the voltage occurring between an earthing system and reference earth.

Earth Surface Potential is the voltage between a point on the earths surface and reference earth. Touch voltage is the part of the earthing voltage, which can be shunted through the human body, the current path passing from hand to foot (horizontal distance from touchable part about 1 m) or from hand to hand. Step voltage is that part of earthing voltage, which can be shunted by a person with a stride 1 m long, the current path passing from foot to foot. Potential control consists in influencing the earth potential, in particular the earth surface potential, by earth electrodes. Earth Fault is electrical connection between one conductor of the normal electrical circuit and the earth or an earthed part due to a defect. The electrical connection can also be by way of an arc. Earth Fault current is the current passing to earth or to earthed parts when an earth fault exists at only one point at the site of defect. Earthing current is the total current flowing to earth by way of the earthing impedance. TRANSIENT ENCLOSURE VOLTAGE (TEV): Very fast transient phenomena, which are found on the grounded enclosure of GIS systems. The phenomenon is also known as transient ground rise (TGR) or transient ground potential rise (TGPR). VERY FAST TRANSIENT (VFT): A class of transients generated internally within GIS characterized by short duration and very high frequency. VFT is generated by the rapid collapse of voltage during breakdown of the insulating gas, either across the contacts of a switching device or line-to-ground during a fault

Fig 6 : Typical Earthing Arrangement of GIS 6.2 Codes and Standards on Earthing DIN VDE 0141/7/7.76 VDE specification for earthing systems in a.c. installations for rated voltages above 1 kV

DIN VDE 0151/6.86. Materials & Minimum dimensions of earth electrodes with regards to corrosion. IEC 621-2A/1981. First supplement to publication 621-2(1978). Electrical installations for outdoor sites under heavy conditions (including open cast mines and quarries). Part 2: General Protection requirements. IEC 364-5-54/1980. Amendment No.1 July 1982. Electrical installations of buildings. Part-5: Selection & erection of electrical equipment. Chapter 54: Earthing arrangements and protective conductors. IEC-479-1/1994. Effects of current passing through the human body. IEEE std 80-2000 IEEE Guide for Safety in AC Substation Grounding IS:3043-1987 Code of Practice for Earthing 6.3 Materials for Earthling Systems Earth electrodes (underground) and earth conductors (above ground) must conform to certain minimum dimensions for mechanical strength and to withstand possible corrosive attack. Selection of material, size of conductors, primary electrodes depends on following factors: Material should have sufficient conductivity It must carry and dissipate fault current without excessive temperature rise. Material should have high resistance to corrosion.

The enclosures of GIS should be properly designed and adequately grounded so as to limit the potential difference between individual sections within the allowable limit of 65-130 V during faults. 6.4 Dimensioning of Earthling Systems The cross-section of earth electrodes and earthing conductors must be such that the materials strength is not reduced in the event of a fault current. 6.5 Measurements for Earthling Systems The specific resistance of the soil is important for calculating earthing systems. 6.6 Design Principles The earthing system of the sub station buildings, especially of buildings with gas insulated switchgear (GIS) must be capable to carry power frequency short circuit currents (earth fault currents) and high frequency currents from switching and lighting. The requirements from the high frequency currents determine the layout of the earthing system, which can be characterized as a meshed network (or a cage shaped network) in order to give low impedance across it. Some special conductors of this cage are rated to fulfill the power frequency requirements. GIS are subjected to same magnitude of ground fault current. The basic requirements of an earthing system of GIS are similar to that of Air Insulated Switchgear .However due to availability of less area (about 10-20%) as compared to conventional outdoor substations and compactness of the equipment, it is difficult to achieve the adequate grounding by conventional methods. In the fault condition or during the normal operation, due to high frequency currents generated, electric breakdown in the insulating gas occurs. This electric breakdown generates very high frequency transients (Transient Enclosure Voltage (TEV)), which can be passed on to the GIS grounding system. These transients may have to be considered in the design of the grounding system.

In addition to these transients, external magnetic field produced by the main circuit current of large capacity GIS is strong. This magnetic field creates problems like local overheating of structures around the GIS, electromagnetic vibration, increased induction to control cables etc. Excessive currents should not be induced into adjacent frames, structures or reinforcing steel, establishment of current loops via other station equipments. In GIS the use of cathodic protection may be required. Normally it is required to protect facilities external to the GIS e.g. pressurized type cables etc. 6.7 Touch & Step Voltage in GIS Touch & Step voltages have to be considered mainly in outdoor substations. GIS buildings have an elaborated meshed earthing system, which comprises all metal parts like foundation earth electrode, earthing conductors and the GIS housing. In case of a power frequency earth fault the total of this earthing system assumes an earth potential rise versus the distant references earth. However the voltage differences between the metal parts of the building are very small. Dangerous touch or step voltages do not exist. 6.8 Grounding of Enclosures Because of availability of return path for induced currents in continuous enclosure type design of GIS, a shielding to internal field exists. However, under asymmetrical faults, the D.C component is not shielded and causes an external voltage drop due to enclosure resistance. To limit the undesirable effects caused by circulating currents the following precautions should be taken: Grounding of the enclosure should ensure that significant voltage difference does not exist between individual enclosure sections Particular attention should be given to the metallic enclosures of the GIS assembly. All metallic enclosures should be grounded properly through the base frame of the GIS so as to ensure the minimum flow of circulating currents. To avoid the circulation of enclosure currents beyond regular return path, power cable sheath should be grounded directly without involving the enclosure in the grounding path. To facilitate this isolation, design of cable terminations should be such that an isolating air gap or proper insulating elements are provided. Proper care should be taken to ensure that current transformers mounted on GIS should not carry the enclosure return current. Wherever there are discontinuities in enclosures / changes in the medium e.g. at cable terminations or transformer connections, special care should be taken to limit very fast transient over voltages and to prevent circulating currents in circuit breakers and transformer tanks. GIS cable terminations and other discontinuities in the enclosures are significant sources of Transient Ground Potential Rise phenomenon. The isolation between the directly grounded power cable sheath and the enclosure may give rise to Transient Ground Potential Rise phenomenon. Particular attention should be given to limit the Transient Ground Potential Rise phenomenon

6.9

Grounding of GIS Foundation In the GIS substations, concrete foundations may cause irregularities in current discharge path. In this respect, a simple monolithic concrete steel reinforced slab is advantageous, both as auxiliary grounding device and for seismic reasons

CHAPTER- 7 ESSENTIAL AUXILIARIES Essential auxiliaries required for GIS application are normally same as that of AIS. The testing equipment however, is required to a limited extent due to reduced maintenance. Essential auxiliaries have been listed below but their application is not discussed in detail since these have been covered in the manual elsewhere. The list for the user is as under:

DC supply : This has already been discussed in General Information in clause No.12.1 Essential Parameter. AC supply This has already been discussed in General Information in clause No.12.1 Essential Parameter. Gas handling cart- vacuum, filling and purification. Testing instruments

Air / gas humidity tester, Gas. purity detector for SO2,H2O,CF4,AIR etc., Gas leakage tester, Breaker timing measurement kit, Set of equipment for pressure measurement and gas density meter

Crane for erection -Selection of Crane capacity depends upon weight of heaviest part of the GIS modules to be lifted. EOT crane2.5 Ton is required for indoor type GIS and mobile crane for outdoor type GIS.

CHAPTER- 8 GAS HANDLING SYSTEM 8.1 Sulfur Hexafluoride SF6 gas is an inert, stable, colorless, odorless, nontoxic, nonflammable gas approximately five times heavier than air and will displace air in confined area Gas contains no oxygen and will not support life. Confined areas must be force ventilated when working with SF6 gas The Occupational Safety and Health Administration (OSHA) regulation on air contaminants, 29 CFR 1910.1000, establishes that SF6 gas has no adverse effects when inhaled in the air at a Threshold Limit Value (TLV) of 1,000 ppm. Extremely stable gas with high dielectric strength and excellent arc-quenching properties The Environmental Protection Agency has identified Sulfur Hexafluoride as a greenhouse gas with a global warming potential 23,900 times the effect of an equal mass of Carbon Dioxide and an atmospheric lifetime of 3,200 years.

Fig 7: Quenching Capacity of SF6 8.2 GAS Specification Pure Sulphur hexafluoride is absolutely non-toxic. The bi-products arising during production of the gas are removed during subsequent purification operations IEC 60376 provides recommendations for SF6 gas as under: SF6 > Air < CF4 < H2O < Mineral oil < Acidity,in terms of HF < Hydrolysable fluorides, In terms of HF < 99.90 % by weight 500 ppm by weight (0.25 vol.-%) 500 ppm by weight (0.1 vol.-%) 15 ppm by weight (0.012 Vol-%) 10 ppm by weight 0.3 ppm by weight 1 ppm by weight

Wherever gas is handled there must be no open place or welding or hot metal surfaces such as infrared equipment. Eating, drinking and smoking while working with gas is to be strictly avoided. Although this gas is recognized as physiologically safe, certain precautions have to be taken in order to ensure safe handling of gas. 8.3 Permanent Gas Treatment Devices Means shall be provided inside each enclosure for treating the SF6 gas by the use of desiccants, driers, filters, etc. to remove impurities in the gas. All gas compartments shall be fitted with static filter material containers that will absorb residual and entering moisture inside the high voltage enclosures. Filters inside the breaker compartment shall also be capable of absorbing gas decomposition products resulting from the switching arc. SF6 Gas Monitoring Devices All gas compartments must have their own independent gas supervision and alarm systems. Each gas supervision circuit shall be equipped with a temperature compensated pressure gauge, test connection point and maintenance connection point and the same shall be easily accessible. One should monitor at least the following locally and on remote Gas Refill Level-This will be used to annunciate the need for gas refilling. Breaker Block Level-This is the minimum gas density at which the manufacturer will guarantee the rated fault interrupting capability of the breaker. At this level the device contact shall trip the breaker and block the closing circuits. Over pressure alarm level-This alarm level shall be provided to indicate abnormal pressure rise in the gas compartment. It shall be possible to test all gas monitoring relays without de-energising the primary equipment and without reducing pressure in the main section. Disconnecting type plugs and sockets shall be used for test purposes; the pressure/density device shall be suitable for connecting to the male portion of the plug. Two potential free electrical contacts shall be provided with each and every alarm. 8.5 SF6 Gas Maintenance Plant

8.4

Fig 8 : Gas Maintenance Plant

The entire plant necessary for filling and evacuating the SF6 gas in the switchgear equipment shall be supplied with the contract to enable erection and maintenance work to be carried out. This shall include all the necessary gas receivers for temporarily storing the evacuated SF6 gas as well as any other gases which may be used in the process. The capacity of the temporary storage facilities shall at least be sufficient for storing the maximum quantity of gas that could be removed from the largest compartment of GIS and associated compartments on either side +10% extra SF6 gas when carrying out maintenance or repair work on any pieces of the switchgear and associated equipment. The necessary compressor to remove SF6 gas from the compartments, vacuum pump to create vacuum inside the compartment before SF6 gas filling operation, dust and moisture filter shall form a part of the plant. The plant shall have facilities for drying air and SF6 gas or any other gases with which the switchgear compartment may be temporarily filled during the process of filling with SF6 gas. Each of the gas compartments shall be fitted with permanent valves through which the gas is pumped into or evacuated from the compartments. The Mobile auxiliary plant complete with necessary hoses and couplings etc. for purifying (SO2,H2O,CF4,AIR) and drying SF6 gas in the switchgear gas compartments . The auxiliary gas purification and drying plant shall be combined as a single unit with the gas filling and evacuating plant.

8.6 SF6 Gas Detecting Instruments The portable SF6 gas detector shall be light weight and provided with long flexible probe to enable detection of SF6 gas leakage from hard to reach areas.

8.7 Maintenance of Gas Handling System The life of GIS largely depends on the quality of gas. Depending on the size of switchgear appropriate equipment with sufficient storage capacity and performance is selected based on two largest gas section capacity in the installation. The handling equipment for the gas should consist of main components such as SF6 compressor, vacuum pump , storage tank , evaporator and filter unit which are connected together with valves and fitting . Every component within closed cycle are to undergo dry running and therefore, absolutely oil free without a chance of gas getting contaminated. The built in filters provide for the drawing and cleaning of gas during each gas operation. In fact gas valves couplings and fittings themselves ensure a high degree of leak tightness and operational safety. When selecting equipment, it should be ensured that couplings are self closing type in order to avoid air and moisture penetrating due to the lines. Maintenance equipment with automatic sequences could be the state of art and should be referred because of its high degree of operational safety. It is necessary, to monitor parameters of gas filled in the GIS.

CHAPTER- 9. LIST OF STANDARDS, MANUALS, CODES AND REFERENCES The design, material and commissioning of switchgears and accessories shall comply with all currently applicable statutes, regulations and safety codes in the locality where these will be installed. The material shall also conform to the latest applicable standards. Except as noted, all the equipments offered shall conform to the requirements of the latest editions of relevant standards. National standards will be acceptable only if they are established to be equal or superior to referred standards. In all such cases, copies of English translation of all such standards shall be enclosed with the bid. In the event of any conflict between the codes and standards referred to in this specification and the requirement of this specification, the latter shall govern. Sr. No. 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. Title International Electro-Technical Vocabulary High Voltage Alternating Current Circuit Breakers Gas-Insulated Metal-Enclosed Switchgear For Rated Voltages Above 52kv Insulation Co-Ordination Cable Connections For Gis For Rated Voltages OF 72 kV & above High Voltage Test Techniques Recommendation For Heat Teated Aluminium Alloy Busbar Material Of The Aluminium-Magnesium-Silicon Type Alternating Current Disconnectors And Eathing Switches Bushing For Alternating Vatages Above 1000 Volts Current Transformers Voltage Transformers Electrical Relays Low Voltage Fuses Low Voltage Motor Staters Specification And Acceptance Of New Sulphurefxafloride First And Second Supplement To Iec Pub-376 (1971) Synthetic Testing Of High Voltage Alternating Current Circuit Breakers Guide For Checking Sf-6 Gas Taken From Electrical Equipment Artificial Pollution Test On Hv Insulators To Be Used On Ac System Gas Insulated Metal Enclosed Switchgear For Rated Voltages Of 72.5 Kv And Above Classification Of Degree Of Protection Provided By Enclosures Common Clauses For HV Switchgear And Controlgear Standards Addl. Requirement For Enclosed Switchgear And Control Gear Form 1 Kv To 72.5 Kv For Use In Severe Climate Conditions Guide For Selection Of Insulators In Respect Of Polluted Conditions Gas Insulated Mental Enclosed Switchgear For Rated Voltages 72.5 Kv And Above Requirements For Switching Of Bus Charging Circuit By Disconnectors Standard Reference IEC-60050 IEC-62271100 IEC 62271203 IEC 60071 IEC 60859 IEC-60060 IEC-60114 IEC-62271102 IEC-60137 IEC-60044 IEC-60186 IEC-60255 IEC-60269 IEC-60292 IEC-60376 IEC-60376 A&B IEC-60427 IEC-60480 IEC-60507 IEC-60517 IEC-60529 IEC-60694 IEC-60932 IEC-60815 IEC-61259

26. 27. 28. 29. 30. 31. 32. 33. 34.

IEEE Recommended Practice For Seismic Design Of Substations High-Voltage Switchgear And Controlgear Part 2: Seismic Qualification For Rated Voltages Of 72,5 Kv And Above Specification For Transportable Gas Containers. Seamless Steel Containers IEEE Guide For Safety In Ac Std.80 Substation Grounding Circuit Breakers Quality System-Model For Quality Assurance In Final Inspection And Test European Standard-Cast Aluminium Alloy Enclosures For Gas Gilled High Voltage Wrought Aluminium And Aluminium; Alloy Enclosure For Gas Filled High Voltage Switchgear And Control Gear Welded Composite Enclosures Of Cast And Wrought Aluminium Alloys For Gas Filled

IEEE 693 IEC 62271-2 BS-5045-1 ANSI/ IEEE ANSI-C37 ISO-9003 EN-50052 EN-50064 EN-50069

CHAPTER- 10 EMERGING TECHNOLOGIES This chapter shall incorporate based on inputs from the manufacturers in respect of following 10.1 i) HYBRID SWITCHGEAR Hybrid switchgear-With the advent of GIS a number of manufacturers have come out with models which are in between AIS and GIS. This could be a very healthy and cost economic solution to the users in terms of constraints they face during application of substations /pothead yards. Combining disconnector, breaker and CTs is commonly available but tailor made configurations to meet the site specific needs are very much possible. Compaction of AIS is, therefore, possible with the use of hybrid switchgear.

Fig 9 : Hybrid Switchgear Hybrid solutions combines five functions of traditional AIS substation namely Circuit Breaker, Disconnector, Earth Switch, Current Transformer and Voltage Transformer into one module encapsulated in SF6 gas compartment. It has following advantages: Space saving upto 50-60% can be achieved compared to an AIS station. It has high reliability due to usage of mature GIS components & technology. Installation is fast and easy. It allows flexibility in layout design.

Fig-10

In order to adopt this course of option the bidding documents will have to be functional without detailing of items and single line diagram along with space available could be supported with the system requirements in terms of electrical parameters. In such cases the optimum use of hybrid switchgear could be possible and the specification would not be limited to a manufacturer 10.2 Controlled switching requirements Experiments have proven that in case of switching on i.e. closing of CB is carried out on or near zero crossing of sine waves, the switching surges would be extremely small and it will be providing least stress on insulation of GIS as well as other connected equipments. Similarly tripping of CBs could also be ensured near zero crossing to limit the over-voltages. The implementation of control switching needs an operating drive immune to variations in operating voltage and frequency. The thyristor control for point of wave switching would be a phenomenon required to achieve control switching requirements. It is understood that relays are available for the purpose but information to what extent CB manufacturers/GIS manufacturers are geared up to adopt this technology remains a point of concern. Gas Mixtures extent and possibility Impurities in GIS are yet to be proven. In fact in chambers where no switching is required as also in gas bays transmission lines experiments are going on and mixing of nitrogen is being considered increasingly feasible. Trials are going on where SF6 and nitrogen mixtures are also being used for switching functions and insulation in electrical equipment. The major application however is commercially limited to gas insulated transmission line as far as SF6 and nitrogen mixtures are concerned. Enclosure options The GIS enclosure should essentially be divided into several sections separated by gas tight barrier insulators. Each section should be provided with the necessary piping and valves to allow isolation, evacuation and refill of gas without evacuation of any other section. Location of gas barriers insulators is to be clearly discernable outside the enclosure by a band of distinct colour normally used for safety purpose. Two options further could arise in respect of enclosure provision i.e. bus disconnectors arranged in a separate gas compartment or arranged in a common gas compartment. Although manufacturers of both technologies are available but majority is towards separate gas compartment. In fact the user need not necessarily stress upon the enclosure options as outage of bus bar disconnectors would in any case lead to outage of bus bar. Similarly the leakage of the gas from the main bus bar chambers are also of little relevance once monitoring equipment is installed. Bus trunking. -Trunking at bus bars is used for extending the GIS connection out of a building or to meet specific lay out requirements. In fact truncated bus sections once clearly shown provide advantageous flexibility in optimizing space and cost with special reference to outdoor applications of GIS. Co-ordination with pot head yard equipment and integration with the lay out is an advantage which alone bus trunking gives with respect to high voltage cables. Composite bushings- Along with