GAS INSULATED SUBSTATION & SWITCHGEAR Asif Eqbal: Electrical & Electronics Engineer Contents 1. Introduction and governing standard 2. Conventional substation (AIS) 3. Limitation of AIS 4. Need for GIS 5. Introduction to GIS 6. Service condition of GIS as per IEEE 7. Services offered by GIS user & GIS supplier 8. Technical requirements & design features 9. Maintenance philosophy of GIS switchgear 10. Comparison of GIS and AIS substation 11. Drawbacks of GIS 12. Growth of GIS substation 13. Future trends in GIS 14. Conclusion Introduction and governing standard This presentation covers the technical requirements and design aspects of a gas-insulated substation (GIS) as per governing standards of GIS substations. Governing standard of GIS substation are: 1. IEEE Standard C37.123-1996 (Rev-2002)- IEEE Guide to Specifications for Gas-Insulated, Electric Power Substation Equipment. 2. IEEE Standard 1300–1996, Guide for Cable Connections for Gas-Insulated Substations. 3. Components inside GIS switchgears (CB, CT, PT and Disconnecters) are governed by their respective IEC standard. Prominent among them is IEC 62271-203, 200, 100 4. ASTM D2472–1992, Specification for Sulfur Hexafluoride. 5. NEMA CC 1–1993, Electric Power Connectors for Substations. Conventional substations (AIS) A:Primary power lines' side B:Secondary power lines' side 1.Primary power lines 2.Ground wire 3.Overhead lines 4.Transformer for measurement of electric voltage 5.Disconnect switch 6.Circuit breaker 7.Current transformer 8.Lightning arrester 9.Main transformer 10.Control building 11.Security fence 12.Secondary power lines Conventional substations (AIS)……continued Power generating stations may be hydro, thermal or nuclear. Depending upon the availability of resources these stations are constructed at different places. These places may not be nearer to load centers where the actual consumption of power takes place. So it is necessary to transmit these huge power blocks from generating station to their load centers. Long and high voltage transmission networks are needed for this purpose. Power is generated at medium voltage level. It is economical to transmit power at high voltage level. Distribution of electrical power is done at lower voltage levels or as specified by consumer requirement. For maintaining proper voltage levels at transmission and distribution level and for providing greater stability a number of electrical transformation and switching setups have to be created in between generating station and consumer ends. These transformation and switching setups are generally known as electrical substations. There are various ways to classify a substation. Based on purpose substation may be transmission substation, distribution substation etc….Based on nature of dielectric medium a substation is of two types AIS and GIS Conventional substations (AIS)……continued Air used as a dielectric. Normally used for outdoor substations. All the equipment like circuit breaker, CT, VT and disconnecters are outdoor Equipment are mounted on lattice or pipe structure and connected with rigid tubes or flexible conductors Easy to expand (in case space is not an issue) Excellent overview, simple handling and easy access. Limitations of AIS………….continued 1. Site selection of AIS substation is a complex process and requires consideration of lots of factor like: Technical factor. Technical factor consist of mainly area requirement and pollution. For deciding the area requirement of a substation its voltage level(s), number of feeders, requirements of step-up/ down transformers & reactors, infrastructural facilities like housing etc., for present and future expansion on a 10-15 year scenario are to be planned. After such an assessment, estimate of required area is made. Substation location should be away from the polluted area as far as possible. As pollution levels increase, the insulator creepage distance of equipment will also increase, which may increase the cost of the equipment. In extreme cases, in heavily polluted area cleaning facilities or the use of protective products may be necessary. The risk of failure of equipment increases with the pollution level. Limitations of AIS Physical factor. Physical factor mainly consist of Topographical, Geological and Geographical fcators. Topographical uniformity is maintained through standardization of supporting structures and uniform equipment terminal height, AIS land needs to be levelled. Wherever, high cost of leveling and retaining walls is anticipated, various terraces needs to be formed. AIS substation needs to be located on the level around 0.5 m higher than highest recorded flood level. It is preferable to select the site as even as possible to save not only time and cost of leveling but also cost of civil foundation . Limitations of AIS…………..continued Physical factor. Physical factor mainly consist of Topographical, Geological and Geographical fcators. Most important geological factor is the type of soil & its bearing capacity, which needs investigation. Soil should be suitable for construction of roads and foundation. A high water level may require special kind of treatment for foundation. Hence cost of substation may vary depending upon nature of soil. Similarly, if the natural soil has a high resistivity, the earth mat becomes costly accordingly. Geographically a AIS substation should be selected away from the hazardous area like mines, land slides, flood prone areas. They should be away from airports and aeronautic al installation as there are usually restrictions on the maximum height of structures and due to possible disturbance on navigation equipment. The orientation of substations should be selected keeping in view the outgoing line orientations. Limitations of AIS…………….continued Infrastructural factor. Easy access, amenities availability, reliable power & water supply constitute the infrastructural factors. Other amenities such as medical, communication facilities etc. should be reasonably available. The availability of reliable power supplies for substation auxiliaries, and construction purpose should be examined. Water should be available be construction of substation as well as for drinking purpose. Social & Environmental factor. Land accusation and forest encroachment constitute the social and environmental factors. With strict laws on land acquisition, resettlement of PAPs (Project Affected Persons), greater attention is required for acquiring an AIS site. It is preferable to go for a govt. land rather than private land as the process may take less time. Out of various options available for locating an AIS substation, the choice which involves minimum forest encroachment by lines needs to be preferred. Low noise transformers and reactors should be installed wherever station is near residential areas. Commercial factor. While selecting substation site, cost of substation considering land cost & its development needs to be examined with respect to the cost of transmission lines. Limitations of AIS……………….continued 2. Large dimensions due to statutory clearances and poor dielectric strength of air. For a typical 400/220 kV Substation, the area requirement for switchyard is around 30 to 35 acres with I-type layout for 400 kV system and double main and transfer bus arrangement for 220 kV system. 3. Life of steel structures degrades. Saline and other types of industrial pollution cause corrosion in supporting structures and protective coating may be required. 4. Seismic instability. All the AIS equipment needs to be type tested individually for respective seismic zone under which AIS site falls. 5. Large planning & execution time. For above mentioned site selection process large planning and thereafter execution time is required. 6. Regular maintenance of the substation and its equipment is required. Need for GIS 1. Non availability of sufficient space. It is very much required to establish a substation at load center. Establishing a substation at load center is quite economical and profitable in following ways: Reduction in length of feeders Improvement of the quality of voltage regulation due to short length feeders Generally main load center of any place is situated at very congested place where, sufficient land for establishing conventional AIS is very hardly available. This problem can be solved by using gas insulated switchgear technology. Total space required for a GIS is 10% of that needed for a conventional substation. 2. Difficult climatic and seismic conditions at site, like high altitude and atmospheric pollution 3. Aesthetically “superior” to air insulated substations 4. The higher the voltage, the more favorable gas insulated technology becomes. The footprint of 765kV conventional substation is enormous, and GIS technology allows a significant size reduction. GIS technology can be used for installations in areas where the cost of real estate is appreciable. 5. Overcomes or decreases the magnitude of limitations of AIS site selection. Introduction to GIS GIS was first developed in various countries between 1968 and 1972. After about 5 years of experience, the use rate increased to about 20% of new substations in countries where space is limited. In other countries with space easily available, the higher cost of GIS relative to AIS has limited use to special cases. For example, in the U.S., only about 2% of new substations are GIS. International experience with GIS is described in a series of CIGRE papers (CIGRE, 1992; 1994; 1982). Introduction to GIS…………continued The atmospheric air insulation used in a conventional, AIS requires meters of air insulation to do what SF6 can do in centimeters. Compact, multi-component assembly. Enclosed in a ground metallic housing. Sulphur Hexafluoride (SF6) gas – the primary insulating medium. (SF6) gas- superior dielectric properties used at moderate pressure for phase to phase and phase to ground insulation Preferred for voltage ratings of 72.5 kV, 145 kV, 220 kV and 420 kV and above. Various equipment like Circuit Breakers, Bus-Bars, Isolators, Load Break Switches, Current Transformers, Voltage Transformers, Earthing Switches, etc. housed in metal enclosed modules filled with SF6 gas. Phase conductor is almost always of Aluminium. The outer enclosure is also of mild steel, although Aluminium is also available. Introduction to GIS…………continued Properties of SF6 Gas Sulfur hexafluoride is an inert, nontoxic, colorless, odorless, tasteless, and nonflammable gas consisting of a sulfur atom surrounded by and tightly bonded to six fluorine atoms. It is about five times as dense as air. SF6 is used in GIS at pressures from 400 to 600 kPa absolute. The pressure is chosen so that the SF6 will not condense into a liquid at the lowest temperatures the equipment experiences. SF6 decomposes in the high temperature of an electric arc, but the decomposed gas recombines back into SF6 so well that it is not necessary to replenish the SF6 in GIS. Lifetime – Very long (800 to 3200 years!). Even if some reactive decomposition byproducts formed because of the trace presence of moisture, air, and other contaminants, the quantities formed are very small. Molecular sieve absorbents inside the GIS enclosure eliminate these reactive byproducts. Service condition of GIS As per IEEE 122, IEEE 123 and IEC62271 Indoor or outdoor : As per customer requirement Elevation above sea level : As per site location Design ambient temperature : 40°C As per IEC 62271 Maximum ambient temperature : 105°C As per IEC 62271 or lesser than temperature at which self sustaining reaction may start between Sf6 and desiccants Service condition of GIS Minimum ambient temperature Enclosure design pressure Seismic As per manufacturer recommendation as SF6 liquefies at a sufficiently low temperature for a wide range of power engineering applications. Enclosures shall be designed to withstand an external pressure of one atmosphere (with a 1.5 safety factor) with zero internal pressure absolute and shall be designed and tested to withstand an internal test pressure of 1.5 times the design pressure. Service offered by GIS user and GIS supplier User Unless otherwise specified or agreed upon, the user should receive, install, and field test the apparatus specified herein. Installation and field testing of the GIS should be completed under the supervision of the supplier to ensure correct installation. Unless otherwise specified, the following equipment and services should also be furnished by the user: Concrete foundations Power transformers or reactors, including bushings Surge arresters outside the GIS The station ground grid, below ground, and vertical connection risers of adequate length Ac and dc auxiliary power to furnished equipment Hoist or crane Supervisory control and data acquisition (SCADA) equipment Conduit or troughs and wiring to supplier-furnished equipment Gas handling equipment Batteries and battery chargers User Service offered by GIS user and GIS supplier……….continued Protective relaying systems Transmission-line dead-end terminations Control and power cable trenches Erection labor, with the supervision and assistance of the GIS supplier Control house and related equipment or building in case of indoor installation, including crane, if applicable Job-site unloading and storage Field welding, if required, with the supervision and assistance of the GIS supplier Test equipment, coordinated with the GIS supplier Specific tertiary bus and other related equipment High-voltage power cables and terminations Anchor bolts and/or embedded steel, if part of foundation pouring Service offered by GIS user and GIS supplier……….continued Supplier GIS supplier should design, manufacture, test, deliver, and guarantee the following services as recommended by IEEE: The complete gas-insulated switchgear, including connections to power transformers and line with associated circuit breakers, disconnect and grounding switches, voltage transformers, and surge arresters. All metal-enclosed gas-insulated buses for interconnecting various switchgear assemblies, including flexible joints to ensure service continuity during thermal cycling and vibration. All cable connections, including SF6 enclosures, terminator support structures, and mounting insulators should be as specified in IEC859 : 1986 and IEEE Standard 1300–1996 . All transformer and reactor bushing sulfur hexafluoride (SF6) enclosures. All coordination with power transformer, reactor, cable, and cable termination suppliers to assure proper electrical and mechanical interface, in accordance with IEC859 : 1986 and IEC1639 : 1996. All auxiliary equipment, for emergency control and local supervision, including interlocks; operating mechanisms; and control, monitoring and protective devices, installed in suitable cabinets. Ground buses and ground connection pads for connection to the ground grid. Gas density monitors, pressure relief devices, and gas-filling connections. New gaskets, sealant, and desiccant for permanent sealing of all field assembled joints Design features & Technical requirement COMPONENTS OF GIS: 1. Bus bar 2. Circuit Breaker 3. Disconnecter (line or bus) 4. Earthing switch (line or bus) 5. Current transformer (feeder / bus) 6. Voltage transformer (feeder/ bus) 7. Feeder Disconnecter 8. Feeder Earthing switch 9. Lightning / Surge Arrester 10. Cable termination 11. Control Panel. Design features & Technical requirement Design features & Technical requirement Design features & Technical requirement Circuit Breaker: Under short circuit conditions, however, the current may reach tens of thousands of amperes at a power factor as low as 0.1. It is duty of a circuit breaker to interrupt such currents as soon as possible to avoid equipment damage. Duty cycle O-0.3s-CO-3 min-CO. In case of HV circuit breakers duty cycle (ANSI) or operating sequence (IEC) refers to how rapidly breaker operations can be sequentially performed. Breaker has a 0.3 second delay between the first opening and the next close-open sequence, and then it must wait 3 minutes until the next close-open operation. It "shows" that you have a breaker meant for rapid reclosing duty. The first delay gives the spring operating mechanism time to recharge, the second delay gives the contacts time to cool down. Design features & Technical requirement Disconnecter: Disconnectors or isolators are used for electrical isolation of circuit parts Disconnect switches can be three-pole, group-operated, or single-pole operated, They are slow acting and operating at off load Disconnectors must be carefully designed and tested to be able to break small charging current without generating too-high over voltage. Design features & Technical requirement Voltage transformer: Variable location on feeder and busbar. Integrated disconnecting facility for GIS and power cable testing without dismantling and gas handling. Flexible gas compartment allocation for optimal service oriented gas supervision. The following should be specified and considered in selection of the voltage transformer. I. II. III. IV. V. VI. Number of secondary windings Number of taps in each secondary winding Ratio of primary voltage to each secondary winding voltage Thermal rating of each secondary winding Thermal rating of primary winding Accuracy class Design features & Technical requirement Local control cubicle: LCC is the interface cubicles to all secondary systems of a substation which are represent a station control and protection. LCC includes control and alarm functions as well as the correct distribution of auxiliary power supply for the relevant GIS bay. In general, each cabinet should contain the following equipment for control, indication and protection of switches, circuit breakers, and associated components: 1. 2. 3. 4. 5. 6. One control switch for each three-phase circuit breaker. One remote-local switch for each three-phase circuit breaker. One open-close control switch for each motor-operated grounding switch. One open-close control switch or push-button for each motor-operated grounding switch. One or two red light-emitting diodes or mechanical semaphore and one green for each circuit breaker, each disconnect and grounding switch, or contact position indication on the mimic diagram. Control switches for ac and dc supply to each compartment. Design features & Technical requirement Local control cubicle 7. A mimic diagram showing connections of all furnished equipment and showing location of all gas zones. 8. An annunciator panel with retransmit contacts. 9. Terminal blocks and terminations for each gas density relay. 10. Terminal blocks and terminations for electrical interlock contacts. 11. Terminal blocks and terminations for alarm and miscellaneous remote-control circuits. 12. Terminal blocks and terminations for all instrument transformer leads. A lead should be installed and terminated for each tap of a multi-ratio current transformer and all secondary winding taps for voltage transformers. 13. Terminal blocks and terminations for all required spare contacts. Design features & Technical requirement Current transformer: In the single phase enclosed Core of CT is located outside the enclosure & inside for three phase gas compartment to reduce access of moisture and to suppress gas-tight bushings for secondary connections. CT current rating is 120%of rated primary current Number of cores is decided based on metering and protection requirement Design features & Technical requirement Cable connection: The following should be specified and considered in design of the cable connection, and values should be chosen from IEEE Standard 48-1990. I. Voltage—kV rms phase-to-phase II. Basic insulation level—kV peak III. Continuous current—A rms IV. Short-time current—kA rms for 1 s or 3s V. Peak short circuit current—kA peak VI. Maximum dielectric fluid pressure for cable system—kPa VII. Power frequency factory withstand voltage—kV rms phase-to-ground Design features & Technical requirement Dielectric medium SF6 The leakage rate of SF6 gas from an individual gas compartment shall not exceed 1% per year. The total leakage rate from the GIS system shall not exceed 0.5% per year. Each gas zone should be furnished with a gas monitoring device capable of signaling two adjustable, independent alarms. The user should specify the signaling requirement as it pertains to the user's protection and control system. Typically, the signaling is done by two sets of adjustable, electrically-independent contacts that operate at the alarm levels as follows: I. First alarm—refill gas density normally 5–10% below the nominal fill density II. Second alarm—minimum gas density to achieve equipment ratings Maintenance philosophy Time based Maintenance and condition based Maintenance The criteria for time-based maintenance are aging of material based on environmental conditions of installation To maintain the correct equipment at the right time after lapse of certain condition is condition based maintenance. With this type of maintenance one can exchange just the components or parts that have been subjected to the most wear over their many years of reliable operation Comparison of AIS & GIS Drawbacks of GIS substation Excessive damage in case of internal fault. Diagnosis of internal fault and rectifying takes very long time (high outage time). SF6 gas pressure must be monitored in each compartment. Reduction in the pressure of the SF6 gas in any module results in flash over and faults. SF6 causes ozone depletion and global warming. SF6 is a gas specifically mentioned in Kyoto protocol. Search is on for a replacement gas or gas mixture. Currently, 80% used by Electrical Power industry. I. II. 7000 metric tons/year in 1993. Reached 10,000 metric tons/year by 2010. Drawbacks of GIS substation Transient enclosure voltage TEV or Transient ground potential rise TGPR can be a very serious EMC and personnel safety problem. Voltage rise on grounded shields of several kV at distances up to several km have been observed in early days. Such transient voltages on the “grounded” enclosure arise from an internal collapse of voltage in the SF6 gas, internal re-strikes across circuit breaker or disconnect switch contacts, or flashover of external insulation close to GIS, e.g., and air-SF6 bushing. Internal voltage collapse produces travelling waves, in both directions, from the point of breakdown. Such transients are often called VFTO (very fast transient over voltages). At the points of discontinuity (changes in surge impedance) these VFTO waves get reflected and refracted. Such points are junctions of transmission lines, air/SF6 bushing, GIS/cable connections, ground leads connecting the enclosure to the earthing grid/mat/plate, or a ZnO arrester. Growth of GIS substation Future trends in GIS Small quantities of SF6 in N2 can improve dielectric strength drastically. All of the dielectric strength of SF6, nearly, can be achieved by adding less than 20% SF6 into N2. SF6/N2 mixtures less susceptible to effects of field non uniformity than pure SF6. Compact design of switch gear by using three phase encapsulated design for higher voltages. Development of DC GIS for incorporating into expanding national/international HVDC systems. Conclusion GIS generate no noise & have no radio interference, being located closure to load centers, easy solution for mountain areas where ice & snow are major problems and due to many other advantages described in this presentation GIS is necessary for Extra HV & Ultra HV substations.