Gas Insulated Substation
GAS INSULATED SUBSTATION A SEMINAR REPORT Submitted in partial fulfillment of the Requirements for the degree of
BACHELOR OF TECHNOLOGY in
ELECTRICAL ENGINEERING DEPARTMENT by VISHAL NIGAM Under the guidance of
Mr. SATYENDRA SINGH
INSTITUTE OF ENGINEERING AND TECHNOLOGY, LUCKNOW, 226021
I hereby, declare that the work which is being presented in this seminar report entitled “Gas Insulated Substation” submitted in partial fulfillment of the requirements for the degree of Bachelor of Technology to the Department of Electrical Engineering, Institute of Engineering and Technology Lucknow, Uttar Pradesh, INDIA, under the guidance and supervision of Mr. Satyendra Singh ,Department of Electrical Engineering, IET LUCKNOW. To the best of my knowledge, the matter embodied in this seminar report has not been submitted for the award of any other degree or diploma in any other Institute/University. Date: Place: Lucknow
CERTIFICATE This is to certify that the above statement made by the candidate is correct to the best of my knowledge. Date: Place: Lucknow
(SATYENDRA SINGH) Department of Electrical Engineering. IET LUCKNOW
ACKNOWLEDGEMENT I would like to extend my heartfelt thanks and deep sense of gratitude to all those who helped me in preparing this report directly or indirectly. I would like to express my sincere thanks to my respected sir SATYENDRA SINGH for his expert guidance and suggestion which helped me to make this report. It gives me immense pleasure in conveying thanks to my faculties who helped a lot in completing this seminar report. I would also like to express my thanks to my friends. I am extremely indebted to them for providing valuable support and co-operation.
VISHAL NIGAM 1205220051 EE 3rd YEAR
ACKNOWLEDGEMENT 1. INTRODUCTION 1.1. FEATURES OF GAS INSULATED SUBSTATION
2. INTRODUCTION TO SF6 3. COMPONENTS OF GIS. 3.1 DESCRIPTION OF COMPONENTS OF GIS 3.1.1 SF6 CIRCUIT BREAKER. 3.1.2 DISCONNECTOR. 3.1.3 EARTHING SWITCH. 3.1.4 INSTRUMENT TRANSFORMER. 3.1.5 LIGHTINING ARRESTORS.
4. ADVANTAGES OF GAS INSULATED SUBSTATION (GIS) OVER AIR INSULATED SUBSTATION (AIS).
5. DISADVANTAGE OF GIS.
6. APPLICATION OF GIS.
1.0 INTRODUCTION A gas insulated substation (GIS) is a high voltage substation in which the major structures are contained in a sealed environment with sulfur hexafluoride gas (SF 6)as the insulating medium. GIS technology originated in Japan, where there was a substantial need to develop technology to make substations as compact as possible. The clearance required for phase to phase and phase to ground for all equipment is much lower than that required in an air insulated substation; the total space required for a GIS is 10% of that needed for a conventional substation.
Gas insulated substations offer other advantages in addition to the reduced space requirements. Because the substation is enclosed in a building, a GIS is less sensitive to pollution, as well as salt, sand or large amounts of snow. Although the initial cost of building a GIS is higher than building an Air insulated substation(AIS), the operation and maintenance costs of a GIS are less.
Fig.1.0 An illustration of High Voltage Gas Insulated Substation.
Gas insulated substations (GIS) have been used in power systems Over the last three decades because of their high reliability, easy Maintenance, small ground space requirement etc. In India also, a few GIS units are under various stages of installation. The basic insulation level (BIL) required for a gas insulated substation (GIS) is different from that of the conventional substation because of certain unique properties of the former. Gas insulated bus has a surge
impedance (70Ω) more than that of the conventional oil filled cables, but much less than that of an over head line (300Ω - 400Ω). Further, the average bus run for a compact GIS is much less than that for the conventional station. In addition, the GIS is totally enclosed and therefore is free from any atmospheric contamination. Hence, in general the GIS permit lower BIL rating than the conventional one. However the life of GIS is affected by several factors such as: conductive particles, particle discharges and contamination (decomposition products, moisture, etc.). Conductive particles inside the enclosure are known to reduce the breakdown level of Gas insulated systems. Partial discharges can develop from conductive particles, contamination, and defects during the manufacturing process, etc. The GIS require less number of lightning arresters than a conventional one. This is mainly because of its compactness. The basic consideration for insulation coordination is V-T characteristic. The V-T characteristic of SF6 is considerably flat compared to that of air. Air can withstand to very high voltages for very short time. However, as the duration of voltage increases, the withstand voltage falls off considerably. On the other hand, SF6 exhibits a flat characteristic, thus the ratio of basic lightning impulse level is close to unity for GIS, whereas for the conventional substations this ratio varies between 0.6 and 0.86. Although GIS has been in operation for several years, a lot of problems encountered in practice need further understanding.
Some of the problems studied are: Switching operations generate very fast transient over voltages (VFTOS). VFTOS may cause secondary breakdowns inside a GIS and Transient Enclosure Voltages (TEV) outside the GIS. Prolonged arcing may produce corrosive / toxic Byproducts. Support spacers can be weak points when arc Byproducts and metallic particles are present. From the reliability point of view, partial discharge detection is important.The methods of detection are of acoustic system and electric systems etc. These methods lack quality control.
1.1 FEATURES OF GAS INSULATED SUBSTATION The modular of design of GIS offers a high degree of flexibility to meet layout requirements of both substations, as well as power station switchgear, making efficient use of available space. GIS technology has reached a stage of application and a wide range of GIS equipment up to highest voltage of 800 kV is available with many unique features. They are: Wide spread application of aluminum enclosure materials for standardized component models for all voltage ranges. The light weight enclosures have good conductivity, low eddy-current losses and a high resistance to corrosion.
Easy handling, as well as reduced stresses on foundation and support structure are additional features. Standard arrangements can be easily modified and extended with good co-ordination between the manufacturer and the user. A gas- tight barrier insulator in switchgear serve for the separation of gas compartments and prevents neighbouring switchgear parts from being affected during maintenance.
2. INTRODUCTION TO SULPHUR HEXAFLUORIDE(SF6) Sulfur hexafluoride (SF6) is an Inorganic, colorless, odorless, non-flammable, extremely potent greenhouse gas which is an excellent electrical insulator. SF6 has an octahedral geometry, consisting of six fluorine atoms attached to a central sulfur atom. It is a hypervalent molecule. Typical for a nonpolar gas, it is poorly soluble in water but soluble in nonpolar organic solvents. It is generally transported as a liquefied compressed gas. It
has a density of 6.12 g/L at sea level conditions, which is considerably higher than the density of air (1.225 g/L).
Fig.2.0 Molecular view of SF6
Characterstics of SF6 : SF6 is colorlessly, odorless and a chemical neutral (inerted) gas. SF6 is 5x heavier than air, is not toxic and has no dangerous components inside. SF6 is no hazardous material. SF6 has no eco-toxic potential. SF6 has no impact for the ozonosphere. SF6 is a potent greenhouse gas ( GWP* CO2).
(* Global Warming Potential; ** according to EU-F-Gas regulation )
Copyright© Siemens AG( SF6 has excellent electrical characteristics. SF6 has been used as an insulating medium inelectronic devices, power apparatus, and HVDC converter stations. Its excellent properties make it ideally suited both as an insulating and as an arc-quenching agent.
Its dielectric strength is greatly superior to that of air, and it is close to 100 times as effective as air in quenching an electric arc.
SF6 is used in the electrical industry as a gaseous dielectric medium for high-voltage circuit breakers, switchgear, and other electrical equipment, often replacing oil filled circuit breakers (OCBs) that can contain harmful PCBs. SF6 gas under pressure is used as an insulator in gas insulated switchgear (GIS) because it has a much higher dielectric strength than air or dry nitrogen. This property makes it possible to significantly reduce the size of electrical gear. This makes GIS more suitable for certain purposes such as indoor placement, as opposed to air-insulated electrical gear, which takes up considerably more room. Copyright© Siemens AG
Fig.3.0 SF6-emission in the lifecycle process of switchgear.
3.0 COMPONENTS OF GIS
Fig. 4.0 structure of GIS
Gas Insulated Substations(GIS) is a compact, multicomponent assembly enclosed in a ground metallic housing which the primary insulating medium is compressed sulphur hexaflouride(SF6) gas. It generally consists components Of:
Bus bar Circuit Breakers Disconnecting switches Earthing switches Current transformers Voltage transformers Cable and boxes Gas supply and gas monitoring equipment and Local control
The figure4.0 illustrates the structure of GIS with some of the marked components like. “BB” refers to BUS BAR. “CB” refers to CIRCUIT BREAKER “VT” refers to VOLTAGE TRANSFORMER “CT” refers to CURRENT TRANSFORMER
Fig. 5.0 sectional
view of GIS
3.1 DESCRIPTION OF COMPONENTS OF GIS 3.1.1 SF6 CIRCUIT BREAKERSThere are mainly three types of SF6 CB depending upon the voltage level of application1. Single interrupter SF6 CB applied for up to 245 KV(220 KV) system. 2. Two interrupter SF6 CB applied for up to 420 KV(400 KV) system. 3. Four interrupter SF6 CB applied for up to 800 KV(715 KV) system. Working of SF6 Circuit Breaker :
The working of SF6 CB of first generation was quite simple it is some extent similar to air blast circuit breaker. Here SF 6 gas was compressed and stored in a high pressure reservoir. During operation of SF6 circuit breaker this highly compressed gas is released through the arc in breaker and collected to relatively low pressure reservoir and then it pumped back to the high pressure reservoir for re utilize. The working of SF6 circuit breaker is little bit different in modern time. Innovation of puffer type design makes operation of SF6 CB much easier. In buffer type design, the arc energy is utilized to develop pressure in the arcing chamber for arc quenching.
Fig 6.1 shows working
of SF6 CIRCUIT BREAKER.
Here the breaker is filled with SF6 gas at rated pressure. There are two fixed contact fitted with a specific contact gap. A sliding cylinder bridges these to fixed contacts. The cylinder can axially slide upward and downward along the contacts. There is one stationary piston inside the cylinder which is fixed with other stationary parts of the SF6 circuit breaker, in such a way that it can not change its position during the movement of the cylinder. As the piston is fixed and cylinder is movable or sliding, the internal volume of the cylinder changes when the cylinder slides. During opening of the breaker the cylinder moves downwards against position of the fixed piston hence the volume inside the cylinder is reduced which produces compressed SF 6gas inside the cylinder. The cylinder has numbers of side vents which were blocked by upper fixed contact body during closed position. As the cylinder move further downwards, these vent openings cross the upper fixed contact, and become unblocked and then
compressed SF6 gas inside the cylinder will come out through this vents in high speed towards the arc and passes through the axial hole of the both fixed contacts. The arc is quenched during this flow of SF6 gas. During closing of the circuit breaker, the sliding cylinder moves upwards and as the position of piston remains at fixed height, the volume of the cylinder increases which introduces low pressure inside the cylinder compared to the surrounding. Due to this pressure difference SF6 gas from surrounding will try to enter in the cylinder. The higher pressure gas will come through the axial hole of both fixed contact and enters into cylinder via vent and during this flow; the gas will quench the arc.
3.1.2 DISCONNECTOR : The basic element of the device is a moving contact performing both the disconnector and earthing switch functions. The design prevents simultaneous closing of both switches. Three positions are available for the contacts: Disconnector closed and earthing switch open. Disconnector open and earthing switch open. Disconnector open and earthing switch closed. The disconnectors are able to switch the capacitive charging currents of the busbars and bus transfer currents in double busbar substations. The disconnectors/earthing switches are operated by an electrical operating mechanism directly attached to the equipment.
3.1.3 EARTHING SWITCH As no arc quenching technique is provided in isolator it must be operated when there is no chance current flowing through the circuit. No live circuit should be closed or open by isolator operation. A complete live closed circuit must not be opened by isolator operation and also a live circuit must not be closed and completed by isolator operation to avoid huge arcing in between isolator contacts. That is why isolators must be open after circuit breaker is open and these must be closed before circuit breaker is closed. Isolator can be operated by hand locally as well as by motorized mechanism from remote position. Motorized operation arrangement costs more compared to hand operation; hence decision must be taken before choosing an isolator for system whether hand operated or motor operated economically optimum for the system. For voltages up to 145KV system hand operated isolators are used whereas for higher voltage systems like 245 KV or 420 KV and above motorized isolators are used.
3.1.4 CURRENT AND VOLTAGE TRANSFORMER Current transformer The current transformers are of the single-phase inductive type with one or more cores and preferably located on the outgoing side of the circuit breaker. They can, however, be located at any point within the bay or substation. he high-voltage conductor forms the primary winding. The cores with the secondary windings are located on a grounded electrode and are designed to comply with the requirements in terms of accuracy, class, and power rating. Different ratios can be achieved via taps in the secondary winding accessible in a terminal box. The pressurized SF6 gas between the high-voltage conductor and electrode serves as the primary insulation. The cores are completely metalenclosed which makes for very high reliability in terms of electromagnetic compatibility (EMC).
Voltage transformer/RC-voltage divider Each single-phase inductive voltage transformer is encapsulated in its own housing and thus forms a separate gastight module. Each voltage transformer consists of the following main components: The primary winding One or more secondary windings (forming one coil) An iron core The pressurized gas inside the enclosure in combination with the film insulation provides insulation against high voltage. The high-voltage connection to the switchgear is established via the primary conductor, which is supported by a gastight bushing. The secondary connections are routed via a gastight bushing plate to the terminal box.
3.1.5 SURGE ARRESTOR OR LIGHTINING ARRESTOR the electrical surge also can be generated from the system itself. Actually during switching operation there may be a chance of current chopping. If during normal operation, if electrical isolator is opened on load. Sudden open circuit is occurred in the system. In addition to these, the basic arc-quenching techniques of SF6 circuit breaker and vacuum circuit breaker may give rise to current chopping and multiple reignition sometimes. As we know that sudden current chopping give rise to the di/dt. [di/dt = rate of change of current with respect to time]. As the electrical load is generally inductive, there is a transient voltage, expressed by L(di/dt) where L is the inductance of load of system. This voltage is induced across the opening contacts, and travels towards load and reflects in similar manner of lightning impulse. lightning arrestor or surge arrester are provided at the end of the transmission line to withstand the surge voltage.
4. ADVANTAGES OF GIS OVER CONVENTIONAL AIR INSULATED SUBSTATION(AIS) The application of GIS during the last fifteen years has been very rapid. The rapid growth in GIS application is due to the following special advantages: Area and volume saving in construction for over or underground applications. Therefore they offer saving in land area and construction costs.
Insensitivity to external influences because of grounded metal enclosures. Greatly improved safety and reliability due to earthed metal housing of all high voltage parts and much higher intrinsic strength of SF6 gas as insulation. Short on site erection times, based on large factory assembled and tested shipping units. Fulfillment of aesthetic requirements with indoor applications High service reliability due to non-exposure of the use of high voltage parts to atmosphere influences Reduction in radio interference with the use of earthed metal enclosures. Use as mobile substations for transportation to load centers on standard tracks. These substations can be located closer to load centers thereby reducing transmission losses and expenditure in the distribution network. More optimal life cycle costs because of lesser maintenance, down time and repair costs. It is not necessary that high voltage or extra high voltage switchgear has to be installed out doors. 5. DISADVANTAGES OF GIS Although GIS has been in operation for several years, a lot of problems encountered in practice need fuller understanding. Some of the problems being studied are: Excessive damage in case of internal fault. Long outage periods as the repair of damaged part at site may be difficult. Requirement of cleanliness is very stringent. Dust or moisture can cause internal flashovers. Such substations are generally indoor, so they need a separate building. This is generally not required for conventional outdoor substations.
Procurement of gas and supply of gas to site is problematic, adequate stock of gas must be maintained. Project needs almost total imports including SF6 Gas. Spares conventional substation is totally indigenous up to 400 kV.
6. APPLICATION OF GAS INSULATED SUBSTATION The primary applications for gas insulated substations include: High voltage installations 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. Urban Installations GIS technology can be used for installations in areas where the cost of real estate or aesthetic appeal is a significant consideration. Indoor Installations Building an air insulated substation indoors is usually impractical, but a GIS can easily go inside buildings. Environmentally Sensitive Installations GIS technology is popular in desert and arctic areas because it can be enclosed in a building with environmental control. Gas insulated substations also contain the electrical components within a Faraday cage and are therefore totally shielded from lightning.
7. CONCLUSION Gas insulated Substations have found a broad range applications in power systems over the last three decades because of their high reliability Easy maintenance, small ground space requirements etc.,. In our country also few GIS units have been in operation and a large number of units are under various stages of installation. GIS are some important areas to be studied include more conservative designs better particle control and improved gas handling and decomposition product management techniques Achieving and maintaining high levels of availability requires a more integrated approach to quality control by both users and manufactures.
8. REFERENCE 1. “GAS INSULATED SUBSTATION ” by P.S NAIDU ,
I K International Publishing House Pvt. Ltd (8 September 2008). 2. P Glaubitz, C Wallner - EPA's 2012 Workshop on SF6emission reduction …, 2012 - epa.gov. 3. http://electrical4u.com/gis-or-gas-insulated-switchgear 4. http://electrical4u.com/sulfur-hexafluoride-sf6-gasproperties/ 5. www.alstom.com/.../GIS%20Global %20Offer/F35%2072.5%20-%20145.. 6. http://betaengineering.com/enus/areasofexpertise/gasinsulatedsubstations.aspx
8.1 IEEE REFERENCES 1. Bolin, P.; Koch, H., "Gas insulated substation GIS," Power Engineering Society General Meeting, 2006. IEEE , vol., no., pp.3 pp.,, 0-0 0 doi: 10.1109/PES.2006.1709113 U.R.L - http://ieeexplore.ieee.org/stamp/stamp.jsp? tp=&arnumber=1709113&isnumber=36065 2. Meinecke, H., "High voltage gas insulated switchgear: an overview," GIS (Gas-Insulated Switchgear) at Transmission and Distribution Voltages, IEE Colloquium on (Digest No.1995/203) , vol., no., pp.3/1,3/8, 14 Nov 1995 http://ieeexplore.ieee.org/stamp/stamp.jsp? tp=&arnumber=494815&isnumber=10737