A Seminar Report on
“GAS INSULATED SUBSTATION” Submitted in the partial Fulfillment of requirement For the award of the Degree of
Bachelor of Technology In Electrical & Electronics Engineering (UP Technical University)
Under the supervision of Name of Guide
Submitted by Name of Student
Department Department of Electrical & Electronics Engineering IMS Engineering College, Ghaziabad CERTIFICATE
This is to certify that Mr/Ms ………………………………………………… a student of B. Tech (Electrical & Electronics Engineering) …… Semester has submitted His/her seminar report
(EEE-651)
entitled
“………………………………………………………………………………”
Under
my
supervision.
Name of the Guide Designation of the Guide
ACKNOWLEDGEMENT
I am thankful to my supervisor, H.O.D, faculty members of the department, whose encouragement, guidance and support from the initial to the final level enabled me to develop an understanding of the seminar topic.
I offer my regards and blessings to all of my colleges, who supported me in any respect during the completion of the seminar. Lastly, I am thankful to the college for providing excellent infrastructure & lab facilities.
Name of student
INTRODUCTION GAS INSULATED SUBSTATION:- Gas Insulated Substation uses sulfur hexafluoride (SF6) gas which has a superior dielectric properties used at moderate pressure for phase to phase and phase to ground insulation. In Gas Insulated Substation the high-voltage conductors, circuit breaker interrupters, switches, current transformers, voltage transformers and lightning arresters are encapsulated in SF6 gas inside grounded metal enclosures. Gas Insulated Substations (GIS) is a compact, multicomponent assembly enclosed in a ground metallic housing which the primary insulating medium is compressed sulphur hexafluoride (SF6) gas. GIS generally consists components Of
1. Circuit Breakers 2. Operating mechanism of circuit breaker 3. Current transformers 4. Disconnector 5. Maintenance Earthing switches 6. Fast acting Earthing switches 7. Voltage transformers 8. SF6 Bushing 9. Gas supply and gas monitoring equipment 10. Bus Bar 11. Voltage Transformer 12. Gas supply and Monitoring eqipment
Why we need GIS:- Gas Insulated Substations are used where there is space for providing the substation is expensive in large cities and towns. In normal substation the clearances between the phase to phase and phase to ground is very large. Due to this, large space is required for the normal or Air Insulated Substation (AIS). But the dielectric strength of SF6 gas is higher compared to the air, the clearances required for phase to phase and phase to ground for all equipments are quite lower. Hence, the overall size of each equipment and the complete substation is reduced to about 10% of the conventional air insulated substation.
Extremely high dielectric properties of SF6 have long been recognized. Compressed SF6 has been used as an insulating medium as well as arc quenching medium in electrical apparatus in a wide range of voltages. Gas Insulated Substations (GIS) can be used for longer times without any periodical inspections. Conducting contamination (i.e. aluminum, copper and silver particles) could, however, seriously reduce the dielectric strength of gas-insulated system. A metallic particle stuck on an insulator surface in a GIS will also cause a significant reduction of the breakdown voltage. 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... Because of the entire equipment being enclosed in enclosures, filled with pressurized SF6 gas, installation is not subject to environmental pollutions, as experienced along coastal areas or certain types of industries. a) Such installations are preferred in cosmopolitan cities, industrial townships, etc., where cost of land is very high and higher cost of SF6 insulated switchgear is justified by saving due to reduction in floor area requirement. It is not necessary that high voltage or extra high voltage switchgear to be installed out doors. b) Since most of the construction is modular and the assembly is done in the works, one site erection time both for supporting structures and switchgear is greatly reduced.
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Locations where Gas Insulated Substation is preferred:-
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Large cities and towns Under ground stations Highly polluted and saline environment Indoor GIS occupies very little space Substations and power stations located Off shore Mountains and valley regions
Explaination:- The SF6 Gas Insulated Substation (GIS) contains the same compartments as in the conventional outdoor substations. All the live parts are enclosed in metal housings filled with SF6 gas. The live parts are supported on cast resin insulators. Some of the insulators are designed as barriers between neighboring modules such that the gas does not pass through them. The entire installation is sub divided into compartments which are gas tight with respect to each other. Thereby the gas monitoring system of each compartment can be independent and simpler.
The enclosures are of non magnetic materials such as aluminum or stainless steel and are earthed. The gas tightness is provided with static ‘O’ seals placed between the machined flanges. The ‘O’- rings are placed in the grooves such that after assembly, the ‘O’-rings are get squeezed by about 20%. Quality of the materials, dimension of grooves and ‘O’-seals are important to ensure gas tight performance of Gas Insulated Substation.
Gas Insulated Substation has gas monitoring system. Gas inside each compartment should have a pressure of about 3kg/cm2.The gas density in each compartment is monitored. If the pressure drops slightly, the gas is automatically trapped up. With further gas leakage, the low pressure alarm is sounded or automatic tripping or lock-out occurs.
SF6 Circuit Breaker:- A circuit breaker is an automatically-operated electrical switch designed to protect an electrical from damage caused by overload or short circuit.
Its basic function is to detect a fault condition and, by interrupting continuity, to immediately discontinue electrical flow. Unlike a fuse, which operates once and then has to be replaced, a circuit breaker can be reset (either manually or automatically) to resume normal operation. Current interruption in a high-voltage circuit-breaker is obtained by separating two contacts in a medium, such as SF6, having excellent dielectric and arc quenching properties. After contact separation, current is carried through an arc and the arc is interrupted & cooled by a gas blast of sufficient intensity. CURRENT TRANSFORMER Current transformer (CT) is used for measurement of electric currents. Current transformers are also known as instrument transformers. Current transformers are commonly used in metering and protective relays in the electrical power industry.
When current in a circuit is too high to directly apply to measuring instruments, a current transformer produces a reduced current accurately proportional to the current in the circuit, which can be conveniently connected to measuring and recording instruments.
VOLTAGE TRANSFORMER Voltage transformers (VTs), also referred to as “ Potential transformers " (PTs), areused in high-voltage circuits. They are designed to present a negligible load to the supply being measured, to allow protective relay equipment to be operated at lower voltages, and to have a precise winding ratio for accurate metering
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The single pole inductive voltage transformer is connected to switch gear with the connecting flanges with a barrier insulator.
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The primary winding is insulated with SF6 gas & connected to high voltage terminal.
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The primary winding is wounded on the top of the core & secondary windings.
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The secondary winding is connected to the terminals in the external terminal box through a gas tight multiple bushing.
INTER CONNECTING TRANSFORMER[ICT]:-
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An autotransformer is an electrical transformer with only one winding. The winding has at least three electrical connection points called taps. The voltage source and the load are each connected to two taps. One tap at the end of the winding is a common connection to both circuits (source and load). Each tap corresponds to a different source or load voltage. An autotransformer for power applications is typically lighter and less costly than a two-winding transformer, up to a voltage ratio of about 3:1 beyond that range a two-winding transformer is usually more economical. In an autotransformer a portion of the same winding acts as part of both the primary and secondary winding
DISCONNECTOR AND EARTHING SWITCHES:Disconnector and Earthing switches are safety devices used to open or to close a circuit when there is no current through them. They are used to isolate a part of a circuit , a machine, a part of an overhead line or an underground line so that maintenance can be safely conducted. The opening of the line isolator or busbar section isolator is necessary for safety, but not sufficient. Grounding must be conducted at both the upstream and downstream sections of the device under maintenance. This is accomplished by earthing switches.
Disconnect switches are designed to continuously carry load currents and momentarily carry short circuit currents for a specified duration. They are designed for no-load switching , opening , or closing circuits where negligible currents are made or interrupted (including capacitive current and resistive or inductive current , or when there is no significant voltage across the open terminals of the switch. FAST ACTING EARTHING SWITCH:-
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Fast earth switch and maintenance earth switch are the two types of earth switches used for gas insulated sub-station systems.
Fast earth switch is used to protect the circuit-connected instrument voltage transformer from core saturation caused by direct current flowing through its primary as a consequence of charge stored online during isolation / switching off the line.
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Use of fast earth switch provides a parallel (low resistance) path to drain the residual static charge quickly, thereby protecting the instrument voltage transformer from the damages that may otherwise be caused. The basic construction of these earth switches is identical.
Advantages of GIS:- A GIS has many advantages over a conventional substation, including a space saving and flexible design, less field construction work resulting in quicker installation time, reduced maintenance, higher reliability and safety, and excellent seismic withstand characteristics. Aesthetics of GIS are far superior to that of a conventional substation, due to its substantially smaller size.
A GIS can even be installed in a building, if desired. When all of these advantages are taken into consideration, a GIS is a cost effective alternative to a conventional substation that offers many benefits to the end user. i. ii.
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Special Features:- Enhanced insulating properties and reduced long-term operational costs by means of sealed metal enclosure filled with SF6 gas. . Reliability:-Extensive experience in designing optimum phase and feeder spacing dimensions according to site conditions enable compact dimensions that reduce space requirements to less than 20% of conventional air insulated substations Compact Design:-Ensured personnel safety by earthed enclosure, numerous interlocks and lockout devices. Increased stability during earthquakes with a low center of gravity. Maintenance :-Virtual elimination of long-term maintenance costs and contamination of critical components by means of SF6 gas-filled metal enclosures, automatic monitoring of operating mechanisms and SF6 gas system. Efficient Installation:-Assembly at factory and shipment in one complete bay dramatically reduces installation time and customer's costs. Environmental:-Minimized operation noise levels allow installation in urban and sub-urban indoor substations. Elimination of radio interference problems and individual painting of enclosures with the color of customer choice.. Space Saving:- SF6 switchgear installations take up only 10% of the space required for the conventional installations. Economical: - Initial high investment is required for installation but the cost can be comparable for the less maintenance, reliable, safe operation against conventional substation. Ability to interrupt out-of-phase Compatibility:- It is incombustible, non toxic, colourless and chemically inert. Low Weight:- due to aluminum enclosure, corresponds to low cost foundations and buildings. The dielectric strength of SF6 gas at atmospheric pressure is approximately three times that of air. It has arc-quenching properties 3 to 4 times better than air at equal pressure. Ability to interrupt short-line faults
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Overvoltage is kept to a minimum. Minimum gas leakage (less than 0.1% per year). The use of SF 6 gas as an insulating medium in switchgear reduces the clearance distance between active and non-active parts of a switchgear facilitating the following advantages of gas insulated applications compared to air insulated applications:
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Less space requirements - especially in congested city areas Less sensitivity to pollution, as well as salt, sand or even large amounts of snow Less operation & maintenance costs
Disadvantages with Air Insulated Substations:-
a) b) c) d) e) f)
It requires huge amount of area . Cost is higher compared to Air Insulated Substation or conventional substation. Maintenance cost is more. Normally this type of substations are indoor type and requires separate building. Each and every component of substation is exposed to air and pollution. Maintaining Cleanliness is very important. Dust or moisture inside the compartment causes the flash overs so frequent flashovers and breakdown occurs. When fault occurs internally, the outage period will be very long. The damage effect will also be severe. Installation time is also more..
ADVANCEMENT IN GIS TECHNOLOGY:(For Further Space reduction) • • •
Gas Insulated Transformer (GIT) Instead of Oil Immersed Transformer (OIT). SMART GIS - Integration of Electronic CT’s & PT’s Combined Earthing Switch & Disconnector
INTERNAL STRUCTURE OF GAS INSULATED TRANSFORMER
MERITS IF GIT OVER OIT:•
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Nonflammability – Gas insulated transformers , using incombustible SF6 gas as insulation and cooling medium, enable to remove a fire fighting equipment from transformer room. Non Tank – explosion - Pressure tank enables to withstand the pressure rise in case of internal fault. Compactness – Since conservator or pressure relief equipment is not necessary, height of transformer room can be reduced approximately 2 – 2.5 meters. Easy installation – oil or liquid purifying process is not necessary in case of gasinsulated 60 transformer. Easy inspection and maintenance work -Only SF6 gas pressure shall be basically monitored during periodically inspection.
SMART GIS - Integration of Electronic CT’s & PT’s •
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The Combined sensors are the Rogowski coil for current measurement and the capacitive divider for voltage measurement A combined current and voltage sensor has been developed to replace the conventional current and voltage transformers in GIS.
1. Serial Optic Link 2. Voltage sensor 3. Rogowski Coil Why Combined Voltage & current Sensor ?
Advanced CT’s without a magnetic core (Rowgowski coil) & Capacitive sensor have been developed to save space and reduce the cost of GIS. The output signal is at a low level, so it is immediately converted by an enclosure mounted device to a digital signal • • • • • •
Small size - Helps to optimize the use of space in the switchgear Lighter weight means less material usage and lower life cycle costs (LCC) Large dynamic range - permits minimization of number of sensor types needed and improvement of some protection functions. Protection and measurement functions combined. Lower losses mean lower LCC (Life Cycle Cost)
UI SENSOR- CURRENT & VOLTAGE MEASURMENT
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Current Measurement – Current Sensor instead of Current Transformer The current sensor is based on a Rogowski coil (a coreless inductive current transformer). Voltage Measurement – Voltage Sensor instead of Voltage Transformer
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The voltage sensor is based on a capacitive electrical field sensor (Capacitive ring sensor). The capacitive ring, which acts as a voltage sensor, also has a linear characteristic and is very simple in terms of the insulation.
COMBINED DISCONNECTOR/EARTHING SWITCH The DSES incorporates the two functions of a disconnector and a maintenance earthing switch as a result saving the space in GIS. This is achieved by a sliding contact characterized by three defined positions: disconnector open / earthing switch closed • disconnector closed / earthing switch open • disconnector open / earthing switch open • The use of one moving contact for the disconnector and the earthing switch inhibits simultaneous closed position of both switches. Combined disconnector and earthing switch is mounted at the front, and acts via bevel gears and an insulating shaft on the three parallel contact pins. Depending on the direction of movement the contacts act as disconnector or earthing switch (maintenance earthing switch). By means of a crank handle, manual operation of the combined disconnector and earthing switch is also possible.
