Gas Insulated Substation Report

GAS INSULATED SUBSTATION

INDEX

PAGE NO.

I.INTRODUCTION

2

2 NEEDS OF GIS

5

3.SF6 CIRCUIT BREAKER

8

4.ELECTRICAL CONNECTION DIAGRAM

15

5.CURRENT TRANSFORMER

16

6.GAS INSULATED TRANSFORMER

19

7.ADVANTAGES OF GAS INSULATED TRANSFORMER

20

8.INTER-CONNECTION TRANSFORMER

21

9.DISCONNECTOR AND EARTHING SWITCHES

22

10.INTERNAL STRUCTURE OF GAS INSULATED TRANSFORMER

31

11.V-I SENSOR CURRENT &VOLTAGE MEASUREMENT

33

12.SURGE ARRESTER CVT –WAVE TRAP

34

13.ADVANTAGES OF GIS

35

14.DISADVANTAGES OF GIS

37

15.CONCLUSION

39

16.REFERENCE

40

MARUDHAR ENGINEERING COLLEGE

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INTRODUCTION: Gas Insulated Substations are high voltage Substations that are compact, requiring little maintenance when compared to air-insulated conventional Substations. Compressed Gas Insulated Substations (CGIS) consist basically a conductor supported on insulators inside an enclosure which is filled with sulfur hexafluoride gas (SF6). The compactness is with the use of SF6 gas, which has high dielectric strength. The voltage withstand capability of SF6 Busduct is strongly dependent on field perturbations, such as those caused by conductor surface imperfections and by conducting particle contaminants. The contaminants can be produced by abrasion between components during assembly or operations. Electrical insulation performance of compressed gas insulated Substation is adversely affected by metallic particle contaminants. Free conducting particles, depending upon their shape, size and location, may lead to serious deterioration of the dielectric strength of the system and also one of the major factors causing breakdown of the system and leading to power disruption. These particles can either be free to move in the Gas Insulated Busduct (GIB) or they may be stuck either to an energized electrode or to an enclosure surface. The presence of contamination can therefore be a problem with gas insulated substations operating at high fields. If a metallic particle crosses the gap and comes into contact with the inner electrode or if a metallic particle adheres to the inner conductor, the particle will act as a protrusion on the surface of the ii electrode. Consequently, voltage required for breakdown of the GIS will be significantly decreased. Several methods have been used to reduce the effect of conducting particles, including electrostatic trapping, use of adhesive coatings, and discharging of conducting particles through radiation. Dielectric coating of a metallic electrode surface affects the particle charge mechanism.

The charge acquired by a particle, the equation of motion, the bounce and the drag are discussed by several authors. The present work makes use of the equation proposed by H. Anis, K.D.Srivastava and M.M.Morcos, it also includes the concept of random motion along


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axial direction. The random motion is due to the cross sectional irregularities of the metallic particles.

Present work analyses the movement pattern of metallic particles in Gas Insulated Substation (GIS) or Gas Insulated Busduct (GIB). In order to determine the particle trajectories in a GIB, an inner diameter of 55 mm and outer enclosure diameter of 152 mm were considered. Aluminum, Copper and Silver particles of 0.2 mm/12 mm (diameter/length) were considered to be present on the enclosure surface. The motion of the metallic particle was simulated using the charge acquired by the particle, the gravitational force on the particle, field intensity at the particle location, drag force, gas pressure, restitution co-efficient and the Reynold‟s number. The distance traveled by the particle, calculated using the appropriate equations, is found to be in good agreement with the published work for a given set of parameters. The results are also presented for other set of parameters.

In order to determine the random behavior of moving particles, the calculation of movement in axial and radial directions was carried out by Monte-Carlo technique. Typically for Aluminum particle for a given Busduct voltage of 100 kV RMS, the movement of the particle (0.25 mm/12 mm) for 1.5 s was computed to be 30.839 mm in radial and 841.12 mm in axial directions. Similar calculations are also extended for other types of voltages. Typical results for aluminum, copper and silver particles are presented in this thesis.

The effect of various parameters like radii and length of particles, co-efficient of restitution, pressure in the Busduct and the applied voltage has been examined and presented. Different metallic contaminants viz., Al, Cu and Ag have been considered for the above study. Typically a GIB of 55mm/152mm (inner conductor diameter is 55mm and outer enclosure diameter is 152mm) has been considered for a 132 kV system.

The thesis presents the movement pattern of metallic particles at different operating voltages in a Gas insulated Busduct (GIB) which has been simulated with and without enclosure coating. The purpose of dielectric coating is to improve the insulation performance. Free conducting particles situated inside the GIS enclosure decrease high local fields caused by conductor roughness. The coating reduces the charge on the particle colliding with the coated enclosure, which in turn reduces the risk of breakdown due to increase of the lift-off field of particles. The movement of a particle has been carried out not only by its electric field effect MARUDHAR ENGINEERING COLLEGE

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on the particle but also considers electromagnetic field and image charge effects. The simulated results are presented at Power frequency, lightning/switching impulse superimposed on power frequency, triangular, square and asymmetric voltages. Particle trajectories obtained for various voltages of aluminum, copper and silver particles are presented and duly discussed. 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


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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:-

i.

Large cities and towns

ii.

Under ground stations

iii.

Highly polluted and saline environment Indoor GIS occupies very little space

iv.

Substations and power stations located Off shore Mountains and valley regions


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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.


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SF6 Circuit Breaker:


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Sulfur Hexafluoride (SF6) is an excellent gaseous dielectric for high voltage power applications. It has been used extensively in high voltage circuit breakers and other switchgears employed by the power industry. Applications for SF6 include gas insulated transmission lines and'gas insulated power distributions. The combined electrical, physical, chemical and thermal properties offer many advantages when used in power switchgears. Some of the outstanding properties of SF6 making it desirable to use in power applications are: V High dielectric strength V Unique arc-quenching ability V Excellent thermal stability V Good thermal conductivity

General Information: Elimsan SF6 circuit breaker is equipped with separated poles each having its own gas. In all types of the circuit breakers, gas pressure is 2 bars (absolute 3 bars). Even if the pressure drops to I bar, there will not be any change in the breaking properties of the circuit breaker due to the superior features of SF6 and Elimsan's high safety factor for the poles. During arcing, the circuit breaker maintains a relatively low pressure (max 5-6 bars) inside the chamber and there will be no danger of explosion and spilling of the gas around. Any leakage from the chamber will not create a problem since SF6 can undergo considerable decomposition, in which some of toxic products may stay inside the chamber in the form of white dust. If the poles are dismantled for maintenance, it needs special attention during removal of the parts of the pole. This type of maintenance should be carried out only by the experts of the manufacturer. (According to ELIMSAN Arcing Products and Safety Instruction for Working on SF6 Circuit Breakers)

Operation of Circuit Breaker: In general, the circuit breakers consist of two main parts, the poles and the mechanism. The poles consist of contact and arc-extinguishing devices. The mechanism is the part to open or close the contacts in the poles at the same time instantaneously (with max. 5 milisec. Tolerance). The closing and opening procedures are performed through springs which are charged by a servomotor and a driving lever. In the system, the closing springs are first charged. If "close" button is pressed the opening springs get charged while the contacts get


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closed. Thus, circuit breaker will be ready for opening. The mechanical operating cycle of the circuit breaker is (OPEN-3 Min CLOSE/OPEN-3 Min- CLOSE/OPEN) or (OPEN-0.3 secCLOSE/OPEN-3 Min CLOSE/OPEN). The second cycle is valid when the circuit breaker is used with re-closing relay. In that case, after the closing operation, the closing springs are charged by the driving lever or by driving motor (if equipped). Thus, the circuit breaker will be ready for opening and re-closing.

Mechanical Life and Maintenance of The Mechanism: Elimsan breaker mechanism can perform 10.000 opening-closing operations without changing any component. The mechanical life of the circuit breaker is minimum lO'.OOO operations. However, it needs a periodical maintenance depending on its environment. In ideal working conditions, lubrication once a year or after every 1000 operations is sufficient. In dusty and damp environment, the mechanism should be lubricated once every 3 - 6 months or after every 250 - 500 operations. Thin machine oil and grease with molybdenum must be used for lubricating. Owing to mechanism's capability of operating between -5°C and + 40 °C, it does not require a heater.

Auxiliary Switch: The auxiliary switch mounted on the circuit breaker has 12 contacts. One of them is for antipumping circuit, four of them are allocated for opening and closing coils. The remaining 7 contacts are spare. Three of them are normally opened and four are normally closed. When it is necessary, the number of the contacts can be increased.

Rapid Automatic Reclosing: The circuit breaker which opens due to a short circuit failure, can be re-closed automatically after a pre selected time by arc closing relay, assuming the fault is temporary. Thus, we avoid long time power loss in case of temporary short circuits. But, if the fault lasts after re-closure, the protection relay will trip to open the circuit breaker again.

What to Specify on The Order: 1- Rated voltage of the circuit breaker 2- Rated current of the circuit breaker 3- Rated short circuit breaking current MARUDHAR ENGINEERING COLLEGE

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4- Voltages of opening and closing coils 5- Motor supply voltage (if equipped)

Closing and Opening Operation Of the Circuit Breaker: When manual or motor-drive is used, the circuit breaker will be ready to close. The closure can be actuated pressing the closing button located on the circuit breaker. It is recommended to close it using remote control system for secure operations. The opening can be performed either by opening button or remote controlled opening coil. In case of a fault, the relay signal actuates the opening coil and circuit breaker opens. (This is mechanically a primary protection system). In addition, there is an anti-pumping relay for preventing the re-closing and opening of the circuit breaker more than one cycle (O - C - O) and for preventing possible troubles created by remote closing button.

Commissioning: The outer surfaces of epoxy insulating tubes of the poles are to be wiped out with a clean and dry cloth. The wiring and connections of the auxiliary circuit are to be carefully examined. DC voltage should be checked to see whether it is suitable for coil and motor or not (if equipped). The opening-closing coils are to be operated 15-20 times and the accuracy of the MARUDHAR ENGINEERING COLLEGE

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relay circuit is to be checked before energizing the circuit breaker. The circuit breaker is to be mounted with two MI2 bolts through its anchoring shoes. It should not move during operation. No excessive load should be exerted to the poles and if possible flexible cables should be used. The incoming and outgoing contacts must have clean surfaces and their contact resistance should be as low as possible. When connecting the circuit breaker to protection system and auxiliary supply, the cable cross sections should be according to the table given. The circuit breaker must be grounded through at least 16 mm steel tape (by cable shoe). After all, the following procedure must be performed: 1. Open the isolator of circuit breaker, 2. Prepare the circuit breaker for closing operation by driving mechanism, 3. Close the isolator of circuit breaker firmly, 4. Send the closing signal to the circuit breaker,

The Maintenance Of Circuit Breaker During Operation: Normally, at least once a year or after every 500 operations, the circuit breaker must be maintained. During maintenance, the moving parts of the mechanism must be lubricated carefully. The insulating parts are to be wiped out by a clean and dry cloth. When maintaining, the circuit breaker should be open and high voltage sides must be grounded. Auxiliary power supply should also be disconnected. On saline areas near seaside, the insulating parts of the circuit breaker must be carefully cleaned, at least once every two months. If not, the microscopic salt particles drawn by wind from the sea will create conductive layers on the insulating surfaces and may cause surface flashover. Before maintenance, first circuit breaker, then isolator should be opened and grounded carefully. The maintenance of circuit breaker must be done after checking the open position of isolator contacts by eye.


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MAIN DIMENSIONS (IN mm):


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ELECTRICAL CONNECTION DIAGRAM:


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Current tansformer: Current transformers are used in electrical grids for measurement and protective applications to provide signals to equipment such as meters and protective relays by stepping down the current of that system to measurable values. Their role in electrical systems is of primary importance because the data sent by current transformers represent the heartbeat of the entire system. RHM International‟s proprietary dry type Current Transformers are unique as they provide a rugged, reliable option for high voltage metering and protection operations up to 550 kV.

Our high quality Current Transformers are based on a unique U-shaped bushing design for the primary winding. The bushing is a very fine capacitance graded insulator. In a new approach to dry designs, our advanced proprietary HV DryShieldTM composite insulation system uses reliable materials like PTFE (Polytetrafluoroethylene) and silicon rubber to provide a low stress, uniform field distribution between the conductor and the outside structure.

The primary and secondary windings are independently sealed resulting in a totally weather resistant design. Oil or gas is not required for insulation. Therefore, our environmentally friendly Current Transformers completely eliminate the risk of explosion and toxic leaks.

Considering the difficulty for our customers to schedule circuit outages for routine upgrades and maintenance, the high cost of maintenance, and environmental issues, RHM International‟s Current Transformers provide exceptionally low cost of ownership and offer you peace of mind as they are truly maintenance free.

Customer benefits: • Totally safe –

no risk of explosion

• Totally maintenance free – exceptional low cost of ownership • Environmentally friendly – no risk of toxic leak and recyclable insulation materials • 2 to 3 times lighter than conventional oil or gas based products • Mature and Innovative with unmatched quality records – not a single failure in 20 000 HV DryShieldTM equipped products in the field MARUDHAR ENGINEERING COLLEGE

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• A new approach to dry design without the limitations of conventional dry Current Transformers • All products are developed and manufactured in an ISO 9001 certified plant and compliant to IEC and IEEE international standards.

Main Characteristics: 1.Performance: Highest voltage for equipment: 40.5-550 kV

Rated frequency: 50, 60 Hz

Rated primary current: 5-5000 A (up to 8000 A under specific conditions – please contact us) Rated secondary current: 5 A, 1 A Rated output: 10-50 VA Accuracy class for measuring current transformer: 0.2S, 0.2, 0.5S and 0.5 class Instrument security factor: 5, 10 Accuracy class for protective current transformer: 5P, 10P and TPY (TPS, TPX and TPZ on demand) Accuracy limit factor for protective current transformer: 5,10, 15, 20, 30, 40 Secondary cores: 1-8

2. Environmental Condition: Places of operation: Indoors and outdoors Environmental temperature: -45°c [-49F] / +45°c [113F] Altitude: <1000m above sea level (when higher than 1000m, specific design available. Please contact us) Pollution level: fully compliant to Class IV environments (IEC category – very heavy pollution level)

3. Type tests and special tests include: • Measurement of capacitance and Dielectric Dissipation under Um/√3 and 10kV, the Dielectric dissipation factor (tanδ) is less than 0.004 • Short-time current tests: Thermal short-time current (Ith): 50 kA, 3s Rated dynamic current (Idyn): 125 kA (peak value) MARUDHAR ENGINEERING COLLEGE

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• Temperature-rise test: 75 K • Mechanical tests (see table 2) • Determination of errors • Insulation

and thermal stability test (36h according to standard but tested 72h for 550kV)

4. Routine tests: Before leaving the factory the following routine tests are carried out: on primary winding: • Power-frequency withstand test • Power-frequency withstand test between sections of primary • Partial discharge measurement • Capacitance and dielectric dissipation factor measurement • Verification of terminal markings

on secondary windings: • Power-frequency

withstand test

• Power-frequency withstand test between sections of secondary windings • Inter-turn over-voltage test • Determination of errors


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GAS INSULATED TRANSFORMER:

Introduction: Since gas insulated transformer does not need the conservator, the height of transformer room can be reduced. In addition, its non-flammability and non tankexplosion characteristics can remove the fire fighting equipment from transformer room. As a result, gas insulated transformer, gas insulated shunt reactor and GIS control panels can be installed in the same room. With such arrangement, a fully SF6 gas insulated substation can be recognized

Specifications and Ratings: Rising demand for electric power in large cities has encouraged large-scale substations to be tucked away underground in overpopulated urban areas, leading to strong demand for incombustible and non-explosive, large-capacity gas insulated transformers from the view point of accident prevention and compactness of equipment. In line with this requirement, several types of large-capacity gas insulated transformer have been developed.

The gas-forced cooling type was considered to be available for up to approximately 60MVA, while all other gas insulated transformer with higher ratings are liquid cooled. But the liquid cooling type has the disadvantage of a complex structure for liquid cooling. Thus, TOSHIBA began development of gas forced cooling type gas insulated transformer, making best use of accumulated experience, latest analyzing technique and the results of innovative research activities. As a result, TOSHIBA has delivered 275kV- 300MVA gas cooled and gas insulated transformer, of which its structure is as simple as the oil immersed type and is the largest capacity gas insulated transformer in the world.

Realization of gas insulated transformer: Since heat capacity of SF6 gas is much smaller than that of insulating oil, the following measures are taken into account. 1. Raise the SF6 gas pressure to 0.5MPa 2. Produce as large flow as possible by optimizing the layout of gas ducts in the windings 3. Develop high capacity gas blower with high reliability 4. Apply highly thermal-resistant insulating materials to raise the limit of winding temperature rises


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Internal structure of gas insulated transformer:

ADVANTAGE OF GAS INSULATED TRANSFORMER: 1. Non-flammability: Gas insulated transformers, using incombustible SF6 gas as a insulation and cooling medium, enable to remove a fire fighting equipment from transformer room. 2. Tank-explosion Prevention: Pressure tank enables to withstand the pressure rise in case of internal fault. 3. Compactness: By directly coupling with gas-insulated switchgear, substation space can be minimized as the result of compact facilities. 4. Easy installation: Oil or liquid purifying process is not necessary in case of gas insulated transformer. 5. Easy inspection and maintenance work:

Only SF6 gas pressure shall be basically

monitored during periodically inspection. 6. Environmentally Friendly: The use of SF6 gas abolishes the risk of oil leakage.


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INTER CONNECTING TRANSFORMER[ICT]:-



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


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DISCONNECTOR AND EARTHING SWITCHES:Disconnectors and earthing switches are used to protect personnel while working on operational equipment and must therefore be very reliable and operationally safe even under adverse climatic conditions. Disconnectors and earthing switches are often offered as a combination of both. Disconnectors have to isolate downstream operational equipment – i.e., de-energised equipment – from the connected circuits. Thus they establish a visible isolating distance in air towards downstream operational equipment. The task of an earthing switch is to earth de-energised parts of the switchgear and – in the case of multi-pole earthing switches – to short-circuit them at the same time.


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Application: Disconnectors and earthing switches are suitable for indoor installations up to 36 kV. Due to their cast-resin ribbed insulators, the disconnectors and earthing switches can also be used with high air humidity and occasional condensation, e.g., in tropical areas.

The devices are protected against corrosion. Steel parts are either galvanised and yellowpassivated, or electrostatically coated with epoxy-resin powder over a phosphate layer.

The switching devices can be installed in any position with horizontal shaft. Designs for installation with the shaft in vertical position are also available.


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Endurance: Normally, disconnectors and earthing switches are operated very rarely. Therefore they are not designed for a high number of operating cycles. The mechanical endurance and the contact endurance are: – 5,000 operating cycles for the disconnector – 1,000 operating cycles for the earthing switch.

Functions of the switching devices: 3DA/3DC disconnectors have the following functions: – Opening or closing circuits when either negligibly small currents have to be switched off/on or when there is no significant voltage difference between the circuits to be disconnected or connected. – Establishing an isolating distance between the terminals of each pole in the open position.


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The task of 3DD/3DE earthing switches is to earth deenergised parts of the switchgear and – in the case of multi-pole earthing switches – to short-circuit them at the same time.

Short-circuit capability: The short-circuit capability of closed disconnectors and earthing switches is tested according to VDE. Due to the loopless circuit, the disconnectors need not be interlocked against being opened by short-circuit forces.

Earthing switches built on disconnectors or used as independent devices have to be interlocked in presence of peak withstand currents above 50 kA if the earthing switch is installed with the peak withstand current flowing through the earthing switch in direction 2 according to the drawing shown on the right. In this direction, strong opening forces are effective.

Sufficient interlocking is guaranteed for motor operating mechanisms as well as for selfblocking manual operating mechanisms (e.g., spherical joint mechanism). For earthing switches built on a disconnector, the mechanical interlock between the disconnector and the earthing switch is a simple means to exclude the disadvantages of the energy direction with opening force effect.

Description

Operating mechanisms

Motor operating mechanism: The motor operating mechanism – provided for disconnectors and earthing switches type 3D – mainly consists of a DC compound-wound motor, degree of protection IP00, which drives the eccentric shaft of a free-wheeling mechanism via a single-step spur gearing. The freewheeling mechanism makes the crank (2) rotate counter-clockwise. The crank is linked with the drive lever through a short drive rod, and the drive lever is connected with the operating shaft (4).


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A 180° turn of the crank produces a switching angle of 90° at the operating shaft. In the end positions of the switching device, the drive motor is de-energised via built-in position switches. If an AC motor operating mechanism is required, a rectifier is installed additionally.

The time from initiation of the command until reaching the end position or arrival of the feedback (total operating time) is 3 s as a maximum at the lowest value of the operating voltage.

Manual operating mechanisms: Instead of being operated by a motor, the operating shaft can also be actuated manually. Operation by means of a switching rod depends on the mounting position and the accessibility. Switching rods are made of glass-fibre reinforced polyester tube and can be MARUDHAR ENGINEERING COLLEGE

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used in switchgear with rated voltages above 1 kV. These switching rods are used to actuate the switching rod lever (available as an accessory) mounted on the operating shaft.

Switching rod levers made of insulating material are always used where the necessary minimum distances are not reached. For fixing in the end positions, an elastic latch is always provided for switching rod actuation (see interlocks).

Interlocks Latch: For disconnectors and earthing switches a latch can be supplied, which latches tight in the end positions in an elastic way. Such a latch must be provided when these switching devices are operated manually with a switching rod.


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Mechanical interlocking: Disconnectors with built-on earthing switch can be equipped with a mechanical interlock if the earthing switch is actuated by means of a switching rod.

Power-operated switching devices must be interlocked with the means belonging to the operating mechanism, i.e., actuation must be prevented. For this purpose, the part without power operating mechanism requires an auxiliary switch.

If neither the disconnector nor the earthing switch are power-operated, mechanical interlocking is possible in connection with an electromechanical lockout. The electromechanical lockout is then mounted on the disconnector.

Electromechanical lockout: Electromechanical lockout devices can be installed on all disconnectors and earthing switches without power operating mechanism. The lockout devices block the switching devices in the end positions when the solenoid is not excited. In the intermediate position (faulty position) the lockout is not effective. The magnet coils are suitable for continuous operation.


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Interlocks for motor operating mechanisms:

Via switchgear interlocking system: For operation in connection with a switchgear interlocking device 8TJ2, a poled relay is required to prevent maloperation. Interlocks on the disconnector can be omitted. For operation in connection with a switchgear interlocking device 8TK, no other auxiliary contactors are required.

Via auxiliary contactor: With an auxiliary contactor (with or without command execution) and pushbuttons, additional protective measures must be taken against impermissible switching operations.

Via changeover switch: The simplest possibility of control is a changeover switch. However, adequate protective measures against impermissible switching operations must be taken here as well.


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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.


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INTERNAL STRUCTURE OF GAS INSULATED TRANSFORMER:

MERITS IF GIT OVER OIT:

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.


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

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: 

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.

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)


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UI SENSOR- CURRENT & VOLTAGE MEASURMENT:



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



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.


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SURGE ARRESTER-CVT-WAVE TRAP: Capacitor Voltage Transformers convert transmission class voltages to standardized low and easily measurable values, used for metering, protection and control of the high voltage system. Additionally, Capacitor Voltage Transformers serve as a coupling capacitor for coupling high frequency power line carrier signals to the transmission line.

Lightning Arresters or Surge Arresters are always connected in Shunt to the equipment to be protected, they provide a low impedance path for the surge current to the ground

Line trap also is known as Wave trap. It traps Hi-frequency communication signals sent on the line from the remote substation and diverting them to the telecom/ tele protection panel in the substation control room (through coupling capacitor 38 and LMU). This is relevant in Power Line Carrier Communication (PLCC) systems for communication among various substations without dependence on the telecom company network. MARUDHAR ENGINEERING COLLEGE

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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.

Special Features:- Enhanced insulating properties and reduced long-term operational costs by means of sealed metal enclosure filled with SF6 gas.

ii.

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

iii.

Compact Design:-Ensured personnel safety by earthed enclosure, numerous interlocks and lockout devices. Increased stability during earthquakes with a low center of gravity.

iv.

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.

v.

Efficient Installation:-Assembly at factory and shipment in one complete bay dramatically reduces installation time and customer's costs.

vi.

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..

vii.

Space Saving:- SF6 switchgear installations take up only 10% of the space required for the conventional installations.

viii.

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

ix.

Compatibility:- It is incombustible, non toxic, colourless and chemically inert.


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x.

Low Weight:- due to aluminum enclosure, corresponds to low cost foundations and buildings.

xi.

The dielectric strength of SF6 gas at atmospheric pressure is approximately three times that of air.

xii.

It has arc-quenching properties 3 to 4 times better than air at equal pressure.

xiii.

Ability to interrupt short-line faults

xiv.

Overvoltage is kept to a minimum.

xv.

Minimum gas leakage (less than 0.1% per year).

xvi.

The use of SF6 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: ● 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


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Disadvantages with Air Insulated Substations:a)

It requires huge amount of area .

b)

Cost is higher compared to Air Insulated Substation or conventional substation. Maintenance cost is more.

c)

Normally this type of substations are indoor type and requires separate building. Each and every component of substation is exposed to air and pollution.

d)

Maintaining Cleanliness is very important. Dust or moisture inside the compartment causes the flash overs so frequent flashovers and breakdown occurs.

e)

When fault occurs internally, the outage period will be very long. The damage effect will also be severe.

f)

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

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


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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.


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Conclusion GIS are necessary for EHV&UHV and some important areas to be studied include more conservative designs better particle control&improved gas handling&decomposition product management techniques Achieving&maintaining high levels of availability requires a more integrated approach to quality control by both users and manufactures


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REFERENCE I develop my this seminar report on “ GAS INSULATED SUBSTATION” by using following web sites.

Web Sites: www.wikipedia.com  www.scribd.com

 www.google.com


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Gas Insulated Substation Report

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