GIT

GAS INSULATED TRANSFORMER

1. INTRODUCTION: A transformer is a device that transforms electrical energy from one electrical circuit to another electrical circuit through the medium of magnetic field and without a change in the frequency. Thus transformer is an electromechanical energy conversion device. Transformer is the most widely used device in both low and high current circuits. Low power transformers are air cooled whereas large power transformers are immersed in oil for better cooling. In oil cooled transformers, the oil serves as a coolant and also as an insulating medium. In a gas insulated transformer, gas is used as an insulating as well as cooling agent. SF6 is an important gas used in these types of transformer. The main advantage of gas insulated transformers is that they are environment friendly in nature and can be recycled easily. These gases are non-flammable in nature and thus are able to remove fire fighting equipment from the transformer room. And thus the liquid or oil purifying process is not used in the gas insulated transformers, the substation space is also minimized, gas abolishes the risk of oil leakage. Conventional substations requires, small installation size, protection against atmospherics pollution and moisture, noiseless operation, non-explosive and flame resistant, reduced maintenance, minimal radio interference, but totally enclosed substations using SF6 gas as insulation that are also known as GIS is now in widespread use in the electrical power industry . The first delivery in Japan of SF6 gas-insulated transformers in 1967 (four units of 66 kv - 3000 kva), Toshiba predicted immense future prospects for this type of transformer that utilizes the features of SF6 gas known for its non flammability and coordinative ability with environmental conditions. Since then, coupled with incessantly continued research and development activities, Toshiba has accumulated abundant experience in delivering large-capacity gas insulated transformers-for example, two units of 77kv -20 MVA. SF6 gas-insulated transformers are characterized by their outstanding non flammability, insulation effects, and safety. On the other hand, SF6 gas-insulated switchgear incorporates the most advanced SF6 gas application techniques. Fully gas-insulated substations, adopting a combination of these transformers and switchgear, offer extra ease for safety assurance, accident prevention, and inspection/ maintenance. Concurrently, substations of this type are ideal for applications involving the needs for prevention of environment pollution, resistance to surrounding conditions, or compactness of installation spaces. Such substations are expected to lead the future trend. MEPPI's Gas Insulated transformers are designed to reduce fire hazards and have less impact on the environment SF6 gas insulated type transformers are the most clean and safe to operate.

CoreThe core is made of high-grade, cold-rolled, grain-oriented, silicon-steel sheets that have been given an inorganic-film treatment to help the core maintain its excellent characteristics. Making full use of the superior characteristics of grainDept. Of Electrical Engineering

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oriented silicon-steel sheets, the core slots adopt the frame construction. The core legs are tightened firmly to avoid characteristics degradation due to loosening. In large-capacity transformers having a core of large sectional area, cooling ducts are provided top re vent overheating in the core.

CoilMEPPI uses a high series-capacitance (hisercap) winding for the high-voltage coil. This coil has a special turn arrangement which significantly increases the series capacitance of the coil.

InsulationMEPPI SF6 gas-insulated transformers have a coil construction highly resistant to impulse voltage and are provided with insulating barriers and static shields that moderate and make uniform the electric-field strength.

2. HISTORY: Since the first delivery of SF6 gas-insulated transformers in 1967, TOSHIBA predicted immense future prospects for this type of transformer that utilizes the features of SF6 gas known for its non-flammability and coordinative ability with environmental conditions. Since then, coupled with incessantly continued research and development activities, TOSHIBA has accumulated abundant experience in delivering large-capacity gas-insulated transformers. SF6 gas-insulated transformers are characterized by their outstanding non-flammability, insulation effects and safety. Additionally,SF6 gas-insulated switchgear incorporates the most advanced SF6 gas application techniques. Fully gas-insulated substations, adopting a combination of these transformers and switchgear, offer extra ease for safety assurance, accident prevention and ease of inspection/maintenance. Concurrently substations of this type are ideal for applications involving the needs for prevention of environment pollution, or compactness of installation spaces. Such substations are expected to lead the future trend. Dept. Of Electrical Engineering

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3. 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.

4. FEATURES OF A GAS INSULATED TRANSFORMER: Toshiba's SF6 gas-insulated transformers offer excellent insulation/cooling characteristics and thermal stability. Additionally, these transformers possess the following features resulting from containing the active parts in a tank sealed with non flammable, harmless, and odourless SF6 gas.  High-level stabilityEven should the actual transformer develop an accident, or should a fire break out on the installation environment, combustion or an explosion will not occur. Since all live parts are housed in grounded metal cases, maintenance and inspection can be achieved easily and safely.  Outstanding accident preventive characteristicsNon flammable structure employing no insulation oil contributes to minimizing the scope of associated accident-preventive facilities such as fireproof walls, firefighting equipment, or oil tanks.  Compactness of substationBy directly coupling with gas-insulated switchgear, substation space can be minimized as the result of compact facilities.  Simplified maintenance and long service lifeBecause the transformers are completely sealed in housing cases, no contact exists with exterior atmospheric air, thereby eliminating problems of degradation or contamination triggered by moisture or dust accumulation. Constant enveloping of components with inactive, dry SF6 gas results in minimizing aging deterioration of insulating materials and prolonging transformer service life.

Dept. Of Electrical Engineering

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 Easy, clean installationSF6 gas can be quickly sealed into the transformer tank from a cylinder. Installation work never contaminates surrounding areas, and ensures maintenance of a clean environment.

 Ideal for high voltage systemsBy increasing the seal pressure, SF6 gas transformers offer insulation performance comparable to that of oil-insulated types, being ideal for high voltages of 22 kV to 154 kV.

5. SF6-AN OVERVIEW: SF6 is the main gas commercially used in gas insulated transformer. SF6 gas is used instead of conventional oil both as an insulating oil and as an cooling agent.  SF6 is Sulphur hexafluoride formed by chemical reaction of molten sulphur and fluorine.  It is colourless, odourless, non flammable, non-corrosive and chemically stable.  High dielectric strength.  Quenches arc effectively and has excellent cooling properties.  Good thermal conductivity and excellent thermal stability.  It’s not poisonous and eco-friendly.  It can be recycled. 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 transform. 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 coo linger have been developed. 5.1 IntroductionSF6 is a combination of sulphur and fluorine its first synthesis was realized in 1900 by French researchers of the Pharmaceutical Faculty of Paris. It was used for the first time as insulating material, In the United States about 1935. In 1953, the


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Americans discovered its properties for extinguishing the electric arc. This aptitude is quite remarkable.

.

5.2. Physical propertiesIt is about five times heavier than air, and has a density of 6.1 4kg / m3. It is colourless, odourless, and non-toxic. Tests have been carried out replacing the nitrogen content of air by SF6 (the gaseous mixture consisted of 79 % SF6 and 24 % oxygen): five mice were then immersed in this atmosphere for 24 hours, without feeling any ill effects. It is a gas which the speed of sound propagation is about three times less than in air, at atmospheric pressure. The interruption of the arc will therefore be less loud in SF6 than in air. The dielectric strength of SF6 in on average 2.5 times that of air, and, by increasing pressure, it can be seen that the dielectric strength also increases and then around 3.5 bar of relative pressure, SF6 has the same strength as fresh oil.

 The principal characteristics of the gas are as follows: Molar mass

146.078

Critical temperature

45.55°C

Critical pressure

37.59 bars

In short, SF6 at atmospheric pressure is a heavier gas than air, it becomes liquid at - 63.2°C and inwhich noise propagates badly.

 SF6 on the market: Dept. Of Electrical Engineering

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SF6 which is delivered in cylinders in liquid phase, contains impurities (within limits imposed by IEC standards No. 376). Carbon tetra fluoride

(CF4) 0.03 %

Oxygen + nitrogen

(air) 0.03

Water

15 ppm

CO2

traces

HF

0.3 ppm

SF6 is therefore

99.99 % pure

5.3. Chemical propertiesSF6 is a synthetic gas which is obtained as we have just explained by combination of six atoms of fluorine with one atom of sulphur:

You can see therefore that this reaction is accompanied by an important release of heat. This approximately similar to coal combustion. Given that the energy released during synthesis is the same as is needed in order to dissociate the final element, it can immediately be seen that: -

SF6 is a stable gas.

-

524 k. calories are necessary for molecular breakdown, we can therefore already expect that it will be a powerful cooling agent:

 The dissociation products before interruption of the arcDept. Of Electrical Engineering

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At normal temperature, the gas is stable, and does not react with its environment. In contact with the parts where electric currents circulate, the gas is heated to temperatures of around four hundred degrees SF6 gives the following decomposition products: Thionyl fluoride

SOF2

Sulphur fluoride

SO2F2

Sulphur tetra fluoride

SF4

Sulphur daca fluoride

S2F10

Thionyl tetra fluoride

SOF4

SF6 also reacts with the materials that are found in its environment: With water (impurity in the gas),it gives hydrofluoric acid HF, With air dioxide (impurity in the gas), it gives sulphur dioxide SO2,With carbon dioxide (impurity in the gas), it gives carbon tetra fluoride CF4, With the araldite casings which are high in silicon dioxide, it gives silicon tetra fluoride SF4.  The dissociation products after interruption of an arcAn electric are develops high temperatures which can reach 15000°C.At these temperatures, many dissociation products that we have previously studied disappear. It is thus that, besides the impurities of the gas (water, air, carbon, and dioxide), there only remain: Sulphur fluoride

SO2F2

Carbon tetra fluoride

CF4

Silicon tetra fluoride

SIF4

Sulphurous anhydride

SO2

You can therefore see that a large number of products have been dissociated by the electric arc. The importance of the remaining products may be lessened by adding a powder (alumina silicate). All these gases are heavier than air, and May, under certain conditions is poisonous. 5.4. SF6 Safety precautionsDept. Of Electrical Engineering

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Today there is no known dielectric and breaking agent combined better than SF6 gas.  Initial stateIn its initial state, before it has undergone thermal stress (usually the electric arc); SF6 is perfectly safe in normal conditions: -It is non-toxic. - It is un inflammable. - It will not explode. This does not mean that no precautions need to be taken: because of its lack of oxygen, this gas will not support life. However, the concentration of SF6 would have to be high, since the International electro technical Commission (IEC) has shown that five mice left for 24 hours in an atmosphere of 79 % SF6 and 21 % oxygen will not only remain alive but will show no signs of abnormal behaviour. Man dies when the oxygen level of the gas he is breathing falls below 12 %.  Precautions and hygieneThe first recommendation is not to smoke when SF6 gas is around. The heat given off by the cigarette may decompose the gas. Your cigarette would then take on a very strange taste also avoid operating combustion engines in this gas. When the work positions are indoors, have ventilation and or a system for detecting this halogen placed at the lowest points of the installations. Remember thatSF6 is a very heavy gas. This device will warn you any gas leaks.  Post-breaking stateAs we seen at the beginning of this Chapter, the heat from the arc modifies the SF6.This creates gaseous and solid decomposition products. It is these products that need to be spoken about. Certain of these gases are medically defined as being violent irritants of the mucous membranes and of the lungs. In extreme cases, they may cause pulmonary enema. The solid decomposition products (whitish powder) an aggressive when the react with the humidity of the mucous membranes and of the hands.


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Following this rather unpleasant description of the SF6 after breaking we may reassure ourselves on two counts: - For reasons of quantity. - For reasons of probability.

 Quantity-

The volume of decomposed is microscopic. This means that dangerous thresholds are rarely reached, thanks in part to the molecular sieve which regenerates the decomposition products to form pureSF6. This sieve is present in all extinguishing chambers. Regeneration time is short, but depends on the number of ampere being broken. The presence of hydrogen sulphide, noticeable through its sickening smell, makes an excellent alarm signal. The smell detection threshold is ten times lower than the toxicthres hold.

 Probability-

In normal operation, electric Switchgear using SF6 has a leak rate guaranteed to be less than 1 % of the mass per year. This makes any danger impossible in normal operation The abnormal situation is the risk of an appliance exploding. This is fortunately extremely infrequent. And if by chance such an incident accrued, the putrid smell would make us aware of it immediately.

 Precaution and hygiene-

If you were to find yourself in contact with decomposed SF6 gas, you must leave your post and ensure that the gas is eliminated by means of powerful ventilation. Once the polluted gas has disappeared (when the smell becomes bearable) you are still in contact with solid decomposition products. Operations on the equipment must be carried out with a gas mask, gloves and appropriate clothing. All this - together with the powders themselves - shall be sent to a factory for dealing with dangerous products. Any damage to the hands caused by these powders can be neutralized by limewater.


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6. LAYOUT: 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 sulphur hexafluoride gas (SF6). The compactness is with the use of SF6 gas, which has high dielectric strength. The voltage withstand capability of SF6Busduct 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.

6.1.

The actual layout-


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 Layout example of gas insulated substation-

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. SF6 gas-insulated transformers are characterized by their outstanding non-flammability, insulation effects and safety. Concurrently, substations of this type are ideal for applications involving the needs for prevention of environment pollution, or compactness of installation spaces. Such substations are expected to lead the future trend.

(Layout of a GIT in a GIS)


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7. COOLING: To place the cooling problem in perspective, the accepted rule of thumb is that the life expectancy of insulation in all electric machines including all transformers is halved for about every 7°C to 10°C increase in operating temperature, this life expectancy halving rule holding more narrowly when the increase is between about 7°C to 8°C in the case of transformer winding cellulose insulation. 7.1.

Cooling apparatus-

Employs a cooling medium in liquid phase such as a Refrigerant R113 otherwise known as tirfluorotrichloroethane or C2F3Cl3.Thus SF6 is used as a second cooling medium.


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7.2.

Cooling type-

Different types of cooling processes can be classified as:

1. Forced gas circulated natural-air cooled type 2. Forced gas circulated forced-air cooled type 3. Natural-cooled type


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1. Natural-cooled type:


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2. Forced gas circulated natural-air cooled type-


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3. Forced gas circulated forced-air cooled type-


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8. ACCESSORIES: Toshiba's SF6 gas-insulated transformers are manufactured under thorough going quality control systems. To enable users to fully utilize these transformers, the following protective/monitoring devices are provided as standard accessories. Table 2 lists these standard accessories.  Dial thermometer  Pressure gauge  Temperature compensating pressure switch 8.1.

SF6 gas temperature indicator (Dial thermometer)-

Measures temperature of SF6 gas sealed in transformer tanks. Gas is measured by the heat sensing probe of a thermometer inserted into the protective cylinder provided in the tank or on the cover. Since this protective cylinder maintains air tightness of the gas, the temperature indicator itself can be removed. The temperature indicator is provided with alarm contacts and a pointer for indicating maximum temperature.


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8.2.

SF6 gas pressure gauge (compound gauge)-

This gauge is used to measure the pressure of SF6 gas sealed in the transformer tank. The gauge is a compound type that measures both positive and negative pressure, capable of measuring the positive pressure up to 3.0 kg/cm 2 and the negative pressure up to 760 mmHg. Generally, only the positive pressure is indicated during operation. Since vacuum suction is conducted when sealing SF6 into the tank, the graduations for negative pressure are provided for use during this gas sealing. The pressure gauge is provided with alarm contacts that actuate at the upper limit of normal pressure during operation.

8.3.

Temperature compensating pressure switch-

Leakage is detected of SF6 gas sealed in the transformer tank. Pressure in the transformer tank is compared with pressure in the reference pressure chamber inserted into the protective cylinder provided in the tank or on the cover. Therefore, regardless of temperatures in the transformer, SF6gas leakage is accurately detected and the alarm contacts are actuated.


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9.INTERNAL STRUCTURE TRANSFORMER:

OF

GAS

INSULATED

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 viewpoint of accident prevention and compactness of equipment. In line with this requirement, several types of large-capacity gas insulated transformer have been developed.


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10.COMPARISON WITH OIL-IMPREGNATED TRANSFORMERS:


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11. ADVANTAGE OF GAS INSULATED TRANSFORMER:

 Non-flammabilityGas insulated transformers, using incombustible SF6 gas as an insulating and cooling medium, enable to remove fire fighting equipment from transformer room.

 Tank-explosion PreventionPressure tank enables to withstand the pressure rise in case of internal fault.

 CompactnessBy directly coupling with gas-insulated switchgear, substation space can be minimized as the result of compact facilities. Thus it acquires less space area.

 Easy installationOil or liquid purifying process is not necessary in case of gas insulated transformer making the installation process easier.

 Easy inspection and maintenance workOnly SF6 gas pressure shall be basically monitored during periodically inspection.

 Environmentally FriendlyThe use of SF6 gas abolishes the risk of oil leakage. And also these gases can be recycled easily.

 ReliabilityExtensive 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.


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

 MaintenanceVirtual 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.

 EconomicalInitial 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.

 EnvironmentalMinimized operation noise levels allow installation in urban and sub-urban indoor substations. Elimination of radio interference problems and individual painting of enclosures with the colour of customer choice.

 Standard capacity application range  Quality specification-

The following specifications are provided to ensure safe operation of gasinsulated transformers.  Withstand voltage during zero gas gauge pressureNo problem is caused by operation under normal operating voltage.  Permissible load under zero gas gauge pressureNo problem is caused by 50% load continuous operation.  Permissible load under 1-series operation when 2-series coolers are providedNo problem is caused by 75% load continuous operation.


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12. DISADVANTAGES OF GAS INSULATED TRANSFORMER: 

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 is 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 over’s 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.

13. APPLICATIONS OF GAS INSULATED TRANSFORMER:  Hospitals, underground shopping centres, substations where a high level of fire safety is required.  Water treatment plants and hydroelectric power plants where waterways must be kept unpolluted.  Thermal power plants, chemical plants, public works facilities etc where high reliability is required.  Locations where safety against fire is essential Buildings such as hotels, department stores, schools, and hospitals Underground shopping areas, underground substations Sites close to residential areas, factories, chemical plants.


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 Locations where prevention of environment pollution is specifically demanded Water supply source zones, residential quarters, seaside areas Water treatment stations.  Locations where exposure exists to high-level moisture or dust accumulation inside tunnels, industrial zone.

(On-site installation status)


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14. CONCLUSION:  Incessant research and continuous developments have made construction of large capacity gas insulated transformer possible.  With the eco-friendly aspects of SF6 gas and considering the huge advantages of the gas, long strides can be made in this regard to further utilize it and increase the efficiency of the transformer and making the power system more reliable and safer.  GIT 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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15. REFERENCE: I develop my seminar report on “GAS INSULATED TRANSFORMER” by using following web sites.

Web Sites:   

www.scribd.com www.wikipedia.com http://www.meppi.com/Products/Transformers/Pages/SF6Gas.aspx http://www.deccanherald.com/content/59128/states-first-gas-insulatedtransformer.html  http://www.toshiba-tds.com/tandd/products/trans/en/gitrans.htm


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GIT

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