repot of gas insulated

8/2/2019 Repot of Gas Insulated

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CHAPTER-1

1.1 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 firefighting

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 oilleakage. Conventional substations requires, small installation size, protection against atmospheric

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 nonflammability 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 gasinsulated transformers-for example, two units of 77kV -

20 MVA. SF6 gas-insulated transformers are characterized by their outstanding nonflammability,

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 installationspaces. Such substations are expected to lead the future trend.


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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.

Core -

The 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 grain-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 to

prevent overheating in the core.

Coil -

MEPPI 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.

Insulation -

MEPPI 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.


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1.2 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 applicationsinvolving the needs for prevention of environment

pollution, or compactness of installation spaces. Such substations are expected to lead the future

trend.


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1.3Realization 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

1.4 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 nonflammable, harmless, and odorless SF6 gas.

1. High-level stability

Even 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.

2. Outstanding accident preventive characteristics

Nonflammable structure employing no insulation oil contributes to minimizing the scope of

associated accident-preventive facilities such as fireproof walls, fire-fighting equipment, or oil tanks.

3. Compactness of substation

By directly coupling with gas-insulated switchgear, substation space can be minimized as the result

of compact facilities.


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4. Simplified maintenance and long service life

Because 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.

5. Easy, clean installation

SF6 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.

6. Ideal for high voltage systems

By 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.


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Chapter-2

2.1 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, nonflammable, non-corrosive and chemically stable.

High dielectric strength.

Quenches arc effectively and has excellent cooling properties.

Good thermal conductivity and excellent thermal stability.

Its 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 largecapacity 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 coolinger

have been developed.


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2.1.1Introduction:-

SF6 is a combination of sulfur 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 Americans discovered its properties for

extinguishing the electric arc. This aptitude is quite remarkable.

Physical properties

It is about five times heavier than air, and has a density of 6.1 4kg / m3. It is colorless, odorless 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 aboutthree 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 than 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 45.55C

Critical pressure 37.59 bars

In short, SF6 at atmospheric pressure is a heavier gas than air, it becomes liquid at - 63.2C and in

which noise propagates badly.

SF6 on the market-


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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

C02 traces

HF 0.3 ppm

SF6 is therefore 99.99 % pur.

Chemical properties-

SF6 is a synthetic gas which is obtained as we have just explained by combination of six atoms of

fluorine with one atom of sulfur:

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 there fore already expect that it

will be a powerful cooling agent:

The dissociation products before interruption of the arc-


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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

Sulfur fluoride SO2F2

Sulfur tetra fluoride SF4

Sulfur deca 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 sulfur 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 arc-

An 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:

Sulfur fluoride SO2F2

Carbon tetra fluoride CF4

Silicon tetra fluoride SIF4

Sulfurous 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.


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SF6 Safety precautions:-

Today there is no known dielectric and breaking agent combined better than SF6 gas.

Initial state-

In 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 uninflammable,

- 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 behavior.

Man dies when the oxygen level of the gas he is breathing falls below 12 %.

Precautions and hygiene-

The 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 that

SF6 is a very heavy gas. This device will warn you any gas leaks.

Post-breaking state-

As 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 edema. The solid decomposition products


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(whitish powder) an aggressive when the react with the humidity of the mucous membranes and of

the hands.

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 pure

SF6. 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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2.2 GAS INSULATED SUBSTATION:-

Theactual layout

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 tank-explosion characteristics can remove thefirefighting equipment from transformer room. As a result, gas insulated transformer, gas insulated

shunt reactor and GIS controlpanels 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.


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2.3 ADVANTAGE OF GAS INSULATED TRANSFORMER:-

1. Non-flammability

Gas insulated transformers, using incombustible SF6 gas as an insulating and cooling medium,

enable to remove firefighting 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. Thus it acquires less space area.

4. Easy installation

Oil or liquid purifying process is not necessary in case of gas insulated transformer making the

installation process easier..

5. Easy inspection and maintenance work

Only SF6 gas pressure shall be basically monitored during periodically inspection.


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6. Environmentally Friendly

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

2.4 Standard capacity application range

1. Quality specification :-The following specifications are provided to ensure safe operation of

gas-insulated transformers.

2. Withstand voltage during zero gas gauge pressure :-No problem is caused by operation

under normal operating voltage.

3. Permissible load under zero gas gauge pressure :-No problem is caused by 50% loadcontinuous operation.

4. Permissible load under 1-series operation when 2-series coolers are provided :-No

problem is caused by 75% load continuous operation.


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Chapter-3

3.1 COOLING APPARATUS:-

Employs a cooling medium in liquid phase such as a Refrigerant R113 otherwise known as

Trifluorotrichloroethane or C2F3Cl3.Thus SF6 is used as a second cooling medium.


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3.2 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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Chapter-4

4.1 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.

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 airtightness 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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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.


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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, SF6

gas leakage is accurately detected and the alarm contacts are actuated.


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Technical Data - SF6

TABLE 9.1.

SF6 Industry Standards

ComponentASTM 2472

DIEC 376 ASG TYPICAL

Sulfurhexafluoride (by wt.) 99.8% 99.8% 99.9%

Water (vol. %) 8 ppmv 15 ppmv 5 ppmv

Dew Point -62C -40C -65C

Hydrolyzable Fluorides (HF) 0.3 ppmw 1.0 ppmw 0.3 ppmw

Air (wt. %) 500 ppmw 500 ppmw 200 ppmw

Carbon Tetrafluoride (CF4)

(wt. %) 500 ppmw 500 ppmw 200 ppmw

Water content, maximum dew point, 62C

Sf6 specifications standard (guaranteed minimum standards)

TABLE 9.2.

1. Mass Content of Sulfur Hexafluoride (SF6) (%) 99.95%

2. Oxygen (O2)


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4.2 APPLICATIONS OF SF6 GAS INSULATED TRANSFORMER

Hospitals, underground shopping centers, 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.

4.3 COMPARISON WITH OIL-IMPREGNATED TRANSFORMERS:-


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4.4 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.

4.5REFERENCE:-

http://www.meppi.com/Products/Transformers/Pages/SF6Gas.aspx

http://www.deccanherald.com/content/59128/states-first-gas-insulated-transformer.html

http://www.toshiba-tds.com/tandd/products/trans/en/gitrans.htm

http://www.alibaba.com/showroom/gas-insulated-transformers.html

http://www.projects-forum.com/Thread-GAS-INSULATED-TRANSFORMER

http://seminarprojects.com/s/gas-insulated-transformer

http://www.whdotek.com/sdp/1425792/4/pd-5940383/9533737-

2373434/Gas_Insulated_Transformer.html
http://www.meppi.com/Products/Transformers/Pages/SF6Gas.aspxhttp://www.deccanherald.com/content/59128/states-first-gas-insulated-transformer.htmlhttp://www.toshiba-tds.com/tandd/products/trans/en/gitrans.htmhttp://www.alibaba.com/showroom/gas-insulated-transformers.htmlhttp://www.projects-forum.com/Thread-GAS-INSULATED-TRANSFORMERhttp://seminarprojects.com/s/gas-insulated-transformerhttp://www.whdotek.com/sdp/1425792/4/pd-5940383/9533737-%20%20%20%20%20%20%20%20%20%20%20%20%20%202373434/Gas_Insulated_Transformer.htmlhttp://www.whdotek.com/sdp/1425792/4/pd-5940383/9533737-%20%20%20%20%20%20%20%20%20%20%20%20%20%202373434/Gas_Insulated_Transformer.htmlhttp://www.whdotek.com/sdp/1425792/4/pd-5940383/9533737-%20%20%20%20%20%20%20%20%20%20%20%20%20%202373434/Gas_Insulated_Transformer.htmlhttp://www.whdotek.com/sdp/1425792/4/pd-5940383/9533737-%20%20%20%20%20%20%20%20%20%20%20%20%20%202373434/Gas_Insulated_Transformer.htmlhttp://www.whdotek.com/sdp/1425792/4/pd-5940383/9533737-%20%20%20%20%20%20%20%20%20%20%20%20%20%202373434/Gas_Insulated_Transformer.htmlhttp://seminarprojects.com/s/gas-insulated-transformerhttp://www.projects-forum.com/Thread-GAS-INSULATED-TRANSFORMERhttp://www.alibaba.com/showroom/gas-insulated-transformers.htmlhttp://www.toshiba-tds.com/tandd/products/trans/en/gitrans.htmhttp://www.deccanherald.com/content/59128/states-first-gas-insulated-transformer.htmlhttp://www.meppi.com/Products/Transformers/Pages/SF6Gas.aspx

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