A dielectric gas or insulating gas, is a dielectric material in gaseous state. Some examples of insulating gases are Sulfur hexafluoride, Nitrogen, Air, Ammonia, Carbon dioxide, Carbon monoxide, Hydrogen sulfide, Oxygen, Chlorine, Hydrogen, Sulfur dioxide, Nitrous oxide, 1,2-Dichlorotetrafluoroethane(R-114), Dichlorodifluoromethane(R-12), Trifluoromethane, 1,1,1,3,3,3-Hexafluoropropane (R-236fa), Carbon tetrafluoride (R-14), Hexafluoroethane (R-116), 1,1,1,2-Tetrafluoroethane(R-134a), Perfluoropropane (R-218), Octafluorocyclobutane(R-C318), Perfluorobutane (R-3-1-10), 30% SF6/70% air, Helium, Neon, Argon and it’s combination. Depending on the particular function of the gas-insulated equipment, the selection and properties of gas are significant vary. Its main purpose is to prevent or rapidly quench electric discharges. For practical application, a good dielectric gas should have high dielectric strength, good heat transfer properties, high thermal stability and chemical inertness against the construction materials used, non-flammability, non-explosive, low toxicity, low boiling point and low cost. Dielectric gases are used as electrical insulators & cooling medium in high voltage applications, e.g. transformers (distribution and power range), high voltage switchgear. Present most frequently used insulation gasses are sulfur hexafluoride and nitrogen.

DESIGN CONCEPTS OF SF6 GIT

GIT are classified into two types based on the tank design pressure. They are low gas pressure type and high gas pressure type. Insulating and cooling capacity of SF6 gas depends on SF6 gas pressure. That is, the higher the gas

pressure, the better the insulating and cooling characteristics. But higher pressure may not be efficient due to the high resisting strengths of tank and cooler, which will increase the weight and manufacturing cost. Therefore, the rated pressure should be chosen carefully considering the rated voltage, power capacity, temperature rise and economical efficiency.

High pressure GIT has following characteristics over low pressure GIT:

1. The gas pressure is higher than conventional GIT (low gas pressure type) to improve the insulation and cooling performance.

2. Better insulation capability yields lower clearances and a more compact design; hence no need to reinforce or modify the existing foundation.

3. The tank is a pressure vessel with cylindrical shape; hence the weight is lighter compared with that of rectangle shape (conventional low pressure type).

Tank Design For structural design of a transformer, the tank strength under working pressure is very important. Since the pressure of the ransformer, the structural trength evaluation using f shown in Fig. 14. Hydro ressure test using 1.5 times working pressure was conducted by IEC 60517, and it has been shown that the designed transformer has good structural integrity even under the internal pressure increase caused by an internal fault during operation.

The future trends of gas insulated transformers are to be towards the development of underground substation application i.e. Gas Insulated Switchgear.

GITs are also expected to find wider application outdoors beyond conventional underground substations. In such sites as hydro power stations from which leaks of insulation oil might have a great effect on water resources, as the importance of giving consideration to environment grows, demand to replace oil-filled transformers with gas insulated models in likely to rise.

A gas-insulated transformer is a transformer whose magnetic circuit and windings are enclosed with an insulating gas. Sulfur hexafluoride (SF6) gas is generally used. Such a transformer is sometimes referred to as a gas-insulated (GIS) transformer.

Abstract: SF6 gas transformer (GIT) classification, the GIT with the traditional oil-

immersed transformer characteristics were compared, in his discussion of the

advantages of GIT on the basis of the last of the GIT in our application.

Keywords: GIT; development; advantage; application prospects

At present, the transformer primary to the development of two aspects: first, to

the development of large high pressure area, the other is the energy-saving, small

size, low noise, high impedance, explosion-proof development. The former are

generally used in large power station or power transmission, the latter, mainly in

small and medium sized products. With China\\ s urbanization, large urban

population is more dense, high-rise buildings everywhere, electricity consumption

increased dramatically, an increasing number of transformers, the traditional

large-capacity oil-immersed transformer oil, once the fire due to failure, will rise

construction and people\\ s lives and property posed a serious threat. Therefore, it

was non-flammable transformer is also increasing emphasis on research and

application. Non-combustible insulation transformer according to the different

media can be divided into silicon transformer, cast resin transformers, complex

and sensitive insulating liquid dielectric transformers, SF6 gas insulated

transformers (gas-insulatedtransformer, GIT), which GIT its unique advantages by

the people concern, since the since the 1960s in Japan, Europe, application and

development more widely.

1 GIT classification

GIT according to their different cooling medium can be divided into gas-insulated,

gas cooling and gas-insulated, liquid-cooled two types. Capacity less than 60MVA

of GIT, its heat loss is small, generally use SF6 gas cooling of cooling method, this

type of GIT with the traditional oil-immersed transformers in the structure there

are many similarities in the design can learn from. And volumes greater than

60MVA of the GIT, most of the liquid (C8F16O or C8F18) cooling and SF6 gas-

insulated separate structure, the structure of these products and oil-immersed

transformers are very different, usually layered cooling, foil winding of the GIT.

2 GIT compared with ordinary oil-immersed transformer

Medium to large capacity transformers are essentially oil-immersed transformer.

As cities increase in electricity load and concentration, increasing the number of

substations, transformer capacity and a corresponding increase in the number. In

this process, some parts of oil-filled transformers have been gradually replaced by

GIT. GIT compared with conventional oil-immersed transformers, the fundamental

difference is that GIT uses SF6 gas for insulation and cooling, and thus both the

structural characteristics of both the similarities, but also somewhat different.

For the GIT, the greater the pressure tank, the cooling features and the better the

insulation. But the high pressure of the transformer while the body strength to

withstand the pressure of higher demands, this will increase the weight and

shape of the transformer requirements.

In the cooling system, GIT and oil-immersed transformer primary difference is the

cooling medium and cooling treatment differences. GIT using SP6 or C8F16O as a

cooling medium, and oil-immersed transformer is used transformer oil as a

cooling medium. Three kinds of physical properties of the cooling medium to

C8F16O features the best in its small viscosity, heat a large, and the boiling point

close to the body of the transformer operating temperature, high heat of

vaporization, and thus the cooling effect Ye Hao. In order to obtain the same with

oil-immersed transformer cooling characteristics, GIT using the air compressor,

and its circulation rate than forced oil cooling type oil-immersed transformer

several times the cycle rate is high. On the other hand, a number of small auto-

cooled GIT has also been widely used. In China, a noise-free either natural cooling

process can also be compulsory dual-mode air-cooled type of the GIT has also

been put into use.

In the insulation structure, according to different operating voltage and capacity,

GIT use a variety of foil pie winding and winding. Between high and low voltage

windings, winding insulation to ground insulation between the main strength is

mainly determined by the dielectric strength of SF6 gas, and uniformity of the

electric field SF6 highly dependent, so the design must be strictly controlled in

the gas in the electric field strength. In insulating materials, oil-immersed

transformer insulation paper used, GIT is used with high mechanical strength and

insulation of high airtight plastic film as a copper wire insulated cloth.

Protection, GIT removed gas relay, the relay installed and gas equivalent to the

sudden pressure relay, the relay measures the rate of increase of gas pressure in

order to determine the occurrence of internal faults and gives a trip signal. Since

the GIT gas capacity is large, unless the fault energy is very large and very long

time to failure, internal failure caused by body pressure will not rise to the

danger, therefore, GIT does not need to install some oil-immersed transformer

pressure relief devices.

Oil-immersed transformer oil in a pillow, its role is when the transformer oil

volume change with temperature when the oil reservoir and fill to ensure the fuel

tank filled with oil. GIT is not such a monster SF6 gas will expand and contract

with temperature changes, but these changes in volume directly into a pressure

change, which does not require an additional oil pillow, thus reducing the height

and area of ??the transformer costs, and eliminate moisture due to breathing

silica replacement transformer is not in a timely manner may cause damp

problems, improve safety and reliability.

GIT using SF6 gas insulated vacuum-load tap, switch it off using a vacuum load

switch current institutions, and the use of roller-type contact system and non-

lubricated bearings, to prevent decomposition of the SF6 gas arc caused by the

transformer body effects. Oil-immersed transformers in the transformer oil cut off

the current, and the use of sliding contact.

3GIT advantage

And compared with ordinary oil-immersed transformer, GIT has the following

advantages:

3.1 insulation and cooling effect

SF6 has a good electrical properties, mainly in the arc goes out of its insulating

properties and characteristics. As the negative charge of SF6 (ie, the adsorption

capacity of the electron), it has excellent dielectric insulation properties. In the

uniform electric field, SF6 insulation strength is about 2.5 times the air; When the

gas pressure of 0.2MPa when, SF6 gas insulating oil dielectric strength and quite.

Extinguish the arc at the same time and instantaneous SF6 discharge

temperature range (1500 ~ 5000K) has excellent heat transfer characteristics.

So, GIT has a very good insulation properties and cooling effects.

3.2 is not flammable

SF6 gas is inert gas, colored, odorless, non-toxic and non-flammable, and its

molecular structure is very stable at room temperature, it will not come into

contact with the material and chemical changes, which greatly simplifies the

configuration of fire fighting facilities; When the GIT and GIS

(gasinsulatedswitchgear) combined, the entire gas-insulated substation in the

environment, thus enhancing the entire substation is not easy flammable. In

terms of proof, when the transformer arcing occurs when increased pressure

within the SF6 gas volume will be offset by changes, so do not need additional

pressure relief devices GIT.

3.3 easy to install, flexible layout

GIT at the factory fully assembled, SF6 gas is injected into them, at the

installation site without vacuum, and SF6 gas from the tank directly to the

transformer tank, basically do not need any tools, so clean when loading quickly.

Also, because the density of SF6 gas insulating oil density is only 1 / 60, and the

viscosity is low, so in cooling the pressure drop is very small, so the cooler can be

installed level, from the transformer can be mounted vertically, so that it layout is

quite flexible

3.4 simple, lightweight

GIT pillow and do not need the oil pressure relief devices such attachments

without the wall, while SF6 gas density and more dense than the transformer oil,

which seems simple, lightweight.

3.5 low noise

SF6 gas density than the density of transformer oil, sounds transmitted through

the slow, intermediate core sound rarely reached the tank, so, GIT-noise ratio

produced by oil-immersed transformer is smaller.

3.6 is easy to maintain inspection

The same environment, SF6 gas than oil-immersed transformer transformer oil

consumption is much slower. GIT without the use of live-load tap changer oil

filter, but also reduces the oil pressure relief pillow and equipment, to bring the

convenience of maintenance and inspection.

3.7 covers an area of ??less

GIT its simple structure, light, reduced height, the corresponding underground

substation roof also reduced, and without the wall, a small installation area, the

entire space has been fully utilized, greatly reduces the capital cost of substation .

Especially when used in conjunction with GIS GIT, these advantages become

more pronounced.

4GIT prospects of application in China

Compared with ordinary oil-immersed transformer, GIT advantage of self-evident,

therefore, its application prospects should be very attractive. GIT has been more

widely used in Japan, but in our present application is still limited. Although as

early as 1988, China\\ s high-voltage electrical appliances plant in Guangzhou,

Beijing transformations, Changzhou Transformer Factory have been developed,

including Beijing and two changes have been small batch production, in the last

ten years have also introduced Japan\\ s Toshiba and Mitsubishi were made

110kVGIT more than 20 units, but the total application is not very optimistic. GIT

application in China is limited mainly has the following disadvantages:

1) basically rely on imported and expensive;

2) 110kVGIT run over the internal pressure up to 180kPa;

3) SF6 gas transmission capacity than the transformer oil potential difference, in

the case of non-uniform electric field strength, dielectric strength will decline;

4) SF6 gas in the hot metal will be broken down a kind of very toxic SP4. The

most uncertain factor is the SF6 gas, which is a greenhouse gas, and its

decomposition products toxic. If the manufacturing process is not good, after the

leak would be disastrous environmental consequences. In today\\ s increasingly

environmentally conscious, it also must be considered. Therefore, many experts

believe that in the domestic manufacturing process reach a certain level should

not promote the development of such a transformer in China.

For these reasons, GIT application in China are still in a kind of exploration and

early stages, only a few plants or engineering use. However, this does not mean

GIT in the Chinese market into a dead end, on the contrary, GIT application

prospects in China are a lot of potential, but also very broad.

First, according to the ratio of the transformer and power generation equipment

demand calculated by 1:11, \"15\" period, the estimated annual average installed

capacity of 15600MW, \"15\" average annual total transformer required

171600MVA. The \"15\" period, the small transformer continues to energy saving,

small size, low noise, high impedance, explosion-proof development. In this case,

the gas-insulated transformer (SF6 transformer) itself has certain requirements.

The most important point, as mentioned earlier with the accelerated process of

urbanization in China, more and more urban high-rise buildings, while at the

same time, the city\\ s surge in demand for electricity, the existing 35kV and

below the power supply voltage level facilities have been difficult to meet the

power requirements of load growth, 110kV power supply has become a preferred

regional cities program. Meanwhile, the people of the power supply security,

reliability, power quality and the increasingly high demand for urban

environmental protection, especially for fire, explosion, pollution prevention, and

pay more attention to noise reduction. Rapid growth in electricity distribution

centers closer and closer to people\\ s activity center, a large number of large

transformers in the city center, safety is the first time, while the general

emergence of some oil-immersed transformer accidents, such as fire, explosion,

serious threatening people\\ s lives and property. In this case, people and

development of the GIT application corresponding to pay attention to it.

The greenhouse effect of SF6, the relevant parties to the impact of atmospheric

SF6 was investigated, that the whole man-made greenhouse effect of SF6 on the

relative impact is negligible, no effect on ozone depletion, as long as appropriate

measures to recycle, can ensure personal safety, the environment is concerned,

SF6 is still the best insulation and arc-quenching medium.

GIT structure design and manufacturing processes with traditional oil-filled

transformers have many similarities. Transformer plant without a lot of

equipment can go into research and development. We can note that at present

some of China\\ s transformer manufacturers are also actively developing GIT

product introduction and technology. Such as change from Baoding Tianwei

Electric Co., Ltd., Japan\\ s Mitsubishi Electric Corporation, Mitsubishi Electric

(China) Co., Ltd. a joint venture established Baoding Transformer Co., Ltd. Paul

Ling full use of Tianwei change mature oil and core-type transformer advanced

level of new rail balance traction transformer technology, Mitsubishi has

introduced the international advanced level of SF6 gas-insulated transformers,

shell-shell-type transformer and shunt reactor technology to produce low noise,

low power, low temperature rise, high resistance short circuit capacity power

products. Changzhou Toshiba Transformer Co., Ltd. in order to meet the needs of

building the capital, is also actively developing GIT, is expected to deliver projects

to use in 2004. Similarly, in March 2003, Shunde Special Transformer Factory

and Germany\\ s Siemens (SIEMENS) also closed SF6 gas-insulated medium

voltage switchgear technology cooperation, and reached consensus and signed a

technical cooperation agreement, authorize the use of Siemens Shunde Special

Transformer Factory technology for product manufacturing, can be expected in

the near future, from Shunde Special Transformer Factory-made with the

international advanced level of SF6 gas-insulated medium voltage enclosed

switchgear will be provided to the city network distribution system, will provide

customers with high quality of the GIT. Founded in 1991, Sino-US joint venture

enterprise in Chongqing Transformer Co., Ltd. has introduced three rivers of the

United States, Japan and other advanced transformer manufacturing technology,

the company also produces GIT. Through this series of facts, with the \"15\" plan

implementation and development of western development, GIT in the Chinese

market will be good, its application will also be more extensive.

QUESTIONS AND ANSWERS ABOUT SF6 GAS

What Is SF6? Sulphur hexafluoride.

SF6 is a gas that is used in electrical power equipment. It is colourless, odourless, non-flammable and chemically stable. This means that at room temperature it does not react with any other substance. Stability comes from the symmetrical arrangement of the six fluorine atoms around the central sulphur atom. And this stability is just what makes the gas useful in electric equipment. SF6 is a very good electrical insulator and can effectively extinguish arcs, which makes high and medium voltage apparatus filled with SF6 highly popular. SF6 can be found in millions of electric apparatus all over the world; electrical equipment containing SF6 is a large export article.

SF6 is formed by a chemical reaction between molten sulphur and fluorine. Fluorine is obtained by the electrolysis of hydrofluoric acid (HF). Pure SF6 is not poisonous. The gas is not dangerous to inhale, provided the oxygen content is high enough. In principle you can inhale a mixture of 20% oxygen and 80% SF6 without danger. SF6 is about 6 times heavier than air. That means that it may collect in cable ducts or at the bottom of tanks. The gas is not dangerous to inhale but if it does accumulate where people work, there is a risk of suffocation due to the lack of oxygen. SF6 is a non-flammable gas that is used in electrical apparatus. SF6 is not poisonous.

Where and How Is SF6 Used?

SF6 is used as an insulating gas in substations, as an insulating and cooling medium in transformers and as an insulating and arc quenching medium in switchgear for high and medium voltage applications. These are all closed systems which are extremely safe and unlikely to leak.

In electrical power systems, high and medium voltage switchgear is required to cut off the power in case of a fault, in order to protect people and equipment. When power is switched, an electric arc strikes between the circuit-breaker contacts. Breakers filled with SF6 are electrically insulating and effectively control arcing. Gas isolated substations are mainly found in urban areas where you want them to take up as little room as possible and often integrate them into buildings. These substations reduce the magnetic field and remove the electrical field completely. This is a real advantage for installers, maintenance personnel and people who live in the vicinity of substations.

SF6 also is used in other ways. Mixed with argon, it can be used in insulated windows. SF6 is used in the metal industry, for example, when casting magnesium. Eye surgeons use SF6 as a cooling agent in operations. SF6 can also be used as a fire extinguishing agent because it is non-flammable and cooling. In electrical applications, SF6 is only used in sealed and safe systems which under normal circumstances do not leak gas. SF6 is collected and recycled if a piece of equipment or a substation needs to be opened.

What Is the Benefit of SF6?

There are two reasons for using SF6 in electrical equipment: SF6 provides extremely good electrical insulation and very effectively quenches electric arcs. These properties of SF6 make it possible to build electrical equipment and apparatus that are compact, use a small amount of material, are safe and will last a long time. At normal atmospheric pressure, SF6 has a dielectric withstand capability that is 2.5 times better than air. Usually the gas is used at 3-5 times atmospheric pressure and then the dielectric properties are ten times better than for air.

SF6 insulates so well because it is strongly electronegative. This means that the gas molecules catch free electrons and build heavy negative ions, which do not move fast. This is effective against the creation of electron avalanches which may lead to flashovers. SF6 effectively controls circuit-breaker arcs because it has excellent cooling properties at temperatures (1500-5000 K) at which the arcs extinguish (the gas uses energy when it dissociates and therefore produces a cooling effect). High-voltage switchgear with SF6 is in principle alone on the market and the volume of medium-voltage switchgear with SF6 is large. Complete gas insulated substations, GIS or RMU, are used where space is restricted or the environment is severe. They are virtually maintenance-free. Electrical equipment filled with SF6 has been in use for about 40 years and the service experience is very good. SF6 has a very high dielectric withstand capability. SF6 effectively quenches arcs in circuit breakers. SF6 apparatus is compact and almost maintenance-free. SF6 equipment is extremely safe in operation and for users.

Are there Any Alternatives to SF6?

High-voltage circuit breakers filled with SF6 are able to cope with higher unit voltage than any other technique. Therefore, SF6 circuit breakers require fewer breaking units in series than air or oil-filled breakers. SF6 circuit breakers for medium voltage have considerable advantages compared to the alternatives. SF6 substations can be built as totally closed systems which provide the highest levels of operational and user safety.

The designs require only little material; all SF6 apparatus are compact. Compared with electrical apparatus without SF6, the environmental impact is low. Especially if you take into consideration every environmental impact “from the cradle to the grave“, that is, from manufacturing, installation, operation and maintenance all the way to final scrapping (when scrapping equipment , all the SF6 is recovered). So SF6 has many important advantages while currently available alternatives represent a return to older and inferior technology.

Without SF6, we would have a bigger impact on the environment as the alternatives require more space, are less safe and affect the environment more for installers, service personnel and people who live in the area. Compared to designs that do not use SF6, the impact on the environment is low. Without SF6, we would have a less safe working environment for installation, operations and service personnel.

Is SF6 Dangerous?

Since SF6 came into use, nearly 50 years ago, a small amount of gas has leaked into the atmosphere. Currently there area round 0.000´000´000´003 parts (by volume) of SF6 in the atmosphere. The stability of the gas means it will stay in the atmosphere for a long time.

Some gases that are released destroy the ozone layer. Thinning of the ozone layer means that more ultraviolet light can get to the ground, increasing the risk of skin cancer. The gases that affect the ozone layer all contain chlorine. SF6 does not contain any chlorine and does not affect the ozone layer. The term “greenhouse effect“ is used to describe that the atmosphere is slowly warming up due to man-made gas emissions. Some gas molecules in the atmosphere, principally carbon dioxide (CO2) and methane (CH4) reflect long wave-length heat radiation from the earth so that heat remains trapped in the atmosphere instead of disappearing into outer space.

The SF6 molecule is very reflective and contributes to the greenhouse effect. But the concentration of the gas is extremely low (0.000´000´000´003 by volume). This means that the contribution of SF6 to the man-made greenhouse effect is very little, less than 0.1 percent of the total effect. This should be compared with carbon dioxide CO2 which contributes about 60%. SF6 does not destroy the ozone layer. The contribution of SF6 to the greenhouse effect is less than 0.1 percent of the total that mankind generates.

Can SF6 Create Any Dangerous Substances?

Despite the fact that the SF6 gas is very stable, it will partly decompose in association with electric discharges and arcs, for example, in a breaker. Then, gaseous and solid decomposition products are produced. Normally the level of gaseous decomposition products is kept low through the use of absorbers built into the switchgear.

In large concentrations, the decomposition products are corrosive and poisonous. Therefore, there are established routines for service personnel when opening SF6 filled equipment for maintenance or scrapping. The solid decomposition products are mainly metal fluorides in the form of a fine grey powder. The powder only appears where arcing has occurred, for instance in used circuit breakers. The powder can be easily taken care of as separate waste.

The decomposition products are reactive, which means that they will decompose quickly and disappear without any long-term effect on the environment. SF6 is however mostly in hermetically closed systems. That means the gas is contained for the entire life time of the equipment, there is no need to open the equipment for servicing and no gas can escape. In SF6-filled switchgear-decomposition products are created – they can be handled safely and are harmless to the environment. Most SF6 gas is collected and recycled.

Today‘s SF6 apparatus is very gas-tight and in normal operation only a small amount of the gas can leak out. The gas is monitored continuously, and any leakage would be discovered at an early stage. SF6 apparatus does not need much maintenance. In the rare case a gas-filled section of apparatus needs to be opened, the gas is pumped into a container. After filtering, the gas can be used again and put back into the apparatus. There is also equipment which allows us to handle gas in large quantities (GIS substations) and also in smaller quantities, for example, in single circuit breakers.

Contaminated SF6 can be cleaned and used again. If the gas is not required any more, it can be destroyed by heating it together with limestone in a high temperature kiln. In this process it is being transformed into the environmentally harmless and non-toxic natural products gypsum and fluorspar. It is usual policy that no SF6 gas should be released into the atmosphere. This applies to every time SF6 apparatus is serviced, maintained or scrapped. Most equipment is also designeds so that gas handling is as easy as possible; we recommend suitable equipment and routines for the handling of the gas; and we offer to take care of the gas when equipment with SF6 is to be scrapped. SF6 can be cleaned and used again. SF6 can easily be destroyed.

Most SF6 users policy is that no SF6 gas should be released when equipment is erected, serviced or scrapped.

Responsibility for the Environment.

SF6 is a gas that, amongst other things, is used in electrical equipment. The gas is very stable and very effective for electrical insulation and arc quenching and is totally harmless to the environment. SF6 is used in electrical power equipment all around the world; SF6 contributes to ensuring that the consumer gets electricity safely and cheaply. It is the stability of SF6 that makes it so useful in electrical apparatus. That stability also means that SF6, when released, could contribute to the greenhouse effect. But very little is released so the contribution to the greenhouse effect is extremely low. Calculations show that SF6 contributes less than 0.1 percent to the total greenhouse effe

Dielectric gas is a form of gas used in industrial applications as an electrical  insulator. Common types of gasses used include air, nitrogen, and sulfur hexafluoride. Various types of electrical components such as transformers and circuit breakers require the presence of adielectric gas to prevent damage to a circuit in the case of an electrical discharge. In routine applications, air is often the dielectric gas of choice because it doesn't require a pressurized, sealed system, and is ubiquitous.The type of dielectric gas used depends largely on the voltage level of the device and circuit, as well as fundamental properties of the gas, such as its inert chemical nature, and thermal properties, such as its boiling point and ability to transfer heat. The level of toxicity and flammability of the dielectric gas under certain conditions must also be considered. An electrical short circuit can cause a component like a high-

voltage transformer to physically degrade to the point that the gas is released into the surrounding environment. For this reason, air and nitrogen gasses are often used as they are largely inert and nonreactive.

Sulfur hexafluoride is used as a dielectric gas in high-voltage switchgear such as industrial circuit breakers that connect generators to step-up voltage transformers. It is also used in areas of high voltage electric power systems that require gas insulators, such as transmission lines, transformers and substations. Around 80% of all the sulfur hexafluoride manufactured is commonly used in electrical power plants and substations throughout the world due to its superior insulating qualities and ability to suppress radio wave and sound wave transmission from electrical equipment. It also has the highest level of breakdown voltage for any insulating gas, which is the level of voltage necessary for a dielectric gas to begin conducting current and fail to act as an insulator.

Disadvantages to using sulfur hexafluoride as a dielectric gas are significant, however, and, for this reason, attempts are being made to combine it with safer gasses, such as nitrogen, carbon dioxide, or perfluorocarbon compounds. It is estimated that sulfur hexafluoride is 22,800 to 23,900 times more of a contributing factor to global warming when released into the atmosphere as compared to an equivalent amount of carbon dioxide. It also endures in the atmosphere as a stable greenhouse gas for much longer than other greenhouse gasses as well, lasting for 800 to 3,200 years before it degrades. The compound also poses serious health risks upon human exposure, such as causing respiratory problems, and it often combines with other compounds when released into the air that can lead to fluoride contamination of the body and a variety of ailments.

Insulating gasesElectronegative gases make good insulators since the ions rapidly combine with the ions produced in the spark. However, they tend to be corrosive. Some gases though, dissociate only where the discharge is (or wants to be), making them particularly good insulators.

Gases with electronegative species (i.e. halogens such as chlorine) make good insulators, hence the popularity of SF6, which is not only dense (breakdown voltage is roughly proportional to density) but is mostly Fluorine, a highly electronegative element. The halogenated hydrocarbon refrigerants are also a popular insulator. CCl4, CCl2F2, CCl3F, and C2Cl2F4

Unfortunately, the cost of insulating gases has greatly increased in the last few years largely due to the various treaties regulating halocarbon refrigerants. The traditional Freons (R-12, R-22) are not being produced any more, and are quite expensive. Since the regulatory thrust eliminated chlorinated alkanes, modern refrigerants are relying more on fluorinated or per-fluoro hydrocarbons (e.g.HC-134a) . Unfortunately, plant capacity is limited, and plants that used to make SF6 are now making fluorinated hydrocarbons resulting in much higher prices for SF6. In the mid 1980's SF6 was about $3-4/lb. Now, in the mid 90's, it is about $100/lb. Since a pound is only about 10 liters, filling up a large insulating tank with SF6 has become a very expensive proposition.

The breakdown voltage of most gases can be increased by increasing the absolute pressure. In the case of some gases, there is a limit imposed by the liquefaction point at normal operating temperatures (i.e.

Freon 12 liquifies at 5 atmospheres). Mixtures of gases can overcome some of these issues and a mixture of Freon 12 and Nitrogen was popular.

One disadvantage of the halogenated compounds is that the dissociation products are highly corrosive, so it is important that operating voltages remain well below corona starting voltages. Even air forms highly reactive nitrogen oxides and other corrosive compounds, particularly if there is any water vapor present. High pressure air can also support combustion due to the oxygen content. Pure Nitrogen seems to not have these disadvantages, although its breakdown is only about 15 % better than air.

Air - approximate breakdown is 30 kV/cm at 1 atm. = 30 + 1.53d where d in cm. The breakdown of air is very well researched, to the point where the breakdown voltage of a calibrated gap is used to measure high voltages.

Freons- The vapor pressure of CCl2F2 (R-12) is 90 psi at 23C, where the breakdown is some 17 times that of air at 1 atm. An even higher insulating strength can be obtained by adding nitrogen to the saturated CCl2F2 to bring the total pressuire to the desired value. The saturated vapor pressure of C2Cl2F4 at 23C is 2 atm abs, at which condition it has a relative dielectric strength of 5.6 times N2 at 1 atm

Sulfur Hexafluoride (SF6) - Sulfur Hexafluoride is probably the most popular insulating gas, although its cost has risen dramatically recently.

Hydrogen - Hydrogen gas is not a particularly good insulator (65% of air) from a breakdown voltage standpoint. Its very low viscosity and high thermal capacity make it an insulating gas of choice for high speed, high voltage machinery such as turbo generators. There isn't an explosion hazard, provided that the oxygen content in the hydrogen tank is kept below the flammable limit (around 5%). Of course, hydrogen has lots of other handling problems, including hydrogen embrittlement, it leaks through very tiny holes (even the pores in the metal tanks), and perfectly colorless, but very hot, flames.

Relative spark breakdown strength of gases

GasN2 Air NH3 CO2 H2S O2 Cl2 H2 SO2 C2Cl2F4 CCl2F2

V/Vair 1.15 1 1 0.95 0.9 0.85 0.85 0.65 0.30 3.2 2.9

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POST-BREAKDOWN PHENOMENA AND APPLICATIONS

This is the phenomenon which occurs after the actual breakdown has taken place and is of technical importance. Glow and arc discharges are the post-breakdown phenomena, and

there are many devices that operate over these regions. In a Townsend discharge see Fig. (1) The current increases gradually as a function of the applied voltage. Further to this point (B) only the current increases and the discharge changes from the Townsend type to Glow type (BC). Further increase in current results in a very small reduction in voltage across the gap (CD) corresponding to the normal glow region. The gap voltage again increases (DE), when the current is increased more, but eventually leads to a considerable drop in the applied voltage. This is the region of the arc discharge (EG). The phenomena that occur in the region CG are the post-breakdown phenomena consisting of glow discharge (CE) and the arc discharge (EG).

Glow DischargeA glow discharge is characterized by a diffused luminous glow. The colour of the glow discharge depends on the cathode material and the gas used. The glow discharge covers the cathode partly and the space between the cathode and the anode will have intermediate dark and bright regions. This is called normal glow. If the current in the normal glow is increased such that the discharge covers the entire cathode surface, then it becomes abnormal glow. In a glow discharge, the voltage drop between the electrodes is substantially constant, ranging from 75 to 300 V over a current range of 1 mA to 100 mA depending on the type of the gas. The properties of the glow discharge are used in many practical applications, such as cold cathode gaseous voltage stabilized tubes (voltage regulation tubes or VR tubes), for rectification, as a relaxation oscillator, and as an amplifier.

Arc DischargeIf the current in the gap is increased to about 1 A or more, the voltage across the gap suddenly reduces to a few volts (20—50 V). The discharge becomes very luminous and noisy (region EG in Fig. 1 This phase is called the arc discharge and the current density over the cathode region increases to very high values of 103 to 107A/cm2. Arcing is associated with high temperatures, ranging from 1000°C to several thousand degrees Celsius. The discharge will contain a very high density of electrons and positive ions, called the arc plasma. The study of arcs is important in circuit breakers and other switch contacts. It is a convenient high temperature high intensity light source. It is used for welding and cutting of metals. It is the light source in lamps such as carbon arc lamp. High temperature plasmas are used for generation of electricity through magneto-hydro dynamic (MHD) or nuclear fusion processes.

Fig. (1) d.c. voltage-current characteristic of an electrical discharge with

electrodes having no sharp points or edges

PRACTICAL CONSIDERATIONS IN USING GASES ANDGAS MIXTURES FOR INSULATION PURPOSES

Over the years, considerable amount of work has been done to adopt a specific gas for practical use. Before adopting a particular gas or gas mixture for a practical purpose, it is useful to gain knowledge of what the gas does, what its composition is, and what the factors is that influence its performance. The greater the versatility of the operating performance demanded from an insulating gas or gas mixture, the more rigorous would he the requirements which it should meet. These requirements needed by a good dielectric do not exist in a majority of the gases. Generally, the preferred properties of a gaseous dielectric for high voltage applications are:(a)     high dielectric strength,(b)     thermal stability and chemical inactivity towards materials of construction,(c)     non-flammability and physiological inertness, and environmentally non-hazardous,

(d), low temperature of condensation,(e)   good heat transfer, and(f)     ready availability at moderate cost.

Sulphur hexafluoride (SF6) which has received much study over the years has been found to possess most of the above requirements.Of the above properties, dielectric strength is the most important property of a gaseous dielectric for practical use. The dielectric strength of gases is comparable with those of solid and liquid dielectrics see Fig. (2.).It is clear that SF6 has high dielectric strength and low liquefaction temperature, and it can be used over a wide range of operating conditions. SF6 was also found to have excellent arc-quenching properties. Therefore, it is widely used as an insulating as well as arc-quenching medium in high voltage apparatus

Fig (2) d.c. breakdown strength of typical solid, liquid, gas and vacuum insulations in uniform, fields

SF6 and Other Gas MixturesSF6 is widely used for applications in power system due to its high dielectric strength and good arc interruption properties. However, SF6 gas has been found to be a green house gas that causes environmental problems. The production and use of SF6 gas has increased steadily and today it is about 10,000 metric tons due to leakages into the atmosphere from the electrical equipment. The concentration of SF6 in the environment has been steadily increasing. The release of SF6 into the atmosphere leads to concentration of large volumes of SF6 gas in the upper atmosphere. SF6 molecules absorb energy from the sun and radiate it into the atmosphere for long duration of time.There has been a large concern for these environmental effects and therefore the electrical industry has been looking for an alternate gas or gas mixture to be used in electrical equipment which presently use SF6 gas, as an insulating and arc interruption medium. The large amount of experimental data that is presently available suggest that 40% SF6/60% N2 mixtures have all the dielectric characteristics that make it suitable for use as insulation in high voltage equipment. Ideally the gas mixture should be suitable for use in the existing equipment as well as in the equipment that will be designed and manufactured in future.

Extensive research work done in SF6 and its mixtures with N2, air and CO2 has given breakdown values which are 80—90% of the pure SF6 values as shown in Table

Lightning Impulse Breakdown Strength of SF6/Other Gas MixturesBreakdown

Strength (kV/cm)

Mixture Ratio

89.0100% SF6 gas

80.01% SF6/99% Nitrogen

78.010% SF6/90% Nitrogen

76.520% SF6/80% Nitrogen

75.640% SF6/60% Nitrogen

76.510% SF6/90% CO2

76.520% SF6/80% CO2

75.540% SF6/60% CO2

77.010% SF6/90% Air

76.520% SF6/80% Air

75.640% SF6/60% Air

The industry is looking for a gas mixture that can replace the pure SF6 gas in the existing SF6 insulated apparatus, requiring no change in hardware, test procedures or ratings. SF6/N2 mixture is the one that has been found to be a good replacement for SF6. SF6/N2 mixtures have been used in Gas Insulated Transmission System and were found to perform well. Also, the work done so far has shown that the ability of SF6/N2 mixtures to quench high current arcs is promising.

The cost of such mixtures is low and is less sensitive to field non-uniformities present inside the equipment. In view of the above, the industry is trying to find out the optimum mixture ratio and the total pressure of the SF6/N2 mixture that would be required for a variety of applications. For many applications, such as Gas Insulated Transmission Systems, cables, capacitors, current transformers and voltage transformers, mixtures with different SF6 concentrations varying from 5% to 40%.

SF6/N2 mixtures show promise as a medium in circuit breakers. It has been found that a mixture containing 69% SF6/31 % N2 gave higher recovery rate than pure SF6 at the same partial pressure. It has also been shown that it is possible to further improve the arc interruption properties of SF6 by using SF6/N2 or SF6/He mixtures.

In summary, it may be said that there is an urgent need to significantly reduce the use of SF6 gas and its leakage from power apparatus. Use of gas mixtures appears to be feasible, but it has to be ensured that there is no loss in the performance of the equipment. Wherefore, further research has to be carried out to identify a suitable gas mixture, its pressure and its arc interruption capability to be used in the existing apparatus and the apparatus that will be designed and manufactured in future.

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ADVANTAGE OF GAS INSULATED TRANSFORMER

1. Non-flammabilityGas 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 PreventionPressure tank enables to withstand the pressure rise in case of internal fault.3. CompactnessBy directly coupling with gas-insulated switchgear, substation space can be minimized as the result of compact facilities.4. Easy installationOil or liquid purifying process is not necessary in case of gas insulated transformer.5. Easy inspection and maintenance workOnly SF6 gas pressure shall be basically monitored during periodically inspection.6. Environmentally FriendlyThe use of SF6 gas abolishes the risk of oil leakage.

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 0.5MPa2. Produce as large flow as possible by optimizing the layout of gas ducts in the windings3. Develop high capacity gas blower with high reliability4. Apply highly thermal-resistant insulating materials to raise the limit of winding temperature rises

Internal structure of gas insulated transformer

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 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.

 

Cooling Type

1.GNAN Type 2.GFAN Type 3.GFAF Type 4.GFWF Type

The IEC has published a new International Standard in the popular IEC 60076 series on power transformers. Dealing with gas-filled power transformers, IEC 60076-15, Power transformers - Part 15: Gas-filled power transformers, is also expected to be a popular part of the series.

A gas-filled transformer is a transformer whose magnetic circuit and windings are enclosed with an insulating gas. Sulfur hexafluoride (SF6) gas is generally used. Such a transformer is sometimes referred to as a gas-insulated (GIS) transformer.Experts say that these types of transformers offer certain advantages when space is in short supply or where they have to be housed internally or underground. Katsutoshi Toda of the Toshiba Corporation, who is TC 14, Power transformers, Project Leader for IEC 60076-15, said: “There has been a rapid increase of gas-filled transformers and reactors, particularly in South-East Asian countries like China, Japan, Korea and Singapore. Plans to construct indoor or underground substations accelerate this trend because of the difficulty in acquiring land for substations in large cities where electric power demand is concentrated. Requirements of security against fire accidents, compactness and total cost reduction are the key factors for these substations. Total gas-insulated substations combining GIS and gas-filled transformers meet these needs.”With the United Nations' forecasting that half the world’s population will live in urban areas by the end of 2008 and about 70% will be city dwellers by 2050, there is likely to be a huge increase in the demand for electricity. To meet this need, authorities and utilities will have to build new substations. Because city land is scarce and expensive, many of the new substations will have to be built indoors or underground. The demand for gas-filled transformers can therefore be expected to increase.IEC 60076-15 is an essential tool for those who design, construct, purchase or operate gas-filled transformers and for those who plan or construct the substations in dense city areas. Australia and the United Kingdom are already among the countries using these transformers, Toda said. Gas-filled transformers (up to 330kV-400MVA class) are currently operating in the field of electric power utilities and industrial firms.IEC 60076-15, Power transformers - Part 15: Gas-filled power transformersFurther information on the work of TC 14, Power transformers is available from the TC 'Dashboard' on the IEC's website. Information includes projects, publications and general contact information.