Power TransformerINTRODUCTION Transformer is a vital link in a power system which has made possible the power generated at low voltages (6600 to 22000 volts) to be stepped up to extra high voltages for transmission over long distances and then transformed to low voltages for utilization at proper load centers. With this tool in hands it has become possible to harness the energy resources at far off places from the load centers and connect the same through long extra high voltage transmission lines working on high efficiencies. At that, it may be said to be the simplest equipment with no motive parts. Nevertheless it has its own problems associated with insulation, dimensions and weights because of demands for ever rising voltages and capacities. In its simplest form a Transformer consists of a laminated iron core about which are wound two or more sets of windings. Voltage is applied to one set of windings, called the primary, which builds up a magnetic flux through the iron. This flux induces a counter electromotive force in the primary winding thereby limiting the current drawn from the supply. This is called the no load current and consists of two componentsone in phase with the voltage which accounts for the iron losses due to eddy currents and hysteresis, and the other 90 behind the voltage which magnetizes the core. This flux induces an electro-motive force in the secondary winding too. When load is connected across this winding, current flows in the secondary circuit. This produces a demagnetising effect, to counter balance this the primary winding draws more current from the supply so that IP.NP = IS.NS Where Ip and Np are the current and number of turns in the primary while IS and NS are the current and number of turns in the secondary respectively. The ratio of turns in the primary and secondary windings depends on the ratio of voltages on the Primary and secondary sides. The magnetic core is built up of laminations of high grade silicon or other sheet steel which are insulated from each other by varnish or through a coating of iron oxide. The core can be constructed in different ways relative to the windings.

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CONSTRUCTION 1- Transformer Core Construction in which the iron circuit is surrounded by windings and forms a low reluctance path for the magnetic flux set up by the voltage impressed on the primary. Fig (1), Fig. (6) and Fig. (7) Shows the core type

Fig (1) core type The core of shell type is sh own Fig.(2), Fig.(3), Fig.(4), and Fig.(5), in which The winding is surrounded by the iron Circuit Consisting of two or more paths through which the flux divides. This arrangement affords somewhat Better protection to coils under short circuit conditions.

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In actual construction there are Variations from This simple construction but these can be designed With such proportions as to give similar electrical characteristics.

Fig (2) shell type

Fig.(3) Single phase Transformer Fig. (4) Single phase Transformer .

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Fig. (5) 3- phase Transformer Shell type

Fig. (6) 3- phase Transformer core type

Fig. (7) Cross section of a three-phase Distribution Transformer (Core Type) Three-phase Transformers usually employ three-leg core. Where Transformers to be transported by rail are large capacity, five-leg core is used to curtail them to within the height limitation for transport. Even among thermal/nuclear power station Transformers, which are usually transported by ship and freed from restrictions on in-land transport, gigantic

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Transformers of the 1000 MVA class employ five-leg core to prevent leakage flux, minimize vibration, increase tank strength, and effectively use space inside the tank. Regarding single-phase Transformers, two-leg core is well known. Practically, however, three leg cores is used, four-leg core and five-leg core are used in large capacity Transformers. The sectional areas of the yoke and side leg are 50 % of that of the main leg; thus, the core height can be reduced to a large extent compared with the two leg core. For core material, high-grade, grain oriented silicon steel strip is used. Connected by a core leg tie plate fore and hind clamps by connecting bars. As a result, the core is so constructed that the actual silicon strip is held in a sturdy frame consisting of clamps and tie plates, which resists both mechanical force during hoisting the core-and-coil assembly and short circuits, keeping the silicon steel strip protected from such force. In large-capacity Transformers, which are likely to invite increased leakage flux, nonmagnetic steel is used or slits are provided in steel members to reduce the width for preventing stray loss from increasing on metal parts used to clamp the core and for preventing local overheat. The core interior is provided with many cooling oil ducts parallel to the lamination to which a part of the oil flow forced by an oil pump is introduced to achieve forced cooling. When erecting a core after assembling, a special device shown in Fig. (8) Is used so that no strain due to bending or slip is produced on the silicon steel plate.

Fig (8)

Fig (9) The steel strip surface is subjected to inorganic insulation treatment. All cores employ miter-joint core construction. Yokes are jointed at an angle of 45 to utilize the magnetic flux directional characteristic of steel strip. A computer-controlled automatic machine cuts grain-oriented silicon steel strip with high accuracy and free of burrs, so that magnetic characteristics of the grain-oriented silicon steel remains unimpaired. Silicon steel strips are stacked in a circle-section. Each core leg is fitted with tie plates

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on its front and rear side, with resin-impregnated glass tape wound around the outer circumference. Sturdy clamps applied to front and rear side of the upper and lower yokes are bound together with glass tape. And then, the resin undergoes heating for hardening to tighten the band so that the core is evenly clamped Fig. (9). Also, upper and lower clamps are connected by a core leg tie plate; fore and hind clamps by connecting bars. As a result, the core is so constructed that the actual silicon strip is held in a sturdy frame consisting of clamps and tie plates, which resists both mechanical force during hoisting the core-and-coil assembly and short circuits, keeping the silicon steel strip protected from such force. In large-capacity Transformers, which are likely to invite increased leakage flux, nonmagnetic steel is used or slits are provided in steel members to reduce the width for preventing stray loss from increasing on metal parts used to clamp the core and for preventing local overheat. The core interior is provided with many cooling oil ducts parallel to the lamination to which a part of the oil flow forced by an oil pump is introduced to achieve forced cooling. When erecting a core after assembling, a special device shown in Fig. (8) Is used so that no strain due to bending or slip is produced on the silicon steel plate. 2 - Winding Various windings are used as shown below. According to the purpose of use, the optimum winding is selected so as to utilize their individual features. 1 - Helical Disk Winding (Interleaved disk winding) In Helical disk winding, electrically isolated turns are brought in contact with each other as shown in Fig. (10) Thus, this type of winding is also termed "interleaved disk winding." Since conductors 1 - 4 and conductors 9 - 12 assume a shape similar to a wound capacitor, it is known that these conductors have very large capacitance. This capacitance acts as series capacitance of the winding to highly improve the voltage distribution for surge. Unlike cylindrical windings, Helical disk winding requires no shield on the winding outermost side, resulting in smaller coil outside diameter and thus reducing Transformer dimension. Comparatively small in winding width and large in space between windings, the construction of this type of winding is appropriate for the winding, which faces to an inner winding of relatively high voltage. Thus, general EHV or UHV substation Transformers employ Helical disk winding to utilize its features mentioned above. 2 - Continuous Disk Winding This is the most general type applicable to windings of a wide range of voltage and current Fig. (11). this type is applied to windings ranging from BI L of 350kV to BI L of 1550kV. Rectangular wire is used where current is relatively small, while transposed cable Fig. (12) is applied to large current. When voltage is relatively low, a Transformer of 100MVA

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or more capacity handles a large current exceeding 1000A. In this case, the advantage of transposed cable may be fully utilized.

Fig. (10)

Fig. (11). Continuous Disk Winding

Fig. (12) Transposed conductor construction Diagram Further, since the number of turns is reduced, even conventional continuous disk construction is satisfactory in voltage distribution, thereby ensuring adequate dielectric characteristics. Also, whenever necessary, potential distribution is improved by inserting a shield between turns. 3 - Helical windings For windings of low voltage (20kV or below) and large current, a helical coil is used which consists of a large number of parallel conductors piled in the radial Direction and wound. Adequate transposition is necessary to equalize the share of current among these parallel conductors. Fig (12) illustrates the transposing procedure for double helical coil. Each conductor is transposed at intervals of a fixed number of turns in the order shown in the figure, and as a result the location of each conductor opposed to the high

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voltage winding is equalized from the view point of magnetic field between the start and the end of winding turn.

Fig. (13) double helical coil 3 - Tank. The tank has two main parts: a The tank is manufactured by forming and welding steel plate to be used as a container for holding the core and coil assembly together with insulating oil. The base and the shroud, over which a cover is sometimes bolted. These parts are manufactured in steel plates assembled together via weld beads. The tank is provided internally with devices usually made of wood for fixing the magnetic circuit and the windings. In addition, the tank is designed to withstand a total vacuum during the treatment process. Sealing between the base and shroud is provided by weld beads. The other openings are sealed with oil-resistant synthetic rubber joints, whose compression is limited by steel stops. Finally the tank is designed to withstand the application of the internal overpressure specified, without permanent deformation.

Fig (14) Power Transformer 30 MVA 132 / 11 KV

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b - Conservator The tank is equipped with an expansion reservoir (conservator) which allows for the expansion of the oil during operation. The conservator is designed to hold a total vacuum and may be equipped with a rubber membrane preventing direct contact between the oil and the air.

Fig. (15)

Fig. (16) 4 - Handling devices: Various parts of the tank are provided with the following arrangements for handling the Transformer. - Four locations (under the base) intended to accommodate bidirectional roller boxes for displacement on rails. - Four pull rings (on two sides of the base) - Four jacking pads (under the base) - Tank Earthing terminals: The tank is provided with Earthing terminals for Earthing the various metal parts of the Transformer at one point. The magnetic circuit is earthed via a special external terminal. 5 - Valves: The Transformers are provided with sealed valves, sealing joints, locking devices and position indicators. The Transformers usually include: 163

- Two isolating valves for the "Buchholz" relay. - One drainage and filtering valve located below the tank. - One isolating valve per radiator or per cooler. - One conservator drainage and filtering valve. And when there is an on-load adjuster: - Two isolating valves for the protection relay. - One refilling valve for the on-load tap-changer. - One drain plug for the tap-changer compartment. 6 - Connection Systems Mostly Transformers have top-mounted HV and LV bushings according to DIN or IEC in their standard version. Besides the open bushing arrangement for direct Connection of bare or insulated wires, three basic insulated termination systems is available. Fully enclosed terminal box for cables Fig. (17&18) Available for either HV or LV side, or for both. Horizontally split design in degree of protection IP 44 or IP 54. (Totally enclosed and fully protected against contact's With live parts, plus protection against drip, splash, or spray water.) Cable installation through split cable glands and removable plates facing diagonally downwards. Optional conduit hubs suitable for single-core or three-phase cables with solid dielectric insulation, with or without stress cones. Multiple cables per phase are terminated on auxiliary bus structures attached to the bushings removal of Transformer by simply bending back the cables.

Fig. (17)

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Fig. (18) HV Side 300 KV

Fig. (19) LV Side (11KV) connection terminal 3-cable for each phase 7 - The dehydrating breather The dehydrating breather is provided at the entrance of the conservator of oil immersed equipment such as Transformers and reactors. The conservator governs the breathing action of the oil system on forming to the temperature change of the equipment, and the dehydrating breather removes the moisture and dust in the air inhaled and prevents the deterioration of the Transformer oil due to moisture absorption. Construction and Operation See Fig. (20) The dehydrating breather uses silica - gel as the desiccating Agent and is provided with an oil pot at the bottom to filtrate the inhaled air. The specifications of the dehydrating breather are shown in Table (1) and the operation of the component parts in Table (2).

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Fig. (20) Dehydrating breather 166

1. Case 2. Peep window 3. Flange 4. Oil pot 5. Oil pot holder 6. Breathing pipe 7.Filter 8. silica-gel 9.Absorbent 10. Oil (Transformer oil) 11. Wing nut 12.Cover 13. Suppression screw 14. Set screw 15. Oil level line (Red

Table - 1Type Weight of desiccating agent 4.5 kg Desiccating agent

FP4.5A

Material --- Silica-gel (Main component SiO2) Shape, Size --- spherical, approx. 4 5 Mixed ratio --- white silica-gel 75% blue silica-gel 25%

Table - 2Item Silicagel Blue silica -gel Action Removes moisture in the air inhaled by the Transformer Or reactor. In addition to the removal of moisture, indicates the Extent of moisture absorption by discoloration. (Dry condition) (Wet condition ) Blue ------ Light purple ----- Light pink Removes moisture and dust in the air inhaled by: the Transformer or reactor. In addition, while it is not performing breathing action, it seals the desiccating agent from the outer air to prevent unnecessary moisture Absorption of the desiccating agent. Absorbs dust and deteriorated matter in the oil pot, to Maintain the oil pot in a good operating condition.

Oil pot

Oil and filter

absorbent

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Bushing Having manufactured various types of bushings ranging from 6kV-class to 800kVclass, Toshiba has accumulated many years of splendid actual results in their operation. Plain-type Bushing Applicable to 24 kV-classes or below, this type of bushing is available in a standard series up to 25,000A rated current. Consisting of a single porcelain tube through which passes a central conductor, this bushing is of simplified construction and small mounting dimensions; especially, this type proves to be advantageous when used as an opening of equipment to be placed in a bus duct Fig. (21).

Fig. (21) 24 KV Bushing Oil-impregnated, Paper-insulated Condenser Bushing

Fig. (22) 800 KV bushing The oil-impregnated, paper insulated condenser bushing, mainly consisting of a condenser cone of oil-impregnated insulating paper, is used

For high-voltage application (Fig. 22&23). This bushing, of enclosed construction, offers the Following features: High reliability and easy maintenance.

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Partial discharge free at test voltage. Provided with test tapping for measuring electrostatic capacity and tan . Provided with voltage tapping for connecting an instrument Transformer if required.

Fig. (23) Bushing type GOEK 1425 for direct connection of 420 KV Power Transformer to gas insulated Switchgear or high voltage cable

Fig. (24) Cut away view of Transformer bushing type GOE Construction of Cable Connection and GIS Connection Cable Connection In urban-district substations connected with power cables and thermal power stations suffered from salt-pollution, cable direct-coupled construction is used in which a Transformer is direct-coupled with the power cable in an oil chamber. Indirect connection system in which, with a cable connecting chamber attached to the Transformer tank, a coil terminal is connected to the cable head through an oil-oil bushing in the cable connection chamber. Construction of the connection chamber can be divided into sections. Cable connections and oil filling can be separately performed upon completion of the tank assembling.

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Fig. (26) Indirect Cable Connection GIS (Gas Insulated Switchgear) Connection There is an increasing demand for GIS in substations from the standpoint of site-acquisition difficulties and environmental harmony. In keeping with this tendency, GIS connection-type Transformers are ever-increasing in their applications. The SF6 gas bus is connected directly with the Transformer coil terminal through an oil-gas bushing. Oil-gas bushing support is composed of a Transformer-side flange and an SF6 gas bus-side flange, permitting the oil side and the gas side to be completely separated from each other.

Fig. (27) Direct GIS Connection

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Buchholz Relays The following protective devices are used so that, upon a fault development inside a Transformer, an alarm is set off or the Transformer is disconnected from the circuit. In the event of a fault, oil or insulations decomposes by heat, producing gas or developing an impulse oil flow. To detect these phenomena, a Buchholz relay is installed. Buchholz Relay The Buchholz relay is installed at the middle of the connection pipe between the Transformer tank and the conservator. There are a 1st stage contact and a 2nd stage contact as shown in Fig. (28). the 1st stage contact is used to detect minor faults. When gas produced in the tank due to a minor fault surfaces to accumulate in the relay chamber within a certain amount (0.3Q-0.35Q) or above, the float lowers and closes the contact, thereby actuating the alarm device.

Fig. (28). Buchholz Relay

The 2nd stage contact is used to detect major faults. In the event of a major fault, abrupt gas production causes pressure in the tank to flow oil into the conservator. In 171

this case, the float is lowered to close the contact, thereby causing the Circuit Breaker to trip or actuating the alarm device. Temperature Measuring Device Liquid Temperature Indicator (like BM SERIES Type) is used to measure oil temperature as a standard practice. With its temperature detector installed on the tank cover and with its indicating part installed at any position easy to observe on the front of the Transformer, the dial temperature detector is used to measure maximum oil temperature. The indicating part, provided with an alarm contact and a maximum temperature pointer, is of airtight construction with moisture absorbent contained therein; thus, there is no possibility of the glass interior collecting moisture whereby it would be difficult to observe the indicator Fig. (30&31). Further, during remote measurement and recording of the oil temperatures, on request a search coil can be installed which is fine copper wire wound on a bobbin used to measure temperature through changes in its resistance. Winding Temperature Indicator Relay (BM SERIES) The winding temperature indicator relay is a conventional oil temperature indicator supplemented with an electrical heating element. The relay measures the temperature of the hottest part of the Transformer winding. If specified, the relay can be fitted with a precision potentiometer with the same characteristics as the search coil for remote indication.

Fig. (29) Construction of Winding Temperature Indicator Relay

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Fig (30) Oil Temperature Indicator

Fig. (31) Winding Temperature Indicator The temperature sensing system is filled with a liquid, which changes in volume with varying temperature. The sensing bulb placed in a thermometer well in the Transformer tank cover senses the maximum oil temperature. The heating elements with a matching resistance is fed with current from the Transformer associated with the loaded winding of the Transformer and compensate the indicator so that a temperature increase of the heating element is thereby proportional to a temperature increase of the winding-over-the maximum- oil temperature. Therefore, the measuring bellows react to both the temperature increase of the winding-over-the-maximum-oil temperature and maximum oil temperature. In this way the instrument indicates the temperature in the hottest part of the Transformer winding. The matching resistance of the heating element is preset at the factory.

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Pressure Relief Device When the gauge pressure in the tank reaches abnormally To 0.35-0.7 kg/cm.sq. The pressure relief device starts automatically to discharge the oil. When the pressure in the tank has dropped beyond the limit through discharging, the device is automatically reset to prevent more oil than required from being discharged.

Fig. (32) Pressure Relief Device Cooling System METHODS OF COOLING The kinds of cooling medium and their symbols adopted by I.S. 2026 (Part 11)-1977 are: (a) Mineral oil or equivalent flammable insulating liquid O (b) Non flammable synthetic insulating liquid L (c) Gas G (d) Water W

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(e) Air A The kids of circulation for the cooling medium and their symbols are: (a) Natural N (b) Forced (Oil not directed) F (c) Forced (Oil directed) D Each cooling method of Transformer is identified by four symbols. The first letter represents the kind of cooling medium in contact with winding, the second letter represents the kind of circulation for the cooling medium, the third letter represents the cooling medium that is in contact with the external cooling system and fourth symbol represents the kind of circulation for the external medium. Thus oil immersed Transformer with natural oil circulation and forced air external cooling is designated ONAF. For oil immersed Transformers the cooling systems normally adopted are: 1- Oil Immersed Natural cooled Type ONAN. Fig. (33 & 34) In this case the core and winding assembly is immersed in oil. Cooling is obtained by the circulation of oil under natural thermal head only. In large Transformers the surface area of the tank alone is not adequate for dissipation of the heat produced by the losses. Additional surface is obtained with the provision of radiators. 2. Oil Immersed Air Blast - Type ONAF Fig. (35 & 36) In this case circulation of air is obtained by fans. It becomes possible to reduce the size of the Transformer for the same rating and consequently save in cost.

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Fig. (33) Oil Immersed Natural cooled ONAN

Fig. (34) Oil Immersed Natural cooled ONAN

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Fig. (35) Oil Immersed Air Blast - Type ONAF

Fig. (36) Oil Immersed Air Blast - Type ONAF 3. Oil Immersed Water Cooled - Type ONWN In this case internal cooling coil is employed through which the water is allowed to flow. Apparently this system of cooling assumes free supply of water. Except at hydropower stations this would off-set the saving in cost when special means have to be provided for adequate supply of water. The circulation of oil is only by convection currents. This type of cooling was employed in older designs but has been almost abandoned in favor of the Type OFWF discussed later. 4. Forced Oil Air Blast Cooled - Type OFAF Fig. (37) In this system of cooling also circulation of oil is forced by a pump. In addition fans are added to radiators for forced blast of air. 5. Forced Oil Natural Air Cooled - Type OFAN Fig. (38) In this method of cooling, pump is employed in the oil circuit for better circulation of oil.

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Fig. (37) Forced-oil, Forced-air-cooled - Type OFAF

Fig. (38) Forced Oil Natural Air Cooled - Type OFAN 6. Forced Oil Water Cooled - Type OFWF In this type of cooling a pump is added in the oil circuit for forced circulation of oil, through a separate heat exchanger in which water is allowed to flow. 7. Forced Directed Oil and Forced Air Cooling -ODAF. 178

It should be remembered that Transformers cooling type OFAF and OFWF will not carry any load if air and water supply respectively is removed. It is quite common to select Transformers with two systems of Cooling e.g., ONAN/ONAF or ONAN/OFAF or sometimes three systems e.g., ONAN/ONAF/ OFAF. These determine the type of cooling upto certain loading. As soon as the load exceeds a preset value, the fans/pumps are Switched on. The rating of a Transformer with ONAN/ONAF cooling may be written, say, as 45/60 MVA. This means that so long as the load is below 45 MVA, the fans will not be working. These are Switched on automatically when the load on the Transformer exceeds 45 MVA. Type of cooling has a bearing on the cost of the Transformer. It shall be appreciated that the ONAN cooling has the advantage of being the simplest with no. fans or pumps and hence no auxiliary motors. On smaller units say up to 10 MVA, saving in price in changing from ONAN cooling to other forms of cooling is negligible. On bigger units not only there is a saving in price but also the reduced weights and dimensions, with other systems of cooling of Transformers, render the transport easy and decrease the cost of Foundations etc. Site conditions sometimes influence the preferred cooling arrangement. For example the advantage of reduced price, dimensions and weight in case of type OFWF can be fully realised only where water supply is readily available. Where special arrangements have to be made for water supply and disposal of the water, the installation costs for OFWF Transformers may increase. INSULATING OIL (SPECIFICATIONS AND DEHYDRATION AT SITE) In Transformers, the insulating oil provides an insulation medium as well as a heat transferring medium that carries away heat produced in the windings and iron core. Since the electric strength and the life of a Transformer depend chiefly upon the quality of the insulating oil, it is very important to use a high quality insulating oil. The insulating oil used for Transformers should generally meet the following requirements: (a) Provide a high electric strength. (b) Permit good transfer of heat. (c) Have low specific gravity-In oil of low specific gravity particles which have become suspended in the oil will settle down on the bottom of the tank more readily and at a faster rate, a property aiding the oil in retaining its homogeneity. (d) Have a low viscosity- Oil with low viscosity, i.e., having greater fluidity, will cool Transformers at a much better rate. (e) Have low pour point- Oil with low pour point will cease to flow only at low temperatures. (f) Have a high flash point. The flash point characterizes its tendency to evaporate. The lower the flash point

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the greater the oil will tend to vaporize. When oil vaporizes, it loses in volume, its viscosity rises, and an explosive mixture may be formed with the air above the oil. (g) Not attack insulating materials and structural materials. (h) Have chemical stability to ensure life long service. Various national and international specifications have been issued on insulating oils for Transformers to meet the above requirements. The specifications for insulating oil stipulated in Indian Standard 335: 1983 are given below.1 2 3 4 5 6 characteristic Appearance Density at 29.5C, Max Interfacial tension at 270C, Min. Flash point Min. Pour Point Max. Corrosive Sulphur (in terms of classification of copper strip). Electric strength (breakdown voltage) Min. (a) New unfiltered oil (b) After filtration Dielectric dissipation factor (tan ) at 90 C Max. Specific resistance (resistivity): (a) At 9 0 C Min. (b) at 2 7 0 C Min. Oxidation stability. (a) Neutralization value, after oxidation Max. (b) Total sludge, after oxidation, Max. Presence of oxidation inhibitor Water content, Max. Requirement The oil shall be clear and transparent and free from suspended matter or sediments. 0.89 g/cm3 0.04 N/m. 104 C - 9 C Non-corrosive.

7

30 kV (rms) 60 kV (rms). 0.002 35 X1012

8 9

/ cm1012

1500 X

/ cm

10

0.4 mg KOH/g 0.10 percent by weight

11 12

The oil shall not contain antioxidant additives. 15 ppm

Gases analysis The analysis of gases dissolved in oil has proved to be a highly practical method for the field monitoring of power Transformers. This method is very sensitive and gives an early warning of incipient faults. It is indeed possible to determine from an oil sample of about one litre the presence of certain gases down to a quantity of a few mm3 , i.e., a gas volume corresponding to about 1 millionth of the volume of the liquid (ppm). The gases (with the exception of N2 and O2) dissolved in the oil are derived from the degradation of oil and cellulose molecules that takes place under the influence of thermal and electrical stresses. Different stress modes, e.g., normal operating

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temperatures, hot spots with different high temperatures, partial discharges and flashovers, produce different compositions of the gases dissolved in the oil. The relative distribution of the gases is therefore used to evaluate the origin of the gas production and the rate at which the gases are formed to assess the intensity and propagation of the gassing. Both these kinds of information together provide the necessary basis for the evaluation of any fault and the necessary remedial action. This method of monitoring power Transformers has been studied intensively and work is going on in international and national organizations such as CIGRE, IEC and IEEE. APPLICATION. The frequency with which oil samples are taken depends primarily on the size of the Transformer and the impact of any Transformer failure on the network. Some typical cases where gas analysis is particularly desirable are listed in the following: 1 - When a defect is suspected (e.g., abnormal noise). 2 - When a Buchholz (gas-collecting) relay or pressure monitor gives a signal. 3 - Directly after and within a few weeks after a heavy short circuit 4 - In connection with the commissioning of Transformers that are of significant importance to the network, followed by a further test some months later. Different routines for sampling intervals have been developed by different utilities and in different countries. One sampling per year appears to be customary for large power Transformers (Rated >= 300 MVA >= 220 kV). The routine that has been used over a long period of time of checking the state of the oil every other year by measuring the breakdown strength, the tan value, the neutralization coefficient and other physical quantities is not replaced by the gas analysis. Extraction and analysis To be able to carry out a gas analysis, the gases dissolved in the oil must be extracted and accumulated. The oil sample to be degassed is sucked into a pre-evacuated degassing column. A low pressure is maintained by a vacuum pump. To assure effective degassing (> 99 per cent), the oil is allowed to run slowly over a series of rings which enlarge its surfaces. An oil pump provides the necessary circulation. The gas extracted by the vacuum pump is accumulated in a vessel. Any water that may have been present in the oil is removed by freezing in a cooling trap to ensure that the water will not disturb the vacuum pumping. The volumes of the gas and the oil sample are determined to permit calculation of the total gas content in the oil. The accumulated gas is injected by means of a syringe into the gas chromatograph, which analyses the gas sample. The result is plotted on a recorder in the form of a chromatogram. Using calibration gases it is possible to identify the different peaks on a chromatogram. Recalculation of the height of a peak to the content of this gas is done by comparison with chromatogram deflections from calibration gases. With the composition of the gas mixture and the total gas content in the oil sample known; the content (in ppm) of the individual gases in the oil is obtained. The following gases are analyzed: 1 - CARBON MONOXIDE CO

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2 - CARBON DIOXIDE 3 - HYDROGEN 4 - ETHANE 5 - ETHENE 6 - ACETYLENE 7 - METHANE 8 - PROPANE

CO2 H2 C2H6 C2H4 C2H2 CH4 C3H6

The detection limits depend partly on the total gas content; for hydrocarbons (except methane) the limit lies below 0,5 ppm, for hydrogen, methane and carbon monoxide about 5 ppm and for carbon dioxide about 2 ppm. This high sensitivity is necessary in those cases where it is desired to determine a trend in the gas evolution at short sampling intervals, e.g., during a heat run test or when oil samples are taken at intervals of only a few days. Identification of faults. The fault types that can and should be identified are corona, electrical discharges, excessively hot metal surfaces and fast degradation of cellulose. It is possible to obtain an idea of the type of fault by using a diagnosis scheme. A number of different schemes of this type have been prepared. To avoid having to deal with the contents of the individual gases, one frequently uses quotients between different gases. Some schemes give an appearance of great precision, but certain care should be observed when making assessments, until all factors influencing the gassing rate are known. GAS ANALYSIS OF TRANSFORMER Type Of Gas Caused By CARBON MONOXIDE, AGEING CO CARBON DIOXIDE, CO2 HYDROGEN, ELECTRIC ARCS H2 ACETYLENE, C2H2 ETHANE, LOCAL C2H6 OVERHEATING ETHENE, C2H4 PROPANE, C3H6 HYDROGEN, H2 CORONA METHANE, CH4 Gas concentration limits used in the Interpretation of DGA data A statistical survey concerning gas concentrations in Transformer Oil using the results of that survey the following limits have been set: 182

H2 CH4 C2H6 C2H4 C2H2 CO CO2

Threshold Limit 20 10 10 20 1 300 5000

Warning Limit 200 50 50 200 3 1000 20000

Fault Limit 400 100 100 400 10

Unit ppm ppm ppm ppm ppm ppm ppm

The limits above are for a Transformer which are open with a breather and have no OLTC or has a separate conservator for the OLTC. If the Transformer tank and the OLTC have a common conservator the warning and fault limits are 30 ppm and 100 ppm respectively for C2H2 Standard IEC 60475 Method of sampling liquid dielectrics IEC 60422 Supervision and maintenance guide for mineral Insulating oils in electrical equipment IEC 60567 Guide for the sampling of gases and of oil from oil filled electrical equipment and for the analysis of free and dissolved gases IEC 60599 Mineral oil-impregnated electrical equipment in Service -Guide to the interpretation of dissolved and Free gases analysis IEC 60296 Specification for unused mineral insulating oils for Transformers and Switchgear ASTM Dl 17-96 Standard guide for sampling, test methods, Specifications, and guide for electrical insulating oils Of petroleum origin ASTM D923-97 Standard practices for sampling electrical insulating liquids ASTM D3613-98 Standard test methods of sampling electrical Insulting oils for gas analysis and determination of Water content ASTM D36 12-98 Standard test method for analysis of gases dissolved In electrical insulating oil by gas chromatography ASTM D3487-88(1993) Standard specification for mineral insulating oil Used in electrical apparatus

PARALLEL OPERATION OF THREE-PHASE TRANSFORMERS

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Ideal parallel operation between Transformers occurs when (1) there are no circulating currents on open circuit, and (2) the load division between the Transformers is proportional to their kVA ratings. These requirements necessitate that any - two or more three phase Transformers, which are desired to be operated in parallel, should possess: 1) The same no load ratio of transformation; 2) The same percentage impedance; 3) The same resistance to reactance ratio; 4) The same polarity; 5) The same phase rotation; 6) The same inherent phase-angle displacement between primary and secondary terminals. The above conditions are characteristic of all three phase Transformers whether two winding or three winding. With three winding Transformers, however, the following additional requirement must also be satisfied before the Transformers can be designed suitable for parallel operation. 7) The same power ratio between the corresponding windings. The first four conditions need no explanation being the same as in single phase Transformers. The fifth condition of phase rotation is also a simple requirement. It assumes that the standard direction of phase rotation is anti-clockwise. In case of any difference in the phase rotation it can be set right by simply interchanging two leads either on primary or secondary. It is the intention here to discuss the last two i.e., sixth and seventh conditions in detail. Connections of Phase Windings The star, delta or zigzag connection of a set of windings of a three phase Transformer or of windings of the same voltage of single phase Transformers, forming a three phase bank are indicated by letters Y, D or Z for the high voltage winding and y, d or z for the intermediate and low voltage windings. If the neutral point of a star or zigzag connected winding is brought out, the indications are Y N or Z N and y n and z n respectively. Phase Displacement between Windings The vector for the high voltage winding is taken as the reference vector. Displacement of the vectors of other windings from the reference vector, with anticlockwise rotation, is represented by the use of clock hour figure. IS: 2026 (Part 1V)-1977 gives 26 sets of connections star-star, star-delta, and star zigzag, delta-delta, delta star, delta-zigzag, zigzag star, zigzag-delta. Displacement of the low voltage winding vector varies from zero to -330 in steps of -30, depending on the method of connections. Hardly any power system adopts such a large variants of connections. Some of the commonly used connections with phase displacement of 0, -300, -180" and -330 (clock-hour setting 0, 1, 6 and 11) are shown in Table ( below) Symbol for the high voltage winding comes first, followed by the symbols of windings in diminishing sequence of voltage. For example a 220/66/11 kV Transformer connected star, star and delta and vectors of 66 and 11 kV windings having phase displacement of 0 and -330 with the reference (220 kV) vector will be represented As Yy0 - Yd11.

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If a pair of three phase Transformers have the same phase displacement between high voltage and low voltage windings and possess similar characteristics (Such as no load ratio of transformation phase rotation, percentage impedance) these can be paralleled with each other by connecting together terminals which correspond physically and alphabetically. Thus taking the case of two three phase Transformers having vector symbols Dd0 and Yy0, these can be put into parallel operation by connecting H.V terminals U1, V1 and W1 of one Transformer to HV terminals U1, V1 and W1 of the other Transformer. Similarly, low voltage terminals U1V1 and of one Transformer should be connected to U1, V1 and W1 terminals of the second Transformer. Sometimes it may be required to operate a three-phase Transformer belonging to one group with another three-phase Transformer belonging to a different group. This is possible with suitable changes in external connections. For example, let us consider a three-phase Transformer with vector symbol Dy1 and see how this can be operated in parallel with a three-phase Transformer of similar characteristics but having vector symbol Yd11. Referring to Table (below) the phasor diagrams of the induced voltages in the h-v and l-v windings of the two Transformers, with the phase sequence of the supply connected to terminals U,V, W of the two being RYB in the anti-clockwise direction are as shown in Figs. (39a) and (39b) respectively.

Fig. (39) Example of parallel operation of Transformers of groups 3 and 4 (Transformers having symbols Dy 1 and Yd 11 operating in parallel

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It may be seen from these diagrams that the phase displacement between the induced voltages in the h-v and l-v windings is -30 in the first Transformer and it is -330 in the second Transformer. However, for the successful parallel operation of these Transformers, the phase displacement must be the same in the two. This can be achieved by interchanging externally two of the h-v connections of the incoming Transformer to the supply, i.e., by connecting 1V to bus B and 1W to bus Y as shown in Fig. (39c) by full lines instead of Connecting 1V to bus Y and 1W to bus B as shown in Fig (39b) by dotted lines.

Vector Group This results in the reversal from anticlockwise direction to clockwise direction of the phase rotation of the induced voltages as shown by arrows in Fig. (39c) and therefore results in a phase displacement of -30 between the induced voltages in the h-v and lv windings [see Fig. (39c)].

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The change in two of external it-v connections of the second Transformer thus brings it -30. The secondary voltages of this Transformer, however, have a phase rotation reversed with respect to that of the secondary voltages of the first Transformer. This can be set right by changing again the two corresponding l-v external connections, i.e., by connecting 2V to bus b and 2W to busy as shown in Fig. (39c) instead of connecting 2V to busy and 2W to bus b as shown in Fig. (39b). Thus Transformers connected in accordance with clock hour No. 1 and 11 can be operated in parallel with one another by interchanging two of the external h-v and also the corresponding l-v connections of one Transformer. Transformers connected in accordance with clock hour No. 0 and 6 however, cannot be operated in parallel with one another without altering the internal connections of one of them as change of external connections only brings about change in phase rotation. The general principle applying to the parallel operation of a three winding Transformer with another three winding Transformer are the same as those for the paralleling of two winding Transformers. However, to obtain the same percentage impedance. Between the three pairs of windings of the two (or more) Transformers (being paralleled) it is imperative that the power ratio of the corresponding windings of the Transformers should be the same, i.e.

( PH )1 ( PM )1 ( PL)1 = = ( PH ) 2 ( PM ) 2 ( PL) 2Where (PH)1 and (PH)2 represent the powers of the h-v windings (say primary), (PM)1 and (PM)2 represent the powers of the medium voltage windings (say secondary) and (PL)1 and (PL)2 represent the powers of the low voltage windings (say tertiary) of the two Transformers labeled 1 and 2. This is proved below. Fig. (40) Shows two 3 winding Transformers (represented by their equivalent circuits) connected in parallel. The currents flowing in the various circuits and windings are shown in the figure.

Fig (40) Shows two 3 winding Transformers (represented)

( ZH )1 ( ZM )1 ( ZL )1 = = ( ZH ) 2 ( ZM ) 2 ( ZL ) 2

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Thus the power ratios of the corresponding windings are similar. This as is evident also fulfils the second condition of same percentage impedance. When Transformers which do not fulfilling this condition are paralleled the operation may be satisfactory without fulfilling the ideal conditions so long as the loads to be carried do not overload either Transformer. Therefore, when new three-phase 3 winding Transformers are to be purchased for parallel operation with existing three-phase 3-winding Transformers the purchase order must specify the power ratings of the various windings of the existing Transformers along with other specifications and indicate that the power ratios of the corresponding windings of the various Transformers must be identical failing which it will be impossible to design Transformers with same percentage impedances for the corresponding windings. Tap Changer The method to change the ratio of Transformers by means of taps on the winding is as old as the Transformer itself. From a very early stage, Transformers with a turn ratio changeable within certain limits have been used for electrical power transmission, since this is the simplest method to control the voltage level as well as the reactive and active power in electrical networks.

Tap-changer with single phase transformer

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At the beginning of the development it was sufficient to have tappings connected to bushings outside the Transformer tank, which were connected according to the necessity of the network. A more comfortable way was to connect the tappings to tap Switches today called "off-circuit" or "no-load tap changers" - which could only be actuated when the Transformer was de-energized. Obviously, this simple device only permitted occasional corrections of the Transformer ratio. It was not possible to control voltage drops caused by load changes in the network. At that stage these parameters could only be controlled at the generating plant. To solve this problem, Switching devices were needed which permitted the change of the turn ratio of Transformers under load condition, i.e. Without interrupting the load current such Switching devices - today called "on-load taps changers" (OLTC) were introduced to Transformers more than 70 years ago. The demand for (OLTCs) came an urgent necessity in the 1920ies, then power consumption took a sharp upward trend, which required the interconnection and expansion of the electrical networks. The very rapid development brought, within a few years, solutions which were quite satisfactory in regards to operating safety and efficiency. The development of (OLTCs) was accelerated over the years due to the steady increase of the transmission voltage and power. The introduction of OLTCs improved the operating efficiency of electrical systems considerably and this technique found acceptance worldwide. In other industrialized countries the situation is comparable. In general the percentage of Transformers equipped with OLTCs is increasing with the increase of the load density and interconnection of electrical networks. In addition. OLTCs applied in industrial process Transformers as regulating units in the chemical and metallurgical industry is another important field of application. These range from some hundred to around 300,000 operations per year while the rated currents range from approximately 50 to 3000 Amps. Today's state of the art OLTC has reached such a high level of reliability that it is safe to state that its mechanical life expectancy is equivalent to that of the Transformer. Exceptions may be applications in industrial process Transformers. However, even on such applications experience shows that with proper maintenance several million operations can be obtained. Table below shows a survey of the typical number of operations for various applications. Transformer No of operation data Power Power Voltage Current OLTC Per Transformer ring ring ring Year MVA KV A Min Mean Max Generator 100 110 100 - 500 3000 10000 -1300 765 2000 Interconnection 200 110 300 - 300 5000 25000 -1500 765 3000 Distribution 15 - 400 60 - 525 50 - 1600 2000 7000 20000 189

Electrolysis Chemistry Arc furnace

10 - 300 20 - 110 50 - 3000 1000 30000 150000 0 1.5 - 80 20 - 110 50 - 1000 1000 20000 70000 2.5 - 20 - 230 50 - 1000 2000 50000 300000 150 0

The problem to be solved when changing taps under load is how to connect the tappings of the Transformer winding successively to the same output terminal without interrupting the load current. During the load transfer operation between to adjacent taps, both taps must be temporarily connected to the output terminal. To avoid a short circuit of the winding transition impedances, which can be reactors or resistors? Are inserted. Two basic principles have been invented and are still used today - the slow motion reactor Switching principle and the high speed resistor Switching principle. Today both principles have been developed into reliable OLTCs. The reactor type OLTC has its development origin in the USA, hut also in Germany inventions were applied for a patent in 1905 and 1906. Because of the fact that the reactor Switching principle causes a 90 degree phase shift between the Switched current and the recovery voltage arising at the Switching distance, the reactor type OLTC is less suitable for large step voltages. In addition to this the costs of transition reactors increase considerably with higher step voltages. Thus the reactor Switching principle over the years has lost the remarkable importance it had in the beginning of the OLTC development. In the late 1940 is many OLTC manufacturers abandoned the production of OLTCs with this Switching principle. However, in the USA the reactor principle is still used in a large scale and reactor type OLTCs are still under production. The high-speed resistor type OLTC has its origin in the invention of Dr. Jansen of a diverter Switch and a tap selector. Which were patented in 1926. The transition impedance is been carried out with ohmic resistor with this principle the current Switched and the recovery voltage are in phase. This lightens the quenching of the arc in the current zero. The transition resistors hake to be dimensioned only for a short-time loading which enables an economic use of OLTCs in case of higher step voltages and power. Though the reactor principle has also proven itself, its application is limited to loner voltages, whereas the resistor principle dominates in the high voltage field or in special applications like HVDC - Transformers, Phase-Shifting Transformers or EHVTransformers. The reactor principle OLTC in these fields can only be applied by mean of booster Transformers. Which make its application more difficult in regards to transport weight, transport size and profile and overall economic considerations compared to the resistance principle OLTC. DESIGN CONCEPTS OF ON-LOAD TAP-CHANGERS With an on-load tap-changer the Transformer voltage ratio can be varied in steps by adding or subtracting turns. For this purpose a Transformer is furnished with a tapped winding and these taps are connected to terminals on the tap-changer. The tap-changer provides two basic functions.

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Fig (2) Basic connection of a star-point linear regulation The first is to select a Transformer tapping connection in an open-circuit condition, the second is to divert or transfer power to that selected tapping without interrupting the through-current. The simplest type OLTC, the selector Switch, combines these two functions into one device. Whereas separate selectors and diverter or transfer Switches are used for higher power requirements. Various tapping winding configurations are possible. The selection function can be without change-over selector (linear). Or with change-over selector (reversing or coarse / fine). A basic connection of a star-point linear regulation is given in Fig (2). The mechanical configuration of the tap selector can be designed as a single or double multiway selector. The transfer of the load current from the connected to the preselected trip is either achieved by means of resistor transition or the alternative method. Mainly used in the USA, reactor transition. In service, the diverter or transfer Switch is required to make and break current at a recovery voltage whose value is in the same order as the voltage between two taps. The power transfer function can be symmetrical or asymmetrical. The former providing similar Switching conditions for advanced or retard power flow from the Transformer. The action of the diverter or transfer Switch can be rotary or oscillatory. All designs of tap-changers maintain direct mechanical synchronism between the tap selector, change-over selector and the diverter or transfer Switch. The transfer of electrical power involves arcing in the oil and therefore contamination of the insulating oil (the exception are OLTCs that use vacuum interrupters as Switching devices). Therefore, the Switching devices are located in their own Switching compartment to separate the contaminated oil from the oil in the transformer main tank. To fulfill this requirement several designs have been developed. Selector Switches are designed for operation within an enclosure inside the Transformer tank (in-tank type) or externally in a separate oil-tilled housing bolted to the outside of the main Transformer tank (compartment type). HIGH-SPEED RESISTOR TYPE OLTC

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The high-speed resistor type OLTC is designed either as a tap selector and a diverter Switch, or as a selector Switch combining the functions of the tap selector and diverter Switch into one device.

Fig (3) Principle scheme of a selector Switch type OLTC The latter is economical to manufacture, but certain inherent limitations reduce the possible applications to small and medium size Transformers with highest voltages of equipment of 132 kV and rated-through currents in the range of 500 A to 600 A.

Fig (4) Principle scheme of a-tap selector and diverter Switch type OLTC This type can only be built in one enclosure as mentioned above and, therefore, the arc products are in contact not only with wearing mechanical parts, but also with insulation subject to high voltages. The selector Switch principle is represented in Fig. (3) The OLTC comprising a tap selector and a diverter Switch lends itself for any application up to the highest Transformer rating. Line-end applications with highest voltages for equipment of 362 kV and rated through-currents of 4500 A have been realized. Figure (4) shows an OLTC comprising a tap selector and diverter Switch. With the tap selector-diverter Switch concept the tap-change is affected in two steps. The tap adjacent to the one in

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service is pre-selected load free by the tap selector. Thereafter the

Fig (4) Switching sequence for tap-changer on Switching from position 6 to position 5. a) Position 6. Selector contact V lies on tap 6 and selector contact H on tap 7. The main contact x carries the load current. b) Selector contact H has moved in the no-current state from tap 7 to tap 5. c) The main contact X has opened. The load current passes through the resistor Ry and the resistor contact y. d) The resistor contact u has closed. The load current is shared between Ry and Ru The circulating current is limited by the resistance of Ry + Ru. e) The resistor contact y has opened. The load current passes through Ru and contact u f) The main contact V has closed, resistor Ru. Has been short-circuited and the load current passes through the main contact V. The tap-changer is now in position 5.

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Fig (45) Three-phase tap-changer type UCBRN 380/600, neutral point design for 21 position with plus/minus Switching

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Fig. (46) Motor-drive mechanism type BUE for UC tap-changer Testing Tap-changers undergo type tests according to the international standards for on-load tap-changers, IEC 214 the first edition of which was published in 1966 and the most recent one in 1976. The tests on the tap-changer itself comprise: 1- Temperature rise of contacts at 1.2 times the maximum rated through-current. 2- Switching tests. 3- Short-circuit current tests. 4- Temperature rise of transition resistors. 5- Mechanical tests. 6- Dielectric tests. And for the motor-drive mechanism: 1- Mechanical load test. 2- Overrun test. 3- Degree of protection of motor-drive cubicle. SF6 Transformer Introduction Demand for effective space utilization is becoming increasingly stronger as a result of grade advancement of commercial/industrial activities and urban life styles. Concurrently, city construction facilities including buildings, underground shopping areas, traffic systems, and public structures are becoming larger in size and gaining in the degree of complications. Since such facilities immensely contribute to improving the efficiency of urban activities, the current trend indicates the possibility of further expansion in the future. On the other hand, accidents involving outbreaks of extensive fire and other troubles are occasionally occurring in these large-sized urban facilities, resulting in the creation of public voices demanding improved fire or accident

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preventive measures.

These construction facilities of cities represent high-valued social assets. However, since a great number of citizens utilize such .facilities day after day, it is quite essential to provide effective means to eliminate outbreaks of fire. To achieve this purpose, it is important to install modern fire-fighting systems capable of coping with various causes of fire. At the same time, Basically it is most important to eliminate the possible causes of fire. The SF6 gas-insulated Transformers are designed to ideally satisfy Non flammability-ensuring plans of power reception and transformation systems installed in these urban facilities. Since no oil for insulation is used, these Transformers can completely free structures or adjacent rivers from oil contamination during new installation work or system operation. In other words, the SF6 gas-insulated Transformers qualify themselves as truly "non flammability-ensuring equipment" usable for power systems required to prevent fires or accidents and eliminate pollution.

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Features The SF6 gas-insulated Transformers offer excellent insulation and 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. 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. Applications The SF6 gas-insulated Transformers are suitable for the following applications: 197

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 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 zones

Specifications and Ratings The SF6 gas-insulated Transformers are manufactured under the following standard specifications. Table 1 Standard specifications

NOTES: 1. Mounting of on-load tap-changer is possible. The voltage adjusting range in this case is 10 % of the rated voltage. 2. As for codes affixed to the primary tap voltage, F indicates full-capacity taps and R indicates rated taps. 3. Consultation regarding ratings other than the above is accepted. Quality specifications The following specifications are provided to ensure safe operation of gas-insulated Transformers. Withstand voltage during zero gas gauge pressure No problem is caused by operation under normal operating voltage. Permissible load under zero gas gauge pressure No problem is caused by 50 % load continuous operation. 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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External Dimensions and Weight Figures below show external dimensions and weight. Since external dimensions are subject to change without notice, please obtain final confirmation from approval drawings. Also,

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Natural-cooled type

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Natural-cooled type

NOTE: In case of 72.5 kV, GIS direct-coupling type, X size (up to bushing Terminal end) becomes "the value in the above Table + 600 mm."

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Forced-gas-circulated, natural-air-cooled type SF6 gas-insulated Transformer

Forced-gas-circulated, forced-air-cooled type

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Accessories SF6 gas temperature indicator (dial thermometer) Measures temperature of SF6 gas sealed in Transformer tanks. Gas temperature 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.

Dial thermometer 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 / 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.cm 2

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Pressure gauge (compound gauge) 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.

Temperature compensating pressure switch SF6 Gas Properties Introduction 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. 204

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 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 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 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:

S 2 + 6 F 2 2SF 6 + 524 KcalYou 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

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

6 F 2 S 2 + 2 SF 6 + 524 Kcal

The dissociation products before interruption of the arc 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. 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.

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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 (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 toxic threshold (1 ppm is detected by smell). 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.

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Any damage to the hands caused by these powders can be neutralized by limewater. Conclusion It is important to point out that sulfur hexafluoride does not bring about an increase in the risks entailed in the work stations. This lack of specific danger is furthermore confirmed by the fact that we have not had to record any accident since 1960, the year in which SF6 was first used as a breaking agent. As a matter of interest SF6 does not harm the ozone layer. This is partly due to its weight. The electric arc The creation of an arc Everyone has noticed that, when placing ones hand near to a television screen, one feels a force which attracts. There exists, in fact, in this apparatus, what one calls an electric field. The latter is the source of an electric current, for it is this that displaces the electrons in the conductors. An electric field appears at the separation of the live contacts. Such a field of a very great intensity will draw electrons at the hot points of contacts. The electric arc has been born. If its own energy is not sufficient, the arc will extinguish rapidly itself. If, on the other hand, it is crossed by a strong current, it draws throughout its own energy, which ensures the survival of the arc. The electric arc: We have seen that the electric field was at the origin of the displacement of electrons. When the contacts separate, the electric field draws electrons to the hot points. These electrons are going to circulate in surroundings which are not conductive, which one calls dielectric, and will cause the temperature of the surroundings to increase, if they are in sufficient number. All bodies, under the influence of temperature, end up by reaching their threshold of ionic dissociation. At this moment, it parts with electrons, and becomes conductive. These electrons themselves, and for the same reasons, will create others. We have an avalanche, that is to say, creation of electrons, which will accelerate. One can reach temperature of 15000 C. The value of the thermal power can be 10MW. The electric arc is thus going to follow the variations of alternating current, and thus, at regular intervals, the arc will disappear and reappear immediately, if the electrons have not been eliminated because in this case, the surroundings remain conductive. In order to eliminate these electrons, one could: - Rid oneself of them by some physical means, like blow-out for example, - use dielectric with a very high speed of recuperation (the case of SF6) - use a process to reduce the temperature of the element (decompression, blowout, etc.) Out-off a current If we perfect a system which allows cooling the arc (turning arc, magnetic blow-out, mechanical or thermodynamic blow-out, etc ...). One can well understand that the arc increasing to temperatures of 1500C. Under the effect of current passing through it, will see a temperature decrease as soon as the alternating current starts its descent towards 0. The temperature will decrease all the more rapidly as:

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- SF6 has two states of conduction, and appearance of the resistive arc will bring about a fall in the intensity, and thus its temperature, - SF6, as we have seen in its physical properties, is a gas which Absorbs large quantities of energy when it dissociates. The blow out of the arc will thus (mean) evacuate a large quantity of energy. This lowering of temperature will make the ionic recombination of the bodies and the dielectric will recover its insulating properties which thus ensure interruption of the current. Lastly the hydrofluoric acids attack all metals giving metallic fluorides which are all very hydroscopic insulating powders.

Fig (1) Disruptive voltage versus pressure

Fig (2) SF6 absolute pressure versus temperature with constant volume mass (density)

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Electrical Substations Electrical Network comprises the following regions: 1 - Generating Stations. 2 - Transmission Systems. 3 - Distribution Systems. 4 - Load Points. www.sayedsaad.com Functions of a Substation 1 - Supply of required electrical power. 2 - Maximum possible coverage of the supply network. 3 - Maximum security of supply. 4 - Shortest possible fault-duration. 5 - Optimum efficiency of plants and the network. 6 - Supply of electrical power within targeted frequency limits, (49.5 Hz and 50.5 Hz). 7 - Supply of electrical power within specified voltage limits. 8 - Supply of electrical energy to the consumers at the lowest cost. www.sayedsaad.com Substation Layouts 1. Switching requirements for normal operation. 2. Switching requirements during abnormal operations, such as short circuits and overloads. 3. Degree of flexibility in operations, simplicity. 4. Freedom from total shutdown and permissible period of shutdown. 5. Maintenance requirements, space for approaching various 6. Safety of personnel. 7. Protective zones, main protection, back-up protection 8. Bypass facilities. 9. Technical requirements such as ratings, clearances, Earthing lightning protection, Noise, radio interference, etc. 10. Provision for extensions, space requirement. 11. Economic considerations, availability, foreign exchange involvement, cost of the equipment. 12. Requirements of network monitoring, power line communication, data collection, Data transmission etc. 13. Compatibility with ambient conditions. 14. Environmental aspects, audible noise, RI, TI etc. 15. Long service life, Quality, Reliability, and Aesthetics. www.sayedsaad.com Essential Features for substation 1 - Outdoor Switchyard having any one of the above. 2 - Bus-Bar schemes. 3 - High voltage Switchgear. Medium voltage Switchgear, Low voltage Switchgear and control room. 4 - Office building.

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5 - Roads and rail track for transporting equipment. 6 - Incoming line towers and outgoing line towers/cables. 7 - Store. 8 - Maintenance workshop (if required). 9 - Auxiliary power supply Low voltage AC. 10 - Battery room and low voltage DC. Supply system. 11 - Fire fighting system. 12 - Cooling water system; drinking water system, etc. 13 - Station Earthing system. 14 - Lighting protection system, overhead shielding. 15 - Drainage system. 16 - Substation lighting system etc. 17 - Fence and gates, Security system etc. www.sayedsaad.com SF6 Gas Insulated Substations (GIS) 1. Introduction SF6 Gas Insulated Substations (GIS) are preferred for voltage ratings of 72.5 kV, 145 kV, 300 kV and 420 kV and above. In such a substation, the various equipments like Circuit Breakers, Bus-Bars. Isolators, Load Break Switches, Current Transformers, Voltage Transformers Earthing Switches, etc. are housed in metal enclosed modules filled with SF6 gas. The SF6 gas provides the phase to ground insulation. As the dielectric strength of SF6 gas provides the phase to ground insulation. As the dielectric strength of SF6 gas is higher than air, the clearances required are smaller. Hence, the overall size of each equipment and the complete substation is reduced to about 10 % of conventional Air-insulated substations. As a rule GIS are installed indoor. However outdoor GIS have also been installed earlier.

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High voltage Gas Insulated Switch gear Type B95 Double Bus-Bar (make Alostom) www.sayedsaad.com

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Single line diagram High voltage Gas Insulated Switch gear Type B95 Double Bus-Bar (make Alostom) 1 Circuit Breaker . 2 Spring Mechanism . 3 Disconnected . www.sayedsaad.com 4 Slow Earthing Switch 5 Make Proof Earthing Switch. 6 Current Transformer. 7 Voltage Transformer. 8 HV cable connection. www.sayedsaad.com The various modules of GIS are factory assembled and are filled with SF6 gas at a pressure of about 3 kg/cm2. Thereafter, they a taken to site for final assembly. Such substations are compact and can be installed conveniently on any floor of a multistoried building or in an underground substation. As the units are factory assembled, the installation time is substantially reduced. Such installations are preferred in cosmopolitan cities, industrial townships, etc., where cost of land is very high and higher cost of SF6 insulated Switchgear (GIs) is justified by saving due to reduction in floor area requirement. They are also preferred in heavily polluted areas where dust, chemical fumes and salt layers can cause frequent flashovers in conventional outdoor air-insulated substations

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GIS bay single Bus-Bar Make Mitsubishi 1- Circuit Breaker 2- Disconnector Switch (GL-Type) 3- Disconnector Switch (GR-Type) 4- Earthing Switch (GRE-Type) 5- 3-ph. Bus-Bar. 6- Current Transformer. 7- Base. www.sayedsaad.com 8- Voltage Transformer. The SF6 Gas Insulated Substations (GIs) contains the same Components as in the conventional outdoor substations. All the live parts are enclosed in metal housings filled with SF6 gas. The live parts and supported on at resin insulators. Some of the insulators are designed as barriers between neighboring modules such that the gas does not pass through them. www.sayedsaad.com The entire installation is sub-divided into compartments which are gas tight with respect to each other. Thereby the gas monitoring system of each compartment can be independent and simpler. The enclosures are of non-magnetic material such as aluminum or stainless steel and are earthed. Static O-seals placed between machined flanges provide the gas tightness. The O-rings are placed in the grooves' such that after assembly, the O-rings are squeezed by about 20 %. Quality of material and dimension of grooves and O-seals are important to ensure gas-tight performance. The GIs has gas-monitoring system. The gas density in each compartment is monitored. If pressure drops slightly, the gas is automatically tapped up with further gas leakage, the low-pressure alarm is sounded or automatic tripping or lock-out occurs www.sayedsaad.com Advantages of GIs and Application Aspects: 1- Compactness. The space occupied by SF6 installation is only about 8 to 10 % of that a conventional outdoor substation. High cost is partly compensated by saving in cost of space. A typical 214

420/525 kV SF6 GIs requires only 920 m2 site area against 30.000 m2 for a conventional air insulated substation. 2 - Choice of Mounting Site. Modular SF6 GIS can be tailor made to Suit the particular site requirements. This results is saving of otherwise Expensive civil-foundation work. SF6 GIS can be suitably mounted indoor on any floor or basement and SF6 Insulated Cables (GIC) can be taken through walls and terminated through SF6 bushing or power cables. 3 - Reduced Installation Time. The principle of building block construction (modular construction) reduces the installation time to a few weeks. Each conventional substation requires several months for installation. In SF6 substations, the time-consuming high cost galvanized steel structures are eliminated. Heavy foundations for galvanized steel structures, www.sayedsaad.com Equipment support structures etc are eliminated. This results in economy and reduced project execution time. Modules are factory assembled, tested and dispatched with nominal SF6 gas. Site erection time is reduced to final assembly of modules. www.sayedsaad.com 4 - Protection from pollution. The external moisture. Atmospheric Pollution, snow dust etc. have little influence on SF6 insulated substation. However, to facilitate installation and maintenance, the substations are generally housed inside a small building. 5- Increased Safety. www.sayedsaad.com As the enclosures are at earth potential there is no possibility of accidental contact by service personnel to live parts. 6 - Explosion-proof and Fire-proof installation. Oil Circuit Breakers and oil filled equipment are prone to explosion. SF6 breakers and SF6 filled equipment are explosion proof and fire-proof.. www.sayedsaad.com Summary of Merits of SF6 GIS Safe Reliable Space saving Economical Maintenance free Operating personnel are protected by the earthed metal enclosures The complete enclosure of all live parts guards against any Impairment of the insulation system. SF6 Switchgear installations take up only 1/10 of the space Required for conventional installations. High flexibility and application versatility provide novel, and economic overall concepts. An extremely careful selection of materials. an expedient design and a high standard of manufacturing quality assure Long service life with practically no maintenance requirement.

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Low weight

Low weight due to aluminum enclosure, correspondingly Low cost foundations and buildings.

Quick site assembly ensured by extensive Shop assembled preassembly and Testing of complete feeders or large units in the factory. Disadvantages of GIS: www.sayedsaad.com 1- High cost compared to conventional outdoor substation. 2 - Excessive damage in case of internal fault. Long outage periods as Repair of damaged part at site may be difficult. 3 - Requirement of cleanliness is very stringent. Dust or moisture can cause internal flashovers. www.sayedsaad.com 4 - Such substations are generally in door. They need a separate building. This is generally not required for conventional outdoor substations. 5 - Procurement of gas and supply of gas to site is problematic. Adequate stock of gas must be maintained. 6 - Project needs almost total imports including SF6 Gas. Spares conventional substation is totally indigenous up to 400 kV. Configuration of GIS: www.sayedsaad.com The GIS installations are assembled from a variety of standard modules. Which are joined together by flange connections and plug contacts on the Conductors. So as to easily permit subsequent disassembly of individual components. Gas-tight barrier insulators in the Switchgear sections prevent neighboring Switchgear parts from being affected by overhauls. Any maintenance and overhaul work on Switch contacts can be done without removing the enclosure. With GIS installations, all basic substation Bus-Bar schemes used, in conventional plant constructions can be realized. Installations with single or multiple Bus-Bar-also alternatively with a bypass bus-can be made with the standard modules, including Bus-Bar sectionalizing with disconnects and Breakers, and Bus-Bar coupling. The two-breaker. One and-a-half circuit breaker and ring-bus systems can also be realized economically. www.sayedsaad.com The essential parts of a GIS are: 1 - Conductors which conduct the main circuit current and transfer power these are of copper or aluminum tubes. www.sayedsaad.com 2 - Conductors need insulation above grounded enclosures. Conductors also need phase to phase insulation, In SF6 GIS these insulation requirements are met by cast resin insulators and SF6 gas insulation. 3 - Gas filled modules have nonmagnetic enclosures. Enclosures are of aluminum alloy or stainless steel. Adjacent modules are joined by means of multi-bolts tightened on flanges. Suitable neoprene rubber O ring gaskets are provided for ensuring Gas-tight sealing joints. www.sayedsaad.com 4 - Various circuit components in main circuit are: CB, Isolator, Earthing Switches for conductors, CTs, VTs, cable-ends, Bushing-ends and Bus-Bars. Each of these main components has its own gas -filled metal enclosed module. 5 - Gas filling, monitoring system. www.sayedsaad.com 6 - Auxiliary LV DC and LV AC supply system, control, protection and Monitoring system. This is air-insulated like in conventional sub-station. 216

The Bus-Bars are conducting bars to which various incoming and outgoing bays are connected. In SF6 GIS the Bus-Bars are laid l longitudinally in GIS hall. www.sayedsaad.com The bays are connected to Bus-Bars cross-wise. Bus-Bars are either with a three-phase enclosure or single phase enclosure. Alternatives of Enclosures, Single three phase and three single enclosures

Three phase Single Enclosures Three phase and three single enclosures The following alternatives are available to the designers for configuration of GIs. 1. Separate enclosure for each phase. This alternative was used for Components and Bus-Bars in early GIs. Now it is used only for EHV and UHV, GIS. The GIS above 420 kV are generally with separate enclosure for each phase. 2. Separate enclosure for components and a common single enclosure For three phase enclosure for Bus-Bars. www.sayedsaad.com This alternative is more widely used now for all GIS 3. Common single enclosure for all three phases for components and For Bus-Bars. The per cent trend is to use single three phase modules for components and Bus-Bars for all GIS. The GIS developed during 1980s are with this philosophy. www.sayedsaad.com Design Aspects The SF6 insulated Switchgear contains the same components as a conventional outdoor substation. Fig (1) illustrates the construction of typical bay.

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Fig (1) Section of a 145 KV SF6 GIS with duplicate bus-bar 1 3- phase Bus enclosure. 2 Isolator. 3 Earthing Switch. 4 C.B puffer type. 5 CT's www.sayedsaad.com 6 Line Isolator. 7 VT. www.sayedsaad.com 8 High Speed Earthing Switch. 9 Cable sealing End. 10 Operating mechanism (cabinet). 11 Conductor tube. 12 Epoxy partition fig. (2). All the live parts are enclosed in metal housing filled with SF6 gas. Live parts are supported on cast resin insulators. Some of the insulators are designed as barriers between neighboring modules such that the gas does not pass through them. The entire installation is sub-divided into compartments, which are gas tight with respect to each other. Thereby the gas monitoring system of each compartment can be independent and simple The enclosures are of nonmagnetic material such as aluminum or stainless steel and are earthed. The gas tightness is provided by static O-seals placed between machined flanges. The O-rings are placed in the grooves such that after assembly, the O-rings get squeezed by about 20 %. Quality of material and dimension of groove are important. Aluminum or stainless steel enclosures surround all live parts. Enclosures are earthed. Pressurized SF6 gas provides internal insulation between conductors and metallic enclosures. Fig (2) below. High grade insulators of Epoxy partition resin give support to active parts inside the enclosures and are also used as barriers between adjacent gas filled compartments.

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Fig(2) Epoxy partition resin Individual compartments (modules) are connected by silver plated Plug contacts for current condu