Transformer
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TRANSFORMER
by : rifkania kambey 070213080
2.56 TRANSFORMER COOLING The core and cooper losses cause heating of transformer. It is necessary to ensure that the maximum temperature rise is less than that which may cause damage to the insulation. In rotating machines the heat dissipation is easier due to the turbulent air flow. Transformers require an elaborate cooling system (inspite of the fact that losses in transformers are less than in rotating machines). Small transformers (up to 25 kVA rating) are of dry type. The air convection current and radiation from transformer tank are sufficient to keep the temperature within limits.
A majority of transformers are oil immersed. The core and the windings are completely immersed in mineral oil. Oil is a good insulating material and provides better heat dissipation than air. These transformers can be further classified as under: 1)Oil Immersed Self-cooled Transformer 2)Oil Immersed Forced Air Cooled Transformer 3)Oil Immersed Water Cooled Transformer 4)Oil Immersed Forced Oil Cooled Transformer
2.56.1 Oil Immersed Self-cooled Transformer The dissipation of heat by the transformer tank to the atmosphere is by natural process. Hot oil flews upwards through ducts (in the core and windings) and then flows down along the inner walls of the tank. The tank surface dissipates heat to the atmosphere. Sometimes the tank surface is corrugated to provide more surface area (a corrugated tank has about 50% more surface area than plain tank). Vertical tubes are welded on the sides of the tank to increase the surface area. Sometimes radiators having finned or fluted surface are also bolted to the tank surface.
2.56.2
Oil Immersed Forced Air Cooled Transformer This transformer is also immersed in oil. The cooling is improved further by an air blast over the outside surface of the tank.
2.56.3 Oil Immersed Water Cooled Transformer Metallic tubes are situated inside the tank, below the oil level. Water is circulated through these tubes to extract heat from oil.
2.56.4 Oil Immersed Forced Oil Cooled Transformer This method uses forced circulation of oil by a pump. Oil is pumped through the ducts and then through external radiators, which are cooled by fans. The oil pumping equipment adds to the cost, but cooling is more effective. The
2.57 TRANSFORMER OIL Oil serves the double purpose of cooling and insulating. Mineral oils alone are suitable for electrical purposes. The main characteristics of transformer oil are: a)Viscosity b)Insulating Property c)Flash Point d)Fire Point e)Purity f) Sludging
a) Viscosity This determines the rate of cooling. A high viscosity is an obvious disadvantage, because of sluggish flow through ducts and tubes.
b) Insulating Property The dielectric strength of transformer oil is generally good. However moisture and dust tend to lower the dielectric strength appreciable. A dielectric strength of 30 kV is considered a good value.
c) Flash Point It is the temperature at which the above an oil surface ignite spontaneously. Flash point of transformer oil should not be less than 140oC.
d) Fire Point The temperature at which an oil will ignite and continue burning is known as fire point. It should be
o
e) Purity Oil should not contain impurities like sulphur and its compounds, which may cause corrosion of metal parts and acceleration of sludge. f) Sludging Sludging means slow formation of semi-solid hydrocarbons, which may get deposited on windings and tank walls. The formation of sludge is due to heat and oxidation. The maximum permissible value of sludge is 0.1%. The deterioration of oil during use can be retarded by the use of antioxidants, particularly oxidation ‘inhibitors’. They are well defined chemical products, which help in retarding the formation of acid and sludge.
2.58 CONSERVATOR AND BREATHER In oil cooled transformers the expansion of oil forces out of the air above it, so that on cooling fresh air is again drawn in thus bringing in oxygen and moisture. A conservator consists of an airtight cylindrical metal drum supported on transformer lid. It takes up the expansion and contraction of oil with changes in temperature. The tank is filled with oil when cold an the expansion is taken up in the conservator. In the absence of conservator condensation of water soluble acids on the under surface of tank lid may occur and acid droplets may fall back into the oil. The displacement of air due to change of oil volume takes place through a breather containing calcium chloride or silica gel, which extracts the moisture from air. Silica gel should be checked regularly and dried or replaced when necessary.
2.59 TRANSFORMER NOISE Most of the transformers are located near residential areas and its noise may be troublesome. The ‘hum’ originates in the core where the laminations tend to vibrate by magnetic forces. This noise is transmitted through oil to tank and thence to surroundings. The essential factors in noise production are the following: 1)Magnetostriction, i.e. very small extension with corresponding reduction in crosssection of sheet steel strips when magnetised. 2)The degree of mechanical vibration developed by laminations depending on the tightness of clamping, size, gauge, etc.
Though magnetostriction is the first cause of hum, much of the noise depends on the natural frequency of vibration of mechanical parts. Any construction changes which remove this frequency outside the audible range will, evidently, mean an improvement.
2.60 NAME PLATE AND RATINGS The specifications of transformers used I power industry are contained in BIS 2026. As per this standard, every transformer must be provided with a name plate giving the following information: Type (power, auto, booster, etc.), year of manufacture, number of phases, rated kVA (for multiwinding transformers rated kVA of each winding), rated frequency, rated voltage of each winding, connection symbol, per cent impedance voltage at rated current (measured value corrected to 75oC), type of cooling, total mass, mass and volume of insulating oil. As per BIS 2026, the preferred kVA rating are 6.3, 10, 16, 25, 40,63, 100, 160, 250, 400, 630, 1000 kVA, etc.
THANK YOU ……………… …… ....
by : rifkania kambey 070213080
2.56 TRANSFORMER COOLING The core and cooper losses cause heating of transformer. It is necessary to ensure that the maximum temperature rise is less than that which may cause damage to the insulation. In rotating machines the heat dissipation is easier due to the turbulent air flow. Transformers require an elaborate cooling system (inspite of the fact that losses in transformers are less than in rotating machines). Small transformers (up to 25 kVA rating) are of dry type. The air convection current and radiation from transformer tank are sufficient to keep the temperature within limits.
A majority of transformers are oil immersed. The core and the windings are completely immersed in mineral oil. Oil is a good insulating material and provides better heat dissipation than air. These transformers can be further classified as under: 1)Oil Immersed Self-cooled Transformer 2)Oil Immersed Forced Air Cooled Transformer 3)Oil Immersed Water Cooled Transformer 4)Oil Immersed Forced Oil Cooled Transformer
2.56.1 Oil Immersed Self-cooled Transformer The dissipation of heat by the transformer tank to the atmosphere is by natural process. Hot oil flews upwards through ducts (in the core and windings) and then flows down along the inner walls of the tank. The tank surface dissipates heat to the atmosphere. Sometimes the tank surface is corrugated to provide more surface area (a corrugated tank has about 50% more surface area than plain tank). Vertical tubes are welded on the sides of the tank to increase the surface area. Sometimes radiators having finned or fluted surface are also bolted to the tank surface.
2.56.2
Oil Immersed Forced Air Cooled Transformer This transformer is also immersed in oil. The cooling is improved further by an air blast over the outside surface of the tank.
2.56.3 Oil Immersed Water Cooled Transformer Metallic tubes are situated inside the tank, below the oil level. Water is circulated through these tubes to extract heat from oil.
2.56.4 Oil Immersed Forced Oil Cooled Transformer This method uses forced circulation of oil by a pump. Oil is pumped through the ducts and then through external radiators, which are cooled by fans. The oil pumping equipment adds to the cost, but cooling is more effective. The
2.57 TRANSFORMER OIL Oil serves the double purpose of cooling and insulating. Mineral oils alone are suitable for electrical purposes. The main characteristics of transformer oil are: a)Viscosity b)Insulating Property c)Flash Point d)Fire Point e)Purity f) Sludging
a) Viscosity This determines the rate of cooling. A high viscosity is an obvious disadvantage, because of sluggish flow through ducts and tubes.
b) Insulating Property The dielectric strength of transformer oil is generally good. However moisture and dust tend to lower the dielectric strength appreciable. A dielectric strength of 30 kV is considered a good value.
c) Flash Point It is the temperature at which the above an oil surface ignite spontaneously. Flash point of transformer oil should not be less than 140oC.
d) Fire Point The temperature at which an oil will ignite and continue burning is known as fire point. It should be
o
e) Purity Oil should not contain impurities like sulphur and its compounds, which may cause corrosion of metal parts and acceleration of sludge. f) Sludging Sludging means slow formation of semi-solid hydrocarbons, which may get deposited on windings and tank walls. The formation of sludge is due to heat and oxidation. The maximum permissible value of sludge is 0.1%. The deterioration of oil during use can be retarded by the use of antioxidants, particularly oxidation ‘inhibitors’. They are well defined chemical products, which help in retarding the formation of acid and sludge.
2.58 CONSERVATOR AND BREATHER In oil cooled transformers the expansion of oil forces out of the air above it, so that on cooling fresh air is again drawn in thus bringing in oxygen and moisture. A conservator consists of an airtight cylindrical metal drum supported on transformer lid. It takes up the expansion and contraction of oil with changes in temperature. The tank is filled with oil when cold an the expansion is taken up in the conservator. In the absence of conservator condensation of water soluble acids on the under surface of tank lid may occur and acid droplets may fall back into the oil. The displacement of air due to change of oil volume takes place through a breather containing calcium chloride or silica gel, which extracts the moisture from air. Silica gel should be checked regularly and dried or replaced when necessary.
2.59 TRANSFORMER NOISE Most of the transformers are located near residential areas and its noise may be troublesome. The ‘hum’ originates in the core where the laminations tend to vibrate by magnetic forces. This noise is transmitted through oil to tank and thence to surroundings. The essential factors in noise production are the following: 1)Magnetostriction, i.e. very small extension with corresponding reduction in crosssection of sheet steel strips when magnetised. 2)The degree of mechanical vibration developed by laminations depending on the tightness of clamping, size, gauge, etc.
Though magnetostriction is the first cause of hum, much of the noise depends on the natural frequency of vibration of mechanical parts. Any construction changes which remove this frequency outside the audible range will, evidently, mean an improvement.
2.60 NAME PLATE AND RATINGS The specifications of transformers used I power industry are contained in BIS 2026. As per this standard, every transformer must be provided with a name plate giving the following information: Type (power, auto, booster, etc.), year of manufacture, number of phases, rated kVA (for multiwinding transformers rated kVA of each winding), rated frequency, rated voltage of each winding, connection symbol, per cent impedance voltage at rated current (measured value corrected to 75oC), type of cooling, total mass, mass and volume of insulating oil. As per BIS 2026, the preferred kVA rating are 6.3, 10, 16, 25, 40,63, 100, 160, 250, 400, 630, 1000 kVA, etc.
THANK YOU ……………… …… ....
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