High Voltage

Topics: Transformer, Dielectric, Circuit breaker Pages: 6 (1058 words) Published: May 19, 2013



Insulating fluids (gases and liquids) provide insulation between phases and grounded parts of electrical equipments. These also carry out heat from the windings of the electrical equipments However, solid insulating materials are used only to provide insulation only. Why insulation is important . Because human safe is important. Power transformer inside is very big current and voltage to moving if don’t take a protection or neglect transformet can be burn or controlling attendant can be exposed very huge voltage.

All electrical systems require some kind of insulation to prevent short circuits and leaking currents.

3 forms of insulators: Solid, liquid and gaseous Performance of these insulators depend on the temperature. Classification according to their temperature rating:

International Electrotechincal Commission has categories various insulating materials depending upon the temperature of operations of the equipments under the following categories.
What insulating materials are used for a particular apparatus depending upon it’s ratings and environmental condition where the apparatus is required to operate. Power Transformers
For small rating, the coils are made of super-enamelled copper wire. For layer to layer, coil to coil and coil to ground (iron core) craft paper is used. However, for large size transformers paper or glass tape is rapped on the rectangular conductors whereas for coil to coil or coil to ground, insulation is provided using thick radial spacers made of pres board or glas fibre. Low power transformer High power transformer

In oil-filled transformers, the transformer oil is the main insulation. However between various layers of low voltage and high voltage winding oil-impregnated press boards are placed.SF6 gas insulated power transformers make use of sheet aluminium conductors for windings and turn to turn insulation is provided by a polymer film. The transformer has annular cooling ducts through which SF6 gas circulates for cooling the winding. SF6 gas provides insulations to all major gaps in the transformer. This transformer is used where oil filled transform is not suitable e.g., in cinema halls, high rise buildings and some especial circumstances: The end turns of a large power transformer are provided with extra insulation to avoid damage to coil when lighting or switching surges of high frequency are incident on the transformer winding.The terminal bushings of large size power transformer are made of condenser type bushing. The terminal itself consists of a brass rod or tube which is wound with alternate layers of treated paper and tin foil, so proportioned, as to length, that the series of condensers formed by the tin foil cylinders and the intervening insulation have equal capacitances, thereby the dielectric stress is distributed uniformly. TRADITIONAL INSULATION MATERIALS

The transformer insulation:

Transformer oil
+ Provides the required dielectric strength and insulation
+ Cools the transformer by circulating itself through the core and the coil structure. + Should be in the liquid state over the complete operating range of temperatures between -40°C and+50°C. + Gets oxidized when exposed to oxygen at high temperatures

-- Formation of peroxides, water, organic acids and sludge.

-- Chemical deterioration of the paper insulation and the metal parts of the transformer.

-- Sludge being heavy, reduces the heat transfer capabilities of the oil, and also forms as a heat insulating layer on the coil structure, the core and the tank walls.

+ The effects of oxidation are minimized by designing them such that access to oxygen itself is...
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