winding of a Transformer

 Windings

The windings are made with copper conductors and are placed on the cores. The winding which is connected to supply is known as primary and the winding which is connected to load is known as secondary winding. The winding which is connected to low voltage is known as low-voltage (L.V) winding and the winding which is connected to high voltage (H.V) is known as high-Voltage (H.V) Winding. According to the construction and arrangement, the windings are mainly classified into:

i)                    Cylindrical type winding and

ii)                   Sandwich type winding.

1. Cylindrical Type winding:  This winding is layered type and is made up of turns helically wounds round with turns close to each to each other. Hence, the height of the winding is equal to the  height of the limb. Since, the low voltage (L.V) Winding is easier to insulate, it is first wound near and over the core and then high voltage winding is wound over the L.V. winding. For improving the cooling of the winding layers, cooling ducts are providing between the two windings. This type of winding is most commonly used in core-type transformers.

2. Sandwich Winding: In This type of winding the L.V. and H.V. Windings are placed one Over the Other alternatively. This arrangement reduces the leakage reactance. The two low voltage coils at the ends have half the turns of a normal low voltage coil. This type of winding is most commonly used in shell-type transformers. As compared to concentric winding they have several drawbacks, such as more difficult to insulate from each other and from the yoke.

Tank:

Transformers are generally housed in a tightly fitted sheet-metal tanks filled with special insulating oil. The core and windings are completely immersed in the oil inside the tank. In case of small transformers, the tanks are made with iron sheets or sheets steel, whereas in case of large transformers the tanks are made with plain boiler plates or cast-aluminium plates the sheets or plates are properly welded and a waterproof gasket being used at the joints, so that oil may not come out of the tank. When the transformer is working, heat is produced in the tank, hence needs cooling. For cooling purpose, cooling tubes are welded around the tank. In case of large transformers separate rotators are welded to the tank which provides better cooling. Eye-bolt is welded to the tank at the top for lifting purpose and wheels or rollers are provided at the bottom for moving the transformer into position. A tank must be able to withstand stresses developed inside and has provision for connecting to the load and supply.

Transformer Oil:

The insulating oil which is used in the tank of a transformer is called transformer oil. It has three functions.

i) It acts as insulating medium between windings and tank.

ii) It provides better cooling by circulating through the cooling tubes

iii) It carries away the heat generated in the core and windings and quench the arc if any.

The Transformer oil should have the following properties.

1. It should have high dielectric strength at least 35kv/mm.

2. It should free from dust and moisture. A very small quantity of water in oil greatly lowers the insulating property. For example even an addition of 8 parts of water in 10,00,000 reducs the insulating quality of oil.

3. It should be free from acid, alkalies and sulphur or its compounds to prevent corrosion of metal parts.

4. It should have the property to quench the arc, if any.

5. It should have good thermal conductivity so that it carries away the heat generated in the core and windings.

The transformer oil is obtained by fractional of crude petroleum. The mineral oil is most commonly used as transformer oil. Vegetable and animal oils are not suitable, because they form fatty acids that attack the fibrous insulating materials (cotton) Used. Now-a-Day synthetic insulating oils know as ASKARELS and PYROCLOR (trade names) are extensively used because they avoid risk of fire and explosion also.

Conservator Tank (or) Expansion Tank

It is a small oil tank mounted above the transformer above the transformer and connected to the main tank by a pipe. The oil level of a Transformer changes with change in temperature of oil which in turn depends upon the load on the transformer. The oil expands with the increase in load and contracts with the decrease in load. When the oil expands, the volume of the oil in the tank increase which flows into the conservator through the pipe. The conservator receives the increased volume of oil during expansion and discharged the oil during contraction and keeps the main tank full of oil. The conservator is not filled completely with oil to facilitate the expansion. As it receives the oil during expansion, it is also called as expansion tank.

Breather

The transformer oil expands when it is heated and contacts when cooled. When the oil expands, the air is expelled out and air is drawn inside under contraction of oil. This process is known as breathing. Thus the oil is in contact with air. During breathing process moisture also enters into the transformer along with the air. This moisture reduces the dielectric, the air entering the transformer is made through an apparatus called breather.

A breather consists of a silica gel crystals or Cacl2 (Calcium chloride) and is connected to the conservator through vent pipe. The silica gel absorbs the moisture from the air and clean moisture less air is allowed into the transformer. Silica gel is blue in colour when dry and becomes whitish pink, it should be replaced. The colour can be observed through the glass window provided infront of the breather.

Terminals and Bushings

The connections to the windings are copper rods or bars insulated from the tank. The windings are connected to the H.V. line L.V. line through insulator bushings mounted on the top of the Transformer tank. The bushings consist of a current carrying element and a porcelain cylinder. Upto 33kv Voltage ordinary porcelain insulators are used and above 33kv oil-Filled or condenser type bushings are used.

Exhaust pipe, Oil Gauge, Temperature Gauge

Many transformers are provided with exhaust pipe made of steel. It is connected to the tank and has some bend at its ends and is provided with a glass disc at top. It protects the tank from the large expansion of the accidental gas formation in large quality and abrupt rise in pressure in case of short circuit etc.  Every transformer is provided with oil gauge and temperature gauges. Oil gauge indicates the level of oil in the tank. The oil gauge is also provided with an alarm which gives an alarm when the oil level has dropped below certain level due to oil leak. The temperature gauge indicates the temperature of the oil.

Cooling system

When current is flowing through the windings, heat is produced which should be dissipated. If the heat is not dissipated property, it may damage the windings and insulation. Hence, cooling is necessary for a transformer, which can be provided by different methods such as natural air cooling, oil immersed forced air, water and oil cooling and air-blast cooling.