The losses in a transformer are small in relative terms, but they are appreciable in absolute terms, especially in large-size units. Therefore, a major task in transformer design is to provide for proper withdrawal of the heat generated in the core and coils to the surroundings. The task grows progressively more complicated as the transformer size increases. Given the magnetic induction in the core and the current density in the coils, the losses in a transformer rise in proportion to its volume (that is, the third power of its linear dimensions), whereas the cooling area increases only in direct proportion to the square of the increase in linear dimensions.
In consequence, the increase in transformer rating entails a rapid increase in the required rate of heat transfer from its surface. This can be done by increasing the cooling surface area of a transformer, or by raising the rate of heat transfer by some artificial means.
On the whole, the cooling systems applied to transformers may be classed into air natural cooling, oil natural cooling, oil-natural air-blast cooling, forced oil natural cooling.
Air natural cooling is applied to dry transformers. In this case, the heat generated in a transformer is directly transferred to the surroundings. Because heat transfer is poor, the temperature distribution in a dry transformer may be all but uniform. Also the low electric strength of air (2.1 MV m -1) impairs the insulation level in dry transformers. Furthermore, account must be taken. of the dust which lodges on the windings and impairs their insulation still more. For these reasons, air natural cooling is mostly used on small-sized, low voltage transformers.
At present the most commonly used type of transformer is an oil immersed transformer. In it the core and coils assembly is placed in a steel tank filled with a carefully purified mineral oil. On absorbing heat, the oil rises in the tank, flows over the core and coils, cools them, transfers the heat thus absorbed to the tank walls, and sinks (this is known as convection). Transformer oil has a high heat capacity, so a higher level of core and copper losses may be tolerated. Also, since transformer oil has a high electric strength, the spacing between the conductors and the core may be substantially reduced.
In transformers up to 20-30 kVA in size, a sufficient cooling surface area is obtained with a plain tank. Larger units (up to 15- 20 MVA) call for the use of tubular tanks (see Fig. 3). For still larger transformers it is usual to employ radiators with air natural or air blast cooling. Units from 90 MVA and bigger in size mostly use radiators in combination with forced oil, air blast cooling.
When a transformer is in operation, the volume of oil in its tank increases or decreases, according as the transformer is heated or cooled. In transformers rated up to 100 kVA at 6.6 kV, there is enough space left under the tank cover to accommodate the expanding oil as it displaces air from the tank through what is called the breather. When the oil contracts, moist ail' inevitably finds its way inside the tank, and the oil picks up moisture. As a result, a layer of water accumulates at the tank bottom with time, and the oil loses a good deal of its electric strength. Also, atmospheric oxygen oxidizes the oil and brings down its electric strength still more. All of this is avoided in large transformers whose tanks are filled with oil to the brim. To accommodate excess oil on thermal expansion, each transformer in this class is fitted with an oil conservator (see Fig .3) which is a sheet-steel cylinder set up on the tank cover and is connected with the tank insides by a tube which stops some distance short of the conservator bottom. The volume of the oil conservator is about 10% of that of the tank. Atmospheric moisture and sludge are mostly collected at tho conservator bottom from which they are removed by means of a drain cock. With a conservator, the oil surface exposed to air is substantially smaller than it is in a transformer without a conservator. Also, since the temperature in the conservator is lower than it is in the tank, the oil is less oxidized.
A major drawback of oil cooling is that transformer oil is combustible (it ignites at around 160°C), so it presents both a fire and an explosion hazard. The gases forming as transformer oil burns can tear the cover off the tank, and expel a large amount of oil , and also cause damage to the tank itself. In large units this is prevented from happening by providing a relief or explosion stack (see Fig.3) fitted with a disc of thin glass. Should a fault inside the transformer entail the evolution of a large volume of gas, it will burst the disc and escape into the atmosphere.
In especially critical applications transformer tanks may be filled with quartz sand or an incombustible synthetic liquid; this liquid and its vapour are toxic.
The use of liquid coolants markedly complicates the use of transformers since this necessitates a constant watch on their condition and a periodic purification or renewal.
Labels: Electricity and Magnetism