Various methods of cooling transformers are adopted in practice, depending upon the size and the local conditions. Very small transformers are cooled naturally by the atmosphere, no special cooling arrangements being necessary. Slightly larger transformers are oil-immersed, being enclosed in a tank for this purpose. The object of the oil is twofold. As an insulator, it is better than air, and it also keeps down the maximum temperature rise by setting up convection currents which tend to equalize the temperature. These convection currents carry the heat away to the walls and lid of the tank, whence it is dissipated into the atmosphere. Small tanks are made with a plain exterior, sufficient cooling surface being obtained in this manner. Rather larger tanks are made with a corrugated exterior, or are provided with fins, to increase the cooling surface. A more popular arrangement, however, is to obtain the desired increase in cooling surface by means of a number of tubes running from top to  bottom on the outside of the tank. The oil immediately in contact with the transformer in the tank is heated by the transformer, and consequently rises. Convection currents cause the oil to flow outwards at the top of the tank and so it enters the tubes at the top. It is now cooled in the tubes, sinks, and re-enters the tank at the bottom. It is now heated again by the transformer, and the cycle of operations is repeated.

The larger the tank, the greater is the number of tubes required. These are now arranged in rows, one behind the other, but very little advantage is gained by adding more tubes when they are already three deep. A fourth row of tubes is so shut in by the tubes on the outside that very little additional cooling surface is obtained. When this stage is reached, the simple tubular design is abandoned, and external radiators are substituted for the tubes.

With this type of cooling the tank surface it now becomes plain again. The external radiators consist of a long horizontal chamber at the top, and another at the bottom, these being joined by numbers of radiator tubes.

Other transformers designed for use with an external oil cooler have no radiators fitted to the transformer tank itself, practically the whole of the heat dissipation taking place in the oil cooler.

Water-cooling is also employed. A number of tubes are arranged in a helical coil inside the top of the transformer tank, but underneath the oil level. A stream of cold water is then passed through this cooling coil. Since the presence of even a minute percentage of water in the oil reduces its insulating properties to an enormous degree, it is extremely important that no water should escape through any leak, should one occur. In order to prevent this, the tank is made oil-tight and the oil is put under pressure. If a leak should develop, oil will leak into the tubes (which do not much matter) instead of the water leaking into the oil.

In confined spaces, and where a supply of water is not available, air-blast cooling may be adopted. The tank is now dispensed with, and a blast of cold air is forced over the transformer windings. Cooling by this means is usually confined to transformers operating on the lower voltages.

Transformer Oil. The oil used for transformer immersion is pure hydrocarbon (mineral) oil, obtained by refining crude petroleum. Its insulating prop­erties are very adversely affected by the presence of even a minute. Proportion of water, and so it must be clean and practically free from moisture. Certain oils tend to form sludge in the course of time, this being due to the slow formation of solid hydrocarbons. If this sludge should form on the windings themselves it tends to produce overheating. Certain high-grade qualities are called non-sludging oils, and these should be used in transformers in which the working tem­perature of the oil exceeds 80° C.

The use of the oil-expansion chamber reduces the tendency of the oil to form sludge, since the access of atmospheric oxygen is effective­ly prevented.   The addition of the breather also keeps the oil dry.

 М.А. Беляева и др. «Сборник технических текстов на англ. языке»