The satisfactory operation of transformers depends so largely on the condition of the oil and therefore devices and methods for keeping the oil clean and dry are of prime importance.
The oil level of a transformer changes with changes in the temperature rise of oil which in turn depends upon the load on the transformer. The oil expands if the load increases and contracts when it decreases. Therefore, provision must be made to take up this expansion and contraction of oil.
Smaller transformers are not totally filled with oil and some space is left between the oil level and the tank cover. This space is taken by air. The tank is connected to the atmosphere through a vent pipe. When the oil expands, the air is expelled out while if it contracts air is drawn in from the atmosphere. This is called breathing of transformer. The air entering the transformer is passed through an apparatus called a breather for the purposes of extracting moisture from it. A breather consists of a small container connected to the vent pipe and contains a dehydrating material like silica gel crystals impregnated with cobalt chloride. The material is blue when dry and a whitish-pink when damp. The color can be observed through a glass window provided in front of the container.
Breathers alone are not sufficient for the protection of large and important power transformers because:
1. These transformers are liable to overloads which may overheat the oil and consequently, there is sludge formation if the air is present.
2. Occasionally such transformers also suffer short circuits and temperature rise becomes very high. This causes vaporization of a part of the oil. The oil vapors form an explosive mixture with air which ignites and can cause considerable damage.
For these reasons, oil is prevented from having contact with air as well as moisture. Conservators are used for this purpose. The function of the conservator is to take up the expansion and contraction of the oil without allowing it to come in contact with ambient air, from which it might absorb moisture.
The conservator is an airtight cylindrical drum mounted on or near the cover of the transformer and connected to it through a small pipe as shown in Fig. The oil is set so that the transformer tank is entirely full with oil and the conservator is about half full. The interior of the conservator above the oil level is connected to the atmosphere through a breather having dehydrating material.
With the use of conservators, interchange of oil between conservator and main tank as a result of temperature changes is slow. Also dry is in contact with a much smaller surface of relatively cool oil. Hence the sludge formation is considerably reduced and whatever sludge is formed remains in the conservator there being no sludge formation in the main tank. This is a great improvement over the ordinary tank with air space above the oil.