Transformer dehydrating breathers are being used for liquid immersed transformers in order to protect the insulating liquid and prevent the harmful moisture absorption from ambient air, which occurs when the load on the transformer fluctuates. The transformer dehydrating breather removes practically all moisture from the air that flows through it into the conservator when the transformer is cooling down. This largely prevents any reduction of the dielectric strength and the formation of condensation in the expansion tank. Thus, dehydrating breathers increases the operational safety of transformers.
The answer is related to the conservator principle of operation. It serves to store the transformer oil when it expands due to the temperature rise and the design is made so that the tank is always full of oil (therefore, there is a minimum oil level in the conservator). Thus, it is not possible to install the dehydrating breather in the tank, because there is no air flow there. It is installed in the conservator, as it is this element that 'inspires' and 'exhales' the air by 'supplying' or 'absorbing' oil from the tank.
Physically conservator is at the top of oil-filled transformer tank to provide space or cushion for expanded oil (due to heating on account of loading in transformer). The upper portion of conservator contains oil-vapor. With breather there at the top while ingress of moisture from outside is stopped, release of oil vapor too is restricted. But if breather is with the tank, it will be filled up with oil that will lead to leakage of transformer oil. Breathing of air / vapor is possible, but not oil which will be leaked.
Satisfactory operation of a transformer solely depends on the condition of the oil. It is required to keep the oil clean and dry. With the rise of temperature, the oil level increases. Rise of temperature depends on the load of the transformer. If load increases, oil expands. If load decreases, contraction of oil occurs. Since smaller transformers are not totally filled by oils, some space is left between tank walls and oil, which is occupied by air.
Satisfactory operation of a transformer solely depends on the condition of the oil. It is required to keep the oil clean and dry. With the rise of temperature, the oil level increases. Rise of temperature depends on the load of the transformer. If load increases, oil expands. If load decreases, contraction of oil occurs. Since smaller transformers are not totally filled by oils, some space is left between tank walls and oil, which is occupied by air.