Moisture contamination is the most common cause of deterioration in the
insulating quality of oil. This contamination can be readily corrected by
purifi cation. A slow but more serious deterioration, the formation of acids
and sludge, is caused by oxidation. Thus, the exclusion of oxygen is of
prime importance. In open-breather transformers, the oxygen supply is
virtually unlimited and oxidative deterioration is faster than sealed transformers.
Atmospheric oxygen and oxygen contained in water are the
sources available for the oxidation of insulating oils. When water is present
in insulating oils, oxidation of the oil will take place. Therefore, leaking
gaskets and seals constitute a very real hazard since a water leak is, in
effect, an oxygen leak. The rate of oxidation also depends on the temperature
of the oil; the higher the temperature is, the faster the oxidative breakdown.
An increase in temperature of 10°C (50°F) generally doubles the rate of
oxidation. The fact points to the importance of avoiding overloading of
transformers, especially in the summertime. Oxidation results in the formation
of acids in the insulating oil and the formation of sludge at a more
advance state of oxidation.
4.2.1.2 Moisture in Oil
Water can be present in oil in a dissolved form, as tiny droplets mixed with the
oil (emulsion), or in a free state at the bottom of the container holding the oil.
Demulsifi cation occurs when the tiny droplets unite to form larger drops,
which sink to the bottom and form a pool of free water. Emulsifi ed water
typically requires vacuum dehydration, as the emulsifi cation cannot typically
be broken by fi ltration or by excellerated gravity (centrifuge). Water in
the free state may be readily removed by fi ltering or centrifugal treatment.
However, dissolved water is not removed by centrifugal treatment; the fi ltration
process can partially remove dissolved water if the fi lter papers are thoroughly
dried before fi ltration, but the effi ciency of the fi ltration process
depends upon