Vaisala MMT330 - Manual (Page 24)

User's Guide _______________________________________________________________________
24 ___________________________________________________________________ M210764EN-F
Transformer Oil
The determination of moisture in oil is an essential part of a
comprehensive transformer maintenance program. Aging and
deterioration increase the capacity of oil to absorb water. The primary
interest in transformers is to measure the water not in oil but in the
cellulosic insulation around the transformer windings. Heating and
cooling have a considerable effect on moisture levels in oil. While
temperature raises, the paper insulation of transformers tends to loose
moisture which is absorbed by the oil surrounding it. Saturation level is
thus a true indicator of moisture present. The MMT330 method provides
for a reliable detection of the aging of oil and possible leakages.
Oil immersed transformers rely on the oil for cooling, protection from
corrosion and as an important component of their insulation. Excessive
moisture contents in oil causes accelerated ageing of the insulation
materials and reduces their dielectric strength. In extreme cases this can
result in arcing and short circuits within the windings. Accurate moisture
measurements can also warn about leaks in the oil system, as water is
absorbed from the surrounding air.
Heating and cooling of a transformer effect moisture levels in oil. This is
due to the fact that the water solubility of oil is temperature dependent. In
general, water solubility increases as temperature raises, see Figure 4 on
page 25. Changes in temperature affect also on water desorption of the
paper insulation around the transformer windings. Desorption of water
from the insulation increases as temperature raises and the surrounded oil
absorbs desorbed water. Moisture level in oil is thus a true indicator of
moisture present in the paper insulation.
In addition, it must be noted that capacity of oil to absorb water depends
both on the chemical structure of the oil and the additives.