Vaisala MMT310 - Manual (Page 75)

Appendix A ______________________________________________________________ Applications
VAISALA ________________________________________________________________________ 75
APPENDIX A
APPLICATIONS
This appendix describes typical applications where MMT310 is used.
Transformer Oils
The determination of moisture in oil is an essential part of a
comprehensive transformer maintenance program. Oil immersed
transformers rely on the oil for cooling, protection from corrosion and as
an important component of their insulation. Excessive moisture content
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 the 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 rises (see
Figure 22 on page 76). Changes in temperature also affect 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 oil’s capacity to absorb water depends
both on the chemical structure of the oil and the additives.
The water concentration of transformer oil is usually 0...80 ppm and the
temperature range of the oil 0...100ºC.