Vaisala MMT162 - Manual (Page 15)

Chapter 2 __________________________________________________________ Product Overview
VAISALA _______________________________________________________________________ 13
It is essential to monitor the water content and to keep it on a suitable
level, meaning below saturation.
When measuring the water content of oil, it is best to measure the water
activity before an oil reservoir and from a pressure line flow. This way
the performance of dehumidifiers can be kept under control to ensure that
no free water reaches the bearings.
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 rises, 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 MMT162 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 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 affect 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 2 on
page 14 below. Changes in temperature also have an effect on water
desorption of the paper insulation around the transformer windings.
Desorption of water from the insulation increases as temperature rises
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.