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The Phenomena of
Moisture in Transformer
Oil
Water in oil appears as an unwanted substance, it is
generally accepted that water in microscopic amounts - not
gallons- is the cause of more electrical breakdowns than any
other impurity.
Moisture constitutes a hazard not only to the
insulating qualities of the oil but also to the insulations
that are immersed in the oil.
Water may be introduced to the oil by leaking gaskets,
poor handling techniques or from the product of natural
insulating paper and oil degradation. As the paper degrades,
it produces Carbon Dioxide and Water and as the insulating oil
ages, water, acids, sludge and other polar compounds are
formed. So its
presence is inevitable during the normal service life of a
transformer.
Water is a polar liquid having a high permittivity or
dielectric constant it is therefore attracted to areas of
strong electric fields.
This sees the internal moisture distributed not
uniformly, but in fact potentially concentrating in the most
dangerous parts of the system.
It is important to note that water is in a continuous
state of movement between the oil and paper insulating system,
caused by internal temperature variations due to load and
ambient conditions.
Water may be present
in four possible forms, they are:
• Free water – That is water
that has settled out of the oil in a separate layer.
It is this water which is indicated by a low dielectric
breakdown voltage.
• Emulsified water – Or water
that is suspended and has not yet settled out into free water
(indicated by “caramel” coloured
oil).
Nb: A high Power
Factor value indicates the possible presence of this suspended
water trapped in oil decay products.
• Water in solution – or
dissolved in the oil.
• Chemically bound water –
Water which is chemically attached to the insulating paper and
which is released when
oxidized.
The destructive
effects of water include:
• Expansion of the paper
insulation, altering the mechanical pressure of the
transformer clamping system.
• Loss of insulating ability
(Dielectric Breakdown Voltage)
• Accelerating paper aging ie
triggering decomposition of the fibres in the
paper
• Increased corrosion of the
core and tank
• Progressive consumption of
oil additives
The most dangerous
and destructive of these effects is the loss of the oils
insulating ability. This may occur from the
following events:
• During periods of high load
and at high ambient temperatures, dielectric breakdowns can
result from the reduced oil strength with high absolute
amounts of water.
• With sudden high loads,
water can boil off conductor surfaces and the vapour bubbles
can cause dielectric failures as they rise to the
top.
• During the cool-down period
after high load, the relative saturation of oil will
increase. At its extreme at 100%
relative saturation, water will precipitate out and greatly
reduce the dielectric strength of the
oil.
If oil is oxidized to any extent, any
water coming into the transformer will partially be absorbed
into the oil decay products (It is this fact which causes old
or highly oxidised oil to dissolve more water than new
oil). As the decay products build
up in the oil, the surface tension of the water or the
interfacial tension between the oil and the water is lowered
dramatically. This heavier decay molecule
will then recirculate throughout the entire transformer and
will find its way into the paper insulation, or into areas of
high electrical intensity thus reducing the insulation
resistance. The water saturated oil decay molecule has a
preference for the coolest part of the transformer (bottom and
fins, leading to corrosion) and areas of highest electrical
stress (leading to arcing).
It has been proven
that insulating paper with 2% moisture content ages three
times faster than one with 1% moisture and thirty times faster
with 3% moisture.
It is thus easy to see the importance of
maintaining low moisture levels within a transformer to ensure
a long and trouble free service
life. | 
