Moisture Measurement
Content
Moisture measurement
The term “moisture” in the transformer
industry is commonly used to indicate water
which is absorbed in the paper or dissolved in the
oil. Occasionally, the terms “water” or “water
content” are used as an alternative way to
describe the same substance.
Water in transformers can be found in different
parts of the insulation system. It can accumulate
in solid insulation, be dissolved in oil, or be
found in the form of liquid water at the core or
bottom of a transformer.
If it has been determined that a transformer has
an excessive moisture content, either
through water-in-oil analysis or dew-point
analysis, the transformer should be dried out.
Otherwise, the presence of moisture in the kraft
paper will definitely shorten the life due to
chemical decomposition through oxidation of the
cellulose.
Drying out a transformer can be accomplished by
untanking the core and coil
assembly and drying it in an oven, but this
would be both expensive and inconvenient.
Fortunately, there are methods that can be
applied in the field that are very effective in
drying out a transformer.
Simple hot oil flush //
A simple hot oil flush is one method, although it
is fairly slow. Oil is pumped through a filter press,
which contains paper filter elements, and then
heated and sprayed onto the core and coils. The
hot oil has a greater affinity for moisture than cold
oil and will pick up some moisture from the core
and coil insulation.
Running the oil through a filter press allows the filter elements to remove some of
the moisture in the oil. Cooling the oil before it goes through the filter press will
enhance the removal of moisture from the oil.
To spray hot oil //
A better method is to spray hot oil into the
transformer under vacuum. The combination of
the heat from the hot oil and the vacuum will
cause the moisture in the insulation to ‘‘flash
off’’ and dry the transformer. A cold trap can
enhance the action of a mechanical vacuum
pump. Once the mechanical pump removes all
the air from the transformer tank, the only gas left
in this space is H2O.
A cold trap using a dry ice and acetone jacket will
instantly freeze any water vapor with the lowtemperature ice having a very low vapor
pressure.
The cold trap thus acts like an auxiliary vacuum pump. The ice provides a direct
measurement of the amount of moisture removed from the cellulose.
The research conducted at Monash University
has shown that, for the accurate moisture
assessment of transformer insulation, the
continuous monitoring of a number of
parameters, including transformer load,
temperature and oil relative saturation, is
required. The measurement of water solubility
characteristic of oil in the transformer is also
required.
A utility may operate tens or even hundreds of
transformers. However, not all of the transformers
require continuous monitoring.
How is it possible to identify a transformer with
moisture concerns for further monitoring? There
is a demand for a classification procedure,
which would rank transformers by moisture
levels.
References //
Power Transformers Principles and
Applications – John J. Winders,
Jr. (Amazon)
Moisture assessment in power
transformers - V. G. Davydov, O. Roizman
The term “moisture” in the transformer
industry is commonly used to indicate water
which is absorbed in the paper or dissolved in the
oil. Occasionally, the terms “water” or “water
content” are used as an alternative way to
describe the same substance.
Water in transformers can be found in different
parts of the insulation system. It can accumulate
in solid insulation, be dissolved in oil, or be
found in the form of liquid water at the core or
bottom of a transformer.
If it has been determined that a transformer has
an excessive moisture content, either
through water-in-oil analysis or dew-point
analysis, the transformer should be dried out.
Otherwise, the presence of moisture in the kraft
paper will definitely shorten the life due to
chemical decomposition through oxidation of the
cellulose.
Drying out a transformer can be accomplished by
untanking the core and coil
assembly and drying it in an oven, but this
would be both expensive and inconvenient.
Fortunately, there are methods that can be
applied in the field that are very effective in
drying out a transformer.
Simple hot oil flush //
A simple hot oil flush is one method, although it
is fairly slow. Oil is pumped through a filter press,
which contains paper filter elements, and then
heated and sprayed onto the core and coils. The
hot oil has a greater affinity for moisture than cold
oil and will pick up some moisture from the core
and coil insulation.
Running the oil through a filter press allows the filter elements to remove some of
the moisture in the oil. Cooling the oil before it goes through the filter press will
enhance the removal of moisture from the oil.
To spray hot oil //
A better method is to spray hot oil into the
transformer under vacuum. The combination of
the heat from the hot oil and the vacuum will
cause the moisture in the insulation to ‘‘flash
off’’ and dry the transformer. A cold trap can
enhance the action of a mechanical vacuum
pump. Once the mechanical pump removes all
the air from the transformer tank, the only gas left
in this space is H2O.
A cold trap using a dry ice and acetone jacket will
instantly freeze any water vapor with the lowtemperature ice having a very low vapor
pressure.
The cold trap thus acts like an auxiliary vacuum pump. The ice provides a direct
measurement of the amount of moisture removed from the cellulose.
The research conducted at Monash University
has shown that, for the accurate moisture
assessment of transformer insulation, the
continuous monitoring of a number of
parameters, including transformer load,
temperature and oil relative saturation, is
required. The measurement of water solubility
characteristic of oil in the transformer is also
required.
A utility may operate tens or even hundreds of
transformers. However, not all of the transformers
require continuous monitoring.
How is it possible to identify a transformer with
moisture concerns for further monitoring? There
is a demand for a classification procedure,
which would rank transformers by moisture
levels.
References //
Power Transformers Principles and
Applications – John J. Winders,
Jr. (Amazon)
Moisture assessment in power
transformers - V. G. Davydov, O. Roizman
