Water has one of the most significant effects on the aging of paper and insulating oil in a power
transformer by reducing the dielectric strength of the oil and physical
deterioration of the strength of the paper at working temperature. It also sets
up the process of hydrolysis which together with heat and an acidic environment
triggers a self-sustaining paper degradation process [1].
Paper insulation and pressboard structures are cellulose
based and are made up of glucose molecules linked together to form chains. The
average number of glucose molecules in a cellulose chain can be measured as a
“degree of polymerization” which for new Kraft paper is usually about 1200. The
larger the chains the more mechanical strength is available to the paper [2].
However water molecules have the ability to split
these chains shortening the length of the chains and thus reducing the
associated mechanical strength.
The influence of temperature
Temperature is a major catalyst of this process and
operating at elevated temperatures speeds up this process. It so happens that
one of the by-products of this process is more water which then adds to the
levels and over time it becomes a self-sustaining process. This is why the
levels of water must always be maintained to acceptable levels as reaching a
critical mass situation will result in rapid deterioration of the paper
properties and invariably the life of the power transformer.
Sudden increases in operating temperature due to
overloading combined with high water content in the insulation and dissolved
particles in the oil can cause bubble formation. Bubbles coming close to
energized parts like the windings can result in dielectric breakdown of the oil
with related discharges that can seriously affect the transformer.
It is found that transformers with high levels of
water that experience many high-loading events can cause the excess water from
the paper insulation to move into the oil. This then results in the relative saturation
of water being very high causing the formation of free water and this may also
occur when a heavily loaded wet transformer rapidly cools down. The collection
of free water affects the dielectric properties especially around the active
parts resulting in discharges that can cause long term damage. Free water also
causes rusting of the metal parts like the tank, pipes and radiators.
How is water formed?
The main sources of
water contamination in a power transformer are:
· Residual
humidity remaining from factory drying process
The factory dry out process will attempt to remove a
much of the water as possible but there will always be a certain amount of
water remaining in the transformer. That is why the quality control process
during winding installation, oil filling and dry out process is very important
in maintaining low water levels.
· Air
from atmosphere during normal breathing
From the time the transformer leaves the factory it
will start absorbing water from the atmosphere. It is important that steps are
taken to reduce the exposure of the internals of the transformer to water.
Usually transformers are transported without oil and are filled under pressure
with Nitrogen or dry air. This must be maintained until it is ready on site for
the oil filling process.
After the transformer is in service the use of
desiccant drying is used for free breathing transformers. Most transformers are now installed with a conservator
air cell / membrane which form a barrier to the external environment and
prevents Air from entering the transformer. Leaks on transformers are another place where water
can enter the transformer. Leaks must be identified as soon as possible and
rectified at the next available opportunity.
Byproducts
of oil-paper decomposition
As
part of of the decomposition of oil and paper the byproducts of the chemical reactions produces water.
Where is most of the water stored?
Mineral oil usually has a much larger mass when
compared to the paper insulation in a power transformer however water has a low
solubility level in oil making majority (> 95 %) of the water to be located
in the cellulose insulation.
Water usually exist
many in three forms
Dissolved in oil - The
amount of water dissolved increases with increasing temperature.
Attaching - to particles
like dirt, fibers from cellulose and acids
As free water - which
usually collect at the bottom of the tank. Free water forms when saturation is
reached with the water dissolved in oil. It has the same effect as “raining”
where free water is formed. Free water also enters the transformer from leaks
or from the air.
How is the level of water measured?
Measurement of water from oil samples are in the range
of a few parts per million (0-10 ppm being acceptable). However direct
measurement of water in the cellulose is usually expressed in percentage and
can range from 0.3 % to 6 % (less than 2% being an acceptable level for
transformers in operation).
Another influence in the accurate measurement or
estimation of water is the temperature of the oil at the time of the sample.
Other influences are how the sample was taken, environmental conditions and
contamination of the sample.
The water content in the cellulose provides a much
more reliable value for condition assessment, as it is barely influenced by
those parameters. A reliable method to measure water content in paper can be performed
using a paper sample by applying Karl Fischer titration, or inferred by using
frequency-domain spectroscopy. Unfortunately, sampling of cellulose for
moisture analysis is a very difficult task as the solid insulation of a power
transformer isn’t easily accessible.
Physical–chemical oil testing analysis is another
common methods used to provide a general condition of water level in the oil.
How to interpret water level measurements?
Oil Samples:
Oil samples analyzed
in the lab usually produce results in ppm (parts per million) in oil. As a
general rule of thumb the following can be used for assessment:
Limits (PPM)
|
|
< 10
|
Normal
|
10 Moisture <
20
|
Monitor Closely and Maintain
|
> 20
|
Oil processing / replacement required / investigated
|
Paper:
To assess the amount
of water in the paper on can use relation charts from the oil samples to infer
the estimated wetness of paper. The following chart can be used.
Oommen Curves for
moisture regions [3]
There are many other
variations that will provide similar assessments.
Limits (PPM)
|
|
< 1.5 %
|
Normal
|
1.5 % < Moisture < 3%
|
Monitor Closely and Maintain
|
> 3%
|
Dry out recommended
|
International Standards
The following
international standards provide guidance on the assessment and measurement of
water or moisture in power transformers:
IEC 60422 - Mineral insulating oils in electrical equipment - Supervision and maintenance guidance
References
1
|
Cropp M, “Energised Dry-outs”, Techcon 2003, Page
66, June-July 2003
|
2
|
R. D. Stebbins, D. S. Myers and A. B. Shkolnik,
"Furanic compounds in dielectric liquid samples: review and update of
diagnostic interpretation and estimation of insulation ageing," Proceedings
of the 7th International Conference on Properties and Applications of
Dielectric Materials (Cat. No.03CH37417), Nagoya, Japan, 2003, pp.
921-926 vol.3.
|
3
|
T. V. Oommen, “Moisture Equilibrium in Paper-oil
Systems,” Proceedings of the Electrical / Electronics Insulation Conference,
Chicago, IL, pp. 162-166, October 3-6, 1983
|
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