Friday, March 27, 2020

How does Water or Moisture affect Power Transformer Health?


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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