Analysis of Paper Insulation
The solid insulation of power transformers consists basically of paper in form of sheets, tapes, and other pressed shapes.
Heat, moisture, and oxygen, as well as oil decay products, adversely affect the life of the paper. Degradation of the paper causes it to lose its tensile strength and results in the release of furans.
Early detection of paper insulation breakdown can prevent major damage or failure.
Gas-in-oil analysis was the only non-invasive test performed on transformers that could indicate internal problems. By monitoring the CO and CO2 found dissolved in the oil, some assertions could be made about the condition of the paper.
The main goal of furan testing is to determine whether the paper in a given transformer has been or is being damaged by heat. Furans produced from temperature buildups are generated in two ways: the first being a localized area of high heat and paper damage, and the second being the general overall heating of the entire insulation system.
Thermal, oxidative and hydrolytic breakdown of paper insulation can be detected through furan analysis. This test, in conjunction with dissolved fault gas analysis, gives you the best overview of the state of your transformer. A furan test should be included with yearly maintenance and trends developed to monitor the condition of the paper.
Presently, there are no limit values or normal values for these compounds. Annual tests should be performed to determine baselines and trending patterns.
Degree of Polymerization
To assess the aging of insulating paper from service-aged power transformers and voltage regulators, samples can be analyzed to determine the degree of polymerization. This, however, is an invasive test requiring a sample of the paper which may present some difficulties such as taking the unit out of service.
Each cellulose fiber of paper is composed by a bundle of cellulose molecules of different lengths, laying side by side. These molecules are held together due to hydrogen bonds between the hydroxyl groups (-OH) existing in their structure. The cellulose molecule is a linear polymer formed by a chain of glucose rings linked by glycosidic bonds. As paper ages, the glycosidic bonds break, and the molecule is shortened. Consequently, paper loses its mechanical stength, and the useful life of the trasnformer is reduced.
New Kraft paper typically has a DP range of 1100 to 1500. Paper at the end of its useful life has a DP for around 150. Partial rewinding occurs around a DP of 250.
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