Tri T. Le

A study of degradation of cellulosic insulation materials in a power transformer, part 1. Molecular weight study of cellulose insulation paper

As a transformer ages, the chemical and physical properties of the cellulose insulation paper will change. For example, the average molecular weight of the cellulose chains decreases with age, as does the strength of the insulation paper. The molecular weight changes can be studied by several methods. However, the GPC method reported in this paper has an advantage over the other methods because it yields the total molecular weight distribution. In order to understand the mechanism of the degradation of the cellulosic insulation materials in a power transformer, a study of the accelerated ageing of cellulosic insulation paper in insulation oil has been performed in the temperature range 129–166 °C, under vacuum. The molecular weight and molecular weight distribution of the cellulose chains present in the insulation paper were measured by GPC techniques. The results provide information on the kinetics of degradation of the insulation paper. The results of the molecular weight study in the accelerated ageing experiments were found to be most useful for monitoring the condition of the insulation paper in an operational transformer.

Electrical and chemical diagnostics of transformers insulation. A. Aged transformer samples

This paper describes the application of two relatively new diagnostic techniques for the determination of insulation condition in aged transformers. The techniques are: (a) measurements of interfacial polarization spectra by a DC method; and (b) measurements of molecular weight and its distribution by gel permeation chromatography. Several other electrical properties of the cellulose polymer were also investigated. Samples were obtained from a retired power transformer and they were analysed by the developed techniques. Six distribution transformers were also tested with the interfacial polarization spectra measurement technique, and the molecular weight of paper/pressboard samples from these transformers were also measured by the gel permeation chromatography. The variation of the results through different locations in a power transformer is discussed in this paper. The possible correlation between different measured properties was investigated and discussed in this paper.

Investigations into effective methods for assessing the condition of insulation in aged power transformers

This paper investigates the effective diagnostic technique(s) for assessing the condition of insulation in aged power transformers. A number of electrical, mechanical and chemical techniques were investigated. Many of these techniques are already used by utility engineers and two comparatively new techniques are proposed in this paper. Results showing the effectiveness of these diagnostics are presented and correlation between the techniques are also presented. Finally, merits and suitability of different techniques are discussed in this paper.

A study of the degradation of cellulosic insulation materials in a power transformer. Part III: Degradation products of cellulose insulation paper

The cellulosic insulation materials in operational transformers undergo a slow thermal degradation during the life-time of the transformer to form furans which dissolve in the insulation oil of the transformer. There are six furans formed in significant quantity in an operational transformer. They are 2-furfuraldehyde, 5-hydroxymethyl-2-furfuraldehyde, 2-methyl-furfuraldehyde, 2-acetyl furan, 2-furoic acid and furfuryl alcohol. The concentrations of these furans can be measured by HPLC analysis of the oil. In this paper the furan products formed by accelerated ageing of cellulose insulation paper in Shell Diala B transformer insulation oil over the temperature range 130–170 °C are investigated, and the results have been correlated with those observed for the changes in the molecular weight and tensile strength of the insulation paper.

Electrical and chemical diagnostics of transformers insulation. B. Accelerated aged insulation samples

This paper describes the analysis of accelerated aged insulation samples to investigate the degradation processes observed in the insulation from aged power transformers. Short-term accelerated ageing experiments were performed on paper-wrapped insulated conductors and on pressboard samples. The condition of aged insulation samples was investigated by two relatively new diagnostic techniques: (a) measurements of interfacial polarization spectra by a DC method; and (b) measurements of molecular weight and its distribution by gel permeation chromatography (GPC). Several other electrical properties of the paper/pressboard samples were also studied. Possible correlations have been investigated among the different measured properties. The GPC results have been used to predict how power transformer insulation molecular weights change with temperature and time.

A study of degradation of cellulosic insulation materials in a power transformer. Part 2: tensile strength of cellulose insulation paper

When cellulose insulation paper ages, its physical and chemical properties change, the paper loses its strength and becomes brittle. A study of the accelerated ageing of cellulosic insulation paper in insulation oil has been performed in the temperature range 129–166 °C under vacuum. The tensile strength of paper that has been subjected to accelerated ageing as well as of paper retrieved from a retired transformer were measured in the cross machine direction (CMD). The results were found to be consistent with the observed changes in the molecular weight of the insulation paper. The relationship between tensile strength and molecular weight has been delineated and correlated with information on the kinetics of degradation of the insulation paper.

The behavior of aged regenerated Bombyx mori silk fibroin solutions studied by 1H NMR and rheology

As part of a project to utilize the regenerated silk fibroin (RSF) membranes as a supporting matrix for the attachment and growth of corneal stem/progenitor cells in the development of tissue engineered constructs for the surgical restoration of the ocular surface, the behavior of the aged RSF solutions has been investigated. The solutions were produced from domesticated silkworm (Bombyx mori) cocoons according to a protocol involving successive dissolution steps, filtration and dialysis. The solutions were kept at 4 degrees C in a refrigerator for a certain period of time until near the gelation time. The changes in molecular conformation were studied by solution-state (1)H NMR, while the flow of the solutions was characterized by rheological method. Upon ageing turbidity developed in solutions and the viscosity continuously decreased prior to a drastic increased near the gelation time. The (1)H resonances of aged solutions showed a consistent downfield shift as compared to the (1)H resonances of the fresh solution. Shear thinning with anomalous short recovery within a certain range of low shear rates occurred in both fresh and aged solutions. While the solutions behave as pseudo-plastic materials, the chain conformation in aged solutions adopted all secondary configurations with beta-strand being predominant.

An ESR study on γ-irradiated poly(vinyl alcohol)

The formation of radicals in poly(vinyl alcohol), PVA, powder irradiated at 77 K by gamma -rays and the transformations of these radicals during photolysis with visible wavelengths and on thermal annealing have been studied. After irradiation a four-line ESR spectrum was observed. It was assigned to a triplet of the C-alpha-radical (38%), with a splitting of 3.27 mT, superimposed on a doublet (62%) with a splitting of 2.7 mT. The doublet appears to be composed of two radicals, one of which is photo-bleachable (58%) and the other which is not photo-bleachable (42%). This suggests that the latter radical is a neutral radical. The photo-bleachable component of the doublet has been assigned to a carbonyl anion radical. but the second doublet due to a neutral radical is unassigned. The total G-value for formation of radicals at 77 K was found to be 2.41 +/- 0.03. Upon illumination with visible light, the anion radicals were removed and the doublet components or the spectrum diminished in intensity, while the three-line spectrum of the C-alpha-radical became more clearly visible. This transition was due to the photo-detachment of electrons from traps which were proposed to be located on carbonyl groups in the polymer resulting from incomplete hydrolysis of the vinyl acetate. The photo-decay of the anion radicals could be satisfactorily described by a two-stage process. The first stage comprised the decay of approximately 80% of the anion radicals present, while the second stage was associated with the decay of the remaining 20%. Subsequent thermal annealing of a photolysed sample to 290 K led to a change in the shape of the spectrum to form a more clearly defined triplet, As the doublet of the neutral radical decays on thermal annealing between 150 and 250K, the C-alpha-radical is formed

Condition monitoring of the bulk insulation of distribution transformers by electrical and chemical techniques

The paper addresses the assessment of insulation condition in distribution transformers. Electrical diagnostic techniques have been applied to several distribution transformers of different ages and ratings. Correlation between dissipation factor and interfacial polarization spectra (IFS) parameters are examined. Gel permeation chromatography (GPC) has been used for observing the change in molecular weight of the cellulose polymer. The correlation between interfacial polarization parameters and peak molecular weights are also examined.<<ETX>>

Aging of Transformer Insulation Paper

Abstract As a transformer ages, the chemical and physical properties of the cellulose insulation materials in the transformer change, and the paper loses its strength and becomes brittle. The average molecular weight of the cellulose chains decrease with age, and degradation products are formed, including water, carbon monoxide, carbon dioxide, and furans. The molecular weight changes in the cellulose have been studied by several methods, but the GPC method for determination of the molecular weight has been shown to offer advantages over other methods because it yields the total molecular weight distribution. The tensile strength of the cellulose insulation in transformers also changes with age as a result of the changing molecular weight of the cellulose. Work carried out in our laboratory on the aging of cellulose insulation is reviewed in this paper. Our studies have included investigations of insulation materials from retired transformers as well as accelerated aging of insulation paper in transformer...