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The applicability of using 2-furfural (2-FAL) and methanol as aging markers for synthetic ester impregnated paper insulation was investigated through laboratory aging experiments at temperatures of 80, 100 and 120 ?C. Methanol in the synthetic ester increased linearly with paper aging until degree of polymerisation (DP) reached 400. During this period concentration of methanol in oil was higher than that of 2-FAL, which proved the concept of using methanol to indicate early paper aging in the synthetic ester. However, the rate of methanol increase against the reduction of DP in the synthetic ester was about three times higher than that seen in mineral oil. This is due to a difference in the partitioning of the marker between oil and paper. Since the synthetic ester is more polar than the mineral oil, it attracted more methanol molecules from the paper, resulting in a higher concentration of methanol in oil for the same reduction of DP. In addition, a reduction in methanol was observed at the later stage of paper aging at 120 ?C. This could be due to either a change in the partitioning between oil and paper, or an esterification reaction which consumes methanol in the system. [ABSTRACT FROM PUBLISHER]

Furfural content in oil is a valuable indicator of cellulose paper degradation and is widely utilized in aging assessment for power transformers. However, with the same aging degree of insulation paper, the actual furfural content in oil is influenced by temperature because furfural partitioning behavior in the oil-paper insulation system of power transformers changes with temperature. Thus, correction of the temperature effect on furfural partitioning is crucial for accurate assessment of the aging of power transformers. In this research, oil-paper samples with different furfural contents were prepared through an accelerated thermal aging test. A furfural equilibrium experiment was conducted at different temperatures to construct a furfural partitioning diagram. The mass fraction of furfural in oil and paper was calculated. Results showed that the mass fraction of furfural in oil increases with temperature rising and aging deepening, which means that furfural tends to migrate from paper to oil at high temperatures and deep aging conditions. The change in furfural partitioning between oil and paper at different temperatures causes a deviation in paper aging assessment. To correct the temperature effect on furfural partitioning, a furfural correction factor was developed to normalize furfural contents in oil at different temperatures to a reference temperature. The furfural correction factor was verified by experimental data and then introduced into the relation between degree of polymerization of paper and furfural content of oil. Verification results showed that the corrected equation can enhance the accuracy of paper aging estimation at different temperatures. The findings of this work indicated temperature effect on furfural partitioning should be considered when determining the furfural thresholds and evaluating the aging condition of power transformers. [ABSTRACT FROM PUBLISHER]

The application of vibration-based condition monitoring methods to power transformers has shown the potential for detecting mechanical transformer failures. The ageing of winding insulation paper, which is a common problem in long-serving power transformers, affects both the electrical and mechanical strength of the windings. This paper is concerned with the vibration features of winding disks with aged insulation paper. The vibration characteristics of single-layer winding disks wrapped with insulation paper are investigated. By examining the vibration frequency responses of winding disks with different degrees of ageing of insulation paper, the effects of aged paper on the vibration behaviors of winding disks are demonstrated. The experimental results show that changes in the vibration responses of winding disks can be related to the ageing of insulation paper, and thus it is possible to detect this failure mode in a power transformer using a vibration-based method. A qualitative explanation for these changes is presented, using the degradation of the mechanical strength of the insulation paper. [ABSTRACT FROM AUTHOR]

The degradation of oil-impregnated paper induced by partial discharge (PD) seriously affects the life of power equipment. In order to investigate the relationship between partial discharge and the degradation of oil-impregnated paper, PD characteristics and trap parameters of the impregnated paper are measured. In this work, a cavity discharge model is adopted and samples with different degrees of polymerization (DP) are fabricated using thermally accelerated aging to simulate transformers with different lifetimes. For each sample, phase resolved partial discharge (PRPD) spectrograms are recorded for various partial discharge times. The characteristics of partial discharge are analyzed, and the trap parameters of specimens are obtained by the thermally stimulated depolarization current (TSDC) method. Following this, the microstructures of the samples are observed using a scanning electron microscope (SEM). It is found that the damaging breakdown process of oil-immersed paper can be divided into three stages: initial stage, developing stage and pre-breakdown stage. There also exist two levels of trap on the surface of oil-impregnated paper: shallow (0.10-0.59 eV) and relatively deep (0.60-0.90 eV) energy. Importantly, the thermal aging makes the trap energy deeper while electrical aging makes the trap energy shallower. [ABSTRACT FROM AUTHOR]

A post mortem analysis of scrapped transformers provides an opportunity to directly measure the properties of in-service aged paper insulation. Investigations on 10 scrapped 275 kV and 400 kV power transformers were carried out, where Tensile Index (TI) and Low Molecular Weight Acid (LMA) of the paper insulation sampled from various locations were measured. Multiple-layer TI profile demonstrated the effect of oxidized oil on paper ageing which was confirmed by laboratory ageing experiments. It is LMA rather than High Molecular Weight Acid (HMA) that accelerates the paper degradation. LMA acts as the paper ageing accelerator and also as the paper degradation by-product, supporting the view that self-accelerated hydrolysis process is the main paper ageing mechanism. Finally, a generic relationship between LMA in paper and TI of paper was revealed based on both in-service and laboratory ageing data, i.e. the lower the TI of paper, the higher value of LMA in paper is measured. It implies that LMA in oil can be a potential paper ageing indicator pro viding a link between LMA in oil and paper is found. [ABSTRACT FROM AUTHOR]

This paper is aimed at the study of aging behavior of cellulosic materials in different dielectric liquids under air and nitrogen atmospheres. Paper for copper conductor and high density transformer board were aged at 130°C up to 1128 h in sealed vessels in presence of mineral oil (MO), natural ester (VO) and synthetic ester (SE). The influence of thermal aging on cellulose and oil characteristics are investigated and compared. It's observed that the paper degradation is associated with the generation of CO2 and CO gases. Whatever the type of oil, this generation is more important with vessel under air atmosphere than that under nitrogen gap indicating a protective effect of nitrogen. In mineral oil, the degree of polymerization (DPv) reaches the end of life criteria at around 200 whereas it remains around 400 with ester oils. This is mainly due to the higher water solubility of esters compared to mineral oil. The measured degree of polymerization in MO is comparable to that calculated through 2-FAL (Furanic compounds) content using Vuarchex equation whereas for esters oil, there is no agreement between the measured and calculated DPv through 2-FAL content. This can constitute a limitation in the use of 2-FAL for the diagnosis of ester-filled transformers. Overall, the insulation cellulose/oil behavior seems better under nitrogen atmosphere. [ABSTRACT FROM PUBLISHER]

Utility asset managers and engineers are concerned about the life and reliability of their power transformers which depends on the continued life of the paper insulation. The ageing rate of the paper is affected by water, oxygen and acids. Traditionally, the ageing rate of paper has been studied in sealed vessels however this approach does not allow the possibility to assess the affect of oxygen on paper with different water content. The ageing rate of paper has been studied for dry paper in air (excess oxygen). In these experiments we studied the ageing rate of Kraft and thermally upgraded Kraft paper in medium and high oxygen with varying water content. Furthermore, the oxygen content of the oil in sealed vessels is low which represents only sealed transformers. The ageing rate of the paper has not been determined for free breathing transformers with medium or high oxygen content and for different wetness of paper. In these ageing experiments the water and oxygen content was controlled using a special test rig to compare the ageing rate to previous work and to determine the ageing effect of paper by combining temperature, water content of paper and oxygen content of the oil. We found that the ageing rate of paper with the same water content increased with oxygen content in the oil. Hence, new life curves were developed based on the water content of the paper and the oxygen content of the oil. [ABSTRACT FROM PUBLISHER]

Dielectric response is a non-invasive diagnostic method with a potential to give information about the status of the solid insulation in a power transformer. To investigate what conditions the method can and cannot reveal, frequency domain spectroscopy has been applied to oil-impregnated paper and pressboard with and without moisture and/or acids added, and to paper at various stages of aging. Water and carboxylic acids are important aging indicators. It is found that moisture is clearly detectable. Also low molecular weight carboxylic acids will affect the dielectric response in a way similar to moisture, but high concentrations are needed for a contribution comparable to that of moisture. It is difficult to tell moisture and acids apart from the responses obtained. High molecular weight carboxylic acids make little or no contribution to the dielectric response of the solid insulation in a paper/pressboard — oil system. It was also observed that keeping paper or pressboard at elevated temperatures had a conditioning effect on the response. [ABSTRACT FROM AUTHOR]