U. Gafvert

Bio: U. Gafvert is an academic researcher. The author has contributed to research in topics: Moisture & Transformer oil. The author has an hindex of 1, co-authored 1 publications receiving 178 citations.

Dielectric response of mineral oil impregnated cellulose and the impact of aging

Abstract: 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

Flexible Electronics Based on Micro/Nanostructured Paper.

Abstract: Over the past several years, a new surge of interest in paper electronics has arisen due to the numerous merits of simple micro/nanostructured substrates. Herein, the latest advances and principal issues in the design and fabrication of paper-based flexible electronics are highlighted. Following an introduction of the fascinating properties of paper matrixes, the construction of paper substrates from diverse functional materials for flexible electronics and their underlying principles are described. Then, notable progress related to the development of versatile electronic devices is discussed. Finally, future opportunities and the remaining challenges are examined. It is envisioned that more design concepts, working principles, and advanced papermaking techniques will be developed in the near future for the advanced functionalization of paper, paving the way for the mass production and commercial applications of flexible paper-based electronic devices.

Understanding the impacts of moisture and thermal ageing on transformer's insulation by dielectric response and molecular weight measurements

Abstract: Properties of oil and paper in a transformer degrade primarily due to thermal ageing and moisture ingress. Dielectric diagnostic tests, such as Recovery Voltage (RV), and Polarizations and Depolarization Current (PDC) measurement are currently being explored as potential tools for insulation condition assessment. A modern chemical analysis tool for paper molecular weight (MW) measurement, Gel Permeation Chromatography (GPC) or the more accurately described Size Exclusion Chromatography (SEC) promises to be useful in assessing ageing condition. However, the issue of separately assessing the impacts of ageing and moisture on oil and paper has been a key issue for many years. In the current research project, a series of experiments have been performed under controlled laboratory conditions with preset moisture content, and at controlled high temperature ageing. Whereas RV and PDC measurement results were found to be more sensitive to the moisture content of the oil and paper insulation, the MW distribution measurement by SEC provided a trend of insulation thermal ageing. This paper first provides a brief description of RVM, PDC and SEC procedures followed by a description of the experimental techniques adopted. Results are then analysed with the view of separately understanding the impacts of thermal ageing and moisture on the condition of oil and paper insulation in a transformer.

Recent Advancements in Functionalized Paper-Based Electronics

Abstract: Building electronic devices on ubiquitous paper substrates has recently drawn extensive attention due to its light weight, low cost, environmental friendliness, and ease of fabrication. Recently, a myriad of advancements have been made to improve the performance of paper electronics for various applications, such as basic electronic components, energy storage devices, generators, antennas, and electronic circuits. This review aims to summarize this progress and discuss different perspectives of paper electronics as well as the remaining challenges yet to be overcome in this field. Other aspects included in this review are the fundamental characteristics of paper, modification of paper with functional materials, and various methods for device fabrication.

Quantitative analysis of ageing condition of oil-paper insulation by frequency domain spectroscopy

Abstract: Frequency domain spectroscopy (FDS) has been used to assess the ageing condition of oil-paper insulation used in transformers. To further understand the ageing process, the reduction in degree of polymerization (DP) of cellulosic paper in itself on the dielectric response was investigated first. It has been found that the reduction in DP itself can alter the dielectric characteristics. The oil-paper insulation was thermally accelerated aged at 110 °C for up to 154 days according to the procedure described in IEEE Guide. Then dielectric characteristics of the thermally aged samples were obtained at different measurement temperatures. The results reveal that the relative permittivity (er') plots of oil impregnated pressboards increase with the reduction in cellulose polymer chain length. er' and tanδ values of oil impregnated pressboards shifts upwards in the lower frequency range (10-3 Hz -10-1 Hz) with different ageing condition, indicating the possibility of utilizing the characteristics of er' and tanδ values at lower frequencies (10-3 Hz-10-1 Hz) to quantitatively characterize ageing condition of oil impregnated pressboard. The parameters including DP, er' and tanδ values at the three characteristic frequencies (10-3 Hz, 10-2 Hz, 10-1 Hz) have been analyzed as a function of the ageing time, showing an exponential relationship based on the best fit. In addition, the er' and tanδ curves of aged oil impregnated pressboard have been observed to be shifted upwards to higher values at lower frequencies when the measurement temperature is increased. A shifting factor αT which describes the relationship between frequency and temperature has been defined. It allows one to shift the master curve of impregnated pressboard with DP obtained at reference temperature to other testing temperatures.

Investigation on dielectric response characteristics of thermally aged insulating pressboard in vacuum and oil-impregnated ambient

Abstract: Dielectric response measurements are non-invasive and promising diagnostic methods that are being gradually used for assessing the insulation and aging condition of transformer oil-paper insulation system. It is known that moisture content in oil-impregnated insulation has significant effect on its dielectric response phenomena, and whereas at present there is few research about the characteristics of aged cellulose without the impact of water. In this work, a series of experiments are designed and carried out in well-controlled laboratory. Accelerated thermal aging is performed on oil-immersed pressboard samples with different time intervals (0, 120, 250 and 400 h). Then these pressboard samples with different aging states are put into petroleum ether to remove the residual oil inside, and dried in a vacuum oven to remove the petroleum ether and moisture. The degree of polymerization (DP) and scanning electron microscope (SEM) are implemented to characterize the aging status of pressboard samples. Two kinds of dielectric response tests, i.e., the polarization and depolarization currents (PDC) and frequency domain spectroscopy (FDS), are performed on each oil/water-removed pressboard sample in vacuum and new-oil-impregnated ambient, respectively. Samples with different aging states show quite different dielectric response characteristics in both ambients. With the aging status more serious, the pressboard sample exhibits higher values of polarization/depolarization currents and complex capacitances. It is confirmed that, besides the influence of water induced by aging, the deterioration of cellulose itself also causes the increasing of pressboard conductivity, and hence greatly affects the PDC and FDS test results. Dielectric response diagnostic techniques have a potential to reflect the aging information of solid insulation.