Pressboard

About: Pressboard is a research topic. Over the lifetime, 1123 publications have been published within this topic receiving 9089 citations.

Aging of cellulose at transformer service temperatures. Part 1: Influence of type of oil and air on the degree of polymerization of pressboard, dissolved gases, and furanic compounds in oil

Abstract: A power transformer is expected to operate up to 40 years. With current cost-benefit calculations, the user is forced to perform maintenance that will bring a further extension of transformer life. The maintenance can only be based on on-site and off-site monitoring with extended analytical and electrical tests, which can define the service condition of the transformer and predict its further life expectancy. The life of a transformer is mainly dependent on the life of its solid insulation, and the life-limit is determined by the thermal degradation of the winding paper. The Kraft paper decreases in tensile strength with the progress of aging and, at some point, can no longer withstand the short circuit stresses. The monitoring of the complex permittivity of transformer oils, as a function of frequency and temperature, may provide information concerning the state of the insulation. In this article, we show the influence of air and oil type on the aging of pressboard under the influence of a considerable amount of moisture. The rate of the degree of polymerization, the development of furanic compounds, as well as the gas-in-oil analysis in comparison with the aging of the pure oil under the same conditions have been investigated.

Propagation of positive and negative streamers in oil with and without pressboard interfaces

Abstract: Inception and propagation of streamers in a point-plane gap, with and without pressboard interface parallel to the field, have been studied. Current and light emission have been recorded. Positive streamers propagate at /spl sim/2 to 4 mm//spl mu/s at voltages to 2/spl times/ the minimum breakdown voltage. Above this voltage, a fast event (>100 mm//spl mu/s) may occur, preceded by streamers propagating with velocities in the 10 to 20 mm//spl mu/s range. The negative streamers have /spl sim/2/spl times/ the breakdown voltage as the positive ones. Even negative streamers may develop into a slower kind of fast event. Solid pressboard parallel to the field does not change the breakdown voltage, but makes inception of fast events easier. The breakdown process in oil shows several similarities to a gas breakdown.

Identification of a chemical indicator of the rupture of 1,4-β-glycosidic bonds of cellulose in an oil-impregnated insulating paper system

Abstract: In this study, headspace gas chromatography/mass spectrometry has been used to assess the volatile by-products generated by the ageing of oil-impregnated paper insulation of power transformers. Sealed-glass ampoules were used to age under oxidative conditions 0.5-g specimens of insulating paper in 9 mL of inhibited mineral oil in a temperature range of 60–120 °C and moisture of 0.5, 1 and 2% (w/w). A linear relationship between one of the oil-soluble degradation by-products, i.e. methanol, and the number of ruptured 1,4-β-glycosidic bonds of cellulose, regardless of the type of paper (ordinary Kraft or thermally-upgraded (TU) Kraft paper), was established for the first time in this field. Ageing at 130 °C of model compounds of the Kraft paper constituents (α-cellulose, hemicellulose and lignin) and two cellulosic breakdown by-products (D-(+)-glucose and 1,6-anhydro-β-d-glucopyranose) confirmed that the α-cellulose degradation was mostly responsible for the presence of this molecule in the system. Furthermore, additional 130 °C-tests with six different papers and pressboard samples under a tight control of initial moisture indicated that at least one molecule of methanol is formed for each rupture of 1,4-β-glucosidic bond of the molecular chains. Stability tests showed that the ageing indicator is stable under the oxygen and temperature conditions of open-breathing transformers. The presence of methanol was detected in 94% of oil samples collected from over than 900 in-service pieces of equipment, confirming the potential for this application. Lastly, the tests have shown that oil-oxidation by-products and TU-nitrogenous agents modify the methanol partitioning coefficients in the paper/oil/air system, which makes their study essential over a range of field conditions encountered by power transformers. Results are presented and discussed in comparison with 2-furfuraldehyde, which is the current reference in the domain.

A survey of aging characteristics of cellulose insulation in natural ester and mineral oil

Abstract: The article makes a case for allowing higher temperatures in transformers filled with natural ester fluids. This article will first provide a review of data from aging studies performed on cellulose (both Kraft paper and pressboard) impregnated separately in mineral oil and BIOTEMP (ABB, USA), a high oleic sunflower based fluid. Next, possible mechanisms that explain the differences in aging behavior will be discussed. The article concludes with recommendations for new temperature limits for BIOTEMP-impregnated transformers in comparison with mineral-oil-impregnated transformers.

Creepage discharge on insulation barriers in aged power transformers

Abstract: This paper presents experimental research of creepage discharge on insulation barriers in power transformers. Using point-to-plate electrode configurations under AC voltages creepage discharge is studied for both single flashover breakdown and partial discharge induced failure modes. It is confirmed that the dielectric strength of oil gap will not be reduced with introducing dry new pressboard surface into the oil gap, indeed the flashover breakdown voltage is hardly compromised by introducing aged pressboard with up to 3% moisture. However, increased moisture content in pressboard reduces partial discharge inception voltage (PDIV) significantly, i.e. ~30% PDIV reduction for pressboard of ~3% moisture as compared with dry pressboard. More importantly, high moisture contents in pressboard increase PD activities in oil pores which allow gasses to be trapped inside to develop gaseous channels which eventually lead creepage discharge to breakdown.