Assessment of space charge behavior of oil-cellulose insulation systems by means of the PEA method

Abstract

Despite recent advancements in material technology, oil-cellulose insulation systems in form of so called Transformerboard combined with paper and mineral oil are still one of the backbones of high voltage technology. This insulation system is widely used in the high voltage industry, especially in high voltage transformers for AC and HVDC applications, e.g. converter transformers. The space charge behaviour of Transformerboard has rarely been investigated, causing the presumption oil-cellulose insulation systems would not collect significant amounts of space charges. This exploratory work investigated oilcellulose insulation in new condition and at different aging stages.Main objective of this series of measurements was to investigate the influence of isothermal aging on the charge behaviour of Transformerboard. To achieve this, a number of oil-impregnated samples were measured with the pulsed electroacoustic (PEA) method. The non-destructive PEA method has the advantage of enabling multiple measurements on the same sample. The surface of the Transformerboard samples has been grinded to eliminate possible surface effects here. These samples had a thickness of 1 mm and they have been impregnated with mineral insulating oil. Space charge profiles were recorded during polarisation and depolarisation at 20°C and 60°C. New samples were recorded first and compared to samples, which were aged for 11 weeks at 135°C. A preliminary explanation for the charge mechanisms involved is given in the analysis.

Progress of Space Charge Research on Oil-Paper Insulation Using Pulsed Electroacoustic Techniques

“…Results from Southampton in [14] show that the moisture content, even within 5%, has a great influence on the charge injection, movement and accumulation. The higher the moisture content of the oil-paper sample, the higher the mobility of the charges, and more positive charge injection and negative charge injection were observed.…”

“…A series of measurements were carried out where an oil-paper insulation system was subjected to different test temperatures (from 15˝C to 60˝C) [11][12][13][14]. The results showed that the temperature had a limited effect on the threshold voltage of the space charge.…”

Calibration of pulsed electroacoustic method considering electrode–dielectric interface status and porosity

“…[6][7][8][9] The PEA method was originally developed to measure space charge distribution in polyethylene, but has since then been applied to a broad variety of substances, such as epoxy resin, 10) polyimide, 11) silicone rubber, 12) and oil paper. [13][14][15][16][17][18][19] With the exception of oil paper, the substances that are used in the PEA method are homogeneous solids unless they are specially treated, for instance by nanoparticles addition. Oil paper is widely used as the main insulation material in convert transformers.…”

“…It is a composite dielectric consisting of porous insulation paper that is permeated by insulation oil. Existing research reports on space charge in oil paper do not address electric field distribution, [13][14][15][16][17][18][19] which stands out compared with other studied materials. The PEA method is based on the elastic wave propagation in a solid and on the hypothesis that the contact interface is perfect; 9) however, these prerequisites are occasionally not satisfied in space charge measurements.…”