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

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 oil-cellulose 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 electro-acoustic (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.