Adhesives for bonding transformerboard: partial discharge and ageing behaviour

Abstract

Large power transformers are usually in operation over several decades. During operation, high temperatures frequently occur. Because of these high temperatures, the insulation materials are subjected to accelerated ageing processes. It is necessary to ensure that all materials used in transformers do not lose their functionality over the decades of operation; for example, the adhesives used in bonded transformerboard must not lose their mechanical strength. With that background, ageing tests were performed with selected adhesives. The aged test samples were used for bending tests at 120 °C. Ageing was carried out with different parameters: The ageing times were either two weeks or two months, and the ageing temperatures were either 120 or 140 °C. During the accelerated ageing process, the test samples were stored in mineral oil. The results of the mechanical tests showed significant differences of mechanical strength after the ageing of the adhesives. Differences of the adhesives with respect to the ageing trends and the temperature dependence of the mechanical tests could be shown. Due to the electrical field, partial discharges can occur in a transformer. Partial discharges may take place in defects of the insulation system like inhomogeneities or pollution, or on sharp edges of structural parts. Consequently, partial discharge activity will reduce the insulation strength of the insulation system (D. Konig, Y.N. Rao, Teilentladungen in Betriebsmitteln der Energietechnik, 1993), thereby reducing the lifetime of the transformer. Hence, partial discharges have to be avoided during the operation of the transformer. In this study, transformerboards bonded with different adhesives have been investigated. The focus was on the partial discharge behaviour of the bonded joint. The investigated adhesives were based on epoxy resin, casein formulation and poly(vinyl acetate). The different adhesives showed a wide range of behaviour with respect to partial discharges. The tests were performed in a homogenous electrical field, up to field strengths of 10 kV/mm.