Effect of Insulation Structure Scale on the Electric Field Characteristics in Oil-pressboard Insulation under DC Voltage

Abstract

The design of oil-pressboard/paper insulation structure is the key to maintain the safe and reliable operation of power transformers, which mainly relies on RC model simulations and verification tests undertaken with simple or equivalent test models, neglecting the potential scale effect and falling short of the physical entity verification conducted in large-scale oil-pressboard insulation structures. Based on Kerr electro-optic effect, the electric field and interface charge characteristics in both small and large scale oil-pressboard insulation structures were obtained to present the impacting mechanism of scale effect. The coaxial large-scale model is based on a converter transformer outlet device entity, which has three layers of pressboards with the thickness of 1 mm to separate out three 5-mm-wide oil spacings, and the model has an inner diameter of 200 mm and a length of 500 mm. As for the small scale model with parallel-plate electrodes, it was scaling down at the same proportion with the large one on the basis of electric field strength equivalence principle. The experiment results indicated that: 1) in the small-scale model, as the size of pressboard increases, the difference between the measured field strength in oil and the RC model one would increase, which could be attributed to the more interface charge accumulation in larger scale model, namely the larger the scale of model, the stronger the weakening effect of more interface charge on electric field in oil; 2) The spatial-temporal distribution characteristics of electric field in the large-scale model are obtained showed that, in the inner and middle oil-spacing, the measured steady-state field strength is larger than that obtained by RC model, with a maximum increase of 77.3%, while in outer oil-spacing, it is smaller than the RC model calculating one, and the maximum decrease is 36.8%; 3) As for the large scale oil-pressboard test model with multi oil-spacings and multi interfaces, the calculation method for charge density on different interfaces was proposed to quantitatively present the impacting mechanism of scale effect on the electric field and interface charge characteristics.