Optimization of the Impregnation Process of Cellulose Materials in High Voltage Power transformers- susceptibility of high-density materials to partial discharge activity

Abstract

The manufacturing process of high voltage power transformers consists of several phases such as design, assembly, drying and impregnation process which are time consuming. Before electrical testing, a transformer needs an extra standing time for further impregnation of cellulose material in particular those of high density. It should be noted that the duration of the standing time depends on the ratings of the transformer. The main reason for considering this standing time is to reduce the probability of failure during electrical testing. This means that the reduction of such standing time is a crucial issue. Therefore, the main goal of this thesis is to optimize the post impregnation process of high density cellulosic material. One of the requirements for shortening the post impregnation standing time is that during the electrical tests of the transformer the PD level should not exceed the PD acceptance given by IEC standard level. In this thesis, it is investigated if the standing time is related to the occurrence of partial discharges in high density materials. This is done by performing a number of partial discharge measurement on different samples. Further on, tan? measurements were performed on different samples in the course of impregnation process to investigate whether the losses decrease in the course of time. In this research, the impregnation processes of two different cellulosic materials, namely transformerboard (PSP) an laminated wood (KP) are studied. Based on that, an empirical model that predicts the time needed to fully impregnate a cellulosic sample is developed. This model relates the impregnation time with sample material, dimensions, shapes, impregnation and temperature. The proposed model shows a good agreement with the measurements