Power transformers

Abstract

A large power transformer, is often, a custom designed equipment that entails a complex and capital intensive manufacturing process. It is an integral part of the power system and the cost of failure during service can be significant. Due to its importance in the power system, transformers should perform reliably under the conditions for which it has been designed while keeping the manufacturing costs to a minimum. Therefore, it is important to optimize the production process. Standing time, or hold time, is the period between the end of tank filling with mineral oil and factory acceptance tests (FAT). During this period, the newly completed transformer is left undisturbed for several days, depending on its design. It is necessary in order to improve insulating properties of the cellulosic insulation, thereby preventing partial discharges (PD) and failure during FAT. Moisture is disadvantageous to any insulation design and is particularly true in the case of cellulosic insulation (widely used in transformer construction). Cellulosic insulation is highly hygroscopic and moisture ingress in small concentrations during the final clamping procedure is inevitable. Based on the research at TU Delft, moisture in cellulosic insulation was found to be the major factor leading to PD and can also have an influence on standing time. This thesis focusses on the moisture ingress into pressboard insulation when dried insulation is exposed to ambient air during manufacture. The aim of the thesis is to investigate the effects of moisture on partial discharges in pressboard insulation and its influence on standing time. The results of this thesis could help optimize the production of transformers. This thesis discusses the results of experiments investigating the effects of moisture in pressboard samples with respect to standing time. 3 different configurations of pressboard samples are subjected to various conditions in a climate chamber (to simulate moisture ingress during clamping) and impregnated with dry mineral oil. PD tests on samples are carried out one by one over 9 days of standing time. The effect of standing time on partial discharge inception voltage and discharge magnitudes on samples with different moisture contents are observed. Additionally, during the course of the thesis, several tests were carried out to study the effect of oil temperature during FAT and the effects of moisture transients in oil. Furthermore, moisture ingress and egress to and from the samples are also analysed. Based on the results of the experiments it can be concluded that the standing time can be reduced considerably. Furthermore, the presence of a temperature gradient along with moisture content greater than 2 % in pressboard can produce discharges which can lead to failure during FAT. A moisture estimation tool is also developed which can be used to vary the vacuum cycle duration in order to control moisture content in pressboard.