Study of anomalous breakdown of paper-oil insulation under unconventional voltages

Many reports are available on behavior of oil impregnated paper insulation under conventional voltages as this insulation has been found to be outstanding in its characteristics in large forced oil cooled transformers. However, the performance of oil impregnated kraft paper (OIP) has not been established under the environment of unconventional voltage which appear in HVDC systems, EHV transformers and others. In the present investigations, the breakdown studies on impregnated and unimpregnated kraft insulating paper of different thickness have been carried out under different field configurations like uniform, moderately uniform and nonuniform fields. The influence of different types of voltages such as ac, dc and combined ac and dc voltages on the performance of insulation paper and oil impregnated paper have been studied. In addition, for the ac system, the influence of additional dc stresses, the range and corresponding voltage as a percentage of applied ac voltage have been analyzed and discussed. Comparison has been presented with respect to field configuration, applied conventional and non conventional voltage. Dependence on other experimental variables like ac component, dc bias, electrode gaps in oil medium and others for both kraft insulating paper and oil impregnated paper (OIP) is also reported. These aspects may provide valuable data for reliable design of large EHV transformers