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Breakdown characteristics of thermally degradated oil-impregnated insulation

The oil-impregnated (OI) insulation has been chosen as the insulating medium of high voltage transformers, power cables and capacitors. When the OI insulation is installed in the component it is expected to withstand operational stresses without breakdown in order to provide reliable operation during whole life-time of the component, i.e. 30-40 years. The chief disadvantage of the OI insulation is its vulnerability to thermal and electrical aging. Once the component is set up into operation the insulation starts to age over time, and withstand properties of the insulation tend to decrease. Therefore, assessment of dielectric properties of insulation is necessary. Often, the breakdown voltage measured according to the IEC or IEEE standards is used. From the experiments it follows that the breakdown voltage of the OI insulation remains constant over the period of thermal aging. On one hand, it can be seen that the breakdown voltage does not yield quantitative information about the insulation condition. On the other hand the life-time of the component is determined based on the life-curve which is usually determined by using information about time-to-breakdown. Therefore, it is interesting to investigate the possible changes of time-to-breakdown of OI insulation characterized by different levels of thermal aging. This contribution will present the results of investigation into breakdown parameters of OI insulation. Particularly, time-to-breakdown and breakdown voltage for samples of OI insulation and changes due thermal aging will be presented.