Performance of non-homogenous insulating oil mixtures under dc conditions

In practice, mixtures of insulating oils are encountered frequently. In many cases, the added new oil is not of the same type of the one used in the electrical equipment. In this communication, we report on an extensive experimental study of the performance of non-homogenous mixtures of insulating oils under dc conditions. Samples of different volumetric percentage ratios (VPRs) of the two oils under study, new Esso and used Diala C, are prepared representing different mixing ratios. The two oils form two distinct layers, non-homogenous mixture. The breakdown voltage (BDV) of each sample is determined experimentally as similar to what is indicated by the IEC 156/1963 publication taking into consideration the experimental limits of the equipment used. Also, the conduction current, of each sample, is recorded at 80% of the BDV of the sample. A current probe and a fast digital oscilloscope are used. Then each sample is stirred to form a uniform mixture of the two oils, homogenous mixture with no boundary layer. The BDV of the sample is then measured. Also, the BDV is measured and the conduction current is recorded as the inclination angle θ between the boundary layer and the axis of the field is changed, for a certain VPR. The polarity effect on the BDV as changed with θ is also studied. We close by a detailed discussion and an explanation of the host of the experimental results presented