Comparison of streaming electrification characteristics between an ester liquid and a mineral oil using rotating disc method

Streaming electrification phenomenon seen in power transformers results from forced oil circulation. This phenomenon has been extensively investigated since it was identified as one of the failure reasons for large power transformers. The majority of the published research focused on the streaming electrification of mineral oils. Taking into account the potential increase in the voltage level and capacity of ester liquid filled power transformers, the present study used a rotating disc system and compared the streaming electrification characteristics of a synthetic ester with those of a conventional mineral oil. The rotational speed was varied between 100 rpm and 1000 rpm, and the oil temperature was varied between 20 °C and 80 °C. Both aged and unaged oil samples were used to form the oil-pressboard interfaces. The measurement results indicate that the synthetic ester exhibits a stronger ability to generate streaming current than the mineral oil. The subsequent experiments show that the streaming current generated in the mineral oil increases as a result of ageing, whereas that in the synthetic ester decreases. The relaxation times of the two liquids have also been estimated. The results suggest that the mineral oil has a much larger relaxation time than the synthetic ester. For both types of oil, ageing results in decreases in their relaxation times.