Experimental research on the streaming electrification of transformer oil under aging

A series of accelerated aging tests of high voltage transformer oil from Karamay of China were completed. All the oil samples were processed by dehydration and degasification firstly, and added with different contents of oxygen and copper, then aged under 120degC in different periods up to 675 hours, which is equal to 30 years on service of transformer oil. Improved mini-static tester was adopted to measure the electrostatic charging tendency (ECT), meanwhile, other traditional parameters, such as dissipation factor, volume resistivity, acid number, were also measured according to standard methods. The results indicate that ECT increases with aging time obviously, addition of copper advances the process of accelerated aging and leads to saturated period occurred in the late part of the aging (450 h-675 h). It can be found from mathematic analysis that correlation between ECT and dissipation factor is positive and the highest. These results are valuable for the life evaluation and condition monitoring of large power transformers.