Experimental study and modeling of static electrification in power transformers

For the last 20 years, static electrification has been suspected to he responsible for failures in power transformers (electric “tree” paths, “worm holes,” etc.). The phenomenon consists in a preferential adsorption of negative ions from the oil (impurities) into the pressboard. This yields, on the one hand, a space charge in the oil which can relax in contact with grounded metallic walls and, on the other hand, a space charge in the pressboard which can accumulate depending on the leakage paths. As part of a research program by Electricite de France in the field of static electrification in transformers, an experiment has been carried out, at the Universite de Poitiers, Poitier, France, to study the most constrained parts in transformers with regard to this phenomenon: the insulated pressboards close to the oil inlet. For this, the experimental loop consists in impinging an immersed oil jet on a pressboard target in which annular insulated electrodes have been inserted to determine the charge in the pressboard. Considering the scale effect, the equipment involves taking oil from an operating transformer and making it possible to study the influence of the flow velocity and the length of the free jet. From a first modeling, compared to the authors´ experimental results, it seems that the local wall current coming from the pressboard is proportional to the local wall shearing stress. Considering the magnitude of the process, it finally turned out that this new configuration increases greatly the charge generation process in comparison to a flow parallel to the pressboard