Static electrification in power transformers: experimental study and modelling of a pressboard submitted to an impinging oil jet

For the last twenty years, static electrification has been suspected as being responsible for failure in power transformers. As a matter of fact internal parts show evidence of electrical discharges: erosion of the pressboard surface (electric tree paths), "worms holes", presence of carbon, etc. The phenomena consists of 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. EPRI started an important research program involving universities, manufacturers and utilities to solve the problems or to minimize the effects on operating apparatus and to define new specifications for the future devices. If on the one hand, all the parameters of influence seem to be clearly identified, on the other hand their critical combination has not been verified. As part of the research program of Electricite de France, an experiment at the university of Poitiers, associating a numerical modeling method developed at the R&D center of EDF with the computational fluid dynamic software ESTET, has been carried out to study the influence of a 3D oil flow geometry encountered in transformers. These investigations should make it possible to determine a critical context which might explain the observed incidents.