Quantitative Evidences of Electrical Charges Creation at the Metal/Liquid Interface in Power Transformer Insulation Systems

The aims of this work is linked to the problematic of electrical charges formation in power transformer insulation systems. After being qualitatively evidenced, this paper focuses on giving quantitative indications of the space charge injection process from metallic materials into dielectrics when these two materials are put in contact under electric stress. The Polarization and Depolarization Currents technique (PDC) is used to assess, in point of view of conduction process, different metal/insulating liquid interfaces. Three electrode systems of diffirent metallic nature (stainless steel, aluminium and brass) are associated with a transformer mineral oil. The metal/liquid/metal structures were submitted to a DC voltages and the electrical current circulating through the dielectric thickness was recorded with high sensitive electrometer. By integration of the measured response, the three cases are compared through the charge quantity involved. The obtained results meet well with the inj ection energetic barrier height at the metal/liquid contact. This is an additional proof of the physical assumption of charge creation, into the insulation system, by the transformer metallic compounds (windings, walls …).