The accumulated charge Q on a well insulated pressboard part being probably the most relevant parameter for flow electrification evolution inside transformers but clearly not measurable in situ, we compare in this paper this parameter with others possible to measure. The two most commonly used are oil parameters: ECT (Electrostatic Charging Tendency) and, to a smaller extent, the continuous aging test for tanS. Another is not so often used because of measurements difficulties, but was considered to he one of the most relevant: the leakage current. Thus in this paper we compare on two facilities (specially made for that) the accumulated charge Q to the ECT and leakage current. We find that, even if sometimes a correct correlation exists between these parameters, several cases show a had correlation. Thus monitoring transformers by analyzing only ECT or leakage current seems to he hazardous.

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Electrification due to oil flow inside transformers induces charge accumulation at the pressboard interface. A part of this charge migrates inside the impregnated pressboard, which leads to a space charge development inside the pressboard. Then this accumulation may be responsible of electrical failure. In order to predict the phenomenon it is important to know the charge evolution and its variation with the temperature gradient and concentration of additives, considered to be used to prevent such failures. An experimental setup has been perfected in which an impregnated pressboard stack is submitted to a potential difference and a temperature gradient. From measurements of potential on special electrodes inserted in the pressboard stack it is possible to obtain the space charge density development. We have made experiments for several potential difference and temperature gradient. The evolution of the space charge density development is observed in terms of time. Then the effect of the Benzotriazol (BTA) concentration has been tested.