Flow electrification in transformers: relevance of ECT and leakage current measurements

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 tan (delta). Another is not so often used because of measurements difficulties, but was considered to be 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 bad correlation. Thus monitoring transformers by analyzing only ECT or leakage current seems to be hazardous.