Electrostatic Hazard in High-Power Transformers: Results of Ten Years of Experience With ^ Capacitive Sensor

In a high-power transformer, oil flowing on pressboard surface is suspected to be responsible of electrostatic hazards and failures. Different methods of risk assessment have been proposed to understand and prevent it: ministatic tester in the Westinghouse protocol, ministatic tester in the spinning disk measurement, monitoring of tangent delta, dissolved gas measurement, etc. At P' Institute of Poitiers, an original sensor was developed and used for quantification of the electric charge generated and of accumulated charge for an oil flow onto the surface of a transformer pressboard insulated from ground. Operational for ten years, this bench has been used to study over a hundred couples of oil/pressboard, pairs of new oil and pressboard, pairs of aged oil and pressboard, pairs of suspicious oil and pressboards, etc. This paper presents a comparative analysis of these ten years of experience. This analysis provides, among other results, a critical electrostatic hazard assessment in transformers and an attempt of discrimination tentative of a suspected transformer.