Electostatic charging tendency of new and aged pressboard samples

The insulation system of modern power transformers is a complex set-up. Design engineers have to cope with several different requirements, which are often diametrical. One of such requirements is a ruggedness design in terms of electrostatic charging tendency (ECT). In the 1970\´s and 80\´s several problems have been reported in literature and countermeasures have been developed. However, recently there have been documented indications in literature, e.g. in [6], that there might be issues with aged transformer materials in terms of ECT. Within this work, a testing device for the screening of materials, based on a spinning-disc principle, has been designed and used successfully. Such a set-up allows the simultaneous testing of oil and pressboard, which gives a good indication of "real life" material behavior. In detail, the influence of oil flow velocity, temperature and surface properties on the electrostatic charging phenomenon have been investigated. Investigations have been carried out for three typical mineral oils and pressboard samples in new and (laboratory) aged conditions.