Monitoring of partial discharges in capacitor models with impregnated polypropylene films

Experiments were made using an aromatic hydrocarbon (benzyltoluene) and a saturated hydrocarbon (n-decane) as capacitor impregnant, to compare their behavior in high fields. The results are used to determine the voltage to apply to these impregnated capacitor models during accelerated electrical aging without discharge occurrence. The measurements indicate that, during an aging test at 4.1 kV, no discharges occur in the capacitor models until the models broke down. When the test was stopped before breakdown, and the applied voltage on the aged model was slowly raised, the model broke down for 5.5-6 kV; no discharges were detected before that. Thus, the evaluation of the discharge threshold voltage shows that, even for low applied voltages without discharges, an aging process exists in impregnated insulation. It can be enhanced by dissolved O₂, temperature, and the electric field (up to a very high value), and causes the breakdown of the model. Liquids used for impregnated capacitors are known for their good gas absorbing properties, which can be evaluated using a normalized method (Pirelli test). It is shown that the same liquids have a high threshold voltage for discharge detection