Analysis on Partial Discharges Variability in Mineral Oil under Long-Term AC Voltage

The paper presents results of measurements and analysis on partial discharge (PD) behaviour in mineral oil under long-term AC voltage. The conventional electrical method is applied for apparent charge measurements. Surface PDs are generated by model source immersed in brand new and 35-year used mineral oil respectively. The high voltage (HV) electrode is 1cm thick brass plate with a diameter of 3 cm, while the grounded (GND) electrode is 1cm thick steel plate with a diameter of 12 cm. There is also a square solid dielectric barrier made of 8 mm thick pressboard oil impregnated paper and the size of 15cm x 15cm placed between the both electrodes. The PD source is powered continuously by the AC voltage with its relative level of 1.3 of the inception voltage (U-i) of the PD source within 168 hours. Signals generated by the continuously occurred PDs within 168 hours are registered. Various qualitative as well as quantitative indicators describing the PD variability in time are assigned and compared to different oil conditions. Furthermore some long-term characteristics of the applied PD model source in mineral oils of different condition are also presented. Finally statistical tools are applied for the results analysis and presentation. From among all assigned indicators there are discriminated those that depend as well as not depend on PD long-term duration. Based on the regression models analysis there are discovered trends that potentially allow to apply the results for modeling of the PD variability in time. Subsequently such approach may potentially support the development and extend the abilities of the diagnostic tools and maintenance policy in electrical power industry.