ELECTRIC FIELDS FOR PREBREAKDOWN PARTIAL DISCHARGES IN LIQUID DIELECTRICS

The existence of prebreakdown collision-ionization processes in liquid dielectrics was observed reproducibly far below breakdown, using a coaxial cylindrical electrode geometry. Preliminary values of the critical electric fields, Epd, for ac prebreakdown partial discharges were measured for three liquid dielectrics: a silicate ester, Oronite Flowcool 180, Epd ≃ 520 ± 100 kV/cm rms; a mineral oil, Wemco C, Epd ≃ 610 ± 30 kV/cm rms; a dimethyl silicone, dc 331, Epd > 1000 kV/cm rms. However, prebreakdown partial discharges were not observed for the coaxial-cylindrical geometry with close spacings when the breakdown electric fields were lower than the above fields.