DISCHARGE DEVELOPMENT IN AIR DURING LIGHTNING IMPULSE STRESS IN UNIFORM ELECTRIC FIELD WITH RODSHAPED PROTRUSION

Technical air (80 % N₂/20 % O₂) is an ecologically friendly insulating medium suitable for gas insulated switchgear (GIS). The discharge development in technical air was investigated at 0.1 MPa, 0.3 MPa and 0.5 MPa in a plate-plate electrode arrangement with 100 mm spacing, distorted by a 15 mm long rod shaped protrusion during lightning impulse (LI) voltage stress of positive polarity. Photomultipliers with distinct narrow spectral sensitivities supplemented the voltage and current measurement. From the experiments, three distinct discharge types can be identified. Type 1 discharges are streamer discharges involving a secondary streamer. Discharge types 2 and 3 are related to the leader type channel phenomena. According to the experimental results, leader type channels seem to be highly conductive. The light emission in narrow IR spectral ranges related to the leader type channel indicate thermal dissociation of oxygen and nitrogen molecules. Therefore, leader type channels show similar characteristics that describe a leader channel in SF6. From the time resolved signals, approximated values for the streamer stability field strength Es were determined at 0.3 MPa and 0.5 MPa. The approximated values show that the stability field strength Es varies with the pressure to the power of 1.5 for pressures up to 0.5 MPa.