Basic Study on Electrical Insulation Property in a New Type Cryogenic Gas Insulated Transmission Line

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An intensive study on electrical insulation property in a cryogenic gas space is required for realizing a gas insulated transmission line and an electric power equipment using a high temperature super conductor. Electrical breakdown characteristics in a non-uniform field gap such as a rod-to-plane electrode which is stressed by lightning or switching impulse voltage are first investigated in a cryogenic gas space of -180 degrees Celsius provided by a cryostat. Secondly, electrical breakdown voltages of a cryogenic and uniform field gap of more than 100 mm whose data have not been so far accumulated are measured using a large cryostat of 1 m^3 in volume. Main results obtained form these studies are summarized as follows.1) The breakdown voltage of a non-uniform field gap in air at room temperature becomes higher in the order, for lightning impulse, switching impulse and dc voltages. On the other hand, the value at cryogenic temperature becomes in the reverse order.2) The breakdown voltage of a non-uniform field gap in nitrogen gas hardly depends on either temperature or voltage waveform. It mainly depends on the tip radius of a high potential rod electrode and it increases with increasing radius.3) The charge produced due to corona discharge strongly depends on the occurrence voltage of the corona. the charge observed at -180 degrees Celsius becomes smaller than that observed at room temperature for the same corona occurrence voltage.4) The breakdown voltage of a sphere-to-sphere gap of 150 mm in diameter increases linearly with increasing gap length up to 100 mm and tends to saturate over the gap length of 100 mmfor all voltage waveforms of dc, ac and impulse. In the region of the linear relationship, it is confirmed that the Paschen's law holds true for cryogenic temperature.