Hiromu Isa

Breakdown process of a rod-to-plane gap in atmospheric air under DC voltage stress

Discharge phenomena were observed under chopped or steady state DC voltage application. A still camera and two photomultipliers were used to record and measure the development of the electrical breakdown in a normal air gap. The breakdown process under both positive and negative DC voltage stress differs from that under impulse voltage. In the case of DC voltage, space charge is produced by corona discharges prior to the complete breakdown, which affects the breakdown process significantly. >

An experimental investigation of electric flashover across solid insulators in vacuum

In this research, we observed the characteristics of the onset of surface flashover in a vacuum for three kinds of test pieces. We also observed the development of surface flashover with good time and space resolution by using an image converter camera. The results obtained may be summarized as follows: (1) The flashover voltage varies by the condition of contact between the test piece and the electrode. In many cases, the flashover voltage sharply decreases after the first flashover. (2) The flashover voltage decreases when the dielectric constant of the test piece is increased. (3) The brightness of the flashover channel varies periodically along the channel. The periodic length is approximately 0.5 mm for our experiment. (4) The cathode spot appears when a large clearance exists between the test piece and the cathode. On the streak photographs, the bright part initiates at the cathode and moves toward mid-gap at a speed of 107 m/s, which is almost independent of pressure and applied voltage. (5) If the clearance between the test piece and the electrode is small, the cathode spot does not appear. In such a case, the flashover initiates at a point about 3 mm from the cathode, and expands toward both the cathode and the anode with a speed of approximately 1.2 × 107 m/s. (6) The calculated velocity of electron hopping on the surface is in good agreement with the expansion velocity of the bright portion.

Degradation of silicone-coated FRP by weathering

To estimate the quality of sample coating materials, a merit index was calculated from the average value of the order in flashover voltages and the contact angle of the sample insulator during exposure. The contact angle is shown to be the most effective factor for the degradation of sample instructors. Under wet conditions, a low contact angle causes creeping discharges resulting in tracking.<<ETX>>