液体ヘリウム中のボイド放電にさらされた高分子フィルムの寿命特性

抄録

Superconducting cable is considered to be one of the promising candidates to transmit the huge amount of electric power in future. To develop superconducting power transmission cable, electrical insulation at low temperature is an important factor. Polymer film wrapped insulation system immersed in high density helium is proposed. In this system, tape butt gap in insulation layer is necessary to obtain flexibility of cable for transportation and installation. The harmful partial discharge may take place in the butt gap when the cable is subjected to over voltage, and limit the life-time of cable system.
This paper describes the electrical-discharge resistance of polymer film in liquid helium. The sample used consisted of three layers of polypropylene film; the middle layer had punch-hole at the center which formed a mid-dielectrics void in the sample. Ac voltage was applied to the sample in liquid helium, and the characteristics of the partial discharge in the void and the life-time to breakdown was measured. The results can be summarized as follows.
(1) The discharge streamer started from the edge of the void and it propagated towards the center.
(2) No trace of degradation of polymer film due to partial discharge was clear. Taking account of the values of shape parameter of Weibull distribution, it is reasonable to consider that the breakdown was caused by streamer attacking the weakpoint of polymer film.
(3) The breakdown-hole of sample appeared mostly at the edge of the void. The positions of breakdown-holes in two layers, above and below the void, were coincident.
(4) Applying “the rovers n low” to the voltage-life time characteristics, 8.3 was obtained as the life time exponent n in liquid helium.
(5) The voltage-life time characteristics of the void sample coincided with a cable sample which consisted of polypropylene tape wounded on metal mandrel.