Abstract
In order to develop SF6 gas-insulated switches with long service life and high reliability for medium voltage ratings, it is necessary to decrease arc erosion of contacts in SF6 gas. In this paper the effects of SK gas pressure and magnetic field for are blow-out on the erosion were studied experimentally in detail using sintered Cu-W contacts. The test current, SF6 pressure and magnetic field were 100 to 400 A, 0.1 to 0.6 MPa and 0 to 50mT, respectively. The results show that the arc erosion rate increased linearly with the SF6 pressure and the magnetic field was effective for the reduction of anode erosion (i.e. the application of a mere 50mT lowered the erosion rate to 1/3 to 1/4). These phenomena may be attributed respectively to the increase of the evaporation rate according to the shrinkage of arcing spot areas by the SF6 pressure and the decrease of the evaporation rate according to high speed movement of the arcing spot on the anode by the magnetic field. But the cathode erosion was insensitive to the magnetic field. The influence of W grain size and composition ratio of Cu to W on the erosion rate was also studied and the smaller W grain at Cu content of 50 to 70 wt % was found to he best for the suppression of erosion under the present test conditions. Furthermore the increase of N2 mixture ratio to SF6 under constant total gas pressure greatly decreased the contact erosion.
