Comparison of Single and Double Needle Tests for Evaluation of Resistance to Treeing Breakdown

This paper describes results on treeing breakdown in organic insulating materials using diverging fields formed by two different types of electrode geometries. These were needle/plane (SNT) and needle/needle (DNT) systems. Characteristic voltage, voltage life, and short-time breakdown tests were performed so that a comparison between SNT and DNT could be made. Low-density polyethylene (LDPE), cross linked polyethylene (XLPE) and epoxy resin (EX) were used. The result on characteristic voltage test shows that the relationship of characteristic voltage is as follows: EX > XLPE > LDPE and DNT > SNT For voltage life tests, the same relationship as the characteristic voltage is found. On the other hand, the relationship of short-time breakdown voltage among three kinds of specimens is given by LDPE > XLPE > EX and DNT > SNT For the voltage life test with SNT and LDPE, the trees spread out with many branches to produce a bushlike shape. With DNT, the trees grow along the electrode direction with fewer branches. The tree shape in XLPE is similar to that in LDPE. For EX, the tree channels seem to be narrow tubes.