New Test Arrangement for Measuring Electrical Treeing in Polymers

Licentiate thesis, 2007

Electrical treeing in high voltage cable insulation has been studied for a number of years and today the mostly used laboratory test set-up is the needle-plane electrode configuration. In this thesis a different type of electrode configuration is described. The new electrode configuration makes use of a thin tungsten wire as the electrode that delivers the high and divergent electrical stress needed to initiate electrical tree growth in insulation material. Multiple electrical trees are formed during the testing, which allows determining the treeing initiation field as well as the rate of tree growth. A comparison between tree initiation conditions in samples prepared of low density polyethylene (LDPE) and crosslinked polyethylene (XLPE) materials indicates that electrical trees start to develop in LDPE material at lower fields than in XLPE. Two different mechanisms of tree initiation are identified in LDPE. Also the improvement of the treeing initiation field in XLPE by adding voltage stabilising additives is illustrated.