Electrical tree growth in polyethylene: complementary chemical and structural characterisation

Electrical treeing is an electric breakdown process of considerable technological importance to the electrical power industry. In the work described here, complementary high resolution analytical techniques were applied to characterise tree structures grown in polyethylene. A combination of confocal Raman microprobe spectroscopy and 3-D optical reconstruction techniques has been used to characterise the physical microstructure and chemical composition of electrical trees. It was found that in tree structures in which partial discharges occurred within the main body of the tree, the dominant Raman characteristic is massive fluorescence. In trees where discharge activity was confined to the tree tips, the Raman bands of polyethylene are seen throughout the body of the tree, but superimposed on pronounced fluorescence. Other than at the tree tips, the G and D bands of sp² hybridised carbon are also seen. The chemical details of both tree types are therefore consistent with the discharge patterns and with the carbon acting to increase tree channel conductivity.