Composition and growth of water trees in XLPE

On the basis of experimental results, a model describing the composition and the structure of water trees is derived. It is concluded that water trees are in fact salt trees, i.e., they consist of tracks of hydrophilic salt groups. These tracks most likely run through the amorphous phase of polyethylene. In water trees in cables, the salts are carboxylate salts and/or inorganic salts. In needle test objects the concentration of the inorganic salts is much higher. The differences in chemical composition appear to correlate with the aging conditions. The degradation products in water trees reflect the processes which took place during aging. In this way the occurrence of oxidation, ion diffusion, and chain scission is confirmed. A water tree growth hypothesis is formulated, which incorporates an electrochemical and an electrophysical effect. The first effect is very important in service-like aging, whereas the latter predominates in needle tests.<
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