Polymer oxidation and water treeing

In order to determine whether or not oxidation of polymer influences water treeing, more than 200 vented and bow-tie trees from 31 field-aged cables were investigated. Micro-IR spectroscopy analysis has not shown any consistent excess of carbonyl content in water trees as compared with the adjacent non-treed regions of the insulation. Although the levels of carbonyl content of the bulk of the polymer within the treed regions are similar to those in untreed regions of the polymer, vented trees are more susceptible to oxidation if subjected to high temperatures in the presence of oxygen. It was observed that vented trees initiate at similar rates in XLPE in either a nitrogen or air atmosphere. This indicates that tree initiation is rather independent of the presence of oxygen. However, the tree growth rate is slower in nitrogen than in air, the actual difference being affected by the type of ionic solution used. This further suggests that some, not yet known, chemical reactions between oxygen, XLPE and ions play an important role during water tree propagation. The IR absorption band at (1585 cm/sup -1/), typical of carboxylates, was detected in some water trees. It should be noted that the large absorption band of water at (1640 cm/sup -1/) often masks the smaller carboxylate band. Under laboratory conditions carboxylate groups were detected on oxidized nontreed XLPE surfaces. These results do not imply that carboxylates are responsible for tree propagation but confirm only that carboxylate groups are formed during XLPE oxidation. Thermal pre-oxidation of XLPE, to the levels measured in typical field-aged cables, has little or no effect on the initiation and growth of vented water trees. Very high levels of pre-oxidation (as determined by the carbonyl content), at least 80/spl times/ the average oxidation level measured in typical field-aged cable, retard the growth of vented water trees. However, these high levels of oxidation negatively affect the dielectric properties of the insulation.