POLYMER OXIDATION AND WATER TREEING

In order to determine whether or not oxidation of polymer influences w ater treeing, more than 200 vented and bow-tie trees from 31 field-age d cables were investigated. Micro-IR spectroscopy analysis has not sho wn any consistent excess of carbonyl content in water trees as compare d 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, vente d trees are more susceptible to oxidation if subjected to high tempera tures in the presence of oxygen. It was observed that vented trees ini tiate at similar rates in XLPE in either a nitrogen or air atmosphere. This indicates that tree initiation is rather independent of the pres ence of oxygen. However, the tree growth rate is slower in nitrogen th an in air, the actual difference being affected by the type of ionic s olution 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(-1), typic al of carboxylates, was detected in some water trees. It should be not ed that the large absorption band of water at (1640 cm(-1) often masks the smaller carboxylate band. Under laboratory conditions carboxylate groups were detected on oxidized nontreed XLPE surfaces. These result s do not imply that carboxylates are responsible for tree propagation but confirm only that carboxylate groups are formed during XLPE oxidat ion. Thermal pre-oxidation of XLPE, to the levels measured in typical field-aged cables, has little or no effect on the initiation and growt h of vented water trees. Very high levels of pre-oxidation las determi ned by the carbonyl content), at least 80x the average oxidation level measured in typical field-aged cable, retard the growth of vented wat er trees. However, these high levels of oxidation negatively affect th e dielectric properties of the insulation.