Morphology and chemical composition of water trees in mineral-filled EPR insulation

The water treeing characteristics of specimens of crosslinked polyethylene (XLPE) clay-filled XLPE, and clay-filled ethylene propylene rubber (EPR) are compared. Specimens are in the form of 15 kV cables, and Rogowski slabs made from the same insulation and shield materials. The cables are tested for up to eight months at 3x rated voltage, and the slabs for up to ten months at 2x rated voltage. The EPR cable specimen has a much lower density of bow-tie trees and vented trees than the XLPE, with the filled XLPE having an intermediate tree density. A characteristic feature of the clay filled cable insulations (and to a lesser extent the slab specimens) is the formation of relatively large (up to 1 mm) bow-tie trees under the chosen test condition. No consistent differences in tree shape or texture between XLPE and EPR are observed by optical microscopy, though the ratio of tree length to width may vary. A feature common to the tree regions of both XLPE and clay-filled EPR is the occurrence of carboxylate anions, as evidenced by infrared microscopy and derivatization reactions.