Water causing a nonlinear dielectric response of water treed XLPE characterized by FTIR microspectrometry

Recently, the nonlinear dielectric response of water treed XLPE cables has been explained by voltage assisted water ingress into water treed regions. The purpose of the work presented here was to characterize the form of water within the water tree structure. This paper presents results from micro-FTIR experiments performed on 100 μm thick water treed XLPE slices microtomed from a 24 kV service aged cable. Water content of up to 2% was found in vented water tree structures. The majority of this was determined to be free liquid water. Some sections of the free branches were, however, found to be hydrophilic, indicating water being bound to the polymer. The water content was highest in the tree branches and lowest at the tip of the trees and in the root region close to the insulation screen. The length of these regions was typically 200-300 μm