Electrical tree formation as a measure of degradation resistance in polymeric materials for HVDC applications

In the framework of elaborating a methodology for testing the resistance to electrical treeing of polyethylene (PE) for HVDC applications, electrical tree inception tests were performed before and after a thermal ageing on XLPE samples. For this purpose test objects of wire-plane electrode geometry were exposed to various voltage regimes. The obtained results show that a DC pre-stress followed by an application of voltage pulses of opposite polarity appeared most successful for a repeatable inception of electrical trees in the studied materials. Thin tree filaments were formed, distributing randomly at the vicinity of the wire electrode (i.e. high voltage electrode). More trees and longer branches were found in the samples having longer thermal ageing period. These results correlate well with results of in parallel performed tree inception tests under AC voltage ramping. It is therefore postulated that an AC electrical tree inception test can successfully be used for evaluating degradation resistance of materials for HVDC applications.