Electrical detection of degradation in specimens of HVDC cable insulation

One of the challenges in laboratory investigation of degradation and ageing of HVDC cable insulation is related to securing, or in other words, imitating the real service environment of the material specimens. So far, the published data refer to experiments conducted in thermo-oxidative conditions, which is not the case during normal cable operation. In reality the cable insulation is protected by a metallic barrier that blocks the transfer of any substances in and out of the construction. By-products from the cross-linking reactions cannot diffuse out and any foreign substances, such as oxygen and water, are blocked from entering into the insulation. Thus, in order to generate results that are valid, these conditions must be replicated in laboratory experiments.This contribution presents a methodology elaborated for performing ageing experiments in a hermetically sealed environment. Degradation of the material is evaluated through measurements of changes in the electrical tree inception voltage and test object capacitance over time. Securing the environmental isolation is primarily accomplished with an isolation system consisting of a glass enclosure with attached metallic electrodes. Indium is used to create a glass-to-metal seal between the glass and the electrodes. The electrode geometry is of needle-plane type and the needle injection process is semi-automated to secure a large degree of repeatability in specimen preparation.