Measuring degradation in cable insulation material under realistic operation conditions

Abstract: 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 practically valid, these conditions must be replicated in laboratory experiments. This contribution presents a measuring system developed for performing ageing experiments in a hermetically sealed environment. The material degradation 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 largely automated to secure a large degree of repeatability in specimen preparation.   Initial measurements utilising four synchronized but independent data streams show that the electrical tree inception voltage can be accurately detected using the developed specimen capsule. The impedance change of the specimen during the test shows to be a particularly useful measure.   In order to further validate the methodology as well as contribute knowledge on the material’s resistance to degradation caused by its exposure to enhanced electrical stress, 40 specimens were prepared and used in an experiment that explores whether multiple joint failures along an HVDC-cable may have any effect on the condition of the cable’s the insulation material. The results indicate that the impact seen in the electrical tree inception voltage is minor and that the insulation has withstood the enhanced stress with negligible consequences.