Measuring degradation in cable insulation material under realistic operation conditions
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.
Douglas Jutsell Nilsson
Chalmers, Elektroteknik, Elkraftteknik, Elnät och komponenter
A challenge that has to be overcome in order to do this accurately stems from the particular nature of the environment found inside a cable. The construction is surrounded by a metallic sheath which means that nothing can enter or escape the cable in-service. In order to generate valid results, these conditions should be replicated. This work explores an idea that combines glass, brass and the element Indium to mimic the environment as well as peripheral methods needed to contain cable specimens.
The work is concluded by an experiment where a large number of specimens are manufactured using the methods developed. These are used in a study that attempts to determine whether a fault that can happen along a cable has a negative effect on its insulation.
Measurement equipment and calculation approaches, elucidated for the detection of degradation in the material, are developed in parallel with the encapsulation techniques and used to generate and evaluate the results.
Elektroteknik och elektronik
Doktorsavhandlingar vid Chalmers tekniska högskola. Ny serie: 4878
Chalmers tekniska högskola
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Opponent: Prof. Erling Ildstad, Department of Electric Power Engineering, NTNU Trodheim