Measuring degradation in cable insulation material under realistic operation conditions
Doktorsavhandling, 2020

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.

Online participation via Zoom. Please e-mail in advance to get the password!
Opponent: Prof. Erling Ildstad, Department of Electric Power Engineering, NTNU Trodheim


Douglas Jutsell Nilsson

Chalmers, Elektroteknik, Elkraftteknik

Modern cables used to transmit power over long distances are insulated with materials that are expected to last 40 years. The cables are a critical part of the electricity network and it is therefore vital to be able to conduct laboratory experiments on the materials used in their construction.

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



Online participation via Zoom. Please e-mail in advance to get the password!


Opponent: Prof. Erling Ildstad, Department of Electric Power Engineering, NTNU Trodheim

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