Electrical detection of degradation in specimens of HVDC cable insulation

Douglas Jutsell Nilsson,Stanislaw Gubanski

doi:10.5324/nordis.v0i26.3276

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 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.

Highlights

Understanding the effects on HVDC cable insulation caused by long-term exposure to in-service stresses is key to evaluating current and emerging materials
To a certain extent, model cables, are costly endeavours. Of a size such that they fit in the palm of a hand, taken from an already produced cable can carry significant advantages
17 min 37.37 sec 17 min 37.42 sec 17 min 37.28 sec the inception voltage can be accurately detected by any of the sources, as is evident by Table 1, further understanding of what transpires in a specimen during an electrical treeing experiment can be gained from calculating the impedance of the specimen during the experiment

Summary

Introduction

Understanding the effects on HVDC cable insulation caused by long-term exposure to in-service stresses is key to evaluating current and emerging materials. Experiments which yield such results can be carried out in a number of different ways, for example, by conducting full scale experiments using actual or model cables, by using cables that have already been in-service or by carrying out laboratory experiments on miniature specimens with artificial defects. A specimen from an already produced cable ensures that there are no differences between the examined material and the insulation in-service. The second relates to the metallic sheath that surrounds a typical cable in-service. Since an actual cable is surrounded by a metallic sheath, these stay inside the insulation. In order for ageing experiments to be valid, the environmental conditions that an actual cable experiences must be replicated

Constraining the specimen

Needle injection and polymer processing

The cell

Measurement setup

Results

Conclusions