A Study of Changes in Resistivity of the Semi-Conducting Layers in High Voltage Direct Current Cables

Sammanfattning

The resistivity of the semi-conducting material in high voltage direct current cables were studied under a variety of conditions. A wide theoretical background was also created, summarizing previous works within the field and providing useful explanations for the characteristics seen in the results. The semi-conducting material works as a protective shield for the insulation of the cable, protecting it from electrical discharge and excessive electrical field concentrations, both of which can damage the insulation and decrease the lifetime of the cables. Pieces of cable were processed in two different ways to create samples designed for measuring both the inner, and outer semi-conducting layers. Measurements were performed using a four-point probe method over durations of 16 hours. The resistivity of the semi-conducting layers was studied under many different conditions. These include: varying measuring temperatures, using a constant measuring current instead of a periodic one, repeatedly measuring the same sample, using different types of semi-conducting material, using cross-linked and non cross-linked samples, using alternate sample designs, and measuring under different values of relative humidity. Additionally, the measurements that initiated the creation of this thesis were also recreated. The experiments produced good results, contributing to a deepened understanding of the behavior of the semi-conducting material under laboratory measurements. The results were discussed, both examining what could be explained from theory and what could come as a result of the methodology. Furthermore, some difficulties with the methodology were also discussed. The ultimate goal of this thesis is to deepen the understanding of the semi-conducting material and improve testing methodology, which in turn could help develop more efficient and cost-effective semi-conducting materials for HVDC cables.