Abstract
In Norway it has been observed that many medium voltage (12 and 24 kV) cable sections with heat shrink joints have suffered from overheating often causing failure during service. The overheating is due to a bad metallic connection between the aluminium conductors in the joint. This is especially observed for XLPE cables installed in the 80’s, which constitutes a significant part of the installed cables in the network.This paper is part of a work to elucidate the mechanisms causing a low insulation resistance of MV cable joints. As a bad metallic connection can result in local high temperatures during normal operation, the properties of a heat shrink stress control tubes commonly used in such joints has been characterized at temperatures around the melting peak of the material that is just above the maximum service temperature of 90 °C for XLPE cables. The main purpose of this paper is to examine the effect of these temperatures on the electrical properties of the field grading material. Here, the electrical properties have been characterized by time domain dielectric response measurements at different temperatures and axial electrical fields. The results show that the conductivity of the stress control tube is temperature dependent. Also, a polarity change in the depolarization current at higher electrical field strengths was observed.
Highlights
Failure statistics for medium voltage cable in Norway show that the overall failure rates for the cable accessories are relatively low [1]
As bad metallic connections is likely to lead to local high temperatures exceeding 90 °C, the maximum service temperature of XLPE cables, the properties of a heat shrink field grading tube has been investigated
When applying a DC voltage across a dielectric material, a polarization current will flow through the insulation as a result of the conductivity and the dielectric displacement
Summary
Failure statistics for medium voltage cable in Norway show that the overall failure rates for the cable accessories are relatively low [1]. From dissections of joints suffered from breakdown, it has been found that some of the failures have been caused by thermal degradation likely due to a too high transition resistance of the metal connector [1] This has resulted in a severe ageing of the joint insulation, leading to embrittlement and cracking of the insulation material initiating partial discharges. When applying a DC voltage across a dielectric material, a polarization current will flow through the insulation as a result of the conductivity and the dielectric displacement. This current can be expressed by: σσ IIpp(tt) = CC0UU εε0 + ff(tt) (1).
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
