Temperature and Field Dependence of Field Grading Tubes for Medium Voltage XLPE Cable Joints

Resumé

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.