Condition Assessment of Medium Voltage Cable Joints: Dielectric Spectroscopy of Field Grading Materials

The electrical properties of a widely used stress control tube for medium voltage heat-shrink joints are examined in this master thesis by using non-destructive laboratory methods. They were determined by use of time domain dielectric response test while changing the DC electric field, temperature and level of humidity. In order to determine the physical and mechanical characteristics of the material at different aging stages and conditions, both differential scanning calorimetry and tensile test measurements were performed on stress control tube samples.Two types of test samples were prepared. For dielectric response tests, Raychem JSCR 42/16 stress control tube was shrunk on a cylindrical rod, consisting of two metal electrodes, separated by a PTFE (Teflon) insulating rod. For differential scanning calorimetry and tensile tests, the stress control tube was shrunk on a cylindrical PTFE rod only, with the same diameter as for the dielectric response tests without the presence of metal electrodes.Tensile test measurements were performed in order to characterize and compare the mechanical properties of the stress control tube material at different aging stages.Differential scanning calorimetry measurements were used to estimate aging degree by assessment of thermal resistivity of the material and antioxidants consumption when exposed to high temperatures for a certain period of time.To calculate the conductivity, dielectric response measurements were performed in time domain at different voltage levels up to 20 kV and charging and discharging time duration of 10800s. They were conducted by placing the test object in a climate chamber in order to examine the influence of temperature and humidity. The measurements were performed on unaged and thermally aged heat shrinkable stress control tubes. The aging process was obtained in heat cabinets at 98°C dry air, 98°C wet air and 150°C dry air conditions.Results from the analysis show that conductivity has not significant dependency of the electric field up to 0.2 kV/mm at different temperatures, aging degree and humidity.It was revealed that conductivity is more temperature dependent, as a significant increase was observed when the temperature was elevated at higher values. Conductivity was found out to be strongly dependent on humidity, especially when combined with high aging degree.