To enable the next generation subsea boosting and processing facilities, high power electrical connectors are strongly needed and considered one of the most critical components of the system.
Electrical tree growth is a precursor to electrical breakdown in high voltage insulation materials. Therefore, the study of the tree growth dependency with hydrostatic pressure is needed to understand the behaviour of the insulation material used in subsea connectors. Silicone rubber (SiR) is used as an insulation material for these applications thanks to its higher viscosity characteristic in comparison with other solid insulation materials used in subsea cables. This property is the main factor that allows the water to be swiped off the connector when a receptacle is mated into the plug of a subsea connector. In addition, the silicone rubber must provide similar electric field control as other insulation materials used in cable terminations and connectors.
The characteristics of partial discharges generated during the electrical tree growth and the light emission from the partial discharge pulses, have been studied under different pressure conditions. SiR samples, with a needle to plate electrode configuration, have been put into a pressure vessel to grow the electrical tree in the material under high hydrostatic pressure conditions. The electrical tree growth has been divided in three stages (initiation, intermediate and final or pre-breakdown stage) and tests have been performed at 1, 20 and 60 bar. A digital NIKON camera and a CCD camera have been used, both attached to a long-distance microscope, to observe in real time the tree growth and light emission, respectively.
Pictures showed a higher growth speed for the electric tree as voltage and pressure were increased. The length of electrical trees pre-grown at lower pressures collapsed faster as the pressure increased, than those pre-grown at higher pressures under the same pressure increasing conditions. As the pressure increased, Pulse Sequence Analysis performed to the partial discharges measured confirmed the partial discharge inception and extinction voltage increase and showed a polarity dependency to space charge generation in addition to other patterns regarding the charge magnitude and phase of occurrence characteristics. Pressure vessel internal reflections have suggested changes to be done in future studies for the light emission measurement. Finally, partial discharge patterns from the electrical tree growth process have been identified to be characteristics from void faults in the dielectric with a spherical void shape.

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.