Cable terminations are essential for connecting a cable to a piece of equipment such as a circuit breaker, transformer, motor, etc. Proper terminations are of great importance in order to provide faultless cable systems. Improper terminations could result in overheating, partial discharges, eventual breakdown, flashover and breakdown in the system. There are several defects that can take place in terminations because of environmental, ageing and imperfections caused during manufacturing and installation . A potential defect is the contamination of oil with water due to sealing failures.

Water contamination can lead to partial discharges that, when produced over the insulation result in accelerated ageing. Typically, online electrical measurements of partial discharges can be used for termination monitoring. However, online monitoring suffers from noise and interferences, and localisation of partial discharges can be a problem. To overcome the challenges of electrical measurements, optical means of detecting partial discharge is researched in this thesis. The main goal of the thesis is to study the feasibility of using the optical PD detection method in oil-filled cable termination. The aim was to detect surface discharges at the rubber-oil interface through an optical sensor.

The identification of parameters for selection and final choice of optical sensor for PD detection application were done. Various setups to produce the surface discharge at the rubber-oil interface were investigated and one of the method was finalized. The simultaneous measurements of PD with electrical and optical sensors were conducted in dry oil and oil with different moisture content. The analysis of data obtained from optical sensor in comparison with electrical sensor based on sensitivity, PD pulse, PRPD pattern and correlation factor are discussed. It is noted that most of the characteristics of optical measurements is comparable to the electrical measurements. Thus, it can be said that this thesis contributes towards initial step of using the optical PD detection for online monitoring of HV components.

At the end of a power transmission line, the cable should be terminated carefully, otherwise it will create problems during the operation. A special equipment called termination is designed to tackle down the problems from cable ends. The main functional part of a termination is a piece of rubber with a cone shape. This rubber cone has a semi-conductive layer which is connected with ground potential. By doing this, the cone smoothens the concentric equipotential lines at the cable end, so the electric field stress there can be decreased. The cone in the termination is called stress cone. However, insulation degradation on the termination stress cone due to water contamination is often found in its operation. This, in the end, will cause operation failures in the power network. This research aims to investigate the influences brought by water contamination on the stress cone, and discuss possibilities of detecting the presence of water content by using electrical methods. By using 3D FEM simulations, the changes of electric field distribution, especially the filed along the oil-rubber interface, were studied. Based on the simulation results, we built up a small scale test setup to observe how water contamination behaves on the oil-rubber interface, and measured the changes of partial discharge signals and tanδ values. As conclusions, we discussed the electrical ways of detecting the presence of water contamination on the stress cone, and gave suggestions for the future work.