Spectroscopic Methods for Understanding Chemical Breakdown of Liquid and Solid Insulation Under Combined Stress Conditions

Abstract

Within the power network there is a requirement to monitor the insulation capabilities of different materials present in different equipment / cables of the power grid. This thesis examines the chemical degradation of insulation materials. The materials to be studied are the liquid dielectric transformer oil, both mineral and natural ester based, and Polyethylene Terephthalate (PET) which is present inside overhead cables to insulate the electrical signal through the cable. Thermally stressed transformer oils were analysed by spectroscopic and electrical breakdown. The results of these are then extrapolated and statistical analysis to establish if a pattern or prediction could be made on the electrical breakdown voltage of the oils based upon the spectroscopic measurement of the linear growth of acid within the transformer oils. The results showed that it was possible to predict the electrical breakdown of the oils with a high degree of accuracy. This method could save time and money on analysis due to the fact that much less sample is required for the spectroscopic tests than the electrical breakdown test. The time to result is also much shorter and there is potential that the partial least squares method could be used to predict the electrical breakdown voltage of transformer oils currently in use in a power transformer and could possibly be performed on location at the transformer if the a portable IR was used. Solid insulation (PET) was subjected to high voltages to determine what changes and differences in the chemical structure could be observed in the insulator where the middle insulating layer had a void compared to a void and vent. The results of these experiments were not as conclusive due to difficulty in obtaining repeatable samples but there does appear to be a greater degree of chemical breakdown observed in the successful samples of the void only circuits when compared to the void with vent samples. A ratio was between the growth of the carbonyl stretch and the hydroxyl stretch was obtained through analysis by FTIR and differences between the unstressed and the stressed samples of PET could be seen. This was further confirmed by Raman and the carbonyl stretch and ester bond were examined to in order to establish if there was relationship between an increase in the carbonyl stretch and a decrease in the ester bond stretch. This relationship was confirmed in the samples analysed but due to issues with repeatability of the samples, further testing may be needed to further prove this relationship.

Date of Award	2023
Original language	English
Awarding Institution	Glasgow Caledonian University
Supervisor	Sheila Smith (Supervisor) & Scott McMeekin (Supervisor)