Dissertação apresentada para obtenção do
Grau de Doutor em Engenharia Física - Física
Aplicada pela Universidade Nova de Lisboa, Faculdade de Ciências e TecnologiaPolymers have been widely used as electrical insulators in power cables. Polyethylene,
initially low density and more recently crosslinked, are one of the more commonly used insulators in medium and high voltage power cables. They suffer electrical ageing in different forms, such as water treeing, electrical treeing and finally dielectric breakdown. The last one leading to costly cable failure. Many research works have been developed on this subject despite of the progress made up to now, results are still sometimes contradictory and difficult
to reproduce. Any new insight into this problem is a step further in preventing failure of the cables and increasing their useful lifetime.
The aim of this thesis was to relate ageing in two different aspects of electrical ageing:
localised damage and changes in bulk properties of the polyethylene. For this different experimental techniques were used. Localised damage was water treeing and breakdown channels since no electrical trees were observed. The methods used for this study were FTIR,estimation of fractal dimension of water trees and statistics of dielectric breakdown. The changes of electric and dielectric bulk properties were accessed using mostly DRS, FTSDC and PEA. From the FTIR results it was possible to find oxidation products (mainly ketones
and carboxylate ions) and chain scission. The values estimated for fractal dimension point to the same underlying microscopic mechanism for water treeing (at least for samples aged at different frequency and temperature). Breakdown statistics point to the important role of manufacture and processing of the insulator, since early breakdown is mostly dependent on defects resulting from these processes. A correlation between DRS, FTSDC and PEA shows
the role played by space charge and trapping on ageing for longer times. The two different
aspects (localised and bulk) are difficult to correlate because the first one has a stochastic behaviour. However the presence of space charge found in bulk studies can be regard as one of the initiator factors for localised damage.Fundação Calouste Gulbenkia
