Ph.D. thesis defence of Mr TCHAMDJIO NKOUETCHA Emeric on 4 August 2021

TCHAMDJIO NKOUETCHA Emeric defended his thesis on Wednesday 04 August 2021 at 10:00 am in the John Ebanja amphitheatre of the UIT of Douala.

Title: Elaboration of vegetable oils and study of the impact of degradation products and moisture on their dielectric rigidity.

The Jury:

Chairman:

Mr NJEUGNA Ebénézer , Full Professor, University of Douala;

Rapporteurs:

Mr AZEBAZE Anatole Guy Blaise , Full Professor, University of Douala;

Mr ELE Pierre , Full Professor, University of Yaoundé 1;

Mr NDZANA Benoit , Associate Professor, University of Yaoundé 1.

Member:

Mr MOUKENGUE IMANO Adolphe (Supervisor), Full Professor, University of Douala

This thesis focuses on the study and analysis of the effects of operating stresses on bioinsulators in power transformers.Two monoester-type bioinsulators based on castor oil and palm trees respectively have been developed.The development process was studied and the monoesters obtained were characterised from the physicochemical point of view, according to ASTM D6871.Dielectric AC performance, including dielectric rigidity and partial discharge initiation voltage (TIDP), were statistically investigated.The impact of degradation products, due to oxidative ageing of bioinsulators, on the dielectric performance of bioinsulators, was studied compared to the dielectric performance of mineral oil under the same conditions.Experiments on the behaviour of dielectric performance of bioinsulators compared to mineral oil, subjected to temperature gradients in transformers, were carried out.Finally, the impact analysis of water contamination of bioinsulators, compared to mineral oil, was investigated.It shows that the methyl esters of palmists and castors have good physicochemical and dielectric properties, in accordance with ASTM D6871, except flash and fire points.However, the flash/flash point of these monoesters remains higher than that of mineral oil.The 18-day accelerated aging tests did not affect the dielectric stiffness of the monoesters but altered the dielectric stiffness of the mineral oil.These tests showed that the TIDP of all insulating liquids is only minimally influenced by the presence of dissolved oxidation products.The tests carried out on the impact of temperature change on dry insulating liquids are negligible.Analysis of the effect of water contamination shows that the behaviour of the dielectric properties of bioinsulators and mineral oil is similar from the point of view of relative humidity.Thus, because they absorb more water, bioinsulators have a better resistance to water contamination than mineral oil.Monoesters can therefore be considered as suitable alternatives for mineral oil in large power transformers.However, further research to improve fire resistance is needed to arrive at a definitive conclusion.

Keywords: dielectric strength, partial discharge initiation voltage, bioisolants, monoester, relative humidity, aging, degradation products.