| For nearly a century the conventional insulation (oil / cellulose complex) was the type of insulation used in the power transformers and most electrical power equipments. But the cellulose paper, the solid part of this insulation has many weaknesses. Indeed, the aging of cellulose paper in power transformers is accelerated by moisture, oxygen, metal catalysts, temperature, etc.). The risk of failures is thereby increased. Another major weakness of cellulose paper is its inability to protect the electrical transformer windings against the harmful effects of corrosive sulfur.;The overall objective of this research was to study the hybrid insulation and to demonstrate its potential use in power transformers. This overall objective has been subdivided into three specific objectives, namely: (i) improving the diagnostic of the condition of solid hybrid insulation and conventional solid insulation; (ii) diagnosing the condition of oils sampled from hybrid, high temperature and conventional insulation and finally (iii) investigating the ability of aramid paper and cellulose paper to protect the copper (electrical windings) against harmful effects of corrosive sulfur.;In order to achieve these objectives, thermal accelerated aging were conducted in laboratory : • according to ASTM D1934 (American Society for Testing and Materials), four different type of insulation samples were considered, namely the oil impregnated hybrid insulation, oil impregnated cellulose insulation, oil impregnated high temperature insulation and paperless oil samples. Following the aging procedure, a local overheating (thermal fault) was applied on the paper sample using an experimental setup designed in our laboratory (first and second specific objectives). • according to the IEC (International Electrotechnical Commission)-62535, for mineral, synthetic, vegetable and silicones oils (third specific objective).;The degree of polymerization by viscosimetry and the determination of the carbon oxides by dissolved gas analysis (DGA) were determined to assess the condition of the paper in conventional insulation compared to that of the hybrid insulation. Our results indicate that cellulose paper in the hybrid insulation is less degraded when compared to the conventional insulation. Since the life of a transformer is directly related to the solid insulation, these results suggest that hybrid transformer insulation has a higher life than conventional ones. Subsequently, a very good correlation between amounts of oxides of carbon and degree of polymerization was established. This relationship might help improving the accuracy when interpreting the results of the DGA for transformers (first specific objective).;Given all the weaknesses of cellulose paper, several studies have been conducted to evaluate the performance of aramid paper, which has better thermal properties. The aramid paper is currently used as high temperature insulation, combined with high fire point oils (synthetic and vegetable oils), mainly in electric traction transformers. The hybrid solid insulation is associated with mineral oil or with high fire point oils; it finds application in transformers of fixed and mobile substations. Manufacturing technology is controlled by manufacturers but operators of electrical networks do not have baseline data (standards) as diagnostic tools, allowing them to monitor the health/condition of the isolation in this new type of transformer.;Regarding the second specific objective, we used four physicochemical diagnosis techniques (dissolved decay products 'DDP', Turbidity, interfacial tension (IFT) and water content) to assess comparatively the quality of oils sampled from the four types of insulation. According to our results, the oil of the hybrid insulation indicated better quality at a certain stage of aging and especially after the application of thermal stress on the solid insulation.;For the third specific objective, a qualitative study followed by a quantitative ones provided the following results: aramid paper better protects copper against corrosive sulfur in mineral oil; synthetic ester oils are not corrosive; the vegetable oil is not corrosive but in the presence of cellulose paper, a degree of corrosiveness is observed and the aramid paper promotes formation of corrosive sulfur in silicone oils.;Based on the obtained results, the feasibility of using hybrid insulation in power transformers is possible.;Keywords : power transformer; hybrid insulation; high temperature insulation; conventional insulation; sub-stations; aramid paper; cellulose paper; degree of polymerization; dissolved gases analysis (DGA); mineral oils; vegetable oils; synthetic oils; corrosive sulfur. |
