For the purpose of using three different types of painting materials for the inner protection of the transformer vats, their behavior was studied under actual conditions of operation in the transformer (thermal stress in electro-insulating fluid based on the natural ester in contact with copper for electro-technical use and electro-insulating paper). By comparing determination of the content in furans products (HPLC technique) and gases formed (by gas-chromatography) in the electro-insulating fluid (natural ester with high oleic content) thermally aged at 130 �C to 1000 hours in closed glass vessels, it have been found that the presence the investigated painting materials lead to a change in the mechanism and kinetics of the thermo-oxidation processes. These changes are supported by oxygen dissolved in oil, what leads to decrease both to gases formation CO2, CO, H2, CH4, C2H4 and C2H6) and furans products (5-HMF, 2-FOL, 2 -FAL and 2-ACF). The painting materials investigated during the heat treatment applied did not suffer any remarkable structural changes affecting their functionality in the electro-insulating fluid based on vegetable esters.

The behaviour of copper and insulation paper in various electrical insulating fluids (transformer oils) exposed to thermal ageing at 110�30C for 1000 hours in closed vessels (without access to atmospheric oxygen) has been studied. The processing of the comparative experimental data revealed in all cases that the concentration of dissolved oxygen in the investigated oils decreases exponentially during the heat treatment. In the presence of the copper foil, the oxygen is almost depleted (the dissolved oxygen concentration is approaching zero), indicating a higher affinity of the copper to oxygen than the affinity to oxygen of the investigated oils. In the presence of the copper foil and / or of the insulation paper, the degradation processes of the mineral oils have a pronounced character, explained by the catalytic activity of the Cu2O film that has been formed and by the paper degradation, respectively. A high thermo-oxidative stability was noticed in the case of natural triglyceride oils, particularly for the synthetic ester-based oil.

The amounts of flammable gases formed in some transformer oils during the long-term storage (1000 hours) at 110 � 3�C in closed containers (limited access to atmospheric oxygen) were determined qualitatively and quantitatively by gas chromatography technique. The comparative experimental results showed that when two types of mineral oils, one synthetic oil and two types of vegetable oils are in simultaneously contact with copper and insulating paper, the total amount of flammable gases formed by thermal aging is about 40 times higher in the mineral oils than in the ester based oils. It has also been established that the electrical-use copper foil catalyses the formation processes of the flammable gases, the maximum effect being for the mineral oils (an increase of 8 times of ethane and 25 times for methane gases). It has further been observed that the contact with the insulating paper during the thermal treatment of the ester based oils does not influence the formation of flammable gases, compared with the mineral oils where the amount of the formed gases is doubled.

The behaviour of some insulating fluids in contact with copper foil and / or insulation paper under thermal stress (110�30C for 1000 hours), in a closed system (the access of atmospheric oxygen being limited), has been studied by determining the changes in viscosity and concentration of CO2 and CO. The experimental data revealed that, following the applied heat treatment, the change in viscosity of the esters-based insulating fluids (both synthetic and vegetable) is approximately 7 times lower than in the case of the investigated mineral oils. It has also been found that, following the thermal ageing, the gas content of the mineral oils is substantially higher than in the esters-based oils (8 times higher for CO2 and 4 times higher for CO, respectively). The experimental results indicate superior values for the thermal stability and compatibility with the insulation paper of ester-based insulating fluids.

The durability and safe operation of electrical equipment and devices with mixed insulation systems (solid/fluid - electro-insulating paper/oil) is determined by the insulation aging under simultaneous and synergic actions of electrical, thermal and chemical stress factors etc. In this context, degradations of insulating paper exposed to thermal aging in 5 different types of electro-insulating fluid have been studied experimentally. Liquid chromatography determinations have shown that the total content in furan products (resulting from cellulose degradation) in mineral oils is substantially higher than in electro-insulating fluid sorts based on of synthetic ester and/or natural ester (vegetable oil). This is due to the temperature between 90 oC and 130 oC when the activation energy of the furans formation process is up to 7.5 times lower in mineral oils than in ester-based oils. Degree determinations of cellulose polymerization (viscosimetric method) before and after exposure to heat treatment indicated that mineral oils degrade the electro-insulating paper much more strongly than ester-based oils (both synthetic and natural). Obtained results by liquid chromatography and by viscosity are in accordance with the images obtained by optical microscopy (at X 100).