From the practical experience regarding maintenance and service life of power transformers, it is important to know the state of degradation (ageing) of the solid insulation and oil transformer. Therefore, it is important to study and analyse how certain factors influence the dielectric properties of insulating materials used in electrical power transformers. This work proposes the experimental study of the behaviour of cellulose type solid materials used to insulate electrical transformers. The aim was to consider the hygroscopicity of cellulose-based products, the influence of temperature, humidity on the insulation resistance, dielectric strength and proof tracking index (PTI) in humid conditions. Experimental determinations were carried out in the framework of the RENAR accredited laboratory within ICMET Craiova. Experimental results have showed that environmental factors (low temperature -40°C and damp heat steady state for 24 hours at 70°C and relative humidity of 95%) strongly negatively influenced some dielectric properties such as insulation resistance and the PTI.
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