Damage assessment of transformer Kraft paper insulation aged in mineral and vegetable oils

Most high-voltage transformers are fluid filled machines whose insulation system is constituted of dielectric oil–solid combination. The insulation oil generally used is mineral oil, however, this fluid has started to be substituted by natural and synthetic esters due to their higher biodegradability and flash point in comparison with mineral oil. The introduction of a new fluid in the insulation system of power transformers requires kinetic models which can estimate the degradation rate of insulation solids, taking into account the properties of the new materials. The aim of this work is to go further in demonstrating that a kinetic model based on damage parameter can be used as a cellulose degradation model for Kraft paper, which is one of the solid materials used in the insulation system of oil-filled transformers. The ageing study has been extended to Kraft specimens immersed in two different oils (mineral and natural ester). The kinetic model that best tracks the ageing patterns was shown to be a function of damage parameter estimated through the energy consumed per unit volume of the failure zone (E_R) in comparison with other mechanical properties such as rupture strength (σ_R) or strain under ultimate strength (ε_cm). Finally, an assessment of the effect of end-of-life criteria on correlation coefficient was carried out, providing further evidence that this criteria established until now (25% retained tensile strength) should be discussed in order to obtain more accurate kinetic models.