Electrical Engineering https://doi.org/10.1007/s00202-018-0709-5 ORIGINAL PAPER Study of kinematic viscosity of mixture oils under thermal aging in power transformer A. Hamdi 1 · D. Mahi 1 · S. Boudraa 1 Received: 17 December 2016 / Accepted: 12 July 2018 © Springer-Verlag GmbH Germany, part of Springer Nature 2018 Abstract This paper presents the experimental results of physicochemical properties studies of mixtures based on mineral oil carried out in the laboratory, under thermal stress at fixed temperature with different intervals for long period. The stability study of thermal aging is an essential tool to estimate the lifetime of insulating oil and therefore for the transformer. The tests results allow to make a comparison between the different mixtures proposed and to select the best. It is shown that the degradation rate of oils represented by kinematic viscosity is changed according to ester rate under thermal stress. We can consider that mixtures are less affected than mineral oil, depending on the ester content in the mixture. Results show the strong correlation between viscosity and dissolved decay product, under thermal stress. The increase in the soluble products in the oils, generated by degradation of oil, affects the physical parameter by increasing the kinematic viscosity. These measures are the main criterion to evaluate the stability of physical parameter of oil in-service. Keywords Mineral oil · Ester oil · Mixing · Thermal stability · Kinematic viscosity · Breakdown voltage · Dissolved decay product · Spectrophotometer UV 1 Introduction Power transformers are critical expensive equipment which ensures the functioning of power grids. It is fundamental that they operate correctly for many years, any failure of this equipment can result in a significant economic loss, and the insulation failure inside is to be accounted as a major reason for accidents transformers [1, 2]. Insulating oil has a fundamental role in power transform- ers that provide both electrical insulation and means for transferring heat losses generated by the coils in the mag- netic circuit. While its performance was exceptionally high, a transformer cannot avoid the energy loss that accompanies any energy conversion, and this lost energy is a dissipated B A. Hamdi h75_ahmed@yahoo.fr D. Mahi mahidjillali@yahoo.fr S. Boudraa saliha_boudraa2@yahoo.fr 1 Laboratoire d’Etudes et de Développement des Matériaux Semi-Conducteurs et Diélectriques, Université de Laghouat, BP 37, route de Ghardaïa, 03000 Laghouat, Algeria form of heat and leads to the obligation of the device cool [3]. A properly organized heat transfer avoids the formation of hot spots that facilitate, so the natural cooling process takes place in a homogeneous manner. On the other hand, oil insulating is important information of the carrier. It can provide information on the degradation of insulating paper, which is very important in the transformer diagnosis; oil contains about 70% of the diagnostic informa- tion. The different variations in oil characteristics, therefore, might be used to identify/detect the kind of incipient failure in the turn [4, 5]. Mineral insulating oil has been used as insulation and cool- ing fluid in power transformers for more than a century, and it is certainly the most important dielectric fluid for this appli- cation. Billions of liters of oil are being used in electrical devices throughout the world. This is due mainly to technical efficiency, availability, low cost compared to other dielectric means, and ability to be reclaimed. The quality of the oil in a transformer plays an important role in performing these functions [6]. Today, esters are being used in transformers, mainly for safety reasons, because they have higher flash points and fire points compared with mineral oils, and for environmental 123