Contents lists available at ScienceDirect Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm Research articles Experimental study of AC breakdown strength in ferrofluid during thermal aging Juraj Kurimský a, ⁎ , Michal Rajňák a,b , Pavol Bartko a , Katarína Paulovičová b , Roman Cimbala a , Dušan Medveď a , Mária Džamová c , Milan Timko b , Peter Kopčanský b a Department Electrical Power Engineering, Technical University of Kosice, Masiarska 74, 041 20 Kosice, Slovakia b Institute of Experimental Physics SAS, Watsonova 47, 040 01 Kosice, Slovakia c Institute of Physics, Faculty of Sciences, P. J. Safarik University, Park Angelinum 9, 041 54 Kosice, Slovakia ARTICLE INFO Keywords: Ferrofluid Mineral oil Thermal aging Magnetic susceptibility Breakdown strength Weibull distribution ABSTRACT A colloidal ferrofluid (FF) with superparamagnetic iron oxide nanoparticles (SPION) has been investigated for the AC breakdown strength during the accelerated thermal aging test. Three volume concentrations of a transformer oil based FF were subjected to the accelerated thermal tests at the temperature 90 °C. AC breakdown strength (BDS) tests were carried out every 200 h period for up to 600 h. The breakdown probabilities were calculated according Weibull distribution function. Measured BDS populations were compared with a base carrier oil. The BDS median of the clear carrier oil has been observed to fall down 1.54 times, however for particular FF samples it dropped 2.31, 2.90 and 3.63 times, respectively, when comparing properties of the samples before testing with the samples after aging. Final BDS probability distributions show that the dielectrics withstand voltage of FF became lower than that of the carrier oil. The long-term thermal load of the particular FF is critical for its colloidal stability, which is deduced to be the main reason of such a significant BDS reduction. The impact of the thermal aging on the AC magnetic susceptibility is briefly documented, too. 1. Introduction Nowadays, oil immersed power transformers are key components of the electrical networks worldwide. There are several certain properties of the insulating liquid for the power transformer which are identified as being very important: low viscosity, low pour point, high flash point, excellent chemical stability, high electrical field strength [1]. Typically, a refined mineral oil (MO) is used as an insulating and cooling fluid. Magnetic fluids, also known as ferrofluids (FF), have been seen as a potential successor of globally used MO due to their improved cooling and insulating properties [2]. A comprehensive review dealing specifically with the transformer-oil based nanofluids have been pub- lished recently [3]. As a dielectric fluid in power transformers, FF en- hances the heat transfer by thermo-magnetic convection due to the interaction between magnetic particles and magnetic field around the windings [4–6]. The crucial requirement for FF insulation systems is to have the improved long-lasting capability and capacity to withstand high elec- trical field levels and the thermal stability (note that the power trans- former insulation must be designed for a lifespan of 40 years or more [1]). In previous studies, they have been investigated either in the certain volume of as-prepared specific fluids [7–11] or in the im- pregnated cellulose pieces [12]. It is clear that for reliable and effective application of FF in power transformers the long term colloidal stability at operating conditions must be guaranteed. In general, the essential approach to determination of colloidal stability criteria consists in the dimensional reasoning. For FF, the interplay between various energies per particle, such as thermal energy, magnetic energy and gravitational energy is crucial for the long term stability [13]. Among the commonly used magnetic nanoparticles in FF, the su- perparamagnetic iron oxide nanoparticles (SPIONs) have been often studied for various potential applications. For the stabilization of SPIONs in FF several types of coating agents can be used. Regarding the MO based FF, the oleic acid belongs to the most frequently used sur- factant besides other ones, such as starch, chitosan, etc. [14]. According to [15], the maximum hottest-spot winding temperature should not exceed 110 °C under a continuous ambient temperature of 30 °C. Re- cently, the analysis of thermal stability of SPIONs has revealed that weight loss is 7.5% and 0.9% for the hydrophylic or hydrophobic SPIONs, respectively, in the temperature 30–120 °C. However, the weight change is related to the evaporation of water adsorbed on the https://doi.org/10.1016/j.jmmm.2018.05.083 Received 8 February 2018; Received in revised form 25 May 2018; Accepted 25 May 2018 ⁎ Corresponding author. E-mail address: juraj.kurimsky@tuke.sk (J. Kurimský). Journal of Magnetism and Magnetic Materials 465 (2018) 136–142 Available online 26 May 2018 0304-8853/ © 2018 Elsevier B.V. All rights reserved. T