**1. Introduction**

The lifetime of the power transformer is a major factor for the reliability and uninterrupted operation of the electricity grid. Liquid insulation, as well as the paper immersed in dielectric liquid, should provide protection of its windings under the influence of electrical, thermal, and even environmental effects to prevent short circuits and leakage currents [1,2]. The mineral oil typically used as dielectric and thermal coolant has certain disadvantages, such as toxicity, high flammability, and reduced lifetime [3–7]. Therefore, the first step towards the improvement of transformer insulation is to turn the attention of research to the study of alternatives, such as natural or synthetic ester oils [7–10]. The use of ester oils entails, beyond improved properties like biodegradability and higher moisture tolerance, benefits for the transformer itself, essentially aging characteristics, enhanced lifetime, and loading capability [3,11,12].

Nanotechnology has already been used for various subsurface applications [13–16]. In liquid insulation, the integration of nanoparticles (NPs) in a base conventional oil volume has been part of

recent research in an effort to achieve enhanced dielectric and thermal performance with optimum insulation quantity to recede the size of the transformer [17]. The term "nanofluid" (NF) was firstly proposed by Choi et al. [18], indicating a mixture where both the NPs and the base oil contribute to the application providing enhanced thermal conductivity. Since then, a number of reports have appeared in the literature regarding the integration of many different types of NPs in mineral or ester oils achieving improvement not only in thermal but also in dielectric properties based on their type, concentrations, shapes, and sizes [1–9,19–22].

It has been noticed that the addition of nitrides mainly leads to improvement of thermal properties [20,21], while Thomas et al. [3] indicated that the integration of CaCu3Ti4O12 NPs in 0.050% vol. concentration could lead to thermal conductivity enhancement of the synthetic ester oil by 10% at room temperature. The integration of metal oxides, on the other hand, could have beneficial effect on dielectric ones. Towards this direction, Zhong et al. [8] and Du et al. [2,22] have reported improvement of AC and Lightning Impulse Breakdown characteristics following the addition of semi-conductive NPs in natural ester and mineral oil volumes, respectively. The addition of magnetic [4,23] and dielectric [19,23,24] NPs, even at low weight fractions, has also shown increase in the dielectric strength with respect to the base oil. Khaled et al. [23] studied the effect of conductive Fe3O4, and dielectric Al2O3 and SiO2 NPs on AC breakdown voltage (AC BDV) of synthetic ester oil reporting 48% improvement with the addition of conductive Fe3O4 NPs (50 nm) at a concentration of 0.4 g/L, while the integration of insulating Al2O3 NPs of 13 nm size at a concentration of 0.05 g/L led to a 35% improvement. Last but not least, the viscosity of the oil is also affected based on the type and the loading of the NPs inside the matrix. Fontes et al. [25] reported a 25% rise in viscosity adding diamond NPs in transformer oil at 0.050 vol.%. Ilyas et al. [26] concluded that the dynamic viscosity of the mineral oil-based alumina NFs had decreased with the rise of temperature.

With regard to the mechanisms leading to the beneficial effects of the NPs on the aforementioned properties, a number of theories have been proposed [27–29]. Most of them consider the operation of NPs as "electron traps" under the application of an external electric field hindering the streamer propagation between the electrodes, adopting different explanations [27,28]. Conductive NPs can capture the fast electrons in shallow traps through charge induction [27,28]. Semi-conductive and dielectric NPs could have the same effect as the conductive ones, because they are polarized under the influence of an external electric field.

Despite the number of publications devoted to the study of the metal oxide NPs' effect on dielectric and thermal properties of the transformer oil, there is limited literature concerning the influence of metal carbides in transformer oils, although they are widely used in other industrial sectors. The main contribution of this article is to compare the dielectric performance of insulating Al2O3 and semi-conducting SiC-ester based NFs in terms of AC breakdown voltage (AC BDV), positive and negative Lightning Impulse breakdown voltage (LI BDV) and partial discharge (PD) activity. The choice of these specific types of NPs, having similar nominal size, is based on the fact that they have similar permittivities, therefore conductivity should play a major role in the dielectric performance of the corresponding NFs.

In this study, insulating alumina and semi-conducting SiC NPs were dispersed into natural ester oil EnvirotempTM FR3TM in concentration of 0.004% *w*/*w*. The choice of this particular concentration is based on previous work [30]. After the preparation of the NF samples, dynamic light scattering was used to assess their agglomeration behavior in the dispersions. AC, positive and negative LI BDV, as well as PDIV, have been measured, analyzed, and compared to the base oil's characteristics in an effort to conclude about their effect on dielectric performance of transformer oil. The results of the conducted measurements have indicated that the NF-containing SiC NPs possesses improved dielectric properties which is attributed to the higher conductivity of these NPs with respect to the Al2O3 NPs for the same nominal size and weight percentages.
