**The Influence of Nanoparticles' Conductivity and Charging on Dielectric Properties of Ester Oil Based Nanofluid**

**Konstantinos N. Koutras 1,\*, Ioannis A. Naxakis 1, Eleftheria C. Pyrgioti 1,\*, Vasilios P. Charalampakos 2, Ioannis F. Gonos 3, Aspasia E. Antonelou <sup>4</sup> and Spyros N. Yannopoulos <sup>4</sup>**


Received: 13 November 2020; Accepted: 9 December 2020; Published: 11 December 2020

**Abstract:** This study addresses the effect of nanoparticles' conductivity and surface charge on the dielectric performance of insulating nanofluids. Dispersions of alumina and silicon carbide nanoparticles of similar size (~50 nm) and concentration (0.004% *w*/*w*) were prepared in natural ester oil. The stability of the dispersions was explored by dynamic light scattering. AC, positive and negative lightning impulse breakdown voltage, as well as partial discharge inception voltage of the nanofluid samples were measured and compared with the respective properties of the base oil. The obtained results indicate that the addition of SiC nanoparticles can lead to an increase in AC breakdown voltage and also enhance the resistance of the liquid to the appearance of partial discharge. On the other hand, the induction of positive charge from the Al2O3 nanoparticles could be the main factor leading to an improved positive Lightning Impulse Breakdown Voltage and worse performance at negative polarity.

**Keywords:** nanofluid; conductivity; alumina nanoparticles; silicon carbide; AC breakdown; lightning impulse breakdown; partial discharge
