**4. Discussion**

It is well known that the presence of conductive and non-conductive solid particles inside insulating oils results in the decrease of breakdown voltage [29–32]. All the relevant studies that have been conducted concern solid particles with sizes of the order of several hundred μm. However, the basic theory can be extended to the nm scale. This effect is also observed in Figure 3, for low and high concentrations of both dispersed nanoparticles in the vicinity of the matrix oil.

However, the increase of BDV of nanofluids for specific concentrations of nanoparticles indicates the presence of an opposite physical mechanism which enhances the dielectric properties. This mechanism was explained satisfactorily in [20,22] and the experimental results of this work are in compliance with the proposed theory as analyzed in the previous section. Nanoparticles should have either high conductivity or higher permittivity than that of pure insulating oil, in order to act as electron scavengers and reduce the speed of streamer. The colMIONs succeeded due to their high conductivity while SiO2 failed to improve the dielectric characteristics of insulating oil. This is also valid for TiO2 nanofluid as its probabilities of BDV are much lower than the matrix oil [14], as can be seen in Table 1.

On the other hand, SiO2 nanoparticles in [19] seem to have a better 50% probability (Table 1) just as other studies [15–17] reveal a better performance of silica nanofluids in contrast to the experimental results of this work. However, in all these studies, mineral oil was used as the base for the synthesis of nanofluids. Mineral oil exhibits lower permittivity (*er* = 2) as compared to natural ester oil (*er* = 3.2) and this fact enhances the ability of silica nanoparticles to capture electrons. Another issue seems to be the

presence of moisture, since SiO2 nanoparticles can absorb large amounts of moisture. Water absorption demonstrates higher conductivity and relative permittivity than SiO2 and thus it enhances the electron trapping capability of SiO2 nanoparticles.

In this study, nanofluid samples were tested shortly after their preparation, and therefore nanoparticles did not have the time to absorb moisture from the vicinity of the oil.

For lower concentrations than optimum, for both nanofluids tested, BDVs are lower than that of matrix oil but with an upward trend with the increase of nanoparticle concentration. For concentrations above the optimum, BDVs are also lower than that of matrix oil but with a steadily downward trend.

As was aforementioned, both conductive and non-conductive nanoparticles are charged when subjected to an external electric field. As the concentration of nanoparticles increases, they will form thin filaments that oscillate across the gap. When the filaments reach a critical length, a conductive bridge path is formed and a breakdown discharge will be triggered. As long as the enhancement mechanism overcomes the negative impact of solid impurities on transformer oil, BDV versus concentration will exhibit an upward trend. When the concentration of nanoparticles exceeds a critical value, the negative impact of the presence of solid particles dominates, and the BDV of the nanofluid rapidly decreases.
