**5. Conclusions**

In this study, the dielectric insulation characteristics for two types of nanofluids based on a natural ester matrix were studied with respect to the influence of electrical conductivity and permittivity of the NPs on the AC dielectric strength of nanofluids. Two types of nanoparticles were studied, oleate-coated colloidal magnetic Fe2O3 nanocrystals (colMIONs) exhibiting high conductivity and permittivity and SiO2 NPs exhibiting low conductivity and permittivity.

The experimental results of this study are in compliance with the proposed theories analyzing the impact of NPs on the dielectric strength of transformer oil-based nanofluids. NPs with higher electrical conductivity or permittivity can exhibit improved dielectric performance by adding them in the matrix oil. Particularly the introduction of colNF consisting of conductive colMIONs, demonstrates an improved AC breakdown voltage of 77.8 kV at 0.012% *w*/*w* concentration as compared with 64.5 kV AC BDV for the base matrix oil. Simultaneously, they exhibit improved field operation reliability taking into account that the U1 breakdown voltage was considerably high, equal to 62.1 kV, which is remarkably higher than the U1 of 37.2 kV of the matrix oil. The aforementioned performance is correlated to the substantially low standard deviation in the range of 6.7 kV. Additionally, the dielectric losses of the colNF studied at 25 ◦C and 100 ◦C in the frequency regime of 10−1–106 Hz behaved similarly to the matrix oil, that is, they reduced as the frequency increased and increased as the temperature increased, but with slightly higher values than that of matrix oil.

In contrast, the AC breakdown voltage of silica NF with SiO2 NPs is reduced as compared to that of the matrix oil for all the ranges of NPs concentrations that were investigated herein. The explanation of the abovementioned dielectric performance is correlated with the low electrical conductivity as well as the permittivity of silica NPs, along with their low capability of capturing free electrons. Furthermore, an in-depth research study is needed which takes into account the electrical conductivity and permittivity in order to discriminate the influence of moisture content on the dielectric strength performance of the nanofluids.

**Author Contributions:** Conceptualization: V.P.C., G.D.P., E.C.P.; Methodology: G.D.P.; Manufacturing of ColMIONs and characterization: A.B., G.D.P.; Preparation of NFs: A.B., G.D.P.; Measurements of BDV: G.D.P.; Measurements of dielectric losses: G.D.P., A.D.P.; Statistical analysis and Analysis of results: V.P.C., G.D.P., A.D.P.; Theoretical Analysis: V.P.C., G.D.P., A.D.P.; Writing: V.P.C., G.D.P., A.D.P.; Review: V.P.C., G.D.P., E.C.P., A.B., A.D.P., I.F.G.; Supervision: E.C.P., I.F.G.

**Funding:** This research received no external funding.

**Acknowledgments:** The authors warmly acknowledge Kevin Rapp Senior scientist of Cargill for the FR3 oil supply during this research.

**Conflicts of Interest:** The authors declare no conflict of interest.
