In this study, three different insulating liquids, natural ester, mineral oil which is currently used by the Public Power Corporation of Greece and a nanofluid of surface coated Fe ₃ O ₄ nanoparticles into a natural ester matrix, were subjected to AC voltage stress and their statistical breakdown voltage was measured and compared. Three different electrode configurations were used during the measurements in order to study the effect of non‐uniform fields on the breakdown voltage distribution. Four different statistical distributions were used for the statistical processing of the experimental results. The two of them were the frequently used normal and Weibull distributions; in addition, Gumbel and generalised extreme value (GEV) function distributions were studied. It was shown that, in most cases, the experimental data followed GEV and Weibull functions rather than normal or Gumbel. Therefore, GEV statistics fits indiscriminately better to the experimental results concerning breakdown voltage, for the three different electrodes configuration and the three understudied insulating oils. The experimental results indicated that nanofluid has better performance, in terms of dielectric breakdown voltage, compared with natural ester oil and mineral oil, hence nanofluid is considered as a potential substitute of the conventional dielectric liquids, with enhanced properties and prospects.

The improvement of dielectric and thermal properties of insulating oils used in High Voltage (HV) equipment has been a key part of research over the last three decades. In this study, a natural ester oil is used as matrix oil and dispersions of two semi-conducting nanoparticles (NPs), titanium oxide (TiO₂) and silicon carbide (SiC), are prepared in 0.004 weight percentage ratio (0.004% w/w). In terms of thermal features, thermal diffusivity and thermal conductivity are measured over a large temperature range between 25 and 90 °C. By means of the dielectric properties, the Lightning Impulse Breakdown Voltage (LI BDV) of nanofluids (NFs) is measured and compared to the corresponding ones of the matrix. Moreover, the dielectric constant and dielectric dissipation factor (DDF) are determined for the NF and matrix samples in the frequency range of 1 to 10⁶ Hz. The obtained results show that the NF containing SiC NPs is characterized from the greatest increase in thermal conductivity, by 58% with respect to natural ester oil’s, as well as the lowest dielectric constant in both 25 and 90 °C, most likely due to their higher permittivity and thermal conductivity. Lastly, the LI BDV of the same NF is the most enhanced, by 10% relatively to the matrix, while a numerical model in COMSOL Multiphysics is developed and is found to successfully verify the LI BDV results.

In this study, the dielectric characteristics of two types of natural esters modified into nanofluids are studied. The AC breakdown voltage was investigated for colloidal Fe₂O₃ and SiO₂ nanoparticles effectively scattered in natural ester oil. The experimental results identify an increase in the breakdown voltage of the nanofluid with colloidal Fe₂O₃ conductive nanoparticles. In contrast, the breakdown voltage was reduced by adding SiO₂ nanoparticles in the same matrix. The potential well distribution of the two different types of nanoparticles was also calculated in order for the results of the experiment to be explained. The dielectric losses of the colloidal nanofluid are compared with the matrix oil and studied at 25 °C and 100 °C in the frequency regime of 10⁻¹–10⁶ Hz. The experimental data and the theoretical study reveal that conductivity along with the permittivity of nanoparticles constitute a pivotal parameter in the performance of nanofluid. Specific concentrations of nanoparticles with different electrical conductivity and permittivity than those of matrix oil increase the breakdown voltage strength. Simultaneously, the addition of nanoparticles having electrical conductivity and permittivity comparable to the matrix oil results in reducing the breakdown voltage.

This work introduces an experimental methodology for time-domain modeling of low-voltage surge protective devices (SPDs), accounting for their sparkover performance as well as their resistive, inductive, and capacitive behavior. The modeling procedure is demonstrated through an application to a combination type SPD connected to the DC side of electric vehicle charging stations. An equivalent circuit model is developed based on experimental records acquired from applied voltages and currents of a wide frequency range and energy content. The developed lumped-circuit model yields results in very good agreement with experimental data regarding sparkover voltage, residual voltage, and energy absorption of SPDs, as illustrated through ATP-EMTP simulations. The proposed methodology can be an effective tool for surge protection and insulation coordination studies.