Comparative Analysis of Selected Insulating Liquids Including Bio-Based Hydrocarbon and GTL in Terms of Breakdown and Acceleration Voltage at Negative Lightning Impulse

Different types of dielectric fluids used in transformers are becoming increasingly prevalent on the market. This article presents a recent comparative study on negative lighting impulse breakdown voltage (LIBV) and acceleration voltage ( ${V}_{a}{)}$ of six insulating liquids of different chemical compositions namely naphthenic uninhibited mineral oil (UMO), naphthenic inhibited mineral oil (IMO), natural ester (NE), synthetic ester (SE), and new modern dielectric fluids such as bio-based hydrocarbon (BIO) and inhibited liquid produced using gas-to-liquid (GTL) technology. Tests are conducted for a point to sphere electrode setup based on IEC 60897 Standard and for two gap distances, 25 and 40 mm. When comparing the results obtained from the experiment the biggest differences between the liquids are noticed not for the LIBV evaluated using Weibull distribution but for ${V}_{a}$ parameter; it is confirmed that both esters considered within the studies are characterized by lower values of ${V}_{a}$ than other analyzed fluids among which IMO, BIO, and GTL are of comparative nature, especially for 40 mm gap. In turn, based on the obtained results of LIBDV and ${V}_{a}$ , the maximum values of the electric field strength are determined by series of finite element method (FEM) simulations. Those simulations confirm stable electric field condition determining breakdown and discharge acceleration in a given liquid considered.