This article presents the results of the studies on lightning properties of two synthetic ester liquids and mineral oil. The measurements concern the standard lightning impulse voltage at both polarities and a point-to-sphere electrode system representing highly divergent electric field distribution. The studies were based on the IEC 60897 Standard wherein the standard recommendations were extended to include light emission registration using a photomultiplier technique. Comparisons of the lightning impulse breakdown voltages (LIBV), times to breakdown (TtB) and light waveforms were made. The results concerning LIBV and TtB showed that at negative polarity both the esters have almost the same lightning characteristics which are slightly below the characteristics of mineral oil. At positive polarity the results indicated an improvement. Regarding the light emission, the mineral oil tested seemed to be more resistant against the appearance of fast light pulses indicating the intense stepwise propagating discharges.

Due to the low biodegradability of mineral oil, intense research is conducted to define alternative liquids with comparable dielectric properties. Natural ester liquids are an alternative in focus; they are used increasingly as insulating liquid in distribution and power transformers. The main advantages of natural ester liquids compared to mineral oil are their good biodegradability and mainly high flash and fire points providing better fire safety. The dielectric strength of natural ester liquids is comparable to conventional mineral oil for homogeneous field arrangements. However, many studies showed a reduced dielectric strength for highly inhomogeneous field arrangements. This study investigates at which degree of inhomogeneity differences in breakdown voltage between the two insulating liquids occur. Investigations use lightning impulses with different electrode arrangements representing different field inhomogeneity factors and different gap distances. To ensure comparisons with existing transformer geometries, investigations are application-oriented using a transformer conductor model, which is compared to other studies. Results show significant differences in breakdown voltage from an inhomogeneity factor of 0.1 (highly inhomogeneous field) depending on the gap distance. Larger electrode gaps provide a larger inhomogeneity at which differences in breakdown voltages occur.

Wax-formation in electrical equipment is usually associated with oil impregnated cables and capacitors. Its formation is generally attributed to high field strength and partial discharges. In context with power transformers x-wax only appeared in the bushings. In recent years, however, there was an increasing number of failures in hermetically sealed power transformers with wax formation in the high voltage windings. Wax formation in transformers may block the cooling ducts and lead to overheating. Especially compact power transformers with synthetic ester and silicone fluid seem to be affected. This investigation sought the necessary conditions under which x-wax is formed in power transformers and how these conditions differ for different types of insulation liquids. A laboratory model of the high voltage insulation of a distribution transformer is used to investigate the influence of temperature, partial discharges (PD) and pressure on the x-wax formation. In hermetically sealed transformers the internal pressure can change depending on the load and ambient conditions. Previous investigations have shown that small reduction of the internal pressure can significantly reduce PD inception voltage and increase the apparent charge of the PDs. This is a transient situation that may happen in wind turbine transformers when the transformer cools down during a calm phase. For this study, different insulation fluids, such as synthetic and natural esters, silicone fluid and mineral oil are stressed with partial discharges for 200 hours at various temperatures both at ambient pressure and at a reduced pressure of approx. 800 mbar. The partial discharges are monitored to allow a comparison between the PD energy and the amount of wax formed during the experiment. The overall behavior of the PD was quite different with the different liquids, requiring an increasing voltage to keep igniting in the case of natural ester. Dissipation factor, permittivity, DC resistance and breakdown voltage of the fluids are measured before and after the PD stress to see if these can be used as indicators for wax generation. The investigation shows, that, when stressed with partial discharges over extended periods of time, all insulation fluids form some sort of solid ageing product, which are being investigated and may or may not be classified as x-wax. The amount of solid and the conditions under which they are formed differs greatly according to type of the fluid.