The paper presents research results of different properties of mixtures of the synthetic ester with mineral oil and synthetic ester with natural ester. For different concentrations of both mixtures, the following properties were investigated: flash point (open and closed cup), fire point (open cup), net calorific value, breakdown voltage, relative permittivity, dissipation factor, conductivity and dissipation factor. In order to compare results, prepared mixtures and base liquids were carefully arranged under uniform climatic conditions. This meant that the prepared samples had the same relative humidity. The obtained results were compared with the results of the base liquids. In addition, results of the investigation were analyzed in terms of fire safety and changes of electrical properties, in the context of the use of synthetic ester/mineral oil and synthetic ester/natural ester mixtures as insulation liquids in power transformers.

The article presents the results of extensive investigations aimed at determining the moisture content in a large population of power transformers (161 units). The project was carried out during the period of 2003-2011 in Poland, Germany, and Sweden. All results were obtained by means of the Frequency Response Spectroscopy (FDS) method. Among the transformers tested were grid transformers, generator step-up (GSU) transformers, and distribution transformers. According to the classification rules established by IEEE, 30% among the investigated transformers could be classified as “wet” (2-4% moisture content). The observed moisture variation with age was found to depend on transformer design and operating conditions. It is suggested that, based on the presented statistical results and knowledge concerning transformer type and age, one may discriminate between probably dry transformers and transformers that require actual moisture analysis and therefore prioritize further condition assessment.

The gases generated by electrical and thermal faults are similar in the case of mineral oil, synthetic ester, and natural ester. However, there are considerable differences in the amount of the generated gases and their solubility in these liquids, what should be taken into account during power transformer diagnostics by means of dissolved gas analysis (DGA) method. The aim of the research was to compare the gases generated as a result of thermal faults (250°aC, 350°C and 450°C) in mineral oil and both esters. The following gases were analyzed: hydrogen, methane, ethane, ethylene, acetylene, propane, propylene, carbon oxide, and carbon dioxide. A qualitative and quantitative analysis of these gases were carried out by means of gas chromatography. A large share of propane in the sum of the gases generated in the esters, mainly in the synthetic ester, was found. Propane and propylene in most of gas interpretation approaches are omitted. The authors proved that in the case of synthetic ester the using of propane as the key gas for thermal faults detection is justified. The obtained results can be used to improve the interpretation of DGA results, in the case of ester liquids used in power transformers.

This work refers to the criterion values used to assess the state of the active part of the transformer based on the analysis of the effective value of the total vibration acceleration and the frequency spectrum. It was proved in the work that the criteria values should be differentiated for transformers of different rated power. Transformers with lower rated power are characterized by lower RMS values of vibration acceleration than units with higher rated power, which cannot remain without impact on the criteria values. Trend analysis of the total a_RMS values may reveal increasing defects before the currently applicable criteria values are exceeded. In addition, the influence of the position of the sensor on the frequency spectrum of the obtained signals was analyzed. It was proved that the sensors should be mounted in the middle of the transformer tank, between its lower part and the cover. The dependence of RMS value of vibration acceleration on no-load losses was also determined for transformers of different types but the same rated power. This relationship will not have a large share in the total vibrations of the transformer, due to slight changes in the value of a_RMS occurring for all analyzed units.

The purpose of this work is to evaluate the influence of the cellulose polymerization degree (DP) on the dielectric response. The research objects were new and aged pressboard samples of the polymerization degree 1435 and 406, respectively. The authors investigated the dielectric response in the time and frequency domains. The research showed that dielectric response characteristics of new and aged samples, of different DP and an identical water content are very close. Their differences are within the limits of statistical dispersion.