Alternative insulating liquids

All insulating liquids, regardless of their chemical composition, are subject to natural aging processes during their use in electrical equipment. If products based on fossil raw materials, such as mineral oil or GTL products, show increased oxidation stability compared to synthetic or natural ester fluids, said ester fluids offer many advantages.

Increased safety standards, complete biodegradability and increased long-term stability, in combination with hermetic operating conditions, aim at revolutionizing the European market. However, it is also a fact that even under hermetic operating conditions, ester liquids cannot be compared with highly refined, saturated hydrocarbons of fossil origin due to their chemical structure, especially with regard to their chemical resistance. However, since insulating liquids in electrical equipment are not only used for electrical insulation, but also as a cooling liquid, and operating times can reach not much less than 100 years, exact analysis and condition assessment are of the utmost importance.

The main objective of this study was the critical assessment of the most important alternative insulating liquids currently on the market and the development and application of alternative analysis methods in order to analyze any aging effects that may occur. For this purpose, we carried out hermetic aging tests. Mineral oil, GTL, synthetic and natural ester products in combination with cellulose-based insulation materials and transformer materials were examined in representative proportions. In addition to HP liquid chromatography (including IEC ion exchange chromatography and SEC size exclusion chromatography), the chemical analysis used included various gas chromatographic methods (including FID flame ionization detection, MS mass spectrometry and TCD thermal conductivity detection), UV/VIS and FTIR analyzes as well as new approaches based on fuel analysis such as the PetroOxy® system.

In addition to material and additive-related differences, we could also determine differences in the formation of aging products and long-term stability between the individual insulating liquids. While ester liquids lead to slower aging of embedded cellulosic materials under all tested conditions, in addition to improved safety aspects, especially at operating conditions ≥ 150°C, we could also identify some disadvantages. In a direct comparison, ester-based insulating liquids show a significantly increased formation of aging and by-products, which increase massively with increased operating conditions ≥ 150°C. Aging under oxygen also shows that natural esters in particular appear completely unsuitable for use under oxygen.