Transformers are used by the electrical industry to transfer electrical energy from one circuit to another. The oil surrounding the coils in a power transformer provide cooling, insulation and protection against corona and arcing. It is normally obtained by fractional distillation and subsequent treatment of crude petroleum. This is why this oil is also known as mineral insulating oil.
Transformer oil use growing globally
Global transformer oil usage is expected to grow substantially by 2020. The Asia-Pacific area, especially China and India, are the largest consumers of transformer oils due to the expansion of electric networks increasing the installation and upgrade of transformers in turn increasing the demand for transformer oil. Small transformers are distribution transformers, these are the largest consumers of transformer oil. Over time this oil is exposed to mechanical and electrical stress as well as to chemical contamination. When the functionality is reduced it can lead to power outages. To maintain and extend the life of the power transformer and to avoid severe breakdowns, regular testing of the transformer oil is very important. The ASTM D971 standard is used to define the general electrical and physical properties of transformer oil.
Important standard for several industries
So what does this standard mean in practice? Well, it determines the possible contaminants of hydrocarbon fluids. The purity of hydrocarbon fluids is important in other industrial areas besides transformer oils such as in aviation, and diesel fuels. For example, jet fuel needs to be highly purified as water or dirt contaminations can cause serious danger in flight safety. Surfactants in jet fuel can cause the lifting of rust in storage tanks as well as absorption of water on coalescing surfaces. The ASTM D971 standard is also a useful indicator of the cetane number of diesel fuels. The cetane number of diesel fuels is used to define the quality of combustion during ignition.
Transformer oil testing by using Du Noüy ring
In this interfacial tension measurement the platium ring is lifted through the water-oil interface, and the force measured is used to calculate the interfacial tension (mN/m) between oil and water. A high interfacial tension value (40mN/m) indicates the absence of undesirable polar contaminants in the hydrocarbon fluid which means that the fluid is immiscible with water. A decrease in interfacial tension occurs for example due to accumulation of contaminants or due to formation of oxidation by-products. The impurities in the hydrocarbon fluid encourage the oil to mix with water.
If you want to learn more about transformer oil testing and see how it is done in practice, see the short webinar below.