An experiment to evaluate the benefits of processing aged transformer oil

The mineral oil/paper insulating medium used in high voltage applications is subject to ageing through electrical, thermal and mechanical stresses. A significant factor in ageing is chemical action through moisture, metal contaminants and oxidation. The stresses change the properties of the paper and oil, which ages the insulation system. Transmission and distribution operators use condition monitoring and maintenance programs in an attempt to prolong the life of the insulation and hence the transmission equipment. Condition monitoring typically involves moisture analysis, dissolved gas analysis and furanic analysis of the oil. Maintenance involves purification of the oil to remove contaminants. Additional monitoring techniques may be used such as infra-red (IR) spectroscopy and dielectric analysis. However, without controlled ageing experiments under laboratory conditions the applicability of condition monitoring particularly for lifetime prediction is questionable. An opportunity arose from the recent scrapping of an aged high voltage transformer to analyze the diagnostic parameters of typical transformer oil aged under service conditions and compare these results with oil aged under control conditions. The aim is to understand the effects of reprocessing and ageing on some physical, chemical and dielectric properties. Some measurable properties of ageing and the benefits of oil maintenance are discussed in this paper