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Large three-phase power transformers normally consist of three cylindrical coil sets (phase windings) assembled side by side in line. The high voltage winding of each phase is generally the outermost coil. The space between the phase windings is called the inter-phase region. Insulating sheets of pressboard, are placed between the phase windings to form dielectric barriers to prevent flashover between...
The materials used as the dielectric media of choice in large transformers continue to be a combination of pressboard and mineral oil. These materials offer a cost effective solution and are backed by a historical knowledge base of performance data and lifetime characteristics. A proven method to prevent inter-phase flashover is to use successive layers of pressboard and oil gaps. However, a number...
Three phase transformers have an insulation system which is a complex three dimensional structure of paper and pressboard surrounding the conductors immersed in a large volume of mineral oil. In core form transformers, a key insulation area is the region between adjacent phases which is often reinforced with vertical pressboard barriers. At the higher voltages, these pressboard barriers provide a...
High voltage equipment (e.g. transformers and switchgear), use mineral oil and paper as a cost effective and well tried insulation medium. However, this insulation is known to age over time and usage. An obvious sign of ageing is the change in the colour of the oil which changes from pale yellow to reddish brown. This colour change can be measured by the technique of ultraviolet/visual (UV/Vis) spectroscopy...
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...
Many experiments have been undertaken and results published on ageing of high voltage transformer insulation. The objective of these experiments has been to quantify how insulation is affected by moisture and temperature. However, the experiments have invariably been performed on small scale bench top equipment. The validity of scaled up results is always questionable and is exacerbated by the lack...
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