Optimum insulation in high voltage transformers

The general knowledge about transformer Insulation System Design is based on the use of cellulose in insulations which is basically used in mineral oil. For more than one hundred years, transformer design and manufacturing methods are based on this knowledge. In insulation design, field stress distribution between oil impregnated solid insulations is taken as the fundamental criteria. This stress is distributed in accordance with the permittivity of insulating materials and the geometry. Furthermore the insulation arrangement is constituted according to the design curves. They are based on decades of experience and depend upon manufacturing, process, workmanship, quality of materials and so on.Deficiency of materials that are essential for producing oil-cooled transformers, such as Grain Oriented Silicone Steel, Copper, Mineral oil, and their increasing and fluctuating prices necessitate finding alternative solutions to reduce the cost, weight and dimensions of the transformers. This thesis aims to demonstrate optimizing process by means of 'lead exits'. The lead exit is an insulation component and connecting element between high voltage winding end and the bottom of the bushing end. High voltage winding end can be positioned at the top or in the middle of the axial height of the coil. Common practice for each lead system is the phase current through a turret to the bushing bottom end. The turret diameter depends on system voltage and design of the lead exit. Determining of the turret diameter will certainly affect the quantity of steel, mineral oil, etc., and this is important for both material cost and the weight and dimensions of the transformers. Lead exit allows reducing the clearances and the diameter of turret therefore provide saving from materials for power transformer manufacturers.The field stress distribution of such a lead exit is highly non-uniform. The geometries of the structures should be arranged to optimize the field stress distribution. For this optimization, it is necessary to comprehend and compare the geometries.Insulation barriers, made from Transformer board, are used for subdividing large oil gaps into smaller oil gaps. Transformer board has pure cellulose fibers that increase the oil strength and provide a higher safety margin. While designing lead exit barriers the important point is to obtain the 'field conform' structure, i.e. electrical field patterns are perpendicular to barriers and equal potential field patterns are parallel with barriers.