Economic design of a 50 kVA distribution transformer. Part 1: The use of liquids of low flammability

Many alternative designs of transformer meet the rating, voltage and impedance specifications of a customer. However, only one design will give minimum total cost defined as purchase price plus capitalisations of core and winding losses. In this study the cost of materials, labour and electrical losses are carefully computed and the purchase price and total cost of alternative designs are plotted to a convenient base of the number of low voltage turns per phase. A curve is obtained for each combination of flux density and copper loss. The minimum cost designs are then selected from each curve while the variations of both purchase price and total cost with flux density and copper loss are determined. The design having minimum total cost will have optimal values of both flux density and copper loss. It has been found that departures from these optimal values have less effect on total cost than does failure to produce the minimum cost design for whatever values of flux density and copper loss are actually used. As expected, the purchase price is found to be more sensitive to small changes in flux density and copper loss than is total cost. Designs optimised for minimum purchase price show an almost linear fall in cost with increases in both copper loss and flux density. The optimal designs for an expensive ester of low flammability (Midel 7131), differs slightly from the optimal design for mineral oil and uses a rounded tank which is not found to be economic for a mineral oil transformer. For minimum total cost of ownership a flux density of approximately 1.45T and copper loss of around 1 kW are optimal when using the ESI capitalisation.