The power transformer design is a non-linear optimization task, where the quotation engineer has to made the solution, wich meets the consumer's needs and has the lowest manufacturing price. The aspects of the selection is specified by the consumer, (see: Loss Capitalization), and presenting the capitalization values to the designers. This task traditionally solved by iteration. The designer makes a basic calculation, and makes lots of variation from this. And chooses one of them, which has the lowest capitalized price. One of the weak points of this process is the elaboration time. There is an other one which is not so trivial. When the engineer makes a basic design, she/he doesn't know that the founded mimima is a local one or a global one. A lot of computer programs have made to improve the quotation calculation process. And lots of them use the above mentioned process. In this work I would like to demonstrate an other method which based on convex optimization and interioir point methods. Before them I show the knowledge, which you need to understand the process.