In this paper we have investigated the area of cost-effectiveness of transition from single to parallel operation for certain transformer models. In order to be able to perform the parallel operation, the transformer must meet the following conditions: the same group of connections, the same transmission ratio, approximately the rated voltages and voltages of the relative short circuit, and the nominal power ratio should not be greater than 2. Furthermore, we have analyzed the losses in transformers , defined the boundary loads and load factors that it is cost-effective to work with a single transformer, two transformers or three transformers in parallel, and draw diagrams accordingly. As in operation of single transformer, as in parallel operation with two or three transformers, for larger transformers, it is more optimal to work with a lower load factor than the smaller power transformers, where it is more optimal to work with a higher load factor. Nowadays, the parallel operation of the transformer is no longer used as a standard form of power supply of the network systems, because higher rated power transformers are used, and there is only a need for parallel operation when converting the transformer through the connection field and in case of failure. To prove the mentioned above, in the last chapter we made an analysis of the TS "Krk" and we drew diagrams per period of time depending on the working and refractive power.