The task of this paper is to analyze the parallel operation of two transformers with different power ratings and short circuit voltages on a specific example in the Petrokemija Plc electrical power system. The analysis was carried out with the aim of determining the impact of such operation on power flows as well as the definition of optimal operating conditions, i.e. the transformer ratios in case of mutual or individual regulation at different transformers main voltages, in which the circulating currents, the reactive power from the network and the losses are minimized. In the theoretical part of the work, the conditions for parallel operation of transformers and the effects of such operation for transformers with different parameters, i.e. the occurrence of circulating currents and the impact on power flows are described. The network modelling is described as a requirement for setting equations to solve load flow problem. The Newton-Raphson method is presented as the best technique for solving the set of various nonlinear equations of load flows. The practical part of the work is based on the simulation of the power system in the engineering software ETAP®, which offers the possibility of analyzing, simulation, monitoring, managing, optimizing and automation of electrical power systems. The obtained values of power flows, voltages and power losses in the system for various operating cases and the transformer ratios are given in the tables, and their interrelations are presented with graphs, where optimum operating conditions of the parallel transformers are defined.