Power transformers are generally designed to be used in conditions where voltage and current are sinusoidal. However, nonlinear loads are increasing in modern power systems with the developing technology. Therefore, line voltages and currents often have harmonically distorted or non-sinusoidal waveforms. In this article, a model has been developed. The mathematically developed model has been proven experimentally and numerically. In this paper, different sub-harmonic content parametric analysis of the loss of transformer under no-load conditions with voltage excitation was performed. For this purpose, the Finite Element Method (FEM) based modeling of the core and windings of the transformer has been developed. An efficient method based on harmonic field model of transformer windings and FEM based modeling of transformer core is used. ANSYS@MAXWELL program, which realizes a solution based on FEM, is used for this. From the results of the analysis, it was seen that the effect of harmonic voltages on the loss of the transformer core is negligible. However, these tensions have been shown to increase winding losses in the unloaded state. This case reveals the importance of harmonics to be taken into account in calculating the losses of power transformers.