In the first part, models of transformers with serial connection of primary (IIIy0-Yd11) and transformer Dy11 were developed. A calculation of the transformer parameters was made according to the data given in the documentation, and the accuracy of the calculated parameters was checked by a short-circuit test on the primary (HV side shorted) and a short-circuit test on the secondary side (LV side shorted). Also, equivalent models of both transformers with parameters referred to the primary side were developed. Based on these equivalent models, models in the dq system were developed. However, the models in the dq system and the calculation of the decoupling signals and feedforward signals required for the control structures are not shown in the paper due to trade secrets. After checking the accuracy of all developed models by comparison with the "original" model of the transformer, control structures were made for both modeled transformers. For the case of a transformer with primaries connected in series, a control structure with a three-level NPC was modeled, which acts as an active power converter for supplying a DC link on each of the LV sides (double AFE control structure). In this case, it is a cascade control structure that has a shared external DC link controller and internal PI current controllers. For the case of a Dy11 transformer, the control structure has only one NPC that acts as an AFE (single AFE control structure). The current is sampled in a fixed abc system. The second part deals with selective harmonic elimination (SHE). The existing algorithm for complete elimination of the 5th, 7th, 11th and 13th harmonics using the Newton-Raphson method was tested, and an algorithm using the fmincon solver for the same application was developed. Also, the same was done in case of partial elimination of these higher harmonics. Due to the appearance of multiple resonances, it was necessary to extend the solution of the nonlinear system of equations for 5th, 7th, 11th and 13th harmonics to the optimization problem where in addition to solving this nonlinear system it was necessary to minimize 29th, 31st, 35th and 37th harmonic. Several optimization problems that have been tried to solve with the fmincon solver have been described. The results of testing the developed algorithms in the control structure with a Dy11 transformer are given.