This final paper discusses the possibility of using automatic voltage regulation at medium voltage level. The paper lists the main parts of the power system and applies the quality of electricity supply to end users. Accordingly, the paper talks about switchyards. The paper gives a detailed description of what a switchyard consists of and describes each element of switchyards in detail. Furthermore, the paper explains the basic operation principle of a transformer, its losses, operating modes and current and voltage relations in different operating modes. Special transformer designs are also explained in the paper. The main part of this paper is the On-load tap changer – OLTC. The paper explains transformers with change of load ratio under load, winding arrangement and load loss of systems with one contact switching. The paper analyses different winding arrangements in OLTC and describes them in detail. Oil type OLTC and vacuum type OLTC are described, the parameters that must be taken into account when choosing OLTC are given. The paper also contains a practical part that was done at HEP ODS Elektra Čakovec. Practical paper deals with the influence of the solar power plant SE Čakovec on the voltage profile in the connection points of the electric power network of Elektra Čakovec. Simulations were done in the ''Neplan'' program. We can see from the diagrams how the automatic control maintains and changes the voltage as needed. We can also see changes in the voltage of SE Čakovec at minimum and maximum load.