In the Croatian medium-voltage network there are several voltage levels (10 kV, 20 kV, 30 kV and 35 kV), what makes purchases rather difficult, production costs higher and requires a number of stand-by transformers. Hrvatska Elektroprivreda d.d. (HEP) is making preparations for introduction of a single 20 kV distribution voltage, which will replace the above-mentioned voltages. Therefore the new network transformers are reconnectable from 10 kV to 20 kV. For the existing 110/36.75/10.5 kV transformers in the connection YNyn0d5, the most efficient solution is reconnection on their mediumvoltage side, so that in the connection YNd5d5 the transformation ratio is 110/21.22/10.5 kV. To be able to earth 20 kV network of such transformers it is necessary to have an earthing transformer, and it would be well to standardize it. In this master’s thesis, such an earthing transformer is defined, its characteristics and applications described, and a survey is given of its possible designs in dependence on various requirements. An analytical computation is made of stray and zero reactances adjusted to various designs of the earthing transformer. Parameters of an earthing transformer that would be suitable for the insulated 20 kV network in Croatia are proposed, and technical data obtained during its design process are presented.