An electric energy conversion apparatus comprising a.d.c. - a.c. conversion unit with a first solid state controllable power switch arrangement and a second solid state controllable auxiliary switch arrangement. Said first and second switch arrangements have their respective outputs connected to the respective ends of a primary winding of a transformer for providing at its secondary winding an a.c. voltage derived by the recurrent on-off switching action of said first power switch arrangement from an input d.c. The switching action performed by said first power switch arrangement is controlled by a control circuit arrangement in such a manner that said first switch arrangement is always switched into its non-conductive state prior to saturation of the core material of said transformer, and is switched into its conductive state directly following the switching action causing said second switch arrangement to become non-conductive. With said d.c.-a.c. convesion unit various alternative embodiments of an electric-energy converter can be assembled. For instance two of said units can be operated in a parallel configuration with a common control circuit arrangement. By inclusion of a rectifying circuit arrangement or a cyclo-converter in the circuitry of said secondary transformer writting d.c.-a.c.-d.c. or d.c.-a.c.sub.1 -a.c.sub.2 converters can be made at option.