This paper presents the current-fed inverter of a TGV-K traction system with thyristor switches using phase control and commutation techniques. The current-fed inverters have two modes of operation which consist of forced commutation and natural commutation. In forced commutation mode, at speed of less than 120km/h, commutation is forced by means of the commutation capacitors and the thyristors. Above 120km/h, the thyristors operate in natural commutation mode. according to the voltages between phases of the motors. In this paper. the power conversion theory of the TGV-K traction system and the control principle of the converter and current-fed inverter are discussed.