Current source converter with gate turn off semiconductor elements and a special commutation mode

The invention provides a converter (2a), and in particular a current source converter (CSC). The converter includes a bridge (14a) having an ac terminal (16a-16c) for each of one or more ac lines, and first and second dc terminals (18a, 18b). A converter arm is connected between each respective ac terminal (16a-16c) and the first dc terminal (18a), and between each respective ac terminal (16a-16c) and the second dc terminal (18b). Each converter arm includes a first power semiconductor switching device (24) capable of being turned 'on' and 'off' by gate control, as a GTO, and having a recovery time. The converter (2a) is adapted to be operated in one or more inverting modes, particularly in an naturally commutated inverting mode, that is, when a second GTO in a brigde arm is switched on, before first, already conducting, GTO in a parallel bridg arm is switched off, when the current through the first GTO has decreasd.