A power transmission system for high-voltage direct current comprises a rectifier station (1) and an inverter station (2), which are interconnected by a dc connection (4), as well as a third converter station (3) connected between the dc connection and an ac network (3c). The third converter station comprises a converter (3a) connected to the dc connection via a direct-voltage converter (5). The direct-voltage converter comprises at least a first and a second converter stage (8:1, 8:2), each with a positive terminal (TP:1, TP:2), a negative terminal (TM:1, TM:2) and an output terminal (TO:1, TO:2) and comprising a voltage-source converter circuit (V, C1, SL) with at least one extinguishable semiconductor valve (T1, T2). The positive terminal of the second converter stage is connected to the output terminal of the first converter stage and the output terminal of the second converter stage is connected to the negative terminal of the first converter stage.