A semiconductor switching string, for use in a HVDC power converter, comprising: a plurality of series-connected semiconductor switching assemblies, each semiconductor switching assembly having a main semiconductor switching element, each main semiconductor switching element being configured to, when switched on, operate in a conducting mode in which a conducting current (i c) flows from a first terminal thereof to a second terminal thereof, each main semiconductor switching element being configured to, while turning off, transition from a reverse recovery mode in which a reverse recovery current (i rr) flows from the second terminal to the first terminal to a blocking mode in which no current flows between the first and second terminals (20, 24), each main semiconductor switching element having an active auxiliary circuit electrically connected between the first and second terminals (20, 24) thereof, and each active auxiliary circuit including an auxiliary semiconductor switching element and a resistive element connected in series with one another; and one or more control units operatively connected with each auxiliary semiconductor switching element, the or each control unit being configured to switch the respective auxiliary semiconductor switching element into a conducting state to divert current through the corresponding resistive element, and the or each control unit being further configured to switch on all of the auxiliary semiconductor switching elements of the semiconductor switching string in their conducting state simultaneously at least while each main semiconductor switching element is operating in one of its reverse recovery mode or its blocking mode, whereby when a given main semiconductor switching element transitions to operating in its blocking mode the voltage (v, v', v") thereacross is determined solely by the voltage across the resistive element in the corresponding active auxiliary circuit electrically connected between the first and second terminals (20, 24) of the said given main semiconductor switching element. a plurality of series-connected semiconductor switching assemblies, each semiconductor switching assembly having a main semiconductor switching element, each main semiconductor switching element being configured to, when switched on, operate in a conducting mode in which a conducting current (i c) flows from a first terminal thereof to a second terminal thereof, each main semiconductor switching element being configured to, while turning off, transition from a reverse recovery mode in which a reverse recovery current (i rr) flows from the second terminal to the first terminal to a blocking mode in which no current flows between the first and second terminals (20, 24), each main semiconductor switching element having an active auxiliary circuit electrically connected between the first and second terminals (20, 24) thereof, and each active auxiliary circuit including an auxiliary semiconductor switching element and a resistive element connected in series with one another; and one or more control units operatively connected with each auxiliary semiconductor switching element, the or each control unit being configured to switch the respective auxiliary semiconductor switching element into a conducting state to divert current through the corresponding resistive element, and the or each control unit being further configured to switch on all of the auxiliary semiconductor switching elements of the semiconductor switching string in their conducting state simultaneously at least while each main semiconductor switching element is operating in one of its reverse recovery mode or its blocking mode, whereby when a given main semiconductor switching element transitions to operating in its blocking mode the voltage (v, v', v") thereacross is determined solely by the voltage across the resistive element in the corresponding active auxiliary circuit electrically connected between the first and second terminals (20, 24) of the said given main semiconductor switching element.