A semiconductor switching string, for use in a HVDC power converter, comprising: a plurality of series-connected semiconductor switching assemblies (10; 70; 110), each semiconductor switching assembly (10; 70; 110) having a main semiconductor switching element including first and second connection terminals (12, 14) between which current flows from the first terminal to the second terminal when the main semiconductor switching element is switched on, the main semiconductor switching element having an auxiliary semiconductor switching element (28; 72; 112) electrically connected between the first and second connection terminals thereof (14, 16); and a control unit operatively connected with each auxiliary semiconductor switching element (28; 72; 112), the or each control unit being configured to switch on a respective auxiliary semiconductor switching element (28; 72; 112) to selectively create an alternative current path (32; 114, 116) between the first and second connection terminals (14, 16) associated therewith whereby current is diverted to flow through the alternative current path (32; 114, 116) to reduce the voltage across the corresponding main semiconductor switching element, the or each control unit being further configured to switch on the said respective auxiliary semiconductor switching element (28; 72; 112) when the voltage across the corresponding main semiconductor switching element differs from a real-time varying voltage reference equivalent to the average voltage across all of the main semiconductor switching elements in the semiconductor switching string, and the or each control unit being still further configured to control the amount of current diverted to flow through a respective alternative current path (32; 114, 116) so that the voltage across the corresponding main semiconductor switching element approaches said real-time varying voltage reference. a plurality of series-connected semiconductor switching assemblies (10; 70; 110), each semiconductor switching assembly (10; 70; 110) having a main semiconductor switching element including first and second connection terminals (12, 14) between which current flows from the first terminal to the second terminal when the main semiconductor switching element is switched on, the main semiconductor switching element having an auxiliary semiconductor switching element (28; 72; 112) electrically connected between the first and second connection terminals thereof (14, 16); and a control unit operatively connected with each auxiliary semiconductor switching element (28; 72; 112), the or each control unit being configured to switch on a respective auxiliary semiconductor switching element (28; 72; 112) to selectively create an alternative current path (32; 114, 116) between the first and second connection terminals (14, 16) associated therewith whereby current is diverted to flow through the alternative current path (32; 114, 116) to reduce the voltage across the corresponding main semiconductor switching element, the or each control unit being further configured to switch on the said respective auxiliary semiconductor switching element (28; 72; 112) when the voltage across the corresponding main semiconductor switching element differs from a real-time varying voltage reference equivalent to the average voltage across all of the main semiconductor switching elements in the semiconductor switching string, and the or each control unit being still further configured to control the amount of current diverted to flow through a respective alternative current path (32; 114, 116) so that the voltage across the corresponding main semiconductor switching element approaches said real-time varying voltage reference.