Semiconductor switching string

In the field of power converters, such as high voltage direct current (HVDC) power converters, a semiconductor switching string (10) comprises a plurality of series-connected semiconductor switching assemblies (12). Each semiconductor switching assembly (12) has a main semiconductor switching element (14) that includes first and second connection terminals (16, 18) between which current flows from the first terminal (16) to the second terminal (18) when the main semiconductor switching element (14) is switched on. The main semiconductor switching element (14) has an auxiliary semiconductor switching element (32) electrically connected between the first and second connection terminals (16, 18) thereof. The semiconductor switching string (10) also includes a local control unit (34) which is operatively connected with each auxiliary semiconductor switching element (32). The or each local control unit (34) is programmed to control switching of a respective auxiliary semiconductor switching element (32) to selectively create an alternative current path (36) between the first and second connection terminals (16, 18) associated therewith whereby current is diverted to flow through the alternative current path (36) to reduce the voltage across the corresponding main semiconductor switching element (14). The or each local control unit (34) is so programmed to selectively control switching of a respective auxiliary semiconductor switching element (32) in a fully-on mode in which the auxiliary semiconductor switching element (32) is operated with its maximum rated base current or gate voltage, and one or both of a pulsed switched mode in which the auxiliary semiconductor switching element (32) is turned on and off and an active mode in which the auxiliary semiconductor switching element (32) is operated with a continuously variable base current or gate voltage. The semiconductor switching string (10) still further includes a higher level control unit (52) that is arranged in communication with the or each local control unit (34) and is programmed to selectively implement: (i) a first model based control methodology to collectively operate via the or each local control unit (34) each auxiliary semiconductor switching element (32) in the fully-on mode; and (ii) a second active control based control methodology to selectively and collectively operate via the or each local control unit (34) each auxiliary semiconductor switching element (32) in one or both of the pulsed switched mode and the active mode.