A semiconductor switching string including a series-connected switching assemblies. Each assembly has a main switching element including first and second connection terminals which current flows between when the main switching element is on. The main element has an auxiliary element between the connection terminals. The string includes a local control unit connected with each auxiliary element which are programmed to switch an auxiliary element to create an alternative current path between the connection terminals that diverts current through to reduce the voltage across the corresponding main switching element. The local unit is programmed to control switching an auxiliary element to a fully-on mode in which the auxiliary element is at maximum rated base current, a pulsed mode which turns the auxiliary element on and off and/or an active mode operating the auxiliary element with a continuously variable base current. The string includes a higher level control unit programmed to implement the modes.