A method of controlling a power converter system comprising: a power converter comprising a first converter including first and second direct current DC terminals connected to a DC circuit and a plurality of alternating current AC terminals, the first converter including a plurality of semiconductor devices, each semiconductor device including at least a controllable semiconductor switch having a threshold voltage, which is the minimum gate voltage that is required to put the controllable semiconductor switch in an on-state, and a gate voltage for normal on-state conduction during a normal switching operation of the power converter; wherein the method comprises: pre-heating the semiconductor devices by supplying current to the first converter and enabling a short circuit state of the first converter, for a certain period of time, or at least until a temperature that is associated with the power converter exceeds a temperature threshold, by controlling semiconductor switches of the first converter to create at least one short circuit path between the first and second direct current DC terminals through the first converter that carries supplied current; and operating the power converter in a normal switching operation where the controllable semiconductor switches of the first converter are controlled to switch between on- and off-states to covert a DC voltage at the DC terminals to an AC voltage at the AC terminals, or vice versa; wherein at least one of the semiconductor switches in at least one of the short circuit paths is operated with modified on-state conduction in order to increase conduction losses by applying to the at least one of the semiconductor switches a modified gate voltage that is less than the gate voltage for normal on-state conduction, and wherein the modified gate voltage is equal to the threshold voltage plus x volts, where x is in the range of about 100 mV to about 4 V; and characterised in that the DC voltage at the DC terminals of the first converter is controlled to be lower during the pre-heating than during normal switching operation of the power converter. a power converter comprising a first converter including first and second direct current DC terminals connected to a DC circuit and a plurality of alternating current AC terminals, the first converter including a plurality of semiconductor devices, each semiconductor device including at least a controllable semiconductor switch having a threshold voltage, which is the minimum gate voltage that is required to put the controllable semiconductor switch in an on-state, and a gate voltage for normal on-state conduction during a normal switching operation of the power converter; wherein the method comprises: pre-heating the semiconductor devices by supplying current to the first converter and enabling a short circuit state of the first converter, for a certain period of time, or at least until a temperature that is associated with the power converter exceeds a temperature threshold, by controlling semiconductor switches of the first converter to create at least one short circuit path between the first and second direct current DC terminals through the first converter that carries supplied current; and operating the power converter in a normal switching operation where the controllable semiconductor switches of the first converter are controlled to switch between on- and off-states to covert a DC voltage at the DC terminals to an AC voltage at the AC terminals, or vice versa; pre-heating the semiconductor devices by supplying current to the first converter and enabling a short circuit state of the first converter, for a certain period of time, or at least until a temperature that is associated with the power converter exceeds a temperature threshold, by controlling semiconductor switches of the first converter to create at least one short circuit path between the first and second direct current DC terminals through the first converter that carries supplied current; and operating the power converter in a normal switching operation where the controllable semiconductor switches of the first converter are controlled to switch between on- and off-states to covert a DC voltage at the DC terminals to an AC voltage at the AC terminals, or vice versa; wherein at least one of the semiconductor switches in at least one of the short circuit paths is operated with modified on-state conduction in order to increase conduction losses by applying to the at least one of the semiconductor switches a modified gate voltage that is less than the gate voltage for normal on-state conduction, and wherein the modified gate voltage is equal to the threshold voltage plus x volts, where x is in the range of about 100 mV to about 4 V; and characterised in that the DC voltage at the DC terminals of the first converter is controlled to be lower during the pre-heating than during normal switching operation of the power converter.