the current source inverter (csi) is widely used in medium voltage drives due to its simple configuration, reliable short-circuit protection, and low dv/dt issue. conventional series-connected current source inverters (sc-csi) are constituted of several current source inverters (csis) connected in series through multi-winding transformers to enhance the power capacity. it is very promising in high-voltage applications. switching losses of the semiconductor switches significantly impact the power efficiency of sc-csis. therefore, inverter design must consider these losses, which depend on factors, such as switching frequency, modulation schemes and semiconductor parameters. this thesis aims to reduce the switching losses of conventional sc-csis by decreasing the switching frequency from 540 hz to 360 hz and eventually to 60 hz. in chapter 2, the switching frequency is reduced to 360 hz for the switching loss reduction. the active damping control is implemented to mitigate the increase in filter capacitance caused by the lower switching frequency. chapter 3 is to further reduce the switching losses by reducing the switching frequency to 60 hz. however, this strategy results in a significant increase in the passive components. to address this issue, the multi-winding transformer is replaced with a phase-shifting transformer, as the active damping method applied in chapter 2 is no longer effective at this reduced frequency. the switching losses and the passive components of sc-csis are investigated with switching frequencies of 540hz, 360hz and 60 hz. simulations are conducted using matlab/simulink to verify the investigation.