This thesis describes an effort to investigate the use of gate-commutated thyristors(GCTs) in cascaded converters. Cascaded converters, such as modularmultilevel converters (M2Cs) and cascaded H-bridge converters (CHBs), haveproved to be especially suitable in high-voltage, high-power applications. Allof the most important advantages of cascaded converters, e.g. redundancy andscalability, can be attributed to the modular structure. Of special interestregarding the choice of semiconductor power devices is the reduced requirementon the switching frequency of individual devices. This brings a shift in thetrade-off between switching and conduction losses, where the latter has moreimportance in cascaded converters than in other topologies. This shift favorsthyristor-type devices like the GCT, which can achieve very low conductionlosses.To quantify the potential gain in the application of GCTs in cascadedconverters the losses have been calculated and a comparison between differentsubmodule implementations has been presented. The comparison has shownthat GCTs can provide 20-30% lower losses compared to insulated-gate bipolartransistors (IGBTs) in a typical HVDC application. In order to verify the lowlosses of GCT-based submodules, extensive work has been put into buildingand testing full-scale submodules employing GCTs. A resonant test circuithas been developed in which the submodules can be tested in steady-stateoperation which allows calorimetric measurements of the losses. The calorimetricmeasurements have verified that the loss calculation was reasonableand not lacking any important components.A drawback of GCTs is that the gate-drive units require much more powerthan gate-drive units for comparable IGBTs. In order to employ GCTs inhigh-voltage cascaded converters some means of supplying this power in thesubmodule must be provided. One option is to take this power from thesubmodule dc-link, but this requires a dc-dc converter capable of high inputvoltages. A tapped-inductor buck converter with a novel, autonomous highsidevalve was developed for this application. The autonomous operation of thehigh-side valve allows reliable operation without galvanic isolation components.A converter with a high-side valve with series-connected MOSFETs capable ofan input voltage of 3 kV has been presented.