Diss. Stockholm : KTH School of Electrical Engineering, 2015

This thesis describes an effort to investigate the use of gate-commutated thyristors (GCTs) in cascaded converters. Cascaded converters, such as modular multilevel converters (M2Cs) and cascaded H-bridge converters (CHBs), have proved to be especially suitable in high-voltage, high-power applications. All of the most important advantages of cascaded converters, e.g. redundancy and scalability, can be attributed to the modular structure. Of special interest regarding the choice of semiconductor power devices is the reduced requirement on the switching frequency of individual devices. This brings a shift in the trade-off between switching and conduction losses, where the latter has more importance in cascaded converters than in other topologies. This shift favors thyristor-type devices like the GCT, which can achieve very low conduction losses. To quantify the potential gain in the application of GCTs in cascaded converters the losses have been calculated and a comparison between different submodule implementations has been presented. The comparison has shown that GCTs can provide 20-30% lower losses compared to insulated-gate bipolar transistors (IGBTs) in a typical HVDC application. In order to verify the low losses of GCT-based submodules, extensive work has been put into building and testing full-scale submodules employing GCTs. A resonant test circuit has been developed in which the submodules can be tested in steady-state operation which allows calorimetric measurements of the losses. The calorimetric measurements have verified that the loss calculation was reasonable and not lacking any important components. A drawback of GCTs is that the gate-drive units require much more power than gate-drive units for comparable IGBTs. In order to employ GCTs in high-voltage cascaded converters some means of supplying this power in the submodule must be provided. One option is to take this power from the submodule dc-link, but this requires a dc-dc converter capable of high input voltages. A tapped-inductor buck converter with a novel, autonomous high-side valve was developed for this application. The autonomous operation of the high-side valve allows reliable operation without galvanic isolation components. A converter with a high-side valve with series-connected MOSFETs capable of an input voltage of 3 kV has been presented.