High frequency IGCT operation for DC transformer

In spite of the dominance of ac technology for the vast majority of power transmission and distributionsince the late 19th century, the past decades have seen an increase in use of dc electrical power. Inparticular, this has been the case both in high voltage transmission, and in low voltage islanded dcgrids such as shipboard power distributions systems, or dc buildings. The increase in interest fordc electrical power is mainly due to its overall decreased losses compared to its ac counterparts,increased flexibility, and ability to more easily integrate renewable generation and energy storage. Inthis context, medium voltage dc grids currently lack in standardisation and are still an active researchtopic.The dc transformer is expected to be a key technology for the operation of future medium voltagedc systems. Essentially, its function is equivalent to the traditional ac transformer in providingan isolated interface between dc buses at different voltage levels. Yet, unlike the purely passivetraditional transformer, the dc transformer is also expected to integrate additional functionality,particularly regarding system protection. Most embodiments of the dc transformer proposed inacademic publications are based on the dual active bridge, with IGBTs being the semiconductor ofchoice and with galvanic isolation provided by a medium frequency transformer. Compared to thissolution, alternative technologies have been somewhat overlooked, in terms of topologies and devices.In particular, resonant conversion for dc transformer applications has not gained much popularity.The focus of this thesis is on a medium voltage dc transformer employing IGCTs as semiconductordevices, in a bidirectional series resonant LLC topology. The principle behind the selection of thistopology and device is the synergy between the IGCT, which contributes the lowest conduction lossesof any actively controlled semiconductor switch, and the series resonant LLC converter principle ofoperation, which provides low switching loss through soft turn-on and low current turn-off. With thisgoal in mind, a significant technical challenge to be overcome is the increase of switching frequencyof the IGCT well beyond the sub-kHz level at which it traditionally finds application, and into themulti-kHz range, targeted by dc transformer applications.This thesis contains three main contributions aiming to acquire sufficient knowledge for the designof dc transformer lab demonstrator. For this purpose, the boundary between zero-voltage and zerocurrentswitching of the IGCT is initially explored to identify the lowest switching loss conditionthrough the variation of turn-off current value. Then, in these low-loss conditions, thermal steadystate operation of the IGCT is demonstrated at the frequency of 5 kHz for the first time, provingthat the IGCT is a device capable of medium frequency operation. Engineering samples of IGCTsoptimised on the technology curve through varying levels of electron irradiation are also exploredin order to quantify potential benefits in the dc transformer application. Finally, medium frequencyoperation of the device is extended to an increased voltage level through series connection of IGCTs,through custom ultra-low capacitance, purely capacitive snubbers, designed for the challenges of5 kHz operation.Ultimately, the thesis demonstrates that while the IGCT has traditionally found use in sub-kHz,hard-switched applications, its rugge