High frequency IGCT operation for DC transformer

In spite of the dominance of ac technology for the vast majority of power transmission and distribution since the late 19th century, the past decades have seen an increase in use of dc electrical power. In particular, this has been the case both in high voltage transmission, and in low voltage islanded dc grids such as shipboard power distributions systems, or dc buildings. The increase in interest for dc 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. In this context, medium voltage dc grids currently lack in standardisation and are still an active research topic. The dc transformer is expected to be a key technology for the operation of future medium voltage dc systems. Essentially, its function is equivalent to the traditional ac transformer in providing an isolated interface between dc buses at different voltage levels. Yet, unlike the purely passive traditional transformer, the dc transformer is also expected to integrate additional functionality, particularly regarding system protection. Most embodiments of the dc transformer proposed in academic publications are based on the dual active bridge, with IGBTs being the semiconductor of choice and with galvanic isolation provided by a medium frequency transformer. Compared to this solution, 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 semiconductor devices, in a bidirectional series resonant LLC topology. The principle behind the selection of this topology and device is the synergy between the IGCT, which contributes the lowest conduction losses of any actively controlled semiconductor switch, and the series resonant LLC converter principle of operation, which provides low switching loss through soft turn-on and low current turn-off. With this goal in mind, a significant technical challenge to be overcome is the increase of switching frequency of the IGCT well beyond the sub-kHz level at which it traditionally finds application, and into the multi-kHz range, targeted by dc transformer applications. This thesis contains three main contributions aiming to acquire sufficient knowledge for the design of dc transformer lab demonstrator. For this purpose, the boundary between zero-voltage and zerocurrent switching of the IGCT is initially explored to identify the lowest switching loss condition through the variation of turn-off current value. Then, in these low-loss conditions, thermal steady state operation of the IGCT is demonstrated at the frequency of 5 kHz for the first time, proving that the IGCT is a device capable of medium frequency operation. Engineering samples of IGCTs optimised on the technology curve through varying