Search

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 levels of electron irradiation are also explored in order to quantify potential benefits in the dc transformer application. Finally, medium frequency operation 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 of 5 kHz operation. Ultimately, the thesis demonstrates that while the IGCT has traditionally found use in sub-kHz, hard-switched applications, its rugge

The need to increase efficiency and reduce operating cost of shipboard DC power distribution networks results in a desire to increase the operating voltage of such installations to the medium voltage levels. Devices such as solid state bus-tie switches are an essential component of such systems, as they allow for system reconfigurations and prevent fault propagation across different sections of the system. This paper presents a scalable solid state bus-tie switch topology designed initially for low voltage power distribution networks, but easily scalable in terms of voltage and current. The use of a single active device greatly increases simplicity and reliability of the solution, and the ability to extend the current and voltage ratings has been verified on the prototypes through extensive experimental tests in parallel and series connection, respectively. Several methods for fault detection are discussed and further verified experimentally.

Recent trends in shipboard power distribution network design, such as the transition from AC to DC result in new challenges on the issue of distribution network protection. Devices such as a solid state bus tie switch provide a first line of defence against propagation of low impedance faults across the distribution system by quickly separating the faulty portion of the network and preventing additional energy to be fed into the fault. This paper proposes a new topology of solid state bustie switch for shipboard power distribution networks. The topology is thoroughly described considering its operating principles, and a small scale prototype has been used to verify its operational performances. Current interruption capabilities are demonstrated, both in open loop and closed loop operation, with fault detection and decision algorithms implemented on a standalone controller.

DC power distribution systems in marine applications are in the process of being established as the preferred solution for shipboard power distribution systems. This paper presents a bus-tie switch topology for the protection of such systems. The device is based on a four quadrant switch with single active semiconductor and is designed to be modular and scalable, so it can be adapted to different interruption current and bus voltage levels. The developed prototype has been experimentally validated for its thermal behaviour in continuous conduction and switching performances, as well as in standalone operation with digital controller added to the power stage. The simplicity of the device topology offers interesting prospects for DC power distribution systems.

The tutorial provides an overview of relevant technologies and concepts for high power DC-DC conversion at MW level, as well as challenges and opportunities in that field. Topics are covered with sufficient level of details to allow for understanding of design trade-offs associated with these conversion systems, expected performances and impact of various technologies on overall solution.

Series connected IGCTs find application in hard-switched commercial power converters at the MW power levels. Expanding the application of these devices to medium voltage dc-dc resonant converters is of great interest due to the IGCTs low conduction losses. This requires a deeper understanding of the devices behaviour in series connection and under switching conditions different than those in hard switched converters. In particular, the voltage sharing between series connected IGCTs must be achieved with significantly reduced turn-off currents, typical for resonant converters. Relying on a versatile IGCT test setup, this paper explores the operational performances of series connected IGCT operation at very low turn-off current and with snubber capacitance in the range of a few tens of nF. The results presented in the paper are obtained from commercially available 4:5 kV 68mm reverse conducting IGCTs (with standard irradiation), as well as two customized engineering samples (high irradiated variants), optimized on their technology curve for high switching frequencies. The IGCTs are successfully operated in series connection at 5 kHz, with device voltage being effectively shared in resonant converter operation.