With the increasing penetration of renewable energy sources and ubiquitous power electronics apparatus, DC power is making a comeback. In the generation aspect, the PV, wind, and energy storage are inherently DC sources. For transmission, the HVDC has become the feasible solution for transmitting large amounts of power over long distances or submarine cables. For distribution, the MVDC is more capable of accommodating a higher level of renewable energy and friendly to the EV charging stations. Therefore, DC grid is highly expected in both academics and industries.

Nevertheless, just like the role of transformer in AC systems, DC grids also require such a device to exchange power between networks with different voltage levels. As DC circuit does not satisfy the law of Electromagnetic Induction, it is not possible to use magnetic transformers to convert DC voltage but has to rely on power electronics technology. Additionally, with the growing of DC system scale, power flow control in meshed DC grid becomes important. DC current flow is determined by line voltage, thus DC power flow controller should also be designed as a DC voltage converter. Although the DC/DC power-electronic converters have been widely studied and applied at low-power applications and a myriad of topologies exist, most of these topologies are not readily scaled up to tens/hundreds of kilovolts and megawatt power ranges, due to the limitations of loss, cost, dv/dt, and ratings of the semiconductors. To overcome these limitations, several novel high-power DC/DC converter topologies have been proposed and demonstrated during the last few years. This tutorial will give a systematic review of the latest development in this filed.

The tutorial will start with an introduction of the applications for high-power DC/DC converters, which is followed by a review of the basics and some widely used converter solutions in this filed. Then the first theme is discussion of DC/DC converters for interconnecting LVDC and MVDC systems, including a ±10kV/±375V 1.5MW solid-state transformer, bipolar/multi-port LVDC distribution, and PV/wind DC collection. The second theme is DC/DC converters for MVDC and HVDC interconnection, which presents a novel converter solution combining the techniques of IGCT-based two-level converter and the MMC, and the converter is further improved for application of all DC offshore wind farms. Furthermore, the third theme will focus on the DC/DC conversion which interconnects HVDC systems with different voltage levels. Particularly, the capacitive energy transfer (CET) principle based DC/DC converters are discussed, which includes a series of new topologies, showing very attractive features of low cost, high efficiency, small footprint, and DC fault-blocking capability. The fourth theme will discuss DC power flow controller, showing its importance to HVDC grid. Finally, summary and outlook are provided. In this tutorial, all these converters and corresponding operation techniques will be explained with simulation or experimental examples.