A Medium-Voltage Multi-Level DC/DC Converter with High Voltage Transformation Ratio

Renewable energy sources are becoming increasingly important these days. The nuclear power phase-out in Germany and the limited amount of fossil energy resources will lead to further expansion of renewables. This might be directly linked with a change in the distribution and transportation grid. As early as in the 1990's ABB introduced a DC grid vision in a Europe 20xx scenario [1]. DC transmission systems are already used in large offshore wind-park installations and offer beside technical advantages (increased efficiency, smaller transformers) also economic benefits [2]. From industry perspective, ABB recently introduced an onboard DC-grid for marine power and propulsion systems with a grid voltage of 1000 V and up to 20 MW of power [3]. But also DC distribution grids in the mediumvoltage level range have come into the focus of research in the last years [4]. According to [1], DC offers significant loss reduction, less visual impact, lower electromagnetic fields. Furthermore it is the only solution for sub-sea connections longer than 60 km.One key component for the required technology is the DC/DC converter, which is also used in solid state transformers [5]. DC/DC converters for small power and low-voltage levels are well known today. The dual-active bridge converter, consisting of two H-bridge or halfbridge converters, offers bidirectional power flow, galvanic isolation and is a well-established topology [6], [7]. The voltage drop over the effective stray inductance of the transformer, which corresponds to the transferred power, is controlled by the phase-shift between both transformer voltages. An overview of the different galvanically isolated DC/DC converters is given in [8] and [9]. Recent research is investigating the potential to optimize the dual-active H-bridge converter using a transformer zero-voltage level [10]. The zero-voltage level is obtained by switching-on either both upper or both lower switches of the H-bridge simultaneously. The principle of realizing a zero-voltage level by phase-shifting both halfbridges of the H-bridge was already introduced with the single-active bridge, also known as the phase-shifted bridge [11].Medium-voltage high-power realisations are very rare. According to Table 2.2 in [12], there is no demonstrator of a galvanically isolated bidirectional DC/DC converter with a voltage rating of at least 6 kV and a minimum power rating of 100 kW known to the author. Few converter concepts exist with similar or higher ratings, but these are bound to research environments and are still under development.In case the classical H-bridge or half-bridge configuration is used, the maximum DC-link voltage is limited by the blocking voltage of the power electronic switches. To overcome this limitation a series connection of dual-active bridges can be used. Alternatively, a direct series connection of switches is conceivable and requires measures to limit statically and dynamically the maximum voltage across each single switch [4]. For example, balancing resistors and RC-snubbers or intelligent gate driver units are used. But altogether losses are produced continuously. The maximum voltage across the switches can be limited by e.g. clamping diodes or active switches leading to multi-level converter topologies for the dualactive bridge. Due to the current flow through the clamping diodes several sub-operations modes can be distinguished. Even with a single three-level NPC (neutral point clamped converter [13]) or ANPC (active neutral point clamped [14]) half-bridge configuration, natural voltage doubling in one-phase configurations is possible. With every additional voltage-level, the degree of freedom for the operation of the converter can be increased. In literature there are only a few publications with an three-level NPC DC/DC converter, but either with an unidirectional power flow or not using the additional degree of freedom [15]. Higher voltagelevel topologies used in a galvanically isolated