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 dual-active bridge configuration could not be found [16]. They are named multi-level DC/DC converters.

A structured analysis of the entire load and operation range of the three-level NPC / two-level one-phase dual-active bridge or of a multi-level DC/DC converter, including all analytical formulae to design and optimize the converter is not known to the author. Multilevel DC/DC converters are investigated in this thesis exemplarily for the three-level NPC / two-level one-phase dual-active bridge DC/DC converter. The derived analytical approach can be applied analogously to more-level DC/DC converter topologies. In this thesis only topologies with clamping diodes are considered. Other multi-level topologies like flying capacitor converters can be used likewise, but they are not considered in this dissertation.

Chapter 2 includes definitions for the comparison of different topologies and formulae for the optimization of the converters. This includes power definitions as well as the derivation of formulae adapted to multi-level DC/DC converters to compare the core losses or transformer utilization of different topologies. Design aspects of the transformer are covered as far as they concern multi-level DC/DC converters. The design of the transformer in general is not part of this chapter due to sufficiently published literature on this topic.

In Chapter 3 the three-level NPC / two-level one-phase dual-active bridge DC/DC converter as an example of a multi-level DC/DC converter is introduced. In contrast to the well-known dual-active bridge with H-bridge or half-bridge converters, a three-level neutral point clamped converter on the high-voltage side replaces the H-bridge. A structured analysis of all main operation modes with an exhaustive analytical description with all it’s formulae is presented. Two operation modes are identified to operate this topology over the full load range.

These two main operation modes are divided into sub-operation modes in Chapter 4. Each sub-operation mode is distinguished by the transformer current at the switching instants. Furthermore, each sub-operation mode is characterized by the power electronic switching behaviour: hard-switched, zero-voltage turn-on switched or near zero-current switched. Formulae to operate the converter in a selected sub-operation mode are given. From these equations the dependence and limitations on the transferred power and voltage transformation ratio can be extracted. This chapter provides the mathematical basis to use the whole optimization potential of the converter.

In Chapter 5 a control structure for multi-level DC/DC converters is proposed. It consists of a feed-forward and closed-loop control. The feed-forward linearises the converter’s behaviour for the control and allows a fast reaction to new reference values. The PI closed-loop controller levels out losses and other errors not covered by the analytical equations. Further topics are the gate signal generation and the steady-state start of the converter to avoid a possible saturation of the transformer core at the PWM start. At the end of this chapter, necessary information will be presented to design and realize a multi-level DC/DC converter in hardware.

Chapter 6 describes a hardware realization of a 3.3 kV. 6 kV, 100 kW galvanically isolated three-level NPC / two-level one-phase dual-active bridge DC/DC converter, which seems to be unique, at least in the university environment [12]. This DC/DC converter is part of a built battery energy storage system and test facility for the characterization of batteries in the kV range. Besides practical design aspects as, for example, the construction of a low-inductance two-layer busbar for the three-level NPC converter, a comparison of measurement and simulation results is given, showing good concordance of the simulation and the analytical description with the real system. The chapter closes with measurement results inside the battery energy storage system, depicting the excellent controllability and sensor resolution of the converter.

Chapter 7 shows the application of the derived analytical approach to a five-level NPC / two-level one-phase dual-active bridge for the first operation mode. An extension to threephases of the three-level NPC / two-level one-phase dual-active bridge is presented, before the N-level NPC / N-level NPC multi-phase dual-active bridge converter is introduced.

The thesis closes with a summary in Chapter 8. This chapter discusses possible future work, for example the influence of the transformer configuration in multi-phase topologies or the comparison between multi-level DC/DC converters and the series-connection of dualactive bridges with H-bridge converters.