Design of a DC/AC-converter based on wide bandgap transistors for Micro-Grid-applications.

Single-phase, utility interfaced, isolated AC-DC converters with power factor correction cover a wide range of applications such as chargers for plug-in hybrid electric vehicles and battery electric vehicles, inverters for multiple renewable energy sources (e.g. photovoltaic modules), as well as interfaces for residential DC distribution systems and energy storage systems. Thereby, bidirectional conversion capability enables the development of smart interactive power networks in which the energy systems play an active role in providing different types of support to the grid. Examples are vehicle-to-grid concepts, smart home concepts, AC microgrids, and residential DC distribution systems (DC nanogrids). 

In the presented work, the main objective is to investigate the feasibility and suitability of a single-stage (1-S) dual active bridge (DAB) AC-DC converter for the realization of the above mentioned bidirectional energy conversions. Compared to the commonly used dual-stage (2-S) systems, the 1-S architecture has the potential to benefit the system performance with regard to efficiency, volume (power density), number of components (reliability), weight, and costs, due to the effective omission of a complete energy conversion stage. In order to validate the presented analyses, a second objective is to realize a state-of-the-art (i.e. regarding efficiency and power density) converter prototype system that is designed in order to meet the requirements for future, mode 1 compatible, on-board electric vehicle battery chargers, interfacing a 400 V DC-bus with the single-phase 230 VAC / 50 Hz mains. Compliance with domestic power sockets results in a nominal (active) AC charging current of 16 Arms and a nominal power of 3.7 kW. 

The main challenge to achieve the above objectives lies in addressing the fundamental limitations of the existing analyses and circuit implementations of DAB converters. These limitations mainly relate to the soft-switching (i.e. by virtue of zero voltage switching, ZVS) modulation schemes available in literature, being especially problematic for DAB converters with large input and/or output voltage variations and large power variations, such as is the case for the 1-S DAB AC-DC architecture at hand. By means of an introductory Chapter (i.e. Chapter 2), the shortcomings in the existing analyses of DAB converters are highlighted, and the selection of the full bridge - full bridge (FBFB) DAB implementation as the most suitable candidate for the considered AC-DC converter topology is motivated. The subsequent chapters discuss the 1-S DAB AC-DC converter in detail:

• Chapter 3 outlines the operating principle of the DAB AC-DC converter. The exact operating range of the DAB DC-DC converter, as the main building block of the 1-S AC-DC architecture, is derived, and a control equation for the DAB input current is obtained. Furthermore, the steady-state analysis of the DAB is presented and commutation inductance(s) are introduced as an essential HF AC-link modification in order to achieve full-operating-range ZVS. Lastly, a novel current-dependent charge-based (CDCB) ZVS verification method is proposed in order to deal with the deficiencies of the existing current-based (CB) and energy-based (EB) ZVS analyses;
• Chapter 4 is devoted to the derivation of full-operating-range ZVS modulation schemes for the DAB converter. Three different approaches are presented, being a numerical approach, an analytical approach, and a semi-analytical approach, all relying on the CDCB ZVS verification method proposed in Chapter 3 in order to assure that soft-switching operation with quasi zero switching losses is obtained within the calculated ZVS regions;
• In Chapter 5, the main functional elements of the DAB AC-DC prototype converter are designed, employing the values for the circuit level variables and the ZVS modulation schemes derived in Chapter 4. State-of-the-art design methods/procedures, models for the component losses, and volume models are combined with custom developed (local) optimization algorithms in order to obtain a high-efficiency and high-power-density converter design that is in compliance with the specified system requirements;
• In Chapter 6, first a DC-DC system characterization of the prototype system is presented in order to validate the theoretical analyses, i.e. the steady-state converter model and the ZVS analysis outlined in Chapter 3, as well as the CDCB ZVS modulation schemes proposed in Chapter 4. Thereafter, the results of an AC-DC system characterization are given, allowing to evaluate the performance of the prototype converter with regard to the reached efficiency and with regard to the quality of the AC input power. Conversion efficiencies higher than 95 % within the major part of the output power range, with a very flat efficiency curve and thus a high partial-load efficiency, are reported. The peak efficiency is around 96 % and the efficiency at nominal power approximately 95.6 %. Moreover, a high power density of 2 kW/liter is obtained. From a brief comparison with several (similar) dual-stage prototype systems found in literature, it is clear that the achieved performance is close to the absolute state-of-the-art;
• Chapter 7 concludes the presented work and provides an outlook regarding future research in the field of DAB AC-DC converters.

Keywords:Convertors, Wide bandgap, DC/AC-converter, MicroGrid

Disciplines:Other engineering and technology

Project type:PhD project