This paper proposed a modulation scheme for the dual-active-bridge (DAB) converter to reduce rms current in wide-range operating conditions. The operating principle of the proposed fundamental duty modulation (FDM) is formulated based on the fundamental component analysis of the DAB converter. By modulating the PWM signals in the fundamental component domain, the optimal operation is implemented with a simple controller structure not requiring an operating mode classification, offline calculation, or current information. Operating characteristics including rms current level and ZVS characteristics are analyzed to compare loss breakdowns of the proposed scheme to those of recent related works. The proposed FDM achieves high efficiency under wide operation conditions due to reduced conduction level and wide ZVS range. Experimental results are obtained under various voltage gain and load conditions to confirm the operation of the proposed modulation scheme. A thorough experimental comparison with other sophisticated modulation schemes has verified the efficiency improvement of FDM.

Fundamental Duty Modulation of Dual-Active-Bridge Converter for Wide-Range Operation

This study presents a comprehensive review of high-power dc-dc converters for high-voltage direct current (HVDC) transmission systems, with emphasis on the most promising topologies from established and emerging dc-dc converters. In addition, it highlights the key challenges of dc-dc converter scalability to HVDC applications, and narrows down the desired features for high-voltage dc-dc converters, considering both device and system perspectives. Attributes and limitations of each dc-dc converter considered in this study are explained in detail and supported by time-domain simulations. It is found that the front-to-front quasi-two-level operated modular multilevel converter, transition arm modular converter and controlled transition bridge converter offer the best solutions for high-voltage dc-dc converters that do not compromise galvanic isolation and prevention of dc fault propagation within the dc network. Apart from dc fault response, the MMC dc auto transformer and the transformerless hybrid cascaded two-level converter offer the most efficient solutions for tapping and dc voltage matching of multi-terminal HVDC networks.

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Review of dc-dc converters for multi-terminal HVDC transmission networks

Market liberalization has significantly changed the energy supply system in Europe, i.e. from a top-down centralized power generation system towards a more decentralized system. In addition, partially due to incentive programs, vast amounts of renewable power generator systems (mostly wind and PV) have been installed. More flexible grid structures are needed to cope with this new landscape of distributed generation. This paper explores the role of state-of-the-art power electronics to realize the required infrastructure. The potential efficiency gains and cost savings that can be realized by using DC-to-DC converters in electronic substations are presented in detail.

Power electronic technologies for flexible DC distribution grids

In this paper, a current-fed modular multilevel dual-active-bridge (CF-MDAB) dc–dc converter is proposed for medium-voltage dc (MVDC) application. The proposed converter inherits favorite characteristics of DAB circuits including soft switching and small passive components. Thereby, high efficiency and high power density can be achieved. Moreover, with direct input and output dc current control, the CF-MDAB is suitable for a breakerless MVDC system since it can realize dc fault ride-though operation. In addition, the dv / dt in the converter is mitigated with the quasi-three-level modulation. In this paper, the proposed converter is applied to integrate the battery energy storage to a MVDC grid as an example to illustrate its operation principles and fault current control capability. The operation principles are presented for both normal and dc fault conditions; the dynamic models are also derived not only under normal operation mode but under dc fault operation mode as well. The control systems under different operation modes are designed, respectively, based on the developed mathematical models. A downscaled 40-kHz 3-kW CF-MDAB prototype was built in the laboratory. The experimental results under both normal condition and dc fault condition verified the analysis as well as the control performance of the proposed converter.

Isolated Modular Multilevel DC–DC Converter With DC Fault Current Control Capability Based on Current-Fed Dual Active Bridge for MVDC Application

This paper investigates the impact of three transformer winding configurations, i.e., the Y–Y, the Y– $\Delta$ , and the $\Delta$ – $\Delta$ configuration, on the performance of a three-phase dual active bridge (DAB) dc–dc converter. For each configuration, equations for the phase currents, power flow, and zero-voltage switching (ZVS) boundaries are derived for all possible switching modes of the three-phase DAB. Thereafter, a comparison is made of the stress on the switches, the transformer, and the filter capacitors for the selected winding configurations. The comparison reveals that the Y–Y and $\Delta$ – $\Delta$ configuration perform equally regarding above aspects, with the only difference of a lower winding current for the $\Delta$ – $\Delta$ configuration. The Y– $\Delta$ configuration shows a constant commutation current for phase-shifts from 0 to 30 degrees. Therefore, this configuration features a wider ZVS region for low output powers. Overall, the Y– $\Delta$ configuration performs best for power levels between 50% and 80%, regarding the stress on the switches, the transformer, and the filter capacitors. The theoretical analysis is supported with measurements obtained from a high-power experimental setup.

Performance Evaluation of a Three-Phase Dual Active Bridge DC–DC Converter With Different Transformer Winding Configurations

In many low-voltage or high-voltage applications, the electrical energy transmission and distribution is enhanced using direct current (dc) already today. To use the advantages of dc also in medium-voltage applications a highly efficient dc-dc converter is needed that has a power capability of several megawatts. At the E.ON Energy Research Center of the RWTH Aachen University a demonstrator for a 5 MW medium-voltage dc-dc converter is constructed, which is presented in this work. Firstly, some general considerations about the three-phase dual-active bridge dc-dc converter are made, with a focus on medium-voltage high-power applications. Whether IGBTs or IGCTs are the optimal switching devices is discussed and a further improvement using a Dual-ICT is evaluated. The construction and the design of the mentioned demonstrator are presented. Besides the demonstrator itself, the medium-voltage high-power transformer that is operated in the ac link is discussed in detail. Finally, the advantages of a three-phase dual-active bridge over the single-phase variant are discussed using the experience gained from the commissioning.

Oscar Apeldoorn

Papers

Development and demonstration of a medium-voltage high-power DC-DC converter for DC distribution systems