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Dual-active-bridge (DAB) converters have been widely recognized as one promising topology for high-frequency-link power conversion applications. However, one critical issue concerning DAB converters is the transient dc-bias current due to the voltage-second imbalance, which may occur frequently in typical dynamic operations of DAB converters such as the start-up, sudden load or output voltage change, and power flow direction change. Due to the high-frequency operation, the dc-bias current of DAB converters will impose more harm to magnetic components because of the limited range of operating flux density and possible magnetic saturation compared with the counterpart low-frequency dc-bias current. Over the past several years, extensive researches have been conducted to address the high-frequency transient dc-bias current of DAB converters under different phase-shift modulation strategies such as single phase shift, extended phase shift, and triple phase shift. In this article, a comparative review of typical dc-bias current mitigation strategies under typical phase-shift strategies for DAB converters is presented. An experimental evaluation of selected strategies under various scenarios is conducted in terms of the settling time, inductor current stress, efficiency, switching number, and implementation complexity. This article ends up with an overview of high-frequency dc-bias mitigation strategies and a discussion for the further optimization. [ABSTRACT FROM AUTHOR]

The dc power transmission system has been obviously attracting more research interests in recent years. In order to satisfy the high-reliability requirement of the dc power transmission system, the power converter should be able to keep uninterrupted operation after multiple unexpected faults. However, high fault-tolerant capability usually leads to the bulky redundant circuit, which increases the cost, volume, and complexity of the power converter. Thus, a fault-tolerant dual active bridge (DAB) converter is proposed to maintain the power transferring ability under multiple unexpected open-circuit faults (OCFs) conditions, which can significantly enhance the reliability of the dc power transmission system. By reconfiguring the central-tapped transformer and two symmetrical auxiliary inductors, the half-bridge conduction branch is built to maintain uninterrupted operation when a single or dual OCF has occurred. The proposed fault-tolerant strategy only requires four extra voltage sensors to detect and locate OCFs for the reconfiguration process. Thus, it significantly improves the system reliability with a low additional cost. Besides, the inductor current, transmission power, and the small-signal models of the proposed fault-tolerant converter have been presented. It proves the proposed fault-tolerant topology can smoothly switch between normal and postfault operation due to the constituency of the inductor current. Finally, the 250-W prototype is designed to verify the advantage of the proposed fault-tolerant DAB converter. [ABSTRACT FROM AUTHOR]

The derivation and implementation of multiple-phase-shift-based switching surfaces for a dual active bridge (DAB) converter is the main focus of this article. First, the mathematical models of multiple natural switching surfaces under different operation states of DAB converters are derived, which lays the foundation to achieve a fast transient response during startup, sudden voltage reference, and load changing conditions. Moreover, in order to improve the overall performance of DAB converters systematically, a minimum-current-stress boundary control (MBC) is proposed that can reduce the inductor peak current stress and achieve fast dynamic response simultaneously by using the multiple-phase-shift-based switching surfaces. The analytical derivation of the proposed MBC is presented together with the simulation and experimental evaluations, which shows the superior performance of the proposed MBC algorithm in terms of the efficiency and dynamic response improvement under various operating conditions. [ABSTRACT FROM AUTHOR]

A transient dc-bias current due to the voltage-second imbalance of isolated bidirectional dual-active-bridge (DAB) converters for the disturbance in line or load may result in the transformer saturation and oscillations in both sides dc currents. This article focuses on the transient dc-bias current elimination by using an improved triple-phase-shift (ITPS) control for DAB converters. The inductor peak current stress optimization is adopted in the proposed ITPS to determine the steady-state phase-shift variables. Originated from the dc-bias current model of DAB converters with the TPS control, the transient phase-shift adjustment strategy can be determined, which has the ability to improve the inductor current changing slope and shorten the settling time. Both simulation and experiments for different conditions are provided to evaluate main dynamic indexes such as the transient period, dc-bias current, and inductor current stress for three different transition cases. The proposed ITPS is proved as a promising solution in eliminating the dc-bias current, minimizing the transient current stress. [ABSTRACT FROM AUTHOR]

As the “brain” of the modern photovoltaic power systems, dc solid-state transformers (SSTs) are playing an increasing important role in integrating different photovoltaic modules, batteries, and supercapacitors. This paper aims at improving the conversion efficiency of dc SST for a wide operating range by minimizing the backflow power of the dc SST system. First, the mathematical model of the backflow power from both input and output sides of dc SST is obtained. Then, the penalty function is derived and an enhanced triple phase shift control (ETPS) strategy is proposed to minimize the backflow power for various transmission power and voltage conversion conditions. A comprehensive comparison of the proposed ETPS with other advanced control strategies was carried out through theoretical analysis and experimental tests. Main experimental results are illustrated to verify the effectiveness of the proposed method. [ABSTRACT FROM AUTHOR]

This paper deals with an optimized three-level modulated phase shift control using particle swarm optimization (PSO) strategy based on the unified phasor analysis with an aim to improve the efficiency of the bidirectional dual active bridge (DAB) converter for the whole operation range. A unified mathematical model based on Fourier transform is built for the DAB converter. All possible operation states under the three-level modulated phase shift control are covered. Accurate complex mathematical expressions for the inductor current, the transmission power, and the reactive power are obtained. Both modulus and angle variables are illustrated with respect to the inner and outer phase shift angle with the phasor diagram. The proposed method is able to achieve the minimum reactive power under three-level modulated phase shift control by obtaining the optimal phase-shift angles directly. The cumbersome process of the optimal operation mode selection for different voltage conversion ratio and load conditions in conventional methods is overcome successfully, thus greatly simplifying the theoretical calculation and implementation difficulty. Simulation and experimental results in terms of the reactive power, soft-switching range, and efficiency are provided to verify the practical feasibility of the proposed method for the bidirectional DAB converters. [ABSTRACT FROM AUTHOR]

As key component for the flexible dc distributed power system, the dual active bridge (DAB)-converter-based solid-state transformer (SST) with high efficiency for a wide operating range is essential. However, with the traditional phase-shift control, high backflow power and current stress will significantly affect the conversion efficiency. In this paper, the backflow power characteristics in both sides of DAB-based SST converters are comprehensively analyzed. On this basis, complete transmission power, backflow power, and peak current mathematical models are established. Then, a minimum-backflow-power-based extended-phase-shift control strategy is proposed with the determination of optimal phase-shift pairs by using the Karush–Kuhn–Tucker function for various scenarios. The backflow power and current stress curves with different algorithms are compared. It shows the proposed control can improve the output power regulation flexibility, minimize the backflow power, and improve the efficiency in wide operating range. Finally, a DAB-based SST prototype was developed and the experimental results verified the effectiveness of the proposed control strategy. [ABSTRACT FROM AUTHOR]

This paper presents a novel reactive-power minimization method under the three-level modulated phase-shift control to improve the efficiency for a wide operation condition. First, in order to unify all operation modes, a mathematic model of the dual-active-bridge converter with the three-level modulated phase-shift control by a harmonic analysis method is obtained. Then, the detailed analysis of the odd-order harmonic components of the active and reactive powers varied with the multiple control dimensions is presented. The soft-switching boundaries with respect to the voltage ratio and unified active power are specified. On this basis, a novel optimal three-level phase-shift (OPS) control strategy for minimizing reactive power is proposed. Experimental results of reactive power and operation efficiency among the proposed OPS method, and other methods are compared to verify the effectiveness of the OPS algorithm. [ABSTRACT FROM PUBLISHER]

Originated from analyzing nonactive power loss, a novel optimization method and modulation solution for bidirectional isolated dual-active-bridge (DAB) dc–dc converters are proposed in order to achieve high efficiency in a wide operating range. A comprehensive nonactive power loss model is developed, including both the nonactive components delivered back to the source and from the load. This paper points out that when the minimum nonactive power loss is achieved, zero-voltage soft switching can be naturally fulfilled. The optimal phase-shift pair obtained by the proposed method can keep low values of both root mean square (RMS) current and circulating power. Rather than using ideal power flow analysis, the nonactive power loss model directly embodies practical nonideal factors, including device voltage drops. Based on the analysis, an extended dual phase shift is proposed, and different operation cases are analyzed with comparison of performance indices. Experimental tests verify the theoretical analysis and show effectiveness of the proposed approach to achieve nonactive power loss minimization and efficiency improvement. [ABSTRACT FROM PUBLISHER]

A current-fed high-frequency alternating-current (HFAC) distributed power system is presented in this paper for medium- or high-voltage gate driving applications. The distributed gate driving modules (GDMs) at the high-voltage side are isolated from the low-voltage-side current-source power supply through magnetic coupling of current transformers (CTs), which are assembled by a high-voltage cable carrying HFAC and passing through the center of toroid magnetic cores. The centralized high-frequency current-source power supply is designed and composed of a two-stage resonant inverter and a double \Gamma-CL resonant network. The resonant network is parameterized by considering the tradeoff between the quality factor and component stress. Different magnetic core samples are tested and compared to construct the CTs for improving transmission efficiency. A 400-W prototype was built and experimentally evaluated with two load conditions, including resistive loads and practical GDMs. It showed significant improvements with regard to sinusoidal current output, system stability, and efficiency. Finally, the proposed current-fed HFAC power supply system was successfully applied to a practical solid-state fault current limiter, demonstrating the advantages of multiple outputs ( n > 10), high-voltage isolation, low harmonic distortion, high efficiency, easy installation, and flexible extensibility. [ABSTRACT FROM PUBLISHER]