High-frequency-link (HFL) power conversion systems (PCSs) are attracting more and more attentions in academia and industry for high power density, reduced weight, and low noise without compromising efficiency, cost, and reliability. In HFL PCSs, dual-active-bridge (DAB) isolated bidirectional dc-dc converter (IBDC) serves as the core circuit. This paper gives an overview of DAB-IBDC for HFL PCSs. First, the research necessity and development history are introduced. Second, the research subjects about basic characterization, control strategy, soft-switching solution and variant, as well as hardware design and optimization are reviewed and analyzed. On this basis, several typical application schemes of DAB-IBDC for HPL PCSs are presented in a worldwide scope. Finally, design recommendations and future trends are presented. As the core circuit of HFL PCSs, DAB-IBDC has wide prospects. The large-scale practical application of DAB-IBDC for HFL PCSs is expected with the recent advances in solid-state semiconductors, magnetic and capacitive materials, and microelectronic technologies.

Overview of Dual-Active-Bridge Isolated Bidirectional DC–DC Converter for High-Frequency-Link Power-Conversion System

Energy storage systems (ESSs) are enabling technologies for well-established and new applications such as power peak shaving, electric vehicles, integration of renewable energies, etc. This paper presents a review of ESSs for transport and grid applications, covering several aspects as the storage technology, the main applications, and the power converters used to operate some of the energy storage technologies. Special attention is given to the different applications, providing a deep description of the system and addressing the most suitable storage technology. The main objective of this paper is to introduce the subject and to give an updated reference to nonspecialist, academic, and engineers in the field of power electronics.

Energy Storage Systems for Transport and Grid Applications

In this paper, the control strategy for a flexible microgrid is presented. The microgrid presented here consists of several line-interactive uninterruptible power supply (UPS) systems connected in parallel. The control technique is based on the droop method to avoid critical communications among UPS units. Thus, a flexible microgrid is obtained to operate in either grid-connected or islanded mode. A small-signal analysis is presented in order to analyze the system stability, which gives rules to design the main control parameters. Simulation and experimental results are presented, showing the feasibility of the proposed controller.

Control Strategy for Flexible Microgrid Based on Parallel Line-Interactive UPS Systems

Today, conventional power systems are evolving to modern smart grids, where interconnected microgrids may dominate the distribution system with high penetration of renewable energy and energy storage systems. The hybrid ac/dc systems with dc and ac sources/loads are considered to be the most possible future distribution or even transmission structures. For such hybrid ac/dc microgrids, power management strategies are one of the most critical operation aspects. This paper presents an overview of power management strategies for a hybrid ac/dc microgrid system, which includes different system structures (ac-coupled, dc-coupled, and ac–dc-coupled hybrid microgrids), different operation modes, a thorough study of various power management and control schemes in both steady state and transient conditions, and examples of power management and control strategies. Finally, discussion and recommendations of power management strategies for the further research are presented.

Overview of Power Management Strategies of Hybrid AC/DC Microgrid

An isolated three-port bidirectional dc-dc converter composed of three full-bridge cells and a high-frequency transformer is proposed in this paper. Besides the phase shift control managing the power flow between the ports, utilization of the duty cycle control for optimizing the system behavior is discussed and the control laws ensuring the minimum overall system losses are studied. Furthermore, the dynamic analysis and associated control design are presented. A control-oriented converter model is developed and the Bode plots of the control-output transfer functions are given. A control strategy with the decoupled power flow management is implemented to obtain fast dynamic response. Finally, a 1.5 kW prototype has been built to verify all theoretical considerations. The proposed topology and control is particularly relevant to multiple voltage electrical systems in hybrid electric vehicles and renewable energy generation systems.

An Isolated Three-Port Bidirectional DC-DC Converter With Decoupled Power Flow Management

Multiport dc-dc converters are particularly interesting for sustainable energy generation systems where diverse sources and storage elements are to be integrated. This paper presents a zero-voltage switching (ZVS) three-port bidirectional dc-dc converter. A simple and effective duty ratio control method is proposed to extend the ZVS operating range when input voltages vary widely. Soft-switching conditions over the full operating range are achievable by adjusting the duty ratio of the voltage applied to the transformer winding in response to the dc voltage variations at the port. Keeping the volt-second product (half-cycle voltage-time integral) equal for all the windings leads to ZVS conditions over the entire operating range. A detailed analysis is provided for both the two-port and the three-port converters. Furthermore, for the three-port converter a dual-PI-loop based control strategy is proposed to achieve constant output voltage, power flow management, and soft-switching. The three-port converter is implemented and tested for a fuel cell and supercapacitor system.

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Transformer-Coupled Multiport ZVS Bidirectional DC–DC Converter With Wide Input Range

A three-port triple-half-bridge bidirectional dc-dc converter topology is proposed in this paper. The topology comprises a high-frequency three-winding transformer and three half-bridges, one of which is a boost half-bridge interfacing a power port with a wide operating voltage. The three half-bridges are coupled by the transformer, thereby providing galvanic isolation for all the power ports. The converter is controlled by phase shift, which achieves the primary power flow control, in combination with pulsewidth modulation (PWM). Because of the particular structure of the boost half-bridge, voltage variations at the port can be compensated for by operating the boost half-bridge, together with the other two half-bridges, at an appropriate duty cycle to keep a constant voltage across the half-bridge. The resulting waveforms applied to the transformer windings are asymmetrical due to the automatic volt-seconds balancing of the half-bridges. With the PWM control it is possible to reduce the rms loss and to extend the zero-voltage switching operating range to the entire phase shift region. A fuel cell and supercapacitor generation system is presented as an embodiment of the proposed multiport topology. The theoretical considerations are verified by simulation and with experimental results from a 1 kW prototype.

Three-Port Triple-Half-Bridge Bidirectional Converter With Zero-Voltage Switching

This paper proposes a line-interactive fuel-cell-powered uninterruptible-power-supply system. A three-port bidirectional converter connects a fuel cell and a supercapacitor to a grid-interfacing inverter. The system can operate in both stand-alone and grid-connected modes. Moreover, an active filtering function is integrated into the system. It is shown that a supercapacitor can serve as both an active and a reactive energy storage and can buffer the periodical low-frequency ripple in the requested power. For connecting the system to the utility grid, a high-performance single-phase phase-locked loop that incorporates an orthogonal filter is presented. Resonant controllers for both the voltage and current regulations eliminate steady-state error and implement selective harmonic compensation. Simulation and experimental results are provided to show the feasibility of the proposed system and the effectiveness of the control methods.

Line-Interactive UPS Using a Fuel Cell as the Primary Source

This paper presents a new multi-input bidirectional dc-dc converter which connects a fuel cell, storage and load by a combination of a dc-link and magnetic-coupling. A boost-dual-half-bridge and a bidirectional direct-connected switching cell are used. The topology is simple and only needs six power switches. The load and the sources are galvanically isolated. Furthermore, the proposed converter draws/injects smooth current from the fuel cell and the supercapacitor. The system is suitable for medium-power applications where simple topology, autonomous operation, compact packaging, and low cost are required. Different control schemes to manage power flow are proposed and compared. Simulation and experimental results that verify the effectiveness of the topology and its control scheme are presented. Moreover, it is shown that the idea of combining the dc-link and magnetic-coupling can be further developed to a general topology for multi-port bidirectional dc-dc converters.

Multi-input bidirectional DC-DC converter combining DC-link and magnetic-coupling for fuel cell systems

This paper presents a zero-voltage-switching (ZVS) three-port bidirectional converter for fuel cell and supercapacitor applications. A simple and effective method to extend the soft-switching range is proposed, especially for the three-port active-bridge topology. By continuously adjusting the duty cycle on the supercapacitor side H-bridge according to the supercapacitor voltage level, soft-switching conditions are achieved over the full operating range due to the equivalent volt-second products applied to the transformer over half the switching period. Detailed analysis is provided for both the two-port and the three-port converters. Furthermore, a dual PI-loop based control strategy is proposed to achieve constant output voltage and precise power flow management, as well as soft-switching. The converter is implemented and tested

Haimin Tao

Papers

Transformer-Coupled Multiport ZVS Bidirectional DC–DC Converter With Wide Input Range

Three-Port Triple-Half-Bridge Bidirectional Converter With Zero-Voltage Switching

Line-Interactive UPS Using a Fuel Cell as the Primary Source

Multi-input bidirectional DC-DC converter combining DC-link and magnetic-coupling for fuel cell systems

A Soft-Switched Three-Port Bidirectional Converter for Fuel Cell and Supercapacitor Applications