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

The solid-state transformer (SST) is an interface device between ac distribution grids and dc distribution systems. The SST consists of a cascaded multilevel ac/dc rectifier stage, a dual active bridge (DAB) converter stage with high-frequency transformers to provide a regulated 400-V dc distribution, and an optional dc/ac stage that can be connected to the 400-V dc bus to provide residential 120/240 V $_{\rm ac}$ . However, due to dc-link voltage and power unbalance in the cascaded modules, the unbalanced dc-link voltages and power increase the stress of the semiconductor devices and cause overvoltage or overcurrent issues. This paper proposes a new voltage and power balance control for the cascaded H-Bridge converter-based SST. Based on the single-phase dq model, a novel voltage and the power control strategy is proposed to balance the rectifier capacitor voltages and the real power through parallel DAB modules. Furthermore, the intrinsic power constraints of the cascaded H-Bridge voltage balance control are derived and analyzed. With the proposed control methods, the dc-link voltage and the real power through each module can be balanced. The SST switching model simulation and the prototype experiments are presented to verify the performance of the proposed voltage and power balance controller.

Voltage and Power Balance Control for a Cascaded H-Bridge Converter-Based Solid-State Transformer

Full-order continuous-time average modeling and dynamic analysis of bidirectional dc-dc dual active bridge (DAB) converters are studied. The transformer current in DAB converter is purely ac, making continuous-time modeling difficult. The proposed full-order continuous-time average model uses the dc terms and first order terms of transformer current and capacitor voltage as state variables, resulting in a third-order model, if capacitor equivalent series resistance (ESR) is not considered, and a sixth-order model if ESR is considered. A control-to-output-voltage transfer function is derived for DAB converters. Experimental results confirm that the proposed model correctly predicts the small-signal frequency response and an even more accurate prediction can be obtained if capacitor ESR is taken into account.

Generalized Average Modeling of Dual Active Bridge DC–DC Converter

Compared to the traditional single-phase-shift control, dual-phase-shift (DPS) control can greatly improve the performance of the isolated bidirectional dual-active-bridge dc-dc converter (IBDC). This letter points out some wrong knowledge about transmission power of IBDC under DPS control in the earlier studies. On this basis, this letter gives the detailed theoretical and experimental analyses of the transmission power of IBDC under DPS control. And the experimental results showed agreement with theoretical analysis.

Power Characterization of Isolated Bidirectional Dual-Active-Bridge DC–DC Converter With Dual-Phase-Shift Control

Solid-state transformer (SST) has been regarded as one of the most important emerging technologies for traction system and smart grid application. This paper presents the system design and performance demonstration of a high-voltage SST lab prototype that works as the active grid interface in smart grid architecture. Specifically, the designs of the key components of the system, including both power stage and controller platform, are presented. In addition, the advanced control system is developed to achieve high-performance operation. Furthermore, integration issues of SST with dc microgrid are presented. Lastly, tests under different scenarios are conducted to verify the following advanced features of the presented SST technology: 1) VAR compensation; 2) voltage regulation; 3) source voltage sag operation; and 4) microgrid integration.

Design and Demonstration of a 3.6-kV–120-V/10-kVA Solid-State Transformer for Smart Grid Application

The solid-state transformer (SST) is one of the key elements in power electronic-based microgrid systems. The single-phase SST consists of a modular multilevel ac-dc rectifier, a modular dual active bridge (DAB) dc-dc converter with high-frequency transformers, and a dc-ac inverter stage. However, due to dc bus voltage and power unbalancing in each module, the modular SST often presents instability problems making its design difficult and causing unpredictable behavior. Moreover, the unbalanced dc-link voltages increase the stress of the semiconductor devices, and also cause high harmonic distortions of grid current, therefore, necessitating the use of a bigger ac filter. This paper presents a novel single-phase d-q vector-based common-duty-ratio control method for the multilevel rectifier, and a voltage feedforward and feedback based controller for the modular DAB converter. With the proposed control methods, the dc-link voltage and power in each module can be balanced. In addition, the low-distortion grid current, unity power factor, and bidirectional power flow can be achieved. Simulation and experimental results are presented to validate the proposed control methods.

Research on voltage and power balance control for cascaded modular solid-state transformer

Single-switch circuit (SSC) in wireless power transfer (WPT) technology shows excellent performance with its simple topology. However, in higher power applications, limitation of power capacity has become the main bottleneck for SSC-WPT technology. Therefore, a research of multiple SSC connected in parallel in WPT systems has received more attention. In this paper, an input-parallel single-switch circuit (IPSSC) based on a compact decoupled coupler is proposed in WPT systems to upgrade the power capacity. Meanwhile, P-LCL compensated network is applied to realize constant voltage output (CVO) independent of the load. Moreover, a novel method of parameter calculation using high-order harmonics approximation is proposed for IPSSC-WPT systems, which can realize precise margin of zero-voltage switching (ZVS). The experimental results of a 1kW prototype with an output voltage of 150V are in agreement with the theoretical analysis well.

Hao Yuan

Papers

Research on voltage and power balance control for cascaded modular solid-state transformer

Research on Input-Parallel Single-Switch WPT System with Load-Independent Constant Voltage Output