There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

The nonresonant single-phase dual-active-bridge (NSDAB) dc–dc converter has been increasingly adopted for isolated dc–dc power conversion systems. Over the past few years, significant research has been carried out to address the technical challenges associated with modulations and controls of the NSDAB dc–dc converter. The aim of this paper is to review and compare these recent state-of-the-art modulation and control strategies. First, the modulation strategies for the NSDAB dc–dc converter are analyzed. All possible phase-shift patterns are demonstrated, and the correlation analysis of the typical phases-shift modulation methods for the NSDAB dc–dc converter is presented. Then, an overview of steady-state efficiency-optimization strategies is discussed for the NSDAB dc–dc converter. Moreover, a review of optimized techniques for dynamic responses is also provided. For both the efficiency and dynamic optimizations, thorough comparisons and recommendations are provided in this paper. Finally, to improve both steady-state and transient performances, a combination approach to optimize both the efficiency and dynamics for an NSDAB dc–dc converter based on the reviewed methods is presented in this paper.

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Overview and Comparison of Modulation and Control Strategies for a Nonresonant Single-Phase Dual-Active-Bridge DC–DC Converter

The CLLC bidirectional resonant converter has significant potential in battery chargers and dc microgrids, due to its bidirectional power transfer capability. To ensure uniform characteristics for bidirectional operation, secondary LC resonant tank components are usually designed to equal the primary LC components after reflection. This conventional design method is regarded as the symmetric design. It is used in applications where wide voltage regulation is not required, such as CLLC dc transformers. However, for bidirectional battery charger applications, the battery has a wide voltage range variation, and gain requirements during charging and discharging are different. These asymmetric characteristics could lead to undesirable large switching frequency range if a conventional symmetric CLLC design is employed. To address this issue, a detailed asymmetric parameters methodology (APM) is proposed in this article. It can design gain curves for charging and discharging modes separately. This enables overlapping of the switching frequency range for both modes, thereby reducing the bidirectional frequency range variation. It brings lower switching loss caused by excessive high frequency, and relieve the extra conduction loss and current stress of power switches as well. Finally, a detailed analysis of the proposed APM is provided and validated with simulations and experiments.

Bidirectional Resonant CLLC Charger for Wide Battery Voltage Range: Asymmetric Parameters Methodology

A linearized hybrid modulation scheme for the dual active bridge (DAB) converter is proposed in this article. For the purpose of minimizing the conduction losses dissipated on the transformer and the power transistors, an optimal relationship function between the two control variables employed in extended phase shift (EPS) modulation can be derived. However, the obtained relationship function is a complex expression, which is not good for simple on-line control. Hence, a linearized modulation scheme is proposed in this article. This modulation scheme can achieve a quasi-minimum root-mean-square (rms) value of the leakage inductance current for the same output power. Meanwhile, the zero-voltage switching (ZVS) of the power transistors can be achieved over the whole power range. The power transfer capability is also kept the same as the optimal EPS scheme. Finally, experiments are conducted on a laboratory prototype to validate the effect of the linearized modulation scheme on the reduction of conduction losses. The experimental results present an improved converter efficiency and the realization of ZVS.

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An Optimized Hybrid Modulation Scheme for Reducing Conduction Losses in Dual Active Bridge Converters

The concept of Energy Internet has emerged from the limitless possibilities of energy sharing networks formed by interconnection of electricity producers cum consumers (prosumers) with renewable energy sources/systems, electric loads, and storage devices. Energy Internet represents a radical transformation of traditional electricity system by orchestrating real-time bidirectional power, communication, and money flows. This transformation is expected to be resultant of ongoing renewable energy transitions and evolution in the energy technologies such as smart grids, storage devices, vehicle-to-grid, etc. Energy Internet will rely on wide spectrum of Information and Communication Technologies such as Internet of Things, Artificial Intelligence, Blockchains, Payment Interfaces, etc. These technologies have achieved a state of evolution to facilitate seamless bidirectional flows in the Energy Internet. This paper has attempted to study the aptness of Energy Internet for a transitioning electricity system by focusing on national electricity systems across the globe. Firstly, study delves into discussion on studies related to smart grids since it is the precursor of Energy Internet, and various other studies which discuss the developments that likely to enable smooth transition of present electricity system to Energy Internet. Secondly, a systematic review of literature related to state-of-the-art of Energy Internet is performed to outline its structure, operational features and energy market mechanisms. We find that while system infrastructures, technologies, and operational principles across globe are adequately ready for the transition to Energy Internet, hindrances put forth by the policies, regulations, government agencies and institutions need to be addressed to realize a nation-wide Energy Internet.

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Smart Grid to Energy Internet: A Systematic Review of Transitioning Electricity Systems

Current stress minimization is one of the most important challenges for the steady-state studies of dual-active-bridge (DAB) dc–dc converter. Triple-phase-shift (TPS) control can minimize the current stress to the utmost extent, and a segmented analytical method-based TPS (SA-TPS) control has been proposed in this paper to minimize the current stress of DAB within the whole operating range. The full model of DAB with TPS control was classified into 12 operating modes by a complete operating mode classification method. On the basis of the analytical expressions of transmission power and current stress in different modes, the current stress minimization issue was transferred to inequality constraints’ problems in different operating modes. The mathematical derivation process of the SA-TPS control method is described in detail, and the analytical expressions of current stress minimization results as a function of voltage conversion ratio and transmission power have been derived by solving the forward and reverse power transmission processes, respectively. The operating mode selection principle and the regularities of minimization results varying with transmission power and voltage conversion ratio have been explored. Experiments under different conditions have been implemented on a laboratory prototype to validate the correctness and effectiveness of the proposed SA-TPS control method.

Current Stress Minimization of Dual-Active-Bridge DC–DC Converter Within the Whole Operating Range

A comprehensive time-domain model of a CLLC class isolated bidirectional dc–dc converter (IBDC) is proposed in this article. Based on the circuit analysis and the solution of the state equations, analytical expressions of the state variables are provided. The model is a piecewise time-domain analytical model and can accurately describe the transformer currents and capacitor voltages under almost all basic types of control strategies, e.g., phase-shift control and frequency modulation control. With the model, the steady-state analysis of the IBDC can speed up evidently (up to 330 times faster compared to MATLAB/Simulink and more than 10 times compared to SIMPLIS and PLECS), which gives great support to parameter sweep and case comparison in the design procedure. By assigning specific parameters, the presented model can also be applied to the analysis of the LLC resonant IBDC or typical DAB IBDC since the LLC converter and DAB converter are similar to the CLLC converter in the circuit topology. The transformation of the model from CLLC to LLC /DAB is provided and several application examples are given. Finally, the simulation and experimental results are presented, verifying the effectiveness of the model.

Design-Oriented Comprehensive Time-Domain Model for CLLC Class Isolated Bidirectional DC-DC Converter for Various Operation Modes

This paper proposed a trajectory prediction control strategy for series resonant dual-active-bridge converter (SR-DAB), achieving extremely fast bidirectional power transient performance when voltages of both DC buses are approximately constant. State-trajectory analysis of LC series resonant tank is here further developed to describe the SR-DAB, which is featured of constant switching frequency, phase-shift operation method, and capability of handling bidirectional power flow. State trajectories of the converter reveal the fastest transient process between different steady states, leading to an intuitive open-loop power transient control strategy. This strategy is implemented by a 2-layer digital controller structure. Simulation and experimental results on a 4-kW prototype converter validated that, through this method, both amount and direction of the average transmission power of SR-DAB can be modified swiftly and smoothly within one switching period in large power range. Besides, impact and oscillation caused by the power transient of resonant tank can also be significantly reduced.

Trajectory-Prediction-Based Fast Bidirectional Power Transient Control for Series Resonant Dual-Active-Bridge Converter

Solid-state transformer (SST) is widely used in many fields and it plays a key role in the development of energy internet. The startup strategy of SST adopted in distribution network has to be designed elaborately because in this application the requirement of functionality, flexibility, and expandability is very high. A number of researches for the startup process of SST were made and several solutions were proposed. In these solutions, the transformer current in the charging process was unable to be observed or controlled, leading to the lack of analytical basis in a startup strategy design. In order to make the charging process observable and controllable, the authors developed a mathematical model of this process. Based on the model, a new startup strategy is proposed to make the peak transformer current in the charging process a constant value. Cases with different conditions are analyzed, indicating that the strategy is insensitive to initial values. Experimental results are provided to validate the accuracy of the mathematical model and the effectiveness of the presented solution.

Startup Strategy With Constant Peak Transformer Current for Solid-State Transformer in Distribution Network

An accurate stray loss calculation method of squirrel-cage induction motors is presented in this paper. The stray losses are divided into the stray rotor copper losses and the stray core losses, which are determined by the air gap magnetic field. The field is supposed to be completely radial and calculated by the product of stator magnetomotive forces and air gap permeance coefficient. The effect of magnetic wedges is reflected in the equivalent stator slot width. A correction factor is added to the field considering the rotor reaction. On the basis of the new stray loss model, the complex method algorithm is used to optimize the motor efficiency. Based on the 1250kW prototype, the detailed optimizing process is illustrated and the validity of the proposed method is proved.

Qing Gu

Papers

Current Stress Minimization of Dual-Active-Bridge DC–DC Converter Within the Whole Operating Range

Design-Oriented Comprehensive Time-Domain Model for CLLC Class Isolated Bidirectional DC-DC Converter for Various Operation Modes

Trajectory-Prediction-Based Fast Bidirectional Power Transient Control for Series Resonant Dual-Active-Bridge Converter

Startup Strategy With Constant Peak Transformer Current for Solid-State Transformer in Distribution Network

An accurate stray loss calculation method of squirrel-cage induction motors for efficiency optimization