In the renewable energy generation systems, the grid-connected inverter with LCL filter (GCI-LCL) is an important device to realize dc-ac power conversion and connect the distributed power generation system and power grid. However, the resonance problems of the LCL filter may bring instability to GCI-LCL. Besides, the grid impedance and the time delay in digital controllers also have a harmful influence on the stability of GCI-LCL. To solve these problems, a dual-current active damping control strategy based on the inverter-side current and grid-connected current feedback for GCI-LCL is proposed in this paper, and the systematic parameters design process is investigated. Compared with the conventional damping strategies, a resonance suppression for the LCL filter, a larger stability margin, and a better anti-interference capability in a weak grid can be achieved under the proposed strategy. In addition, by analyzing the active damping effect of the proposed strategy, a simplified model in a low-frequency band for GCI-LCL is proposed to enhance the anti-interference capability in the weak grid and simplify the parameters design process. Finally, a GCI-LCL prototype is constructed and the experiment results verify the correctness and feasibility of the analysis and proposed strategy.

The dual-active-bridge (DAB) converter is applied for power transmission and voltage conversion in dc power grid. To improve the quality of input current, an input LC filter is cascaded to DAB converter. However, due to the interaction between LC filter and DAB converter, substantial oscillation and instability occur, leading to the excessive electromagnetic interference, large voltage ripple, and power loss and even operation failure. To solve this issue, an active damping strategy is proposed in this article to reshape the small-signal input impedance by parallel virtual impedance. The input voltage is regarded as a control objective, as well as the output voltage, therefore, a triple-closed-loop control structure is established. Besides, all dynamics of controllers are considered for the input impedance modeling of DAB converter. Moreover, compared with traditional methods, an active damping method for cascaded DAB converter with LC filter is adopted to design the virtual impedance and controller, suppressing resonance of LC filter and achieve a larger bandwidth. Therefore, a more stable and rapid dynamics cascaded system composed of DAB converter and LC filter are achieved. Finally, a prototype is set up and the effectiveness and superiority of proposed strategy are verified by experiments.

This study adopts the method of third-order harmonic voltage injection (THVI) to move the peak value of a reference voltage toward the high capacitor voltage and increase the utilization rate of the capacitor voltage. This method can lower the cost or enlarge the capacity of a modular multilevel converter (MMC) when applied in high-voltage direct-current transmission. In this paper, the principle of THVI is introduced. Two calculation methods for modulation waves are studied, and the mathematical expressions of the capacitor voltage under all operating conditions are deduced. Thus, the optimal magnitude and phase of the third-order harmonic voltage are determined under all operating conditions. The mathematical expressions of the optimal magnitude and phase of the third-order harmonic voltage are obtained, and their online calculation burden is reduced. Simulation and experimental results show that the proposed method of THVI is feasible and effective.

Input-series-output-parallel (ISOP) isolated bidirectional direct current (dc)/dc converter (IBdc) becomes a preferred scheme connecting high-voltage and low-voltage bus in dc distribution network. Input-voltage-sharing (IVS) among modules is essential to realize the stable operation of ISOP system. Nowadays, with large-scale access of distributed energy sources and loads in dc grids, the fluctuations in bus voltage and connected load become frequent and great, deteriorating the IVS performance and stable operation of ISOP structure IBdc. To solve this issue, a triple-close-loop IVS strategy is proposed in this article. Compared with the conventional IVS strategy with constant input impedance, the proposed IVS strategy reshapes input impedance to be a full-order model containing high-order components and sensitive to fluctuation of output voltage, and IVS control based on reshaped impedance improves dynamics feature, maintains ideal output power, and avoids false protection and potential instability for ISOP structure IBdc under frequent and large fluctuation. Experimental results verify the correctness and effectiveness of the analysis and proposed strategy, providing a feasible, efficient, and practical control scheme for ISOP system in dc distribution network.

In a DC distribution network, the modular multilevel high-frequency DC converter (MDCC) can achieve the electrical isolation, voltage conversion and power transmission between the low- and medium-voltage DC buses. In this paper, the quasi-square-wave (QSW) modulation, which avoids the dv/dt stress problem and maintains a high DC voltage utilization, is applied in MDCC, and the mathematical model of QSW modulation with a dual-phase-shift (DPS) scheme is established. Based on the model, the influences of staircase rising and falling process in QSW modulation on MDCC performance, including power characteristic, current stress, switching characteristic and efficiency characteristic are analyzed. The influences of QSW modulation under DPS scheme on MDCC performance are also investigated. Moreover, to achieve a comprehensive optimization of MDCC performance, a multi-objective optimized control strategy based on QSW modulation under DPS scheme is proposed. Finally, a MDCC prototype is set up and the experimental results verify the correctness and effectiveness of the analysis and proposed strategy.

This paper provides a comprehensive review of the existing research on the Dual Active Bridge (DAB) DC-DC converter, focusing on modeling methods, modulation strategies, optimization algorithms, and control methods. A comparative analysis of selected methods along with guidelines to assist engineers and researchers in their study of DAB is also presented. Firstly, a comprehensive review of modulation strategies for DAB is provided, ranging from classical phase-shift modulation to the popular asymmetric duty modulation. The intrinsic relationships among different modulation methods are summarized, and a comparison is made based on the difficulty of control and DAB operating characteristics. Secondly, the various modeling methods for DAB are described, including reduced-order modeling, generalized state-space averaging modeling, and discrete-time modeling methods. A comparison is made based on the suitability for different application scenarios, providing recommendations for the adoption of different modeling methods. Furthermore, a survey of optimization algorithms for modulation methods is presented, including classical algorithms, swarm intelligence optimization, and reinforcement learning algorithms. A number of criteria are proposed for different algorithms, and an analysis of the unresolved challenges and future prospects is provided. Finally, the advanced control methods for DAB are summarized based on control effectiveness and applicability. The article concludes with a summary and an outlook on future research directions is also provided.