Extensive solutions have been proposed to damp LCL filter resonance. However, the time delay caused by computation and modulation process aggravates the complexity of current loop analysis. In this paper, a generalized virtual impedance model for six kinds of commonly used active damping methods is proposed, and the physical meaning is established by introducing the metric of “damping factor”. To ensure system robustness in a wide range of grid conditions, a hybrid active damping strategy that combines inverter current feedback and capacitor voltage feedforward is proposed. Importantly, this method only requires inverter current and capacitor voltage sensors, which are the basic variables for over-current protection, power control and synchronization. The superposition theorem is also utilized to analyze the damping capability provided by parasitic resistances, current feedback loop and voltage feedforward loop. Finally, experimental results verify the feasibility and robustness of the proposed damping method.

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.

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.

The LCL-type grid-connected inverter (GCI) is widely adopted between distributed generation (DG) and power grid to realize DC/AC power conversion. However, the underdamped LCL filter will cause a resonance phenomenon near the control stability boundary, which may lead to instability of the GCI system. The traditional passive damping method will cause power loss, while the active damping method may have the need for additional sensors or be sensitive to parameter changes. In this paper, a control scheme based on active disturbance rejection control (ADRC) is proposed for the LCL-type GCI, and the systematic parameters design process is presented through root locus analysis. With the proposed strategy, only the grid-injected current is sensed to achieve the objectives of grid-injected current direct control and robust resonance damping for the LCL-type GCI. In the design process, the influence of digital control delay and grid impedance uncertainty on system stability is explicitly analyzed in detail. Through this, the GCI can work stably over a wide range of the typical inductive-resistive grid impedance. In addition, it is found that the proposed strategy has good adaptability to the filter parameter perturbation. Simulation and experimental results validate the theoretical analysis in this paper.

The isolated bidirectional DC/DC converter (IBDC) plays an important role in DC buses connection, voltage conversion, electrical isolation and bidirectional power transmission for DC distribution networks. In this paper, a multiple modular IBDC scheme based on hybrid- dual-active-bridge (HDAB) for medium-voltage (MV) and high-voltage DC (HVDC) distribution network is proposed. In the proposed HDAB, the switches in diagonal position of full-bridge are replaced by half-bridge sub-modules (SMs), the proposed HDAB maintains advantages of conventional DAB and achieves bidirectional fault handling capability, avoiding the application of large DC-link capacitors and DC circuit breakers, reducing the volume and construction cost. Besides, high-frequency-link (HFL) voltage regulation strategy is also proposed in this paper, adjusting the HFL voltages in HDAB to match the HFL transformer voltage adaptively, thus improving the efficiency and applicability of IBDC under the fluctuation of power flow and DC bus voltage. The topology, operation principle, control structure and corresponding bidirectional fault handling and HFL voltage regulation strategy of proposed IBDC scheme are investigated. The experimental results verify correctness and effectiveness of proposed scheme and control strategy, promoting the adaptability and practicality for both IBDC and DC distribution network.