This article presents a novel reduced order average model for dual-active-bridge converter which can be applied to all modulation methods, such as single-phase-shift modulation, dual-phase-shift modulation, extended-phase-shift modulation and triple-phase-shift modulation. This model considers conduction, inductance, and transformer power losses. Furthermore, the input-output filters are suitable for system performance analysis. Based on the large-signal average model, the small-signal model and the output transfer function are derived. The detailed model for predicting the large-signal transient is established and the small signal analysis in frequency domain is carried out. This model can accurately predict the steady-state and transient-state response of the system. The experimental results agreed well with the simulation results which proved the accuracy and correctness of the model.

This paper presents a novel voltage self‐balancing converter (VBC) applied to DC solid‐state transformer (DCSST), analyzes the operating principle of the proposed VBC, and proves the voltage balance performance. Based on this converter, a novel DCSST topology is proposed, which can be used in medium and low voltage DC distribution application, microgrid and medium and low voltage energy storage system. Compared with the previous work, under the same voltage level, it can effectively reduce the number of transformers and switches, reduce the voltage level of switches, and realize the soft switching of the VBC switches, so as to improve the system efficiency and power density, and then improve the system reliability. Meanwhile, the fault handling ability and fault tolerance ability of DCSST are improved, which makes DCSST more reliable when multiple isolated bi‐directional DC/DC Converters (IBDC) are connected in series. In addition, with this topology, the input voltage self‐balancing control strategy of the IBDC is not needed because of the voltage self‐balancing characteristic of VBC, which simplifies the control complexity and greatly improves the control accuracy of the control system. Experimental results agree well with the accuracy and correctness of the proposed topology.

In order to achieve high and low voltage isolation transformation in DC transmission and distribution networks, a multi module input-series output-parallel (ISOP) system consisting of a buck/boost converter and a CLLLC resonant converter as submodules was studied. This system can ensure that the CLLLC converter operates in the optimal state during frequency changes, achieves a soft switching function, and maintains a high conversion efficiency. This article establishes a mathematical model of a cascaded converter, analyzes its gain characteristics, and proposes a fuzzy PID three-loop control strategy to achieve good input voltage and output current sharing in the ISOP system. A simulation model is built on the MATLAB(R2023a)/Simulink platform to verify the effectiveness and superiority of the proposed control strategy. Finally, by building a prototype platform, the feasibility of the ISOP system and the effectiveness of fuzzy PID three-loop control were verified through experiments.

Direct current transformer (DCT) is a key piece of equipment in direct current (DC) microgrids, and the mainstream topologies mainly include LLC resonant converter (LLC) and dual active bridge (DAB). In this paper, a novel bi-directional buck/boost + CLLLC cascade topology is proposed for the input-series-output-parallel cascade converter system of a DC microgrid. To solve the problem that frequency variation causes the converter to deviate from the optimal operating point, resulting in low efficiency, and the inability to achieve a soft switching function. The CLLLC converter operates near the resonant frequency point as a DCT, only providing electrical isolation and voltage matching, while the buck/boost converter controls the output voltage and the voltage and current sharing of each module. Compared to other cascaded converter systems, the cascaded converter proposed in this paper has high efficiency, simplifies the parameter design, and is suitable for wide input and wide output operating conditions. The system adopts a three-loop control strategy, establishes the small-signal modeling of the system, and its stability is verified by theoretical analysis and simulation. The simulation and experimental results verify the correctness of the proposed cascaded converter based on buck/boost + CLLLC and the effectiveness of the control strategy.

Aiming at the poor dynamic performance of DC solid transformer with dual-loop control, a three-loop control method is proposed for the DC transformer to improve the system dynamic performance. The proposed control method is implemented by introducing an output current loop to the traditional dual-loop control system. Simulation results show the effective of the proposed method.