The Hybrid DC/DC converter is proposed to be used in the condition of bidirectional asymmetric power flow between DC buses. Firstly, this paper analyzes the switching mode of the Hybrid DC/DC converter under Extended Phase Shift (EPS) Control. Then, this paper calculates the converter losses and models the losses of the power and magnetic devices based on soft-switching characterization. The losses of those are not only by their material characteristics, but also closely related to the phase shift ratio. Based on the loss model and transmission power model of the converter, the power optimization allocation control strategy is proposed with the goal of efficiency optimization. Finally, the optimization method is verified through simulation analysis.

The virtual oscillator control (VOC) strategy, as a grid-forming controller, demonstrates substantial potential for application in weak grid scenarios. However, research on voltage support strategies for VOC during grid sag faults remains limited. This paper establishes an Andronov-Hopf VOC (AH-VOC) system with voltage-current dual-loop control, then systematically analyzes the principles of inverter-based voltage support at the point of common coupling (PCC) during grid sag faults. The relationship between voltage support effectiveness, grid impedance ratio, and system short-circuit ratio (SCR) is thoroughly investigated. Specifically for AHVOC, an optimal power distribution strategy and a modified voltage reference scheme are proposed to achieve precise power regulation and optimal voltage support under grid sag fault conditions. The effectiveness of the proposed control strategies is validated through MATLAB/Simulink simulations.

As the core equipment of DC power system interconnection and energy transmission, the improvement of DC solid state transformers’ performances has an extremely important impact on the development and application of DC power system. In this paper, a Partial-Power-Processing (PPP) LLC-DAB DC/DC converter with a bridge arm reusing rectifier is proposed for DC microgrids. The proposed converter topology can not only decrease the switching devices’ quantity, but also increase the control freedom of the converter and improve the light load efficiency. The operation principles, soft switching characteristics and power loss analysis of the designed converter are carried out. Finally, the feasibility of the proposed topology and the correctness of the theoretical analysis were verified by simulation results.

Due to the advantages of low cost and high-reliability, the bidirectional asymmetric power flow (BAPF) DC/DC converters are regarded as the competitive DC/DC candidate for power electronics transformers (PETs) in low-penetration renewable energy systems. For this converter, this paper researches the active operation mode and analyzes the circulating current caused by the parallel connection of two full bridges on the low-voltage (LV) side. Based on that, the circulating current suppression method based on extended-phase-shift (EPS) modulation is proposed in the BAFP converters. The simulation results clearly verify the correctness of theoretical analysis and the effectiveness of the proposed method.

As the core equipment for interconnection and energy transmission of DC power systems with different voltage levels, the improvement of DC transformers' performances has an extremely important impact on the development and application of DC power system. Given the asymmetric characteristics of DCTs' power flow in low-penetration renewable energy networks, a hybrid isolated DC-DC converter with bidirectional asymmetric power flow is proposed. The proposed converter topology can not only satisfy the power flow requirements, but also reduce the number of active power semiconductor devices, decrease the system cost and improve the converter capability. For the proposed converter, the power transfer modes are studied, the operating principles of the converter in different modes are analyzed. Finally, the feasibility of the proposed topology and the correctness of the theoretical analysis are verified by simulation results.

In low-penetration-renewable-energy DC distribution networks, the bidirectional power flows in power electronic transformers are asymmetric. Based on the above power transmission characteristics of power electronic transformers, this article proposes an isolated DC/DC converter topology. The proposed topology not only meets with the power transmission requirements, but also reduces the active device quota and system cost, increases the number of power paths and improves the power density. The operating principles, soft switching characteristics, return power characteristics and device stress of the proposed converter topology are analyzed in detail. The feasibility of the proposed topology is verified by simulation results.

In distribution networks integrated distributed generation, the forward and reverse power of power electronic transformers may be unequal. For this regard, this paper proposes a bidirectional asymmetric-power-flow isolated DC/DC (BAI-DC/DC) converter topology, which not only meets the requirements of bidirectional asymmetric power transmission, but also reduces the active device rating and increases the number of power paths, which can decrease system cost and improve system reliability. Simulation and preliminary experimental results verify the feasibility of the proposed topology.

A modular Solid-State- Transformer (SST), typically consisting of cascaded H-Bridge (CHB) cells and isolated dc-dc converters, is considered as one of the most promising candidates for grid integration of large-scale PV systems (from MW to GW). However, the power imbalance among different modules and phases limits the operation range of SST. In this paper, a new common-dc-bus SST architecture, which consists of boost dc-de converters, isolated dc-dc converters and cascaded H-bridge cells, is proposed to solve the power imbalance issue. Power losses in the conventional and proposed common-dc-bus SST architectures are compared. Considering the characteristics of the proposed architecture, Zero Voltage Switching (ZVS) and the reactive current ratio of the isolated dc-dc converters are studied in detail. Simulation results of a three-phase 7-level large-scale PV system verify the effectiveness and feasibility of the proposed architecture.

The Cascaded H-Bridge (CHB) converter is an appealing solution for large-scale grid-connected photovoltaic (PV) systems. However, the phase power imbalance is one of the main challenges due to the non-uniform irradiance, unequal temperature and parameter mismatch, which may result in imbalanced grid current. The interphase Quadruple Active Bridges (QAB)-based CHB (CHB-QAB) converter is one of the promising converter architectures to overcome the issue of the phase power imbalance. Nevertheless, the existing control strategy for the interphase CHB-QAB PV systems can not ensure the phase power balancing when the mismatch exists in the CHB stage. If the sinusoidal zero-sequence voltage injection (SZVI), the conventional control strategy can exchange the power among three phases to keep grid currents balanced. However, in the condition of high mismatch, the control strategy with SZVI may cause over-modulation. In this paper, a control strategy for the interphase CHB-QAB PV systems is proposed, which can ensure the phase power balance as well as individual MPPT, even in the condition of high mismatch. Based on the derived average and small-signal models of the overall system, the control system is designed. As a consequence of the proposed control strategy, the bandwidth of the dc-link voltage controller can be increased for reducing the dc-link voltage ripple of H-bridge cells. This enables to reduce the dc-link capacitance. Simulation results validate the effectiveness and superiority of the proposed control strategy.

Cascaded H-bridge (CHB) multilevel converters are promising candidates for large-scale grid-connected photovoltaic (PV) systems. In this paper, a new topology concept is introduced based on CHB multilevel converters and three-port dc-dc converters with isolation. This topology can handle the inherent power imbalances through the use of a single dc bus made up of ports in dc-dc converters. Simulation results of a 7-level CHB PV system are presented as a preliminary validation of the proposed topology.