Soft normally open point (SNOP) is an emerging solution for distributed networks (DNs) to address voltage violation and feeder congestion caused by the increasing integration of distributed energy resources and new types of loads. Based on power electronics, SNOPs can substitute traditional tie switches for power flow regulation, voltage adjustment, and power quality improvement, enhancing the DNs’ controllability and flexibility. Existing SNOPs found in demonstration projects and the literature are generally based on the back-to-back voltage-source converter (VSC). Because it requires more full-power-rating VSCs as the number of connected feeders increases, this solution is uneconomical in multifeeder flexible interconnection scenarios due to the high device cost and volume. Alternatively, this article proposes a series–shunt multiport SNOP (S ² -MSNOP) based on a cascaded H-bridge structure and series–shunt arrangement. The proposed topology can easily extend its ports by increasing the number of submodules with small power ratings, reducing the device cost and volume. The operation principles and control strategies of the S ² -MSNOP are elaborated. Verifications on both a 1-MVA simulation model and a 3.3-kVA scaled-down experimental platform prove the feasibility and effectiveness of the S ² -MSNOP.

The multiport power electronic transformer (PET) is a critical equipment in ac/dc hybrid distribution grids, which has the functions of multidirectional power flow, flexible power flow regulation and reactive power control, and fault isolation. However, most proposed PETs have only a single-medium-voltage-ac (MVac) port, limiting their application scenarios in distribution grids. To address this problem, in this article, we propose a PET topology with multiple controllable MVac ports based on the cascaded H-bridge PET. The proposed PET topology can connect to multiple-MVac feeders and supply low-voltage-side loads through multiple terminals, achieving the full utilization of the remaining capacity of connected feeders, thus delaying the distribution capacity expansion and grid upgrade due to the rapid development of energy electrification. Besides, the decoupling control of active and reactive power for each feeder can be achieved. In this article, the operating principle, energy balance mechanism, control strategies, and design and comprehensive comparison of the proposed topology are studied. The simulation and experimental results demonstrate the validity of the proposed topology and control strategy.

SST have gained increasing interest lately. Topologies made of directly coupled MMC submodules (SM) and dual active bridge (DAB) modules have been introduced to form multi-port SSTs to interface different grid entities. Designated as modular SSTs (M-SSTs), such devices can interconnect distribution systems and enable flexible power flow control. However, the large cell capacitors needed to suppress power fluctuations are a major contributor to the SST's poor power density. This paper proposes a power fluctuation delivery (PFD) control for capacitance optimization. Through PFD control, the low frequency fluctuating power in MMC is transferred to DAB's secondary side and are automatically cancelled. And the cell capacitance requirement is significantly reduced. Considering the modified ripple power path, both power semiconductors and high frequency transformers in DABs are affected. Therefore, the design considerations of single cell (1 MMC SM + 1 DAB) with/without PFD control are elaborated for a 2 MVA M-SST example. It is shown that the cell capacitance can be reduced to 10% of its original value, and the power density of the whole cell is increased by 50% with 0.22% drop in overall efficiency. The feasibility of PFD method are verified by simulation and 4.8kVA experimental results.

The lack of high-performance cathode materials is a great challenge for the development of large-scale energy storage sodium ion batteries. Here, a new resource-rich P2-type Na_0.44Mn_0.6Ni_0.4−xCu_xO₂ (0 ≤ x ≤ 0.3) sodium-ion battery (NIBs) cathode material was designed and synthesized by a sol–gel method. Na ions occupy the Na_f sites and the Na_e sites in a proportion of 1 : 1, which maintains the high symmetry and stability of the P2-type structure. The charging/discharging tests show that Na_0.44Mn_0.6Ni_0.3Cu_0.1O₂ has a high initial capacity of 149 mA h g⁻¹ and 80% capacity retention after 50 cycles at 0.1C, showing a high energy density of 469 W h kg⁻¹. The addition of inactive copper enhances the lattice spacing, obviously reducing the irreversible reaction resistance (R_f) during the intercalation and deintercalation of Na⁺, thereby reducing the internal resistance of the battery and improving the cycle performance. In order to maintain the P2-type structure, the voltage is controlled at 1.5–4.0 V during charging and discharging, which inhibits the phase transition of P2-O2, leading to the improvement of cycling performance. Therefore, the Cu-substituted Na_0.44Mn_0.6Ni_0.3Cu_0.1O₂ possibly serves as a promising high capacity, high energy density and stable cathode for sodium ion battery applications.

Possessing the capability to handle high voltage, the bidirectional input-series–output-series (ISOS) modular dc–dc converter is suitable for use in dc grids. Regarding topology selection of the constituent modules, the open-loop controlled series-resonant dual active bridge (SR-DAB) converter exhibits superior efficiency performance and the advantage of simple control. However, it has no regulation capability. Aimed to reap the advantages of SR-DAB and achieve flexible control, this article presents a hybrid ISOS converter composed of SR-DAB and nonresonant phase-shift controlled dual active bridge (PS-DAB) modules. The SR-DAB modules process most of the power to ensure high-efficiency conversion, whereas the PS-DAB is responsible for flexible control. At the output stage of the ISOS system, embedded nonisolated resonant dual active half-bridge (NR-DAHB) converters are integrated between every two adjacent modules without adding any switches. The transformer property of the open-loop controlled SR-DAB and NR-DAHB circuits facilitates natural voltage sharing at both the input and output sides. The hardware experiment has been conducted to verify the operating principles and performance of the proposed ISOS system.

Modular multilevel converter (MMC)-based solid-state transformers (M-SSTs) show promise in hybrid ac/dc distribution grids where there is need for a medium-voltage dc (MVdc) interface. It can be used to interconnect several distribution networks with different voltage forms and levels and enable flexible power flow among them. In an M-SST, the MVdc short-circuit fault is a critical issue due to the rapidity of the fault current, which can cause device damage and power failure of the low-voltage (LV) grids. Aiming at addressing this issue, this article proposes a fault-mode control strategy applied to a hybrid M-SST topology for uninterrupted power supply of its LV ports. The topology consists of a hybrid MMC with both half- and full-bridge submodules and isolated bidirectional dc–dc converters. Through the proposed control, the fault current is eliminated when an MVdc short-circuit fault occurs. The hybrid M-SST can keep uninterrupted power interaction between the MVac port and the LVdc port during the fault, thereby improving power supply reliability. Moreover, an improved capacitor voltage balance control and an optimized modulation scheme are developed and included in the uninterrupted operation scheme to maintain the harmonic performance of the ac port. The feasibility and effectiveness of the proposed topology and control are verified by simulation and experimental results.