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.

Impedance spectrum is a key signature of a fuel cell stack (FCS). The variations of the impedance spectrum reflect the internal status of an FCS. Enabling electrochemical impedance spectroscopy (EIS) with the onboard power conditioning converter (PCC) provides an attractive approach for in situ diagnostics and condition monitoring of an FCS in end applications, such as heavy-duty vehicles. Although a few previous attempts were made with the PCC being controlled as the source of ac perturbations, the issue of how to properly produce a wide frequency range of perturbations to a high-power FCS has not been well recognized and addressed. In particular, the high-frequency portion is limited by the converter switching frequency and controller bandwidth. Different from existing approaches that fall short of being practical solutions for a high-power FCS, this article proposes the use of PI plus resonant controllers for the PCC to generate quality high-frequency perturbations without any additional hardware. Enabled by the proposed EIS method, an example application showing the effective detection of the impedance changes of an emulated FCS is also presented. The design and implementation of the scheme and the considerations of response measurement and impedance calculation are given in detail. Experimental verifications on a scaled-down laboratory setup demonstrate the validity and possibility of the proposed methods.

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.

A convolutional neural network fault diagnosis method based on frequency attention mechanism was designed for the problem that the traditional method cannot adaptively extract effective feature information in rolling bearing fault diagnosis and the diagnosis effect of rolling bearing is poor under strong environmental noise interference. Firs, the Mel-frequency cepstral coefficient (MFCC) of the bearing vibration signal was extracted. Second, to solve the problem of the channel attention mechanism adopting global average pooling (GAP) and neglecting channel internal characteristic information, the GAP was extended in the frequency domain, and a two-stage frequency component selection criterion was designed. The results show that the MFCC method can extract fault-sensitive features in industrial noise environments, improve the existing channel attention mechanism using frequency domain attention mechanism, and overcome the information loss caused by GAP of convolutional layer features in channel attention mechanism. Identification accuracy, recall rate, and F1-score are 100% on the rolling bearing simulation fault datasets of Case Western Reserve University and Central South University. Compared with the convolutional block attention module, the accuracy of the method combining spatial attention mechanism and channel attention mechanism is improved by 0.34 and 0.24%, respectively, and compared with other front-bearing fault diagnosis methods, it also offers significant improvement.