Recently more occasions with high input voltage and/or high output voltage applications are emerging. However, the existing switching devices are limited by the voltage stress, e.g., the available maximum blocking voltage of IGBTs is 6.5 kV. Therefore, several methods are provided to solve this issue, including series connection of switches, multilevel converters and modular series-parallel structures, where the modular series-parallel systems are the most popular choice due to the advantages of low cost, simple design process, high modularity, reliability and redundancy. The main issue of series-parallel systems is to ensure the sharing of input/output voltage/current. Based on this, this paper provides a comprehensive review on the available voltage/current sharing methods for series-parallel systems, which include input-series output-parallel (ISOP), input-parallel output-series (IPOS), and input-series output-series (ISOS) structures. Moreover, the relationships between the input voltage/current sharing and output voltage/current sharing of series-parallel systems are explained.

A hybrid dc transformer (DCT) combining multiple series resonant converters (SRCs) and dual-active bridge (DAB) converters with input-series-output-parallel (ISOP) configuration is proposed in this paper. The proposed DCT is suitable for connecting medium-voltage dc (MVDC) and low-voltage dc (LVDC) distribution networks, which combines SRCs as the majority modules for power transmission and DAB converters as the minority modules for terminal voltage regulation or power regulation. Benefiting from the hybrid circuit topology, the proposed DCT can achieve a higher efficiency than the ISOP DAB converters with a reduced complexity in the communication and control, while maintaining the controllability of the dc voltage/power. The operation principles of the proposed DCT are analyzed and key parameters design are presented. Then a 1 MW, 10 kV/750 V hybrid DCT is simulated and a DCT prototype of 2 kW, 400 V/100 V is built to verify the theoretical analysis.

The utilization of power converters in a distribution network could give rise to stability problems, especially when the penetration is high. The existing literature studies the modeling and stability of converters with specific control or synchronization methods. However, the types or the control schemes of converters are normally different in an actual grid, and moreover the line impedance plays an important role on the network stability. In this regard, this paper aims at studying the modeling and stability issues of a two-bus electric network with both grid-forming and grid-following converters. The main purpose is to provide design guidelines and solution of two-bus system with converters for stable operation. The interactions between the converters in various scenarios will be investigated by considering the effect of line impedance. More importantly, a filter-based stabilized control strategy will be proposed for the grid-forming converters to mitigate the oscillation in the network. Simulation and experimental results are provided to validate the effectiveness of the theoretical analysis and control strategy.

It is well known that there is a design tradeoff between voltage regulation and load current sharing in conventional linear droop control for several parallel-connected distributed sources in a DC microgrid. To improve both performance areas, this article proposes a design method to identify the nonlinear droop curve coefficient with the desired bus voltage regulation and current sharing accuracy in a specified heavy load range, while a linear droop function with a negative droop resistance is used to tighten the bus voltage regulation in the light load range. The design procedure of load boundary and the effect of negative droop resistances on the system stability are analyzed in detail. Also, a boost-up and curve-fitting method can further improve the voltage regulation and achieve smooth transition between light load and heavy load. In this way, no additional voltage compensation loop is needed. Two parallel-connected DC converters are finally built to verify the theoretical analysis.

Fault localization is one of the most important issues for the MMC consisting of lots of switches. This paper proposes a switch open-circuit fault localization strategy for the MMC, where a sliding-time window (STW) based features extraction algorithm (FEA) is proposed to extract the features of the MMC based on the feature relationship between neighboring STWs. Based on the extracted features, the fault in the MMC can be easily located with the two-dimensional convolutional neural networks. The proposed STW-FEA based fault localization strategy can constructs concise low-data-volume features samples for the MMC in both time domain and frequency domain, and accordingly it can locate the fault with short time and high accuracy for the MMC. In addition, it does not require the creation of complex mathematical models and manual setting of empirical thresholds. Simulation and experiment are conducted, and the results confirm the effectiveness of the proposed strategy.

It is well-known that the low-frequency negative input impedance of the constant power load (CPL) is the major cause of the cascaded system instability, and the heavier the power, the worse the stability. In this article, a power adaptive load-side parallel virtual impedance (PALPVI) control strategy is proposed to improve the stability of the cascaded dc system with buck-type CPL. First, a parallel impedance with power adaptability is derived followed up with the derivation of the corresponding compensation controller transfer function to realize it virtually. Considering that the compensation controller is highly dependent on the circuit parameters and needs extra current sensors to acquire the power information, a simplification of the compensation controller is made based on the open-loop characteristics of the buck-type CPL. The final PALPVI control strategy does not require the circuit parameters or any current sensor and has almost no side effect on the dynamic performance. Finally, a 48–24–12 V cascaded dc system is fabricated to verify the feasibility and effectiveness of the proposed PALPVI control strategy.