In this paper, the issue of the safety and power supply capability for the DC low-voltage distribution systems is investigated. The safety capability encompasses safe current threshold and safe voltage level. Based on the Chinese design standards, three key points concerning the safe current threshold, safe voltage level, and power supply capability are studied based on the practical data and the comparisons of DC systems with AC systems. For the safe current threshold, the hazards of DC and AC shock currents in dry environments are compared to derive appropriate values; for the safe voltage level, the sampling data of voltage/human resistance is adopted to deduce the safety voltage; while for the power supply capacity, the current-carrying capability of cables is analyzed to evaluate power delivery under different supply voltages. This paper can be referred to the practical applications of DC low-voltage distribution systems.

With the large-scale integration of grid-forming photovoltaic (PV)-storage systems in power systems, the transient stability during its mode transition is a great challenge. In this paper, a virtual synchronous generator (VSG)-based integrated control approach is proposed to realize the smooth switching from the off-grid mode to the grid-connected mode for the grid-forming PV-storage systems. The innovation lies in the elimination of the need for phase-locked loops (PLL) by adjusting the frequency and voltage of VSG for the realization of smooth switching operation while controlling the battery energy storage system via the voltage-current control so that the reliability of the power exchange can be guaranteed at the same time. Simulations validate the effectiveness of this paper with the advantages of reducing the inrush current in the process of grid synchronization and the better stability of the whole system.

An improved virtual impedance droop control (VIDC) strategy based on sliding mode control (SMC) is proposed in this paper to effectively suppress the inter-phase circulating current of parallel three-phase grid-connected inverters. Unlike the traditional VIDC method, the proposed strategy is discussed and improved in four aspects: bus voltage stability, Q-U loop performance, P-f loop performance, and virtual impedance regulation. The generation mechanism of the inter-phase circulating current is emphatically analyzed, and the output voltage difference and power distribution are studied and integrated with the improved VIDC strategy to suppress the circulating current. The simulations are carried out to demonstrate that the improved VIDC strategy is effective in restraining the circulating current in the system while also significantly improving the system's response speed and output accuracy.

In this paper, taking the typical buck type in paralleled DC/DC converter as an example, a mathematical model of the parallel buck converter is established in the continuous domain, and a sliding mode control (SMC) system based on the master-slave control is designed for improving the output performance of the paralleled system. In order to adapt to practical engineering applications, the SMC system is further discretized by the zero-order holder (ZOH) to obtain the corresponding discrete control system, and the constraint conditions of the sampling period of the discretized SMC system for the paralleled buck converter is derived by analyzing the sufficient stable conditions of the discrete SMC system. Finally, the influence of discrete controller parameters and the superior performance of the designed controller on the SMC system is further discussed and verified by simulations.