Large disturbance scenarios such as pulse power load operation and load switching are commonly seen in the operation of DC microgrids. The mixed potential theory-based large-signal stability (LSS) analysis is a simple and practical method for investigating the LSS of DC microgrids. However, this method is characterized by conservatism, and produces conservative stability criterion results. When they are applied to the parameter designs of DC microgrids, they create excessive control parameter redundancies; when used for running status estimations, they may cause the system to be misjudged as unstable, and trigger unnecessary protective actions when the system runs into the conservative region. This paper presents a comprehensive analysis of the conservatism, revealing its main causes to be the idealization of the load converter's response characteristics, and the lack of refined models. It then proposes a novel, improved analysis method for transient response characteristics of the load converter. Experimental and simulation results using the improved method have validated it by showing that the conservatism of the traditional method had been eliminated, and the stability criterion obtained was more accurate.

An input-independent and output-series (IIOS) modular DC-DC converter with multiple input ports and high voltage conversion ratio can apply well to medium-voltage DC (MVDC) interface of distributed photovoltaics (PV). However, PV power mismatch will result in output voltage imbalance of the submodules (SMs), which is contrary to the standardized multi-module design. In this paper, a multiport modular DC-DC converter with low-loss series LC power balancing unit (PBU) is proposed to address this problem. An isolated DC-DC converter is employed as the SM, which consists of full-bridge (FB) circuit on the PV side and half-bridge (HB) circuit on the grid side. Independent maximum power point tracking (MPPT) can be realized by controlling the phase shift between the FB and the HB, while voltage balancing (or power balancing) of SMs can be achieved by regulating the phase shift between adjacent HBs. Only one inductor and one capacitor are used in the PBU and all the switches of SM turn on with zero-voltage switching (ZVS), which contributes to the low-cost and low-loss characteristic of the converter. Simulations and experiments are carried out to verify the validity of the theory.

Single-phase single-stage non-isolated photovoltaic grid-tied inverters mainly suffer from issues of common-mode leakage current and double-line-frequency power oscillation. Aiming at above issues, this paper designs a new single-phase photovoltaic grid-tied inverter with only two switches. The proposed inverter structure allows the neutral line of the AC grid directly connected to the negative pole of the photovoltaic panel so that the common-mode leakage current is completely eliminated. Additionally, a power decoupling method is proposed to suppress the double-line-frequency power oscillation. Then an improved control strategy is presented in this paper to solve the stability problem of the proposed inverter when it is connected to a weak grid. It should be noted that the designed inverter can also output reactive power, achieve zero-voltage switching, track maximum power point, and step up the output voltage. The validity of the proposed inverter and improved control strategy is verified by simulation and experiment results.

Input-independent and output-series (IIOS) modular converters are characterized by high step-up ratio and independent maximum power point tracking and can therefore be used to integrate renewable energy into medium voltage dc grids. However, the associated power mismatch can lead to voltage imbalance of the submodules(SMs). At present, the only solution using no extra active switch involves the insertion of an LC -branch-based power balancing unit (PBU) between every two adjacent SMs (the SM and PBU share the switches). However, the considerable peak current of the shared switches threatens the security of the switches and increases the power loss. Therefore, this article considered the peak-current problem. A dual active bridge (DAB) converter was employed as the SM of the IIOS converter. The basis for the large peak current associated with the shared switches was analyzed and a peak current reducing method using the DAB inner-phase-shift control was proposed. The contribution of this article is that the current stress can be reduced and the power efficiency can be increased. A two-SM and 1000-W down-scaled experimental prototype was developed for verification of the theoretical analysis results. The peak currents and efficiencies with and without the proposed method were compared with experimental results.

Since electric bicycles (EBs) of different specifications are equipped with different numbers of batteries, it is crucial that the wireless power transfer (WPT) system can output voltages of different specifications to charge EBs. This article proposes a reconfigurable magnetic integrated transmitter-based multimode WPT system that enables charging of different EBs. The transmitter consists of two coupling coils integrated together to change the operating mode of the proposed system. By reconstructing the coupling circuit on the transmitter, the proposed system can selectively operate in a two-coil structure for constant current (CC) mode and two three-coil structures for two constant voltage (CV) modes. Two three-coil structures can output two different voltage specifications, meeting the charging needs of two different specifications of EBs. Therefore, the proposed system is highly versatile. An experimental prototype with 2.2-A/66-V/78-V charging configuration is built to verify the feasibility and practicality of the proposed system.