Due to the differences of line impedance, initial state-of-charge (SoC) and capacities among distributed energy storage units (DESUs), the SoC of the DESUs with different capacities in an AC islanded microgrid will be unbalanced when using the conventional droop control. A droop control based on SoC balancing scheme is introduced in this paper to eliminate the influence of capacity on SoC balancing and maintain a good power quality. The proposed control strategy does not require communications or central controller, and the scalability of system is greatly improved. The effectiveness of the proposed approach has been validated by simulation and experimental results.

Flying capacitor photovoltaic (PV) inverters have been widely discussed in the literature. However, the relevant leakage current issues have not received much attention. In this paper, the modulation techniques for a transformerless three-phase flying capacitor PV inverter are investigated for the leakage current suppression. First, the theoretical analysis of the system common mode model and leakage current is presented. Second, three kinds of conventional carrier-based modulation techniques are evaluated, and results indicate that they fail to effectively suppress the leakage current. Third, a new carrier modulation and logic function is presented. It is able to maintain the constant common mode voltage, and consequently, the leakage current can be significantly reduced. Finally, the experimental tests are conducted on a transformerless three-phase flying capacitor PV inverter to verify the effectiveness of the proposed method.

Owing to the distributed installation position, the electric vehicle (EV) charging pile is usually connected to the grid via transmission cables of different lengths. The resonance of parasitic inductance and parasitic capacitance on the transmission cables might amplify the background harmonics, which result in a serious distortion of the grid-connected current of the front-end Vienna rectifier of EV charging piles. To overcome the resonance issue, this article proposes a harmonic resonance suppression strategy of the Vienna rectifier, which reshapes the closed-loop input harmonic impedance of the Vienna rectifier to be close to the characteristic impedance of the transmission cable by optimizing the traditional dual closed-loop control strategy of the Vienna rectifier based on the dq coordinate frame. The proposed suppression strategy can not only ensure the low ripple output voltage of the Vienna rectifier, but also suppress the background harmonic resonance amplification; thus, the power quality of the Vienna rectifier grid-connected current and the voltage at the point of common coupling will be improved effectively. Both the simulation and experimental results verify the effectiveness and correctness of the proposed strategy.

Compared to the dual-active-bridge converter, the dual-bridge series resonant converter (DBSRC) can widen soft-switching range. To further widen the soft-switching range and improve the circuit performance, a new DBSRC with dual tank based on DBSRC is proposed in this paper. This new converter features two resonant tanks and a tapped transformer, and it can perform better than the DBSRC by optimized tap coefficient x of the tapped transformer. Its operation principle, voltage gain, the soft-switching characteristics, and output power are analyzed in detail, and compared with the DBSRC. Results show that the proposed dual-tank topology has presented higher voltage gain, wider soft-switching region, and larger output power than the traditional DBSRC when the tap-coefficient x is selected reasonably. At last, a 1-kW prototype is built; experimental results verify the feasibility and advantages of the proposed dual-tank converter.

In this article, a new resonant three-port dc–dc converter with dual half bridges plus one full bridge is proposed, which can realize bidirectional power flow between each port. The resonant capacitor has three functions in the circuit: the resonant capacitor can improve the magnetic bias of half bridge converter, and therefore, the transformer saturation can be avoided; the resonant capacitor is introduced to make the inductor current close to sine wave, so that the eddy current loss of the transformer can be reduced; by setting the resonant frequency of one of the three ports equal to the switching frequency, the mutual interference between the other two ports can be reduced, so that the hardware decoupling effect is achieved. First, the operation principle of the three-port converter is analyzed. Second, the steady-state model of the converter is established, and the power transmission characteristics and soft-switching characteristics are analyzed. Third, the cause of the power coupling between ports is analyzed and the hardware decoupling method is proposed. Finally, the number of magnetic elements is reduced through magnetic integration, and the correctness of the theoretical analysis is verified by simulation and prototype experiments.

This paper proposes a new single-stage high-frequency link based single phase Solid State Transformer (SST) topology. Switching tube multiplexing is carried out to reduce the number of switching devices and the corresponding modulation strategy is given. An optimized hybrid control analysis for reducing the average conduction current of IGBT and inductor was performed. By switching different control modes, the average current value of the SST inductor is kept being the minimum, so as to reduce the conduction losses of IGBTs in the SST. As the core unit of the SST is the dual active bridge (DAB) converter, the proposed optimal control is based on extended phase shift control (EPS), instead of triple phase shift control (TPS), that makes the implementation method very simple. And the feasibility of topology and optimized hybrid control scheme is verified through experiments.