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With the serious environment pollution and power crisis, the increasing of renewable energy resource (RES) becomes a new tendency. However, the high proportion of RES may affect the stability of the system when using the conventional droop control with a fixed droop coefficient. In order to prevent the power overloading/curtailment, this paper proposes an adaptive fuzzy droop control (AFDC) scheme with a P-f droop coefficient adjustment to achieve an optimized power sharing. The droop coefficient is adjusted considering the power fluctuation of RES units and the relationship of power generation and demand, which can realize the stability requirements and economic power sharing for the islanded microgrid. What is more, a secondary control is considered to restore the frequency/voltage drop resulting from the droop control. The proposed strategy improves the stability and economics of microgrid with a droop-based renewable energy source, which is verified in MATLAB/Simulink with three simulations which are variations in load, in generation and in load and generation simultaneously. The simulation results show the effectiveness of the proposed control strategy for stable and economic operation for the microgrid.

Although the deployment of alternating current (AC)-busbar plug-in electric vehicle (PEV) charging station with photovoltaic (PV) is a promising alternative, the interaction among subsystems always causes the instability problem. Meanwhile, the conventional generalized Nyquist criterion (GNC) is complex, and it is not suitable for the design of the AC system. Therefore, this paper proposes a modified infinityone-norm (MION) stability criterion based on the impedance method to assess the stability of the foresaid charging station. Firstly, the typical structure and operation modes of the charging station are studied. Furthermore, each subsystem impedance matrix is built by small-signal method, and the MION stability criterion based on impedance method is proposed to assess the charging station stability. Compared with the previous simplified stability criteria based on the norm, the proposed criterion has lower conservatism. Furthermore, the design regulation for the controller parameters is provided, and the stability recovery way is provided by connecting the doubly-fed line and energy storage equipment, which are selected based on intermediate variable, i. e., short-circuit ratio (SCR). Finally, the effectiveness and conservatism of the proposed stability criterion are validated through simulation and experimental results.

Three-phase unbalance issue can cause adverse impacts on the power quality of the solid-state transformer (SST) based AC/DC hybrid microgrid. In this paper, an unbalanced power control strategy for cascaded dual active bridge (DAB) converter is proposed. With the strategy, the transmitted power of each DAB module in each phase is evenly distributed in terms of the three-phase unbalance degree of the three-phase AC grid. Thus, the idea of unequal power sharing in cascaded DAB can be applied to mitigate the three-phase unbalance from the perspective of power control. Moreover, this paper proposed a three-phase unbalance power mitigation algorithm based on the unbalanced power control strategy for the cascaded DAB. The proposed strategy can help the SST realize the function of mitigating three-phase unbalance when it transmits energy to the DC microgrid. Thus, the proposed control strategy can mitigate the three-phase unbalance on the premise that the power demand of the system is met. Finally, the simulations and the experiment results have verified the effectiveness and correctness of the proposed control method.INDEX TERMS Solid-state transformer, microgrid, power control, three-phase unbalance regulation.