For the islanded dc microgrids, the photovoltaic (PV) generators and storages are the main components to provide power and maintain the system stability. To avoid the dc bus voltage rise, the overcharge and overdischarge of storages, the coordination between generations and storages is critically important. In this paper, a decentralized control method is proposed to realize the generation-storage coordination based on the dc bus voltage signaling. First, a novel state-of-charge (SOC) based droop control strategy is designed for storages to provide dc bus voltage support and balance their SOCs, which can prevent the overuse of a certain storage. Second, an adaptive power control strategy is applied to PV generators, which can seamlessly regulate the output power of PV generators off maximum power point tracking mode according to the dc bus voltage. Combining these two strategies through the dc bus voltage signaling, the generation-storage coordination can be realized. Furthermore, the stability of the proposed control method is also analyzed. All the conclusions are verified by the real-time hardware-in-loop (HIL) tests.

In the hybrid ac-dc microgrid, the ac and dc subgrids are connected by bidirectional power converters (BPCs) that play an important role in the load power sharing and power interaction between the ac and dc subgrids. The coordination control and circulating current suppression for the parallel BPC system are very challenging. In this paper, a decentralized coordination control method is proposed for parallel three-phase BPCs, which can suppress the circulating currents, realize proper power interaction, and achieve overall load power sharing in both the grid-connected mode and the islanded mode. The performance of the proposed control methods is verified by the real-time hardware-in-loop tests.

In the grid-connected mode, the dc microgrid is connected to the utility by parallel bidirectional power converters (BPCs), which provide voltage support for the dc microgrid. The coordination control for these parallel BPCs is very challenging. First, for the parallel BPCs, there is circulating current among them, which limits the total system capacity and may damage switching devices. Second, taking the constant power load into consideration, the performance especially the stability of the dc microgrid is degraded if the conventional i _dc - v _dc droop is adopted to share the power. Third, the voltage restoration should be faster for the better quality of dc voltage, and the restoration strategy should be scalable to the changes of the dc microgrid topology for the better flexibility. For these problems, this paper proposes a decentralized control method for the parallel BPCs, which can make the BPCs operate in coordination and provide voltage support for the dc microgrid well. Furthermore, the reliability of the proposed control method is illustrated by the stability analysis. All the conclusions are verified by the real-time hardware-in-loop tests.

Recently, the stability issues when renewable energy sources are connected to the weak grid have obtained great attention. The popular view thinks that the grid impedance is adverse to the system stability. However, in this paper, it is first found that for the photovoltaic (PV) generator, the grid impedance has different influence on the system stability depending on the frequency range when the complete model of the PV generator is taken into consideration. That is, the increase of grid impedance can suppress the low-frequency (<;50 Hz) oscillation, whereas the increase of grid impedance degrades the high-frequency (>300 Hz) stability. Concretely, for the low-frequency range mainly dominated by the power loop and voltage loop of the PV generator, the describing function method is adopted to overcome discontinuity and nonlinearity of the power loop. Then, the low-frequency dynamics such as power oscillation can be analyzed accurately. For the high-frequency range mainly dominated by the phase-locked loop and current loop of the PV generator, the impedance analysis method is adopted to reveal the high-frequency instability mechanism. Through these analyses, the different influence of grid impedance on the low- and high-frequency stability of PV generators can be clearly distinguished. All the conclusions are verified through the real-time hardware-in-loop tests.