Stability Analysis Of Islanded DC Microgrid Under DBS Hierarchical Control

The islanded DC microgrid has the advantages of simple operation, flexible installation location, and reliable power supply in the case of power grid fault. This thesis Aimed at the islanded photovoltaic-storage DC microgrid, adopts the DBS (DC Bus Voltage Signaling)hierarchical control strategy to achieve the objects of multi-source coordination, autonomous energy management and DC bus voltage stability. On this basis, the small-signal stability of DC microgrid is analyzed,the influence factors are discussed,and the passive damping method is adopted to improve the stability of DC microgrid. Specific contents are as follows:Firstly, the DBS hierarchical control strategy is designed. According to the DC bus voltage,the operation states of the islanded photovoltaic-storage DC microgrid are divided into 3 working modes. As it works in mode 1, the load is light, the bus voltage is high. In this mode, the photovoltaic unit works in droop control to limit the output power and maintain the stability of the bus voltage, and the battery unit works in the current limiting charging state to absorb the excess power and prevent overcharge. With the load power increasing, DC microgrid works in mode 2, the bus voltage drops. In this mode,the photovoltaic unit works in MPPT (Maximum Power Point Tracking) control to achieve the maximum power output, and the battery unit works in drooping charge state to absorb the excess power and maintain the stability of the bus voltage.In mode 3, the load is heavy, the bus voltage is low. In this mode, the photovoltaic unit also works in MPPT control to achieve the maximum power output, and the battery unit works in drooping discharge state to supply the insufficient power and maintain the stability of the bus voltage. Simulations and experimental results show that the DBS hierarchical control strategy is effective and feasible, and the DC microgrid can achieve stable operations and smooth switching among 3 modes.Secondly, caused by the interactions between paralleled multi-converters, the stability problem has become a hot topic in the field of the DC microgrid. Under the DBS hierarchical control strategy, the DC microgrid is a multi-point equilibrium system, and thus the stability analysis of the single control mode is not suitable for the DC microgrid. On the basis of determining the structure and control strategy of the islanded photovoltaic-storage DC microgrid, small-signal models under different control modes are established, and the expressions of the closed-loop impedance for every interfaced converter are deduced, and then the equivalent impedance models of DC microgrid operating in 3 modes are obtained. Based on the criterion of GMPM (Gain Margin and Phase Margin) impedance ratio, the small-signal stability of the DC microgrid in 3 modes is determined.Finally, due to that the negative resonance peak in the total load impedance with the DC microgrid operating in mode 2 is not conducive to the system stability, the passive damping method is adopted to cut the resonant peak by adding the passive damping circuit. And thus the load impedance characteristics can be improved, so as to the stability of DC microgrid.Meanwhile, this thesis also makes an analysis on the influences of filter parameters on the stability to guide the design of filter parameters. Simulation results show that the passive damping method can reduce the bus voltage and load current ripple, and thus improve the stability of the DC microgrid and the power supply quality.