Abstract:
Microgrid is considered to be an effective means to embracing renewable energy and improve power quality. A microgrid based on the energy storage inverters requires that the energy storage inverters have the dual-mode function of grid-connection and standing-alone. Such a microgrid can realize the stability of the output frequency of the energy storage inverters and reduce effects on the inverter reactive power control performance caused by the line impedance in the case of an isolated island situation. It also achieve the precise control of the grid-connected active and reactive power in the case of grid-connection and the smooth switching between grid-connected and off-grid operations. To carry out the above multi-dimensional controls, this paper proposes a self-recovery droop control (SRDC) to improve the traditional self-recovery droop control. By introducing the deviation proportional feedforward, the frequency recovery transient performance of the energy storage inverter under a load disturbance in the isolated island mode is significantly promoted, with the frequency dynamic overlimit solved and the steady-state indifference control realized. The additional grid-connected power control loop is applied to realize the precise controls of the active power and reactive power of the inverters in the grid-connected mode and the smooth off-grid switching of a non-planned island. In this paper, the operation mechanism of the proposed improved self-recovery droop control and the whole operation process of grid connection and off-grid are analyzed in detail. Simulation and experimental results verify the effectiveness of the proposed method.