For the purpose of accurate power sharing and the state of charge (SOC) balancing of each energy storage unit (ESU), a novel secondary control scheme which regulates the virtual impedance is proposed herein. However, there is a coupling relationship between the power sharing and voltage restoration process. In this study, the secondary control scheme could eliminate that effect so that the virtual impedance was only influenced by the system distribution and no longer affected by system initialization and noise. The proposed secondary strategy contains four controllers, including the current sharing controller, the SOC balancing controller, the virtual impedance correction controller, and the local reference voltage controller. Through the proposed scheme, the ESU with a higher SOC will give more power, whereas the lower one will give less power. The steady-state analysis for the uniqueness of the droop coefficient is given. Besides, the small-signal analysis of the ESU was thereby performed to ensure stability. Finally, the effectiveness of the proposed strategy was verified through MATLAB/Simulink.
A Distributed Secondary Control Strategy for Power Sharing and Voltage Regulation by Multiple Converters in DC Micro-grids