Consistency Algorithm-Based SOC Balancing Scheme of Retired Non-Equal Capacity Lithium Battery in Virtual Synchronous Generator Controlled Microgrids

Retired electric vehicle lithium batteries can be used in microgrids to fully utilize the remaining capacity of lithium batteries. Due to differences in service life and usage environment, the state of charge (SOC) and capacity of retired lithium battery packs in microgrids are difficult to be completely consistent. SOC balancing is an important means to extend the lifespan of retired lithium battery (RLB). However, the traditional virtual synchronous generator (VSG) control scheme and the existing VSG-based SOC balancing scheme fail to achieve the SOC balancing of non-equal capacity RLB in VSG-controlled microgrids. Therefore, a VSG control-based SOC balancing scheme for non-equal capacity RLB is proposed. This paper analyzes the influence mechanism of traditional VSG control and RLB capacity difference in SOC balancing. The influence of capacity on SOC balancing can be eliminated by correcting the traditional VSG control parameters and line impedance. Then, a SOC balancing factor is added to the VSG to regulate the output active power of inverter and achieve SOC balancing. The consistency algorithm is introduced in the process of calculating the SOC balancing factor, which realizes the SOC balancing of the non-equal capacity RLB under the premise of fewer communications and guaranteed frequency quality. In addition, a small signal model for VSG is established to analyze the influence of control parameters on system stability. Finally, the simulation and experimental results verify the effectiveness of the proposed scheme. Compared with the traditional SOC balancing scheme based on VSG, the proposed SOC balancing difference is only 0.0023, the frequency deviation is only 0.02Hz, and the energy loss is reduced by 2.13%, which reflects the advancement of the proposed scheme.