Hybrid Electric Springs for Grid-Tied Power Control and Storage Reduction in AC Microgrids

The renewable generations (RGs) are conventionally interfaced with the utility grid through battery energy storage systems (BESS). In this cascaded configuration of the grid-connected inverter, battery, and RG, the power of the battery is passively governed by the power difference between the RG and the grid. This compromises the state-of-charge (SoC) control of the battery and increases the storage capacity. To address this issue, a hybrid electric spring (HES), which is integrated with the RGand noncritical load (NCL), is proposed in this paper for the grid-tied power control and reduction of battery storage capacity in ac microgrids. Such an integrated configuration enables a flexible control of the power flow among battery, NCL, and grid. On top of that, the proposed HES can achieve an extended operating region of gridtied power control compared with the conventional BESS and the existing electric springs. The operating principle, steady-state analysis, and control design of the HES are discussed. The functions of the grid-tied power control and battery SoC control are verified experimentally and through simulations.