A Nonlinear Droop Control for Supercapacitor-Hybrid Uninterruptible Power Supplies in DC Microgrids

Fuel cell-supercapacitor and battery-supercapacitor hybrid energy storage systems (HESS) are widely used in uninterruptible power supplies. Integral droop control is employed for managing supercapacitors, while linear droop control methods have been utilized for controlling fuel cells and batteries. However, linear droop control has excessive energy use of supercapacitors and low load-sharing performances. To address these issues, nonlinear droop control can be implemented. Nevertheless, selecting the appropriate nonlinear droop gain is crucial, as it can impact the system stability and dynamic performance of the HESS. Therefore, this conference paper proposes a nonlinear droop gain design method. By utilizing the proper nonlinear droop gain, the proposed method enhances load-sharing performance while maintaining high stability and dynamic performance. The paper demonstrates the effectiveness of the nonlinear control strategy through theoretical analysis and experimental validation.