An Integral Droop for Transient Power Allocation and Output Impedance Shaping of Hybrid Energy Storage System in DC Microgrid

Power allocation in hybrid energy storage systems (HESSs) is an important issue for dc microgrids. In this paper, an integral droop (ID), inspired by the electrical characteristics of capacitor charging/discharging process, is proposed and applied to a cluster of energy storages (ESs) with high ramp rates. Through the coordination of the ID and conventional V-P droop, the transient power allocation in HESSs can be intrinsically realized in a decentralized manner. The high-frequency components of power demand can be compensated by the ESs with ID, whereas the ESs with V-P droop respond to the smooth change of load power. Additionally, the ID coefficient can be designed according to the nominal ramp rate of the ESs with slow response, which helps to extend the lifespan of the HESS. On the other hand, to easily assess the stability of the system feeding constant power loads, a minimum relative impedance criterion (MRIC) is developed. Based on MRIC, it is revealed that the proposed ID can shape the output impedance of the HESS and stabilize the entire system. The feasibility and effectiveness of ID are verified by both simulations and experimental results.