Multilevel Energy Management System for Hybridization of Energy Storages in DC Microgrids

Hybridization of energy storages (ESs) with different ramp rates helps minimization of system bus voltage variation and extension of ESs lifetime in dc microgrids. Hybrid ES system (HESS) control is normally realized with centralized coordination. In this paper, HESS distributed control, which is independent of the communication link, is proposed to enhance system reliability. All ESs are configured as slack terminals to regulate system bus voltage with droop control. System net power decomposition and ESs power sharing are realized with localized low-pass filter (LPF) applied to ESs with low-ramp rates. The relationship between LPF cut-off frequency and ES ramp rate is elaborated in detail. However, bus voltage deviation and power tracking errors are the main drawbacks of HESS distributed control. Multilevel energy management system (EMS) is thus proposed to enhance system control accuracy. HESS distributed control is scheduled as the primary control. Bus voltage restoration and power sharing compensation are applied in secondary control to eliminate the voltage deviation and power tracking errors, respectively. In tertiary control, autonomous state of charge (SoC) recovery is implemented to limit SoC variation of ES with high-ramp rate. A lab-scale dc microgrid is developed to verify the proposed multilevel EMS for HESS control.