A Power Allocation Strategy for Hybrid Energy Storage System with Virtual Inertia Support

The droop control is widely used in the power allocation for hybrid energy storage systems (HESS) comprising both battery and super-capacitors (SC). By modifying the droop coefficients in the control laws for both the battery energy storage systems (BESS) and SC, it is possible to achieve power-sharing among different energy storage components. However, the inertia of the DC microgrid can become quite low due to the modified droop control, which may cause the stability issue. This paper proposes a new power allocation strategy for HESS, where the concept of virtual capacitor is introduced in the SC control law to increase the system inertia while BESS control law remains unchanged. The coordination of BESS and SC control laws can ensure BESS and SC work as an second order low-pass filter and second order high-pass filter, respectively. Therefore, the low-frequency power is allocated to BESS while the high-frequency power is allocated to SC. Further, the proposed control strategy achieves high system inertia and better power allocation performance compared with the modified droop control. Three different equivalent load types are used to verify the proposed control strategy.