Control strategy of automatic charging/discharging of hybrid energy storage systems in DC micro-grid island mode

Aiming at stabilizing the DC bus voltage and optimizing energy storage, this paper presents a control strategy of hybrid energy storage systems in DC micro-grid based on voltage droop method. The control strategy makes use of the super capacitor not only to compensate the high-frequency components of bus power according to the DC bus voltage, but also indirectly to compensate low-frequency components of it through the energy supplement of the batteries connected with the super capacitor, finally to achieve the smooth control of hybrid energy storage and automatically to switch operation mode in terms of DC bus voltage, super capacitor voltage and the state of charge (SOC) of battery. The experiments show that the control strategy can automatically adjust the output of the energy storage system, and maintain light fluctuation of DC bus voltage near to rating value, effectively reduce the times of its charging and discharging and extend its service life.