An effective control algorithm for hybrid superconducting magnetic/battery energy storage systems employed in DC microgrids

DC microgrids are exposed to diverse disturbances that can cause DC bus voltage instability and power quality issues. Renewable volatility and fault occurrence on the power lines are considered as high-impact variations, which jeopardize the resilience of the microgrid. Therefore, the deployment of a battery storage system can improve the robust operation of the microgrid. However, the frequent cyclic activity and the rapid power variations have detrimental effects on battery's lifespan. This study proposes an effective control scheme for a hybrid superconducting magnetic energy storage (SMES)/battery system to protect the battery from rapid lifetime fading and to improve the DC bus voltage stability. Under the proposed method, SMES is the prior-responsive device, while battery's mode of operation (i.e., idling, charging, discharging) depends on SMES current value. Specifically, battery is kept idle as long as SMES current is within a user-defined dead band and is triggered as soon as SMES current deviates from that. In such cases, battery enters smoothly either the discharging or the charging mode following a different droop curve for each mode of operation. Initially, a comparative analysis between the proposed scheme and a battery-only system under two different scenarios is presented, namely PV fluctuation and DC fault. Afterwards, the presented method is compared with a hybrid SMES/battery system controlled with a preceding residual SMES energy-based technique, under a fluctuating PV generation. The simulation results demonstrate the superiority of the novel strategy.