Operation and Control Strategy of a New Hybrid ESS-UPS System

This paper proposes a hybrid energy storage system (ESS) that integrates an ESS with an online uninterruptible power supply (UPS). The power conversion system cost of the proposed hybrid ESS-UPS is reduced, and battery utilization is increased by using the energy of the integrated battery for both demand management and emergency power supply. Further, unlike the conventional online UPS, the proposed hybrid ESS-UPS is capable of supplying emergency power to two types of critical loads, voltage-frequency independent (VFI) load and voltage-frequency dependent (VFD) load. A seamless mode transfer algorithm of the ac-dc converter is proposed for supplying uninterruptible power to the VFD load as well. In addition, an autonomous and seamless mode transfer algorithm of the bidirectional dc-dc converter is proposed to minimize the transient across the dc link that could affect the power quality of the VFD load and the VFI load at the mode transition. To validate the proposed control scheme, experimental results from a 5-kW prototype are provided.