부산대학교 도서관

Supply of contingency power to critical loads during electrical power outages is a crucial requirement in uninterruptible power supplies (UPS) and micro-grid applications. Ultra-capacitor (UC) based Energy storage systems (ESS) are increasingly being deployed in such applications owing to their high power density, higher charge-discharge cycles, and longer lifetime characteristics. A bi-directional dc-dc converter is typically present in the ESS that operates in constant power mode to extract energy from the UC stack during the outage. In this paper, an optimal design of UC stack with power electronic interface is proposed that leads to minimum overall system cost of the ESS. Such a design approach also enhances the utilization of the ESS by optimizing the discharge percentage of the UC stack, for a given power and backup period specifications. An algorithm is presented that provides design parameters such as unit UC cell value, number of cells in series, number of strings in parallel and initial stack voltage that meets the specified energy requirement. Simulation studies and experimental results on a 70 Wh, 4 kW UC stack prototype are presented that validate the design and analysis carried out.