Study on Optimization Design Method of Energy Storage Device for Dynamic Voltage Restorer Based on Ultra-Capacitor

Ultra-capacitor is ideal for use as energy storage device for active distribution network, for instance, in the field of electric vehicle, uninterrupted power supply and dynamic voltage restorer, etc. When the bus voltage rises, the number of ultra-capacitor units increases, which in turn leads to higher costs. Changing the ratio of series-coupled transformer can reduce the injection voltage of the inverter and then reduce the bus voltage, which means that the number of ultra-capacitor units decreases. However, the ultra-capacitor discharge current will increase, which leads to higher withstand current of switching device. The design of transformation ratio need to be carefully considered. In this paper, with the relationship between the transformation ratio and the number of units deduced from the energy formula and the power formula of ultra-capacitor discharging, it is easy to find an appropriate transformation ratio to achieve the optimal performance. The feasibility of the design method is validated by the experimental results with a 200kVA DVR device.