Analysis of generalized parallel-series ultracapacitor shift circuits for energy storage systems

Ultracapacitors have numerous advantages including a remarkably high energy density as compared to conventional capacitors, long life cycle, temperature stability, and require no maintenance, which make them a good candidate to replace batteries as energy storage devices in renewable energy applications. However, ultracapacitors, just like conventional capacitors, inevitably suffer from constant voltage drop during discharging, which may limit their energy utilization, increase the voltage ripple of the DC bus across which the capacitors are connected and consequently increase the VA stresses of the subsequent converter stage. To alleviate the above limitations, parallel-series ultracapacitor shift circuits are employed to improve the energy utilization and minimize the DC bus voltage ripple. Two generalized parallel-series ultracapacitor shift circuits are presented and analyzed, and some design considerations and optimization methods are discussed.