An Isolated Bidirectional Single-Stage Inverter Without Electrolytic Capacitor for Energy Storage Systems

This paper presents a new isolated bidirectional single-stage inverter (IBSSI) suitable for grid-connected energy storage systems. The IBSSI contains no electrolytic capacitor. Therefore, its reliability and lifetime are improved in comparison with the well-known two-stage voltage source inverters without increasing the converter cost. In the IBSSI, a high-frequency transformer (HFT) is used to isolate the energy storage device from the grid. In addition, an active snubber is used to suppress the voltage spikes caused by the leakage inductance of the HFT and improve the converter efficiency. Boosting capability with a high-voltage ratio is also provided by the IBSSI. In this paper, a novel switching algorithm based on the space vector modulation is developed to maintain the volt-second balance on the HFT and generate three-phase balanced currents. The switching algorithm also provides soft switching conditions for the main switches, which are explained in this paper. Moreover, a single-loop control system is employed to regulate the input and output currents. The test results of the 3-kW prototype with the SiC power devices show that the IBSSI can inject/draw three-phase currents to/from the grid with a unity power factor and without using any ac current sensors in the discharging/charging mode.