Five-level bidirectional converter for renewable power generation system

A novel five-level DC/AC bidirectional converter is developed and applied for interfacing renewable energy into the grid. The converter has reduced switching losses, voltage stress, harmonic distortion and electromagnetic interference caused by switching operation of power devices. The bidirectional operation feature allows it to interface with parallel dc-dc PV optimizers that requires a start-up dc link voltage of 200-250Vdc. Two DC decoupled capacitors, a high-frequency three-level converter, a grid-frequency unfolding bridge and a filter compose the proposed converter. The high-frequency converter offers a three level positive voltage output, while the unfolding bridge is responsible for switching the flow direction every half grid cycle. With the proposed control strategy, the converter operates via two modes, i.e. the inverter and rectifier mode. In inverter mode, the output current of the converter generates a sinusoidal current in phase with the grid voltage, while in rectifier mode, grid voltage acts as the source and dc link supplies the PV optimizer with a threshold voltage to avoid false tripping of the optimizer. A hardware prototype is further developed to verify the performance of the topology and control strategy.