Analysis and Suppression of Active Power Backflow of Three-Phase Common DC-Bus Cascaded H-Bridge PV Grid-Connected Inverter During LVRT

Featured with the expandable modular structure, three-phase isolated cascaded H-bridge (CHB) inverters are capable of directly connecting to medium voltage power grid without bulky and heavy line-frequency transformer, which has outstanding advantages applied in large-scale photovoltaic (PV) power plants. However, different from traditional PV inverters, three-phase CHB topology is essentially composed of three single-phase CHB inverters. Affected by negative-sequence grid voltage, the active powers of some phases may inversely flow from ac grid to PV inverter during low voltage ride through (LVRT), resulting in that dc-bus voltages of all H-bridges in these phases continue to rise, and that system will shut down due to over-voltage protection. Aiming at this issue, this article studies the mechanism of active power backflow during LVRT, and deduces the quantitative relationship between active current and reactive current to be necessarily injected under different types of voltage faults and different degrees of voltage sags. In addition, combined with the relevant standard of LVRT, the feasibility of suppressing active power backflow by positive-sequence active power current injection is analyzed in detail. The validity of the proposed method is verified by simulation and experimental results.