Low Voltage Ride Through Control Strategy for Cascaded H-Bridge Photovoltaic Grid-Connected InverterBased on Asymmetric Current Injection

The cascaded H-bridge (CHB) photovoltaic grid-connected inverter shows significant advantages in the field of photovoltaic power generation due to its modular structure, multilevel output characteristics, and independent maximum power point tracking control ability. However, when an asymmetrical voltage dip fault occurs in the power grid, there is an inherent problem of active power backflow in the system. The DC side capacitors will continuously absorb the back flowing energy, resulting in capacitor voltage pumping. Eventually, it may trigger the over-voltage protection of the inverter and cause the inverter to stop operating and disconnect from the grid. The current low voltage ride through (LVRT) standard requires that during a grid fault, the inverter should inject asymmetrical current into the grid to achieve dynamic voltage support. But this requirement will exacerbate the active power backflow problem of the CHB photovoltaic grid-connected inverter. Therefore, this paper deeply analyzes the mechanism of active power backflow in the CHB photovoltaic grid-connected inverter when an asymmetrical voltage dip fault occurs in the grid and asymmetrical current is injected. A LVRT control strategy for the CHB photovoltaic grid-connected inverter based on asymmetrical current injection is proposed, which can effectively suppress the active power backflow. Finally, the effectiveness and feasibility of the proposed control method are verified by building a CHB simulation platform.