An Optimized LVRT Control Strategy of Cascaded Modular Medium-Voltage Inverter for Large-Scale PV Power Plant

Active power backflow is an inherent problem of three-phase cascaded H-bridge (CHB) photovoltaic (PV) grid-tied inverters during low-voltage ride through (LVRT), probably resulting in no balanced operating point of the system, and the inverter will be shut down and off-grid due to overvoltage fault. Aiming at this issue, this article first reviews the existing control methods and analyzes their limitations in application, that is, they cannot effectively suppress active power backflow under the scenario of deeper voltage drop and lower output power. On this basis, an optimized LVRT control strategy for cascaded modular medium-voltage PV power generation system is proposed, which adopts different control methods to suppress active power backflow under three types of asymmetric grid drop conditions: single-phase short-circuit fault with ground, two-phase short-circuit fault with ground, and two-phase short-circuit fault without ground, enhancing the adaptability of CHB PV grid-connected inverters to different output powers and different drop depths during grid voltage failure and then improving LVRT capability of the system. Finally, the effectiveness and feasibility of the proposed control strategy are verified by experimental results.